The Application Of Functional Polymers In Separator/solid Electrolyte For Lithium Metal Batteries

Lithium-ion batteries play an important role in modern electronic products and electric vehicles.However,the current electrochemical technology of lithium-ion batteries cannot meet the increasing energy demand.Lithium metal batteries（LMBs）with extremely high theoretical capacity and extremely low electrochemical potential,have received extensive attention.Before LMBs enter the mainstream market,many obstacles（such as lithium dendrites,flammable electrolytes,etc.）need to be addressed.Polymer is a type of material widely used in current battery systems.The emerging polymer chemistry can achieve better performance and reliability,thereby overcoming the bottleneck encountered by current LMBs.As an important component in liquid LMBs,the separator plays a vital role in isolating the cathode and anode,influencing the diffusion of lithium ions,and inhibiting the dendrite growth.Based on the problem of dendrite growth caused by uneven ion transport in liquid metal batteries,the newly polymer-functionalized separator can effectively regulate the ion flux and improve the performance of LMBs.Moreover,compared with the conventional liquid LMBs system,solid polymer electrolyte（SPE）is expected to replace liquid electrolyte and separator,so as to better solve the problem of flammability and leakage of the liquid electrolyte.Among them,solid-state LMBs using non-flammable inorganic or solid polymer electrolytes（SPE）have become a research hotspot for next-generation energy storage devices.By designing SPE with a unique polymer topology,it can not only eliminate the inherent problems of the narrow electrochemical window and low ionic conductivity,provide high energy density and power density,but also eliminate the safety risks caused by the high flammability of organic solvents.Given this,with the help of polymer chemistry,this work has rationally designed polymer-modified separators and composite polymer electrolytes from the perspective of inhibiting dendrite growth in liquid lithium metal batteries and the safety of flammable electrolytes,respectively,as well as to study its electrochemical performance when used in liquid/solid LMBs.（1）With the help of a polymer functionalized porous polyolefin separator to control the lithium-ion deposition behavior,one-dimensional polypentafluorophenol acrylate（PPFPA）polymer brushes are synthesized by surface initiation-atom transfer radical polymerization（SI-ATRP）method,to obtain a one-dimensional electronegative polymer brush functionalized porous composite separator（PPFPA-g-Celgard）.The electronegative functional group on the hairy PPFPA molecular chain can be used as an efficient lithiophilic site to provide fast ion transmission.In addition,the nanochannels formed by orderly aggranged one-dimensional polymer brushes on the Celgard surface can integrate with lithiophilic groups to achieve uniform lithium ionic flux on the electrode surface at the molecular level,thus enabling uniform nucleation and deposition of lithium metal.Therefore,LMBs with PPFPA-g-Celgard have excellent cycle stability（>900 h）even under high current density(1 m A cm^-2).（2）Based on the controllable ideas of polymer morphology,composition,structure and functional groups,we further developed a strategy of cationic polymerization to improve the interfacial compatibility of composite polymer electrolytes.A unique composite polymer electrolyte was designed by in-situ polymerization of 1,3-dioxy-amyl（DOL）in a self-supported three-dimensional La_0.56Li_0.33Ti O₃（LLTO）fiber framework,denoted as In-situ PLE.Compared with rigid and brittle inorganic electrolytes,In-situ PLE formed by in-situ polymerization of PDOL can not only form a good electrode-electrolyte compatibility interface and reduce the interfacial impedance but also can in-situ form a stable SEI rich in Al complex and Li F on the anode.The SEI enhances the stability of the anode and inhibits the formation of dead lithium.In addition,In-situ PLE exhibits a high lithium-ion migration number（0.6）,a wide electrochemical window（5.5 V）,a high room temperature ion conductivity(6.6×10^-4 S cm^-1)and exhibits excellent safety performance even at high temperatures.Furthermore,In-situ PLE can achieve long cycle stability of about 900 h in a symmetric battery,and significantly improve the cycle performance of the LFP|In-situ PLE|Li full battery at room temperature.