W‐Doping Induced Efficient Tunnel‐to‐Layered Structure Transformation of Na _0.44Mn _{1‐ x} W _x O ₂: Phase Evolution, Sodium‐Storage Properties, and Moisture Stability

Abstract

Na _0.44MnO ₂ is a promising cathode material for sodium‐ion batteries owing to its excellent cycling stability and low cost. However, insufficient sodium storage sites still hinder its practical applications. Herein, a facile strategy to induce the efficient structural transformation from the tunnel to the layered type of Na _0.44MnO ₂ by trace W‐doping for the first time is reported. The W‐doping not only enriches the sodium storage sites but also improves the cycling performance. As a result, the phase‐pure P2‐Na _0.44Mn _0.99W _0.01O ₂ demonstrates an enhanced reversible specific capacity of 195.5 mAh g ⁻¹ and energy density of 517 Wh kg ⁻¹ at 0.1 C, accompanying superior cycling stability with capacity retention of 80% over 200 cycles. Moreover, the W‐doped samples show high structure stability in a moist atmosphere and can still maintain the original electrochemical performance after water treatment. In situ and ex situ characterizations reveal the enhanced structural stability of the P2‐Na _0.44Mn _0.99W _0.01O ₂ electrodes. This work provides a facile strategy on the structural engineering of transition metal oxides to induce the tunnel‐to‐layered structure transformation and could shed light on the design and construction of stable and high‐capacity cathode materials.