Study On New Transition Metal Sulfides Composites As Alkali Metal Ion Storage Materials

With the development of electric vehicles and grid energy storage and the increasing demand of portable electronic equipment,higher requirements have been put forward for the development of a new generation of alkali metal ion batteries.The most widely used anode material for commercial lithium ion batteries is graphite material,but its theoretical specific capacity is only 372 m Ah·g^-1 and it has almost no capacity when used as anode for sodium ion batteries,which is unable to meet people’s demand for high performance alkali metal ion batteries.MoS₂ has a layered structure similar to graphite,which is a unique sandwich structure composed of S-Mo-S layers through weaker van der Waals forces,and its adjacent layer spacing is larger than that of graphite material which is beneficial to the insertion/extraction process of alkali metal ions.In addition,MoS₂ material has a higher theoretical specific capacity than graphite material,so it is a potential anode material for alkali metal ion batteries.Compared to monometallic sulfides,polymetallic sulfides have better alkali metal ions storage capacity due to their complex chemical compositions and synergistic effects.NiCo₂S₄ is a typical bimetallic sulfide,which has higher electrochemical reaction activity than the corresponding monometallic sulfides（NiS_x and CoS_x）,and the material has better stability due to the synergistic effects between different metal atoms during the cycle process.However,both monometallic sulfides and bimetallic sulfides take on poor conductivity and severe volume expansion,which limit their further development.Therefore,this thesis combines the strategies of the construction of heterostructure,the control of special morphology and the introduction of carbon materials to construct a new type of transition metal sulfide composites as advanced anode materials for alkali metal ion batteries.（1）With ZIF-67 material prepared by liquid phase method as the precursor,and the holey carbon box materials were obtained after calcination and acid leaching.The abundant pore structure not only favours the infiltration of electrolyte to the electrode material,but also provides more channels for the transmission of ions and electrons.The holey carbon box materials were used as the growth template,and the MoS₂ nanosheets self-assembled on the surface of the holey carbon box material to form composite materials through the hydrothermal process.The holey carbon box skelon plays important roles not only as conductive matrix to improve the conductivity of the composite material,but also as supporting to buffer the volume changes of the self-assembled MoS₂ nanosheets during the repeated charge/discharge cycle process,so as to ensure better structural stability for the composite material.Due to the advantages of the above-mentioned structure design,the as produced composite materials,especially the one with the flower-like sphere structure,exhibit excellent electrochemical performance when used as anode of both lithium ion batteries and sodium ion batteries.（2）The CoSn（OH）₆ hollow cubes were prepared with the co-precipitation method,the Co₂SnO₄/SnO₂ material can be obtained by subsequent calcination,which retains the cubic morphology of precursor.After that,the MoS₂ particles grew on the surface of the Co₂SnO₄/SnO₂composite materials during the hydrothermal process,and then the Co₂SnO₄/SnO₂@MoS₂ heterostructure composite materials were further successfully constructed.The engineering of the heterostructure can exert the synergistic effect of each component and inhibit the volume expansion of the composite material during the repeated charging and discharging process,so that the material shows better structural stability.Moreover,the internal electric field formed at the heterogeneous interface of MoS₂ and Co₂SnO₄/SnO₂ materials can improve the reaction kinetics of Li⁺/Na⁺ions at the interface of the composite materials.In addition,the introduction of small quantity of pyrolytic carbon from the organic chelating agent used during co-precipitation process can further improve the conductivity of the composite material.Among the prepared Co₂SnO₄/SnO₂@MoS₂composite materials,the as produced one when the mass ratio of molybdenum source and Co₂SnO₄/SnO₂ is 1:1 possesses the best cycle performance and rate properties when used as the anode of both lithium ion batteries and sodium ion batteries.