Cr-LiF as a high energy density conversion-type cathode for Li-ion solid-state batteries

Transition-metal fluorides (TMFs) are attracting attention as alternative lithium-ion battery cathodes, primarily focusing on Fe-based systems. Here, we report chromium as a previously unexplored transition metal (TM) for TMF cathodes in rechargeable lithium batteries. Utilizing a thin-film solid-state platform, we mitigate the common shortcomings of TMF cathodes, such as sluggish kinetics and electrolyte incompatibility. Coevaporation of Cr and LiF produces a heterogeneous thin film of Cr-LiF with a 1.1:2 stoichiometric ratio, delivering an initial capacity of 435 mAh/g and an energy density of 0.71 Wh/g at a C/10 cycling rate. Experimental measurements and first-principles calculations identify CrF2 as the dominant delithiated phase. The cathode maintains a capacity of 208 mAh/g at both 1C and 5C discharge rates after 1500 cycles. Compared to Fe-LiF (FeF2) analogs, Cr-LiF demonstrates a higher rate capability with 0.255 Wh/g at 3.80 W/g. This work introduces chromium fluorides as a new high-energy conversion cathode, expanding the options of viable positive electrode materials for next-generation batteries.