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Chapter 1

Introduction

1.0.1 Framework

Energy is one of the most urgent issues in modern society. It is crucial in every kind

of application. Energy exists in several forms, although often it is not available in

the required one. Thus, suitable devices for energy conversion are needed. Galvanic

cells, or in common usage batteries, are able to convert chemical into electrical

energy, that can be used to perform work. If this process is reversible they are

defined secondary or rechargeable batteries, in opposite to primary ones. [2–6]

Rechargeable lithium ion batteries are one of the most frequent types of galvanic

cells employed in portable electronics because of their superior specific power and

energy. Nowadays commonly used rechargeable lithium ion batteries exploit liquid

electrolytes. However, these devices are characterized by several inherent drawbacks,

like the need of separators that limits their downscaling and the high risk of leakage

that makes these systems prone to fire and explosion. All solid-state batteries could

overcome these problems and find applications in stand-alone systems including

implantable bioengineering.[1–6]

Downscaling of the electrolytes o↵ers higher energy densities, flexibility and longer

lifetime. The reduction of layer thickness leads to higher output power thanks to

smaller di↵usion distances and hence faster charge/discharge process. Thin film

batteries already find application in aerospace and medical fields. These systems

have been found to sustain 10000 cycles without power output or voltage drops.

However, a lot of research is going on to resolve some important drawbacks of these

systems, like the loss of charge retention capacity, loss of material due to volumetric

changes during charge-discharge and the large dependence on surface roughness and

on geometry. [1–6]

1

CHAPTER 1. INTRODUCTION 2

1.0.2 Objectives

Galvanic cells can be schematized in their simplest form as cathode/electrolyte/anode

stacks. The two electrodes are connected electrically by means of an external circuit.

Consequently, the electrolyte should be characterized by low electronic conductivity

to avoid short circuit. In addition a large ionic conductivity of the electrolyte is

crucial to achieve high performance.

This thesis work will aim to the development of novel inorganic thin-film elec-

trolyte materials. Both glasses (e.g. Lithium Phosphorus OxyNitride or, equiva-

lently, LiPON) and crystalline materials (e.g. spinel electrolytes) are under inves-

tigation at the Electrochemical Energy Storage group in IMEC. The research will

be focused on the first category. This class of materials o↵ers: 1) a wide range of

possible compositions 2) straightforward thin film formation and non-flammability

3) higher ionic conductivities than the corresponding crystalline materials as a con-

sequence of the open structure of the glass 4) single ion conduction, which results

in fewer side reactions and wide electrochemical windows.

The main goals of the project are:

1. The creation and characterization of defect-free solid-state LiPON films by

tuning deposition temperature. The dependence of several materials proper-

ties (composition, ionic conductivities, electrical and electrochemical perfor-

mance) on this parameter will be determined. A major focus will be given

to the investigation of the ionic conductivity by electrochemical impedance

spectroscopy (EIS) since this property is a key feature to achieve high perfor-

mance.

2. The study of the ultrastable glass formation capabilities of LiPON at spe-

cific deposition temperatures towards improving materials properties. Several

types of glassy materials, including both metallic and organic substances, are

known to form highly stable(HS) glasses when deposited from the vapour phase

at a deposition temperature around 0.8-0.9 Tg. This phenomenon should re-

sult in higher glass transition temperature of the material because a larger

amount of heat must be provided to displace the molecules from their glassy

configuration. These glasses should be characterized by excellent energetic

and kinetic stability. A positive outcome could lead to significant advances

in the use of LiPON glassy material, which for the moment is characterized

by extremely good electrochemical window and electronic insulation, but also

by a relatively low ionic conduction in comparison to other electrolytes. The

use of highly stable glassy thin films is expected to change the e↵ective ionic

resistance, which in turn will influence the performance of the material.

High temperature deposited LiPON electrolytesfor thin film solid-state batteries

Francesca Criscuolo

CHAPTER 1. INTRODUCTION 3

3. The analysis of the morphology and structure of these complex oxides, whose

complete understanding is still missing, by means of scanning electron mi-

croscopy, Raman spectroscopy and X-ray di↵raction techniques. For all the

aspects described above comparisons with LiPO samples will also be made to

have a complete picture.

4. The testing of these LiPON electrolyte materials deposited at di↵erent tem-

perature in half stacks and complete battery devices in terms of energy density,

cycle life and safety. Half-stack made with Li2

MnO4

(LMO) will be produced

for this purpose because of the excellent properties of this cathode material.

The complete device will be obtained by depositing metallic lithium as anode

material on top of half-stacks to obtain a LMO/LiPON/Li battery. The in-

creased density and thermal stability of eventually formed HS glasses may be

beneficial in terms of performance upon cycling.

To our knowledge, this is the first time that the properties of LiPON deposited are

studied systematically in di↵erent aspects. Moreover, also the HS glass formation

capability has never been investigated for this class of materials.

1.0.3 Thesis outline

Chapter 2 gives a brief introduction to the working principles of lithium ion batteries

and to the materials properties required to achieve high devices performance.

Chapter 3 provides some theoretical background on phenomena occurring in amor-

phous materials, with particular focus to the concept of HS glasses.

Chapter 4 describes the basic principles behind the experimental techniques directly

exploited by the author in this work. All the experimental techniques whose data

were collected indirectly are described in Appendix A.

Chapter 5 is the central part of this thesis and is dedicated to results discussion.

Chapter 6 is used for conclusions and speculations on future advances.


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Chapter 2

Lithium-ion batteries

2.1 Importance of lithium ion batteries

Smart electronic and wireless devices play an increasingly important role in everyday

life. These systems find applications in several fields, including smart medicine,

ambient technology and building control. The availability of an e�cient power

supply and storage is crucial to ensure proper functionality. Nevertheless energy

storage is one of the fundamental issues in the present century. The climate change

and the depletion of fossil fuels gave a significant boost to research in this field,

which could lead to wider applicability of renewable energy sources (wind, solar

power). Consequently, technological breakthroughs in energy storage could have a

strong impact on actual society. [2, 3]

Nowadays batteries are the most used systems to collect energy. Lithium ion

batteries thanks to their high operating voltage and exceptional energy density gov-

ern the market, despite their relatively high cost. They are particularly interesting

for applications in which volume and weight are crucial, like bioengineering and

portable electronics. Moreover, they show superior cycle life and low self-discharge

issues (less than 10% in 30 days). In fig. 2.1 the gravimetric and volumetric energy

densities of di↵erent battery technologies are compared. [7–10]

Commonly used rechargeable lithium ion batteries exploit liquid electrolytes.

Consequently, high risk of leakage and formation of dendrites of lithium are present,

which make the device prone to explosion and fire. In addition, liquid outflow could

also a↵ect neighbouring microelectronic elements. Moreover, constraints concerning

the device design and size must be taken into account due to available electrolytes

and separators. The advent of solid-state batteries surmounts these problems. The

non-flammability and excellent safety of these devices provide noticeable environ-

mental compatibility and solution of security issues. Moreover, the possibility of

creating thin film devices makes them suitable for applications in microelectrome-

4

CHAPTER 2. LITHIUM-ION BATTERIES 5

Figure 2.1: Comparison among di↵erent types of batteries in terms of volumetric and

gravimetric energy density. [10]

chanical system (MEMS) and implantable medical devices. A comparison between

the structures of lithium ion batteries with liquid and solid-state thin film elec-

trolytes is given in fig. 2.2. Each layer is deposited from the vapor phase. A

protective coating is employed to protect the lithium present in the anode from the

atmosphere. It is evident that the absence of separators makes the downscaling

of thin film solid-state batteries much easier. Other advantages include the larger

storage capacity, the reduction in the net volume and weight of the battery, minimal

self-discharge and wear and the large uniformity of the output voltage. However,

solid-state batteries also face many challenges, such as the high ionic resistance at

room temperature, lower power density and high manufacturing costs. In this the-

sis particular attention will be given to inorganic glassy materials, in particular to

lithium phosphorus oxynitride (LiPON) electrolytes, which seem the most promising

because of their wide stability windows and extremely low electronic conductivity.

[2–5, 8, 9, 11, 12]

2.2 Working principle

Batteries can be classified in two categories. Primary batteries are characterized

in general by large capacity, but they cannot be recharged. They include lithium

metal and alkaline batteries. On the contrary, secondary or rechargeable batteries


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(a) (b)

Figure 2.2: Comparison between the structures of lithium ion batteries with liquid (a) and

solid-state thin film (b) electrolytes. [12, 13]

can undergo several charge-discharge cycles. Lithium ion and lead-acid batteries are

part of this category. The first ones have attracted large attention by the scientific

community because of their higher lifetime and large energy density due to the high

redox potential (-3.03 vs SHE) and to the lightness of lithium(density 0.53 g/cm).

[1, 2]

Figure 2.3: Working mechanism of a lithium ion battery. [10]

An example of lithium ion battery system is shown in fig. 2.3. The electrolyte

material is sandwiched between two electrodes that are able to store lithium ions

in their structures. During the charging process lithium ions are forced through

the application of an external potential to move from the positive to the negative

electrode. Electrons are produced at the positive electrode and flow in the external

circuit to the opposite side in order to maintain charge neutrality. During the

discharging process the migration is reversed. The electrons formed at the negative

electrode can be used to perform electrical work in an external circuit before they


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recombine at the positive terminal. [1, 2, 10, 13]

Three main types of lithium ion storage are possible in the electrodes: alloying,

conversion and intercalation. The first mode enables the achievement of incredibly

large capacity, up to 4212 mAh/g. However, large stresses due to volume changes

generate during the process and cause a fast reduction of the functionality with

increasing cycles number. An example of this problem is shown in fig. 2.4. This

storage mode is characteristic of metals like Pt, Zn or Al or of semiconductors,

in general Si. Conversion is typical of nickel oxide. This mechanism is based on

the reaction between lithium and the electrode material to form lithium oxide and

nickel. The reaction is at least partially reversible, although a large energy barrier

is present that causes a large voltage drop between charge and discharge processes.