（3）The hierarchical porous carbon material was constructed,which has not only numerous microporous structures but also rich mesoporous structures,and then the NiCo₂S₄materials were embedded in the pore structure of the hierarchical porous carbon materials by hydrothermal method to obtain the filling-type composite materials.Through optimizing the loading amounts of NiCo₂S₄ material,the NiCo₂S₄ nanoparticles can be evenly embedded in the pore structure of the hierarchical porous carbon material.The as produced NiCo₂S₄@HPC composite material still retains rich pore structures especially meso-pores,which favours the penetration of the electrolyte,increases the effective contacting between the active material and the electrolyte,provides more channels for the transmission of alkali metal ions and electrons,and shortens their diffusion paths.Embedding the NiCo₂S₄ material into the hierarchical porous carbon material can not only exert the synergistic effect of both the two components,but also effectively suppress the volume expansion of NiCo₂S₄ material during the charging and discharging process,thereby bring about excellent structural stability.Therefore,the optimized composite material presents good alkali metal ion storage performance when used as the anode of alkali metal ion batteries.Based on that,the matching performance of NiCo₂S₄ material with a variety of electrolytes for sodium ion batteries were also explored.（4）The NiCo₂S₄/ZnS@C heterostructure composite anode materials were designed and successfully prepared by optimized novel one-step hydrothermal method.Through the urea enabled maneuverability of the interfacial energy,the morphology control of the heterostructure composite anode materials can be realized during self-assembly procedure,and a series of heterostructure composite materials with different morphologies,such as hollow porous core-shell microsphere structure and microflower structure,were successfully engineered.The reasonable design and construction of the heterostructure can not only fully exert the synergistic effect of every components to suppress the volume expansion during the charging and discharging process,but also form internal electric fields at the interface of the composite material to accelerated the surface reaction kinetics of the material.The design and adjusting of the special morphology not only makes the composite materials have excellent structural stability,but also increases the effective contact area between the active material and the electrolyte.In addition,the introduction of heteroatom doped carbon materials further improves the conductivity of the composite material.Under the synergistic effect of these factors,the as engineered NiCo₂S₄/ZnS@C composite materials exhibit excellent electrochemical performance.Particularly,the composite material with hollow porous core-shell microsphere structure shows advanced electrochemical property when used as anode material for sodium ion batteries,which still has as high reversible specific capacity as513.9 m Ah·g^-1 after 100 cycles at a current density of 0.5 A·g^-1,and the discharge specific capacity can still be remained at 243.4 m Ah·g^-1 with high coulombic efficiency after 300cycles even at a high current density of 2.0 A·g^-1.The high capacity and excellent rate performance of the composite material also originate from the pseudocapacitance behavior brought by the special structural design,and the CV test analysis shows that the contribution ratio of pseudocapacitance is as high as 90.32%.Based on the design and optimized construction of the transition metal sulfide composite anode materials,the application of NiCo₂S₄/ZnS@C composite material in the sodium ion full batteries was preliminary explored.The cathode material choosed for pairing is the0.4Li₂MnO₃·0.6NaNi_1/3Co_1/3Mn_1/3O₂material with high capacity and high working voltage.Firstly,the loading proportion of the cathode and anode was optimized for the NiCo₂S₄/ZnS@C|1.0 M NaPF₆ in EC:PC（3:4 V/V）|0.4Li₂MnO₃·0.6NaNi_1/3Co_1/3Mn_1/3O₂,sodium ion full battery and the working voltage range（0.5-3.2 V）was optimized through the CV test.The electrochemical performance of the full batteries assembled according to the two ways with the excess cathode loading and the excess anode loading were explored,respectively.Then,the matching performance of different electrolytes in the above-mentioned sodium ion full batteries were further explored.Among them,the NiCo₂S₄/ZnS@C|1.0 M NaPF₆ in EC:PC（3:4 V/V）|0.4Li₂MnO₃·0.6NaNi_1/3Co_1/3Mn_1/3O₂ full battery shows the most excellent electrochemical performance.The initial coulombic efficiency of the full battery is88.59%,and it still has a discharge specific capacity of 71.5 m Ah·g^-1 after 5 cycles at 0.2 C and subsequent 45 cycles at 1.0 C,implying the potential practical applications.