High-Rate Nanostructured Pyrite Cathodes Enabled by Fluorinated Surface and Compact Grain Stacking via Sulfuration of Ionic Liquid Coated Fluorides.

Metal-polysulfide batteries are attracting broad attentions as conversion reaction systems of high theoretical energy density and low cost. However, their further applications are hindered by low loading of active species, excess conductive additive and loose (nanostructured) electrode network. Herein, we propose that compact grain stacking and surface fluorination are two crucial factors for achieving high-rate and long-life pyrite (FeS2) cathode enabled by sulfurating ionic liquid wrapped open framework fluorides. Both the factors can accelerate the Li- and Na-driven transport across pyrite-electrolyte interface and conversion propagation between adjacent grains. Such an electrode design enables a highly reversible capacity of 425 mAh/g after 1000 cycles at 1C for Li-storage and 450 mAh/g after 1200 cycles at 2C for Na-storage even under high loading of pyrite grains and ultrathin carbon coating (< 2 nm). Its cathode energy density can reach to 800 and 350 Wh/kg for Li and Na cells respectively under a high power density of 10000 W/kg. The cross-linkage between ionic liquid and fluoride precursors appears to be a solution to the reinforcement of surface fluorination.