Guided phase transition for mitigating voltage hysteresis of iron fluoride positive electrodes in lithium-ion batteries

Despite the high capacity attained by conversion-reaction-based metal-fluoride positive 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-type positive 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. The thermodynamically stable rhombohedral FeF₃ involves irreversible phase transitions accompanied by significant structural rearrangement during lithiation. In contrast, the metastable tetragonal FeF₃, electrochemically derived from a LiF-FeF₂ composite, undergoes facile and reversible phase transitions by maintaining structural integrity, enabled by conversion reactions between structurally analogous phases. Our study provides valuable insights into the importance of avoiding irreversible reaction pathways and deliberately guiding them to minimize structural changes in the crystal lattice, which is critical for designing positive materials with high structural reversibility.