Degradation Analysis and Thermal Behavior of Ni‐Rich Cathodes at High Cutoff Voltages With Fluorine‐Free Electrolytes

Nickel‐rich layered cathodes, LiNixMnyCo1−x−yO2 (NMC), are promising materials for next‐generation Li‐ion batteries due to their high energy/power densities; however, capacity fading remains a major limitation. While fluorinated electrolytes have enabled optimal performance, their environmental and safety issues prompt the investigation of fluorine‐free alternatives. Here, we evaluate a fluorine‐free electrolyte based on lithium bis(oxalato)borate in NMC811||Si–graphite full cells operated at low and high cutoff voltages, comparing its performance with that of a LiPF6‐based electrolyte. The reactions at the cathode–electrolyte interface (CEI) are the primary cause of capacity fade, leading to electrolyte decomposition, layer disorder, and irreversible phase transitions. Increasing the cutoff voltage and the use of a fluorine‐free electrolyte exacerbate these reactions, resulting in more rapid capacity loss. Differential scanning calorimetry characterization of harvested electrodes from cycled cells with both electrolytes indicates that the anode exhibits lower thermal stability than the cathode and that the thermal stability of the anode is enhanced when using the fluorine‐free electrolyte. Overall, the results indicate that the fluorine‐free electrolyte performance is limited at high cutoff voltage by the cathode and highlights the need for improved CEI‐based additives or cathode coatings to allow competitive performance of more environmentally friendly electrolytes with fluorine‐free components.