Ligand Field Electronic State Regulation of Monoclinic Prussian White Toward Highly Stable Sodium-Ion Batteries

Manganese hexacyanoferrate (MnHCF) has a high output voltage and is expected to be a promising cathode material for high energy density sodium-ion batteries (SIBs). However, the capacity decay problem caused by the Jahn-Teller effect of high-spin Mn³⁺ restricts its use in sodium-ion batteries. In this study, an elemental modulation strategy is proposed to regulate the electronic state of the ligand field by introducing the nickel element, which can keep the Mn in a low-spin configuration during cycling to inhibit the Jahn-Teller aberration of MnHCF. The nickel-doped MnHCF (NLS2-PW) with low-spin electronic state inhibits the disproportionation and dissolution process of Mn³⁺ and thus exhibits excellent cycling stability, and its capacity retention is close to 80% after 600 cycles at 2 C with a reduction of the Mn dissolution by ≈70%. Combined with theoretical calculations, it is confirmed that the distortion of the MnN₆ octahedron in NLS2-PW is reduced significantly, and the tetragonal phase transition caused by the Jahn-Teller effect during the electrochemical process is also effectively suppressed. This work demonstrates an efficient modulation strategy to enhance the performance of the Prussian blue analog cathode of sodium-ion batteries, being favorable for the design and optimization of relevant PB analogues for SIB applications.