Abstract:

Activating anion redox reactions in P2-type layered oxides is an effective strategy to achieve higher specific capacities for sodium-ion batteries. A common approach to realizing anion redox reactions involves substituting transition metals with elements such as lithium or magnesium. However, this strategy faces challenges due to the inherent irreversibility of the anion redox reaction and kinetic hysteresis. In addition, high-voltage conditions can lead to irreversible oxygen loss and excessive electrolyte decomposition, resulting in rapid capacity decay and a sustained decrease in discharge potential. In this study, W-doped P2-type layered oxide Na_0.6Li_0.27Mn_0.73-x W _x O₂ (NLMWO) was synthesized using a high-temperature solid-state method. The effects of varying W doping levels on the electrochemical performance of the materials were investigated through multiple characterization techniques. The results demonstrate that W doping effectively reduces Na⁺/vacancy ordering, suppresses the P2-OP4-O2 phase transition, and enhances Na⁺ diffusion rates. The capacity retention of Na_0.6Li_0.27Mn_0.72W_0.01O₂ (NLMWO-1) with 1.0% W doping increased by 63.6% after 100 cycles at a rate of 0.5 C in the voltage range of 2.0—4.6 V, compared to Na_0.6Li_0.27Mn_0.73O₂ (NLMO). At 5.0 C, NLMWO-1 delivers an average discharge specific capacity of 92.6 mAh/g, 1.5 times that of NLMO, demonstrating excellent rate capability. These findings suggest that inactive metal doping can effectively mitigate the irreversible anion redox of transition metal layer oxides, providing valuable insights for the design of high-capacity and high-stability cathode materials for sodium-ion batteries.

Key words: sodium-ion battery, layered oxide, sodium manganate, element doping