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-Na_0.7MnO₂. To address these issues, a high-entropy design combined with Cl substitution has been implemented, resulting in the material P2-Na_0.7Li_0.02K_0.02Mg_0.02Fe_0.05Cu_0.05Ni_0.25Ti_0.02Nb_0.02Mn_0.59O_1.93Cl_0.07. 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.