Synthesis And Supercapacitive And Lithium Storage Properties Of Manganese Sulfide Nanosheets And Their Carbon Composites

The rapid economic growth and development accelerate the consumption of fossil fuel.In this regard,it is urgent to develop renewable energy and highly efficient energy storage devices.Although supercapacitors(SCs)and lithium-ion batteries(LIBs)have been commercialized,they can't meet the ever-increasing demand for emerging industries such as renewable energy and electric vehicles in terms of capacity and lifespan.It's of great importance to explore new high-performance electrode materials for SCs and LIBs.Among various electrode materials,transition-metal sulfides have emerged as appealing candidates for electrode materials due to their high theoretical capacitance/capacities and enhanced electronic conductivity compared with those of the corresponding transition metal oxides.Manganese sulfide(MnS)has attracted intensive attention,as manganese element is earth-abundant,environmentally friendly,and low-cost.However,MnS has several shortcomings in practical applications:(1)large volume changes during charging and discharging,resulting in capacitance/capacity degradation;(2)low intrinsic electric conductivity,hampering the rate capability and specific capacitance/capacity;(3)the dissolution of polysulfides in electrolytes,shortening the lifespan.In this thesis,we synthesized precursor Mn(OH)(OCH₃)nanoplatelets in the methanol-manganese acetate system by a solvothermal method,which show a platelet-like morphology.MnS and its composites with carbon materials were obtained using Mn(OH)(OCH₃)nanoplatelets as the precursors.When employed as electrode materials,MnS and its composites demonstrate the high capacitance/capacities,great rate capability,and improved cyclability.The main research contents are shown as follows:(1)MnS nanoplatelets were prepared by calcinating Mn(OH)(OCH₃)and sublimed sulfur.The topological sulfurization of Mn(OH)(OCH₃)results in the formation of MnS with a two-dimensional(2D)structure,and the removal of organic components in Mn(OH)(OCH₃)produces porous surfaces of MnS.The 2D structure and porous characteristic allow the fast transport of ions and electrons.As a result,when applied in SCs,the capacitance of MnS can reach up to 806 F g^-1at a current density of 1.0 A g^-1.We exploited the MnS nanoplatelets as cathode materials and para-phenylene diamine modified reduced graphene oxide(PPD-rGO)as anode materials to assemble a hybrid SC for full-cell tests.The resulting SC operates in a wide potential window of 0-1.7 V,exhibits a high energy density over 92 Wh kg^-1 and a high power density over 8492W kg^-1,and sustains their performance over 10000 charge-discharge cycles.The results suggest that the MnS nanoplatelets are promising alternative materials for high-energy density SCs.(2)When employed as an anode material for LIBs,the bare MnS delivers a specific capacity of 715 mAh g^-1 at 0.2 A g^-1,and shows a specific capacity of 241 mAh g^-1 after 570 cycles at1.0 A g^-1.In order to improve the lithium-storage capacities and cyclability of MnS,reduced graphene oxide(rGO)was introduced to form a MnS/reduced graphene oxide(MnS/rGO)composite.Graphene oxide(GO)and Mn(OH)(OCH₃)were mixed in a surfactant aqueous solution to form a Mn(OH)(OCH₃)/GO composite.The MnS/rGO composite was obtained by calcinating Mn(OH)(OCH₃)/GO.The MnS/rGO composite could deliver a high specific capacity of 1097 mAh g^-1 at 0.2 A g^-1,and keep a specific capacity of 465 mAh g^-1 after 1000cycles at 1.0 A g^-1.These findings suggest that the combination of MnS with rGO greatly enhances the electrochemical properties of MnS for LIBs.(3)The N,S co-doped carbon coating MnS nanoplatelets(MnS@N,S-C)were derived from an intermediate Mn(OH)(OCH₃)/polydopamine(PDA).The Mn(OH)(OCH₃)/PDA composite was synthesized by the in-situ polymerization of dopamine on Mn(OH)(OCH₃).Then during the calcination of Mn(OH)(OCH₃)/PDA with sublimed sulfur,the sulfurization of Mn(OH)(OCH₃)into MnS and the carbonization of PDA occur,giving rise to the generation of MnS@N,S-C.When applied in LIBs,MnS@N,S-C shows a high capacity of 1275 mAh g^-1 at0.2 A g^-1,a great rate capability(679 mAh g^-1),and a long-lifetime cycling performance of 758mAh g^-1 at 1.0 A g^-1 over 450 cycles.The excellent Li-storage performance is most likely due to the carbon shells,which improve the electric conductivity,buffer the volume change,and restrict the dissolution of polysulfides.