Unexpected Exchange\nReactions between LiPF<sub>6</sub> and Additives: Enhancing Thermal\nStability and Mitigating Transition-Metal\nDissolution

To investigate electrolyte/electrode\ninteractions as a way to understand\nand improve the overall stability of bulk electrolytes, electrodes,\nand interfaces, soaking experiments were conducted on an earth-abundant\ncathode active material of 0.3Li<sub>2</sub>MnO<sub>3</sub>·0.7LiMn<sub>0.5</sub>Ni<sub>0.5</sub>O<sub>2</sub> (LMR-NM) as part of an effort\nfor the Argonne-led Deep Dive Cathode Consortium through the Department\nof Energy. It was discovered that electrolyte additives featuring\na tetracoordinated B<sup>–</sup>-(OR)<sub>2</sub>XY structure\n[including lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate,\nand other additives] function through a specific swapping mechanism\nwith early-stage LiPF<sub>6</sub> decomposition. This mechanism, as\nevidenced by NMR, facilitates the formation of thermally stable salts\nof LiPF<sub>4</sub>(OR)<sub>2</sub>, which prevents further electrolyte\ndegradation. LiDFOB was further proven to be an effective additive\nin mitigating transition-metal dissolution of LMR-NM caused by acidic\nelectrolyte decomposition products etching during electrode soaking\ntests due to bulk electrolyte stabilization. Additionally, in an effort\nto improve the stability between electrolytes and electrodes, surface\nmodified electrodes were also tested, showing that both Co doping\n(as well as bulk) and Al(NO<sub>3</sub>)<sub>3</sub> coating can also\nmitigate these adverse electrode/electrolyte interactions. Density\nfunctional theory simulations reveal that Co can increase the formation\nenergy of surface Mn vacancy defects on LMR-NM in the presence of\nH<sup>+</sup> ions, thereby making dissolution more difficult.