Interface Evolution And Engineering For Lithium-Ion Batteries

With the development of the electric vehicles and large-scale energy storage systems,current commercial lithium-ion batteries can no longer meet the demand.In this regard,various novel materials have been proposed to increase the energy density and safety of batteries,such as silicon（Si）anode and sulfide solid electrolyte.However,they suffer from serious interfacial issues including electrode/electrolyte interface instability and lithium（Li）dendritic growth,which hinder their practical applications.To address these issues,this thesis focuses on the electrode/electrolyte interface evolution and engineering with liquid electrolyte and solid electrolyte.To reveal the intrinsic instability of solid electrolyte interphase（SEI）on Si anode in the liquid electrolyte system,modified titration gas chromatography is applied to quantitatively investigate the evolution of SEI and Li_xSi.Combing with X-ray photoelectron spectroscopy（XPS）and cryogenic transmission electron microscopy（cryo-TEM）,we unravel the structure and chemistry evolution of SEI on Si anode.SEI forms continuously during discharging and some components in it,such as Li₂O and lithium ethylene dicarbonate are involved in the electrochemical reactions during charging,demonstrating a“breathing behavior”,which causes capacity loss of Si anode.Adding fluoroethylene carbonate（FEC）into the electrolyte is beneficial for forming SEI rich in inert Li F,which can help stabilize the SEI and alleviate the formation of inactive Li_xSi,thus improving the cycling performance of Si anode.Sulfide electrolytes with high ionic conductivity and facile formability are expected to replace the conventional flammable liquid electrolyte to construct high-energy and safe all-solid-state batteries（ASSBs）.However,the practical use of sulfide electrolytes is mainly obstructed by their high sensitivity to humidity and instability to the high-voltage oxide cathodes.To solve these problems,a surface modification method is proposed for sulfide electrolytes.CO₂ can react with sulfide electrolytes spontaneously forming a Li₂CO₃ layer,which can effectively enhance the stability of sulfide electrolytes against the humid sensitivity and high-voltage cathodes.Coupled with bare Li Co O₂,the ASSB with CO₂-treated electrolyte can cycle for 2100 cycles with a capacity retention of 89.4%at 0.5 C within voltage of 2.6～4.5 V vs.Li⁺/Li.This strategy is proved valid for various sulfide electrolytes and is practically feasible based on current battery fabrication conditions.A composite anode consisting of all-electrochemically-active Si/Al is proposed to construct the high-energy-density ASSBs.Owing to the low electronic conductivity and Li⁺diffusion coefficient of nano Si,the sluggish reaction dynamic can induce short circuit of the ASSBs operated at high current density.Although mixing the nano Si with conductive additives and sulfide electrolyte can accelerate the reaction dynamic in the anode,the low content of active materials and severe electrolyte decomposition will reduce the energy density of the ASSBs.Constructing all-electrochemically-active Si/Al anode not only increases the loading of active materials but also reduces the growth of SEI.In addition,the Si/Al composite anode exhibits high ionic/electronic conductivity and improved density.Coupling Si/Al anode with Li Ni_0.8Co_0.1Mn_0.1O₂,the ASSB can operate stably under 3 C rate and its cycling retention after 145 cycles at 0.5C rate is as high as 92.9%.