LAUSR

Cation-disordered rocksalt (DRX) oxides are a promising new class of high-energy-density cathode materials for next-generation Li-ion batteries. However, their capacity fade during cycling presents a major challenge. Partial fluorine (F) substitution into oxygen (O) lattice appears to be an effective strategy for improving the cycling stability, but the underlying atomistic mechanism remains elusive. Here, using a combination of advanced transmission electron microscopy-based imaging and spectroscopy techniques, we probe the structural and chemical evolutions upon cycling of Mn-based DRX cathodes with an increasing F content (Li-Mn-Nb-O-Fx , x = 0, 0.05, 0.2). We reveal atomic origin behind the beneficial effect of high-level fluorination for enhancing surface stability of DRX. We discover that, due to the reduced O redox activity while with increasing F concentration, F in the DRX lattice mitigates the formation of O deficient surface layer upon cycling. For low F-substituted DRX, the O loss near the surface results in the formation of an amorphous cathode-electrolyte interphase layer and nanoscale voids after extended cycling. Increased F concentration in DRX lattice minimizes both O loss and the interfacial reactions between DRX and liquid electrolyte, enhancing surface stability of DRX. Our results provide guidance on development of next-generation cathode materials through anion substitution. This article is protected by copyright. All rights reserved.