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.

A solid electrolyte of LAGP [Li_1.5Al_0.5Ge_1.5(PO₄)₃] contained 0.5 wt% LiF was prepared by using low-volatile raw materials. The effects of different heat treatment conditions (750–900 °C, 4–10 h) and additive (LiF) on the ionic conductivity, structural morphology, and crystal transformation process were investigated in detail. EIS (electrochemical impedance spectroscopies) showed that the ionic conductivity of LAGP contained 0.5 wt% LiF had a highest value of 3.17 × 10⁻⁴ S cm⁻¹ with low activation energy (0.31 eV) after treating 825 °C for 6 h, more than LAGP of 2.45 × 10⁻⁴ S cm⁻¹. DSC analysis and SEM images indicated that adding a small amount of LiF to LAGP not only can lower the glass transition temperature (from 513 °C to 507 °C) and crystallization temperature (from 622 °C to 605 °C), but also can modify the grain boundary and increase the relative density of LAGP (from 95.8% to 97.7%).