(Invited) Impact of Electrolytes on Li+ Charge Transfer Kinetics at the Electrolyte and Electrode Interface and Rate Performance in Li-Ion Batteries

Abstract

The rate performance of Li-ion batteries remains challenging, especially at temperatures below -20 oC. Apart from engineering improvements, the rate performance of the cell is determined by the rate of Li + ion transfer across the interface, so-called charge transfer kinetics and the rate of Li+ ion diffusion within the electrode. In this paper, we will focus on the Li + ion charge transfer kinetics as it is still poorly understood and can be improved by modifying the electrolyte. Examination of Li + charge transfer kinetics at both a graphite anode and a LiFePO4 cathode, revealed a much higher activation at the graphite anode/electrolyte interface than at the LiFePO4 cathode/electrolyte interface [1]. Values from 50 to 60 kJ mol-1 have been observed for the activation energy at the graphite/electrolyte interface [1-3] whereas, at the LiFePO4 cathode/electrolyte interface, an activation energy of 30 kJ mol-1 has been observed [1]. At either electrode, the solvated Li + ion needs to be de-solvated before diffusing into the electrode. The distinct difference between the graphite anode and the intercalating cathode is that there is a distinct presence of a solid electrolyte interphase (SEI) on the graphite anode while there is no SEI or no discretely defined SEI layer on the cathode. What is not clear is why the SEI on the graphite anode leads to a larger Li + charge transfer activation energy value. Is this due to diffusion through the SEI itself or the slow de-solvation process in the presence of the SEI? Recent studies indicate that the electrolyte composition plays a vital role in determining the electrolyte solvation structure and furthermore the nature of the SEI formed on the graphite anode can be modified by changing solvents, salt concentrations and additives [3-5]. This paper will report our recent exploration of how changing the electrolyte composition can improve the Li + charge transfer kinetics. References T. R. Jow, M. B. Marx, J. L. Allen, J. Electrochem. Soc. 2012, 159 (5), A604-A612. T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 2004, 151(8), A1120-A1123. Y. Yamada, Y. Iriyama, T. Abe, Z. Ogumi, Langmuir, 2009, 25(21), 12766-12770. M. Nie, D. P. Abraham, D. M. Seo, Y. Chen, A. Bose, and B. L. Lucht, J. Phys. Chem. C 2013, 117, 25381−25389. K. Abe, M. Colera, K. Shimamoto, M. Kondo, and K. Miyoshia, J. Electrochem. Soc. 2014, 161 (6) A863-A870.