Fluorinated Rocksalt Cathode with Ultra-high Active Li Content for Lithium-ion Batteries

The key to increasing the energy density of lithium-ion batteries is to incorporate high contents of extractable Li into the cathode. Unfortunately, this triggers formidable challenges including structural instability and irreversible chemistry under operation. Here, we report a new kind of ultra-high Li compound: Li_4+x MoO₅ F_x (1≤x≤3) for cathode with an unprecedented level of electrochemically active Li (>3 Li⁺ per formula), delivering a reversible capacity up to 438 mAh g^-1 . Unlike other reported Li-rich cathodes, Li_4+x MoO₅ F_x presents distinguished structure stability to immunize against irreversible behaviors. Through spectroscopic and electrochemical techniques, we find an anionic redox-dominated charge compensation with negligible oxygen release and voltage decay. Our theoretical analysis reveals a "reductive effect" of high-level fluorination stabilizes the anionic redox by reducing the oxygen ions in pure-Li conditions, enabling a facile, reversible, and high Li-portion cycling.