synthesis and characterization of comb-polymethacrylate

Synthesis and Characterization of

Comb-Polymethacrylate

/Poly(ethylene oxide) Electrolyte for Li-ion

Batteries

JUAN DU

Outline Lithium-ion batteries Solid polymer electrolyte Mechanism of ion conduction in SPEs Mechanism of atom transfer radical

polymerization Experiment procedures Products Characterization (FTIR, NMR, DSC, EIS,

SEM & Galvanostatic cycling) Conclusions

Lithium-ion batteriesWhat’s the

mechanism?

ADVANTAGES

• High energy density

• Light weight

• High potential

• Long cycle life

Solid polymer electrolytes

Type Ion conductivity

Mechanical strength

Safety Price

Liquid Good Poor Unsafe Expensive

Gel Good Good Unsafe Not cheap

Polymer Poor Good Safe Cheap

Glass Poor Good Safe Not cheap

Solid polymer electrolyte is an ionically conducting but electronically insulating solution of a salt in a polymer.

Ionic conductivity SPEs: 10-6-10-9 S/cm(room temperature), 10-4-10-5S/cm (80-100 °C)Liquid electrolyte: 10-2-10-3 S/cm (room temperature)

Mechanism of ion conduction in SPEs

Both cations and anions in polymer electrolyte may contribute

to its conductivity, but their transport mechanisms are different.

Cations-each lithium-ion is complexed to PEO through roughly five ether oxygens. The transport of lithium-ion is connected with the movement of the complexing segment of the PEO chain.

Anions- hopping mechanism between different occupied sites and vacancies, which are large enough to hold the ion.

Mechanism of atom transfer radical polymerization

CuIBr/PMDETA CuIIBr/PMDETA+ + e-

H3CO

CCH

O

CH3

Br

+ e- H3CO

CC

O

CH3H

+ Br-

Br- + CuIIBr/PMDETA+ Br-CuIIBr/PMDETA

Initiator

transition metal catalyst/ligand

Mechanism of atom transfer radical polymerization

H3CO

CC

O

CH3H +O

OH3C

OC

CH

O

CH3

CH2

C O

OH

( R') ( M1) ( R'-M1)

M2 =

CH2C

O OOH6

M3=

CH2C

O OO

CH38.5

R'-M1 + M R'-M1- MR'-M1- M + n M R'-M1- Mn

INITIATION

PROPAGATION M = M1, M2 or M3

Radical coupling: R1-CH2-CXH + HXC-CH2-R2 R1-CH2-CXH-HXC-CH2-R2Radical disproportionation:R1-CH2-CXH + HXC-CH2-R2 R1-CH2-CXH2 + XHC=CH-R2

TERMINATION

Experiment procedures

Remove the

inhibitorsAdd

chemicalsThemostated at

50 ℃ for several hours

Remove the

catalystPrecipitation

ProductsCOMPOSITION

Aimed molecular

weight(g/mol)

Stickiness Succesful solventMethacrylate

content

Poly(ethylene glycol)

methacrylate content

Poly(ethylene glycol methyl

ether methacrylate

content

PMAPEGOH-90-10 90% 10% - 20,000 Somewhat

sticky chloroform

PMAPEGOH-80-20 80% 20% - 20,000 little sticky chloroform

PMAPEGOH-70-30 70% 30% - 20,000 little sticky Cannot

dissolve*PMAPEGCH3

-90-10 90% - 10% 20,000 very sticky chloroform

PMAPEGCH3-90-10 90% - 10% 80,000 very sticky chloroform



Products The PMAPEGOH-70-30 polymer could not be dissolved in

chloroform, even after stirring for 10 days at 50 °C. This can possibly be due to that the increased amount of hydroxyl (-OH) end-group monomers formed much more hydrogen bonds in the polymer, making it more difficult to dissolve.

*PMAPEGCH3-90-10, which had a molecular weight of 20000 g/mol. The polymer was too difficult to handle in any of the following steps due to its severe stickiness and somewhat liquid-like state. Perhaps since no hydrogen bond can form in the polymers with methoxy-group (-OCH3) end-capped side-chains, these hydrophilic side chains present high stickiness.

