Preparation And Study Of Sodium Storage Properties Of Metal Sulfide Composites

By virtue of unique advantages over other energy storage systems,lithium ion batteries（LIBs）have been generally applied in all kinds of electronic products.However,inadequate lithium resources as well as the increasing cost greatly limit the sustainable development of energy industries.In contrast,sodium has more abundant reserves and low cost.Thus,sodium ion batteries（SIBs）as one of the most promising alternatives for LIBs have gained more and more attention.Anode materials,the most important component of SIBs,directly affect the electrochemical performance.Unfortunately,the large diameter of Na⁺make it difficult to insert into the electrode materials,so that the commercial graphite anode applied in LIBs cannot be used as the anode material for SIBs.Therefore,the development of novel high-performance anode materials with high specific capacities and long cycle life spans has become one of the research hotspots in this field.Among various anode materials,metal sulfides have attracted much attention owing totheir high theoretical capacities,easy preparation,abundant reserves and good reversibility.However,some problems in most metal sulfides cannot be ignored,such as the poor electronic conductivity,large volume strain and dissolution of polysulfide ions during cycling.In order to solve these problems,this thesis mainly studies the following two parts:first,preparation of Ti₃C₂T_x/MoS₃ composite and study of sodium storage performance;second,preparation of MoS₃/Cu₂S composite and study of sodium storage performance.The specific works are shown as follows:（1）First,three kinds of Ti₃C₂T_x with different surface functional groups（F/O ratio）are prepared by an acid etching method using Ti₃Al C₂ as the raw material.And then three kinds of Ti₃C₂T_x/MoS₃ composites are prepared by an acid co-precipitation method,making small MoS₃ nanoparticles wrap around the inner/outer surfaces of Ti₃C₂T_x.The effects of different surface functional groups on the sodium storage performances of Ti₃C₂T_x/MoS₃ composites are studied.Among the three composites,the rich-OH groups on the surface of Li OH-treated Ti₃C₂T_x（LF/O-Ti₃C₂T_x）contribute to stronger electrostatic effects between Ti₃C₂T_x and MoS₃ for the formation of a stable sandwich-like structure,namely LF/O-Ti₃C₂T_x/MoS₃.As an anode for SIBs,LF/O-Ti₃C₂T_x/MoS₃ shows outstanding long-term cycling performance,and its reversible capacity can maintain at 611m Ah g^-1 after 1000 cycles at 2 A g^-1.Kinetics analysis and theoretical calculation further explain the excellent performances of LF/O-Ti₃C₂T_x/MoS₃.In addition,LF/O-Ti₃C₂T_xmodified CuS or SnS₂ composites also show similar and excellent electrochemical results for SIBs.The above excellent sodium storage performances are mainly determined by the synergistic effect between high-capacity transition metal sulfides and LF/O-Ti₃C₂T_x.（2）First,the nano-sized Cu₂S hollow spheres as the core are synthesized by the solvothermal method.After that,the nitrogen-doped carbon（NC）coated Cu₂S composites are prepared by the subsequent room temperature polymerization reaction.Finally,Cu₂S@NC@MoS₃ composites are prepared by a facile acid co-precipitation method.The improvement effect of high capacity MoS₃ and bridging nitrogen-doped carbon on the sodium storage performance has been explored.The loaded MoS₃ with a unique amorphous structure can effectively shorten the diffusion path of Na⁺and improve the capacity of the whole composite.The nitrogen-doped carbon layer on the surface of Cu₂S@NC has many defects,which can provide a large number of active sites to connect Cu₂S and MoS₃.It also can suppress the volume strain of the composite,enhance the structural stability and improve the electrical conductivity.Thus,Cu₂S@NC@MoS₃composite shows an exceptional initial discharge capacity of 652 m Ah g^-1 and maintains a high reversible capacity of 545 m Ah g^-1 after 200 cycles at 0.5 A g^-1.It still delivers a remarkable long-term cycle life(94%capacity retention after 2000 cycles at 3 A g^-1),significantly higher than that of pure Cu₂S and Cu₂S@MoS₃.The above excellent electrochemical performance can be attributed to the synergistic effect of Cu₂S,MoS₃ and nitrogen-doped carbon.