A novel NASICON-Na3.4MnV0.2Cr0.2Ti0.6(PO4)3 cathode with ultrahigh energy density and remarkable cycling stability toward practical Na-ion batteries

The pursuit of an advanced cathode material featuring high energy density and long lifespan is of great significance to promote the practical applications of sodium-ion batteries (SIBs). Herein, a novel Na superionic conductor (NASICON)-type Na_3.4MnV_0.2Cr_0.2Ti_0.6(PO₄)₃/C (NMVCTP/C) cathode is developed, the active Ti³⁺/Ti⁴⁺, V³⁺/V⁴⁺/V⁵⁺, Mn²⁺/Mn³⁺/Mn⁴⁺, and Cr³⁺/Cr⁴⁺ redox couples endow the cathode with an extraordinary discharge capacity of 176.7 mAh g⁻¹ (3.1-electron transfer) and a high output voltage of 3.47 V, harvesting a record-high practical energy density of 613.15 Wh kg⁻¹ for the polyanionic cathodes for SIBs. The reversible bi-phase and solid-solution reaction with a small volume change of 5.5 % during the multi-electron charge/discharge process is systematically expounded by in-situ X-ray diffraction, ex-situ X-ray absorption spectroscopy and ²³Na nuclear magnetic resonance analyses. Besides, theoretical computations elucidate that Cr doping could enhance the V⁴⁺/V⁵⁺ reaction reversibility by suppressing the V migration to Na site. Consequently, the NMVCTP/C cathode affords an admirable cycling stability of 92.4 % capacity retention after 2000 cycles. More excitingly, a 3.75 Ah pouch cell is successfully constructed employing the NMVCTP/C cathode and hard carbon anode, demonstrating considerable application prospects. This study upgrades the energy density of polyanion-type Na-storage cathodes to a new level by rationally modulating redox couples.