Interface Optimization And Application Of Silicon Anode For High-performance Lithium-ion Batteries

With the rapid development of electric vehicles,portable electronic devices and large-scale energy storage systems,the demand for developing new anode materials to replace graphite is thus becoming increasingly urgent towards future lithium-ion batteries with higher energy density.Silicon(Si)materials are considered ideal for next-generation anode materials because of their highest theoretical specific capacity(4200 mAh g-1),suitable operating voltage(<0.4 V vs.Li/Li+)and abundant reserves.However,Si anode materials suffer from drastic volume changes(>300%)upon lithiation/delithiation based on the alloying/dealloying mechanism,which inevitably causes instability of the solid electrolyte interface(SEI)film and the pulverization of Si particles.This ultimately leads to the low coulombic efficiency(CE)and dramatic capacity degradation of the Si anode,posing a significant impediment to the commercial application of Si-based lithium-ion batteries.In response to the above problems,nanostructure Si design and material compositing strategies are often used for further electrode modification.However,most of these strategies involve complex preparation procedures and even high cost,and are still far from practical applications.In this thesis,based on commercial Si nanospheres as the active materials,we focus on the perspectives of binder design,electrolyte formulation and surface modification of the Si material,and further on the inner mechanisms behind the enhanced stability.Specifically,functional boron crosslinked polyvinyl alcohol(PVA)-based three-dimensional(3D)network binders are designed in inhibiting the volume expansion of Si,whilst allowing for stabilized interfacial condition with the all-round protection from the crosslinked network structure.Furthermore,a nonflammable and high-performance ether-based localized concentrated electrolyte is designed.A 4-trifluoromethylphenylboric acid(TFPBA)is selected to modify the surface conditions of Si,which significantly improved the interfacial conditions of Si electrode and the performance of Si-based lithium-ion batteries.The main research content of this paper is as follows:(1)A PVA binder-based optimization,where a 3D binder network is devised with organic polyvinyl alcohol(PVA)backbones cross-linked by a functional boric acid(BA).Apart from the improved mechanics,B-OH bond is able to set a chemical bridge between the binder and Si surface by dehydration with Si-OH group,which enables the coherent link with Si surface.More importantly,the electron deficient B element readily accepts electrons during the cell formation and thus facilitates the buildup of an effective SEI film.The obtained lithium borate species significantly reduce the interfacial side reactions and improve the initial Coulombic efficiency(ICE).After optimization of the key parameters,a high ICE(92.76%)with excellent rate capability(2920.73 mAh g-1 at 42 A g-1)is obtained.After 500 cycles,the Si anode retained a capacity of 1883.7 mAh g-1 at 0.84 A g-1,massively outperforming the pure PVA-based Si counterpart(3 1 7.9 mAh g-1 at 0.84 A g-1).In addition,a molecular design of a multifunctional network is presented,created by grafting acrylamide(AAm)monomer onto PVA chains,followed by crosslinking to form a 3D network.In this design,the strong PVA backbone binds tightly to the Si surface with its hydroxyl groups,whereas the highly stretchable polyacrylamide(PAAm)branch endows the binder with adequate flexibility and improved Li+conductivity.After proper optimization,the Si anode using the c-PVA-g-PAAm binder exhibits improved mechanics and surface chemistry.Thus,at a high rate of 84 A g-1,the Si electrode is able to deliver a capacity of 2141.64 mA h g-1.(2)A nonflammable ether-based electrolyte containing dual additives of fluoroethylene carbonate(FEC)and lithium oxalyldifluoroborate(LiDFOB)is used to achieve superior cyclability of the Si anode.A high modulus SEI rich in fluoride and boride was formed on the Si surface,and a high ICE of 90.2%is attained in the Si anode,accompanied with a low capacity-fading rate of only 0.0615%per cycle(discharge capacity of 2041.9 mAh g-1 after 200 cycles at 0.84 A g-1).Full cells pairing the unmodified Si anode with commercial LiFePO4(13.92 mg cm-2)and LiNi0.5Mn0.3Co0.2O2(17.9 mg cm-2)cathodes further show extended service life.It is confirmed that the optimization of electrolyte system is the key technology and path for the industrialized application of Si anode.(3)TFPBA is applied as a multifunctional SEI precursor on the Si surface,which readily polymerizes to form 2,4,6-tris-4-(trifluoromethylphenyl)boroxine(TTFPB)on Si surface during high-temperature drying process of the electrodes.After one-electron reduction,the B-O bonds in TTFPB molecule break where the radical molecules are thus generated,initiating the spontaneous polymerization to form the poly-4-trifluoromethylphenylboronic acid(PTFPBA)with repeated B-O chains.Such polymerized nano-layer not only manifests a desirable artificial SEI for its high robustness and elasticity in accommodating the volume expansions,but also effectively improves the electrolyte absorption rate of the electrode,providing accelerated kinetics for Li+transfer.Under the stable framework formed by PTFPBA,preferential adsorption of LiPF6 molecules is enabled with the electron-deficient boron species,further inducing a continuous and dense SEI rich in benzene rings and inorganic substances.As such,the as-obtained Si@TTFPB anode demonstrates significantly enhanced rate capability and long-cycle performance.After 500 cycles at 0.84 A g-1,the modified Si electrode still delivers a capacity of 1778.7 mAh g-1,while the unmodified control Si sample has almost no capacity.The above work verifies the significance on the electrode/electrolyte interfacial modification of the Si electrodes.Such interface optimization can be realized by means of binder optimization,electrolyte formulation or functional molecule surface grafting.All these strategies serve as key aspects in dealing with the interfacial instability facing Si anodes,and thus have critical implications towards future development and applications of Si-based lithium-ion batteries.