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) is used to study electrolyte degradation processes during initial cycling of DRX cathodes. Comparing outgassing during cycling in six unique voltage windows with upper cutoff voltages of 4.6 or 4.8 V vs Li/Li+ and lower cutoff voltages of 1.5, 1.85, or 2 V vs Li/Li+ reveals the high- and low-voltage processes that separately contribute to degradation at the cathode-electrolyte interface. Charging to high voltages is shown to drive oxidative degradation, consistent with conventional understanding of interfacial reactivity at the cathode-electrolyte interface. Surprisingly, discharging below 2.0 V vs Li/Li+ is also revealed to drive reductive degradation at the same interface, which induces elevated CO2 evolution on the following charge. Subsequently, extended cycling of electrolyte-lean DRX-graphite full-cells shows that performance decay is exacerbated by cycling in voltage ranges that induce interfacial degradation. Post-mortem analysis also indicates notable loss of active Li and 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.