Improving Performance of Cation-Disordered Rocksalt Oxyfluoride Cathodes

Abstract

The development of next-generation rechargeable batteries requires cathode materials with elevated energy and power densities, increased lifespan and safety, as well as reduced cost. Lithium-rich cation-disordered rocksalt (DRX) compounds, composed of a redox-active transition-metal (TM) cation such as Mn3+ and a redox-inactive d 0 TM cation such as Ti4+, Nb5+ and Mo6+, are promising new cathode materials in meeting these requirements. The high capacity in this class of materials originates from the combined redox activities of Mn cations at lower voltages and oxygen anions at higher voltages, typically around 4.5 V. While the Mn redox process is fairly reversible, O redox often has limited reversibility which presents significant challenges in maintaining cathode cycling stability.In this presentation, we report the excellent performance of a new Mn-rich DRX oxyfluoride cathode material based on the Li-Mn-Nb-O-F system. The phase-pure material was synthesized by using a fluoropolymer precursor in the solid-state reactions. The initial capacity contribution of the cathode was primarily attributed to Mn3+/Mn4+ redox with little O redox participation. The unique redox behavior, evidenced by the defined regions on the voltage profile (Figure 1a) and the dominating reduction peak near 3 V (Figure 1b), leads to a high discharge capacity and impressive cycling stability (Figure 1c). Stable cycling with a discharge capacity as high as 250 mAh g-1 was achieved. Combined with the high Mn content, the substitution of O anions with F likely increases the Mn migration barrier and consequently enhances overall stability.Figure 1. Electrochemical performance of the Mn-rich DRX cathode: (a) charge and discharge voltage profile, (b) the corresponding differential capacity versus voltage profile, and (c) discharge capacity as a function of cycle number. Figure 1