Headline Electrochemistry, 84(10), 759–765 (2016)

Electrochemical Evaluation of Active Materials for Lithium Ion Batteriesby One (Single) Particle MeasurementKiyoshi KANAMURA,* Yuto YAMADA, Koji ANNAKA,Natsuko NAKATA, and Hirokazu MUNAKATA

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences,Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan

*Corresponding author: kanamura@tmu.ac.jp

ABSTRACTA one (single) particle measurement was employed to estimate electrochemical parameters of active materials forlithium ion batteries in order to design porous electrodes and cells. A micro electrode was used as a current collectorfor LiCoO2 and graphite particles. In the cases of materials with large expansion and shrinkage during discharge andcharge process, a tweezers-type current collector was developed and applied to the measurement. Si particle as ananode material for post lithium ion batteries was measured by using a tweezers-type current collector to stabilize acontact between active material and current collector. Successfully, the tweezers-type current collector provided astable contact to the active material particle. The electrochemical parameters for various active materials wereobtained from the one (single) particle measurement. Based on these parameters, the porous electrode and lithiumion cell can be designed.

© The Electrochemical Society of Japan, All rights reserved.

Keywords : Lithium-ion Battery, One (Single) Particle measurement, Active Materials, Electrochemical Parameters

1. Introduction

Lithium ion batteries have been utilized as power sources formobile devices and electric vehicles. Recently, lithium ion batterieshave been used in a smart grid system to utilize natural energies,such as solar power and wind power. The wide application needsvarious types of lithium ion batteries which are suitable for eachapplication. Therefore, the artificial design for lithium ion batteriesbecome more important based on battery sciences. There are manypublications for estimation of electrochemical properties of activematerials,1–4 binder materials,5,6 and other materials.7 For example,the exchange current (charge transfer resistance) and diffusioncoefficient of Li+ ion in solid matrix of active material and innon-aqueous electrolytes8,9 have been estimated. In the most ofpublications, a composite electrode consisting of active material,binder and conductive additive is used. The electrochemicalmeasurement using the composite electrode may be influencedby porous nature of composite electrode, chemical compositions,porosity, and thickness. This is a highly complicated system. Dokko

et al. has developed a new measurement technology for evaluationof electrochemical properties of active materials by using one(single) particle.10 In this measurement, there is no effect of porousnature of composite electrode. Another useful method is anelectrochemical measurement with a thin film of active material.11,12

However, some active materials cannot be prepared in a thin filmform due to chemical characteristics of materials. In addition, thethin films of active materials are not exactly same with the particlesof active materials used in practical batteries. The one (single)particle measurement can be applied to the real particles of activematerials. This method has been advanced and applied to variousactive materials for lithium ion batteries in some research groups.Here, a summary of research results obtained in our group isdescribed by using several standard active materials to clarify ausefulness of the one (single) particle measurement. Moreover,a new type of micro current collector is introduced, which canbe applied to new materials having large volume changes duringdischarge and charge processes.13–15

2. Electrodes in Lithium Ion Batteries

Positive and negative electrodes in lithium ion batteries havebeen prepared by a coating process of slurry consisting of activematerial, binder, conductive additive and solvent on Al or Cu currentcollector. After the coating, the slurry is dried to form a porouselectrode on the current collector. Figure 1(a) shows a cross-

Kiyoshi Kanamura (Department of AppliedChemistry, Graduate School of Urban Environ-mental Sciences, Tokyo Metropolitan University,Professor)

5/31/1984 Department of Industrial Chemistry,Graduate School of Engineering, Kyoto University,Doctor Course. 6/1/1984 Department of IndustrialChemistry, Faculty of Engineering, Kyoto University,

Research Instructor. 3/1/1995 Department of Energy and HydrocarbonChemistry, Graduate School of Engineering, Kyoto University, AssociateProfessor. 10/1/1998 Department of Applied Chemistry, Graduate School ofEngineering, Tokyo Metropolitan University, Associate Professor. 4/1/2002Department of Applied Chemistry, Graduate School of Engineering, TokyoMetropolitan University, Professor. Award: 4/1/1992 “Sano” Prize forYoung Researchers from Electrochemical Society of Japan, 5/1/2005Research Award, Energy Technology Division, Electrochemical SocietyInc. Hobby: Golf.

Yuto Yamada (Department of Applied Chemistry,Graduate School of Urban Environmental Sci-ences, Tokyo Metropolitan University, doctorcourse student)

Yuto Yamada was born in 1991. He received masterdegree from Urban Environmental Sciences, TokyoMetropolitan University in 2015. He has been doctorcourse student at same university until now. Hobby:

Piano, Guitar.

