The Synthesis Of Metal Sulfides Composites For The Anode Of Sodium-ion Batteries

“Environmental pollution”and“Energy Crisis”are the serious problems that mankind must face in the twenty-first century.The development and utilization of electric vehicles and large-scale energy storage power grids are the most effective ways to solve such problems.Since the successful commercialization of lithium-ion batteries?LIBs?,various portable devices and electric vehicles have also developed rapidly.In recent years,the rising price of lithium has become a major factor limiting the large-scale application of LIBs to electric vehicles and green grids.However,sodium-ion batteries?SIBs?are the most ideal substitute for LIBs because of their rich resources,low price,and similar chemical properties with lithium element.At present,it is the primary task to find the suitable anode material in order to achieve commercialization of SIBs.It is well known that the commercial LIBs anode material is graphite for its low price and stable cycling performance.However,the successful experience of LIBs can not be copied and applied to the SIBs simply because the radius of the sodium ion is greater than the lithium ion radius.Therefore,this paper will further explore the structure of carbon materials,aiming at obtaining carbon based anode materials with high specific capacity,high rate capacity and long cycle life.In addition,it has found that metal sulfides has good sodium storage properties,which is mainly due to its unique physical and chemical properties.In this paper,WS₂carbon nanofibers and SnS₂/RGO nanocomposites are synthesized on the basis of the existing research.At the same time,some improvements are made on the structure to alleviate the volume expansion effect,and the sodium storage properties are systematically studied.The main contents and innovation points are as follows:?1?In the second chapter,we prepared a N/S codoped graphene material by simple in-situ polymerization which has superior reversible sodium storage performance.The PDMcT/RGO anode delivers high capacities(240 mAh?g^-1 at 500mA?g^-11 and 200 mAh?g^-1 at 1000 mA?g^-1 with capacity retention of 81.8%and 82.2%,respectively)and excellent rate capability(144 mAh?g^-1 at 10 A?g^-1).In addition,the anode material also exhibits outstanding cycling stability(153 mAh?g^-1 after 5000cycles at 5000 mA?g^-1).The excellent cycling performance of the anode material is mainly because the doping of S atoms significantly expands the interlayer distance of graphene,while the S atoms in the layer effectively avoid the stacking of graphene and improve the cycle life.In addition,N atoms improve the wettability and conductivity of the materials which help to raise the specific capacity of the materials.?2?In the third chapter,we introduced the Dimethyldistearylammonium Bromide and prepared a complex?DODA?₃PW₁₂O₄₀0 as the precursor of WS₂.DODA-WS₂ nanofibers was prepared with electrospinning method and used as SIBs anode materials showing excellent electrochemical performance.The DODA-WS₂anode delivers high capacities(362.9 mAh?g^-1 at 200 mA?g^-11 and 318.5 mAh?g^-1 at 200mA?g^-1 after 400 cycles)and excellent rate capability(31 mAh?g^-1 at 10 A?g^-1 and 392mAh?g^-11 after back to 50 mA?g^-1).In addition,the anode material also exhibits outstanding cycling stability(226.5 mAh?g^-1 after 800 cycles at 1000 mA?g^-1).The excellent electrochemical properties of the anode material benefit from the introduction of the surface active agent,which greatly improves the dispersivity of the material.At the same time,it provides certain spaces around WS₂,which can effectively buffer the volume expansion,and on the other hand it can be attributed to the unique structural and mechanical properties of one-dimensional nanofiber.?3?In the fourth chapter,we used a simple hydrothermal method to synthesize SnS₂/RGO?N?nanocomposites.We studied the effects of precursors,heat treatment conditions,and electrolytes on the structure and electrochemical properties of the materials.It is found that the introduction of ethylenediamine to the surface modification of graphite oxide can significantly improve the specific capacity and cycling stability of the material.The SnS₂/RGO?N?anode delivers high capacities(425.9 mAh?g^-1 at 500 mA?g^-1 after 50 cycles with capacity retention of 87%)and excellent rate capability(209.7 mAh?g^-1 at 5 A?g^-1 and 579.6 mAh?g^-11 after back to 100mA?g^-1).In comparison with unmodified SnS₂/RGO nanocomposites,its specific capacity and cycling performance got enhanced.The improvement of electrochemical properties is mainly attributable to the two forms of bonding C-N-Sn and C-N-Na in the reaction process,which not only effectively alleviates the structural damage of the material,but also effectively avoids the aggregation of the discharging products through the action of chemical bonds,thus keeping the material good stability.