Synthesis And Properties Of Polysiloxane-based Polymer Electrolyte For All-solid-state Lithium Ion Batteries

Poly（ethylene oxide）（PEO）is good conductor of lithium ion and is believed a potential material for polymer electrolyte.However,PEO with high molecular weight is easy to crystallize at room temperature which limits their segments movement and results in extremely low conductivity;While,PEO with low molecular weight does not have enough mechanical strength for practical use.The comb like copolymer with crosslinked flexible polysiloxane as the main chain and low molecular weight polyethylene glycol monomethyl ether（MPEG）as side chains might be an effective way to solve the problem.In the previous work of our group,a comb like copolymer with low molecular weight PEG as the main chain and MPEG as the side chain has been prepared by using n-Propyl trimethoxysilane（PTMS）as a bridging agent hexamethylene-1,6-diisocyanate homopolymer（HDI trimer）as the crosslinking agent.However,and the addition of HDI trimer largely restricts the segment movement of the polymer electrolyte,which will limit their application at room temperature.Meanwhile,the time-consuming preparation procedure is also a big problem.In order to improve the above method,polymethylhydrosiloxane（PMHS）is chosen as the main chain,on which MPEG350 grafts through dehydrogenation coupling reaction between Si-H and-OH.Then,PEG with two-OH end groups is used as a crosslinking agent.Finally,bis（trifluoromethane）sulfonimide lithium salt（Li TFSI,EO:Li=20:1）is added and mixed uniformly to obtain a solid state polymer electrolyte（SPE）.Si-O-C bonds formed via the dehydrogenation coupling reaction are more flexible than Si-C bonds formed by hydrosilylation,which is benefit to the movement of side MPEG chain and the conduction of lithium ions at relatively lower temperature.SPE（XM）is used to represent the polymer electrolytes,where X is the ratio of the-OH of MPEG to the Si-H of PMHS.At 20 ^oC,the ionic conductivities of all SPE（XM）are over 1×10^-4S/cm,and the best is 1.81×10^-4S/cm,which can meet the commercial standards of solid electrolytes for lithium ion battery.FT-IR characterization proves that all Si-H functional groups in PMHS have participated in the reaction.The electrochemical window of SPE（0.7M）electrolyte is 5.3V,which is tested via assembling Li/SPE（0.7M）/Li battery at 30 ^oC.The cycle performance of the assembled Li Fe PO₄/SPE（0.7M）/Li half battery is tested at 0.3C and 30 ^oC.The initial discharge specific capacity reaches 159m Ah/g.This result proves that the polymer electrolyte prepared by MPEG-grafted polysiloxane have good segment mobility and high electrical conductivity at room temperature.However,the capacity retention rate of the above Li Fe PO₄/SPE（0.7M）/Li half battery is only 60%after 20 cycles.And it further reduces to 25%after 50 cycles.Even though the electrolytes have great conductivities and wide electrochemical windows,their cycle performance are still bad.Relatively poor mechanical properties of the SPE films might be the key factor of the bad cycle performance.In order to solve the problem,a composite electrolyte based on SPE（0.7M）reinforced by rigid organic or inorganic fillers are prepared.Cellulose acetate（CA）,Si O₂and Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）nanoparticles are used.And their contents are fixed at10 wt.%.It is found that composites electrolyte containing LLZTO nanoparticles has the highest conductivity of 1.44×10^-4S/cm at 30 ^oC.The battery assembled with composite electrolyte 0.8SPE（0.7M）/0.2LLZTO has the best cycle performance.The initial discharge specific capacity is 163m Ah/g,and the capacity retention rates are95%after 20 cycles and 70%after 50 cycles,respectively.It is proven that the addition of LLZTO has largely improved the cycling performance of the composite electrolyte.It is proven that the polymer network synthesized by polysiloxane and low molecular weight polyether have good segment mobility and high ionic conductivity at room temperature.And composite electrolytes with LLZTO inorganic particles that have better mechanical strengths get better cycle performance without reducing the high ionic conductivity.