Preparation of Prussian Blue Submicron Particles with a Pore Structure by Two-Step Optimization for Na-Ion Battery Cathodes

Traditional Prussian blue (Fe₄[Fe(CN)₆]₃) synthesized by simple rapid precipitation shows poor electrochemical performance because of the presence of vacancies occupied by coordinated water. When the precipitation rate is reduced and polyvinylpyrrolidone K-30 is added as a surface active agent, the as-prepared Prussian blue has fewer vacancies in the crystal structure than in that of traditional Prussian blue. It has a well-defined face-centered-cubic structure, which can provide large channels for Na⁺ insertion/extraction. The material, synthesized by slow precipitation, has an initial discharge capacity of 113 mA h g^-1 and maintains 93 mA h g^-1 under a current density of 50 mA g^-1 after 150 charge-discharge cycles. After further optimization by a chemical etching method, the complex nanoporous structure of Prussian blue has a high Brunauer-Emmett-Teller surface area and a stable structure to achieve high specific capacity and long cycle life. Surprisingly, the electrode shows an initial discharge capacity of 115 mA h g^-1 and a Coulombic efficiency of approximately 100% with capacity retention of 96% after 150 cycles. Experimental results show that Prussian blue can also be used as a cathode for Na-ion batteries.