Artificial Solid Electrolyte Interphase As Coating Layer Of Silicon-based Anode Materials And Their Electrochemical Properties

The emerging markets of portable electronic devices and electric vehicles have generated a tremendous demand for lithium-ion batteries(LIBs)with higher energy and power density and longer cycling life.Silicon(Si)has been regarded as one of the most promising alternatives to the current used graphite anode materials for LIBs due to its natural abundance,low discharge potential and high theoretical capacity(4200 mAh g-1 if Li4.4Si).However,Si suffers from extreme volume change(up to 300%)and unstable solid-electrolyte interphase(SEI)during charge-discharge cycling which causes structure degradation,rapid capacity decay and inferior coulombic efficiency(CE).In this thesis,we mainly focus on applying artificial SEI on commercial Si nanoparticles to get high electrochemical performance Si-based anode materials for lithium ion batteries application,and investigation the relationship between artificial SEI and lithium diffusion kinetics in Si-based electrode.The main research contents are described as follows:i)Our investigation indicates that lithium trapping in Si anode of LIBs is one of the key factors to affect the CE and capacity decay during high rate cycling.Here,we demonstrate that LiAlO2 as an SEI on commercial Si nanoparticles can effectively address the lithium trapping issues of Si anode to improve its electrochemical performance.We demonstrate that artificial solid electrolyte interphase layer significantly improves the kinetics of lithium alloying/dealloying process due to its better electrochemical performance comparing to the natural SEI.Owing to the artificial SEI coating,the natural SEI formation is dramatically inhibited,Si@LiAlO2 anode demonstrates a superior rate capability comparing to the Si anode,which offers a specific capacity of 788.3 mAh g-1 at an ultrahigh current density of 8000 mA g-1.Besides,the cycling performance of Si anode is also improved,which shows a specific capacity of 1106 mAh g-1 at a current density of 4000 mA g-1 with capacity retention of 90.9%after 500 cycles.ii)We apply lithium-conducting covalent-organic-framework(COF)as coating layer for Si nanoparticles,which serve as SEI for Si electrode.First,the COF coating layer reduces the electrolyte decomposition,thus the CE and cycling stability of the Si electrodes are dramatically improved.Second,the superior lithium-ion conductivity of COF can enhance the lithium-ion transportation kinetics of Si electrode.Si@COF elecctrodes presented a high specific capacity of 1864 mAh g-1 at a current density of 2000 mA g-1 and high capacity retention of more than 60%after 1 000 cycles.iii)We demonstrate that the double-shell coating of graphene and Li4SiO4 on commercial Si nanoparticles as an effective strategy for improving the anode of lithium ion batteries to overcome the two critical concerns,i.e.rapid capacity decay and inferior CE caused by the large-volume changes.It is proven that the double-shell coating enables the formation of a stable hybrid solid electrolyte interphase,leading to much higher CE and longer cycling stability of the Si anodes.Furthermore,the rate performance of Si is significantly enhanced by the outstanding electrical conductivity of inner graphene layers and the excellent ionic conductivity of Li4SiO4 out-shell.The overall results suggest this new strategy holds promising perspectives in optimizing electrochemical performances of Si anodes,which should promote their practical applications for next-generation lithium ion batteries.iv)By investigating the structure evolution of Si with in-situ Raman and ex-situ XRD measurements,we demonstrate the lithiation/delithiation process of Si electrode.By comparing the lithiation/delithiation process of pristine Si and Si@LiAlO2,we demonstrate that artificial solid electrolyte interphase layer significantly improves the kinetics of lithium alloying/dealloying process,which implies that the better lithium diffusion kinetics are obtained through the artificial SEI coating.Better cycling performance and rate capability is expected through artificial SEI coating due to its better electrochemical and mechanical properties comparing to the natural SEI.This work provides novel approaches to addressing the low CE and rapid capacity decay problems for practical using of commercial Si nanoparticles.With artificial SEI layer coating,not only natural SEI formation is suppressed,but also a better alloying/de-alloying kinectic is achieved.Therefore,the rate capability and cycling performance of Si anode are dramatically improved via reducing its lithium trapping phenomena,providing further insight into Si anode commercialization.