Lithium halide cathodes for Li metal batteries

Lithium halide cathodes potentially offer a high energy density at a low cost for rechargeable batteries. However, these cathodes suffer from quick capacity decay in organic electrolytes, and the failure mechanism remains elusive. Here, we report that liquefying the halogen or interhalogen compounds is a prerequisite for achieving high reversibility for the lithium halide cathodes. The gas or solid halogen can be liquefied by using interhalogen compounds with different electronegativity or changing the temperature. As a proof of concept, reversible LiCl conversion-intercalation chemistry in organic electrolytes is demonstrated by using either redox coupling with less electronegative I/Br to form liquid ICl/BrCl or reducing the temperature to −30/°C. The LiCl-LiBr-graphite cathodes in 1.6 M lithium difluoro(oxalato)borate/1.6 M lithium triflate in diglyme electrolytes achieve a high reversible specific capacity of 250 mAh/g at 3.7 V with an energy density comparable to or higher than that of transition metal oxide cathodes at a much lower cost.