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

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 the smaller particles and relieved distortion in cycling. The shorter diffusion path for Na intercalation due to the fine powder enables a faster charging process. Higher Fe vacancies induced at lower temperature limit Na accommodation, yielding a reduced distortion. The coprecipitation at low temperature enhances the rate performance and cyclability, sacrificing the capacity. Novelty Statement High temperature builds the large particle with low Fe vacancies and high Na amounts. High temperature derives the extended lattice constant due to Co2+ evolution in CoHCF. Evolution of the irreversible redox-inactive Co2+ degrades the specific capacity. Low temperature enhances rate performance due to small powder and relaxed distortion. High rate performance and cyclability at low temperature sacrifices the capacity.