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Ion transport in lithium polymer electrolyte—ionic liquid systems Yogita Oza*,Daniel Gunzelmann, Siti Aminah Mohd Noor, Luke A O’Dell, Maria Forsyth. Institute of Frontier materials, Deakin University, Waurn Ponds Campus, Victoria, Australia School of Chemistry, Monash University, Clayton Campus, Victoria, Australia

Aim: To achieve high ionic conductivity in an ionomer system by creating anion centres on the polymer that are less associated with the corresponding counterions.

Applications:

Introduction: Extensive efforts have been undertaken to develop and optimize new materials for Li-ion batteries to address power and energy demands for electrochemical devices. The development of high ionic conductivity solid state electrolytes is paramount for a range of electrochemical devices including lithium ion batteries and solar cells. Lithium based polymer electrolyte materials used in such battery systems, therefore are considered an important research field. High ionic conductivity in single ion conducting polymer electrolytes is still the ultimate aim for many electrochemical devices such as secondary lithium batteries. Achieving effective ion dissociation in these cases remains a challenge since the active anion tends to remain in close proximity to the backbone charge as a result of low degree of ion dissociation. In this work 7Li solid state NMR measurements together with conductivity and thermal analysis of PAMPS (polyacrylamide-2-methyl-1-propane sulphonic acid) poly (N1222)(AMPS)Li+ and poly(N1124OMe)(AMPS)Li+ with different ratios of ionic liquid and lithium cations are presented in order to determine the ionic conductivity in these polymer electrolyte materials.

Schematic representation of a lithium-ion battery

Results:

Materials and methods: A series of ionomers were prepared by copolymerizing an AMPS monomer and vinyl sulphonate with Li+ and tetra alkyl ammonium cation in different ratios as follows:  PAMPS 33: Poly(N1222)(AMPS)Li+ 50% and 50% tetra alkyl ammonium cation.  PAMPS 34: Poly(N1222)(AMPS)Li+ 90% and 10% tetra alkyl ammonium cation  PAMPS 35: Poly(N1124OMe)(AMPS)Li+ 50% and 50% tetra alkyl ammonium cation  PAMPS 36: Poly(N1124OMe)(AMPS)Li+ 80% and 20% tetra alkyl ammonium cation Analysis of these materials has been carried out by:  Solid state NMR spectroscopy (SSNMR)  Differential scanning calorimeter (DSC)  Ionic conductivity by impedance spectroscopy

Narrowing of the 7Li NMR line widths at temperatures up to 120 °C indicates lithium ion mobility and further broadening indicates possible separation in this material.

The 7Li narrower line widths with increasing temperatures indicates increase in lithium ion mobility with 10% Li+ ion content.

The onset of narrowing of the lines below Tg indicates the mobility is increasing. as it goes further beyond glass transition temperature and phase separation for 50% which is also observed in static NMR.

The broad Tg for PAMPS 33 reflects an inhomogeneous material. PAMPS 33 shows a slightly sharper Tg at a lower temperature.

7Li

spin–lattice T1 relaxation times as a function of temperature indicate the Li ions are more mobile for 10%. than 50% below Tg

The Tg for PAMPS 36 is the lowest of all four samples, suggesting that the Li ions will be more mobile.

Discussion: At the lowest temperature the 7Li NMR line widths are broad and are approaching the rigid lattice line width. These broad line widths arise from distributions in chemical environments which lead to distributions in chemical shifts and quadrupolar interaction parameters, implying that the Li ions are essentially immobile. As the temperature increases, the line widths decrease as a result of increased Li ion mobility and hence averaging of these interactions. A decrease in the FWHM observed well below the glass transition temperature (Tg) shows that the Li ions are mobile below Tg. Temperature dependent 7Li spin lattice relaxation times are also indicative of an increase in Li ion mobility with change in T1 of around an order of magnitude over a temperature range of 150 °C. A single broad Tg peak is observed in the DSC plot for all compositions but with the measured value of Tg varying significantly between different compositions. The preliminary impedance plots show two components, suggesting two distinct conduction process are present.

References: 1)

2)

3)

Sun, J., MacFarlane, D. R. and Forsyth, M., Solid state ionics 47(2002), 333-339. Every, H.A, F Zhou, D.R Macfarlane, Electrochimica Acta 43(1998) 1465-1469. Aminah, M.N, Daniel Gunzelmann, Jiazeng Sun, Douglas R MacFarlane and Maria Forsyth J. Mater. Chem. A 2(2014) 365-374

The PAMPS 36 impedance plots for heating and cooling cycles at each temperature show two semicircles, indicating that two conduction processes are present, possibly due to phase separation which has been observed in similar systems to those studied here.

Future work:

To develop new copolymer electrolyte-ionic liquid systems with higher conductivity in order to meet the requirements for application of this type of materials in secondary lithium batteries.

Contact information yoza@deakin.edu.au luke.odell@deakin.edu.au maria.forsyth@deakin.edu.au Institute for Frontier Materials Deakin University, Waurn Ponds Campus, Geelong, VIC.

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