High-Performance and Cost-Effective All-Solid-State Conversion-Type Iron Fluoride Cathode

The large-scale deployment of lithium-ion batteries requires next-generation cathodes to not contain toxic and expensive elements such as Ni and Co, as well as to provide a higher energy density than existing cathodes. Conversion-type iron fluorides can fulfill these requirements and are being actively pursued by many research groups. Despite being discovered about two decades ago, the commercialization of cost-effective and energy-dense iron fluoride cathodes was obstructed by its low first-cycle redox reversibility, poor cycling stability, and large voltage hysteresis. Here, we demonstrated that these challenges are promising to be overcome by applying halide-based solid electrolytes (SEs). Through detailed ex-situ X-ray diffraction and X-ray absorption spectroscopy investigation, our results showed that halide-based SEs realize the complete intrinsic conversion and reconversion of FeF₂ because of their extinguished electrochemical stability, which cannot be achieved by sulfide SEs. The halide-SE-based all-solid-state FeF₂ cathode demonstrated extraordinary electrochemical performance compared with FeF₂ electrodes in liquid electrolytes, including a high specific capacity (~ 600 mAh/g), a long cycle life (over 100 cycles), a high first-cycle coulombic efficiency (~100%), and high-rate performance. The superior performance can be attributed to the reduced and reversible electrolyte decomposition, avoidance of cation dissolution, and mechanical lockage of electrode particles. Our study also revealed amorphization of FeF₂ cathode with halide SE enhanced the electrode kinetics. Our results demonstrate solidifying the batteries can be used as an effective approach to address the key challenges of iron fluoride cathodes for their practical application.