Synthesis Of Sandwich Spherical Graphene Decorating Tin Sulfide (Oxide) Composites And The Lithium Storage Mechanism

Since the beginning of the 21 st century,environmental pollution and energy shortages have become increasingly serious.It is imperative to develop and use various types of clean and energy-saving energy.Lithium-ion batteries have the advantages of high working voltage,high energy density,wide temperature range,and environmental friendliness,and have become one of the major research directions for new energy materials.In the lithium-ion battery anode material,the theoretical capacity of the commercial carbon material electrode is 372 m Ah g-1,and people can no longer meet the increasing commercial demand for high capacity and high power.The maximum theoretical specific capacity of lithium tin alloy is 993 m Ah g-1,which is more than twice the theoretical specific capacity of currently commercial graphite anode materials and is considered as one of the most promising alternatives to lithium ion battery carbon anode materials.Although tin and tin-based compounds have a high theoretical capacity,they are prone to huge volumetric effects during charge and discharge process,leading to capacity degradation.The new graphene material is a kind of carbon material with a special two-dimensional structure,which has excellent conductivity and electrochemical performance.It is very promising to be used in lithium ion battery electrode materials.In this dissertation,graphene was used to prepare composites loaded with tin-based compounds on the carbon skeleton of sandwich-bonded hollow spheres,so that tin-based compounds could be fixed in the interlayer of graphene hollow spheres,and the morphology of the materials was improved by changing the synthesis process,so that can improve the stability and integrity of the material's morphology,increase the conductivity while suppressing the agglomeration of tin-based compounds,the volume expansion caused by lithium insertion during the discharge process,and the phenomenon of material pulverization.(1)Sn S is widely studied as anode materials since of its superior structural stability and physicochemical property comparing with other Sn-based composites.Nevertheless,the inconvenience of phase morphology control and excessive consumption of sulfur sources during synthesis hinder the scalable application of Sn S nanocomposites.Herein,we report a facile in-situ sulfuration strategy to synthesize sandwiched spherical Sn S/sulfur-doped graphene(Sn S/S-SG)composite.An ultra-low sulfur content with approximately stoichiometric ratio of Sn:S can effectively promote the sulfuration reaction of Sn O2 to Sn S and simultaneous sulfur-doping of graphene.The as-prepared Sn S/S-SG composite shows a three-dimensional interconnected spherical structure as a whole,in which Sn S nanoparticles are sandwiched between the multilayers of graphene sheets forming hollow sphere.The sandwiched sphere structure and high S doping amount can improve the binding force between Sn S and graphene,as well as the structural stability and electrical conductivity of the composite.Thus,a high reversibility of conversion reaction,promising specific capacity(772 m Ah g?1 after 100 cycles at 0.1 C)and excellent rate performance(705 and 411 m Ah g-1 at 1 C and 10 C,respectively)are exhibited in the Sn S/S-SG electrode,which are much higher than that of the Sn S/spherical graphene synthesized by traditional post-sulfuration method.(2)Heterostructures have great potential applications ranging from high-speed electronics to optoelectronics devices due to their interface effects,which is beneficial to the specific charge transfer kinetics owing to the internal electric field at heterointerfaces.In this paper,we successfully synthesize the heterostructured Sn S/Sn O2/spherical graphene(Sn S/Sn O2/SG)composite,in which ultrafine Sn S/Sn O2 nanoparticles with heterostructures are sandwiched between multilayers of graphene sheets which form hollow spherical structure.Detailed electrochemical studies indicate that the molar ratio of Sn S to Sn O2 has great influence on the charge transport efficiency.Theoretical calculation reveals that Sn S and Sn O2 exhibit different work functions and the Fermi level shift is influenced by the Sn S/Sn O2 molar ratio,thus the hybrid with the ratio close to 1.0 owns the most adsorbed lithium ions,which leads to the highest specific charge-transfer kinetics and lowest ion-diffusion resistance than other samples.Electrochemical tests show that the composite with appropriate composition delivers the best lithium storage rate performance(620 and 312.7 m Ah g-1 at 1 C and 10 C).A much stable and high reversible specific capacity of 850 m Ah g?1 is obtained after 200 cycles at 0.1C.The Sn S/Sn O2 nano-heterostructures with appropriate molar ratio and the novel sandwich hollow spherical composite structure are attributed to the excellent electrochemical performances.(3)The Sn/Sn O2/spherical graphene(Sn/Sn O2/SG)composites were synthesized by a simple low-temperature,one-step hydrothermal method.Using polystyrene spheres as a template,by controlling the amount of Na BH4,hydrothermal partial reduction of Sn2+ was performed to obtain Sn/Sn O2 nanoparticles sandwiched between layers of graphene sheets and formed a hollow spherical structure.The highly conductive Sn element and the high-capacity active Sn O2 nanoparticle are uniformly dispersed and isolated from each other.Li2 O generated by the lithium intercalation of Sn O2 can disperse/suppress the Sn agglomeration,maintain the structural integrity,and improve cyclic lithium storage property of the material.the intrinsic existence of the material exists.The tin element,with good electronic conductivity,helps to improve the Sn O2 composition and the overall conductivity of the material.At the same time,the three-dimensionally connected hollow graphene structure can effectively adapt to the strain of the volume change,prevent the aggregation and pulverization of the nanostructure Sn/Sn O2,promote the transmission of electrons and ions throughout the electrode,and further improve the cycle performance of the composite material.Electrochemical tests show that Sn/Sn O2/SG nanocomposites have excellent lithium storage rate performance(633.2 and 355.9 m Ah g-1 at 1 C and 10 C).A stable and high reversible specific capacity of 843.8 m Ah g-1 was obtained after 100 cycles at 0.1 C.