Design And Interface Study Of Fluorine-rich Electrolyte For High Voltage Lithium Metal Battery

With the exponential growth in demand for lithium-ion batteries（LIBs）in portable smart devices and electric vehicles,increasing battery energy density has become an urgent need.High voltage lithium metal batteries based on the combination of high theoretical specific capacity(3860 m Ah g^-1)lithium metal anode and high voltage cathode（>4.3V）are expected to greatly improve the energy density of the battery.However,the commercialization of high-voltage lithium metal batteries still has great challenges,including serious safety issues caused by the high reactivity of lithium metal anodes,the oxidative decomposition of traditional carbonate electrolytes at high voltages,and the very unstable interface problems between traditional carbonate electrolytes and lithium metal anodes and high-voltage nickel-rich cathodes,resulting in reduced cycle performance of high-voltage lithium metal batteries.Electrolyte engineering has been put forward to solve the above problems,especially fluorine-rich electrolyte is considered to be one of the most promising electrolyte to achieve high voltage lithium metal batteries.Therefore,this paper mainly focuses on the structural design,performance optimization and interface regulation of fluorine-rich electrolytes,aiming at obtaining practical high-voltage lithium metal batteries with high energy density.The main research contents of this paper are as follows:（1）Polyvinylidene fluoride（PVDF）was used as the substrate to introduce a rich EO segment on the PVDF segment to dissociate and conduct lithium ions.In order to further improve the conductivity of lithium ions,a crosslinked polymer-in-salt solid electrolyte（PISSE）with multiple ion transport paths was constructed by adjusting the content of Li TFSI.Multiple ion transport paths include:Li TFSI-rich region construction[Li_m⁺TFSI_n^-]（m>n）cluster structure;The EO segment in the cross-linked polymer network forms[Li（EO）_x]⁺complex and[Li（NMP）_x]⁺clusters formed between lithium ions and residual solvent NMP.The synergistic effect of multiple ion transport pathways yielded high lithium-ion conductivity at room temperature(3.03×10^-4 S cm^-1),electrochemical stability window（5.44 V）and mechanical strength（0.811 MP）.The conduction behavior of lithium ions in Li TFSI-rich region conforms to the typical percolation model,thus increasing the concentration of carriers and realizing the rapid transport of lithium ions.The PISSE can form a variety of interfacial phases such as Li F on the surface of lithium metal anode,which ensures the excellent interfacial stability of PISSE and lithium metal anode.Finally,the in situ thermal polymerization of PISSE on the cathode promotes the interfacial compatibility between the cathode and PISSE,which makes the battery based on Li Fe PO₄ and high voltage Li Fe_0.2Mn_0.8PO₄ cathode have excellent cycle performance.（2）In order to realize high voltage lithium metal battery,a fluorine-rich flame retardant deep eutectic electrolytes（DEEs）was developed.The DEEs system is simple,mainly composed of hydrogen bond donor（3-cyano-6-trifluoromethylpyridine,CTFP）and acceptor（Li TFSI）through the interaction of Li⁺with pyridin-N and cyano-N in CTFP to produce deep eutectic effect,achieving high ionic conductivity at room temperature(1.36×10^-4 S cm^-1)and high lithium ion transfer number（0.81）.Among them,the rich-CF₃ and rich-CN groups in CTFP can not only reduce the HOMO energy of DEEs and increase the oxidation potential of DEEs（-4.84 V）,but also form stable rich-Li F and Li₃N SEI and CEI interface layers on lithium metal anode and high voltage cathode,respectively.The DEEs can be matched with a variety of high-voltage cathode materials(Li Fe PO₄、Li Ni_0.8Co_0.1Mn_0.1O₂、Li Mn₂O₄,etc.),and the capacity retention rate of the Li//NCM811 cell can be 71%after 200 stable cycles at 4.5V.Its cycling performance is better than that of traditional carbonic ester based liquid electrolyte.（3）A deep eutectic fluorine-rich gel electrolyte（PMBA-DGE）based on poly（N,N-methylene bisacrylamide）（PMBA）was successfully prepared by in-situ polymerization process.Fluoramide-rich DEEs provide high ionic conductivity and fluorine sources for PMBA-DGE,while PMBA polymer networks can lock DEEs to prevent side reactions with lithium metal anodes and high-voltage cathodes.The synergistic effect of fufluoramide DEEs and PMBA endow PMBA-DGE with high ionic conductivity at room temperature(6.75×10^–4 S cm^–1),high electrochemical stability window（4.88 V）and flame retardancy.PMBA-DGE constructs a robust positive electrolyte interface（CEI）and a stable solid electrolyte interface（SEI）on a nickel-rich cathode and a lithium metal anode.The SEI structure is composed of abundant Li pophilic N-（C）₃,high ionic conductivity Li F and Li₃N to achieve uniform distribution of lithium ions,inducing uniform plating and stripping of lithium ions,while the CEI structure is composed of high antioxidant amide organic matter,high ionic conductivity Li F and Li₃N,reducing electrolyte consumption and structural degradation of the cathode active material.Therefore,the PMBA-DGE based NCM811//Li cell exhibits excellent cycle stability at 4.3 V voltage.In summary,the fluorine-rich electrolyte prepared in this paper can not only obtain high ionic conductivity and electrochemical stability window,but also regulate the interface phase of electrolyte with lithium metal anode and high voltage cathode,so that it can be applied to high voltage lithium metal batteries.Therefore,the fluorine-rich electrolyte developed in this paper can provide an effective solution for the real commercial application of high voltage and high energy density lithium metal batteries.