Study On Aluminum-doped Sodium Vanadate Phosphate As Cathode Material

Polyanionic sodium vanadylphosphate(Na3V2(PO4)3,NVP)is one of the most promising cathode materials for sodium ion batteries,owning to its outstanding stability,moderate redox potential(3.4V)and acceptable specific capacity(117.8 mAh g-1).However,the utilization of two expensive vanadium ions in each NVP molecule puts a big cost challenge for the large-scale application of NVP.In NVP,only one electron could be delivered by each vanadium atom in the redox couple of V3+/V4+(3.4 V),which is not efficient enough since the elemental V owns three available valences of V3+/V4+/V5+.From this perspective,it is theoretically feasible to substitute partial vanadium by another invariant three valence element,enabling a possible valence alternation from V3+to V5+,reaching enhanced voltage and energy density with involvement of less V.The main research contents are as followsIn the first chapter of this paper,the composition and working principle of sodium ion batteries,the types and research status of positive materials of sodium ion batteries are introduced.Finally,the background and research content of this paper are briefly described.In chapter 2,the changes of Gibbs energy of the redox processes of aluminum doped Na3AlxV2-x(PO4)3@C(NAVP@C,x=1/3,1/2,2/3,3/4,1)were simulated,which provides a reasonable structure design for experiments.In the third chapter,the carbon coated NAVPs were synthesized by sol-gel method.The morphology,structure and electrochemical properties of NAVP@C were characterized.Compared with NVP,a new voltage platform appears near 4.0 V for all family members of NAVP.Although Na3AlV(PO4)3 theoretically owns the highest theoretical energy density if V3+ could be fully oxidized into V5+,only around 1.5 Na+can be reversibly extracted/intercalated out/into NAVP,giving a capacity of around 93.0 mAh/g.With 2/3 Al substitute V,NAVP shows thermodynamic stability with two Na+extraction,but only around 1.7 Na+ can intercalate back,with a capacity of 103.6 mAh/g,owing to a shrank crystal size with Al doping.When Al amount is decreased to 1/2 and 1/3,the reversible discharge capacity increased to 106.3 and 113.8 mAh/g,respectively,gradually closer to their theoretical specific capacity of 120.8 and 119.8 mAh/g.Compared with all members of NAVPs,best capacity and rate performance were obtained when A1 amount was 1/3,owing to a suitable Al substitution together with an appropriate of crystal sizeThe fourth chapter summarizes the innovation and shortcomings of this paper,and potential improvements on the future work.