Study On Manganese Doped NASICON Type Sodium Vanadium Phosph Ate Cathode Materials

Sodium ion batteries(SIBs)are considered as a promising alternative for large-scale energy storage systems due to their abundant resources,low cost,and similar working principles to lithium-ion batteries.Among them,the exploitation of cathode material with excellent performance is crucial.Layer transition metal oxides can provide high specific capacity,however,its cycle stability is poor.NASICON(Na+ Super Ionic Conductor)-structured cathode materials have received extensive attention due to their stable structure and high ionic conductivity.The classic NASICON-type Na3V2(PO4)3 exhibits excellent cyclic stability,but V is expensive.Na4 Mn V(PO4)3 achieves high operating voltage and excellent cycle performance at 2.5-3.8 V by replacing V with cheap Mn,but suffers severe capacity decay due to irreversible V4+/V5+ reaction when widening the voltage window to 2.5-4.2V.Therefore,it is helpful to improve the cycle performance to study the mechanism of the reaction under the wide voltage window.Na3+x V2-x Mnx(PO4)3,Na3.25V1.75-x Mn0.25Mx(PO4)3(M = Al,Cr,x = 0.25,0.5)and Na3.5V1.5-x Mn0.5Alx(PO4)3(x = 0.25,0.5)compounds are prepared by a simple sol-gel method,and their structure,morphology and electrochemical properties are characterized.The effects of the Jahn-Teller effect of Mn3+ and the degree of V4+/V5+ reaction on the electrochemical performance are studied by the controlled variable method,respectively.Electrochemical results show that the electrochemical reversibility and stability of Na3+x V2-x Mnx(PO4)3 first increases and then decreases with the increase of Mn content.When the content of Mn is maintained at the same level,more V4+/V5+ reactions can be obtained more by electrochemically inactive element(Al,Cr)doping in Na3.25V1.75-x Mn0.25Mx(PO4)3,but it leads to poor electrochemical stability due to the shrinkage of structure.When the Al content is the same,more high-voltage discharge capacity cannot be obtained with the increase of Mn content,which leads to the reversibility of the V4+/V5+ reaction decreased.Therefore,Na3.25V1.75Mn0.25(PO4)3 with appropriate content of Mn can achieve stable and reversible high voltage capacity,but also take into account excellent cyclic stability(the capacity retention of 82.54%after 500 cycles at 1 C).