Vacancy and Composition Engineering of Manganese Hexacyanoferrate for Sodium-Ion Storage

Manganese hexacyanoferrate (MnHCF) with an open framework is suitable for accommodating sodium ions. However, it suffers from severe capacity decay caused by the large structural deformation resulting from the Jahn-Teller (J-T) distortion of Mn-N6octahedra. By taking advantage of the strong chelation between EDTA and Mn ions, a strategy employing Mn vacancies (VMn) in combination with the introduction of Ni is proposed to suppress the J-T distortion of MnHCF. A Ni-doped MnHCF with VMnand a low crystal water content (K1.95Na0.05Ni0.02Mn0.93[Fe(CN)6] □0.05·0.41H2O) is prepared, which delivered an improved capacity retention of 80% at 0.5C after 100 cycles. This work provides a combined vacancy and composition engineering strategy for effective suppression of J-T distortion in the MnHCF framework toward high-performance sodium-ion batteries.