Stabilized Multi-Electron Reactions in a High-Energy Na4Mn0.9CrMg0.1(PO4)3 Sodium-Storage Cathode Enabled by the Pinning Effect

Na superionic conductor (NASICON)-type Na4MnCr(PO4)(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 Na4MnCr(PO4)(3) to alleviate Jahn-Teller distortions and to prolong the cathode cycling life by virtue of the pinning effect induced by implanting inert MgO6 octahedra into the NASICON framework. The as-prepared Na4Mn0.9CrMg0.1(PO4)(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 approximate to 558.48 Wh kg(-1) based on approximate to 2.8-electron reactions of Mn2+/Mn3+, Mn3+/Mn4+, and Cr3+/Cr4+ 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.

Automated summary

In this study, Mg is used to partially substitute Mn in the Na4MnCr(PO4)(3) cathode to improve its cycling stability. The Mg-doped cathode exhibits high capacity retention, impressive rate capability, and excellent energy density. In situ X-ray diffraction and density functional theory calculations confirm the highly reversible structural evolution and improved electrode process kinetics and electronic conductivity after Mg doping.