Entropy-Mediated Stable Structural Evolution of Prussian White Cathodes for Long-Life Na-Ion Batteries

The high-entropy approach is applied to monoclinic Prussian White (PW) cathode materials for sodium-ion batteries to address the issue of unfavorable multilevel phase transitions, leading to structural degradation and poor cycling stability. A series of Mn-based PWs was prepared, composed of up to six metal atoms sharing the N-coordinated positions (with Mn predominating), endowing the materials with unique structural properties. The high-entropy PW material of composition Na_1.65Mn_0.4Fe_0.12Ni_0.12Cu_0.12Co_0.12Cd_0.12[Fe(CN)₆]_0.92■_0.08 was found to exhibit superior cyclability over medium-entropy, low-entropy and conventional single-metal PWs. In addition to the promising electrochemical performance, we report, to our knowledge for the first time, that the high-symmetry crystal structure (cubic form in this study) is favorable for high-entropy PWs during battery operation. Computational comparisons of the formation enthalpy of high-, medium- and low-entropy materials show that the compositionally less complex samples are prone to phase transitions that negatively affect the cyclability, especially in the deep de-/sodiated state. Based on data from complementary operando and ex-situ characterization techniques, an intrinsic mechanism for the stability improvement of the disordered PW structure during Na⁺ insertion/extraction is proposed, namely the dual effect of suppression of phase transitions and gas evolution.