Lithium Metal Anode Modified By Mg-base Composite Artificial Solid Electrolyte Interphase Membrane

In order to promote the“fourth industrial revolution”represented by intelligent manufacturing,the new energy industry reprensented by smart grid and new energy vehicles has attracted extensive attention from the international community.Today,the specific energy of commercial lithium-ion batteries（LIBs）has reached the theoretical limit,and the future consumer electronics and electric vehicle market is in urgent need to develop lithium metal batteries with high energy density,long cycle life and high safety batteries.Although lithium metal has an extremely high theoretical specific capacity(3860 m Ah g^-1)and an extremely low electrochemical potential（-3.04 V vs standard hydrogen electrode）,its inherent defects such as irregular lithium dendritic,unstable interfaces and infinite volume changes have a serious negative impact on the practical application of lithium metal batteries.So far,the construction of artificial solid electrolyte interface membrane（artificial SEI membrane）is considered to be one of the most effective ways to solve the problems related to lithium negative electrode.This paper focuses on the research of lithium anode protection:1.Construction of organic/inorganic composite artificial SEI film on Li metalAn organic/inorganic composite artificial SEI film（PEG-Mg SEI）in situ polymerized is designed and prepared on the surface of lithium anode by surface coating.The organic polyethylene glycol diacrylate（PEGDA）provided a uniform structure and dense flexible support,which can isolate metal lithium and adapt to the volume change of lithium anode.The inorganic magnesium fluoride（Mg F₂）is in contact with lithium metal and undergoes a displacement reaction to generate Li F and magnesium-rich inorganic substances.It can promote Li⁺transport,inhibit the growth of lithium dendrites,and improve the interfacial stability of SEI film.With three different inorganic contents（1.35 wt.%,0.68 wt.%,0.23 wt.%）composite SEI membranes were assembled with Li||Li symmetrical cells and Li||LFP full cells,in which PEG-0.68 wt.%Mg SEI protected lithium assembled cells exhibit better cycling performance,far exceeding the cycle life and charge-discharge capacity of bare lithium.2.Application of Mg-based composite SEI film to modify lithium metal with different thicknessOn the basis of the experiment in the previous chapter,vinylene carbonate（VC）is introduced to copolymerize with PEGDA to promote the closer bonding of artificial SEI film with lithium metal,thereby effectively increasing the discharge specific capacity and prolonging the life of the battery.Li||LFP full batteries were assembled with PEG-Mg-VC SEI modified lithium metal of different thicknesses（600μm,100μm,30μm）.In comparison with bare Li,SEI modified lithium metal battery maintained longer cycle life and higher charge and discharge capacity under three different positive/negative electrode ratios.This excellent electrochemical performance is mainly due to artificial SEI film,which reduces the side reaction between liquid electrolyte and lithium metal and inhibits the formation of dead lithium.Through SEM and AFM tests,it is proved that after 50 cycles,the metal lithium protected by PEG-Mg-VC SEI still has a metallic luster without darkening.The electrode surface morphology is smooth and dense,and there is no obvious presence of dendrite and dead lithium.The activation energy of artificial SEI and natural SEI showed that Li⁺migrated more freely in artificial SEI film,which was conducive to uniform deposition of lithium ions.Modified lithium metal with a thickness of 600μm is used for Li||NCM622 full cells,tests are carried out at different C rates（0.3 C,0.5 C,1.0 C）.The SEI protected lithium-assembled batteries showed better electrochemical cycling performance and rate performance than bare lithium.