High‐Capacity, Long‐Life Iron Fluoride All‐Solid‐State Lithium Battery with Sulfide Solid Electrolyte

Abstract Metal fluoride–lithium batteries with potentially high‐energy densities are regarded as promising candidates for next‐generation low‐cost rechargeable batteries. However, liquid‐electrolyte metal fluoride–lithium batteries suffer from sluggish reaction kinetics, high voltage hysteresis due to side reactions, poor rate capability, and rapid capacity drop during cycling. Moreover, the research on sulfide all‐solid‐state batteries (ASSBs) with metal fluoride cathode is still lacking. Herein, four kinds of iron fluoride materials are applied to the sulfide all‐solid‐state lithium battery system for the first time to investigate the best cathode and corresponding methods. Electrochemical tests showed the cycling performance at different current densities (0.1, 0.3, and 1 C) and rate performance of the four cathodes with the following rules: FeF₃‐HT > FeF₃‐RT > FeF₃·0.33H₂O > FeF₃·3H₂O. The reversible capacities of FeF₃‐HT AASB are 519.9 mAh g⁻¹ after 120 cycles at 0.3C and still maintains 340.7 mAh g⁻¹ after 400 cycles even at a high rate of 1 C. In addition, electro impedence spectroscopy and cyclic voltammetry tests of the above four cathodes show that different contents of crystal water, morphologies, and particle sizes have a great influence on the lithium storage mechanism of cathode. Moreover, the reason for the FeF₃‐HT cathode's superior specific capacity and rate performance compared with other cathodes at high current densities is revealed, according to cyclic voltammogram tests under different scan rates. The cause is that FeF₃‐HT has the highest proportion of the contribution capacity of the cathode surface control process. The above research opens up a new avenue for FeF₃‐HT, FeF₃‐RT, FeF₃·0.33H₂O, and FeF₃·3H₂O cathodes in sulfide ASSBs.