Preparation And Sodium Storage Properties Of Tin/Cobalt Bimetallic Sulfides

In order to fully meet the energy supply in the fields of large energy storage systems and electric vehicles,sodium-ion batteries（SIBs）with low cost,high safety,and abundant resources are considered as potential substitutes for lithium-ion batteries.As an important part of SIBs,anode materials have drawn rising attention.However,due to the larger radius of Na⁺,it can result in the more challenging intercalation/deintercalation processes which leads to lower reaction kinetics and energy density.Therefore,developing anode materials with stable structures,high charge-discharge capacities,and long cycle life is a significant factor of the commercialization scale and the speed of sodium-ion batteries.Among numerous anode materials in sodium storage,tin/cobalt bimetallic sulfides have attracted a great deal of interest due to their stable mechanical stability,outstanding theoretical capacity,and rich redox reactions.However,the low conductivity and serious volume expansion have caused rapid capacity decay.In view of the above problems,we designed and prepared several Sn/Co-based sulfides of various micro-morphology characteristics coated with carbon materials and successfully obtained composites possessing high capacity and long life.The specific research contents are as follows:（1）Cubic precursor CoSn（OH）₆-GO of 100-300 nm was synthesized by a simple coprecipitation method at room temperature and the proportion of graphene oxide（GO）was also controlled in the process.Then,it was further vulcanized to SnS@Co_1-xS-rGO through the reducing gas H₂S produced by the decomposition of Na HS at 600℃.It can be found from structural characterization tests that particle sizes can stay the same before and after the vulcanization.Adjacent particles maintained a certain distance and connected with each other through graphene.Compared with samples without graphene,the agglomeration phenomenon had been significantly improved.Additionally,with the increase of GO content,the lamellar graphene was gradually stacked on the surface of metal sulfides.In electrochemical tests,the SnS@Co_1-xS-rGO-2 composite with the addition amount of GO was 0.5 mg m L^-1delivered the highest capacity and the most stable cycle performance.It exhibited the first discharge capacity of 810.97 m Ah g^-1 at 0.1 A g^-1 with the first coulomb efficiency up to 83.66%and maintained 571.23 m Ah g^-1 after 80 cycles.It can also keep a stable rate capacity of 352.92 m Ah g^-1 as the current density up to10.0 A g^-1.Therefore,coated with graphene oxide has been proved to perform a significant effect on improving the accumulation and conductivity of metal sulfides particles.（2）Based on the reasonable structural design of two metal sulfides SnS₂and CoS₂,multi-group control trials for the synthesis of SnCoS₄material was designed by adjusting the solvent type,reaction temperature,reaction time,and molar ratio of raw materials.According to comprehensive structural characterizations and electrochemical tests,solvent glycol can provide the most suitable viscosity and guidance for the growth of the layered structure.After adding tin and cobalt raw materials with the same molar ratio and conducting solvothermal reaction at 180℃for 8 hours,the optimized product possessed most complete lamellar structures,which showed three-dimensional flower balls of 1-1.5μm.Three-dimensional flower balls of SnCoS₄were self-assembly through two-dimensional lamellas.The mutual lamellas can not only provide strong support but also restrict the further stack in space of each other,leading to the excellent structural stability and mono-dispersity.In addition,the rich nanosheets offered sufficient active sites and reserved a large amount of space to adapt to volume expansion.The as-synthesized composite maintained a reversible capacity of 434.54m Ah g^-1 after 100 cycles at 0.1 A g^-1 and could deliver a discharge specific capacity of 346.78 m Ah g^-1 at a current density of 2 A g^-1.This design ideas and straightforward method provide ideas for the follow-up experiment.（3）We successfully synthesized the SnCoS₄@rGO composite through the one-step solvothermal method.The results showed that when the content of GO was 1.25 mg m L^-1（SnCoS₄@rGO-2）,GO can induce the growth of SnCoS₄nano-layers and co-generate a unified whole with a lamellar network structure.The rich redox reactions between bimetallic sulfides and the interconnected cross-linking network with rGO fully enhanced the diffusion kinetics of sodium ions.In addition,the overlap between nanosheets with each other formed the hierarchical porous structure of micropores,mesopores,and macropores,which can also provide large specific surface areas and abundant pores for ion transport and electrolyte permeation.All in all,the special structure of SnCoS₄@rGO-2 contributed to the inspiring performance in capacity and cycle stability(it can remain 704.81 m Ah g^-1 after 100 cycles at 0.1 A g^-1 with the capacity retention rate close to 100%and keep an ultra-stable rate capacity of427.98 m Ah g^-1 as the density up to 10 A g^-1).