High-Energy Na₄MnCr(PO₄)₃@C Cathode for Solid-State Sodium Metal Batteries

Developing high-energy cathode for sodium ion batteries (SIBs) that enables 3-electron reaction is the most effective way to reduce high cost and solve range anxiety and safety hazards. Here, a novel NASICON-type Na₄MnCr(PO₄)₃ (NMCP) is synthesized as a high-energy cathode for SIBs. The as-prepared 6.4 wt% carbon-coated NMCP (NMCP@C) cathode annealed at 700 ℃ delivers a discharge capacity of 107.3 mA h g⁻¹@0.1C, high rate capability up to 15C, and well capacity retention of 69.3% after 500 cycles at 5C within 1.4 ~ 4.3 V. Meanwhile, it also provides an exciting reversible capacity of 164.3 mA h g⁻¹@0.1C within 1.4 ~ 4.6 V and an ultrahigh energy density of 523.6 Wh kg⁻¹. Moreover, high sodium ion diffusion coefficients (D_Na+, 10⁻¹² ~ 10⁻⁹ cm² s⁻¹) demonstrate the superior electrode process kinetics, and both solid solution and 2-phase reactions are involved in the sodium ion extraction/insertion process through ex situ x-ray diffraction analyses. Na_3.3La_0.3Zr_1.7Si₂PO₁₂ with 3 wt% Na₂B₄O₇ additive (NLZSP(NBO3)) ceramic electrolyte is utilized to assemble NMCP@C//NLZSP(NBO3)//Na solid-state sodium metal batteries for the first time, which reveals an initial discharge capacity of 99.1 mA h g⁻¹ (1.4 ~ 4.3 V) and 121.7 mA h g⁻¹ (1.4 ~ 4.6 V) at 0.1C and an excellent cycling performance (capacity retention of 75.8%/72.6% after 100 cycles at 0.2C) at 30 ℃. All these confirm that NMCP@C is a high-energy cathode with great practical potential for all-solid-state SIBs.