Capacity up to 700mAh/g can be reached. The most used lithium ion storage mode

is intercalation, i.e. the hosting of ions in the interstices of the electrode crystal

structure. In this case the achievable capacity is one order of magnitude lower,

but high potential versus lithium and a smaller volume expansion are possible. The

occupation of the interstitial sites can exploit a solid solution mechanism or can lead

to a phase transformation. In the first case a sloped voltage will result depending

on the state of charge of the electrode. On the contrary a flat voltage is obtained

for phase transformation. Example of intercalating electrodes are LiM2

O4

(LMO),

LiCoO2

(LCO), Li4

Ti5

O12

(LTO) and graphite. For more details see section 2.3. [1,

2, 10]

Figure 2.4: Volume expansion associated to alloying with lithium for a silicon negative

electrode. [14]

In this work LiM2

O4

was employed as cathode, LiPON as electolyte and metallic

lithium as anode. The half reactions occurring at the electrodes surfaces during


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discharge are

Li1�xMn

2

O4

+ xLi+ + xe� ! Li1

Mn2

O4

(2.1)

Li ! Li+ + e�. (2.2)

The overall reaction is

Li1�xMn

2

O4

+ xLi ! Li1

Mn2

O4

(2.3)

During this process the oxidation number of manganese goes from +4 to an average

of 3.5, which means that a mixture of +3 and +4 values is obtained. The opposite

reaction occurs during the charging process. [1]

It is important to notice that in current batteries discussions the positive electrode

is typically defined the cathode, while the negative one the anode. However, this

nomenclature is correct only during discharge, because during the charging process

the redox reactions are reversed.

The choice of the electrode materials is crucial since they determine the lithium

ion charge capacity Qtot that can be stored in the battery and the cell output voltage.

In the charged state, a large voltage di↵erence is present between the positive and

negative electrodes. The cell output voltage can be expressed as:

Vcell = �Velectrodes(Q)� ⌘tot(I,Q)� IRint (2.4)

where I is the cell operating current, ⌘tot(I,Q) is the overpotential over the two

electrodes and IRint is the drop due to the internal resistance of the cell. The first

term in eq. (2.4) �Velectrodes(Q) gives the open circuit voltage of the system and

can be expresseed in terms of the di↵erence between the electrochemical potential

of the two electrodes:

�Velectrodes(Q) =(µc � µa)

nF(2.5)

where F is the Faraday’s constant (F = 96485C/mol) and n is the number of

electrons transferred during the reaction (in this case 1 since for each lithium ion

only 1 e� is exchanged). [1, 15–18]

The passage of an electrical current in the cell causes deviations from the equilib-

rium in the electrode reactions. This phenomenon is called polarization. It causes

an overpotential ⌘tot(I,Q), that is a di↵erence between the polarized (Epa and Epc)

and the equilibrium (E0a and E

0c) electrode potentials. Typically in a galvanic cell

the anodic potential is less negative (⌘a = Epa � E0a > 0), while the cathodic po-

tential is less positive (⌘c = Epc � E0c < 0), resulting in lower energy supply than

what predicted by thermodynamics. With increasing current density the overpo-

tential becomes larger. Polarization includes three di↵erent types of contributes,

corresponding to the kind of resistance that limits the reaction rate:


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• activation polarization (included in ⌘tot(I,Q) in eq. (2.4)), that is realted to

kinetics of charge transfer

• concentration polarization (included in ⌘tot(I,Q) in eq. (2.4)), that is related

to the kinetics of mass transfer

• internal resistance (ohmic) polarization (last term in eq. (2.4)). [1, 15–18]

⌘tot(I,Q) is thus the sum of several contributions. In particular, it includes the

activation polarizations due to charge transfer resistance and the concentration po-

larizations, both at the anode and at the cathode:

⌘tot(I,Q) = [(⌘ct)a + (⌘ct)c] + [(⌘conc)a + (⌘conc)c] (2.6)

The activation overpotential is the potential di↵erence with respect to the equilib-

rium value needed to produce current due to the presence of an activation energy

barrier for the redox reaction to occur. More specifically, it refers in general to

the activation energy for the charge transfer at the electrode-electrolyte interface,

which is typically the slowest step in the process. For this reason the activation

overpotential is also called charge transfer overpotential. Tafel equation expresses

the dependence of the activation overpotential with the current density:

⌘act = ±�logi

i0

(2.7)

where the tafel constant � is given by:

� = log2.3RT

↵nF. (2.8)

The concentration overpotential is due to the depletion of charge-carriers at the

electrode surface with respect to the bulk electrolyte when the reaction is su�ciently

rapid. Therefore, a concentration gradient in the electrolyte is formed in a region

close to the electrode surface. This is defined a boundary layer. In this case the

reaction rate is determined by the mass transport, in particular by the easiness of

the charge carriers to reach the electrode surface. It can be demonstrated that the

concentration overpotential ⌘conc can be expressed by the following equation:

⌘conc =2.3RT

nFlog1� i

iL(2.9)

where il is the limiting current density, i.e. the one that is reached when the trans-

port of charge carriers to the surface is maximum. Both activation and concentration

e↵ects isolate the electrode from the electrolyte, limiting the charge transfer between

the two. Consequently, the reduction potential and the reaction rate diminish, while

a part of the electric current is converted into heat, instead of contributing to the


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electrochemical work. The generated heat can alter the electrode material, for ex-

ample by accelerating dendrites formation. [1, 15]

The IR drop term in eq. (2.4) includes the e↵ects of electric resistance of electrodes

and current collectors, electrolyte ionic resistance and the possible formation of films

at the electrode surface. In the case of liquid electrolyte Rint is mainly due to

the presence of a solid-electrolyte interface (SEI), while in solid-state systems the

most important contribution to the impedance response is related to the low ionic

conductivity of the electrolyte itself. [1, 15]

The dependence of the output potential on the activity of the lithiated and

delithated form of the cathode can be obtained by using Nernst equation for the

cell. By using the reaction in (2.3) we obtain:

�Ecell = E0

cell �RT

xFln⇣ aLiMn2O4

aLi1�x

Mn2O4

⌘(2.10)

where

E0

cell = E0

Li1�x

Mn2O4/LiMn2O4� E0

Li. (2.11)

If we define the lithiated fraction as X and hence the delithiated one as 1-X, it is

possible to rewrite eq. (2.10) as:

�Ecell = E0

cell �RT

xFln⇣ X

1�X

⌘. (2.12)

Eq.(2.12) shows that during charging, when the lithiated fraction X increases, the

output voltage lowers since the argument of the logarithm becomes larger. The

resulting discharge curve is given in graph 1. Similar discharging curves will be

found in this work.

In summary, the two electrodes fix the maximum lithium storage capacity and

output voltage of the battery, hence determining the energy of the battery:

E =

Z t

0

Pdt =

Z t

0

dQ

dt�V dt = Qtot�V (2.13)

where t is the time required for full charging, �V is the voltage and Qtot the total

charge flowing during charging. It is important to notice that the potential di↵er-

ence between the two electrodes is dependent on the state of charge of the electrode.

The energy is typically expressed in Wh/g with respect to battery or electrode

weight, thus obtaining a specific energy, or as an energy density in Wh/cm3. From

equation (2.13) it is clear that the performance of the battery is described by the

output potential and the storage capacity (expressed in mAh, corresponding to a

charge of 3.6 C). Other important parameters are cycle life (expressing the number

of charge-discharge cycles that the system can undergo before loosing its capacity),

self-discharge(a reduction of capacity over time even in the absence of battery usage)


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Graph 1: Discharge curve showing the dependence of the potential on the fraction X of

lithiated LMO. The trend was computed using Nernst equation.

and cost. It is evident that the desired characteristics will depend on the specific

applications. for instance primary batteries could be employed for implantable bio-

engineering or smart cards, while aerospace devices would need functionality up to

104 cycles. [1, 8, 19]

Multi-cells systems can be used to obtain the desired voltage or capacity, but at

the expense of large heating and lower energy density. [1]

2.3 Materials

2.3.1 Historical overview of lithium ion batteries

The advances in rechargeable lithium ion batteries found their major boost from

three di↵erent fields: portable electronics, electric vehicles and implantable medicine.

The first solid-state lithium ion batteries were not rechargeable. Only in 1972 the

first primary lithium ion device was developed. This system was made of a metallic

lithium anode, a metal-iodide cathode and a lithium-iodide electrolyte. In the same

period several inorganic intercalation compounds were discovered and the mecha-

nism underlying this phenomena was deeply investigated. Since that moment sev-

eral materials have been proposed for their use in solid-state batteries. In 1979


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Goudenogh used for the first type LCO as intercalating cathode material. Four

years later he discovered, together with Thackeray, the intercalation properties of

spinel structured LMO. [6, 10]

The idea of solid-state thin film devices was introduced by Hitachi Co. in Japan in

1982. They employed Li3.6Si0.6P0.4O4

as amorphous electrolyte, TiS2

and metallic

lithium as positive and negative electrode materials, respectively. In the meanwhile

dry polymer electrolytes were investigated to substitute the liquid ones in the so-

called Li solid polymer electrolyte (Li-SPE) batteries. However, this technology

needed very high temperatures. Li hybride polymer electrolytes (Li-HPE), batteries

were proposed as alternatives. These devices were based on a polymer matrix swollen

in a salt and a liquid solvent. The major issue was related to hazardous lithium

dendrites formation. [6, 10]

In 1991 Sony introduced on the market the first commercial lithium ion secondary

battery based on C, LCO and a liquid electrolyte. The energy storage of this device

was double with respect to conventional systems available at that time with same

mass and dimensions. [6, 10]

Lithium Phosphorus Oxynitride glass (LiPON) prepared by RF sputtering was

proposed for the first time for its use as a solid electrolyte by a group in Oak

Ridge National Laboratory in USA in 1993. This material is nowadays considered

a standard materials for applications in solid-state lithium ion batteries because of

its exceptional stability and electronic insulation. The research in LiPON batteries

have been very exciting in the last years. However, still many issue needs to be

solved before this technology will be able to have a strong impact on everyday life.

[6, 10]

Nowadays materials science is fundamental for further improvements in battery

technology. The key properties required for each battery components are given in

fig. 2.5. In the next section, the state-of-art of materials employed in each part of

a galvanic cell is described briefly.

2.3.2 Electrodes

The choice of electrode materials is crucial to determine the output voltage and

lithium ion charge capacity, which in turn impose the energy density of the battery.

They must be characterized by proper potential, large lithium storage capacity,

reactions reversibility, structural stability, low cost and toxicity. [2, 6]


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Figure 2.5: Scheme of di↵erent battery components and corresponding required material

properties.

