Decoding Li⁺/H⁺ ion exchange route toward low-temperature synthesis of layered oxide cathode materials for lithium-ion batteries

The synthesis of layered oxide cathode materials by the traditional high-temperature ceramic method usually requires calcination and annealing at temperatures in the range of 700–1000 °C, with high energy consumption and serious cation mixing problems. Herein, we present a novel hydrothermal Li⁺/H⁺ exchange method for the preparation of layered oxide cathodes at temperatures as low as 200 °C. In contrast to the widely reported Li⁺/Na⁺ exchange method using sodium-containing precursors, layered oxide cathodes can be directly synthesized by hydrothermal reaction between commercial hydroxide precursors and LiOH·H₂O. The reaction pathway consists of two steps. (1) The hydroxyl oxide intermediate is obtained by oxidizing the hydroxide precursor. (2) The layered oxide product is obtained by the Li⁺/H⁺ exchange reaction of the hydroxyl oxide with Li⁺ in solution. Through studying the time-resolved structural evolution, we reveal that the mechanism of material formation during Li⁺/H⁺ ion exchange is in situ crystallization, and the ion exchange process is accompanied by lattice distortion caused by internal diffusion of ions. These findings not only provide valuable insights into the Li⁺/H⁺ exchange process, but also provide a new paradigm for the low-temperature synthesis of advanced cathode materials.