Understanding High Voltage Electrolyte Reactivity on Cation-Disordered Rock Salt Cathodes

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Li-ion battery demand is expected to increase dramatically to meet rapidly increasing energy storage demands. To address the expense and scarcity of cobalt and nickel used in current layered oxide cathodes, alternative cathode materials that rely on earth-abundant transition metals must be explored. Promising such materials are cation-disordered rocksalt (DRX) oxides, particularly those that are comprised of manganese and titanium, along with lithium excess to allow for high reversible capacities (>300 mAh/g). A substantial fraction of extractable lithium capacity in DRX materials occurs at high voltages, creating high energy density, but also leading to deleterious degradation processes. Recent studies have observed reactivity of the DRX surface when cycled to high voltages: >25% of transition metals migrating out of the cathode and continuous evolution of CO2 after 150 cycles [1]. 13C labeling has identified the carbonate electrolyte as the primary source of the CO2 after the first cycle [2]. This reactivity leads to a sharp increase in impedance and rapid capacity fade [1]. However, the exact cause of high voltage reactivity is still not fully understood for the electrolyte – DRX interface.In this study, we quantify the surface reactivity of select electrolyte – DRX systems using operando gas measurements and inductively coupled plasma – optimal emission spectrometry (ICP-OES). Through this method, we can identify what electrolyte and DRX properties have the largest influence on surface reactivity. To monitor surface reactivity of the electrolyte, differential electrochemical mass spectrometry (DEMS) was used to quantify evolution of CO2 and other gaseous products during cycling. To track the surface reactivity of the DRX, ICP-OES was used to quantify transition metal dissolution. We show that ethylene carbonate oxidation is particularly problematic above 4.4 V, and that processes involving the conductive carbon, as well as Mn and O redox are culpable. The results of this study provide insight into the primary causes of capacity fade in DRX-based batteries, informing future designs of the electrolyte, DRX, and other cathode materials.[1]: Matthew J. Crafton et al 2024 J. Electrochem. Soc. 171 020530[2]: Manuscript in Preparation: Tzu-Yang Huang et al 2024. Chem. Of Mat. (Manuscript ID: cm-2024-007563)