Designable Structures And Interphases Of Lithium/Sodium Metal Anodes And Their Electrochemical Performances

High energy density and safe chemical storage systems play critical roles in various kinds of emerging fields of technology.As the electric vehicle market continues to expand and the demand for energy storage continues to grow,emerging industries have increasingly higher requirements for battery energy density,and it is imperative to develop the next generation of advanced materials for high energy density battery systems.Lithium/sodium metal anodes that have a high theoretical capacity,high energy density,and low oxidation-reduction potential have been widely studied.Nevertheless,the lithium/sodium metal anode in practical application is still hindered by formidable challenges.Specifically,the uneven electrodeposition of lithium/sodium ion during the cycling processes leads to the formation of dendritic structures,which can bridge the inter-electrode space and thereby produce an internal short circuit in cells.Moreover,the volatile lithium metal anode can react with the active electrode and electrolyte,causing severe capacity decay.Therefore,to solve the above problems,the main objective of this thesis is to build a stable multifunctional electrode/electrolyte interface layer,design a three-dimensional solid electrolyte interface layer（solid electrolyte interphase,SEI）,or through combining constructing a host frame and modifying the interphase layer to improve the electrochemical performance of lithium/sodium metal anode.The main research content and results are as follows:（1）By modifying the SEI layer on the surface of the lithium metal,a chemical reaction between lithium and Al F₃forms a multifunctional artificial SEI layer on the surface of the lithium metal.The main components of the SEI layer are Li-Al alloy and LiF.DFT calculations show that Li-Al alloy has high lithium-ion conductivity,which can increase the lithium-ion conductivity of the SEI layer and inhibit the formation of lithium dendrites.LiF can improve the electrochemical stability of the SEI layer and slow down the side reactions between the lithium metal electrode and the electrolyte.Through in-situ observation of the charging process of the modified lithium metal electrode,it was found that the growth of lithium dendrites was significantly inhibited.At the same time,the coulombic efficiency of the modified lithium metal negative electrode has also been significantly improved.The symmetrical battery and full-cell performance tests on the modified lithium metal electrode show that the modified lithium metal electrode exhibits excellent electrochemical performance.In addition,the modified SEI layer can also improve the air stability of lithium metal.The modified lithium metal negative electrode was exposed to the air（relative humidity of 25%）for more than 24 hours and showed excellent electrochemical performance compared with the control group.（2）An artificial SEI layer with a three-dimensional（3D）structure was constructed by an in-situ reaction between graphene/lithium metal composite electrode and electrolyte-containing TiF₄.The main components of the SEI layer are Li₂TiF₆and LiF,which have high mechanical strength and electrochemical stability.In-situ TEM results show that the SEI layer with high mechanical strength can withstand the huge volume change and maintain its structural integrity during cycling process.Inhibiting the continuous decomposition and formation of the SEI layer and alleviate the consumption of active lithium and electrolyte.In addition,the electrode with a special structure has a high specific surface area,which can reduce the current density on the electrode surface during the charging process and inhibit the formation of lithium dendrites.The test results of the symmetrical cell show that the electrochemical stability of the modified composite electrode is significantly improved,delivered a stable cycle of 4000 h.The full cell was assembled with the LiFePO₄electrode,it also shows excellent cycling stability.This work puts forward a new structure of lithium metal anode and provides a new strategy for the study of lithium metal anode.（3）By reacting the graphene sodium metal composite electrode with an electrolyte containing SnCl₄,an artificial hybrid SEI layer is constructed on the surface of the 3D composite sodium metal electrode.The main components of the artificial SEI layer are NaCl,Sn,and Sn-Na alloy.The DFT calculation results show that the SEI layer has excellent sodium ion conductivity,and the AFM results also show that it has high mechanical strength.The composite sodium metal electrode with a special structure prepared by combining the 3D host frame with the modification of the layer can systematically solve the problems of the sodium metal electrode.The modified composite sodium metal electrode has a higher specific surface area,which can significantly reduce the exchange current density on the electrode surface,realize the uniform deposition of sodium ions,and inhibit the growth of sodium dendrites.In addition,due to the high mechanical strength of the SEI layer,it can maintain its stable structure during charging and discharging and inhibit the continuous side reaction between the active sodium and the electrolyte.The symmetrical cell assembled by the electrode with the special structure can realize a stable cycle of 500 h,and the coulombic efficiency is also significantly improved.When assembled with NaV₂（PO₄）₃cathode to form a full battery,the test results also show better cycle stability and rate performance.