High-Entropy Cl Substitution Promotes High Specific Capacity and Specific Energy Release from Sodium Manganate for Sodium-Ion Batteries

Poor electronic conductivity, low potential plateau, rapid capacity decay, and poor rate capability are critical challenges hindering the development of layered transition metal oxides such as P2-Na0.7MnO2. To address these issues, a high-entropy design combined with Cl substitution has been implemented, resulting in the material P2-Na0.7Li0.02K0.02Mg0.02Fe0.05Cu0.05Ni0.25Ti0.02Nb0.02Mn0.59O1.93Cl0.07. Hall effect tests reveal a transition from hole-dominated to electron-dominated conduction, significantly improving the Na+ transport kinetics, enhancing electronic conductivity, and facilitating the release of a high specific capacity and specific energy. Notably, the bulk carrier concentration increases from 1.63 × 1011 to 2.63 × 1012 cm–3, the surface carrier concentration rises from 2.47 × 1010 to 2.16 × 1011 cm–2, and the mobility improves from 322.7 to 455.9 cm2 (V s)−1. Consequently, the electronic conductivity undergoes a remarkable 20-fold increase from 8.44 × 10–6 to 1.915 × 10–4 S cm–1. The material delivers a reversible capacity of 196.80 mAh g–1 and a specific energy of 587.07 Wh kg–1 at a current rate of 0.05 C.