Design Of Sn-based Nanocomposites As High-performance Anode Materials Or Lithium-ion Batteries

With the advent of information age and the rapid development of electric vehicle industry,the demand for lithium-ion batteries with higher capacity,better rate capability and longer cycle life is urgent.The key to meet these requirements lies in the development of anode and cathode materials,especially anode materials.However,the current commercial graphite is difficult to meet present power market due to its low specific capacity(373m Ah/g).SnO₂,as one of the alternative anode materials for lithium-ion batteries,has caused great attention due to its low discharge potential(<1.5V)and natural abundance.Moreover,SnO₂possesses large theoretical capacity of 783 m Ah/g based on the alloying reaction(Sn+4.4Li⁺+4.4e^-(?)Li_4.4Sn),which can achieve 1494 m Ah/g if the conversion reaction(SnO₂+4Li⁺+4e^-?Sn+2Li₂O)is fully reversible.However,the low electrical conductivity and large volume expansion(300%)during lithium-ion insertion/extraction results in the poor cyclability and rate capability of SnO₂,limiting its practical applications.Graphene,a single sheet of graphitic carbon,has been widely employed to fabricate SnO₂/graphene nanocomposites,due to its excellent conductivity,large surface area and high mechanical flexibility.Graphene can act as an insertion host for Li⁺,a conducting agent and a buffering matrix,which are very helpful in improving the electrochemical performances of Sn-based anodes.However,the graphene sheets are prone to stack together during drying,resulting in that the electrolytes are difficult to penetrate into the electrodes.And there is no strong interaction between the graphene sheets and the electroactive materials in these graphene-containing composite,which might result in the capacity fading of the electrodes arising from the aggregation and detachment of active materials after repeated cycling.In this study,L-ascorbic acid was used as a spacer to inhibit the stacking of graphene sheets and a coupling agent to firmly immobilize Sn-based nanoparticles on rGO sheets to improve the structural stability.In addition,a SnS₂/SnO₂heterostructure is constructed to improve the electrochemical reaction kinetics,which can effectively enhance the capability under high current density.The main contents of this paper are as following.?.A SnO₂@C/rGO nanocomposite was prepared by a facile one-step hydrothermal method using Sn Cl₂,L-ascorbic acid and GO as reactants.The rGO,C/rGO,SnO₂/rGO,SnO₂@C samples were also synthesized and compared.In the SnO₂@C/rGO nanocomposite,the L-ascorbic acid can be polymerized and carbonized to form an amorphous carbon layer to encapsulate the SnO₂ nanoparticles with grain size of ca.2-3 nm.The FT-IR and XPS results show that L-ascorbic acid can also act as a coupling agent to enhance the strength of C-O-Sn bonds between the SnO₂ nanoparticles and rGO sheets to strongly anchor SnO₂ nanoparticles on the rGO sheets.SnO₂@C/rGO has a specific surface area of 296 m²/g,which is much higher than that of SnO₂/rGO and C/rGO.The L-ascorbic acid-derived amorphous carbon can serve as a spacer to inhibit the stacking of rGO sheets to provide more micropores and mesopores for Li⁺transport.As a result,the as-obtained SnO₂@C/rGO nanocomposite shows better cycle stability than that of SnO₂/rGO,delivering a specific capacity of 731 m Ah/g after180 cycles at a current density of 0.1 C(1 C=783 m A/g)and a high reversible capacity of 410m Ah/g at a large current rate of 5 C.?.A novel SnS₂/SnO₂@C/rGO nanocomposite was synthesized by in-situ H₂O₂oxidation of SnS₂@C/rGO that was prepared via a hydrothermal reaction using Sn Cl₂,thiourea,L-ascorbic acid and GO as reactants.It is found that the L-ascorbic acid play multiple roles during reaction,including as a chelating agent to inhibit the hydrolysis of Sn⁴⁺to promote the stable formation of SnS₂,a carbon source to generate a cross-linked amorphous carbon layer wrapped on the SnS₂/SnO₂nanoparticles,a reductant to reduce GO to form rGO,a spacer to inhibit the stacking of rGO sheets,and a coupling agent to firmly immobilize SnS₂/SnO₂heterostructured nanoparticles on rGO sheets.Besides,the N and S heteroatoms are co-doped into the rGO sheets by using thiourea as N and S sources,which can enhance the electrical conductivity and provide more active sites for Li⁺insertion/extraction.Compared with the SnS₂@C/rGO,SnS₂/SnO₂/rGO and SnS₂/Sn S/SnO₂@C samples,the as-obtained SnS₂/SnO₂@C/rGO composite exhibits a better cycle stability and a higher rate capability.The obtained SnS₂/SnO₂@C/rGO nanocomposite exhibits a specific capacity of 1249 m Ah/g after 120 cycles at a current density of 0.1 C(1C=783 m A/g)and a high reversible capacity of 1070 m Ah/g after 100 cycles at a large current rate of 0.5 C.The superior electrochemical performance of SnS₂/SnO₂@C/rGO could be attributed to not only the fast electron and Li⁺transfer rate at the tightly contacted SnS₂/SnO₂heterojunctions,but also the electrical interconnection and structural stability guaranteed by the cooperation of amorphous carbon and rGO.The amorphous carbon can strongly anchor SnS₂/SnO₂nanoparticles on the rGO sheets and inhibit their aggregation and shedding during lithiation and delithiation process.Together with the amorphous carbon coating,the rGO sheets act as elastic buffer to release the stress generated from the volume change of SnS₂/SnO₂nanoparticles.