Improving Sodium Manganese Hexacyanoferrate with Optimized Synthesis Conditions, Effective Drying, and Small Amounts of Nickel

Prussian Blue Analogs are a promising class of positive electrode materials for sodium-ion batteries that can be synthesized at low temperatures using only Earth-abundant elements like sodium, iron, and manganese. Their open framework structure allows them to sustain high current densities but also makes them prone to absorption of moisture. We improve the specific capacity of sodium manganese hexacyanoferrate (MnHCF) by optimizing synthesis and processing conditions, enabling a material-level energy density of 562 Wh kg −1 , which is on par with lithium iron phosphate. We remove interstitial water from these materials by rigorous drying. We also demonstrate a factor two improvement in cycling life of MnHCF by substituting only 3 at. % Ni for Mn and leaving some vacancies, which leads to 80% capacity retention after 3,500 h (∼5 months) of cycling in Na half cells at 0.2 C between 2.0 and 4.1 V and an ability to retain >85% capacity at a high current density of 10 C.