The Performance Optimization Of Polymer Composite Solid Electrolytes And Their Interface Stability Modification Of Lithium Metal Batteries

Lithium-ion batteries（LIBs）have attracted extensive attention in the field of energy storage,because of their series of advantages such as excellent cycle stability,no memory,and high voltage platform.However,conventional LIBs with flammable organic liquid electrolytes lead to safety hazards.Meanwhile,the complex structure makes their energy density close to the theoretical limit,which can’t meet the high energy density requirements.Metallic lithium anodes have received extensive interests owing to their merits of high theoretical specific capacity(3860 m Ah g^-1)and low redox potential（-3.040 V vs.standard hydrogen electrode）.Solid polymer electrolytes（SPEs）exhibit excellent safety,low density,good flexibility,and superior machinability,which are suitable for the processing and manufacturing of solid-state lithium metal batteries（SLMBs）.Therefore,SLMBs are considered to be the most ideal next-generation high energy density and safety energy storage devices.However,the low ionic conductivity of SPEs limits their practical application.Furthermore,the alkali lithium metal with high reducibility can react with the SPEs,resulting in detrimental interface resistance and rapid Li dendrite permeation,while each of these issues hinders the cycle lifespan seriously.Therefore,the development of SPEs with high ionic conductivity and superior comprehensive performances is critical for the practical application of the SLMBs.Based on the pressing requirements,the present research focuses on exploring new strategies to improve the comprehensive performance of SPEs.Through modification,the SPEs with high room temperature ionic conductivity and good interface stability were developed.It provides new strategies for the preparation of SPEs with high comprehensive performances.Firstly,based on the idea of structure design,a sandwich-like nanofibrous membrane-reinforced poly-caprolactone diol and trimethyl phosphate（TMP）solid polymer electrolyte（SPE）has been designed.As the supporting framework,the nanofiber membrane can increase the mechanical strength of SPEs and prevent lithium dendrites from piercing the electrolyte membranes.Specifically,the flame-retardant TMP is employed as a plasticizer that can reduce the crystallinity of the polymer and improve the ionic conductivity effectively.The as-prepared PPT-SPE exhibits superior comprehensive performances in terms of high room temperature ionic conductivity of 2.15×10^-4 S cm^-1,wide electrochemical window of 4.9 V,and superior thermal stability.Furthermore,the assembled Li/Li Fe PO₄ SLMBs with PPT-SPE show good rate performance and outstanding cycling stability at 30°C.This study proves that the support of nanofiber membrane and modification of plasticizer can effectively improve the ionic conductivity while ensuring the mechanical strength of SPEs.The SPEs are easy to be reduced by alkali lithium metal,resulting in the irreversible decomposition reaction and poor cycle life of SLMBs.Inspired by the idea of interface design,we selected the ionic liquid（IL）of 1-butyl-1-methylpyrrolidinium bis（trifluoromethanesulfonyl）imide（BMP-TFSI）as the additive for PCL-based SPEs.The addition of IL can improve the comprehensive electrochemical performance of PCL-based SPEs and improve the electrochemical stability between SPEs and lithium metal effectively.Time-of-flight secondary ion mass spectrometry（TOF-SIMS）and X-ray photoelectron spectroscopy（XPS）reveals a stable solid electrolyte interface（SEI）with multiple interphases of Li F,Li₂Sx,and Li₃N is in-situ formed between the interface of SPEs and Li metal.SEM images demonstrate that the constructed SEI can effectively promote homogeneous Li deposition.As a result,the as-prepared Li Fe PO₄/Li SLMBs demonstrated an ultra-long cycle lifespan.This work provided an effective strategy for constructing a stable SEI protective layer between SPEs and Li metal.Ionic liquid（IL）shown a significant effect in improving electrochemical stability,but the low ionic conductivity of the SPEs limited their application.To further improve the ionic conductivity,a flexible composite solid electrolyte（CSE）membrane consisting of the ceramic conducting Li_1.3Al_0.3Ti_1.7（PO₄）₃（LATP）fillers,polyethylene oxide（PEO）matrix,and IL was designed.In particular,the addition of IL can not only decrease the interface impedance between the polymer and LATP ceramic fillers and improve the ionic conductivity,but also prevent the adverse reaction between Ti⁴⁺in LATP and Li metal and further enhance the interface stability.The obtained organic-inorganic PLI-CSE membrane achieves an excellent ionic conductivity of 2.42×10^-4 S cm^-1 at 30^○C and a wide electrochemical stability window of 5 V（vs.Li⁺/Li）.Moreover,the SLMBs assembled with PLI-CSE membrane demonstrate outstanding cycling stability.Finally,to simplify the preparation process of SPEs,the polyethylene glycol dimethyl ether methacrylate（PEGMEA）based SPEs were prepared by in-situ polymerization method,which can avoid the use of toxic organic solvents.To further improve the ionic conductivity,Succinonitrile（SN）was employed as the plasticizer.The obtained PEGMEA₁-SN_1.2-SPE shown a high ionic conductivity of 1.3×10^-3 S cm^-1.In addition,the lithium fluoroacetate borate（Li DFOB）was selected as the lithium salt that can effectively inhibit the adverse interface reaction between SN and lithium metal.Due to the good comprehensive performances of the PEGMEA₁-SN_1.2-SPE,the assembled SLMBs exhibit excellent electrochemical performance and good applicability in a wide temperature range（-5～45^○C）.In addition,the P（PEGMEA）₁-SN_1.2-SPE also exhibits good stability with high-voltage NCM622 cathode,which shows a great application prospect in the field of high energy density SLMBs.