Modification Of Lithium Metal Anode By Magnetron Sputtering

With the rapid development of electronic equipment and electric vehicles,higher energy density is urgently required for electrochemical energy storage devices.The lithium?Li?metal anode is considered as the preferred anode material to replace graphite due to its ultra-high theoretical capacity?3860 mAh/g?and the lowest redox potential?-3.045 V vs.standard hydrogen electrode?.However,at this moment,some shortages hinder its practical application.Firstly,Li metal anode easily suffers from Li dendrite growth,which could lead to short circuit of the battery and the safety risk.Secondly,Li metal is also very unstable toward the organic electrolyte,so the reaction with the electrolyte results in Li consumption,which greatly shortens the battery life.In addition,infinite volume change of Li metal anode is also a thorny problem in cycling.Therefore,the modification of Li metal anode is imperative.Among many modification methods,interface modification has been widely studied because of its simplicity and effectiveness,together with minor negative impact on the properties of Li metal.In this thesis,the surface of Li metal anode is modified by using an economical and commercially applied way-namely magnetron sputtering.?1?ZrO₂ as coating layer was sputtered on the surface of Li metal.Then,the surface morphology and electrochemical properties were systematically investigated.The results show that the interfacial stability and electrochemical performance were improved obviously.Li metal anode coated with 37 nm thickness ZrO₂ thin film exhibits the best cycling performance,and the symmetric Li-Li cell can stably cycle over 900 h under the current density of 0.5 mA/cm².By using the modified Li foil as the anode in Li₄Ti₅O₁₂ cell,the capacity retention rate was promoted up to 92.9%,which was far higher than that of using pristine Li foil?27.0%?,and the polarization was also more stable.Further SEM observation revealed that the surface and cross-sectional morphology of the Li electrode coated with 37 nm thick ZrO₂ is still dense and uniform after 100 cycles,which means that few mossy Li,dendritic Li and SEI by-product are generated.The reason is that chemical inertness of ZrO₂ prevents the electrolyte from corroding Li metal electrode.In addition,the uniform and flat ZrO₂ coating layer provides a better matrix for Li deposition.?2?Li-Al alloy has higher Li ion diffusion coefficient(6.0×10^-10 cm²/s^-1)than that of pristine Li(5.69×10^-11 cm²/s^-1).Thus,we deposited Al thin film?74 nm?onto the surface of Li foil with magnetron sputtering technique.The Al-coated lithium metal electrode creates high Al atomic concentration zone?Li-Al alloy layer?on the Li surface.The in-situ formed Li-Al alloy layer can effectively passivate the interface preventing Li anode from reacting with organic electrolyte.Moreover,the alloy layer contributes to alleviating dendrite formation owing to the high Li⁺diffusion coefficient in Li-Al alloy,which facilitates the reduced Li atoms to rapidly diffuse into the Li-Al alloy layer.The Li-Li cell with Al-coated Li metal electrode can endure for⁹50 h at a current density0.5 mA cm^-2 in conventional carbonate electrolytes.Therefore,after 200 cycles,the surface of Li sheet coated with Al still remains smooth and compact showing metallic luster.?3?The Li-Zn alloy has a lithium-ion diffusion coefficient of up to 4.7×10^-8 cm²s^-1,which could better meet the requirement under high current density.Thus,a layer of600 nm thick Zn film was deposited on the surface of Li metal by sputtering,then the as-deposited Zn can spontaneously react with Li to form Li-Zn alloy layer on the Li surface.Further,the surface morphology and electrochemical properties were systematically characterized.The cyclic life-span of symmetric Li-Li cell under the current density of 1 mA/cm² is more than 500 h,which is more than 3 times of that for the pristine Li electrode?¹50 h?.Under the condition of a large current of 3 mA/cm²,the cyclic life is still more than 200 h.The solid battery with modified Li foil and PEO-based solid state electrolytes can cycle over than 920 h at 0.1 mA/cm² and 0.1mAh/cm²,which increased by more than 6 times comparing with that of bare Li.This indicates that the high Li⁺ion diffusion coefficient of the Li-Zn alloy is favor of the uniform deposition of Li metal and reduces the dendrite growth.In conclusion,the Li-Zn alloy formed on the surface Li metal can inhibit the parasitic reaction with the electrolyte,and prolong the cycle life of the battery.