Ethylene Carbonate Free Electrolytes with High Oxidative Stability for High Voltage Lnmo Spinel Cathode in Li-Ion Cells

Abstract

The increasing implementation of lithium-ion batteries (LIBs) for the fast growing electric vehicle market calls for further improvements in energy density and drives research and innovation to investigate beyond state-of-the-art cell chemistries [1]. Currently, the capacity of the commonly employed transition metal oxide cathodes (NMC series) is approaching their theoretical limit. Thus, to further promote the energy density of LIBs, the most promising strategies are the adoption of a high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) cathode in combination with a high energy density anode. While commercial grades of LNMO are now available, the main challenge toward their implementation is the identification of suitable electrolyte systems with high oxidative stability enabling safe operation of LNMO cathodes at voltages beyond 4.7 V vs Li/Li+, which are also compatible with advanced anode materials operating at low potential. Indeed, electrolytes represent a crucial component in LIBs, and hence a great deal of research has been focusing on the next generation electrolyte systems. These should present a wide electrochemical stability window, high ion conductivity over a wide range of temperature, good thermal stability, and the ability to form stable interphases.[2] In this work we present results on a range of novel electrolytes, based on previous publications [3-5] including super concentrated systems with unconventional solvents to improve the anodic stability, beyond LiPF6 salts to mitigate transition metal dissolution at the cathode, and phosphorous and boron-based additives to improve the stability of the solid/electrolyte interphase (SEI) at the anode. The electrolytes are comprehensively characterised in terms of ionic conductivity over a range of temperatures (see Fig. 1a), viscosity, and thermal stability. Electrochemical impedance spectra (EIS) has been carried out in symmetrical lithium // lithium cells to investigate the interphase stability evolution upon time. Linear sweep voltammetry has been conducted to investigate the anodic stability (see Fig. 1b). The systems have been then investigated in combination with high voltage LNMO cathode, graphite anodes as well as a novel silicon/graphite composite anode. The results will highlight the results achieved so far toward the identification of suitable electrolytes for high energy density Li-ion cells, while discussing the remaining challenges and strategies to mitigate them.[1] Energies 2017, 10(9), 1314, G. Berckmans, M. Messagie, J. Smekens, N. Omar, L. Vanhaverbeke, J. V. Mierlo.[2] Chem. Soc. Rev., 2021, 50, 10486, X. Fan, C. Wang.[3] J. Am. Chem. Soc., 136 (2014) p. 5039, Y. Yamada, K. Furukawa, K. Sodeyama, K. Kikuchi, M. Yaegashi, Y. Takeyama & A. Yamada.[4] Nature Communications, 7 (2016) 12032, J. Wang, Y. Yamada, K. Sodeyama, C. H. Chiang, Y. Tateyama & A. Yamada[5] J. Mater. Sci: Mat. Elec. , 30 (2019) p. 5098, H. Zhou, B. Liu, D. Xia, C. Yin & J. Li Figure 1