Preparation And Electrochemical Properties Of Functional Coatings For Lithium/Zinc Metal Anodes

Since the 21st century,with the rapid development of lithium-ion batteries(LIBs)technology,LIBs have been widely used in fields such as portable electronic devices and electric vehicles.However,the existing LIBs are close to their inherent energy density limit and cannot meet the demand for battery energy density due to technological development.Therefore,researchers generally believe that the development of advanced battery systems other than LIBs is an inevitable trend in the development of electrochemical energy storage technology.Compared with ion batteries,metal batteries represented by lithium-sulfur(Li-S),lithium-air(Li-Air),and zinc-air(Zn-Air)batteries have significant advantages in energy density and become recognized as the next generation high specific energy battery.Among them,lithium metal anode and zinc metal anode have become the preferred anode materials for organic systems and aqueous metal batteries due to their ultra-high theoretical capacity and suitable electrode potential.Unfortunately,lithium and zinc metals suffer from uncontrollable dendrite problems during the deposition process,resulting in a significant reduction in the cycle life and Coulombic efficiency of the battery.When the dendrite grows seriously and pierces the separator,the battery will suffer from safety hazards such as short circuit or even explosion,which seriously hinders the practical application of lithium and zinc metal batteries.In response to the above problems,this paper uses the method of coating modification to suppress the generation of dendrites from the three aspects of electrode,current collector and separator,thereby improving the overall electrochemical performance of lithium and zinc metal anodes.According to the above research ideas,we took electrodes,current collectors and separators as research objects,designed four corresponding coating strategies,and obtained lithium and zinc metal electrodes with excellent performance.Further combined with in-depth experiments and theoretical analysis,the effects of different coatings on the deposition of metal ions were studied,and the mechanism of action was clarified.Finally,the improved metal anodes were matched with commercial cathode materials to assemble metal lithium and metal zinc full batteries,verifying the potential of different functional coatings in practical applications.Based on the above research ideas,the main research results of this paper are as follows:(1)An organic/inorganic composite artificial lithium metal protective film SPF with a single-sided concave structure was successfully prepared by the phase inversion method.We coated SPF on the surface of lithium metal anode.The electrochemical performance and modification mechanism of it as a negative electrode interface protective layer were studied.Due to the addition of organic/inorganic additives,the SPF protective film has the advantages of high ionic conductivity,high Young's modulus,and high flexibility.In addition,the special single-sided recessed structure enables lithium ions to deposit not only in the recesses on the SPF surface,but also on the negative electrode surface through the SPF.This special deposition mechanism increases the specific surface area for lithium ion deposition and provides a deposition host for lithium metal,so that dendrites are effectively suppressed.At the same time,the depressions on the surface can also keep the SPF-protected lithium metal interface in a stable state quickly and for a long time.The lithium metal symmetric battery protected by SPF has a long cycle life of more than 1000 hours in either ether electrolyte or carbonate electrolyte.The assembled SPF@Li||LFP full cell can be stably cycled for500 cycles at a rate of 4 C with a capacity retention rate of 97.5%.The SPF@Li||S battery can also be cycled stably for 100 cycles at a rate of 0.2 C,with a capacity retention rate of 86%.(2)Two-dimensional monolayer Ti₃C₂T_x nanosheets were coated on the surface of the copper foil current collector by a simple doctor blade method.The electrochemical performance and modification mechanism of it as a current collector-induced coating were investigated.The current collector modified by Ti₃C₂T_x coating has a flat surface,which can induce the horizontal deposition of lithium ions and inhibit the formation of dendrites.We further combine the experimental and theoretical calculation results to elucidate the deposition mechanism of the horizontal growth of Li-ion epitaxy from the atomic point of view.The results show that lithium firstly forms hcp-Li layer on the surface of Ti₃C₂T_x coating through ionic bond.With the increase of the number of deposited layers,the ionic bonds are continuously weakened,and lithium continues to deposit bcc-Li layers in the form of metallic bonds.In addition,the small lattice mismatch number between the Ti₃C₂T_x substrate,hcp-Li layer and bcc-Li layer enables Li metal to grow densely towards the(110)level.The Ti₃C₂T_x modified current collector can still cycle stably for 160 cycles and maintain 90%Coulombic efficiency at a high current density of 5.0 m A cm^-2.The assembled Li-Ti₃C₂T_x||LFP full cell can operate stably for 500 cycles at 4 C rate and maintain a high specific capacity of 120m Ah g^-1.(3)The organic-inorganic composite thin film was prepared by ion-layer epitaxy to modify the surface of the current collector,and its electrochemical performance and modification mechanism as an affinity coating for the current collector were studied.The as-prepared zinc oxide/oleic acid(Zn O@OA)modified layer has a thickness of only 80 nm and exhibits a uniform and dense morphology.Among them,Zn O can react with lithium to form a lithium-zinc alloy during the deposition of lithium metal,thus playing a strong lithophilic effect.In addition,the results of first-principles calculations indicate that Li,Zn,and Zn O and OA all have larger binding energies.Therefore,the Zn O@OA coating with good lithiophilic and zincophilic properties drives the uniform distribution of Li and Zn ions on the surface of the current collector during the deposition process,thereby effectively inhibiting the formation of dendrites.The electrochemical performance of the modified current collectors is significantly improved in both symmetric cells and half cells.The assembled Li-Zn O-OA@Cu||LFP full cell can be cycled stably for 600 cycles at 1 C rate in carbonate electrolyte with a capacity retention rate of 90.9%.The Zn O-OA@Zn||Mn O₂ full cell can operate for1000 cycles at a current density of 1 A g^-1 in aqueous Zn SO₄ electrolyte and maintain a high specific capacity of 257.3 m Ah g^-1.(4)Electronegative covalent organic framework(COF)thin layers were in situ coated onto glass fiber(GF)separator surfaces using a solvothermal method.The electrochemical performance and modification mechanism of negatively charged separators in organic/inorganic electrolytes were systematically studied.We successfully prepared a nanometer-thick COF coating and transformed the surface positive charge of the commercial GF separator into negative charge,thereby accelerating the transport rate of lithium and zinc ions and effectively improving the overcurrent at the electrode interface.In addition,first-principles calculations indicate that the Li,Zn and COF coatings have larger binding energies,which is favorable for the uniform deposition of Li and Zn ions,thereby suppressing the formation of dendrites.The cycling performance of lithium and zinc half cells and symmetric cells assembled with COF@GF separators is greatly improved.When matched with commercial cathode materials,the assembled Li||COF@GF||LFP full cell can stably cycle for 500 cycles at 3 C rate in carbonate electrolyte.Zn||COF@GF||Mn O₂ full cell can run for 700 cycles at 1 A g^-1 rate in aqueous Zn SO₄ electrolyte.In summary,this paper takes metal battery electrodes,current collectors and separators as the research objects,and successfully designs and prepares four coating materials to effectively inhibit the formation of lithium and zinc metal dendrites.The anode is applied to the metal full battery system to obtain excellent electrochemical performance.The effects of surface and interface coatings on the deposition behavior and electrochemical performance of metal ions in electrodes,current collectors and separators are summarized.Theoretical and experimental results are combined from the molecular and atomic perspectives to analyze and verify the mechanism of action of different coatings.These modification strategies of metal electrodes provide theoretical guidance.