Study Of Vanadium-based Phosphates Cathode Materials For Sodium Ion Batteries

Realizing multielectron reactions and more than one guest ions(de)intercalation in the electrodes is crucial to improve the capacity and energy density of batteries.Vanadium based phosphates attract extensive attention as potential high energy density cathode materials due to the accessibility of multiple oxidation states of vanadium ion(+2?+5).The mechanism of multielectron reactions and new compounds with potential multielectron reactions are explored here.A series of NASICON-type Na3V2-xrCrx(PO4)3 are synthes,ized via sol-gel method and served as model materials for the investigation of V3+/V4+/V5+ reactions.XRD is employed to investigate the structure and the effect of Cr3+ substitution on the cell parameters of Na3V2-xCrx(PO4)3.CV and galvanostatic charge/discharge reveal the V3+/V4+ and V4+/V5+activity in Na3V2-xCrx(PO4)3,Moreover,1.5-e reactions with inferior reversibility and cyclability are realized in Na3VCr(PO4)3 at 30 ?.Ex situ XAFS and 51V NMR,and in situ XRD discover that the inferior electrochemical performances are caused by irreversible long-range and local structural changes.The irreversible structural changes are further proposed to be resulting from the transformation of the metastable Na2-xVCr(PO4)3.1.5-e V3+/V4+/V5+reactions with excellent reversibility and cyclability are finally realized by suppressing the irreversible structural changes of Na2-xVCr(PO4)3 at low temperature.The reversibility of Na3VCr(PO4)3 during cycling is further investigated through in situ low/room temperature lab XRD,and V K-edge XANES.Firstly,a phase transition between Na3VCr(PO4)3 and Na2.5+sVCr(PO4)3 is revealed,which can indicate the reversibility of the multielectron reactions of Na3VCr(PO4)3.Secondly,Na3VCr(PO4)3 shows excellent reversibility of electrochemical reactions and structural evolutions during cycling if setting the voltage to 3.8 V.Thirdly,when the upper cut-off voltage is 4.3 V,excellent reversibility of electrochemical reactions and structural changes during cycling are also revealed at low-temperature.Lastly,the capacity loss accompanying continuously structural changes of Na3VCr(PO4)3 in the first several cycles is discovered.Moreover,23Na SS-NMR spectra reveal a new Na position with weaker Fermi contact effect after cycling.Nevertheless,the lost electrochemical performance and changed structure can be partially recovered by discharging Na3VCr(PO4)3 under 1.7 V.The deterioration of the electrochemical performances of Na3VCr(PO4)3 at room temperature is tentatively ascribed to the distortion of the lattice or migration of V to Nal sites which needs further investigation.The synthesis of Na3V(PO4)2 at a relatively lower temperature(700 0C)is researched through solid state method.The starting materials are NH4VO3,(NH4)2HPO4,Na2CO3,and acetylene black.A monoclinic C2/c structure is confirmed by the combining refinement of XRD and ND.BVS map shows 2D Na diffusion paths in the structure.The elemental composition of Na3V(PO4)2 is determined to be Na:V:P = 2.83:1:1.89,which fits well with the theoretical value.XANES and XPS further determine that the oxidation state of V ion in Na3V(PO4)2 is +3.The effect of the content of Na,V,and calcination temperature on the phase composition of the product is analyzed on the basis of the Na2O-V2O3-P2O5 ternary system.CV and galvanostatic charge/discharge reveal that Na3V(PO4)2 can uptake-1 Na with a plateau at around 3.4 V,corresponding to the capacity of 67 mAh/g.Na3V(PO4)2 could further extract Na at 4.1 V with inferior reversibility.Ex situ XRD reveal drastic structural changes upon Na(de)intercalation,which may relate to the capacity fading.The synthesis,characterization,electrochemical performance and mechanism of a new compound Na3V3(PO4)4 are investigated for the first time.Na3V3(PO4)4 with high purity is synthesized through sol-gel method using NH4VO3,H2C2O4·2H2O,NaH2PO4-2H2O,(NH4)2HPO4,and glucose as starting materials.XRD refinement and BVS map reveal that the new compound exhibits a unique layered structure with 3D Na diffusion paths.The morphology and elemental composition of the compound via SEM,ICP-OES,and XANES.The effect of the content of glucose,V,P and oxalic acid,calcination temperature,and the atmosphere on the phase composition of the product is analyzed.Effect of the oxalic acid and the decomposition product on the formation of Na3V3(PO4)4 is researched using TG-MS.CV and galvanostatic charge/discharge reveal that Na3V3(PO4)4 could be used as both of the cathode material and anode material.One Na can be reversibly uptaken with 3.9 V plateau when used as cathode.In addition,when used as anode material,two Na can be reversibly cycled with two plateaus at 1.5 V and 1.0 V.