Study On Preparation Of Tin-based Sulfur-containing Complex At Room Temperature And Its Conversion Reaction For Lithium Storage

The lithium intercalation capacity of tin-based compounds is contributed by alloy reaction and conversion reaction.The alloy reaction is easier to proceed,while the conversion reaction is more difficult,and it was once considered an irreversible reaction.Some studies have found that the conversion reaction of tin-based materials is reversible when the crystal particles have very fine size（<10 nm）and good conductivity.The number of electrons corresponding to one Sn atom increases from4.4 to 8.4,and the theoretical specific capacity increases by 91%.Nano-tin is easy to agglomerate,its recrystallization temperature and melting point are very low,the compound’s conductivity is poor,and the conversion reaction needs to overcome a higher energy barrier,which are all important factors that inhibit the continuous reversibility of the conversion reaction.In order to improve the reversibility of the conversion reaction,it is necessary to optimize the above problems,separate the active substances from each other,avoid agglomeration,improve the conductivity of the materials,increase the reaction interface,and avoid the loss of electrical contact of the active materials.Increasing the degree of dispersion of tin-based materials is an effective means to alleviate the volume effect,thereby improving the cycle stability of the tin-based materials,and also conducive to improving the reversibility of the conversion reaction,thereby increasing the specific capacity.Tin atom is the smallest unit of electrochemical reaction.For the first time,this article proposes to use coordination bonds to disperse active materials,so that Sn ions are dispersed at the atomic scale to improve the reversibility of the conversion reaction.In order to verify this idea,three representative organic compounds were selected to disperse tin ions,which were ethyl xanthic acid（EtX）with linear chain structure,1,3,5-Triazine-2,4,6-trithiol trisodium（TMT）with cyclic conjugated structure and polyethyleneimine xanthic acid（PEX）with chain polymer structure.New type of tin-based anode materials with reversible conversion reaction were prepared at room temperature.Stannous ethyl xanthate（Sn（EtX）₂）was prepared by dispersing tin ions with ethyl xanthate at room temperature.The structure of Sn（EtX）₂ was verified by IR,Raman,X-ray photoelectron spectroscopy,solid-state NMR and elemental analysis.Sn（EtX）₂-graphene（SEG）composite was prepared by introducing graphene into the growth process of the material.By comparing the capacity voltage differential（dQ/dV）curves,cyclic voltammetry（CV）curves and X-ray photoelectron spectroscopy analysis,it is found that the conversion reaction has good reversibility,and the capacity retention rate of the conversion reaction reaches 82%after 400 cycles.This indicates that the tin-based materials with this structure can be used for electrochemical reaction and is conducive to the conversion reaction.The contribution of lithium-ion diffusion behavior and capacitance behavior to the capacity was distinguished in kinetics.Conductive carbon material is helpful to increase the proportion of capacitance behavior.The apparent diffusion coefficient(D_Li)was calculated by galvanostatic intermittent titration technique（GITT）to be about 10^-11 cm² s^-1.However,when the current density increases to 500 mAg^-1,the capacity of SEG decreases continuously,and the cycle performance of SEG at high current density needs to be improved.TMT has a structure similar to a benzene ring,and its conjugatedπbond can provide delocalized electrons,which is more conducive to electron conduction than ethyl xanthogenic acid.Sn₃（TMT）₂ was synthesized by replacing the organic matter with 1,3,5-Triazine-2,4,6-trithiol trisodium,and its structure was confirmed by FT-IR,Raman,XPS,SSNMR and ICP analysis.Compared with Sn（EtX）₂ electrode,the ohmic resistance（R_Ω）and charge transfer resistance(R_ct)of Sn₃（TMT）₂ electrode are reduced to 8.2Ωcm² and 64.5Ωcm²,respectively.The reversibility of the conversion reaction of Sn₃（TMT）₂ electrode is weak without the addition of conductive carbon material,which lasts only about 30 cycles.For the conversion reaction,the conductivity among particles is more important than the structure of ligand.CNTs-Sn₃（TMT）₂（CST）composite was synthesized by introducing carbon nanotubes（CNTs）into the growth process to enhance the conductivity among particles.Compared with the spontaneous nucleation,the size of the active particles is reduced by 10 times,the lithium-ion transport path is shortened,the accessibility of the active materials is increased,and the volume effect is alleviated.Through the analysis of dQ/dV curve,CV curve and XPS spectrum,the conversion reaction of CST also has good reversibility.At the current density of 500 mAg^-1,the reversible capacity reaches 713mAh g^-1 after 500 cycles.At the high current density of 1.0 Ag^-1,the reversible capacity is 511 mAh g^-1 after 500 cycles.Compared with SEG electrode,CST electrode has a significant improvement in its cycle performance at high current density,but the cycle life needs to be further extended.Tin-based materials were dispersed by chain polymer PEX,which contains a large number of dithiocarboxylic groups and can disperse metal ions into the polymer at atomic scale.Stannous polyethyleneimine xanthate and CNTs composites（PSC）were prepared.The cycle life of the PSC electrode is further extended,and the remaining capacity reaches 560 mAh g^-1 after 1000 cycles at a high current density of 1.0 Ag^-1.This is because the chain polymer has a certain degree of ductility and deformation recovery ability,which can adapt to the volume change in the process of lithium-ion intercalation and deintercalation.Through the analysis of dQ/dV curve and CV curve,X-ray energy spectrum（EDS）and XPS spectrum after the cycle,it is found that the conversion reaction of PSC electrode also has excellent reversibility,and the active material does not appear excessive agglomeration after the reaction.The mechanism of the reaction is speculated.The presence of organic groups can effectively block the excessive agglomeration of tin elements.The advantages and disadvantages of the three electrode materials of SEG,CST and PSC are compared and the optimal loading of conductive carbon materials is analyzed.The introduction of organic will form more solid electrolyte interface（SEI）films,and the initial coulomb efficiency（ICE）of SEG,CST and PSC electrodes are 61%,56%and 55%respectively.The lower ICE is also one of the problems to be solved in the follow-up research.In addition,the mechanism of conversion-type anode materials is the same as the conversion reaction of tin-based materials,so they face the same problems as that of tin-based materials.Using Ni²⁺,Co²⁺and Mn²⁺to replace Sn²⁺,the conversion-type anode material was prepared by the method of preparing CST and have excellent electrochemical performance,indicating that this type of material is not only suitable for alloy-type tin-based materials,but also suitable for conversion-type anode materials.