Investigation On Sn-based Li Alloy For Advanced Li Metal Anode

The boom in electric vehicles and electronic products has expanded the need for better performance of energy density,charging time,endurance,flexibility,and other aspects.Li metal becomes an ideal anode material due to its excellent advantages of high specific capacity and light mass density.However,the deficiencies of the Li anode such as Li dendrite growth and safety problems,still cannot be ignored.Moreover,the large thickness of commercial Li foils prepared by mechanical methods leads to low effective utilization of electrode capacity.Therefore,it is critical to inhibit the growth of Li dendrites,improve the performance of the Li anode and prepare ultra-thin Li anode with good performance.Li-Sn alloy is an ideal material for modification of Li anode,due to its good lithiophilicity,fast Li-ion transportation capacity,low nucleation potential and high exchange current density with respect to Li metal.In this dissertation,Li-Sn alloy is used in both bulk and surface modification for the improvement of Li anode performance.In the bulk modification,a Sn-containing Li-rich alloy dual-phase anode with three-dimensional（3D）rod-like Li₂₂Sn₅ alloy skeleton and a composite anode of Sn-containing Li-rich alloy and carbon cloth with hierarchical bamboo shoot-like lithiophilic framework is formed by melting method and thermal infusion method.In the surface modification,an artificial solid electrolyte interphase（SEI）layer containing Li-Sn alloy is ex-situ or in-situ formed on the surface of Li anode by using the Sn Cl₄ as one of the components of the coating solution or the electrolyte additive.The 3D lithiophilic alloy framework and coating layer with rapid Li ion transport capability can regulate Li deposition/dissolution behavior,improve the electrode/electrolyte interface,enhance the mechanical stability of the negative electrode,reduce the Li nucleation/deposition overpotential and prolong the cycling lifespan of the battery.（1）Sn-containing Li-rich dual-phase alloy is prepared by melting method.Several Li-rich Li-Sn dual-phase alloys with different atomic ratios are prepared by melting at500°C,following with a cooling process.Sn-containing Li-rich dual-phase alloy contains two phases,i.e.,excess free Li metal and Li₂₂Sn₅ alloy.Different from other proportions,Sn Li90 forms a rod-like Li₂₂Sn₅ alloy skeleton structure during the phase separation process.The excess of free Li and the higher redox potential of Li₂₂Sn₅ guarantee the good electrochemical"inertness"of the Li₂₂Sn₅ alloy skeleton during the charge and discharge process.The three-dimensional structure design of lithiophilic Li₂₂Sn₅ alloy,which has the kinetic advantage of inducing Li deposition on its surface,possesses functions of reducing the effective current density and inducing a conformal deposition of Li.Since the deposition/dissolution behavior of Li is improved,the cyclic lifespan of the Sn-containing Li-rich dual-phase alloy anode is three times that of the pure Li anode.The excess of Li metal in the Sn-containing Li-rich alloy also retains the advantage of high specific capacity.（2）In order to further improve the performance of three-dimensional skeleton,such as the continuity,specific surface area,mechanical strength and stress release ability,a foreign framework is adopted for further modification of the Sn-containing Li-rich alloy anode.Flexible carbon cloth is an ideal skeleton since the stress in the electrochemical deposition/stripping can be easily released,benefiting for Li dendrites suppression.The wettability of the carbon cloth toward molten Li is greatly enhanced in terms of the following three aspects:the dual affinity of Sn to Li/C;an increase of wettability by doping;the formation of nano-cracks and a larger specific surface area by high-temperature treatment of carbon fibers.A three-dimensional lithiophilic skeleton with a hierarchical structure is prepared by thermal infusion.The flexible and continuous carbon fibers network as the primary skeleton and the lithiophilic bamboo shoot-like particles anchored on the carbon fibers as the secondary skeleton have the effects of improving the specific surface area of the 3D current collector,reducing the current density and inducing uniform deposition of Li,which further increases the electrochemical properties of the battery.（3）The 3D skeleton has limited protection of the electrode surface.Therefore,an artificial SEI is very important to improve the Li anode/electrolyte interface.And a coating layer is particularly important in ultra-thin Li anode since it has a lower content of Li due to the ultra-thin thickness.Firstly,the wettability between molten Li and stainless-steel substrate is improved by doping copper atoms.And the thickness of the molten Li-Cu alloy is controlled by a scraper.The ultra-thin Li copper alloy with a thickness of 30μm is prepared by melting and casting method.Then the Li₂₂Sn₅/Li Cl hybrid coating layer is formed on the surface of the ultra-thin Li-Cu alloy by the reaction between the Sn Cl₄ solution and Li.Li-Sn alloy is used for the surface modification of ultra-thin Li-Cu anode.Ultra-thin Li-Cu alloy has a rich well conductive lithiophilic Li Cu_x nanowire skeleton.The hybrid coating layer has fast Li ion transport,good mechanical strength and a low Li ion diffusion barrier.Under the synergistic effect of bulk modification of Li-Cu alloy and surface modification of Li-Sn alloy,the ultra-thin Li composite anode reaches a long cycle life of unmodified thick Li foil.（4）The ex-situ coated protective layer might gradually break and fail due to the accumulated stress during the electrochemical cycling.In order to repair the protective layer timely,the Sn Cl₄ additive is introduced into the ester-based electrolyte which contains the fluoroethylene carbonate（FEC）additive.During the electrochemical cycling process,a hybrid SEI layer containing Li₂₂Sn₅/Li Cl/Li F is in situ formed by the reaction between the mixed additive and Li.Herein,Li-Sn alloy plays a positive role for the surface modification of the Li anode,where the surface and cross-section morphology at the 1^st cycle and 50^th cycle cannot be identified indicating that the coating layer possesses good stability.The rich grain boundaries/interfaces formed among Li₂₂Sn₅,Li Cl and Li F components can assist Li ion transport,thereby greatly reducing the polarization and prolonging the lifespan of the Li anode.The Li-rich Sn Li90 anode is further matched with the mixed additive electrolyte system,i.e.,the Li-Sn alloy is applied for both the bulk modification and surface modification of the Li anode.Consequently,the life of the battery is significantly prolonged.