Electrochemistry Received: June 26, 2016Accepted: July 8, 2016Published: October 5, 2016

The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.84.759

759

sectional scanning electron micrograph of a typical porous electrodeused in lithium ion batteries. The electrochemical reaction takingplace in this electrode involves several elemental reaction processes,as described below.(1) Charge transfer process at an interface between active material

and electrolyte(2) Diffusion or migration of Li+ ions in active material(3) Diffusion or migration of Li+ ions in an electrolyte(4) Other side reactionsFigure 1(b) shows a schematic illustration of electrochemical

reaction taking place in the porous electrode. Usually, the thicknessof porous electrode is 30–100 µm with 40–60% porosity. Theporous matrix includes 90–98% active material, 1–8% conductivematerial and 1–8% binder material depending on preparationprocess of porous electrode. Processes (1)–(3) depend on the porousnature of electrode. In addition, a particle size of active material alsoinfluences on electrochemical measurement results. Various sidereactions may occur in the porous electrode. A typical one is anelectrochemical reaction of electrolyte. For example, electrochem-

ical reduction of electrolyte occurs on a graphite particle surface toform a solid electrolyte interphase (SEI).16,17 This phenomenon isvery important to realize a reversible discharge and charge behaviorof graphite anode. Process (4) also influences the electrochemicalmeasurement results. The porous electrode is necessary to obtainhigh capacity of electrode. However, when using the porouselectrode to evaluate electrochemical parameters of active material,the porous nature of electrode influences measurement results. Thisis due to current (potential) distribution in the porous electrode.18,19

One of useful method to eliminate the effect of porous nature is autilization of thin electrode which should be less than 10µm. In thiscase, the current distribution (potential) distribution is not so large,so that the electrochemical properties of active materials can beevaluated more precisely. However, there are still some effectsfrom binder and conductive materials. One of solutions for theseproblems is provided by one (single) particle measurement.

3. One Particle (Single) Particle Measurement

Figure 2 shows typical particles used in lithium ion batteries asanode and cathode materials. The particle size is 5–30 µm, so that theparticles can be observed with an optical microscope. By the way, amicro electrode system has been developed to observe electrochem-ical reaction without IR drop in low ionic conductive electrolytes.The effect from a diffusion process is also diminished by using themicro electrode system. In one particle measurement, the microelectrode is used as a micro current collector. Figure 3 shows thescanning electron micrograph of a typical micro current collector forone particle measurement. The micro current collector is connectedto one particle of active material under observation with an optical

Curr

ent

colle

ctor

e-

(3) Diffusion or migration of Li+ ionsin the electrolyte

(1) Charge transfer process

e-

Li+

(2) Diffusion or migration of Li+ ionsin the active material

e-

Li+

(4) Side reactions

Porous electrode

(a)

(b)

Figure 1. (a) Scanning electron micrograph of cross-section ofa typical porous electrode used in lithium ion batteries and (b) aschematic illustration of electrochemical processes taking place inporous electrode.

Figure 2. Scanning electron micrographs of typical particles usedin lithium ion batteries as anode and cathode materials.

Koji Annaka (Department of Applied Chemistry,Graduate School of Urban EnvironmentalSciences, Tokyo Metropolitan University)

3/31/2014 Department of Applied Chemistry,Graduate School of Urban Environmental Sciences,Tokyo Metropolitan University, Master Course.Hobby: Golf, Stock Investment, Single particlemeasurement.

Natsuko Nakata (Department of Applied Chem-istry, Graduate School of Urban EnvironmentalSciences, Tokyo Metropolitan University)

3/31/2009 Department of Applied Chemistry,Graduate School of Urban Environmental Sciences,Tokyo Metropolitan University, Master Course.Hobby: Music appreciation.

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microscope by using a manipulator. The schematic illustration of anelectrochemical cell used in this measurement is shown in Fig. 4.The cell can be tightly sealed to prevent volatilization of electrolytesolvent and the temperature of the cell can be controlled from¹10°Cto 80°C by using Peltier device. Two electrode system is employedfor electrochemical measurement in this system due to low currentmeasurement leading to negligible IR drop between working (microcurrent collector) and counter electrode. By using this system,LiCoO2 particle was measured. The counter electrode was Li metaland the electrolyte was a mixed solvent of ethylene carbonate andethylmethyl carbonate (3:7 in volume) with 1.0mol dm¹3 LiPF6.Figure 5 shows the cyclic voltammogram of LiCoO2 particle at1mV s¹1. The measurement was conducted at 30°C in argon glovebox. The anodic and cathodic current peaks were clearly observedwhich corresponds to Li+ ion extraction and insertion from/intoLiCoO2 matrix with some structural changes. This result showsthat the one particle measurement is very useful to characterizeelectrochemical reaction of active material for lithium ion battery.

