Unexpected Exchange Reactions between LiPF6 and Additives: Enhancing Thermal Stability and Mitigating Transition-Metal Dissolution

Abstract

To investigate electrolyte/electrode interactions as a way to understand and improve the overall stability of bulk electrolytes, electrodes, and interfaces, soaking experiments were conducted on an earth-abundant cathode active material of 0.3Li₂MnO₃·0.7LiMn_0.5Ni_0.5O₂ (LMR-NM) as part of an effort for the Argonne-led Deep Dive Cathode Consortium through the Department of Energy. It was discovered that electrolyte additives featuring a tetracoordinated B^–-(OR)₂XY structure [including lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate, and other additives] function through a specific swapping mechanism with early-stage LiPF₆ decomposition. This mechanism, as evidenced by NMR, facilitates the formation of thermally stable salts of LiPF₄(OR)₂, which prevents further electrolyte degradation. LiDFOB was further proven to be an effective additive in mitigating transition-metal dissolution of LMR-NM caused by acidic electrolyte decomposition products etching during electrode soaking tests due to bulk electrolyte stabilization. Additionally, in an effort to improve the stability between electrolytes and electrodes, surface modified electrodes were also tested, showing that both Co doping (as well as bulk) and Al(NO₃)₃ coating can also mitigate these adverse electrode/electrolyte interactions. Density functional theory simulations reveal that Co can increase the formation energy of surface Mn vacancy defects on LMR-NM in the presence of H⁺ ions, thereby making dissolution more difficult.