| Silicon has been identified as one of the most attractive high-energy anode materials for lithium ion batteries due to its low working voltage and high theoretical specific capacity of 3579mAh g-1.However,a number of challenges inhibit the commercialization of Si electrode relative to graphite.Many of these challenges rise from severe volume change(≈275%)associated with the(de)alloying reaction of silicon with lithium,resulting in structural degradation and instability of solid electrolyte interphase(SEI),causing significant irreversible capacity loss and subsequent rapid capacity fade.In this study,the synergetic usage of additives and concentrated electrolyte were used to construct organic-inorganic complex SEI layer on the surface of silicon materials,so as to improve silicon electrolyte/electrode interface properties,and enhance the cycling stability of Si-based electrode.Firstly,the concentrated electrolyte was designed and the basic physical and chemical properties were studied.In order to obtain a stable,homogeneous liquid state at high concentrations,the choice of Li salt and solvent was based on the thermodynamic stability(thus a melting point)of these Li salt solvates.Four types of solvent with high dielectric constant,that is propylene carbonate(PC),ethyl acetate(EA),three glycol dimethyl ether(TEGDME)and acetonitrile(AN)as representatives of carbonate,carboxylic acid esters,ethers and nitriles,were chosen to prepare the concentrated electrolyte,respectively.In addition,two lithium salts of lithium bis(fluorosulfonyl)imide(LiFSI)and Lithium bis(trifluoromethanesulphonyl)imide(LiTFSI)with low symmetry and high flexibility were chosen.The compatibility of different solvent-lithium salt systems with silicon nano-particles(SiNP)anode was investigated,and the concentrated electrolyte system LiFSI/PC is provided.The effects of electrolyte concentration on the ionic transfer properties,electrochemical stability and solution structure were studied.The system can be divided into three types according to the solution structure,dilute electrolyte such as LiFSI-(PC)8,concentrated electrolyte such as LiFSI-(PC)3 and highly-concentrated electrolyte such as LiFSI-(PC)2.Sencondly,the electrochemical performance of the three types of electrolyte were investigated in the SiNP anodes.The SiNP anodes in the electrolytes of LiFSI-(PC)n(n=8,3and 2)can deliver initial lithiation capacity of 3361.8,3296.1 and 3060.9mAh g-1 at a rate of 0.1 C(350 mA g-1).After 100 cycles,the corresponding lithiation capacity are 257.4、1439.7 和 1997.9mAh g-1 with capacity retention of 7.5%,42.7%and 63.5%,respectively.Besides,the average coulombic efficiency between 2 and 100 cycles are 97.25%,98.07%and 99.57%,respectively.This results show that(highly)-concentrated electrolyte,such as LiFSI-(PC)3 and LiFSI-(PC)2,can be used to improve the cycling performance of SiNP anodes.Based on the analysis of the Si particle morphology,electrode structure and the composition of SEI layer.the mechanism of concentrated electrolyte to improve the cycling stability of SiNP anodes was revealed.In concentrated electrolyte,most of PC solvents and FSI anions are complexed by Li+ to form a specific solution structure like fluid polymeric network.The reduction of FSI anions starts to play important role due to the increasing concentration of contact-ion pairs(CIPs)or aggregates(AGGs).The LiF,insoluble sulfur-based compounds and other species derived from FSI anions as well as reduction products of PC are credited to the formation of a more mechanically robust and chemically stable organic-inorganic complex SEI layer.The modified SEI layer can effectively suppress the morphology evolution of Si particles,and self-limit the excessive growth,which mitigate the crack propagation of Si electrode and the deterioration of kinetics.Then,on the basis of concentrated electrolyte,the application of additives such as lithium fluoride boric acid oxalate(LiDFOB),fluorinated ethylene carbonate(FECi)and three(trimethylsilane)boric acid ester(TMSB)can further improve the cycling performance of SiNP anodes.In the baseline concentrated electrolyte LiFSI-(PC)3,the reversible capacity is just 574.8mAh g-1 after 300 cycles with the initial lithiation capacity of 3296.1mAh g-1.By contrast,the silicon electrode with 3%LiDFOB-,3%FEC-and 3%TMSB-containing electrolytes can deliver reversible capacity as high as 1142.9、1863.6 and 1852.2mAhg-1,respectively.Based on the study of electrochemical properties,the surface mophorlogy and bulk structure of electrode and chemical composition of the electrode surface,the mechanisms of additives to improve the cycling performance of SiNP anodes are revealed.,the Li(BF2O)n polymer generated in subsequent cycles contributes to maintain structural integrity,suppress the electrical contact loss of pulverized particles,and further mitigate the capacity decay after 20 cycles.The ring-opening reaction LiDFOB coupled with the ring-opening reaction of PC can quickly form SEI layer containing a large number of inorganic components,which helps to maintain the integrity of the electrode structure,improve the utilization of active materials,and suppress the capacity degradation caused by kinetic factors.The FEC derived SEI layer which is rich in LiF and poly(VC)can suppress the particle size decrease and the excessive growth of SEI layer.Through the breakup of B-O-Si group,TMSB can derivatize a stable SEI layer with good reaction kinetics on the Si to suppress the decomposition of the electrolyte on the SiNP electrode and the deformation of Si particles to some extent.More importantly,LiDFOB,FEC,and TMSB involve in the SEI formation,and the content of inorganic components in the SEI layer is increased,thereby increasing the stability of the SEI film,which is the fundamental reason for suppressing the capacity rade of the SiNP electrode.Finally,considering that the commercial applications of silicon/carbon composite material begin to hit the market,so the compatibility of concentrated electrolyte with silicon/carbon anode was examined.The PC-based concentrated electrolyte can effectively inhibit the lithium-PC co-intercalation in graphite so that the silicon/carbon anode in the PC-based electrolyte can deliver a theoretical capacity.Besides,the cycling stability of the silicon/carbon anodes can be further improved through further increasing the lithium salt concentration.Moreover,the cycling stability of silicon/carbon anodes was further improved when LiDFOB,FEC or TMSB is added.Among them,the electrochemical performance of the silicon/carbon electrode in the FEC-adding concentrated electrolyte is best.These conclusions are consistent with those obtained from SiNP anodes.Since there is a lot of room to improve the cycling stability of the silicon/carbon anode,the properties of the SEI layer on the surface of the Si-based materials can be further improved by the mixed additives.It is expected that the synergistic application of FEC and TMSB with concentrated electrolyte can further improve the stability of SEI layer. |
