Single-crystalline Mg-substituted Na 4 Mn 3 (PO 4 ) 2 P 2 O 7 nanoparticles as a high capacity and superior cycling cathode for sodium-ion batteries.

Mn-based mixed phosphate Na 4 Mn 3 (PO 4 ) 2 (P 2 O 7 ) (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 Mn 3+ Jahn-Teller effect. Herein, a Mg-substituted NMPP (NM 2.7 Mg 0.3 PP)@C composite was constructed via modified solution combustion and subsequent calcination treatment. The obtained NM 2.7 Mg 0.3 PP 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 Mg 2+ incorporation, and the prepared NM 2.7 Mg 0.3 PP@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 2.7 Mg 0.3 PP@C cathode. In addition, NM 2.7 Mg 0.3 PP 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.