Positive electrode

Intercalation cathodes are in general the most used for rechargeable batteries. They

are characterized by operating voltages in the range 3-5 V vs Li+/Li and by gravi-

metric capacities around 100-200 mAh/g. Typically used cathode materials are

lithium-based phosphates, vanadium oxides and transition metal oxides. The prop-

erties of the most common compounds are summarized in table 2.1. [14]

Lithiated electrode (LCO, LMO) needs annealing to form the active crystalline

structure. They are able to provide output voltages around 4 V vs Li+/Li. On the

contrary, transition metals oxides or sulfides (V2

O5

, TiS2

) do not require annealing

because they can directly be produced in the active form(crystalline for V2

O5

or

amorphous). Their operation voltages are around 3 V vs Li+/Li, hence they need a

lithiated negative electrode. [1, 6, 14, 20]

The olivine-structured LiFePO4

(LFP) is probably the most important materials

among lithium-based phosphates. It is capable to provide high power with good

thermal stability, but its low output potential is a significant drawback. [6, 14]

In the same category another important example of commonly used cathode ma-

terial is LiMnPO4

(LMP). This compound has the same crystal structure as LFP,

but it is able to provide a slightly higher average voltage (about 0.4 V larger then the

counterpart). Therefore, it is possible to achieve higher gravimetric energy, although

the conductivity is much lower than for LFP. [6, 14]

V2

O5

was the most investigated vanadium oxide. It is made of square pyramids of

VO5

that form a layered structure. Several deposition methods have been proven to

be successful for this material. However, its usage is decreasing in the years because

of the relatively low capacity and output voltage.

Transition metal oxides include very common cathode materials like lithium cobalt

oxide (LiCoO2

or LCO), lithium manganese oxide (LiMn2

O4

or LMO) and lithium


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nickel oxide (LiNiO2

or LNO). LCO have been probably the most successful from a

commercial point of view thanks especially to Sony. It has a layered structured with

hexagonal symmetry, where octahedral sites are occupied alternatively by cobalt

and lithium ions. Its quite large capacity combined with high operating voltage

and cycling performance, made him very attractive for energy storage applications.

However, it will probably need to be replaced by materials with lower costs and

higher thermal stability. [6, 14]

LNO has same structural order and similar gravimetric capacity as LCO, but it

is much cheaper. The main disadvantages that prevent this material to be widely

used is related to the lower operating voltage and thermal stability.

Positive

electrode

material

Structure Gravimetric

capacity

(mAh · g�1)

Volumetric

capacity

(mAh · cm�3)

Potential

(V vs

Li/Li+)

Reference

LiMn2

O4

(LMO)

spinel 148-296 600-1200 4.1 [1, 21]

LiCoO2

(LCO)

layered 274 1373 4.2 [6, 14]

LiNiO2

(LNO)

layered 275 1270 3.8 [14]

V2

O5

layered 118 400 3.4 [6]

LiFePO4

(LFP)

olivine 170 589 3.4 [14, 21]

LiMnPO4

(LMP)

olivine 171 567 3.8 [6, 14]

Table 2.1: Comparison among commonly used positive electrode materials. [1, 6, 14]

In the following section focus will be given to LMO since this was the electrode of

choice for this work. It is important to underline that the negative electrode should

not have a potential close to metallic lithium that, at high current, could result

in the formation of dendrites leading to short circuit of the device. Consequently,

to improve the battery energy density one strategy is the use of cathode materials

with high potential, although in general cathodes have lower capacity (around 150

mAh/g) then negative electrodes (about 300 mAh/g). Structures like LMO able to

intercalate two lithium ions can also be employed to improve the storage capacity.

[1, 2, 22]

LMO was chosen as positive electrode material because of its excellent intercala-

tion properties, low toxicity and large availability. Despite a slightly lower capacity


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with respect to LCO (about 10%), it is characterized by better rate performance

and no tendency to oxygen evolution. This material can be deposited by means of

RF magnetron sputtering. This method enables the production of dense, flaws-free

and amorphous thin films with deposition rates around 1-2 µmh�1 [12]. LMO has a

spinel crystal structure (fig. 2.6) with three cross-linked di↵usion channels in three

di↵erent directions, which enable the high rate performance of this material. Spinels

have a nominal formula AB2

O4

with a close-packed disposition of the oxide ions.

The manganese is at the centre of a tetrahedron of oxygen atoms. The lithium ions

occupy the tetrahedral interstitial sites in between manganese and oxygen. The

main benefit of a three dimensional crystal structure with respect to layered mate-

rials like LNO and LCO is related to the smaller volume expansion during lithium

storage, although this occur at the expenses of lower compositional reproducibility

and of an approximately 10% reduction in capacity with respect to LCO.

LMO can intercalate lithium ions both in tetrahedral and octahedral sites, defined

8a and 16c respectively. The occurring reaction is the following:

Mn2

O4

+ Li+ + e�4V�! Li

1

Mn2

O4

+ Li+ + e�3V�! Li

2

Mn2

O4

(2.14)

Each process has a storage capacity of 148 mAh/g, leading to a total capacity of 296

mAh/g or 1.2 Ah/cm3. When lithium ions are intercalated in octahedral sites at 3 V,

a transformation from a cubic to a tetragonal structure is induced due to the e↵ect

of Mn+3 ions. This process lead to a volume expansion 5% and to a change in ratio

between the crystal axis c/a of 16%. It is generally called Jahn-Teller distortion.

For this reason LMO is typically employed in the 4 V region, because the expansion

in the 3 V region causes the formation of significant cracks and damages in the

electrode. However, also the cyclability at 4 V needs improvement, for example by

means of coating, nanostructuring and doping of the material. Doping include the

use of elements like iron, cobalt, nickel and zinc in order to remove the Jahn-Teller

distortion by increasing the valence number of manganese and improve the electronic

conductivity, thus enhancing the reversibility. In this work both 3 V and 4 V regions

were exploited. [1, 2, 8, 22]

Negative electrode

Anodes are typically made of lithium metal, carbon-based materials (in general

graphite), lithium-based oxides and alloy of Sn, Pb, Sn, Al and Zn. The most

representative properties of some of them are reported in table 2.2. [1, 2]

Elemental lithium has the lowest possible weight (molecular mass=6.94 g/mol,

density=0.53 g/cm3) and potential, enabling the achievement of extremely high en-

ergy densities. This element is thus a very interesting electrochemical reservoir.


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Figure 2.6: Crystal structure of LMO. [10]

When the lithium content is decreased, higher potentials, lower cell voltages and

larger weight are inevitably obtained. However, the use of lithium metal as an-

ode material in rechargeable lithium ion batteries is not trivial, mainly because of

phenomena occurring in metallic elemental electrodes during the charging process.

In fact macro- and microstructural instability of the growth interface and thermal

issues can happen. The formation of dendrites that could lead to short circuit of

the battery is very common. Moreover, liquid electrolytes are extremely unstable

in the presence of high lithium activities. In fact, they react with lithium to form

crystalline or amorphous products that typically form a layer on the electrode sur-

face. Part of the deposit can be insulating and shedding can occur. The reaction is

extremely exothermic and can lead to ignition. This process is commonly defined as

thermal runaway. Safer anode materials have been investigated to solve this problem

when liquid electrolytes are employed. [1, 2, 7, 8]

Carbon is a largely abundant and low cost material. Moreover its low electronic

resistance and potential made hit a very attractive candidate for negative electrodes,

although its gravimetric capacity remains one order of magnitude lower with respect

to elemental lithium. [2, 14]

Among lithium-based oxides, the most important compound is Li4

Ti5

O15

(LTO).


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This material was able to find even commercial applications because of good thermal

stability and cyclability, while maintaining a relatively high volumetric capacity. The

main drawbacks are the high cost, high potential relative to Li and reduced storage

capacity. [2, 6, 14]

Several elements that where found to be able to alloy with lithium have been

widely investigated in the last years. These materials are able to achieve superior

gravimetric and volumetric capacity, up to a few thousands of mAh · cm�3. Among

them, silicon was the most promising candidate because of low potential, moderate

cost and large availability. TiN was discovered to have similar properties, but its

easy fracturing and lower capacity is a major concern. However, all these incredibly

capacitive materials su↵er of enormous volume changes during alloying with lithium,

thus leading to extremely poor cycle life. An example of this phenomenon is shown

for Si in fig. 2.4.

Negative

electrode

material

Structure Gravimetric

capacity

(mAh · g�1)

Volumetric

capacity

(mAh · cm�3)

Average

potential

(V vs

Li/Li+)

Reference

Li4

Ti5

O12

(LTO)

spinel 175 600 1.55 [14]

Graphite layered 372 164 0.19 [1, 6]

Lithium bcc 3860 7228 0 [6, 10]

Li2

Sn5

tetragonal 790 2023 0.2 [6, 14]

Li2

Si5

tetragonal 2012 2374 0.4 [6, 14]

LiAl diamond 790 1383 0.1 [6, 14]

Table 2.2: Comparison among commonly used negative electrode materials.

2.3.3 Electrolyte

Electrolytes should be characterized by a large ion transference number (contribution

of the ions to the total electric current in the electrolyte), which means they must be

good ionic conductors, but electronically insulating to avoid leakage currents that

contribute to self-discharge. Moreover, also high chemical, mechanical and thermal

stability are crucial. In addition, a good rate performance is important. Nowadays

pulsed laser deposition and RF sputtering are the most used techniques for thin

films deposition. Common solid electrolytes for both lithium and non lithium based

batteries include lithium-based glasses and ceramics, inorganic polymer composites


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and silver-, peroviskites- or sodium-based systems. In this section only lithium-based

electrolytes will be considered. The most important properties of some common

materials are given in table 2.3. [23, 24]

Solid electrolyte materials shows many benefits with respect to liquid systems,

including a wider electrochemical stability window, easy design, low risk of leakage,

non-flammability, resistance to vibration and shock. The main drawback is the still

low electronic conductivity. [23, 24]

Solid electrolytes can be both crystalline or amorphous. Glassy electrolytes are in

general preferable because of their open structure, wide compositional range, absence

of grain boundaries, isotropicity, simple fabrication and downscaling. In addition

they have lower ionic resistance than their crystalline counterparts thanks to their

open structure. On the contrary, crystalline electrolytes requires annealing, limiting

the possible choices for multi-layer stacks to materials with limited di↵erence in

thermal expansion. In addition intermixing between di↵erent battery components

might occur at high temperature, eventually causing electronic conduction. [23–25]

One of the most used candidates for solid-state batteries are sulfur- based glassy

electrolytes because of their high ionic conductivities, with values that can reach

10�3 S/cm at ambient temperature. The main drawbacks are the sensitivity to

humidity and the easy degradation. [1, 15, 23, 24, 26]

Both perovskite (Li3x

La2/3�x

TiO3

, also abbreviated with LLTO) and garnet struc-

tured (Li6.75La3Zr1.75Nb0.25O12

) materials have also shown interesting ionic conduc-

tion properties (around 10�3-10�4 S/cm), but they have a narrow stability window

and they require high temperature annealing that could lead to lack of electronic

insulation as a consequence of interdi↵usion phenomena. [1, 23, 24]

In order to solve this problem LiPON electrolytes can be employed. These mate-

rials were ideated by Bates et al. [27]. LiPON can be deposited by RF sputtering

a Li3

PO4

target under a nitrogen atmosphere, but also other fabrication methods

are possible, such as e-beam evaporation, ion-beam assisted deposition (IBAD) and

pulsed laser deposition (PLD). The first technique is the most used one because of

high reproducibility and formation of good contacts and dense layer thanks to high

particle energies. The main drawback is the low deposition rate (about 4 nm/min).

