Design, Synthesis And Electrochemical Properties Of Cobalt-based Vanadate Anode Materials

Recent years,lithium-ion batteries（LIBs）have become the main energy source for electronic devices.And it is potential to become a high-energy density energy storage device in our daily life with the further improvement of lithium-ion battery technology.Positive and negative electrode materials play decisive roles in the performances of lithium ion batteries.Transition metal vanadates have attracted attentions because of their high theoretical capacity lately.However,it still has problems such as large volume expansion and poor conductivity during charge and discharge processes,which leading to poor cycle and rate performances.In this paper,from the micro-and nano-structure design of vanadates,cobalt vanadate materials with different morphologies were prepared via various methods.Their composition and morphology were characterized by XRD,SEM,TEM,BET,and AFM etc.Combined with battery performance testing results,the structure-activity relationship among its composition,structure,and properties was explored,revealing the essential reason for improving the electrochemical performances of transition metal vanadates.Co₂V₂O₇ hexagonal prisms have been successfully synthesized via a hydrothermal process,and then,designed a unique hydrothermal etching process with the help of H₂O₂ and CO（NH₂）₂ to in-situ construct hollow structure.Uniform Co₂V₂O₇ hollow cylinders were obtained after optimizing the temperature,etching time and molar ratio of H₂O₂ to Co₂V₂O₇ hexagonal prisms.Then the formation mechanism of Co₂V₂O₇ hollow cylinders was discussed based on the time-dependent experiments.Compared with Co₂V₂O₇ solid structure,Co₂V₂O₇ hollow cylinders showed significantly improved lithium-storage performances.When used as anode material,the initial discharge capacity of the hollow cylinder is 1595 m A h g^-1and the first charge capacity is 1160 m A h g^-1at 0.4 A g^-1,leading to the first Coulomb Efficiency to 73%.By contrast,the first Coulomb Efficiency is less than 60%for solid structure.In addition,the stable capacity at 0.4 A g^-1of Co₂V₂O₇ hollow cylinders is 500 mA h g^-1after 180^thcycle,about 2.5 times of hexagonal prisms not etched.This study express constructing hollow structure can effectively improve the lithium ion storage capacity of materials.Co₂V₂O₇·3.3H₂O large and thin nanosheet precursors were prepared via reflux method,and then topologically converted into Co₂VO₄ nanosheets.In order to solve problems of the poor conductivity for Co₂VO₄ material and the stacked structure formed during the manufacturing process for nanosheets in the electrode discs,the Co/Co₂VO₄ composite material with novel 2D-2D structure was obtained via in-situ phase separation method based on Co₂VO₄ material.The composite Co nanosheets and its 2D-2D structure can facilitate the lithium ions and electrons migration behavior in the material effectively.The Co/Co₂VO₄ material outperformed Co₂VO₄material in electrochemical properties.At a current density of 1 A g^-1,the Co/Co₂VO₄material have a high capacity retention rate of 100%until 100^thcycle（57.6%for Co₂VO₄ material）.When the current density were 0.5,1,2,3,and 5 A g^-1,the corresponding capacities are 860,710,664,600,and 409 m A h g^-1,respectively,then when the current density reverted to 1 A g^-1,the capacity can still to be 680 mA h g^-1,showing an excellent rate performance.Furthermore,according to electrochemical analysis and microstructural characterizations,the "active center transfer-capacity compensation" mechanism was proposed,which can explain the fundamental reason of the excellent cycle performance of the material logically.