Stabilized Lithium Metal Anode And Mechanism

Recent years have seen the rapid development of the portable electronic devices such as mobile phones and laptops,as well as the electric vehicles;the growing demands for the energy storage have led to the development of rechargeable batteries with higher energy density than the lithium-ion batteries.In this regard,the lithium metal battery becomes one of the most promising candidates,due to the high theoretical capacity(3860 m Ah g^-1)and low redox potential（-3.04 V vs.SHE）of the lithium metal anode.However,the practical application of the lithium metal anode is impeded with problems such as the growth of lithium dendrites,the large volume variation during cycling,and the unstable lithium/electrolyte interfaces.Aiming for the above problems,this thesis focuses on enhancing the lithium plating/stripping stability by way of improving the electrolyte and current collector.The lithium deposition potential is often found drop sharply after some lithium deposition in the cells using the carbonate-based electrolytes.Considering the importance of the carbonate electrolyte in the practical secondary lithium batteries and the severe issues that the potential dropping will bring about,such as the decreases of the energy conversion efficiency and the energy density of a full cell,we studied the reason for the potential dropping in the commercial Li PF₆-EC/DMC electrolyte（EC for ethylene carbonate and DMC for dimethyl carbonate）.It is clarified that the repeated formation and decomposition of the organic species such as ROCO₂Li and ROLi in the solid electrolyte interphase（SEI）layer during lithium plating/stripping cycling are responsible for the sharp potential dropping.LiNO₃ as an additive has been proved effective in improving the cycling stability of the lithium metal anode,but its low solubility in the carbonate electrolytes makes this strategy impractical for the long-term cycling.A dual-salt carbonate electrolyte containing Li PF₆–LiNO₃ was developed for a stable SEI film by modifying the Li-ion solvation structure,thereby enhancing the cycling stability of the lithium plating/stripping.The replacement of NO₃^-for PF₆^-facilitates the enrichment of NO₃^-in the solvation shell and suppresses the decomposition of PF₆^-.The highly Li⁺-conductive and stable SEI film effectively tailors the lithium nucleation,suppresses the formation of lithium dendrites,and improves the cycling performance of the lithium metal anode.These improvements were demonstrated in a Li||Cu cell that ran stably for210 cycles with Coulombic efficiencies over 97%.Application of the lithium-wetting（lithiophilic）metals or lithium alloys as the current collector is an effective strategy to improve the cycling stability of the lithium batteries.However,the lithium plating/stripping behaviors are found highly dependent on the substrate materials.Principles are still lacking for the rational selection of the substrate materials.We studied the phase transition process of a series of lithiophilic materials and the lithium plating/stripping behaviors on them.A close correlation was found between their binary phase diagrams and the lithium cycling behaviors.These metals/nonmetals can be classified into two categories according to their binary phase diagrams.The selection principles of the substrates were proposed.The substrate that only forms solid solutions with lithium（such as Ag and Mg）shows high structural stability during repeated alloying and dealloying because the lithium-removed porous particles can guide the lithium deposition and support the SEI layer.In contrast,the materials（such as Au,Al,Zn,Si,and Sn）that form lithium intermetallic compounds become lithium-saturated,on which free metallic lithium is plated.Therefore,they lose their ability to guide the lithium deposition once covered with the free lithium metal.The alloy particles crumble or fall off after repeated alloying/dealloying due to the large structural change of the alloys,resulting in the poor lithium plating/stripping cycling stability.Density functional theory（DFT）calculations further verify that the above phase transitions and lithium plating/stripping behaviors are determined with the thermodynamic factors of the substrate materials.