Guided phase transition for mitigating voltage hysteresis of iron fluoride cathode materials in lithium-ion batteries

Despite the high capacity attained by conversion-reaction-based metal-fluoride cathode materials in lithium-ion batteries through multiple electron storage, the large voltage hysteresis and low structural reversibility constrain their use. Herein, we propose guided phase transitions for designing conversion cathode materials that undergo minimal structural changes upon lithium-ion storage. This approach reduces the compositional inhomogeneity, a culprit of the voltage hysteresis, while providing high structural reversibility. Unlike the thermodynamically stable rhombohedral FeF3 (R-FeF3), which suffers from irreversible phase transitions accompanied by drastic structural evolution, tetragonal FeF3 (T-FeF3), a thermodynamically metastable phase guided by fluoride-ion incorporation into FeF2 from the electrochemical splitting of LiF, undergoes facile and reversible phase transitions during intercalation and conversion reactions by sustaining its structural integrity upon charge and discharge. Our research provides valuable insights into the significance of avoiding an irreversible reaction pathway and inducing it to minimize changes in the crystal structure for the design of conversion cathode materials with low voltage hysteresis and excellent cycle stability.