4. Rate Capability of LiCoO2 and Graphite Particles

Figure 6 shows the discharge and charge curves during initialthree cycles and those at various current densities. The current of3 nA corresponds to 2.3C rate (discharge duration is 26min). At the

first charge and discharge, small irreversible capacity was observedwhich corresponded to electrolyte decomposition on LiCoO2 particleand current collector (Pt) surface. At the second discharge andcharge cycles, a reversible behavior was clearly observed. TheLiCoO2 particle exhibited high electrochemical performance ascathode material. The discharge curves at various current from 3 nAto 100 nA corresponding from 2.3C to 75.8C are shown in Fig. 7(a).As increasing C rate from 2.3C to 37.9C, the discharge capacityof LiCoO2 particle changed from 1.32 nAh to 1.25 nAh. Thiscorresponds to 5% decrease in the discharge capacity. In the case ofcomposite electrode with 50µm thickness, the discharge capacityalso decreased as increasing discharge current. However, even at the5C rate discharge, 5% of decrease in the discharge capacity wasobserved as shown in Fig. 7(b). This result is far from that for oneparticle measurement. The main reason for this difference is an effectof porous nature of composite electrode and influence of conductiveand binder materials. In order to evaluate electrochemical parametersof active material particle, composite electrodes are not suitable.One particle measurement provides more precise results.

Another example is the graphite anode used in lithium ionbatteries. Figure 8(a) shows the discharge and charge curves ofgraphite particle with 24 µm diameter. The well-defined dischargeand charge curves were observed and the discharge and chargebehavior was so reversible and 1.55 nAh discharge capacity wasobtained, which was smaller than that estimated from particle size.This is due to density of graphite particle and some error inassumption of spherical shape of graphite particle. Figure 8(b)shows the discharge curves at various current (various C rates). Atthe 100 nA discharge (64.5C rate discharge), the discharge curveshifted less than 200mV from the discharge curve at 7 nA dischargeto more cathodic potential. The shape of discharge curve did not

1st2nd3rd

2.0

1.0

1.5

0.5

0

- 0.5

- 1.0

- 1.5

Cur

rent

/ 10

-8A

E / V vs. Li/Li+3.6 3.8 4.0 4.2 4.2

Figure 5. Cyclic voltammogram of LiCoO2 particle at 1mV s¹1.

1st2nd3rd

4.2

4.0

3.8

3.6

3.4

3.2

3.0

E/ V

vs.

Li/L

i+

Capacity / nA h0 0.5 1.0 1.5

Figure 6. Charge and discharge curves of LiCoO2 particle duringinitial three cycles at 3 nA corresponding to 2.3C rate (duration:26min) in a mixed solvent of ethylene carbonate and ethylmethylcarbonate (3:7 in volume) with 1.0mol dm¹3 LiPF6.

Hirokazu Munakata (Department of AppliedChemistry, Graduate School of Urban Environ-mental Sciences, Tokyo Metropolitan University,Assistant Professor)

3/25/2004 Department of Materials Chemistry,Graduate School of Engineering, Osaka University(Ph.D.). 4/1/2004 Department of Applied Chem-istry, Graduate School of Engineering, Tokyo

Metropolitan University, Postdoctoral Researcher (JST-CREST). 9/1/2008Department of Applied Chemistry, Graduate School of Urban EnvironmentalSciences, Tokyo Metropolitan University, Assistant Professor. Award: 2011“Sano” Prize for Young Researchers from Electrochemical Society of Japan.Hobby: Sake-tasting, Cooking.

Figure 4. Electrochemical cell used in the one (single) particlemeasurement.

Figure 3. Scanning electron micrograph of a micro currentcollector for one (single) particle measurement.

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change so much. The discharge capacity did not change withchanging discharge current from 7 nA to 50 nA corresponding to4.5C (Discharging duration: 797 sec) 32.3C (Discharging duration:107 s). Only the operation potential of graphite particle changedfrom ³150mV to ³250mV vs. Li/Li+. From this result, it can beseen that the diffusion process of Li+ ion in graphite particle doesnot influence on Li+ ion insertion process and mostly controlled byinterfacial resistance at the graphite and electrolyte interfaceinvolving charge transfer resistance and surface film resistance ongraphite particle. From the discharge curves in Fig. 8(b), the pseudoTafel plot can be drawn to determine the exchange current densityand diffusion coefficient of Li+ ion in graphite particle. Figure 9shows the Tafel plot of graphite particle at discharge capacity of0.15 nAh (State of Charge (SOC) = 90%), as shown by dot line inFig. 8. The potential and current were obtained according to thisdot line. When the electrode reaction kinetics is controlled by thecharge transfer resistance with a small surface film resistance, therelationship between electrode potential and current obeys to theTafel equation. With increasing current, the measured values aredeviated from the Tafel line. This behavior indicates an influence ofLi+ ion diffusion process in graphite particle. The deviation beganat 64.5C rate discharge. The minimum time constant of diffusionprocess of Li+ ion can be determined from the starting current of thedeviation. 64.5C rate corresponds to 49.7 sec. By using this time asa time constant, the diffusion coefficient can be calculated accordingto the following equation:

