Theoretical and Experimental Optimization of P2-Type Sodium-Ion Battery Cathodes via Li, Mg, and Ni Co-Doping: A Path to Enhanced Capacity and Stability

Abstract

Understanding the oxygen-redox reactions within Mn-rich layered cathode materials is an important strategy to improve the capacity of sodium-ion batteries (SIBs) while satisfying the demand for low cost and the use of abundant resources. Nonetheless, the P2-type Na_y[A_xMn_1-x]O₂ compositions (where A = electro-inactive elements) exhibit poor capacity retention and low operation voltage along with a broad voltage hysteresis. In addition, Na_y[TM_xMn_1-x]O₂ (where TM = transition metal) still suffers from low capacity in the absence of anion redox activity. This investigation introduces Li, Mg, and Ni into the P2-layered Na_xMnO₂ matrix to explore diverse compositional dynamics engineered by density functional theory and ab initio molecular dynamics. The P2-Na_0.7[Li_0.1Mg_0.05Ni_0.15Mn_0.7]O₂ configuration is optimized, exhibiting enhanced structural and electrochemical stabilities. Operando X-ray diffraction analyses affirm the preservation of the P2 structure throughout de/sodiation, and comprehensive structural analyses unraveled the complex charge-compensation mechanisms facilitated by Ni²⁺/Ni⁴⁺, Mn³⁺/Mn⁴⁺, and O²⁻/(O₂)ⁿ⁻ redox pairs. Neutron diffraction and nuclear magnetic resonance techniques elucidate the Li migration phenomena within the TM and sodium layers. This research underscores the pivotal role of Li, Mg, and Ni co-doping in the development of cathode materials, paving the way for SIBs with enhanced electrochemical performance.