Modification And The Electrochemical Performance Of Tin Disulfide/Graphene Composites

Due to the prominent problems of environmental energy,lithium-ion batteries（LIBs）have been widely used as new energy storage equipment,and their anode materials have also become a research hotspot.Tin disulfide（Sn S₂）is a transition metal sulfide material used in lithium ion batteries in recent years.It has high theoretical capacity(1231 m Ah g^-1),and has unique advantages such as low environmental toxicity,chemical stability,and sufficient resources.Its special interlayer structure is composed of two layers of sulfur atoms interposed by a layer of tin atoms.This crystallographic feature is conducive to lithium ion（Li⁺）transport.However,the lithium sulfide（Li₂S）generated from the first lithium storage of Sn S₂ can produce a large irreversible capacity,and the initial coulombic efficiency is low.During the lithiation/delithiation process of Sn S₂,large volume change causes structure collapse and formation of solid electrolyte interface films,which results in poor electrochemical performance.Therefore,in order to address these issues,this paper tends to combine Sn S₂ with graphene materials to stabilize the structure,and modify it by doping heteroatoms and metal atoms to improve the electrochemical performance of the materials.Therefore,the research work in this paper mainly reflects in the following two aspects:（1）Used as the source of nitrogen（N）and sulfur（S）,L-cysteine combined with the oxygen-containing functional groups on the surface of graphene,and at the same time Sn S₂ hexagonal nanoplates prepared in advance were dissolved and dispersed with an isopropyl alcohol solution.The two parts of the homogeneous solution were mixed and then the Sn S₂@NSG composite material can be obtained by a simple solvothermal method.According to the material characterization,it can be seen that Sn S₂ nanoplates with a diameter of 200-500 nm are uniformly embedded in the graphene,and the N and S elements are uniformly distributed in the graphene.N,S codoped graphene provides more active sites for Li⁺,accelerates the surface charge transfer speed,improves the electrical conductivity of the material,and thus improves its electrochemical performance.In addition,the graphene can reduce the structural collapse of Sn S₂ during electrochemical cycling and stabilize the cycle performance.When Sn S₂@NSG is used as the negative electrode material of the battery,it exhibits good cycle performance and rate performance.A reversible capacity of 878.7 m Ah g^-1 can be obtained at a current density of 0.1 A g^-1 after 200 cycles,which is higher than specific discharge capacity of Sn S₂@G(785.9 m Ah g^-1).And even at a high current rate of 5 A g^-1,a reversible capacity of 668.2 m Ah g^-1can also be achieved.（2）Based on a single tin source（SnCl₄·5H₂O）,a small amount of cobalt source（CoCl₂·6H₂O）was introduced as a metal doping source,and then an interlayer structure of Co-Sn S₂@rGO composite material was obtained by a simple one-step hydrothermal method.By adjusting the different metal ratios of cobalt（Co）and tin（Sn）,and preparing the samples as negative electrodes,the electrochemical performance and lithium storage mechanism under different metal ratios were investigated.Finally,the optimal ratio was obtained.According to the morphological and structural characterization of the composite,it can be seen that the Co-Sn S₂ sheets are evenly embedded in the multilayer graphene,forming an interlayer structure,and the skeleton structure of graphene provides mechanical stability for the material.Its excellent electrochemical performance is attributed to the doping of cobalt atoms,which not only synergizes with the Sn metal,improves the conductivity of the material,but also increases the crystal lattice of Sn S₂,thereby increasing the active sites of Li⁺.In addition,the interlayer structure provides buffer space for Co-Sn S₂ sheets,thus improving the electrochemical performance of the material.According to the experimental results,it can be known that the Co-Sn S₂@rGO-0.1 composite shows a high reversible specific capacity.After 100 cycles,the capacity is stable at 938.6 m Ah g^-1.