Preparation And Modification Of Li₃V₂（PO₄）₃ Aqueous Zinc Ion Hybrid Battery Cathode Materials

Due to their high safety and low cost,aqueous zinc ion batteries have been attracted,which is attributed to the zinc anode having the high theoretical and volumetric capacity（820 m Ah/g and 5855 m Ah/cm³）and low redox potential（-0.76V vs.Standard hydrogen electrode）,abundant in resources,and the advantages of good stability in oxygen and humid atmosphere.However,the development of aqueous zinc ion batteries is still in its early stage and there are a numer of problems that need to addressed,such as structural degradation of cathode materials in aqueous solution,low energy density,and complex zinc storage mechanism leading to unsatisfactory electrochemical performance.Therefore,the development and optimization of cathode materials is still the focus of current research.The polyanionic compound M₃V₂（PO₄）₃（M=Li,Na）is one of the most promising cathode materials for zinc ion hybrid batteries due to features such as VO₆octahedron and PO₄ tetrahedron sharing oxygen atom vertices to form a 3D open frame structure,which can provide large gap space and redox-active metal sites.However,Li₃V₂（PO₄）₃（LVP）has poor conductivity,structural degradation and other problems that tend to occur in aqueous solutions,leading to poor rate performance and cycling performance.To address theses problems,this paper improves the electrochemical performance of the material in three ways:by improving the preparation method,carbon cladding and manganese doping.The specific studies are as follows:（1）Hydrothermal-assisted sol-gel synthesis of micronano-particles LVP.The hydrothermal assisted sol-gel method combines the homogeneous properties of sol-gel synthesised materials up to the sub-micron,or even molecular level with the advantages of high purity,small particle size and no agglomeration in the hydrothermal synthesis.The effect of p H on the structure,morphology,and electrochemical properties of LVP was investigated.The influence of p H on the structure,morphology and electrochemical properties of LVP was investigated.The XRD results showed that all the samples synthesized belonged to monoclinic P2₁/n LVP.The crystallinity of the material improved as the p H increased,with the maximum peak intensity,sharp peak shape and best crystallinity at p H=7.SEM characterization analysis revealed that the special needle-like morphology increased significantly with increasing p H,showing a large number of randomly oriented needle-like nanoparticles at p H=7,which facilitates accelerated electron/ion transport.However,as the p H increases to 8,the needle-like special morphology decreases significantly.Electrochemical analysis showed that the first discharge specific capacities of the LVP-p H5,LVP-p H6,LVP-p H7,and LVP-p H8samples were 43 m Ah/g,54 m Ah/g,67 m Ah/g,and 55 m Ah/g in that order as the p H increased.Meanwhile,the LVP-p H7 sample had a higher discharge specific capacity at different current densities than the other samples and returned to a capacity retention of approximately 97%after 50 cycles at 200 m A/g,showing good rate performance.CV analysis yielded an ion diffusion coefficient of 1.299×10^-12 cm²/s for the LVP-p H7sample,which was higher than that of the other samples,mainly due to the presence of a large number of needle-like morphology and uniform distribution of micro-nano particles,the presence of the needles reduces the aggregation between the lateral structures,increasing the contact surface area with the electrolyte and accelerating the ion/electron transport.Results from density flooding theory calculations show that the energy-voltage relationship for adsorbed/desorbed ions in LVP materials remains consistent with experimental electrochemical test findings.The ex-situ XRD results verify that Li⁺can be reversibly de-embedded during the LVP charging and discharging process.（2）Freeze-drying assisted sol-gel method for the synthesis of LVP@C cathode materials.The carbon coating was used to refine the particle size of the material,improve the electrical conductivity and enhance the rate performance of the material.In particular,in order to suppress the agglomeration phenomenon caused by nanoparticles during high-temperature drying while maintaining a well-dispersed and homogeneous morphology,the freeze-drying assisted sol-gel method was used to synthesize the materials.XRD analysis showed that the synthesised samples belonged to the ideal monoclinic P2₁/n LVP,and no diffraction peaks containing carbon were detected,indicating that the carbon was wrapped on the surface of the material in an amorphous free state.SEM analysis revealed that the carbon encapsulated particles were more uniformly dispersed compared to the as-built LVP.In particlual,the LVP@C-05 sample has a finer and more uniformly dispersed paticle shape,and the TEM results show that the material is covered with a thin later of carbon.The electrochemical results show that the appropriate amount of carbon coating can effectively improve the electrochemical properties of the material.Especially,the LVP@C-05 sample of 2000 m A/g had a capacity retention of approximately 70%（relative to 200 m A/g）and a capacity retention of approximately 97%for 50 cycles,indicating that the material has good rate performance,which is attributed to the improved conductivity of the material due to the amorphous carbon coating on the surface and the ability to stabilise the main material structure in an aqueous electrolyte.The results of the EIS results show that the Rct value of LVP@C-05 is 189.5Ω,and CV analysis results show that the ion diffusion rate of LVP@C-05 is 2.59×10-12 cm²/s,and at a scan rate of0.5 m V/s the electrodes exhibit up to 96%pseudocapacitive charge storage,further validating the fast and stable ion migration and electron transport.The ex-situ XRD confirms the ability of Li⁺to achieve reversibly de-embedded in different charge and discharge states,indicating good structural stability of the material.（3）Preparation of Li₃V_2-xMn_x（PO₄）₃（x=0,0.02,0.04,0.06,0.1）by freeze-drying assisted sol-gel method.Manganese doping was used to improve the ion/electron transfer rate and rate performance.The XRD results showed that the Li₃V_2-xMn_x（PO₄）₃ products all belonged to the monoclinic（P2₁/n）Li₃V₂（PO₄）₃ structure and no manganese diffraction peaks were found,indicating that the crystal structure of the material was not altered by the appropriate amount of manganese doping.SEM results demonstrated that Li₃V_1.94Mn_0.06（PO₄）₃ sample was more dispersed and smaller in size.Meanwhile,EDAX also shows that the elements（V,Mn,P,O）in the Li₃V_1.94Mn_0.06（PO₄）₃ sample are uniformly distributed in the material,and the XPS results further indicate that Mn is present in the material in the form of positive divalent.The electrochemical results show that the Mn-doped samples exhibit better electrochemical performance compared to the undoped LVP.x=0.06 samples have a specific capacity of 106 m Ah/g for the first discharge compared to the doped x=0.02,0.04,0.1 samples,and capacity retention of about 92%after 2000 m A/g charge/discharge（versus 200 m A/g）and about 98%after 50.EIS results show that the Rct value of Li₃V_1.94Mn_0.06（PO₄）₃ is 233.6Ω.CV results show that the ion diffusion rate of Li₃V_1.94Mn_0.06（PO₄）₃ is 5.8×10^-13 cm²/s,which is higher than that of the other samples,which is consistent with the rate performance results.The ex-situ XRD results show that no heterogeneous peaks of Zn²⁺were detected in LVP,indicating that Zn²⁺was not embedded in the LVP lattice structure.Secondly,the analysis of XRD under different charging and discharging states showed that the Mn doping did not affect the crystal structure of the material.The LVP materials synthesized by the three modifications in this paper provide a reference idea for the application of AZHBs cathode materials.