Rational Design of Phosphate Cathodes with Improved Na-storage Performance

Abstract

As a typical Na-superionic conductor (NASICON), the Na_3+xV_2–xMn_x(PO₄)₃ cathodes have a high capacity of over 120 mA g^–1 owing to the multielectron reaction. However, these cathodes always show structural degradation and sluggish Na-ion diffusion kinetics derived from the Jahn-Teller distortion of Mn. Here, Al³⁺ was first used to replace Mn²⁺ in Na_3.5V_1.5Mn_0.5(PO₄)₃, which could significantly reduce the Jahn-Teller distortion through regulating the MnO₆ local crystal structure and enhancing Mn–O bond strength. Additionally, Al³⁺ could also increase the electronic conductivity and the Na-ion migration ability. As a result, the as-prepared Na_3.3V_1.5Mn_0.3Al_0.2(PO₄)₃ cathode achieved a high-rate capacity of 96.8 mAh g^–1 at 20 C and a high-capacity retention of 88.8% after 3000 cycles at 10 C. The application prospects of Na_3.3V_1.5Mn_0.3Al_0.2(PO₄)₃ cathode were further confirmed by a scale-up synthesis, and a 21700-type cylindrical cell showed an excellent cycling performance of over 80% capacity retention after 4000 cycles at 1 C.