Effect of Fluoride Doping on Lithium Diffusivity in Layered Molybdenum Oxide

The effects of cation doping on cathode performance has been extensively studied; however, the field of anion doping has historically received much less attention. Fluoride doping can greatly increase the initial diffusivity of the layered MoO₃ system. The first discharge cycle of the layered α-MoO₃ and MoO_2.8F_0.2 phases were investigated and compared using the galvanostatic intermittent titration technique (GITT) in a lithium ion cell. The analysis revealed that a slight reduction of the oxide by fluoride doping to form a fluorobronze (MoO_2.8F_0.2) eliminated a slow electrochemical process observed in α-MoO₃. Galvanostatic cycling studies show that while α-MoO₃ has a higher initial capacity, it exhibits a first cycle Coulombic efficiency of only 86% with rapid capacity fade which has been associated with lithium trapping within the MoO₃ layer. In contrast, the Li⁺ intercalation process in the fluorobronze was found to have a 94% Coulombic efficiency on the first cycle. By the third cycle Coulombic efficiencies greater than 99% were observed for five cycles. A thorough investigation of the synthesis of MoO_2.8F_0.2 is also presented. Under mild hydrothermal conditions, the fluorination of α-MoO₃ to form MoO_2.8F_0.2 is topotactic, while a competing reaction in solution forms MoO_2.4F_0.6 (ReO₃ structure). Methods to prevent the solution phase reaction from occurring are discussed.