Preparation Of V₃S₄-based Composites For Lithium/sodium-ion Storage

Since Sony commercialized lithium-ion batteries(LIBs)based on the Li Co O₂/C system in 1991,LIBs have been widely used in portable electronic devices and electric vehicles.However,the low theoretical specific capacity of commercial graphite anodes and the lack of lithium resources limit the further development of LIBs.Sodium and lithium are located in the same main group,have similar physical and chemical properties.Besides,sodium resources are abundant,widely distributed,and low cost.Therefore,sodium-ion batteries(SIBs)have become the most promising alternative to LIBs.However,commercial graphite anodes usually exhibit poor capacity and stability as SIBs anodes,which is due to the narrow lattice spacing(0.34 nm)of graphite that is not suitable for the intercalation and deintercalation of a large number of sodium ions.Therefore,it is urgent to develop LIBs and SIBs anode materials with high specific capacity and high rate performance.In recent years,transition metal sulfides(TMSs)have received extensive attention due to their high redox reversibility,good electrical conductivity,and excellent theoretical capacity.Based on the intercalation reaction and conversion reaction of V₃S₄,it is considered as an ideal anode material for realizing high-performance LIBs and SIBs.However,the low electrical conductivity of V₃S₄ and the severe damage to the electrode structure due to volume expansion during cycling hinder its commercialization.The study found that the construction of nanostructures(one-dimensional,two-dimensional and three-dimensional,etc.)or the introduction of buffer matrices(carbon materials,metals and conductive polymers,etc.)can effectively improve the diffusion and transport of lithium/sodium ions in electrode materials,and alleviate the volume and structural changes of materials in LIBs and SIBs.At the same time,the conductivity of the electrode material is also improved.This paper focuses on the application of V₃S₄ anode materials in LIBs and SIBs.The conductivity and stability of V₃S₄ electrode materials are improved by designing three-dimensional(3D)nanostructures and effective strategies for compounding,thereby enhancing the lithium and sodium-ion storage properties of V₃S₄.The main work contents are as follows:1.The 3D nanostructured V₃S₄ was constructed and the flower-like V₃S₄ was prepared by a simple solvothermal post-calcination method.The effect of different heat treatment temperatures on the physicochemical properties of the flower-like V₃S₄ was explored.The results show that when the heat treatment temperature is 400°C,the as-prepared V₃S₄(VS-400)has a larger specific surface area(37 m~2 g^-1),which is beneficial to providing more active sites for lithium and sodium-ion storage.When VS-400 is used as the anode of LIBs,the reversible specific capacity reaches 1099 m Ah g^-1 after 100 cycles at a current density of 0.1 A g^-1.At a current density of 2 A g^-1 and 5A g^-1,the reversible specific capacity reaches 669 m Ah g^-1 and 589 m Ah g^-1,respectively.When VS-400 is used as the anode of SIBs,the reversible specific capacity reaches 499 m Ah g^-1 after 100 cycles at a current density of 0.1 A g^-1.At a current density of 2 A g^-1 and 5 A g^-1,the reversible specific capacity reaches 348 m Ah g^-1 and289 m Ah g^-1,respectively.However,the specific capacity and rate performance of nanoflower-like V₃S₄ are not very ideal because the pure V₃S₄ electrode material is prone to volume expansion during the process of lithium and sodium ions de/intercalation,resulting in poor structural stability.2.Combining with conductive polymers can effectively buffer the volume and structure changes of electrode materials during charging and discharging.Therefore,V₃S₄/PPy composites were prepared by introducing polypyrrole(PPy)as a buffer matrix.Compared with general carbon