The Li3

PO4

targets are in general made in several steps, including powder calci-

nation, binder addition, drying, sieving, ball milling, cold/hot pressing, sintering.

However, after the first uses, the g phase forms instead of the less stable b and con-

sequent cracking can occur. In some cases it can thus be beneficial to use powder

target. Moreover, high deposition rates are possible because of the lower energy re-

quired to remove atoms from a surface with respect to a densely packed target. This

materials show a higher ionic conductivity (highest value are typically around 1-2


Francesca Criscuolo


10�6 S/cm at room temperature) and electrochemical stability than LiPO. In addi-

tion, no phase transformations occur over a wide temperature range and electronic

conductivity is very low (10�1310�14 S/cm). [24, 28–36] Many papers attribute this

improvement in materials properties due to the substitution of oxygen atoms in P-

O-P and P=O bonds by nitrogen. In this process P-N=P and P� N <P

P

are formed

and a more crosslinked structure is obtained. [24, 37–40] Another explanation that

is typically found in literature is related to the decrease of the ratio BO/NBO oxy-

gen. [41] The use of thinner films can be useful to improve ionic conductivity and

cyclability, although a certain limit exists due to loss of electronic insulation. [1]

Electrolyte ma-

terial

Type Ionic conductivity

(Scm�1)

Electronic

conductivity

(Scm�1 )

Reference

1M LiPF6

in

EC/DMC

liquid 11 10�17 [1, 6]

1M LiClO4

in

PC

liquid 5.6 10�17 [1, 6]

LiPON solid amor-

phous

1 · 10�6 10�13 � 10�14 [1, 6]

LiNbO3

solid crys-

talline

8 · 10�7 10�11 [6, 14]

Li2

AlZr(PO4

)3

(NASICON)

solid crys-

talline

3 · 10�3 10�11 [4, 6]

Li10

GeP2

S12

solid crys-

talline

2.2 · 10�2 10�11 [9, 14]

Li3x

La2/3�x

TiO3

(LLTO)

solid crys-

talline

5 · 10�5 10�8 � 10�9 [1, 14]

Table 2.3: Comparison among commonly used electrolyte materials.

2.3.4 Substrate

The substrate must be compatible with the operating and process conditions for

battery layers deposition. Moreover, they must block lithium di↵usion out of the

battery stack and avoid the entrance of harmful species. For instance, if a silicon

wafer is used as substrate for deposition, a di↵usion barrier for lithium is necessary.

This can be for example a layer of TiN or Pt, that can act also as a current col-

lector. In fact, the oxide used as electrode materials have a typically low electronic

conductivities, therefore current collectors are required. [19]


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2.4 New concepts: why thin films - why 3D bat-

teries

Figure 2.7: E↵ect of the use of di↵erent architectures on battery performance. [1]

Thin-film solid-state batteries show some advantages related to the easy minia-

turization and to the capability to better accommodate stresses and strains, thus

improving cycle performance. This is also possible because the variation in volume

is smaller with respect to thick systems. In addition, a good electrical contact is

easily obtained as compared to liquid batteries that exploit composite electrodes

with carbon black to improve the contact among di↵erent particles. However, the

absence of a composite electrode in solid-state batteries also causes a limitation in

film thickness. In liquid batteries, the electrolyte can penetrate in the electrode ma-

terial facilitating lithium ion di↵usion. This is not possible in solid-state batteries

because currently employed electrodes have low di↵usion constants for lithium ions

(10�9 � 10�11 cm2/s). Consequently, the charging time t for thicker electrode films

becomes significantly high [1, 6, 42]:

t =l2

2D. (2.15)

Thin films electrodes lead to lower charge discharge time and higher power output

(since it is the work done per unit time), but limits the total capacity of the system.

The use of multi-cells stacks and 3D designs can be used to achieve higher capacities


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(fig. 2.7). [6] Several concepts has already been proposed in this regard: architec-

tures based on aerogels [43, 44], on microchannel plates on templated depositions

[45] and on microchannel plates. [46] In fact, the use of high aspect ratio structures

leads to an increase in the amount of material and consequently to a larger capac-

ity. However, the higher electrode surface to volume ratio decreases the di↵usion

distance of lithium ions, leading to lower charging time. Consequently, both high

power output and capacity can be achieved. It is evident that the pattern density of

the 3D structure must be high enough to ensure a su�cient amount of material for

lithium ion storage and hence reach large capacity. Di↵erent types of systems can

be employed, like silicon pillars, anodized aluminum, etched trenches and meshes,

with large pattern density to increase the amount of electrode materials without

using higher thicknesses. [1, 47]

The conformal deposition of electrode materials and of an ultrathin electrolyte

layer in order to provide electronic insulation between the two electrodes is crucial

to obtain functional 3D batteries. ALD can be used to obtain conformal deposition,

although the creation of defect free electrolytes is still a huge challenge. Moreover,

this technique is time-consuming and expensive. Furthermore, it was found that

at the nanoscale rapid self-discharge can occur also in the presence of defect-free

electrolyte layers. This phenomenon can be explained in terms of the large electric

field present in the electrolyte, that leads to the beginning of space-charge limited

electronic conduction. This results in large electronic current in the electrolytes and

short circuit of the battery. [19, 47, 48]

The creation of a working 3D solid-state batteries has not been reached yet. In

fact large electric fields are generated that can lead to dielectric breakdown of the

oxide thin film. However the main limitation is related to the incompatibility of

materials and processes. ALD could be an important method to create barrier,

seed, bu↵er and pinhole free ultra-thin electrolyte layers that could be essential

for the production of a functional device. Moreover, the investigation of proper

electrolyte/electrode combination is crucial. This last aspect is part of this work.

[19]


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Chapter 3

Ultrastable glasses

3.1 Supercooled liquids and glasses

Glasses are amorphous solids that lack the long range order typical of crystalline

materials. Equivalently, they can be thought as frozen liquids. This statement

can be explained by considering the plot of the specific volume as a function of

temperature (fig. 3.1). When a liquid is cooled, crystallization can start when the

crystallization temperature of the material Tm is reached. This is a first order phase

transition, i.e. it gives rise to a discontinuity in the first derivative of the Gibbs

free energy, in particular in the specific volume, which in most cases shows a drop.

However, it can happen that a liquid is able to reach temperatures below Tm without

organizing itself in a crystal structure. This system is called a supercooled liquid.

Its thermodynamic behaviour can be described by simple extrapolation from the

properties of the normal liquid. [49]

As the temperature lowers, its viscosity increases rapidly, leading to lower molec-

ular mobility. Di↵usive motions of particles is very di�cult to be achieved. Conse-

quently, the molecules do not have time to rearrange and find equilibrium positions.

Hence, the specific volume starts to diverge from the equilibrium value. At a cer-

tain point the system freezes forming a glass. With further cooling the volume of

the system keeps lowering, but at a lower rate since the glass thermal expansion

coe�cient is smaller with respect to the liquid and the supercooled liquid ones. [49]

3.2 Glass transition

It is crucial to understand that the glass transition is not a first-order transition

like the crystallization process. In fact, it is essentially kinetic in nature since it is

related to the time scale for molecular rearrangement to follow the rapid changes

22

CHAPTER 3. ULTRASTABLE GLASSES 23

Figure 3.1: Plot of the specific volume versus temperature showing the glass transition

phenomenon. Its variation with the cooling rate highlights its kinetic nature.

occurring in the experimental observation. [49]

The glass transition temperature can be described in the simplest way as the mate-

rial softening point or in semi-crystalline substances as the melting of the amorphous

regions. However, its definition is arbitrary, hence not unique. It is a range of tem-

perature over which the material deviates from an equilibrium state. A commonly

used definition is the onset temperature of the increment in the heat capacity dur-

ing heating at 10 K/min. This jump in heat capacity results in the formation of an

endothermic step in a DSC curve (fig. 3.2). The heat capacity controls the entropy

variation of a system. An overshoot as the one in figure can be present during heat-

ing and can be correlated to the excess enthalpy needed for the process. This spike

is an indication of the stability of the material. [49, 50]

3.3 Relaxation phenomena

A complicated highly dimensional potential energy surface can be used to represent

the potential energy of a supercooled liquid or a glassy system. This is a function

of all atomic positions. Each point corresponds to a possible instantaneous con-

figuration. This representation is called the potential energy landscape. [49, 51,

52]

Crystals and supercooled liquids are equilibrium and metastable states, respec-

tively. Therefore they are stable systems. On the contrary, when a material is in

a glassy state their properties varies with time since relaxation towards a close free

energy minimum (i.e. towards a more stable situation) occurs. The understanding


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Figure 3.2: Experimental evidence of glass transition phenomenon during a DSC scan

both during heating and cooling. The overshoot during the heating scan is related to the

excess enthalpy release and is an indication of the glass stability.

of glass behaviour is still not complete because of the complexity of these relaxation

phenomena and kinetics they can undergo. The time scale of these phenomena is

extremely wide, from atomic vibration in the picosecond range to densification and

aging processing that might take thousands of years. As a consequence of these

transformations the material changes, therefore leading to di↵erent behaviour. [49,

51, 52]

In general the structural relaxation time can be defined as the time needed for

the liquid to go back to an equilibrium situation after a small perturbation. It can

be derived experimentally by dielectric relaxation spectroscopy or dynamic neutron

scattering. Several types relaxation phenomena can in general occur. ↵ relaxation

is the most important relaxation mode and it is typically evident at low frequencies

in a isothermal relaxation dielectric spectrum (fig. 3.3). This relaxation mechanism

is strictly connected to glass transition and viscosity. Below the glass transition

temperature this mode is frozen and � relaxation phenomena are the dominant

ones. At the beginning these mechanisms were thought to be related to side chains

and functional groups rotations in polymeric materials. However, this belief was

soon contradicted by the occurrence of these phenomena in materials free of side

chains and functional groups, observed in the ‘60s and ‘70s by Johari and Goldstein.

