Rational Design of Phosphate Cathodes with Improved Na-storage Performance

As a typical Na-superionic conductor (NASICON), the Na 3+ x V 2– x Mn x (PO 4 ) 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, Al 3+ was first used to replace Mn 2+ in Na 3.5 V 1.5 Mn 0.5 (PO 4 ) 3, which could significantly reduce the Jahn-Teller distortion through regulating the MnO 6 local crystal structure and enhancing Mn–O bond strength. Additionally, Al 3+ could also increase the electronic conductivity and the Na-ion migration ability. As a result, the as-prepared Na 3.3 V 1.5 Mn 0.3 Al 0.2 (PO 4 ) 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 3.3 V 1.5 Mn 0.3 Al 0.2 (PO 4 ) 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.