Structure Design Of Lipophilic Polymer Interfacial Layer And Its Protection For Lithium Metal Anode

Lithium metal is known as the"Holy Grail"electrode due to its ultra-high theoretical specific capacity(3860 m A h g^-1)and extremely low electrochemical potential(-3.040 V,relative to standard hydrogen electrode).Lithium metal batteries are considered as one of the promising candidates for the next generation high energy density energy storage systems.In spite of these advantages of lithium anodes,there are still a series of bottlenecks impeding their wide applications as electrodes for lithium metal batteries:(1)Uneven plating/stripping of lithium results in break of SEI during charge and discharge process.(2)Lithium dendrites and the volume expansion cause low coulombic efficiency,capacity degradation and safety risks of the battery.Therefore,how to effectively inhibit the lithium dendrites and ensure the long-term cycle of the lithium metal anodes is the key for the commercialization of lithium metal batteries.In view of above points,from the perspective of the interfacial layer design,this thesis designs polymer interfacial layers with lithiophilic functional groups on the surface of the electrodes and optimizes their structures,thus to control the deposition behavior of lithium metal,inhibit the growth of lithium dendrites and improve the cycle performance of batteries.We believe our work will provide a new strategy for the protection of lithium metal anodes.The main research contents are as follows:Poly(vinylidene fluoride)/poly(methyl methacrylate)(PVDF/PMMA)composite nanofiber interlayer was introduced on the surface of lithium anode by electrospinning technique.The composite nanofiber network with a large number of lithiophilic groups(C-F)have good thermal stability,and could provide uniformly dispersed pathways for regulating Li⁺flux and accommodating volumetric change.The electro-chemically active polymer-PMMA can gradually dissolve in the electrolyte during cycling to manipulate the deposition behavior of lithium ions,thus effectively inhibiting the growth of lithium dendrites and guiding the uniform lithium deposition.As a result,the lithium anode protected by PVDF/PMMA composite interlayer delivers a high Columbic efficiency of 98.2%for 320 cycles at a current density of 0.5 m A cm^-2.Symmetric cells can stable cycling over 900 h with a small voltage hysteresis of 65 m V.In addition,when coupled with Li Fe PO₄,the full cells exhibit superior rate and cycling performance with high capacity retention of 78%at 1 C rate after 400 cycles.In addition,an artificial SEI consisting of polydopamine-polyacrylamide(PDA-PAM)polymer brushes was constructed on the Cu current collector by a simple UV-initiated polymerization.PAM polymer brushes with lithiophilic functional groups of amide O,can induce the uniform distribution of Li nucleation sites on the surface of Cu foil and increase the possibility of nucleation.The space limitation of the nano-brush structure can homogenize ion mass transfer,and the ultra-small space between each brush can be used as a channel for the transport and normalization growth of lithium,thus leading to the uniform lithium deposition.Based on the synergistic effect of homogenization and normalization of lithium deposition,the Coulomb efficiency of the Li-Cu cell with PDA-PAM polymer brush-modified Cu foil can still reach 98%after 120 cycles at a current density of 1 m A cm^-2.It also shows low voltage hysteresis and excellent cycle stability in the symmetric cells test.