Enhanced lifespan and rate capability of cobalt hexacyanoferrate in aqueous Na-ion batteries owing to particle size reduction

Cobalt hexacyanoferrate (CoHCF) suffers from low rate capability and lifespan because of the dissolution and structural instability of Co during cycling. A smaller active material can relieve the stress caused by volume change during cycling and enable shorter transport pathways. Here, particle size reduction is performed by reducing the mixing time or citrate concentration in the coprecipitation reaction to improve the rate capability and lifespan of CoHCF. The reduced mixing time produces smaller particles with enlarged active sites, thus improving the capacity. Upon coprecipitation at a low citrate concentration, particle precipitation at reduced sizes is achieved by preferential nucleation versus growth. Although lowering the citrate concentration enlarges the external surface area, the reduced redox-active Fe degrades the capacity. All CoHCF samples with small sizes achieved enhance rate capability and stability. In particular, the sample synthesized with a short mixing time (0 h) achieve a specific capacity of 122 mAh g⁻¹ with improved rate capability (79.4%) and stability (79.3%) over 500 cycles. Structural distortions are relaxed because of the shortened transport pathways and relieved distortion owing to particle size reduction. This study paves the approach for fast rechargeable and long-lasting aqueous Na-ion batteries using CoHCF electrodes.