Application Research Of Electrodeposition Technology In Lithium Metal Secondary Battery And Trivalent Chromium Hard Chromium Electroplating

Over the past few decades,with the continuous expansion of the research scope of electrochemistry,and constant improvement and innovation of electrochemical technology,the electrochemical industry has undergone a series of developments and reforms.Traditional electrochemical industries,such as electroplating industry,chlor-alkali industry and electrometallurgy industry,are mainly faced with the challenges of energy saving,consumption reduction,emission reduction and efficiency improvement.Emerging electrochemical industries,such as battery industry,organic electrosynthesis industry,and high-end electronics manufacturing industry,face the challenge of developing original systems and leading technologies.Electrochemical deposition is an indispensable key technology for many emerging and traditional electrochemical industries,and it also plays an important role in high-energy-density secondary batteries and green electroplating.In the present study,based on the theory and method of electrodeposition,we study two challenging problems in the fields of energy and environment:1)dendrite-free lithium electrodeposition in lithium metal secondary batteries,and 2)hard(thick)chromium deposition in trivalent chromium plating.Lithium(Li)metal is considered an ideal anode material for future high-energy rechargeable batteries.However,the extremely unstable interfaces between lithium anode and electrolyte induce uncontrollable dendrite formation and low Coulombic efficiency,retarding its practical applications in Li metal batteries(LMB).In chapter 3,we propose a facile and feasible strategy of introducing 12-Crown-4 ether as a leveling additive into the electrolyte to inherently eliminate the self-amplification behavior of dendrite growth.The Li⁺–additive complexes with low electron-accepting ability are preferentially adsorbed on the initial protuberant tips of Li metal surface,which can effectively regulate the local polarization resistance and improve the current distribution uniformity.Based on this principle,a self-leveling electrolyte comprising fluoroethylene carbonate(FEC)solvent and 12-Crown-4 additive is developed.Dense and dendrite-free Li deposition,as well as enhanced interfacial stability is achieved in such electrolyte,rendering a significantly improved Coulombic efficiency(97.24%)in Li||Cu cells and a superior cycling stability in Li||Li cells.Moreover,a Li metal full-cell with high-loading Li Fe PO₄ cathode(12.5 mg cm^-2)retains 98%capacity after 100cycles at 0.5 C.This self-leveling electrolyte approach opens up new perspective on the mechanistic and methodological aspects for designing safer lithium metal batteries.Trivalent chromium electroplating is considered a greener alternative to hexavalent chromium electroplating.However,the electrochemical reduction of Cr(III)in aqueous solution is extremely unstable and the thickness of the coating can hardly grow beyond 10 micrometers,which limits its industrial applications.In chapter 4,we report a trivalent chromium bath containing ternary complexing agent,which yields bright and uniform chromium coatings with thickness exceeding 30?m,as well as high hardness and corrosion resistance.The electroplating behaviours and first principal calculations reveal that the Cr(II)intermediate plays a vital role in sustained electroplating in Cr(III)baths.The composition of complexing agents has a profound effect on the geometry and electron accepting ability of the Cr(II)complex ions.The failure to grow thick Cr coating is attributed to the accumulation of inactive hydroxo bridged complexes.In the bath with ternary complexing agent,the Cr(II)complex ions are dsp³ or dsp² hybridized with high electron affinity,which can be readily reduced to metallic Cr.The bath also shows strong competitive ligand-binding effect that renders Cr(II)to preferentially bind with organic ligand rather than hydroxy ligand.Furthermore,the planar geometry of dsp² hybridization does not favour the formation of hydroxo bridged complexes because of the steric hindrance effect.The above merits of the ternary complexing agent lead to sustained electroplating and thick Cr coatings are obtained.