Research On Interface Regulation And Electrochemical Performance Of Lithium Metal Anodes

Lithium metal shows a theoretical specific capacity of 3860 m Ah/g and the lowest electrode potential.Hence,it is called the "holy grail" of energy and is an ideal negative electrode material for next-generation lithium batteries.Lithium metal batteries using lithium metal as a negative electrode exhibit a huge advantage in energy density.However,the higher activity of lithium metal will cause many problems such as dendrite growth and electrolyte consumption,which will lead to the fast capacity fading and safety hazards.Therefore,the protection of lithium metal anodes has become a hot topic in this research field in recent years.Regulating the lithium metal interface or using a three-dimensional supporting host are often used to improve the electrochemical performance of lithium metal anodes.These methods can achieve the purpose of suppressing lithium dendrites and improving the cycle life of lithium metal battery.Current research results show that it is difficult to achieve highly efficient protection of lithium metal only through a single strategy,and it is necessary to consider the synergy of multiple protection methods in order to achieve better protection effects.Therefore,this dissertation focuses on achieving effective suppression of dendrites and significantly increasing cycle life through various interface regulation and collaborative protection strategies such as ex-situ artificial protective film,in-situ protective film,electrolyte additives,and in-situ film on threedimensional host.The purpose is to provide theoretical support and technical reference for the practical advancement of lithium metal anodes.First,an organic/inorganic composite bilayer ex-situ protective film was prepared.Zn O nanorods were prepared by low-temperature solid-state method,and was uniformly scraped on the surface of lithium metal to form a buffer layer,and then the buffer layer was covered with a Zn O/PVDF-HFP composite film obtained by spin coating to form a double-layer ex-situ protective film.The buffer layer is used to alleviate the solid-solid interface contact,reduce the interface impedance from hundreds of ohms to 26.2 Ω,thereby reduce the voltage mutation caused by the untight contact between lithium and protective film.This double-layer Zn O/PVDFHFP@Zn O protective film enabled lithium-copper cells to obtain a Coulombic efficiency of about 94 % in the ester electrolyte and maintained 100 cycles without attenuation.In addition,when this kind of protective film was used in symmetrical Li-Li cell,it could also make the cell run stably for 500 h,and after 200 cycles,the surface of the protected lithium in the symmetrical cell was smooth without dendrite growth,and the rod-like structure of Zn O powder maintained well.Secondly,the dielectric effect in the protective film and its influence on lithium deposition were explored.The theoretical models under galvanostatic and potentiostatic modes were established using computer software to simulate the distribution of lithium ion flow on the lithium metal surface that the protective film with different dielectric constants covers.When the lithium metal is deposited,the concentration polarization electric field,the dielectric polarization electric field and the external electric field are simultaneously present in the protective film.The dielectric effect of the protective film caused by the concentration polarization electric field and the external electric field can induce a reverse electric field,which suppresses the sudden change of the current in the film,and then plays the role of homogenizing the lithium ion flow.In the case of galvanostatic conditions,the simulation results can provide an optimal relative permittivity range of 5～13 to make the ion current uniform.A series of protective films with different relative dielectric constants were prepared,and the simulation results were preliminarily verified by the stability test of the symmetrical Li-Li cells.Thirdly,in order to solve the problem of non-close interface contact caused by the ex-situ protective film,this dissertation used solid-state anodic oxidation to prepare the in-situ protective film on the surface of lithium metal.Using amorphous aluminum oxide film as the intermediate solid electrolyte,an in-situ protective film was prepared on the surface of lithium metal under a voltage of 10 V,and was rich in carbon and lithium compounds.This in-situ protective film could make the symmetrical Li-Li cell run stably for 1500 h,and make the lithium cobalt oxide full cell run 300 cycles.This method can not only improve the cycle life of lithium metal in the full cell,but also realize the repair of the dendritic layer at the interface of the lithium negative electrode.The fluorinated in-situ protective film modified with hydrofluoroether can improve the capacity retention rate of the lithium cobalt oxide full cell,reaching a value of 83.18% after 200 cycles.Finally,the mechanism of the non-sacrificial lithium perchlorate additive and the preparation method of the three-dimensional carbon host were studied respectively.The interface regulation effect of lithium perchlorate additives on carbon host surface has been explored,which can further improve the cycle stability of lithium metal anodes.The hollow structure carbon fiber formed by the carbon diffusion effect can enhance the lithium metal cycle performance.When combined with lithium perchlorate additives,lithium intercalation occurs on the surface of the host to form a carbon-containing surface layer.The synergistic effect increased the Coulombic efficiency of the lithium-carbon cell to 99.6 %,and the cell stably run for800 cycles.Through the combination strategy of lithium perchlorate additive and carbon host,the capacity type NCM full cell cycled stably for 250 cycles,which breaks through the cycle life limit of the full cell caused by the rapid consumption of traditional sacrificial additives on the host with large specific surface area.