Dialing in the Voltage Window: Reconciling Interfacial Degradation and Performance Decay for Cation-Disordered Rocksalt Cathodes

Abstract

Li-excess, cation-disordered rocksalt (DRX) cathode materials possess promising electrochemical properties and resource-friendly compositions, making them attractive Li-ion cathode materials. A key drawback of DRX materials is high interfacial reactivity that leads to electrolyte degradation, which ultimately causes a decay in cell performance. In this work, differential electrochemical mass spectrometry (DEMS) was used to study electrolyte degradation processes during initial cycling of DRX cathodes in different voltage windows, demonstrating the manner in which high- and low-voltage processes separately contribute to degradation at the cathode-electrolyte interface. Subsequently, extended cycling experiments were employed to evaluate the performance retention of electrolyte-lean DRX-graphite full-cells cycled in the same set of voltage windows, showing that performance decay is exacerbated by cycling in voltage ranges that induce interfacial degradation. Finally, post-mortem analyses of the full-cells after cycling were conducted to examine the cell degradation that underlies performance decay, revealing the loss of active Li and the dissolution of Mn and Ti from the DRX cathode. Collectively, these analyses demonstrate a clear link between electrolyte degradation and performance decay during cycling of DRX materials. This work highlights the necessity of voltage window optimization to maximize DRX cycling performance and the importance of cell design when evaluating cycling stability.