Entropy-assisted honeycomb-layered oxide without undesirable P3-O1 phase transition: A high-performance cathode for wide-temperature sodium-ion batteries

Abstract

Layered O3-type transition metal oxide materials (Na_xMO₂) attract substantial attention because of their affordability and high safety. Na₃[Ni₂Sb]O₆ is an O3-type layered oxide with honeycomb cation ordering in transition metal layers, which experiences the complicated phase transitions (O3-P3 and P3-O1) upon sodium ions de-insertion/re-insertion in the course of electrochemical reactions. Consequently, the material displays a staircase-like charge/discharge profiles with several voltage plateaus during the electrochemical process, leading to noticeable capacity decay and voltage drop with repeated cycling. The entropy-increase strategy via the replacement of multiple elements into the transition metal layers significantly improves the electrochemical behaviors of O3-type layered oxide cathode materials. The high-entropy O3-Na_7/8[Ni_0.44Cu_0.06Li_0.06Mg_0.06Zn_0.06Sb_0.32]O₂ (HEO) was obtained by a sol–gel method in this work. Structural characterizations indicate the O3-type HEO maintains the original honeycomb-type structure. In-situ structural tests show that the undesirable P3-O1 phase transition at higher potentials in O3-type layered oxides accompanied by large volume variation is successfully eliminated in our HEO, and the material undergoes an O3-O3/P3-P3 phase transition along with Na⁺ extraction and a reversible P3-P3/O3-O3 phase transition on Na⁺ re-insertion during the electrochemical process. Electrochemical measurements indicate the HEO displays smooth curves with no noticeable voltage plateaus and indicates a capacity of 112 mAh g⁻¹ with significantly enhanced cycling capability. More importantly, the material can function appropriately within a broad temperature ranging from −35 °C to 55 °C, enabling this material appropriate for practical applications. This work introduces an encouraging strategy to modify and optimize the honeycomb-ordered layered cathode materials for sodium ion batteries.