Structure And Electrochemical Properties Of Na₃V₂（PO₄）₃ Electrodes For Sodium-Ion Batteries

Na3V2(PO4)3(NVP)has been considered as one of the most promising electrode materials for sodium ion batteries considering its suitable sodium storage plateau,high theoretical capacity and good structure stability.The inherent open NASICON framework provides three-dimentional(3D)channels for fast sodium ion transport.Interestingly,NVP can work as both cathode and anode material,due to the structural advantage of capability of accomodating a very wide range of Na content associated with different potentials.However,severe capacity degradation and inferior rate capability resulting from low electronic conductivity,as well as the poor electrochemical reversibility of the incorporation of the fifth Na+ into NVP,have hindered its implementation.To solve these problems,this work demonstrates effective strategies of lattice doping at V site to remarkably boost the electrochemical performance of Na3V2-xMx(PO4)3(M = Al,Ti,Ca)electrode materials.The detailed electrochemical properties of Na3V2-xMx(PO4)3 electrodes for sodiuim-ion batteries,lattice doping effects on the crystal/electronic structure and charge transfer features,as well as the relationship between the structure and electrochemical performance were systematically studied theoretically and experimentally.On the other hand,high sodium storage performance in NVP was also realized by optimizing the particle morphology.At last,to further demonstrate excellent electrochemical performance of as-prepared NVP material for practical applications,the symmetric and asymmetric sodium-ion full cells were constructed,and their electrochemical properties are investigated.The main contents of this thesis are as follows:High sodium storage performance of NVP was achieved by optimizing the particle morphology(2D thin nanoplates assembled 3D microflower-like micro-nano structure)and surface engineering(coated by the N doped carbon layer).The unique nanosheets interconnected 3D porous structure facilites not only electrolyte soak in so as to provide large contact area but also the sodium ion transportation by shortening the diffusion path distance.The obtain 3D porous NVP/NC microflower material shows high reversible capacity(117.2 mAh g-1 at 0.1 C),excellent rate capability(79.1 mAh g-1 at 200 C)and ultra-long cycling life(73.3%of capacity retention over 10000 cycles at 100 C)as cathode for SIBs.The effects of A1 doping on the electronic structure and properties of ion dynamics of Na3V2-xAlx(PO4)3 cathode materials were systematically investigated through first principles calculations.It was found that the substitution of Al for V in Na3V2(PO4)3 can decrease the band gap and change the material from indirect to direct band gap,thus endowing an enhanced electronic conductivity.However,Al doping increases the migration energy,and consequently,is detrimental to the Na ionic conductivity.The optical measurement confirms experimentally the influence of Al substitution on the band gap change.The opposite effects of A1 doping on the electronic and ionic conduction leads to the occurrence of optimal A1 doping amount where Al-doped Na3V2(PO4)3 shows the best electrode reaction kinetics.The experimental results indicate that the optimum A1 doping level is x = 0.2.The Na3V1.8Al0.2(PO4)3 sample exhibits the smallest electrode polarization and the best rate performance.Additionally,due to the enhanced structural stability,Na3V1.8Al0.2(PO4)3 electrode displays superior cycling performance compared to the pristine NVP in a wide temperature range.The strategy of Ti doping at V-site was employed to boost the rate performance and cycling behavior of Na3V2(PO4)3 cathode.The Ti substitution for V enhances the electronic conductivity and Na+ diffusion coefficient,reduces the electrode polarization and therefore noticeably improves the rate performance and cyclability of Ti-NVP@C electrodes.The extra redox couples(Ti4+/Ti3+ and V4+/V5+)guarantee the high reversible capacity of the Na3V2-xTix(PO4)3@C electrodes by providing some extra capacity.The Na3Vi.9Ti0.1(PO4)3@C delivers a high discharge capacity(116.6 mAh g-1 at 1 C),an exceptional high-rate performance(93.4 mAh g-1 at 400 C),and ultra-stable long-term cyclability(69.5%of capacity retention over 14 000 cycles at 100 C).Low-cost Ca doping was demonstrated as an effective strategy to dramatically improve the electrochemical properties of NVP material during both cathodic and anodic processes.Outstanding rate capability and cycling performance of Na3V1.95Ca0.05(PO4)3@C nanocomposite used as both anode and cathode material originate from the multifunction of Ca doping:1)The mixed valence V4+/3+ in NVP lattice,generated from aliovalent Ca2+ substitution for V3+,increases the intrinsic electronic conductivity in the bulk;2)Ca2+ cations with bigger ionic radius enlarge Na+ migration channels and facilitate the sodium ion diffusion in the lattice;3)Structural stability is greatly enhaIced by Ca2+ doping due to the stronger Ca-O bond strength,leading to the excellent cyclic stability.When serving as a cathode material,Na3V1.95Ca0.05(PO4)3@C electrode displays unprecedented rate capability(93 and 84 mAh g-1 at 400 and 500 C,respectively)and long-term cycling stability(75.8%and 43.6%of capacity retentions over 10000 cycles at 100 and 200 C,respectively).In addition,when used as an anode material,Na3V1.95Ca0.05(PO4)3@C electrode demonstrates high reversible capacity of～180 mAh g-1 at 0.1 C,a surperior rate cabability with a specific capacity of 86.8 and 71.0 mAh g-1 at 400 and 500 C,respectively,as well as an ultra-stable cycling stability(67%and 50%of capacity retentions over 10000 cycles at 100 and 200 C,respectively).To further demonstrate excellent electrochemical performance of as-prepared NVP material for practical applications,the symmetric Na3V1.95Ca0.05(PO4)3@C//Na3V1.95Ca0.05(PO4)3@C and asymmetric NVP/NC//hard carbon sodium-ion full cells were constructured and well investigated.The symmetric Na3V1.95Ca0.05(PO4)3@C//Na3V1.95Ca0.05(PO4)3@C full cell demonstrates attractive rate capability(102.2 mAh g-1 at 50 C),cycling performance(0.02%capacity decay per cycle over 2000 cycles at 10 C rate)and high energy density of～166 W h kg-1.The asymmetric NVP/NC//hard carbon sodium-ion full cell shows advanages of high working voltage and low material cost.