Metal-polysulfide batteries are attracting broad attention as conversion reaction systems of high theoretical energy density and low cost. However, their further applications are hindered by the low loading of active species, excess conductive additive, and loose (nanostructured) electrode networkss. Herein, we propose that compact grain stacking and surface fluorination are two crucial factors for achieving high-rate and long-life pyrite (FeS2) cathodes enabled by sulfurating ionic liquid wrapped open-framework fluorides. Both of the factors can accelerate the Li- and Na-driven transport across the 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 1 C for Li storage and 450 mAh/g after 1200 cycles at 2 C for Na storage, even under a 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.
Keywords: alkali-metal batteries; conversion cathode; fluorinated carbon layer; ionic liquid; open-framework fluoride; pyrite FeS2.