Electrochemical Performance of Large-Grained NaCrO₂ Cathode Materials for Na-Ion Batteries Synthesized by Decomposition of Na₂Cr₂O₇·2H₂O

The solid-state reaction has been widely employed as the standard procedure to prepare oxide cathode materials for sodium-ion batteries. However, it involves multiple steps and consumes much energy. In this work, we report a facile method to synthesize a large-grained O3-NaCrO₂ cathode by directly reducing sodium dichromate dihydrate (Na₂Cr₂O₇·2H₂O) under a hydrogen atmosphere. Owing to its unique large particle morphology, the as-prepared NaCrO₂ exhibits a high tap density of 2.55 g cm^-3. The compact NaCrO₂ shows excellent electrochemical performance with a high reversible capacity of 123 mAh g^-1 at 0.1C, a high capacity retention of 88.2% after 500 cycles at 2C, and an outstanding rate capability of 68 mAh g^-1 at 20C. The performance is attributed to a stable structure from the distinctive morphology with small specific surface area to suppress interfacial side reactions and rapid Na-ion diffusion channels with a highly (110)-oriented crystal structure. Ex situ X-ray diffraction and cyclic voltammetry tests demonstrate the consecutive and reversible phase transition mechanism with facile Na⁺ migration. Importantly, the obtained cathode material exhibits an excellent performance in sodium-ion full cells with hard carbon as the anode.