L ¼ ð6DtÞ1=2

where L, D and t are the diffusion length of Li+ ion corresponding tothe particle radius, the diffusion coefficient of Li+ ion and the time

required for Li+ ion diffusion from the center to surface of particle,respectively. The diffusion coefficient was estimated to be 4.8 ©10¹9 cm s¹1. The diffusion resistance estimated from this value andparticle size is smaller than that from the diffusion coefficient of Li+

ion in electrolyte and diffusion length corresponding to thickness ofporous electrodes.

The apparent exchange current density can be also obtained fromthe Tafel plot shown in Fig. 9. The exchange current (i0) for thegraphite particle was estimated to be 0.31mAcm¹2. The surfacearea was calculated from diameter of graphite particle assumingspherical shape. In addition, an open circuit potential was obtainedat each SOC. Figure 10 shows a summary of exchange currentdensity at each SOC. The exchange current density changeddepending on SOC, especially at the end of discharge. In the

0.5

1

1.5

2

2.5

E / V

vs.

Li/L

i +

Capacity / nA h

7 nA10 nA20 nA

50 nA100 nA200 nA

400 nA

500 nA

750 nA

1000 nA

2000 nA

SOC = 90%

(b)(a)

0.5

1

1.5

2

2.5

0 0.5 1 1.5 20 0.5 1 1.5 2 2.5

E / V

vs.

Li/L

i +

Capacity / nA h

1st2nd, 3rd

Figure 8. (a) Discharge and charge curves of graphite particle with 24 µm diameter at 7 nA corresponding to 4.5C and (b) discharge curvesat various C rates in ethylene carbonate with 1.0mol dm¹3 LiPF6 at 20°C.

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0 0.2 0.4 0.6 0.8 1

log

i / A

cm

-2

E / V vs. Li/Li+

Equilibrium potential: 0.093 Vi0

Figure 9. Quasi Tafel plot of graphite particle at SOC = 90%.

4.2

4.0

3.8

3.6

3.4

3.2

3.0

E/ V

vs.

Li/L

i+

Capacity / nA h0 0.5 1.0 1.5

3 nA5 nA

15 nA

30 nA

100 nA

75 nA

50 nA

40 nA

20 nA

10 nA

4.2

4.0

3.8

3.6

3.4

3.2

3.0

E/ V

vs.

Li/L

i+

Capacity / mA h g-1

0 100 120 14080604020

5 C

1 C

2 C

0.5 C0.2 C

(a) (b)

Figure 7. (a) Discharge curves of LiCoO2 particle at various currents from 3 nA to 100 nA corresponding from 2.3C to 75.8C and (b) thoseof LiCoO2 composite electrode with 50 µm thickness at various C rates.

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region of SOC = 80%–30%, the exchange current density changed,slightly. By using the same method, the exchange current of LiCoO2

particle was estimated to be 0.81mAcm¹2.In this way, the electrochemical parameters can be obtained by

using one (single) particle measurement and Tafel analysis. Table 1shows a summary of diffusion coefficients and exchange currentsfor various materials. These values are important to design porouselectrodes for lithium ion batteries. One particle measurement with amicro current collector as shown in Fig. 2 can be applied to cathodeand anode materials with a small volume expansion. However, it isvery difficult to apply one (single) particle measurement for particleswith a large volume expansion.

5. Tweezers-type of Micro Current Collector

Figure 11 shows the discharge and charge curves of Si anodeparticle by using a micro current collector. A large irreversiblecapacity was observed at the first discharge and charge cycle. Thisresult is not only due to material nature, but also current collectorproblem. Figure 12 shows a new type of current collector which is atweezers-type current collector. Tips of stain-less steal tweezers areelectroplated by Cu metal to avoid electrochemical reactions takingplace on stain-less steal. Figure 13 shows cyclic voltammograms ofthe tweezers-type current collector with and without Cu electro-plating. The Cu electroplating is very useful to suppress a background current corresponding to the reduction of electrolyte on astain-less steal current collector. Figure 14 shows the discharge andcharge curves of Si particle obtained by one (single) particlemeasurement with a tweezers-type current collector. The dischargeand charge curves were very different from those in Fig. 11. The

Table 1. Summary of diffusion coefficients and exchange currents for various materials.

