Understanding the Fluorination of Disordered Rocksalt Cathodes through Rational Exploration of Synthesis Pathways

摘要

We have designed and tested several synthesis routes targeting a highly fluorinated disordered rocksalt(DRX)cathode,Li_(1.2)Mn_(0.4)Ti_(0.4)O_(1.6)F_(0.4),with each route rationalized by thermochemical analysis.Precursor combinations were screened to raise the F chemical potential and avoid the formation of LiF,which inhibits fluorination of the targeted DRX phase.MnF2 was used as a reactive source of F,and Li6MnO4 LiMnO2,and Li2Mn_(0.33)Ti_(0.66)O3 were tested as alternative Li sources.Each synthesis procedure was monitored using a multi-modal suite of characterization techniques including X-ray diffraction,nuclear magnetic resonance,thermogravimetric analysis,and differential scanning calorimetry.From the resulting data,we advance the understanding of oxyfluoride synthesis by outlining the key factors limiting F solubility.At low temperatures,MnF2 consistently reacts with the Li source to form LiF as an intermediate phase,thereby trapping F in strong Li-F bonds.LiF can react with Li2TiO3 to form a highly lithiated and fluorinated DRX(Li3TiO3F); however,MnO is not easily incorporated into this DRX phase.Although higher temperatures typically increase solubility,the volatility of LiF above its melting point(848 ℃)inhibits fluorination of the DRX phase.Based on these findings,metastable synthesis techniques are suggested for future work on DRX fluorination.