UV-Cured Polymer-based Quasi-Solid Electrolytes And Their Application In Lithium Metal Batteries

The utilization of lithium metal anode combined with high voltage/capacity cathode is an effective way to improve the energy density of lithium batteries.However,poor stability of commercial electrolytes limits the practical applications of high capacity lithium metal batteries.Instead of liquid electrolytes,solid-state electrolytes with high mechanical strength and electrochemical stability are expected to achieve high safety for lithium metal batteries.However,it is difficult to balance the relationship among ionic conductivity,mechanical strength and interfacial wettability for the current solid-state electrolytes.Constructing quasi-solid electrolytes with good wettability,mechanical properties and high ionic conductivity make it possible to realize high performance lithium metal batteries.In this thesis,polymer-based quasi-solid electrolytes were designed and prepared by UV-curing.Ionic conductivity,electrochemical/thermal stability,mechanical properties of the obtained quasi-solid electrolytes and their compatibility with Li-metal were systematically studied.The main work and results are as follows:Firstly,quasi-solid polymer electrolytes based on semi-interpenetrating network(SNP)were constructed by trimethylolpropane ethoxylate triacrylate(ETPTA)and poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP).Based on the rigid/flexible coupling between ETPTA crosslinking network and PVDF-HFP chains,SNP shows good mechanical strength and shape versatility.Thanks to the good liquid confinement ability and segment mobility of the semi-interpenetrating network,SNP exhibits excellent ionic conductivity(>1 mS cm^-1)and Li⁺transference number(0.53).The LiFePO₄/Libatteries based on SNP-30(with 30 wt%ETPTA)electrolyte exhibit excellent cycling performance(450 cycles at 2.0 C without obvious degradation).Furtherly,oxidation resistant succinonitrile(SN)was introduced into the UV-cured electrolyte mentioned above,and deep eutectic polymer quasi-solid electrolytes(DPE)with wide electrochemical stability window(?5.3 V vs.Li⁺/Li)were prepared.The deep eutectic structure formed by SN and lithium salt can effectively improve the ionic conductivity,Li⁺transference number and safety of electrolyte.In addition,the stability of DPE to Li-metal is greatly enhanced by the synergistic effect of crosslinking network and lithium difluoro(oxalato)borate(LiDFOB)additive.The Li/DPE-LiDFOB/Cu cells can be uniformly plated/stripped for more than 500 cycles at 0.5 mA cm^-2 with an average coulombic efficiency of 97.6%.Moreover,high-voltage NCM622/Liand NCM811/Lilithium metal batteries based on the DPE-LiDFOB electrolyte demonstrate good cycling performance.Finally,in view of the flexibility and stability of DPE,DPE is expected to improve the problems of volume expansion and transition metal dissolution in ferrous fluoride(FeF₂)lithium free cathode.DPE interphase layer was in-suit polymerized on the surface of FeF₂ particles to alleviate the contact between FeF₂ and electrolyte,and meanwhile inhibit the related side reactions.The DPE modified FeF₂/Libatteries exhibit enhanced capacity and cycling stability.