Characterizations (FTIR&1H NMR)

Characterizations (1H NMR)

The structures of the polymers are somewhat complex and

contain several protons which have similar chemical shift coupled with each other. Furthermore, they are random copolymers, i.e., the protons are placed in slightly different chemical environments when their neighbors are different, which may lead to different chemical shifts of the protons even if they originate from similar functional groups. These factors make it difficult to analyze the NMR spectra in more detail.

Characterizations (DSC)

Tg (°C)PMAPEGOH-90-10-

polymer 5.2PMAPEGOH-80-20-

polymer 4.1PMAPEGOH-90-10-polymer electrolyte 1.8PMAPEGOH-80-20-polymer electrolyte 6.7

Tg (°C)PMAPEGCH3-90-10-polymer -13.9PMAPEGCH3-80-20-polymer -24.8PMAPEGCH3-60-40-polymer -28.9PMAPEGCH3-90-10-polymer

electrolyte -25.2PMAPEGCH3-80-20-polymer

electrolyte -32.7PMAPEGCH3-60-40-polymer

electrolyte -17.2

Characterizations (DSC)The influences on Tg come from several factor:

the lithium ions in the salt (LiTFSI) leads to the formation of cross-links in the PEO part, and should thus increase Tg due to increased rigidity;

the anions (TFSI-) from the salt is a common plasticizer and will decrease Tg;

while high amounts of methacrylate monomers and hydrogen bond formation in the polymers will raise the Tg.

All of the factors are work at the same time, and it is difficult to say which is stronger or weaker. There is no obvious trend in changes in Tg when the salt is dissolved in the polymer matrix.

Characterizations (EIS)The impedance of the cell was measured by applying a sinusoidal potential excitation in the frequency range between 1×10-2 Hz and 1×107 Hz, and the AC current signal was recorded. The AC root-mean-square voltage was set to 1V, and the measurements were performed at room temperature, 30 °C, 40 °C, 50 °C and 60 °C. Thereafter, the AC frequency (F), and the real (Cp’) and imaginary (Cp’’) parts of the capacitance is given by the computer.

Characterizations (EIS)PEO

polymer electrolyt

e

PMAPEGOH-90-10 polymer

electrolyte


electrolyte

PMAPEGCH3-90-10

polymer electrolyte

PMAPEGCH3-80-20

polymer electrolyte

PMAPEGCH3-60-40

polymer electrolyte

Room temperat

ure4.33×10-7 2.04×10-7 7.57×10-7 1.99×10-7 1.48×10-6 7.12×10-6

30 °C 9.58×10-7 6.25×10-7 1.45×10-7 5.12×10-7 5.47×10-6 1.09×10-5

40 °C 8.16×10-6 1.47×10-6 3.87×10-6 2.17×10-6 9.12×10-6 2.14×10-5

50 °C 7.94×10-5 2.75×10-6 8.05×10-6 6.03×10-6 2.31×10-5 3.80×10-5

60 °C 1.92×10-4 4.07×10-6 1.51×10-5 1.48×10-5 4.62×10-5 7.02×10-5

Characterizations (EIS)

PEO polymer

electrolyte


electrolyte


electrolyte

PMAPEGCH3-90-10

polymer electrolyte

PMAPEGCH3-80-20

polymer electrolyte

PMAPEGCH3-60-40

polymer electrolyte

Room temperat

ure4.33×10-7 2.04×10-7 7.57×10-7 1.99×10-7 1.48×10-6 7.12×10-6

30 °C 9.58×10-7 6.25×10-7 1.45×10-7 5.12×10-7 5.47×10-6 1.09×10-5

40 °C 8.16×10-6 1.47×10-6 3.87×10-6 2.17×10-6 9.12×10-6 2.14×10-5

50 °C 7.94×10-5 2.75×10-6 8.05×10-6 6.03×10-6 2.31×10-5 3.80×10-5

60 °C 1.92×10-4 4.07×10-6 1.51×10-5 1.48×10-5 4.62×10-5 7.02×10-5

The ionic conductivity of all of the polymer electrolytes increased with increasing temperature. This is accordance with the VTF equation, which is commonly used to describe the variation of conductivity with temperature for amorphous polymer electrolyte systems below the melting point.