Therefore, it has been assumed that intermolecular motions are at the base of this


Francesca Criscuolo


mode of relaxation. [51]

Figure 3.3: Typical dielectric spectrum of a glass-forming liquid. [51]

According to Stillinger, � modes are related to movement of the system between

neighbouring potential energy minima. Typically these relaxation phenomena are

cooperative, but reversible because the atoms are moving in a short range and

enclosed by the neighbouring ones. On the contrary, ↵ relaxations involve non -

reversible cooperative rearrangements between macroscopic minima in the potential

landscape. They are characterized by large activation energies. These di↵erences

are illustrated schematically in figure 3.4. [51, 53]

In general it is possible to state that the time of experimental observation is much

larger than the relaxation time typical of liquid phases, but much smaller than the

one of glassy systems: ⌧ liquidR << tobs << ⌧ glassR (fig. 3.6). [50]

3.4 Classification of glassy materials

Glasses can be classified in terms of the sensitivity of their structures to thermal

variations. In particular, strong and fragile behaviours can be identified by means

of the so-called fragility index m:

m =

d(log⌘)

d�TgT

�!

T=Tg

=

d(log⌧R)

d�TgT

�!

T=Tg

(3.1)

in which ⌘ is the viscosity and ⌧R the relaxation time. This classification was first

introduced by Angell. Glasses with low m values (typically below 16) are defined as


Francesca Criscuolo


Figure 3.4: Two-dimensional scheme of � and ↵ relaxations with the related potential

energy representation. Atoms prone to movement are shown as open circles. [51]

strong, while the ones with high m (larger than 200) are fragile. [3, 54]

In other words, the temperature dependence of viscosity and relaxation times

of supercooled liquids can be expressed by the the so-called the Vogel-Tammann-

Fulcher or VTF equation (fig. 3.5):

⌘ = ⌘0

exp⇣ B

T � T1

⌘(3.2)

⌧R = ⌧0

exp⇣ B

T � T1

⌘(3.3)

Supercooled liquids with strong glassy behaviour have T1 that approaches zero.

In this case the above equation reduces to an Arrhenius dependence. Supercooled

liquid with open networks have typically a strong glass behaviour, i.e. they exhibit

an exponential dependence on temperature of the viscosity and relaxation time.

They show a certain resistance to structural changes when thermally excited with

respect to fragile liquids. Their energy landscape have a low amount of minima with

large energy barriers. [52, 54]

On the contrary fragile liquids have a positive value of T1. At this temperature

their relaxation time becomes infinitely large, giving rise to a non-exponential de-

pendence of the material response to several perturbations. At Tg they are close


Francesca Criscuolo


Figure 3.5: Angell plot showing the di↵erence between the behaviour of strong and fragile

glasses with respect to thermal perturbations.

to structural collapse. Therefore, they can be represented in the potential energy

landscape as materials with large density of minima, but low barriers between them.

Their viscosity grows rapidly when the glassy state is reached. Also the specific heat

shows a sharp change at Tg. They include materials with non directional coulombic

or Van der Waals interactions with many i electrons. The most fragile liquids are

polymeric materials. [54]

The fragility index can also be expressed in terms of the activation energy required

for structural relaxation of the glass, i.e. for molecular motion:

m =Ea

ln(10RTg)(3.4)

Lithium-rich phosphate glasses show activation energy around 400 kJ mol-1, so they

can be considered strong glasses (small m). [3]

The variation of the activation energy value with temperature typical of fragile

liquids, evident from fig 3.5, seems to indicate a higher degree of cooperation in the

relaxation mode. [52]


Francesca Criscuolo


3.5 The entropy crysis

The entropy of a system can be calculated by the following relationship:

S(T2

)� S(T1

) =

Z T2

T1

CP (T )

T@T (3.5)

Its dependence on the temperature is shown in fig.3.6. As explained earlier, the glass

transition is a kinetic e↵ect due to di↵erences in the time scales of the experimental

and molecular rearrangement process. If it would be possible to suppress the occur-

rence of this transition, the entropy of the liquid would be able to keep diminishing

with same slope. Since the heat capacity of the liquid is higher than the one for

the crystal, at a certain temperature, defined Kauzmann temperature, the entropy

of the liquid would equal the one of the crystal. Upon further cooling, its value

would become even lower than the one for the crystal. This e↵ect is not forbidden

by thermodynamics, until it leads to negative entropy values at temperature higher

than the absolute zero. This phenomenon is unphysical and is called Kauzmann

entropy crisis. [49, 54]

Typically fragile liquids are employed by researcher to examine this e↵ect since

they are characterized by a marked di↵erence between the specific heat capacity

of the liquid and the crystal. In fact, as said before, supercooled liquids are char-

acterized by a large density of energy minima in the potential energy landscape,

leading to a high rate of entropy change with temperature. Consequently, the slope

of the curve S vs T is large and the Kauzmann temperature of these systems is high,

leading to easier experimental investigation. [49, 54]

Going back to the potential energy landscape representation, the ground state for

an amorphous packing, i.e. the state corresponding to the lowest minimum, would

be obtained if the cooling rate would be so small that the supercooled liquid can

reach the Kauzmann temperature without falling out of equilibrium. As intuitive,

this situation is impossible to be achieved experimentally. [50, 54]

Several phenomena have been proposed to solve the entropy crisis. They are

illustrated in fig. 3.7 and include:

• a first order thermodynamic transition in between Tg and TK (red curve)

• the formation of an ideal glass, i.e. a glassy material with entropy equal to

the one of the corresponding crystal (blue dashed line)

• the absence of any phase transitions and continuous liquid behaviour (green

curve)


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Figure 3.6: Plot of the entropy versus temperature for liquid, supercooled liquid, glassy and

crystalline phases. Tm represents the melting point, Tg the glass transition temperature,

TK the Kauzmann temperature. The kinetic nature of glass transition phenomenon is

highlighted by the e↵ect that di↵erent cooling rates have on it.

3.6 Highly stable glasses

Recently, glassy materials with improved thermodynamic and kinetic stability have

been discovered. These glasses correspond to deep positions in the energy landscape,

thus they could lead to clarity in the Kauzmann entropy crisis thanks to their

proximity to the ideal glassy state. They are characterized by an increase in the

glass transition temperature with respect to the corresponding quenched glass or

equivalently by a decrease in the fictive temperature. The fictive temperature is

defined as the temperature at which the supercooled liquid falls out of equilibrium,

leading to the creation of a glass. Lower Tf values are an indications of lower

locations in the potential energy landscape. Empirically Tf can be determined by

considering the intersection of the experimental enthalpy versus temperature curve

and the extrapolation of the supercooled liquid enthalpy (fig. 3.8). It is important

to remind that the enthalpy versus temperature can be obtained by integrating the


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Figure 3.7: Representation of possible solutions to the Kauzmann entropy crisis: a first

order thermodynamic transition in between Tg and TK (blue curve); the formation of an

ideal glass, i.e. a glassy material with entropy equal to the one of the corresponding crystal

(red line) ; absence of any phase transitions and continuous liquid behaviour (green curve).

heat capacity [51, 55–57]:

�H =

Z T2

T1

CPdT. (3.6)

Highly stable(HS) glasses show smaller enthalpy, heat capacity and less evident

hygroscopic behaviour. Moreover, they are typically characterized by higher densi-

ties and strength than normal glassy materials. The packing is so e�cient that the

transformation into liquid needs the presence of high surface mobility and interfaces

to start, even at temperatures largely above the glass transition temperature. In

addition, more evident overshoots of the heat capacity versus temperature can be

present due to increased thermal stability. They are correlated to release of the

excess enthalpy. [51, 55, 58]

HS glasses are obtained from the vapour phase on substrates at about 0.8-0.9

Tg. The exceptional properties of these systems have been related to increased

surface mobility. Several classes of materials have already been found to have such

behaviour, including both inorganic and organic substances. A correlation with the

fragility index has been proposed (fig. 3.9). An interrelationship with the strength

of the � relaxation is also possible. [51, 56, 57, 59–63]


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Figure 3.8: Representation of di↵erent fictive temperatures Tf , defined as the temperatures

at which the supercooled liquids fall out of equilibrium, leading to the creation of glassy

phases.

Figure 3.9: Correlation between fragility index and improved kinetic stability of HS glasses,

expresses by �Tg

Tg

). [51]


Francesca Criscuolo

Chapter 4

Experimental techniques

4.1 Introduction

In this section some theoretical background about the experimental techniques em-

ployed in this work is given. Only methods directly used by the author are discussed

here. For more details on the techniques regarding indirectly obtained data refer to

appendix A.

4.2 Physical vapour deposition(PVD)

Physical vapour deposition (PVD) processes enable the formation of films through

the transfer of atoms from a source to a substrate in the vapour phase. All LiPO and

LiPON thin films analysed in this work were deposited by means of physical vapour

deposition, more specifically by using RF sputtering. Nitrogen was used as reactive

gas in the chamber in order to produce lihtium phosoporus oxy-nitrides films from a

LiPO target. This process is commonly called reactive sputtering. Moreover, metal-

insulaor-metal (MIM) capacitors for impedance measurements (see section 4.4.1)

were obtained by thermal evaporation of 70 nm gold dots using a shadow mask. [64,

65].

Evaporation was the most popular deposition method up to the 1960s because

of cleaner atmosphere, higher deposition rates and large applicability to di↵erent

classes of materials. However, as soon as high purity targets and gases became

available and RF and magnetron sputtering were introduced, it was soon replaced

by sputtering. These techniques allow larger control on conformal growth and enable

the deposition of thin film compounds with precise stoichiometry. Moreover, any

material compatible with vacuum environment can be deposited. [64, 65].

