Irreversible Mechanical Deformations Associated with the Formation of Cathode-Electrolyte Interface on Lithium Iron Phosphate Cathodes

Abstract

The demand for rechargeable batteries for energy storage in electrical automobiles is rising presently due to their key role in reducing carbon emissions. However, the increasing demands require improvement in batteries lifetime. interfacial instabilities associated with the formation of cathode-electrolyte interface and intercalation-induced volumetric expansions are the primary reasons behind the capacity fade. Although there have been great efforts to understand structural stress/strains in cathodes, there is a lack of knowledge about the formation mechanism of cathode-electrolyte interface (CEI) layers. To fill the gap, we investigate the role of electrolyte chemistry on the interfacial deformation of lithium iron phosphate (LFP) cathodes by employing digital image correlation (DIC), spectroscopy, and electrochemical techniques. DIC was utilized to study the dynamic physical behavior of the electrode materials and irreversible strains associated with the electrolyte decomposition on the electrode surface [1]. Therefore, the overall objective of this study is to elucidate the mechanical deformations in the LFP cathodes of battery electrodes by investigating the physical response of the electrode and dynamic changes on the electrode – electrolyte interface.To be able to understand the different effects of the electrolyte solution, electrolytes were made of either LiPF6, LiClO4, or LiTFSI salts dissolved in 1 M EC:DMC. When cycled in the LiClO4 and LiTFSI salts containing electrolytes, the volumetric contraction is observed in the electrode because of the extraction of lithium and associated phase transformations during the first delithiation. However, positive strain generation was observed only during the first cycle in LiPF6 containing electrolyte, although delithiation results in reducing the lattice parameters. Control experiments were conducted where the electrode was cycled in LiClO4-containing electrolyte for one cycle and electrolyte was changed to the LiPF6-containing electrolyte for the subsequent cycles. A similar irreversible mechanical behavior was also observed in the control experiments where positive strain generation is recorded during the subsequent first delithiation cycle in LiPF6-containing electrolyte. To further analysis, electrochemical impedance spectroscopy measurements were performed on each system to identify surface resistance of the electrodes in a voltage range between 3 - 4.4 V. Increasing in the surface resistance of the electrodes were associated with CEI layer formations [2, 3]. Chemical composition of the CEI layers were characterized by using x-ray photoelectron spectroscopy. Our study provides a pathway to investigate the formation of the cathode-electrolyte interface on the cathode electrodes by monitoring irreversible physical behavior of the electrode – electrolyte interface during cycling.This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (Award numberDE-SC0021251). References Çapraz, Ö. Ö., Rajput, S., Bassett, K. L., Gewirth, A. A., White, S. R., & Sottos, N. R. (2019). Controlling expansion in lithium manganese oxide composite electrodes via surface modification. Journal of The Electrochemical Society, 166(12), A2357.Liu, Y., & Xie, J. (2015). Failure study of commercial LiFePO4 cells in overcharge conditions using electrochemical impedance spectroscopy. Journal of The Electrochemical Society, 162(10), A2208.Bassett, K. L., Çapraz, Ö. Ö., Özdogru, B., Gewirth, A. A., & Sottos, N. R. (2019). Cathode/electrolyte interface-dependent changes in stress and strain in lithium iron phosphate composite cathodes. Journal of the Electrochemical Society, 166(12), A2707.