Characterizations (EIS)PEO

polymer electrolyt

e


electrolyte


electrolyte

PMAPEGCH3-90-10

polymer electrolyte

PMAPEGCH3-80-20

polymer electrolyte

PMAPEGCH3-60-40

polymer electrolyte

Room temperat

ure4.33×10-7 2.04×10-7 7.57×10-7 1.99×10-7 1.48×10-6 7.12×10-6

30 °C 9.58×10-7 6.25×10-7 1.45×10-7 5.12×10-7 5.47×10-6 1.09×10-5

40 °C 8.16×10-6 1.47×10-6 3.87×10-6 2.17×10-6 9.12×10-6 2.14×10-5

50 °C 7.94×10-5 2.75×10-6 8.05×10-6 6.03×10-6 2.31×10-5 3.80×10-5

60 °C 1.92×10-4 4.07×10-6 1.51×10-5 1.48×10-5 4.62×10-5 7.02×10-5

In the PEO electrolyte, there is a relatively large enhancement of ionic conductivity from 40 °C to 60 °C. This is due to that the melting point of the PEO electrolyte is around 52 °C, i.e., when the temperature gets close to 50 °C, the mobility of the system increases significantly, resulting in much higher ionic conductivity values.

Characterizations (EIS)The ionic conductivities of PMAPEGOH-90-10 and PMAPEGCH3-90-10 are lower than for the PEO electrolyte for all investigated temperatures, probably due to the low content of the PEO-based monomer – only 10 %. This also means that there is much lower salt concentration in the co-polymer electrolyte.

Characterizations (EIS)When comparing the two systems, it can be seen that the methoxy-group end-capped generally show higher conductivity. This could well be due to that they have longer PEO-side chains, hence higher salt concentration.Furthermore, the hydroxyl-group can form hydrogen bonds in the systems, which then decrease the mobility of the side-chains and thus result in lower ionic conductivity.

Ionic mobility The ionic conductivity of a polymer

electrolyte can be calculated from the equation:

Ionic mobilityThe overall ionic mobility can be estimated from:


The lithium-ion mobility is very low in PEO system at room temperature, but relatively high above its melting temperature (Tm = 52°C) at 60 °C. The reason is that the ion mobility is much higher in the liquid state than in the solid state.


The ion mobility for the bipolar system is much higher than the PEO system, because the dual polarity of the SPE system promotes a nano-scale separation and ordering of the macromolecular constituents, thus offer structures which have shown to significantly promote ionic transport.


The ion mobility increases with higher portion of the PEO-based monomers because the ion transport generally occurred in the amorphous portions of PEO.


The PMAPEGCH3 system higher ion mobility, due to its longer EO units side-chains and no hydrogen bonds formed in this system.

Characterizations (SEM)

uncoated LiFePO4

PMAPEGCH3-60-40 electrolyte coated LiFePO4

Characterizations (SEM)

SEM images of cross-section view of polymer electrolytes (PMAPEGOH-80-20, left; PMAPEGCH3-80-20, right) coated onto LiFePO4

electrode.

Characterizations (Galvanostatic cycling)

Conclusions The special characteristic of this polymer is that it comprises both

hydrophobic polymethacrylate backbones, and short (6 or 8.5 EO units) PEO side-chains, which are hydrophilic. The bipolar structure could result in a nano-scale phase-separation, which can offer higher ionic conductivity than linear amorphous PEO, as suggested in previous Molecular Dynamic studies.

From the results, it is seen that the lithium-ion mobility is comparatively low in a conventional PEO electrolyte system at room temperature, but relatively high above its melting temperature (Tm = 52°C) at 60 °C. The ionic mobility is much higher in the synthesized bipolar systems at ambient temperatures, in accordance with previous MD simulation studies.

Problems with the half-cell batteries: pinholes & contamination

Thank you very much!Supervisor:

Daniel BrandellTim BowdenSemra Tan

synthesis and characterization of comb-polymethacrylate

Documents