32

CHAPTER 4. EXPERIMENTAL TECHNIQUES 33

4.2.1 Evaporation

Evaporation is the controlled deposition of atoms on a substrate based on the dis-

lodgement of atoms from a source by means of target heating. In the ’80s the

scientist Hertz proposed the following expression to describe the evaporation rate:

�e =↵eNA(Pe � Ph))p

2(4.1)

where ↵e is the evaporation coe�cient, NA is Avogrado’s number, Pe and Ph are the

equilibrium and hydrostatic pressures of the evaporating species, M is the molecular

mass, R the ideal gas constant and T the temperature. This equation is valid for

both liquid and solid sources. It is possible to notice that the highest evaporation

rate is achieved when the evaporation coe�cient equals unity and Ph is null. [64–66]

In a small temperature range, it can be demonstrated that the following relation

between the vapour pressure of the material and temperature is valid:

lnP = ��He

RT+ I (4.2)

where �He is the molar heat of evaporation and I is an integration constant. How-

ever, for wide temperature ranges the dependence of �He on temperature must be

considered. [64, 65]

4.2.2 Sputtering

Sputtering is, together with evaporation, one of the most widely used PVD tech-

niques for thin films. These depositions typically involve the formation of plasmas

by power supply. This is a partially ionized, but neutral gas, characterized by the

presence of a large number of free carriers (ions and electrons). These carriers un-

dergo electromagnetic interactions because of their large density and a↵ect strongly

the physical properties of the medium, which behaves like a fluid. As a consequence

of the presence of a large amount of carriers, plasmas are highly conductive, typically

with anisotropic behaviour. [64, 65]

Figure 4.1 shows a simple sputtering system. The target is made of the material

that needs to be deposited. It is called the cathode since it is connected to the

negative terminal of the radio frequency (RF) or direct current (DC) power supply.

Typically the powder of target materials is produced by wet chemical methods and

consequently processed by cold or hot pressing to obtain a high density crystalline

product with desired stoichiometry. Di↵erent sizes or shapes are possible. A cooled

backing plate with high thermal conductivity, frequently made of metal-filled epoxy

cements, is used as support to avoid thermal damage. [1, 65] Vacuum is created in

the chamber before a gas, normally argon, is introduced to sustain the discharge (the


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Figure 4.1: Schematic representation of a magnetron RF sputtering system.

pressure ranges from a few to hundreds of mTorr). The application of a su�ciently

high voltage leads to the breakdown of the gas and the formation of a plasma

discharge. The created positive ions are accelerated by the present electric field

and hit the cathode ejecting neutral target atoms by momentum transfer. These

atoms di↵use through the gas and deposit onto the substrate where they form a

film. During the process also secondary electrons, negative ions and radiation are

generated as a consequence of the interaction between the plasma and the target

material. Secondary electrons are accelerated by the electric field and help sustaining

the discharge by collision with gas atoms and consequent generation of new ions. It

is evident that the modelling of the process is very complex since a large number of

species are involved and several configurations are possible. [65, 66]

It is possible to define a sputtering yield SY as the number of atoms or molecules

displaced from the target divided by the number of incident ions. According to

Sigmund’s theory, based on collision cascade (4.2), SY can be written as:

SY =3↵

4⇡2

4M1

M2

(M1

+M2

)2E

1

Eb

(E1

< 1keV ) (4.3)

SY = 3.56↵Z

1

Z2

Z2

1

/3 + Z2

2

/3

M1

M1

+M2

Sn(E)

Eb

(E1

> 1keV ) (4.4)

where ↵ describes the e�ciency of momentum transfer and Sn(E) represents the


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loss in energy per unit length due to nuclear collisions. This last quantity depends

on the atomic number and mass of the species involved. It varies with increasing

energy until it reaches a constant value. [65]

Figure 4.2: Collision cascade of the sputtering process. [64]

There exist several sputtering techniques. In this work particular attention will

be given to RF sputtering, which was used for LiPON thin film deposition.

RF sputtering

RF sputtering was ideated to overcome the impossibility of depositing insulating thin

films by DC method. This in fact would need too high voltages to work. Moreover,

the charge deposited onto the insulating target surface by the colliding ions must

be neutralized. High frequency arc discharges, typically in the range between 1 and

30 MHz, are used for this purpose. In fact the inversion of the potential during the

second part of the RF cycle attracts electrons to the target. In this way the charge

accumulated during one-half cycle can be neutralized by the electron bombardment

in the next cycle. [64] In the presence of reactive gases, typically mixed with the inert

one, thin films of compounds can be formed on the substrate surface. For instance,

nitrides can be formed in the presence of ammonia or nitrogen gas. Also the sputter

power can be modulated to change film properties. Moreover, it is generally found

that the deposition rate in nm/s increases linearly with increasing power. However.

attention must be paid with extremely high power to avoid target cracking as a

consequence of fast thermal expansion due to plasma heating. [1, 65]

Another important factor to be considered in the deposition rate and in the control

of the sputtering process itself is preferential sputtering. In this regard an important

parameter to look at is the kinematic factor Kr, defined in equation (4.5):

Kr =4M

1

M2

cos2(�)

(M1 +M2)2(4.5)

where M1

and M2

are the masses of the incident and sputtered ions respectively

and � is the sputtering angle. Kr represent the amount of energy transferred during


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collision from the incident to he sputtered atoms. Its highest value, and hence also

the largest sputtering rate, is found for M1

= M2

. This phenomenon explains the

lower deposition rate of lithium in comparison to other materials like aluminum

(mLi = 0.15mAr while mAl = 0.68mAr). [1, 67–69]

During this work, a magnetron setup was used. This is based on the presence of

permanent magnets that generates a magnetic field in proximity of the target. In this

way the electrons ejected during the deposition process are blocked in a circular path

close to the target. The increased plasma density enhances the collision probability.

Consequently, a larger amount of ions can be produced and deposition can be up to

ten times faster than in a normal setup. [1, 66]

4.3 Compositional, structural and morphological

characterization

4.3.1 X-ray di↵raction (XRD)

Introduction

When radiation is focused onto the sample, scattering or absorption phenomena

are possible. If the scattering is elastic, i.e without energy losses, the radiation

wavelength of the scattered beam is equal to that of the incident one. XRD is a

an extremely widely used technique based on coherent scattering of X-rays from a

crystalline samples along specific direction. It is mainly used to obtain information

about the phases proportion and the structure of a material, although a large variety

of other applications are possible. [70–72]

Physical principle

Crystals are, in their simplest definition, periodic structures characterized by the

repetition of a unit cell in space. These can be seen as made of two di↵erent com-

ponents: a Bravais lattice and a basis. The first one is a mathematical definition of

regular points in space described by the vector:

~n = n1

~a1

+ n2

~a2

+ n3

~a3

(4.6)

where n1

, n2

and n3

are integer numbers and ~a1

, ~a2

and ~a3

are the basis vectors (in

di↵erent planes) of the primitive cell. The complete crystal structure can then be

reproduced by decorating every primitive cell in the lattice with the basis. This is

a set of atoms that represent the elementary building block from which the whole


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Figure 4.3: Construction of two di↵erent crystals from the union of a Bravais lattice and

the two di↵erent associated basis.

crystal can be built by simple translation along the basis vector directions. An ex-

ample of this process of construction/deconstruction of the crystal is given in fig.

4.3. These regular arrays of atoms can be considered as a di↵raction grating that,

when hit by a wave with wavelength of dimension comparable to the interatomic

spacing in the lattice, can give rise to constructive interference along defined direc-

tion from the rays scattered by the crystalline planes. The condition for this process

to occur for a certain family of planes {h, k, l} is expressed by Bragg’s law:

n� = 2dhklsin✓hkl (4.7)

where n is an integer expressing the di↵raction order, � is the wavelength of the

incident radiation, dhkl is the distance between the planes of the family and ✓hkl is

the di↵raction angle, i.e. the angle between the incident beam and that family of

planes. This equation is valid under the assumption that crystalline planes behave

as specula mirrors. A schematic of the process is given in fig. 4.4. [1, 70–73]

This phenomenon leads to a di↵raction spectrum showing the intensity of the

scattered radiation versus the scattering angle. Both peaks positions and intensities

give important information about the material structure. The first one is described

by Bragg’s law and determined by the type and dimension of the lattice. On the

contrary, the intensity of the di↵raction peaks is related to the positions of atoms


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Figure 4.4: Representation of XRD phenomenon by a crystalline structure.

in the unit cell, or , in other words, to the basis. [70–72]

Experimental apparatus

A di↵raction experiment typically requires four main components: a source, a

di↵ractometer assembly, a sample in the proper form and a detector. X-rays are

produced in a vacuum tube by striking a metal target with highly energetic elec-

trons. In this way holes are created in the target materials. Higher energy electrons

can occupy these vacant places emitting X-rays. The function of the di↵ractometer

assembly is essentially the beam alignment and the correct positioning of detector

and specimen. The most common type of X-ray detector is the proportional counter,

based on the ionization of a low pressure gas by the incident X-ray phonons. This

process creates clouds of ions that can be detected as current pulses. [70–72]

Specific apparatus enables also the recording of XRD spectra during sample heat-

ing at a controlled rate. This technique is typically defined as in-situ XRD. In some

cases di↵erent background atmospheres can also be used during the measurement.

An example of data obtained with this method is given in fig. (4.5). In fig. (4.5b)

the 2D plot is given. This can be considered as a top view of (4.5b). The black line

is using to convert time into temperature (and viceversa) by simple multiplication

(or division) by the heating rate. [70]

Working conditions

XRD is a fast, accurate, economic and non destructive. It requires only small quan-

tity of material and it is applicable to almost all types of samples. Grazing incidence

XRD can be used to improve the measurement quality and surface sensitivity. In

fact, this method enhances the signal thanks to the long path of the radiation in the

sample. [1, 70–72]


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(a) (b)

Figure 4.5: (In-situ XRD data obtained at a heating rate of 10C/min in helium athmo-

sphere for a LiPON sample: (a) 3D plot; (b) 2D plot.

Achievable information

XRD is a powerful techniques used for many di↵erent purposes. The most common

is probably phase identification and quantification by analyzing peaks position and

relative intensity respectively. Other possible employment include the distinction

between amorphous and crystalline materials, structural studies of materials, deter-

mination of crystal orientation, to name just a few. In addition, in-situ techniques

provide information about chemical reactions and phase transformation kinetics

thanks to their time-resolved data recording. [70–72]

4.3.2 Scanning electron microscope (SEM)

Introduction

The scanning electron microscope (SEM) is an imaging instrument based on the

interaction of a focused electron beam with the sample under examination, which

generate di↵erent types of signals (fig. 4.6). These technique allows the observation

of microscopic morphology of both organic and inorganic samples surfaces. Analyt-

ical investigations are derived from the signals recorded by suitable detectors, which

may be of di↵erent types depending on the di↵erent responses of the samples to

electronic excitations. In particular, detectors for secondary(SE) and backscattered

electrons (BSE) enable the reconstruction of an image of the region scanned by the

primary beam. This picture is related to both surface topography and atomic num-

bers of the chemical elements present on it. The processing of these signals allows for

a wide range of information, including morphology, composition and structure. The

SEM shows quite e↵ective analysis of the sample chemical composition and crystal-


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lographic orientation, and allows precise local and areal analyses, both qualitatively

and quantitatively. [70, 71, 74, 75]

Figure 4.6: Di↵erent signals generated after the interaction between the incident electron

beam and the sample.

