Investigation Of Vanadium Phosphates As Cathode Materials For Aqueous Zinc-ion Batteries

Aqueous zinc-ion batteries（AZIBs）,with its advantages of rich resources,high safety and eco-friendliness,have been considered as a promising large-scale energy storage system.The cathode material is important for the overall performance of the AZIBs.However,the further development of AZIBs is limited by the problems such as low specific capacity and operating voltage,poor rate performance and poor cycling stability.Therefore,the design and synthesis of high-performance cathode materials is the key to promote the development of AZIBs.The vanadium phosphates with stable open skeleton structure become the ideal cathode materials for AZIBs due to its advantages of high operating voltage,high specific capacity and excellent cycling stability.In this thesis,two kinds of new vanadium phosphate cathode materials were designed and synthesized,and their physical properties,electrochemical performance and charge/discharge mechanism were investigated,which provided new electrode material system for the development of AZIBs.Zn₃V₄（PO₄）₆ has a stable three-dimensional skeleton structure and large transport channel for Zn²⁺,which can realize the rapid transfer of Zn²⁺.Moreover,Zn₃V₄（PO₄）₆contains Zn²⁺and is suitable to be used as cathode material for rocking-chair-type AZIBs.However,the storage performance of Zn²⁺is not studied for the poor electronic conductivity.In order to improve the electrical conductivity of Zn₃V₄（PO₄）₆,a double-conducting carbon network Zn₃V₄（PO₄）₆@C/BP composite was designed and synthesized.Benefiting from the two-electron reaction of vanadium and the co-intercalation of Zn²⁺/H⁺,Zn₃V₄（PO₄）₆@C/30%BP delivers a specific capacity as high as 120 m Ah·g^-1 at 0.04 A·g^-1.In addition,Zn₃V₄（PO₄）₆@C/30%BP has excellent electrochemical stability,with a capacity retention rate of over 80%after 400 cycles at 1.0 A·g^-1.This work not only provides a new type of vanadium phosphate cathode material for AZIBs,but also gives a new strategy to increase the specific capacity of phosphate cathode material.Metal oxalate-phosphate open frameworks（A₂[（VO）₂（HPO₄）₂（C₂O₄）]（A=Na,K））,which contain both organic oxalate and inorganic phosphate,are suitable for the intercalation of Zn²⁺for the large distance between layers.In this paper,Na₂[（VO）₂（HPO₄）₂（C₂O₄）]·2H₂O（NVPCO）and K₂[（VO）₂（HPO₄）₂（C₂O₄）]（KVPCO）have been synthesized by a simple hydrothermal method.Both of them show two-dimensional layer structures,lamellar morphology and large reversible Zn²⁺transport channels.Thanks to the polyanion effect of the anionic group and the large ion transport channels,they all present good electrochemical performance.NVPCO delivers a high average voltage plateau（～1.3 V）,a moderate specific capacity(90 m Ah·g^-1),and excellent cycle stability（100%after 100 cycles）.KVPCO has a higher voltage platform of 1.4 V,a moderate specific capacity of 84 m Ah·g^-1 and excellent cycling stability（100%after 550 cycles）.In-situ XRD and ex-situ XPS results show that Zn²⁺has a reversible（de）intercalation reaction during the charge/discharge process.This work suggestes that A₂[（VO）₂（HPO₄）₂（C₂O₄）]possess certain practical value as cathode material for high voltage AZIBs,and this research also provides a new direction for the design of cathode materials for AZIBs.