Research On The Electrochemical Performance And Prelithiation Strategy Of SiO-based Anode

Silicon monoxide(SiO)is considered to be the most promising next-generation anode material as a result of its high specific capacity,suitable lithiation voltage and better cycle stability compared to Si.However,the non-negligible volume expansion of SiO and the generation of irreversible components during the charge-discharge process lead to poor cycle performance and low initial Coulombic efficiency.In order to improve the cycle stability and initial Coulombic efficiency of SiO-based electrodes,we studied and analyzed the size-dependent electrochemical performance of SiO@C,and carried out functional prelithiation of SiO-based materials from the perspectives of powder and electrode to improve its electrochemical performance.Finally,we introduce our industrialization research on high-performance SiO-based materials.The innovation points and specific contents of this thesis are as follows:(1)Study the electrochemical performance and mechanism of SiO@C-based electrodes from the perspective of particle size.The size-dependent electrochemical behavior of SiO@C was investigated from the aspects of irreversibility/reversibility of electrochemical lithiation/delithiation,particle interface/electrode stability,and interaction between SiO@C and graphite particles in hybrid electrodes.The effects of particle size on electrochemical activity,volume change on deactivation of LixSi and continuous side reaction consumption on particle surface were investigated.Reducing SiO@C particle size can improve the overall capacity display,inhibit the deactivation of LixSi,but aggravate the occurrence of interface side reactions.For graphite/SiO@C hybrid electrodes,graphite can buffer the volume change of SiO@C with low addition ratio to a certain extent,but the volume effect of SiO@C with high addition ratio will lead to the deactivation of hybrid electrodes.Reducing the particle size of SiO@C can make the distribution of the components of the electrode more uniform and improve the cycle stability,but the side reaction of electrolyte decomposition associated with it will hinder the lithiation/delithiation process of graphite.(2)Realize prelithiation from the perspective of powder materials.A LiCl moltensalt-induced thermochemical prelithiation strategy is employed to tune the electrochemically active Si/O ratio of SiO to enhance the initial Coulombic efficiency.Using Li/NH3 solution to coat LiNH2 on the surface of SiO,followed by thermal reaction of molten LiCl to realize prelithiation.The bulk SiO particles are transformed into pomegranate-like cluster composites(M-Li-SiO),whose inner core is SiO and outer is nanoscale aggregates composed of Li2Si2O5,SiO2 and Si.Through the analysis of the reaction intermediates,with molten LiCl as the medium and Li2Si2O5 as the active site,SiO2 precipitates from the inside of SiO particles in the form of nanotubes.The initial Coulombic efficiency of M-Li-SiO can be increased to 88.2%,and the initial Coulombic efficiency of graphite/M-Li-SiO(8:2)hybrid electrode can be increased to 91.79%.The energy density of the graphite/M-Li-SiO‖LiFePO4 full cell is increased by 37.25%.Ex-situ X-ray photoelectron spectroscopy,Raman spectroscopy,X-ray diffraction and related electrochemical measurements reveal that the SiO core and Si of M-Li-SiO are involved in lithiation,and the garnet-like structure of the particles and the presence of Li2Si2O5 reduce the lithiation reaction resistance and SEI film impedance.(3)Realize prelithiation from the perspective of electrodes.The initial Coulombic efficiency of SiO-based electrodes is improved by using Li-1-naphthalenecarbonitriletetrahydrofuran prelithiation system,and a multifunctional interfacial film is constructed on the electrode surface by using the prelithiation reagent.X-ray photoelectron spectroscopy,electron energy loss spectroscopy,nuclear magnetic resonance spectroscopy and atomic force microscopy prove that the Li-1naphthalonitrile-tetrahydrofuran prelithiation reagent integrates the SEI film:1)The outer layer is dense with nitrogen-containing organics and the inner layer Gradient distribution of LiF-rich inorganics;2)Uniform horizontal distribution of components,mechanical properties and surface potential.The initial Coulombic efficiency of SiO@C electrodes and hard carbon/SiO hybrid electrodes can be increased to 100%,and the energy density of hard carbon/SiO@C(9:1)‖ LiCoO2 full cells has increased by 62.3%.Moreover,the prelithiation system achieves higher Coulombic efficiency during cycling and suppresses Li dendrite formation in the overcharged state.We also designed a Li-2-fluorobiphenyl-2-methyltetrahydrofuran prelithiation system.Under the premise of effectively improving the initial Coulombic efficiency of SiO@C anode and graphite/SiO@C hybrid anode,the SEI film rich in LiF and organic oligomers is constructed in-situ to improve the stability of the electrode interface.The fluorine group is removed from the organic molecule and combined with lithium to produce homogeneous LiF on the electrode surface during the prelithiation,thereby achieving long-term cycle stability of the electrode.X-ray diffraction,X-ray photoelectron spectroscopy and high-resolution liquid mass spectrometry demonstrate that the electrode generates reducing species rich in LiF and organic oligomers after prelithiation.After the prelithiated electrode contacts the electrolyte,the SEI film rich in LiF,ROCO2Li and organic components is formed,and the SEI film has a high degree of rigidity and uniformity.Measurements such as nuclear magnetic resonance spectroscopy,time-of-flight secondary ion mass spectrometry and constant voltage discharge have proved that high-quality SEI films can reduce electrolyte consumption and improve electrode interface stability during battery operation.In addition,the initial Coulombic efficiency of the LiCoO2‖graphite/SiO@C full battery increased from 77.49%to 92.01%.This prelithiation process has also been successfully applied to the anode of the laminated battery and the anode of the wound battery.(4)Based on the previous research work,we take high reversible capacity,high initial Coulombic efficiency and cycle stability as the fundamental indicators,focused on the material coating method,the design of the reaction system and the process improvement of industrial scale-up,and realize the ton-level production of the firstgeneration SiO@C products whose various performance indexes have reached the leading domestic and international progressive levels.In addition,on the basis of the first generation of products,we have overcome the technical difficulties of the prelithiation scale-up experiment through system design and improvement of the reaction device,and realize the Kg-grade production of the second-generation highinitial-Coulombic-efficiency SiO.