materials,PPy can improve the pseudocapacitive process on the surface of V₃S₄ electrode,which is beneficial to improve the reversible specific capacity of V₃S₄ at high current density.When V₃S₄/PPy is used as the anode of LIBs,the reversible specific capacities reach 853 m Ah g^-1 and718 m Ah g^-1,respectively,at current densities of 2 A g^-1 and 5 A g^-1.When V₃S₄/PPy is used as the anode of SIBs,the reversible specific capacities reach 585 m Ah g^-1 and 535m Ah g^-1,respectively,at current densities of 2 A g^-1 and 5 A g^-1.The calculation shows that when V₃S₄/PPy is used as the anodes of LIBs and SIBs,the proportion of pseudocapacitance contribution can reach 86%and 84%at a scan rate of 1.0 m V s^-1,respectively.Although the lithium and sodium-ion storage properties of V₃S₄/PPy composites are greatly improved compared with V₃S₄,their first Coulombic efficiency still needs to be further improved.3.The 2D layered Ti₃C₂T_x MXenes with open ion transport channels and high specific surface area can provide a large number of active sites for ion storage.Therefore,it has attracted much attention in the field of energy conversion and storage.V₃S₄/Ti₃C₂T_x composites were prepared by ultrasonic mixing,solvothermal and high temperature calcination.The results show that the prepared V₃S₄/Ti₃C₂T_x has the largest specific surface area(156 m~2 g^-1)when the content of Ti₃C₂T_x is 5%(relative to the mass of the vanadium source).When V₃S₄/Ti₃C₂T_x MXene was used as the anode for LIBs and SIBs,the first Coulombic efficiencies reached 78%and 80%,respectively.Compared with V₃S₄/PPy(lithium storage:77%;sodium storage:79%),the first Coulombic efficiency of V₃S₄/Ti₃C₂T_x is improved,which is mainly attributed to the synergistic effect of both V₃S₄ and Ti₃C₂T_x MXenes.On the one hand,the highly conductive Ti₃C₂T_x MXene improves the electron/ion transport ability of V₃S₄ and suppresses its volume and structure changes during charging and discharging.On the other hand,V₃S₄ nanosheets can effectively increase the interlayer distance of Ti₃C₂T_xMXene,providing more space for the storage of lithium and sodium ions.Due to the strong van der Waals interactions and hydrogen bonds between the adjacent nanosheets of 2D Ti₃C₂T_x,it is easy to aggregate and stack,which will limit the movement of ions in the electrolyte to a certain extent and hinder the full utilization of the V₃S₄/Ti₃C₂T_xMXene surface structure,which will affect its electrochemical performance.4.To address the stacking problem of 2D Ti₃C₂T_x MXenes,3D sea urchin-like alk-Ti₃C₂T_x MXenes were prepared by a two-step method.Then it was compounded with nanoflower-like V₃S₄ to obtain V₃S₄/alk-Ti₃C₂T_x electrode material.The results show that the 3D alk-Ti₃C₂T_x MXene is tightly bound to the V₃S₄ nanoflowers,forming a 3D network structure with excellent electrical conductivity.When V₃S₄/alk-Ti₃C₂T_x is used as the anode of LIBs,the reversible specific capacity reaches 1403 m Ah g^-1 after 50cycles at a current density of 0.1 A g^-1.At current densities of 2 A g^-1 and 5 A g^-1,the reversible specific capacities reach 1118 m Ah g^-1 and 1030 m Ah g^-1,respectively.When V₃S₄/alk-Ti₃C₂T_x is used as the anode of SIBs,the first Coulombic efficiency reaches81%.At current densities of 2 A g^-1 and 5 A g^-1,the reversible specific capacities reach620 m Ah g^-1 and 565 m Ah g^-1,respectively.Compared with V₃S₄,V₃S₄/PPy and V₃S₄/Ti₃C₂T_x prepared in previous chapters,V₃S₄/alk-Ti₃C₂T_x has the best rate performance for lithium and sodium-ion storage.In addition,V₃S₄/alk-Ti₃C₂T_x has the highest specific capacity and first Coulombic efficiency for sodium ions storage.The calculation shows that when V₃S₄/alk-Ti₃C₂T_x is used as the anodes of LIBs and SIBs,the proportion of pseudocapacitance contribution can reach 87%and 87%at a scan rate of 1.0 m V s^-1,respectively,which is beneficial to its lithium and sodium storage.