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

Abstract

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 (NM_2.7Mg_0.3PP)@C composite was constructed via modified solution combustion and subsequent calcination treatment. The obtained NM_2.7Mg_0.3PP 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 NM_2.7Mg_0.3PP@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 NM_2.7Mg_0.3PP@C cathode. In addition, NM_2.7Mg_0.3PP adopts an optimal 3Mg_0.1–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.