Alternative Solid-State Synthesis Route for Highly Fluorinated Disordered Rock-Salt Cathode Materials for High-Energy Lithium-Ion Batteries

Abstract

Fluorination has been identified as a key element for enabling the stable cycling of earth-abundant manganese-based disordered rock salt (DRX) cathodes. However, fluorination in the DRX bulk remains a challenge for scalable solid-state synthesis. In this study, a tailored reaction pathway is proposed to synthesize a highly fluorinated DRX. It is demonstrated for the first time that the unconventional precursors, Li₆MnO₄, MnF₂, and TiO₂, can avoid the formation of Mn-based intermediates (such as Li₂(Mn,Ti)O_3, LiMnO₂, and Mn₃O₄), which, once formed, persist until the synthesis temperature reaches close to or above that required for fluorine volatility. Therefore, this method can form a highly fluorinated DRX with a composition of Li_1.23Mn_0.40Ti_0.37O_2−yF_y (y = 0.29–0.34) at a low temperature (800 °C) relative to that required for conventional DRX solid-state reactions (≥900 °C). Li_1.23Mn_0.40Ti_0.37O_2−yF_y (y = 0.29–0.34) delivers a specific capacity above 300 mAh g⁻¹ and a specific energy of 980 Wh kg⁻¹ at 30 °C. Detailed characterization reveals that this DRX phase reversibly utilizes Mn^2+/3+ redox in the low-voltage region and Mn^3+/4+ redox in the middle-voltage range, whereas reversible oxygen redox is observed at high potentials.