Energy storage in composites of a redox couple host and a lithium ion host

The quest for new positive electrodes for rechargeable lithium-ion batteries has been escalating in recent years. Until now, candidates of positive electrode were limited to crystals that contain both redox-active element (usually transition-metal) and lithium ion in the open framework with few exceptions. Here, we demonstrate lithium-free compounds, a material with little activity by itself, can be activated electrochemically by addition of LiF after the first charging. This general strategy is exemplified in various lithium-free iron compounds. Reversible lithium ion extraction and reinsertion take place for Fe ²⁺F ₂, Fe ²⁺SO ₄, and Fe ₂ ²⁺P ₂O ₇, when blended with LiF in nanoscale, in which a simultaneous valence change of Fe ^2+/3+ occurs above 3 V. FeF ₂-LiF could deliver 190 mAh g ^-1 (∼3.53 V) at 50 mA g ^-1 which is even higher energy density than that crystalline LiFePO ₄ can offer. Various combinations of blending are possible using this approach, which can bring a new branch of material group for positive electrodes in lithium-ion batteries.