Reversible Conversion Reactions with Lithium in Bismuth Oxyfluoride Nanocomposites

For the first time, the effect of the oxygen anion substitution on the reversible electrochemical activity of metal fluorides was investigated through the use of bismuth oxyfluorides. We demonstrate that a reversible conversion occurs in nanocomposites with the formation of , and/or LiF during lithiation and the reformation of during subsequent delithiation as opposed to a two-phase mixture of . We also show evidence that in BiOF and the fluoride component reacts first at higher voltage during lithiation to form , LiF, and . The newly formed is then subsequently reduced into and on a second plateau at lower voltage. During delithiation the process is reversed, reacting first at low voltage followed by LiF. Although cycling is presently poor, our results show that a relatively low oxygen content can significantly improve the electrochemical activity of the metal fluoride, suggesting that optimized metal oxyfluorides could be an attractive alternative to metal fluorides for applications as positive electrode materials in lithium batteries.