CNT-Decorated Na4Mn2Co(PO4)2P2O7 Microspheres as a Novel High-Voltage Cathode Material for Sodium-Ion Batteries.

The Mn-based mixed polyanion is expected to be a promising cathode material for sodium-ion batteries applied to large-scale smart grid energy storage systems due to its stable three-dimensional crystal structure, low cost, and high energy density. Herein, a novel carbon nanotube (CNT)-modified mixed-polyanion material (Na4Mn2Co(PO4)2P2O7) with a high voltage of 3.86 V is synthesized by a facile spray-drying method. The well-designed Na4Mn2Co(PO4)2P2O7/C-CNTs microsphere has excellent electronic and ionic conductivity by virtue of the carbon nanotube conductive skeleton. The as-prepared Na4Mn2Co(PO4)2P2O7/C-CNTs composite exhibits a reversible initial discharge capacity of 96.1 mA h g-1 and an energy density of 371 Wh kg-1 at 0.1 C. Furthermore, Na4Mn2Co(PO4)2P2O7/C-CNTs and hard carbon are assembled into a full battery, which delivers an initial discharge capacity of 88.8 mA h g-1, a working voltage of 3.85 V, and a promising energy density of 249.9 Wh kg-1 at 0.1 C. Therefore, the outstanding performance makes the Na4Mn2Co(PO4)2P2O7/C-CNTs material a potential candidate for large-scale applications of sodium-ion batteries.