Physical principle

The analysis is performed by scanning an electron beam which is focused above the

specimen. The interaction between energetic electrons and matter leads to the emis-

sion of electrons and photons (fig. 4.6) that can be captured by suitable detectors.

The emitted electrons can be divided into two main groups: the BSE, that, as a con-

sequence of the interaction with the sample nuclei, are deflected or reflected without

significant loss of energy (this is called elastic scattering); and the electrons that

undergo inelastic scattering, i.e. when the interaction with matter causes transfer

of energy from the electrons to the sample atoms. A further distinction between

these two classes of electrons is based on the value of the energy E possessed by the

electrons themselves: the secondary electrons are characterized by values of energy

lower than 50 eV, while backscattered electrons have higher energies. [70, 71]

As said before, both elastic and inelastic scattering phenomena are possible as

a consequence of the the interaction between the primary beam and the material

under observation. The sample region from which di↵erent interaction signals are

generated is generally called volume of interaction (fig. 4.7) In this volume the

electrons of the primary beam show an irregular path, with a progressive loss of


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Figure 4.7: Penetration volume and volume of flight for di↵erent signals in SEM.

energy as a result of di↵erent interactions. The volume of interaction is a↵ected

mainly by two parameters:

• the di↵usion depth (XD), that is, the depth to which it can be assumed that the

electrons follow purely random paths, without any influence from the original

beam direction.

• The interaction depth (XR), that is, the depth at which the electrons have

energy equal to the thermal energy kT of the material.

By varying the material characteristics and the measurement parameters, the volume

of interaction changes. In particular, it becomes larger with increasing incident beam

energy and with smaller atomic numbers of the elements present in the sample. It

can be observed from fig 4.7 that di↵erent regions of the penetration volume give rise

to signals of di↵erent nature (backscattered electrons, secondary electrons, X-rays).

In particular, the volume from which a given signal comes is called the volume of

flight of that signal. [70, 71, 74]

It is possible to define two important numerical factors: the yield of backscattered

electrons ⌘BSE and of secondary electrons �SE,

⌘BSE =iBSE

io(4.8)


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�SE =iSE

io(4.9)

where i0

, iBSE and iSE are the currents associated to the primary beam, to all

the backscattered and secondary electrons, respectively. The first one is strongly

dependent on atomic number Z. Consequently, large values of Z will correspond to

bright zones of the SEM image. On the contrary, the yield of secondary electron

does not show any dependence with Z. However, it is influenced by the energy and

the incidence angle of the the electron beam. In particular, it increases as the values

of these parameters fall down. [70]

Experimental apparatus

(a) (b)

Figure 4.8: (a) Operating scheme of a SEM. (b) Typical system of lenses of a SEM. [70]

A schematic representation of the SEM operating system is shown in fig. 4.8a.

The flow of electrons needed to perform the measurement is produced by an electron

gun. Its task is to produce an electron beam as much as possible concentrated, with

a focused area that can go down to a few nanometers. The processes exploited to

obtain electron emission are two: thermionic e↵ect and field emission. [70, 71]

In the first case a filament of a suitable material is heated so that the electrons

acquire a kinetic energy greater than the work function of the material, or, equiva-

lently, the minimum amount of energy necessary so that an electron can move away

from the material itself. Use is made of tungsten filaments, metal that allows the

emission of su�ciently large amounts of electrons at temperatures that reach 3000

K. An alternative is represented by a crystal of lanthanum hexaboride LaB6

: such


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a crystal has a work function smaller than the one of tungsten, therefore, it can

work at lower temperatures and ensure increased density of electrons within the

beam. However, lanthanum hexaboride needs to operate at vacuum levels higher

than tungsten in order to have good stability and su�ciently long lifetime. [70, 71]

A second type of electron source is the one based on field emission, that exploits

pointed metal structures, typically made of tungsten, with radius of curvature in

the order of 0.1 mm or less. An electric field of considerable intensity is applied in

this area. The pointed shape helps to concentrate the electric field lines of force

in correspondence of the maximum radius of curvature. All these means enable

electrons emission from the metal also for lower temperature values. The electrons

bundle is highly directional and the emission e�ciency is higher. The disadvantages

associated with this phenomenon are the high cost of the instrument, the higher

vacuum level required and, sometimes, the need to hold the tip by means of materials

that exhibit a relatively low work function (such as zirconium oxide). [70, 71]

The beam is focused by means of a system of magnetic lenses, that take advantage

of the magnetic fields to force deviations in the electrons path (fig. 4.8b). In fact,

the latter are charged particles and therefore tend to interact with a magnetic field

by means of Lorentz force . The magnetic lenses are coils with cylindrical symmetry

traversed by a known current. By varying the current, it is possible to control the

magnetic field associated with the coil and thus change the electrons path. The

all system can be schematized as if it were constituted by two parts: in the upper

section, the closest to the electron source, condenser lenses are placed, which allow

to focus the beam and control the total current that arrives on the sample. In

the lower zone there are objective lenses which contribute to more focusing of the

electron beam and determine the final resolution. [70, 71]

The scanning of the electron beam on the sample surface is obtained by means

of two pairs of coils. Each of them generates a magnetic field which deflects the

direction along which the beam moves, in such a way that they are responsible for

the translation of the beam along two axes of a suitable orthogonal reference system.

In this way it is possible to scan the material surface point by point. [70, 71]

The electrons coming from the sample reach the detector, where the recorded

information is converted into a digital signal and sent to a computer. From fig.

4.9 it is possible to observe that, for the same beam energy, the number of formed

SE is much greater than the number of BSE. To obtain an image by means of

BSE exclusively, use is made of a metal grid placed between the sample and the

detector: on the grid a potential of about 50V is applied in such a way that only the

BSE have enough energy to overcome it. The drawback of this detection system is

that it o↵ers a very small solid angle of collection. To overcome the problem, it is


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possible to employ a detector placed directly above the sample in order to increase

the collection angle. [71, 75]

Figure 4.9: Number of electrons produced as a consequence of the interaction between the

primary beam and the sample as a function of the electron energy. [75]

Working conditions

Electrons show an extremely small mean free path in the presence of gas molecules.

Consequently, it is necessary to achieve vacuum levels of at least 10�6-10�7 Torr. If

this is not the case, the direction of motion of the electrons is significantly influenced

by the gas contained in the chamber and does not yield reliable information. The

SEM is suitable for all samples that are able to withstand vacuum conditions. Both

conductive and insulating materials can be investigated. In the last case, it is

necessary to deposit a thin layer of carbon, gold or another metallic species over the

sample in order to avoid the accumulation of surface charges which would distort

the measurement. [70, 71]

A recurrent problem in SEM measurements is surface contamination, in particular

the development of a carbon layer. This results in images with darker colour. It

is believed that these contaminants originate from the air pumping system or from

the microscope components. [71]


The SEM technique can give several interesting information concerning the sample,

depending on the specific signal that is used:

• ⌘BSE depends on the atomic number Z of the tested species: the increase of

Z leads to a larger yield. This results in a strong contrast between areas with

di↵erent compositions. Phases made of heavy atoms are clear, while regions


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rich in light atoms appear dark. Therefore, the analysis of BSE images pro-

vides, therefore, important information regarding the distribution of chemical

species in the sample.

• �SE does not change significantly with varying atomic number Z, but it is

a↵ected by the measurements parameters. In fact, �SE falls down with in-

creasing incident beam energy and rises with decreasing angle of incidence.

This last dependence can be explained by considering that SEs, because of

their low energy, can emerge from the sample only if they come from a region

close to the surface. Consequently, �SE depends strongly on the inclination

of the sample surface with respect to the incident beam, and, hence, on its

morphology.

• The analysis of X-rays emitted from the sample is useful to perform chemical

analyses. However, due to the lightness of lithium , these measurements are

inaccurate for LiPON thin films investigated in this thesis. therefore, they will

not be discussed in this section.[70, 71, 74]

4.4 Electrical and electrochemical characterization

Electrical and electrochemical properties of the materials involved in thin- film solid-

state lithium ion batteries are essential to obtain e�cient systems. In general, high

electronic conductivity is required for the electrode material, while the electrolyte

must not conduct electrons. In the present work the ionic conductivity of LiPON

thin films was investigated by means of impedance spectroscopy. Both dry and wet

measurements were employed.

In addition, solid electrolytes must have wide enough electrochemical stability

window and low electronic conductivity, while electrode materials should show high

rate performance and capacity for Li-ion intercalation. [1]

The experimental part of this thesis includes also the investigation of the be-

haviour of half and complete battery stacks through cylic voltammetry and charge-

discharge measurements. Both wet and dry methods were employed.

4.4.1 Electrochemical impedance spectroscopy(EIS)

Introduction

The impedance of a system is an indication of its resistance to the flow of electrical

current. AC impedance or impedance spectroscopy is based on the measurement of

impedance and phase response of a cell to the application of a sinusoidal potential


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input and on the recording of the resulting current. In particular, the frequency

dependency is analyzed to obtain information on the ongoing chemical processes and

to distinguish electric and dielectric contributions. The response is then fitted by

means of a modeling procedure that exploits an equivalent circuit made of di↵erent

electric components to represent the single contributions to the general behaviour

of the material. This method can be used to study the ionic conductivities and

activation energy values of a material. Both wet and dry measurements are in

general possible. [76]

Physical principle

When a monochromatic sinusoidal potential input V (t) = Vosin(!t) is applied to

a cell, a current with a certain phase di↵erence angle ✓ can flow in the material.

This can be expressed as I(t) = Iosin(!t + ✓). ✓ is null only for purely resistive

behaviour. The impedance Z is defined as the ratio of the applied voltage to the

flowing current and shows a larger generality with respect to resistance because it

includes also the phase di↵erence e↵ect. It is in general a complex quantity that

depends on the frequency w :

Z(!) = Z 0(!) + iZ 00(!) = |Z|exp(i✓). (4.10)

It can be represented in a complex plane by means of rectangular (Z’ and Z”) or

polar (|Z|,✓) coordinates, that can be easily converted by means of the following

equations [9, 76]:

Re(Z) ⌘ Z 0 = |Z|cos(✓) (4.11)

Im(Z)) ⌘ Z 00 = |Z|sin(✓) (4.12)

✓ = tan�1(Z 00/Z 0) (4.13)

|Z|=⇥(Z 0)2 + (Z 00)2

⇤. (4.14)

When Z(!) is represented in a complex plane, the so-called Nyquist plot is obtained.

