Single-crystalline Mg-substituted Na4Mn3(PO4)2P2O7 nanoparticles as a high capacity and superior cycling cathode for sodium-ion batteries

Mn-based mixed phosphate Na4Mn3(PO4)2(P2O7) (NMPP) is a promising cathode for high-potential, low-cost and eco-friendly sodium-ion batteries. However, this material still faces some bottleneck issues in terms of low conductivity, disturbance of impure crystalline phase, micron-sized agglomerated particles and the Mn3+ Jahn-Teller effect. Herein, a Mg-substituted NMPP (NM2.7Mg0.3PP)@C composite was constructed via modified solution combustion and subsequent calcination treatment. The obtained NM2.7Mg0.3PP presents a highly pure phase and single-crystalline characteristics. It is noteworthy that the sample shows a smaller particle size of 100-300 nm due to the Mg2+ incorporation, and the prepared NM2.7Mg0.3PP@C cathode exhibits considerable discharge capacity (119 mA h g-1), an improved rate capability and excellent long cycling stability of 1000 cycles. A series of measurements indicated that the Mg-substitution enhanced the electronic conductivity and ion diffusion rate, and effectively relieved the lattice distortion influenced by the multiphase transition from the Mn Jahn-Teller effect of the NM2.7Mg0.3PP@C cathode. In addition, NM2.7Mg0.3PP adopts an optimal 3Mg0.1-Mn1-Mn2-Mn3 crystal structure based on density functional theory (DFT) calculations and refined X-ray diffractometry results. These findings provide new insight into the design of highly stabilized and high-conductivity polyanionic cathodes for sodium-ion batteries.