Study Of Morphology And Electrochemical Reversibility Of Metallic Lithium And Silicon-based Negative Electrode Materials With High Specific Capacity

Both metallic lithium and silicon are promising anode candidates for secondary lithium batteries because of their high theoretical specific capacities(3860 mAh g^-1 for lithium and 4200 mAh g^-1 for silicon).However,lithium-and silicon-based anode materials have suffered from several intrinsic problems that impede their application.Both lithium and Li-Si intermetallic compounds are highly reactive to electrolyte and they all involve volumetric changes during charge and discharge,which hinder the formation of a stable solid electrolyte interphase?SEI?film.Upon cycling,the electrolyte would be continuously decomposed at the interfaces,leading to a low coulombic efficiency.For lithium metal electrode,excessive lithium dendrite will be formed during repeated lithium deposition/dissolution,which may cause internal short circuits or other safety hazards,e.g.firing or even explosion.For silicon-based anode materials,the drastic volume changes above 300%during lithiation and delithiation may cause the collapse of the electrical conducting network and rapid capacity loss.Recently,silicon materials with various nanostructures have been prepared by template method and chemical vapor deposition?CVD?technique.The problems of huge volumetric change and low electronic conductivity of silicon have been solved well.However,the preparation of these functional nanostructures often involves complicated processing or/and expensive raw materials,either/both of which present a challenge for the successful implementation in industrial productions.In order to address above-mentioned problems,two approaches were explored in this dissertation:?1?Based on in situ improving the properties of SEI film,the favorable morphology of deposited lithium and enhanced electrochemical reversibility of lithium electrode were achieved by designing new electrolyte solutions compatible with metallic lithium.?2?A technique,with the use of low-cost raw material and facile large-scale processing route,was developed to propel the enlarged preparation of silicon anode materials.The obtained silicon-based materials exhibited good electrochemical performances.The main results of this dissertation are summarized as follows:1.A novel dual-salts electrolyte consisting of LiTFSI and LiFSI as dual-lithium salts and DOL-DME mixture as solvents was designed and investigated.The cycling results of lithium electrode showed an extremely stable and high coulombic efficiency of approximately 99%and SEM images revealed smooth,uniform and dendrite-free Li deposit.Furthermore,the excellent cycling stability and favorable lithium morphology were retained even at a high current density of 10 mA cm^-2.The remarkably enhanced performance of lithium electrode could probably be attributed to the effective protection of a unique SEI layer,formed under the synergistic effect of LiFSI and DOL.2.To improve the anodic stability of the above electrolyte?only up to³.8 V vs.Li/Li⁺?,co-solvent 1,4-dioxane?DX?was explored.Combined with the merit of LiFSI in SEI film formation,two new ether-based electrolytes?DOL-DME-DX/LiFSI and DX-DME/LiFSI?with extended electrochemical window and ability of changing lithium morphology were explored.The influences of high salt concentration up to 3M LiFSI in DOL-DME-DX on the electrochemical performance of lithium electrode were investigated as well.With the introduction of co-solvent DX,the anodic stability potential of DOL-DME-DX/LiFSI and DX-DME/LiFSI electrolyte solutions was extended up to 4.3 V and 4.87 V?vs.Li/Li⁺?,respectively.DOL-DME-DX/1M LiFSI electrolyte presented good compatibility with lithium and a high coulombic efficiency of almost 98%was achieved.However,the rate performance of this electrolyte was limited.The dynamic and cycling performance of lithium electrode was improved dramatically by increasing the concentration of LiFSI from 1M to 3M in DOL-DME-DX solvent,which was probably attributed to the abundance of lithium ion at negative electrode and the formation of a SEI film with lower interfacial impedance and more compact structure.Nevertheless,high lithium salt concentration often means increased cost.Compared with other ether solvents,symmetrically structured DX was much more stable against lithium,which reduced the parasitic reaction between electrolyte and lithium.Therefore,a stable SEI layer dominated by LiF was formed with further optimized DX-DME/1M LiFSI electrolyte,resulting in a high reversibility of Li deposition/dissolution?coulombic efficiency is about 98%?.Particularly,this electrolyte exhibited excellent rate performance even at 1 M lithium salt concentration and lithium dendrites were effectively suppressed at current density of 5 mA cm^-2.3.Nanoporous silicon?pSi?with sponge-like morphology was successfully derived from low-cost and earth-abundant natural clinoptilolite?NCLI?through one-step high-energy mechanical milling?HEMM?pretreatment and magnesiothermic reduction reaction.After surface carbon coating,the electrochemical performances of pSi-C composite was enhanced significantly.With facile and only one-step HEMM pretreatment,the side-reaction and agglomeration of middle-products in magnesiothermic reduction reaction were prevented successfully.The specific charge capacity of pSi at 1^st cycle was increased from 754 mAh g^-1?without HEMM pretreatment?to 1768.5 mAh g^-1?with HEMM pretreatment?.The cycling stability of pSi pretreated with HEMM was also improved greatly.After surface carbon coating of pretreated pSi?pSi-C?,the cycling stability was further improved with capacity retention of 87.5%after 200 cycles.Moreover,the pSi-C composite also showed good rate performance.The rich porosity and nano-scale primary particles of pSi-C composite suppress the volume changes of silicon and a uniformly thin carbon layer maintained the conducting net-work of the electrode and enhanced the structural stability.4.KCl was explored as a compatible heat absorbent for magnesiothermic reduction reaction and a rotational reactor was designed for the preparation of nano-silicon material at enlarged scale.The results showed that the species of heat absorbents greatly influenced the particle size and morphology of the product.The spherical nano-silicon?50⁶0 nm in particle size?synthesized with KCl heat absorbent exhibited superior electrochemical performance.To dissipate the heat accumulation in enlarged production of nano-silicon material,dynamically rotational magnesiothermic reduction reactor was designed and implemented.Nano-Si?⁴0 nm in particle size?prepared via rotational reactor demonstrated the improved electrochemical performance compared with commercial one.After surface carbon coating via the CVD process,the cycling stability was further enhanced.