The Preparation,Modification And Electrochemical Properties Of Vanadium-Based Phosphate Cathode Materials

Sodium-ion batteries show excellent potential for large-scale application in new energy electric vehicles due to their rich resources,inexpensive price,and high safety.Cathode materials with superior performance play a key role in the development of sodium-ion batteries.Na₃V₂（PO₄）₃（NVP）,as one of the potential cathode materials has the advantages of high working voltage,high capacity,and stable structure.Nevertheless,the inferior intrinsic conductivity and slow Na⁺diffusion kinetics of the NVP lead to subpar electrochemical performance.In view of that fact,the reasonable structural/component design of NVP is carried out in this thesis,which improves its inherent conductivity to ensure high working voltage and stable long-life cycling performance.The specific research results are as follows:（1）To address the problem of low electronic conductivity of Na₃V₂（PO₄）₃（NVP）,a typical hydrothermal process coupled with the subsequent calcination was used to rationally design the structure for building 3D hierarchical Na₃V₂（PO₄）₃@C micro-flowers（NVP MFs）,which were self-assembled with 2D single-crystalline nanoflakes.The amorphous nano-carbon layers and single-crystal structure with fixed orientation ensure efficient electron transport rates between and within NVP grains.The large surface area of nanosheets provides conditions for electrolyte infiltration,thus promoting charge transfer at the electrolyte/electrode interface.The micro-scale aggregate superstructure self-assembled by the low-dimensional nanometer building blocks provides elastic cushioning,reducing the strain caused by volume changes during Na⁺（de-）insertion,and providing feasibility for the long-term cyclic performance.The sodium intercalation cathode of NVP MFs provides superior rate performance(maintaining a reversible capacity of～92.9 m Ahg^-1 at 8 C)and excellent cyclic stability（4 C,maintaining capacity retention of～93.6%after 2000 cycles）.（2）In response to the demerit of slow kinetics,the tiny Cr doped single crystal micro-flowers cathodes Na₃V_1.94Cr_0.06（PO₄）₃@C（i.e.,NVCP-6）were synthesized employing scalable component design.The tiny Cr doping can tune the electronic structure of NVP,the occupation of Na（2）sites,and energy barriers of Na⁺migration simultaneously,and further improve the structural stability and the electronic/ionic conductivity of the NVP for efficient sodium storage.Based on these competitive advantages,the NVCP-6 manifests considerable rate capacity(～99.8 m Ah g^-1,even at a high rate of 200 C)and capacity retention of～82.2%over 7300cycles at 50 C.In addition,the full cell based on the NVCP-6 assembled with commercial hard carbon（HC）anode shows remarkably high rate capacity and long-duration cycle performance at a wide temperature range（-20-50°C）.（3）In order to improve the working voltage of NVP,the carbon-free and high-entropy Na₃V_1.94（Cr,Mn,Co,Ni,Cu）_0.06（PO₄）₂O₂F（NVPOF-HE）fluorophosphate cathode materials were designed and prepared by the introduction of high-entropy strategy.Due to the high-entropy effect of the advantages,the discharge capacity of NVPOF-HE is suppressed in the low-voltage region,the discharge proportion in the high-voltage region is increased,and the adverse influence of the discharge process in the low-voltage region is eliminated.The in-situ testing proves that the NVPOF-HE cathode has an excellent reversible phase transition behavior and a low volume change during the charging and discharging process.Therefore,NVPOF-HE reveals a high reversible rate capacity of～73.9 m Ah g^-1（at 50 C）and superior long-cycle performance（capacity retention of～90.2%after 4000 cycles）.In addition,the device matched with the HC anode shows superior sodium storage performance.