Rational Design of Phosphate Cathodes with Improved Na-storage Performance

As a typical Na-superionic conductor (NASICON), the Na3+xV2–xMnx(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 Na3.5V1.5Mn0.5(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 Na3.3V1.5Mn0.3Al0.2(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 Na3.3V1.5Mn0.3Al0.2(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.