Sample Measurement conditionsExchange current density

(mA cm¹2)Diffusion coefficient

(10¹9 cm2 s¹1)

LiCoO2Particle size: 20 µm, SOC = 50%, 30°C1mol dm¹3 LiPF6/EC:EMC = 3:7 in vol.

0.81 1.4

LiFePO4Particle size: 24 µm, SOC = 50%, 30°C1mol dm¹3 LiClO4/EC:PC = 1:1 in vol.

0.27 2.7

GraphiteParticle size: 24 µm, SOC = 90%, 20°C1mol dm¹3 LiPF6/EC

0.31 4.9

Graphite(MCMB)

Particle size: 25 µm, SOC = 90%, 30°C1mol dm¹3 LiClO4/EC:PC = 1:1 in vol.

0.41 23

Li4Ti5O12Particle size: 26 µm, SOC = 50%, 45°C1mol dm¹3 LiClO4/EC:PC = 1:1 in vol.

6.5 35

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7

E / V

vs.

Li/L

i+

Capacity / nA h

1st

2nd

3rd

Figure 11. Discharge and charge curves of Si particle measuredby using a micro current collector.

Figure 12. Photograph of a tweezers-type current collector.

0

50

100

150

200

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

Rct /

Ω c

m2

i 0 / m

A cm

-2

SOC / %

Figure 10. Exchange current density and charge transfer resist-ance at each SOC.

-1000

-500

0

500

1000

0 0.5 1 1.5 2 2.5 3

I / n

A

E / V vs. Li/Li+

pristine

Cu-electroplated

Figure 13. Cyclic voltammograms of a tweezers-type currentcollector with and without Cu electroplating.

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stable discharge and charge behavior was clearly observed. Thisresult indicates that the tweezers-type current collector is suitablefor materials with a large volume expansion during discharge andcharge cycle. Figure 15 shows the optical microscope images beforeand after the electrochemical Li+ ion insertion. In this measurement,the optical microscope is employed to fix the current collector toactive material particle. This microscope is also very useful to in situobservation of particle volume expansion and shrinkage.

6. Material Characterization by Using One (Single) ParticleMeasurement

One (single) particle measurement is useful to characterize moreprecise electrochemical properties of active material. Recently, thismethod has been applied to one composite particle which consistsof binder material, active material, and conductive material. Thismeasurement provides information on binder and conductivematerial. The composite particle consists of anode or cathodeparticles with binder and/or conductive material. The size ofcomposite particle is usually 30µm, so that the effect of Li+ iondiffusion does not influence on electrochemical kinetics ofcomposite particle. Therefore, the similar analysis can be applied

to obtain electrochemical parameters. In this case, the effect ofbinder or conductive material on electrochemical characteristics ofactive materials used in lithium ion batteries can be evaluatedmore clearly. For example, LiCoO2 one particle measurement wasconducted at 60°C. At room temperature, LiCoO2 particle showsgood performance, as shown in Fig. 7. At 60°C, a degradationof discharge capacity was observed. On the other hand, LiCoO2

particle with PVdF binder showed a less degradation as shown inFig. 16. These results indicate an important function of PVdF binderon stability of LiCoO2 cathode material at high temperature. In thisway, one (single) particle measurement is not only applied to anevaluation of electrochemical parameters, but also to an investigationof other materials.

7. Conclusions

One (single) particle measurement has been introduced asanalytical method for active material and other materials usedin lithium ion batteries. The electrochemical measurement forcomposite electrodes is still very important to evaluate electrodeperformance. However, the estimation of electrochemical parame-ters should be done very carefully. The physical meaning ofobtained parameters is sometimes very complicated when usingporous electrodes. On the other hand, the one (single) particlemeasurement provides more precise electrochemical parameters dueto less influences of porous electrodes. A needle-type micro currentcollector can be applied to active material particles without largevolume expansion and shrinkage during discharge and chargecycles. However, some particles expand or shrink during dischargeand charge cycles. The contact between active material and currentcollector becomes poor. In this study, a tweezers-type currentcollector was newly developed and applied to Si particle anode. Thedischarge and charge curves were successfully measured. The one(single) particle measurement was also applied to evaluate functionof binder. Thus, the one (single) particle measurement is very usefulto obtain more precise electrochemical parameters and evaluatefunction of battery materials.

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