Stabilized Multi-Electron Reactions in a High-Energy Na 4 Mn 0.9 CrMg 0.1 (PO 4 ) 3 Sodium-Storage Cathode Enabled by the Pinning Effect.

Na superionic conductor (NASICON)-type Na 4 MnCr(PO 4 ) 3 has attracted extensive attention among the phosphate sodium-storage cathodes due to its ultra-high energy density originating from three-electron reactions but it suffers from severe structural degradation upon repeated sodiation/desodiation processes. Herein, Mg is used for partial substitution of Mn in Na 4 MnCr(PO 4 ) 3 to alleviate Jahn-Teller distortions and to prolong the cathode cycling life by virtue of the pinning effect induced by implanting inert MgO 6 octahedra into the NASICON framework. The as-prepared Na 4 Mn 0.9 CrMg 0.1 (PO 4 ) 3 /C cathode delivers high capacity retention of 92.7% after 500 cycles at 5 C and fascinating rate capability of 154.6 and 70.4 mAh g -1 at 0.1 and 15 C, respectively. Meanwhile, it can provide an admirable energy density of ≈558.48 Wh kg -1 based on ≈2.8-electron reactions of Mn 2+ /Mn 3+ , Mn 3+ /Mn 4+ , and Cr 3+ /Cr 4+ redox couples. In situ X-ray diffraction reveals the highly reversible single-phase and bi-phase structural evolution of such cathode materials with a volume change of only 6.3% during the whole electrochemical reaction. The galvanostatic intermittent titration technique and density functional theory computations jointly demonstrate the superior electrode process kinetics and enhanced electronic conductivity after Mg doping. This work offers a new route to improve the cycling stability of the high-energy NASICON-cathodes for sodium-ion batteries.