Preparation Of Metal Sulfide MS₂?M=Co,Mo?/Carbon Composites And Their Sodium Storage Properties

Lithium-ion batteries are widely used in wearable electronic devices,electric vehicles,and smart grids due to their high energy density and long cycle life.However,the growing market of lithium-ion batteries leads to the shortage and uneven distribution of lithium resources,which makes sodium ion batteries with low cost,abundant resources and similar charging and discharging principles to lithium-ion batteries become the best candidates.The larger radium of sodium ions compared to that of lithium ions restricts the host materials commonly explored for LIBs.Finding suitable electrode materials is a major challenge to improve the performance of sodium ion batteries.Among the anode materials,conversion-type metal sulfides with high theoretical capacity suffer from large volume expansion during sodiation/desodiation process,resulting in poor cycling stability.To address above issues and aiming at high-performance sodium ion battery anodes,this dissertation presents the hybridization of transition metal sulfides CoS₂and MoS₂with two-dimensional graphene?rGO?and three-dimensional carbon nanofibers?CNFs?porous networks respectively to prepare CoS₂/rGO and MoS₂/CNFs sodium ion battery anode materials with high capacity and long cycle life.Furthermore,the effect of carbon content on the performance of sodium storage was explored.The contents and results are listed as follows:CoS₂/rGO composites with particle size of 240 nm were prepared by hydrothermal method.The effects of carbon contents on the sodium storage properties were investigated.The results showed that CoS₂displayed a nano-octahedral structure,and pure CoS₂was prone to agglomeration after hydrothermal reaction.The rGO can effectively inhibit the agglomeration and the growth of CoS₂,forming smaller CoS₂/rGO nanoparticles.Three groups of samples with carbon content of 3.23%,7.98%and 10.64%were labeled as CoS₂/rGO-20,CoS₂/rGO-40 and CoS₂/rGO-60,respectively.The electrochemical performance of CoS₂/rGO-40 composite was the best.In the voltage window of 0.1-3.0 V,the initial discharge specific capacity was 912 m Ah g^-1at a current density of 0.1 A g^-1,and the corresponding coulombic efficiency was 96%.After 40 cycles,the specific capacity still maintained 825 m Ah g^-1,while the capacities of CoS₂,CoS₂/rGO-20 and CoS₂/rGO-60composites were decreased to 569,773 and 723 m Ah g^-1,respectively.Even at a high current density of 1 A g^-1,the initial capacity of CoS₂/rGO-40 composite was 788 m Ah g^-1,and 658m Ah g^-1was delivered after 150 cycles,with capacity retention of 83.5%.The excellent electrochemical performance of CoS₂/rGO-40 composite was attributed to the pseudocapacitive behavior.At the sweep rate of 2.0 m V s^-1,the capacitance contribution was77%,which was higher than 69%of CoS₂.Three-dimensional carbon frameworks derived from high-temperature carbonization of bacterial cellulose confined MoS₂composites were obtained via a hydrothermal method.The results showed that MoS₂microflowers were assembled by nanosheets,while MoS₂/CNFs manifests few-layered MoS₂nanosheet decorated on CNFs network structure.The content of CNFs in MoS₂/CNFs-1,MoS₂/CNFs-2 and MoS₂/CNFs-3 were 5.14%,10.24%and 15.20%,respectively.The introduction of CNFs with porous three-dimensional conductive network framework,not only improved the conductivity of the material and facilitates the diffusion of sodium ions,but also effectively alleviated the volume expansion during sodiation/desodiation,thus enhancing the sodium storage performance.As anodes for sodium ion batteries,the initial discharge capacity of the optimized MoS₂/CNFs-2 composite was 510m Ah g^-1at the current density of 0.1 A g^-1within the voltage window of 0.1-3.0 V.After 40cycles,MoS₂/CNFs-2 delivered a specific capacity of 478 m Ah g^-1with capacity retention of93.7%,which was significantly higher than that of MoS₂?62.5%?,MoS₂/CNFs-1?84.1%?and MoS₂/CNFs-3?86.3%?.Moreover,a reversible capacity of 495 m Ah g^-1was achieved at the current density of 2 A g^-1,and MoS₂/CNFs-2 electrode still maintained 401 m Ah g^-1after 200cycles.The charge transfer resistance of MoS₂/CNFs-2 composite was 18?.The kinetic analysis revealed that the capacitance contribution was as high as 83.7%at the sweep rate of2.0 m V s^-1.