Preparation Of Derived Porous Carbon Nanosheets / TiN Nanocomposites And Their Applications In Lithium-sulfur Batteries

The traditional commercial lithium-ion battery with an energy density of 200 Wh kg^-1and a power density of 350 Wh kg^-1,is already unable to meet people's higher demand for high energy density energy storage devices.Lithium-sulfur batteries with ultra-high energy density of 2800 Wh L^-1and ultra-high theoretical specific capacity of 1675 m Ah g^-1have become a hot topic in lithium-ion battery field.Compared with traditional graphite,carbon-based composites can afford a high specific capacity,along with outstanding conductivity and mechanical properties when used as sulfur-loaded materials for lithium-sulfur batteries.The thesis adopts two-dimensional MOFs obtained under H₂O system as precursor,to prepare Derived Carbon Nanosheets(DCNSs).The materials show sheet structure with a thickness of 150-200 nm.By adjusting the stirring reaction time of the mixed solution consisted of zinc nitrate and 2-methylimidazole as well as the carbonization temperature of MOFs,we conclude the influence laws on the morphology and structure of MOFs and also the lithium-sulfur batteries performances of sulfur-loaded DCNSs.The MOFs present regular sheet structure when stirring reaction time is extended to 24 h,and the sulfur-loaded DCNSs give the best electrochemical performances when the carbonization temperature is 800?.At a rate of 0.1 C,the materials can discharge a high specific capacity of 843 m Ah g^-1.After 100cycles of charging and discharging,the discharge specific capacity can still maintain at 712m Ah g^-1,showing a capacity retention rate as high as 84.4%.We adopt KOH activation technology to etch the DCNSs and make pore,and then study the relevance between porous structures and lithium-sulfur batteries performances.By adjusting the mass ratio of C/KOH,we conclude the influence laws on the morphology,pore structures and the lithium-sulfur batteries performances of Porous Derived Carbon Nanosheets(PDCNSs).The material obtained under the mass ratio of 3/1 shows the best structure features with a high specific surface area of 3042 m²g^-1and a pore volume 2.405cm³g^-1.The porous structures focus on micropores with some mesopores and a little macropores.When the sulfur-loaded amount is 60%wt.,the material shows the best performances with a high specific capacity of 1481 m Ah g^-1for the first cycle and a high capacity retention rate of 92%after 100 cycles.The material also has superior rate performance with high specific capacities of 1059,888,869 and 803 m Ah g^-1at the rates of0.2 C,0.5 C,1 C and 2 C respectively.These results indicate that the electrochemical performances of sulfur-loaded DCNSs have been greatly improved after KOH activation to make pore.We coat PDCNSs with crystalline TiN particles.A microwave heating method is used for growing TiN particles on PDCNSs rapidly,and the influence of the molar ratio of PDCNSs to TBOT and the lithium-sulfur batteries performances has been focused on.As the increase of TiN loading amount,the electrochemical performances of the materials increase first and then decrease.When the molar ratio of PDCNSs to TBOT reaches 100/1,the material shows the best performances with a high specific capacity of 975.1 m Ah g^-1for the first cycle and a high capacity retention rate of 65.2%after 350 cycles,which also gives a high specific capacity of570.5 m Ah g^-1at a high rate of 2 C.Compared to the PDCNSs without coating TiN,the cycle performance at low rate(0.1 C)increases greatly,but the performances at high rates(0.5 C-2C)decrease.The result can be interpreted as the conjecture that polar TiN can restrain the shuttle effect of polysulfides effectively,however,which may make the surface structure unstable.