Synthesis Of Manganese/Iron Sulfides@Carbon Nanocomposite Materials Using Microgel Template And The Study Of Lithium-Storage Performance

Power lithium-ion batteries with high energy density over 200 Wh kg^-1 have become the research focus nowadays.It is necessary to develop new anode materials instead of carbon-based materials to eahance the energy density.Fottunately,transition metal sulfides?TMS?has higher energy density.For example,the theoretical specific capacities of manganese sulfide?MnS?and iron disulfide?FeS₂?add up to 616 m Ah g^-1 and 894 m Ah g^-1,respectively.This renders them to be one of important choices as the anode material for lithium-ion batteries.However,the TMSs anode materials,which based on the chemical conversion reaction mechanism,have some disadvantages such as large volume change during charge and discharge,low initial coulomb efficiency?ICE?and poor rate capability.Therefore,researches on the modification of TMS anode materials are urgently needed.Anode nanomaterials have short lithium-ion diffusion distance,high intercalation surface and small volume change.Synthesis of nanomaterials has become an important means to improve the electrochemical performance of TMS anode materials.How to maintain nanoscale morphology and suppress its aggregation during cycling are the key issues to improve high performance of the TMS materials.To solve this problem,polymer microgels with three-dimensional meshes and polar functional groups are used to synthesize the anode materials by optimizing their synthesis condition and adsorbing performance.That is,transition metal ions absorbed in the microgel spheres are hydrolyzed in-situ,and transform to TMS nanoparticles were generated within the microgels to from the precursors.After the precursors are anneled at a high temperature,TMS@C nanocomposites are prepared due to pyrolyse of the polymer matrix.The main contents were listed as following:1. Study on the adsorption properties of P?AM-co-AA?microgel for manganese and ferrous ionsThe P?AM-co-AA?microgels were synthesized by an inverse emulsion polymerization method.Their adsorption performance for manganese and ferrous ions under different conditions was studied,and the optimal conditions for their adsorption performance were obtained.The optimal adsorption conditions included:the initial ion concentration 100 mg L^-1,temperature 50 ^oC,adsorbed time 8 h.The A1,A2 and A3 samples prepared with different monomer molar ratios had mesoporous structures,and showed the BET specific surface areas of 18.6 m² g^-1,20.5 m² g^-1,and 22.3 m² g^-1,respectively.The absorption process of A3 sample followed the quasi-second-order 33.3 mg g^-1 and 37.8 mg g^-1,respectively,with an equilibrium time of 8 hours.2. Synthesis of MnS@C composite material using the P?AM-co-AA?template and its electrochemical propertiesUsing the P?AM-co-AA?microgels as the precursor and thiourea as the sulfur resource,the MnS@P?AM-co-AA?composite materials were prepared by a solvothermal method.The MnS@P?AM-co-AA?composite materials were carbonized to form the MnS@C nanocomposites.Half cells assembled with the MnS@C electrode and lithium electrode were utilized to perform electrochemical tests.The results showed that the pure MnS electrode could deliver a specific discharge capacity of 780 m Ah g^-1 in the first time at 0.1 C rate,along with an initial coulomb efficiency of 75.6%,and retained 40%of the initial capacity after 200 cycles.The initial discharge specific capacities of the MnS@C-1 and MnS@C-2 samples were 822 m Ah g^-1 and 795 m Ah g^-1,respectively,along with the corresponding initial coulombic efficiency of 82.5%and 79.3%.The corresponding capacity retention after200 cycles were 59.2%and 54.7%.Among them,the MnS@C-1 sample presented the best rate capability.It could deliver the specific capacities of 822 m Ah g^-1,543 m Ah g^-1,489 m Ah g^-1 and 435 m Ah g^-1 at the rates of 0.1 C,0.5 C,1 C and 5 C,respectively.3. P?AM-co-AA?template synthesis of FeS₂@C composite material and its electrochemical propertiesUsing the P?AM-co-AA?microgels as the precursor and thiourea as the sulfur resource,the FeS₂@P?AM-co-AA?composite materials were prepared by a solvothermal method.The FeS₂@P?AM-co-AA?composite materials were carbonized to form the FeS₂@C nanocomposites.Half cells assembled with the FeS₂@C electrode and lithium electrode were utilized to perform electrochemical tests.The results showed that the specific discharge capacity of the pure FeS₂ electrode reached895 m Ah g^-1 at 0.1 C rate in the first time,along the initial coulombic efficiency was78%.Its capacity retention after 200 cycles was 62.9%.The FeS₂@C-1 and FeS₂@C-2 samples could deliver the initial specific capacities of 1050 m Ah g^-1 and969 m Ah g^-1 respectively,along with the corresponding initial coulombic efficiencies of 83.1%and 79.8%.And the corresponding capacity retentions after 200 cycles were75.3%and 73.8%.Among them,the FeS₂@C-1 sample presented the best rate performance.At the rates of 0.1 C,0.5 C,1 C and 5 C,the discharge capacities were1050 m Ah g^-1,706 m Ah g^-1,610 m Ah g^-1 and 513 m Ah g^-1,respectively.The results suggested that the size of TMS nanoparticles that in-situ formed within the microgels can be controlled by the network pores.Their growth can be effectively suppressed by the limited space within polymer matrix during the formation and annealing processes.Carbon matrix from the pyrolysis of the polymer chains constructs three-dimensional?3D?channels for electron transport while the porous structure within the composites forms 3D channels for ion transport,leading to 3 D ion/electron hybrid conductive networks.Therefore,the electrochemical performances of the TMS nanoparticles can be improved significantly.