Structural Design And Property Optimization Of Solid Electrolyte Interfaces For High-performance Lithium Metal Anodes

With the advancement of China’s carbon peak and carbon neutrality targets,green energy has become an important object of development in the next decades.As new energy storage device,lithium-ion batteries has been an important part of the large-scale application of green energy.However,the capacity of lithium-ion batteries based on graphite anodes is approaching its theoretical limit.Among different anode candidates,Li metal has attracted numerous attentions due to its high theoretical specific capacity(3860 m Ah g^-1)and the lowest redox potential（-3.04 V）.However,the poor reversibility and Li dendrite growth hinder the commercial applications of Li metal anode.Because of the highly active chemical properties,Li metal can react with solvents and Li salts in the organic electrolytes to form a thin solid electrolyte interface（SEI）layer on surface to prevent the continuous side reactions.However,the fragile SEI film cannot accommodate the huge volume changes during the charge/discharge processes,and the cracks in the SEI will aggravate the parasitic reactions between the fresh Li metal and electrolyte,resulting in the rapid deterioration of Coulombic efficiency（CE）and cycle life of the Li metal batteries（LMBs）.Besides,the non-uniformity of the native SEI film in structure and composition will induce the uneven deposition of Li ions,which will lead to the generation of Li dendrites and even safety hazards.These concerns can be greatly accelerated when the LMBs are operated at elevated current densities and areal capacities.To solve the above problems,the main research of this thesis is to design and construct a stable lithium/electrolyte interface to achieve uniform Li⁺deposition enhanced cycle stability of the lithium metal anode at high current density and high areal capacity.The main research results of this thesis are displayed as follows:（1）A dense and uniform inorganic SEI layer composed of ZrO₂,Li₂O,Li₃N and Li N_xO_y is constructed on surface of Li metal via the spontaneous reaction between Li metal and ZrO（NO₃）₂ solution.The abundant grain boundaries in the artificial SEI created by the multicomponent enable the rapid diffusion of Li ions at the interface.In addition,the dense and high mechanical strength of the inorganic SEI layer facilitates the inhibition of side reactions and dendrite growth at the interface.As a result,the Li metal anode treated with zirconyl nitrate（Li ZrO（NO₃）₂@Li）delivers a stable cycle performance of over 550 h at a high current density of 10.0 m A cm^-2 and a high areal capacity of 10.0 m Ah cm^-2.When paired with high-loading Li Co O₂ cathode(19.0 mg cm^-2),the Li ZrO（NO₃）₂@Li anode shows much enhanced rate performance and long-term cycle stability without Li dendrite formation.（2）To further improve the ionic conductivity of the artificial SEI,a phenoxy radical Spiro-O8 is proposed as an artificial protection film for Li metal anode owing to its excellent film-forming capability and remarkable ionic conductivity.A spontaneous redox reaction between the Spiro-O8 and Li metal results in the formation of a uniform and highly ionic conductive organic film in the bottom.Meanwhile,the phenoxy radicals on surface of Spiro-O8 facilitate the decomposition of Li salt upon exposed to the ether electrolyte and lead the formation of Li F film on the top.Arising from the synergistic effects of inner high ionic conductive film and outer rigid film,stable Li plating/stripping can be realized at a high current density(10.0 m A cm^-2)and a high areal capacity(10.0 m Ah cm^-2)for 700 h with an ultrahigh Li utilization rate of 54.6%.（3）To achieve a balance between the high ionic conductivity and mechanical properties of artificial SEI,a dynamic gel with reversible imine groups,which was prepared via a cross linking reaction between flexible dibenzaldehyde-terminated telechelic poly（ethylene glycol）（DF-PEG-DF）and rigid chitosan（CS）,has been proposed to fabricate protective layer for Li metal anode.The as-prepared CS/DF-PEG-DF film shows combined merit of high Young’s modulus,strong ductility and high ionic conductivity.When the CS/DF-PEG-DF film was fabricated on a Li metal anode,the thin protective layer shows a dense and uniform surface owing to the interactions between the abundant polar groups and Li metal.Besides,the polar groups in the CS/DF-PEG-DF layer can homogenize the distribution of Li⁺at the electrode/electrolyte interface and facilitate the even deposition of Li⁺ions.As a result,unprecedented cycle stability over 3200 h under a high areal capacity of 10.0 m Ah cm^-2 and a high current density of 10.0 m A cm^-2 has been obtained for the protected Li metal anodes.Moreover,greatly enhanced cycling stability and rate capability has also been achieved in the full batteries when paired with high-loading Li Fe PO₄(～20.0 mg cm^-2)or sulfur/carbon cathode(～7.0 mg cm^-2),showing the great potential for the practical Li metal batteries.（4）To improve the solubility of nitro in carbonate electrolytes,a new electrolyte additive（PCL-ONO₂）with nitro groups is synthesized by acylation of both ends of polycaprolactone diol（PCL-idol）and substitution of nitro groups.The additive can rapidly construct a dual-layered SEI with outer organic-rich layer and inner inorganic-rich layer such as Li₂O and Li₃N.The stable SEI can ensure rapid Li⁺transport,homogenize Li⁺deposition as well as inhibit the growth of dendrites.Benefit from the incorporation of PCL-ONO₂ additive,the symmetric cells in carbonate electrolyte can demonstrate an unprecedented cycle stability over 1400 h at a high current density of 10.0 m A cm^-2 and of 10.0 m Ah cm^-2.In addition,the cycle stability and rate capability of Li metal batteries are greatly improved when used with high-loading Li Fe PO₄ or Li Ni_0.5Co_0.2Mn_0.3（NCM523）cathodes,indicating a great potential for practical applications.By constructing stable SEI on the surface of lithium metal,the above work achieves the cycling stability of lithium metal anode under high current density and high areal capacity.The mechanism of the ionic conductivity of the SEI on the stability of the lithium metal anode and the influence of the mechanical properties on the interfacial structure of the lithium metal anode are also revealed.These woks provide a new way for the realization of efficient and stable cycling of LMBs.