In this plots the impedance can be represented as a vector with length equal to its

modulus (fig. 4.10) that forms an angle ✓ with the real axis. It is intuitive to

state that in most cases the impedance decreases as the frequency becomes higher.

Consequently, low frequency points corresponds to the left part of the plot, while

high frequency data will appear on the right side. Another possible representation

is the so-called Bode plot, in which both the impedance modulus and its phase shift

is plotted against the logarithm of the frequency. However, in this work only the

first type will be used. [1, 76, 77]


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Figure 4.10: Represenation of Z(!) in a Nyquist plot.

The first and most important step in impedance data processing is the selection

of a proper equivalent electrical circuit able to fit the material response. There are

no unique rules for its choice, but some guidelines can be given:

• combine models previously used in literature with physical intuition

• choose the circuit that provides good fit with the lowest amount of elements in

order to avoid di�cult physical interpretation of di↵erent components (com-

plex models give accurate fitting, but they lack physical meaning)

• check the consistency of the values of fitting parameters

• compare the results with other experimental techniques

• perform an electric breakdown to compare the value of the resistor representing

cables and contacts with the slope of the linear I-V curve obtained from the

experiment. [1, 76, 77]

Some common elements used for impedance fitting are given in table 4.1. It is

possible to see that he impedance response of a resistor does not depend on the

frequency and that it is a real value. Consequently the current is always in phase

with the applied potential di↵erence. On the contrary, the response of an inductor is

an imaginary value and depends of frequency. It gives rise to a 90o shifts between the

current and the voltage. Also the capacitor dose not have a real part. Its impedance

lowers as the frequency increases. In this case the phase shift between current and

voltage is -90o. Very often in reality capacitors does show an ideal behaviour. For

this reason constant phase elements(CPE) must be introduced. Their impedance


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Equivalent element Abbreviation Impedance

Resistor R R

Inductor L i !L

Capacitor C 1

i!C

Constant phase element CPE 1

Q0!n

exp

✓� in⇡

2

◆

Warburg element W �!� 12 � i�!� 1

2

Table 4.1: Comparison among ionic conductivities of LiPON samples deposited on Pt and

TiN substrates. Samples deposited at various deposition temperature and thicknesses are

considered.

response can be expressed as:

ZCPE =1

Q0

!nexp

✓� in

⇡

2

◆. (4.15)

Q0

and n are two fitting parameters. n represents the degree of ideality and varies

from 0 to 1. For n=0 the equation becomes a simple resistor, without any imaginary

component. On the contrary, for n=1, an ideal capacitor is represented by:

ZCPE =1

i!C. (4.16)

For n=0.5, equation (4.15) reduces to the so-called Warburg element. This is gen-

erally used to model di↵usion-limited motion at the lowest frequencies and is repre-

sented by the following impedance equation:

ZW = �!� 12 � i�!� 1

2 . (4.17)

s is the Warburg coe�cient and is dependent on the di↵usion constant and the

concentration gradient. The Warburg impedance gives rise in the Nyquist plot to a

straight line with 45o slope. [1, 76, 77]

If we consider an RC circuit as the one in fig. 4.11a, the resulting Nyquist plot is

a semicircle (fig. 4.11b). This system is of very common use in equivalent circuits.

Its impedance can be written as:

ZRC =1

R+

1

i!C. (4.18)

In real systems, very often several types of these responses are present and partially

overlap, therefore only part of each semicircle can be seen.[76]

Working conditions

EIS is useful for highly resistive materials analysis. Small amplitudes are required

in general in order to minimize the perturbation of the system and reduce possible


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(a) (b)

Figure 4.11: RC circuit (a) and its impedance response (b).

measurements errors. It is a non-destructive technique. [76, 77] Measurements can

be done in dry conditions or in a liquid electrolyte. The most important distinction

between wet and dry measurements is the sensitivity to electronic conduction: the

wet method, thanks to the electronically insulating behaviour of the liquid solution,

enables the characterization also of conductive materials. [1, 8, 76, 77]


Impedance spectroscopy is a useful method for the determination of the ionic con-

ductivity of solid-state electrolytes from proper fitting of materials response. Also

interface phenomena can be investigated. Moreover an indication of the number of

mobile carriers and of the activation energy for ionic motion can be obtained. In

addition, it provides information on the chemical processes that occur in the mate-

rial. The contribution of electric and dielectric components to such phenomena can

be separated. Attention must be paid to avoid wrong interpretation of the data,

for example when good fitting is achieved with equivalent circuits with no physical

meaning. [8, 76, 77]

4.4.2 Cyclic voltammetry

Introduction

Cyclic voltammetry is one of the most common electrochemical analyses. In this

method the voltage of a working electrode in a cell is swept between two values at

a fixed rate. [1, 9, 78, 79]


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Physical principle

The current that results from the application of a linear voltage input is measured.

After the arrival to a certain cuto↵ voltage, the scan is reversed. A cyclic voltammo-

gram can be obtained by plotting the flowing current versus the applied potential.

[1, 9, 78, 79]

The electrochemical stability of an electrolyte sample is defined by the region

with zero current. An example of a cyclic voltammogram for LiPON is given in fig.

(4.12a). No peaks are present since intercalation is not possible.

(a) (b)

Figure 4.12: (a) Electrochemical stability window of LiPON determined by cyclic voltam-

metry measurements. [80] (b) Cyclic voltammogram of LMO in the voltage range 3.1-4.5

V vs Li/Li+. [79]

On the contrary electrode materials are able to intercalate lithium ions and con-

sequently show spikes representing these phenomena at specific voltage values. An

example of a cyclic voltammogram for LMO in the voltage range 3.1-4.5 V vs Li/Li+

is given in fig. 4.12b. The peak positions can di↵er from the thermodynamic po-

tential value because of overpotential e↵ects, that depends on electrolyte resistance,

charge-transfer reactions and di↵usion processes. This e↵ect is in generally lower as

the scan rate is decreased. Release of lithium ions gives rise to a positive current,

while ions insertion to a negative one. [1, 9, 78, 79]

In an ideal capacitor the potential sweep causes the flowing of a current I given

by:

I =dQ

dt= C

dV

dT(4.19)

where dVdT

is the scan rate and C the capacitance. Very slow scan rates allow the

occurance also of slow processes. However, they can be very time-consuming. For

this reason higher scan rates can be employed, although these will show lower ca-

pacitance values. [1, 9, 78, 79]


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Experimental apparatus and working conditions

Cyclic voltammetry is performed in a proper solution typically using a three-electrode

setup: the working electrode, i.e. the one under investigation; the reference elec-

trode, that is an electrode with constant electrochemical potential; the counter elec-

trode, typically inert and present in the electrochemical cell in order to complete

the circuit. A potentiostat is used to apply and control the voltage on the working

electrode. In general, for batteries testing instruments with current ranges in the

order of mA or more are needed. [1, 9, 78, 79]


Cyclic voltammetry is a useful potentiodynamic electrochemical technique to inves-

tigate redox processes and electron transfer kinetics. Information about the formal

potential of both oxidation and reduction half reactions can be obtained when the

formed products are stable during all the measurement. It also provides information

about the reversibility of the of a redox reaction and the kinetics of heterogeneous

charge transfer phenomena. It can be applied also to the study of complex elec-

trode processes. When applied to the study of capacitors properties, it provides

information about voltage window, capacitance and cycle life. [1, 9, 78, 79]

4.4.3 Charge-discharge measurements

Introduction

Electrode intercalation performance is typically evaluated through charge-discharge

cycles that simulate real battery operation.

Physical principle

Charge-discharge measurements are based on the recording of the potential when

a constant current input, typically expressed in C-rate, is applied. Therefore the

C-rates expresses the rate at which the battery is discharged. A C-rate of 1C cor-

responds to the current necessary to fully recharge the electrode to its theoretical

capacity in one hour. Therefore, a value of 2C leads to full recharge in half an hour.

In some cases, the current is expressed in C/R, where R indicate the number of

hours to reach complete discharge of the nominal capacity. For instance, if a bat-

tery with a nominal capacity of 3 Ah is discharged at a rate of C/10, the discharge

current is 0.3 A. If it is discharged at a C/3, the corresponding current is 1 A. The

measurement data are typically shown in a graph of the potential versus the lithium

ion capacity. [1, 9, 79]


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Figure 4.13: Di↵erent types of potential behaviour in discharge curves. [8]

Experimental apparatus and working conditions

The experimental setup is identical to the case of cyclic voltammetry described in

section 4.4.2.


Lithium insertion and extraction occur at specific values of the potential and give

rise to characteristic plateau in the graphs potential versus capacity. These potential

values correspond to the possible operating potential of the electrode vs Li+/Li. In

the ideal case, the operating potential are stable. This corresponds to a completely

flat plateau. Di↵erent types of potential trends in discharge curves are given in fig.

4.13. [1, 9, 79]

The loss of capacity with increasing number of cycles can be easily obtained with

this method. It represents one of the most important parameters to estimate the

performance of batteries. The Coulombic e�ciency expresses the fraction of charge

capacity that is accessible during the discharge process. In other words, it is the

ratio of the capacity during discharge and the one after charging. [1, 9, 79]

Charge-discharge analysis can also be used to evaluate the rate performance of

the system. This is done by simply tuning the C-rate, although for low values these

measurements are very time-consuming. As intuitive, the C-rate strongly a↵ects

both the output voltage and the charge capacity. For low rates, the equilibirum

situation is well approached and the voltage is close to the open circuit value. It

is intuitive that the best trend is a flat voltage during discharge in order to limit

the complexity and costs of the circuits regulating the potential. As the discharged


Francesca Criscuolo


Figure 4.14: E↵ect of the C-rate on discharge curves. [9]

current is increased the whole curve shifts to lower values. this is due to resistive

drops over the entire curve. Moreover, at low and high depth of discharge (DOD)

other phenomena overlaps, in particular losses due to charge transfer kinetics occur

and mass transport limits further the performance, respectively. Therefore, at high

C rates the whole system capacity is not accessible and the delivered energy (rep-

resented by the area under the discharge curve) lowers. This e↵ect is schematically

represented in fig. (4.14), where the cell voltage is represented against the DOD. [1,

9, 79]


Francesca Criscuolo

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