Effect of valence state of cobalt in cobalt hexacyanoferrate coprecipitated at different temperatures on electrochemical behavior

摘要: Cobalt hexacyanoferrate (CoFe(CN)6; CoHCF) is coprecipitated as a cathode material in an Na‐ion aqueous battery with varying solution temperatures of 5°C, 25°C, 50°C, and 70°C to tailor the particle size and defect content. At a low temperature, the small cubic particle is prepared with high Fe vacancies and low Na amounts. Conversely, an elevated temperature results in the enlarged particle with lower Fe vacancies and high Na amounts. Meanwhile, the lattice parameter is extended at a high temperature based on the Co2+ evolution with a larger ionic radius than that of the Co3+ ion at the Co site. Contrary with the expectations, a lower specific capacity is obtained for the highly crystalline structure at high temperature due to the evolution of the irreversible redox‐inactive Co2+. The low temperature of 5°C confers an excellent rate capability, capacity retention of 89% at 20 times faster charging rate, based on …