Preparation Of PEG-based Polymer Electrolytes And The Electrochemical Properties Of All-solid-state Lithium Ion Batteries

All-solid-state polymer lithium ion batteries have become the most promising secondary energy storage equipment for lithium-ion batteries because of their excellent performance（high safety,high energy density and wider operating temperature）.Polyethylene glycol（PEO）is widely studied as polymer electrolyte matrix material because of its strong ability to dissolve lithium salt.PEO is a semi-crystalline material,PEO with high molecular weight can be easily crystallized,which may hinder the migration of lithium ions in the amorphous phase;and PEO with low molecular weight has no mechanical strength.To this end,PEO must be modified,such as by blending,copolymerization,doping and other methods to reduce the crystallinity of PEO and also improve the conductivity of lithium ions while ensuring a certain degree of mechanical strength.In this paper,a self-cross-linked comb-like copolymer was synthesized by copolymerization and crosslinking through the"one-pot"method.Triphenylmethane4,4`,4``-triisocyanate（TTI）,polyethylene glycol monomethyl ether（MPEG）and polyethylene glycol（PEG）are used as raw materials,wherein TTI not only acts as bridging agent to form PEG backbone and side chain by reacting with hydroxyl groups of PEG and MPEG,but also acts as crosslinking agent to form a cross-linked structure which can provide enough mechanical strength by reacting with the terminal hydroxyl groups of the copolymer chain under heat treatment.The synthesized polymer was characterized by FT-IR and H¹NMR.The results of DSC and TG indicate that the PEG-based polymer with high PEG content is non-crystallizable and exhibits good thermal stability.PEG-based polymer electrolyte was prepared by mixing the above polymer with the lithium salt LiTFSI.The electrochemical window of SPE1000（EO:Li=20:1）reached 4.71V at 60℃.High ionic conductivity of about 2.2×10^-5 S·cm^-1 at room temperature is obtained and 10^-4 S·cm^-1 is got at 60℃.The PEG-based comb-like copolymer electrolyte synthesized in this paper exhibits good thermal stability,good mechanical properties,larger electrochemical windows and higher ionic conductivity,which provide a potential candidate for lithium-ion battery polymer electrolyte.The charging and discharging properties of all-solid-state polymer lithium ion battery assembled in order of commercial cathode,the synthetic polymer electrolyte and lithium metal were measured.At 60°C,the initial discharge specific capacity is 125 mAh·g^-1 at 1C current density,and about 42%of capacity is maintained and the coulombic efficiency is above 95%after 300 cycles.SEM images of cross-section of battery showes that the cathode is cracked after cycled.In addition,the effects of running temperature and cathode thickness on the specific capacity of all-solid-state lithium ion battery were investigated.The results show that the battery specific capacity decreases along with the decrement of running temperature,but when the running temperature rises back,the specific capacity picks up again.The thicker the cathode is,the larger extent of the decrement in the specific capacities can be found.Pre-coating the cathode nanoparticles is proved to be an effective way to enhance the capacity.In this work,the synthetic non-crystalline and non-cross-linked PEG-based polymer electrolyte with high ionic conductivity and good electrochemical performance is used to pre-coat LiFePO₄ nanoparticles to prepare the composites cathode and the SPE membrane is prepared by the same cross-linked polymer electrolyte.Therefore,there is no additional interface between the cathode and SPE film.The charging and discharging properties of all-solid-state polymer lithium ion battery assembled in order of composite cathode,the SPE membrane and lithium metal were measured.First,it is researched that the optimum content of polymer electrolytes in composite cathode was fifteen portion（LiFePO₄:C:PVDF:Polymer=60:20:5:15）and the most suitable content of lithium salt LiTFSI in polymer electrolyte was 25wt%（EO:Li=20:1）.The cycle performance of battery demonstrated under above conditions is that:at 60℃running temperature and at 1C current density,the initial discharging specific capacity is 138 mAh·g^-1,the capacity retention ratio is over 90%and the columbic efficiency keeps more than 98%after 300 cycles.SEM image of the cross-section of battery shows no crack emerges both inside of composite cathode and at the interface of cathode and SPE.What’s more,the charge and discharge performance at large current density shows that the capacity remains 50 mAh·g^-1 when the current density increases to 11C.And only 27.5%of the specific capacity decays when composite cathode thickness increases from100μm to 200μm,which is much lower than that of un-coated cathode.This is mainly due to the formation of bi-continuous pathways of acetylene black and polymer electrolytes within the composites cathode,which ensure both electrons and lithium ions can be transferred easily and the specific capacity of the battery can be largely enhanced.The composite cathode of LiFePO₄ coated by the PEG-based polymer electrolytes produces more contact areas between active particles and SPE,and generates more lithium ion transferring paths from active particle to anode through polymer electrolytes,which results in higher capacity of all-solid-state lithium ion battery.Furthermore,the PEG-based polymer electrolyte encapsulated LiFePO₄ active particles serves as a buffer layer to relieve the volume change of the active particles during the process of charging and discharging,which contributes to the good cycle performance.In conclusion,for all-solid-state lithium ions battery,pre-preparing composite cathode with nanoparticles encapsulated by polymer electrolytes might be an effective way to increase both capacity and stable cycling performance,and the all-solid-state lithium-ion battery in this article is expected to be applied in power battery system.