Electrochemical Construction And Characterization Of Lithium Metal Anodes

Lithium metal is considered as a most promising metal anode for the next generation high-energy-density rechargeable batteries owing to its lowest potential and highest theoretical specific capacity among other possible anode candidates.However,intrinsic and induced dendrite growth upon charge-discharge cycling and formation of unstable solid-electrolyte interphase(SEI)with low Coulombic efficiency(CE)are the major obstacles to impede realization of its practical application.Looking into the origins of dendrite growth,the rough surface morphology and inhomogeneity of SEI are detriment factors that promote dendrite growth.To alleviate the above-mentioned problems,strategies including an electrochemical polishing method based on potentiostatic stripping-galvanostatic plating processes and the atomic-scale design of Cu current collector surface lithiophlilcity are proposed in this work.Meanwhile,three plasmon-enhanced strategies based on different plasmonic sources are also developed for in situ electrochemical SERS study on the Li/electrolyte interfaces.The main accomplishments are outlined as follows:1.We develop a general non-conventional electrochemical approach for the first time to create near-perfect Li metal anodes,based on electrochemical polishing of the Li metal surfaces down to atomic-flatness as well as manipulation of electrolyte reduction processes to construct ultra-smooth ultra-thin(USUT)SEI with designable structure.The key factors including anodic stripping potentials,cathodic plating current densities,and types of salt and solvent systems are further investigated in detail based on the understanding of electrode reactions taking place during polishing.In particular,the importance of considerations on the mutual constrains between electropolishing and SEI formation and,thus,the necessity of fine control of potential and/or current is elucidated.The Cu Li cell using Li metal anodes with USUT SEI can run for over 300 cycles under 2 mA cm-2(1 mAh cm-2)with an average CE of-99%.To further probe the potential application of USUT SEI prepared in DOL-based electrolyte in Li metal batteries involving carbonate-based electrolyte,Li LiCoO2 full cells in the carbonate ester electrolyte were conducted.The cell using Li metal anodes with USUT SEI exhibits a promising reversibility.The electrochemical polishing approach can be extended to Na metal and K anodes for creating atomic-flatness surface as well as smooth SEI and improving electrochemical performances depending on their features,respectively.Especially for Na anodes,long-term cycling stability was achieved for the Cu Na cell,which can run for at least 550 cycles with Columbic efficiency close to 100%.The potentiostatic stripping-galvanostatic plating electrochemical polishing method can be used as a simple and universal method for construction of stable alkali metal anodes.2.Precise multi-scale characterizations were conducted for revealing the structure and properties of USUT SEI on alkali metal anodes.For Li metal,AFM force probing with corroborative in-depth XPS profile analysis reveal that the molecular-scale USUT SEI can be designed to have alternating inorganic-rich and organic-rich/mixed multi?layered structure,which offers mechanical property of coupled rigidity and elasticity.The thickness of USUT SEI is range from 10~30 nm,which have considerably smaller SEI resistance as well as charge transfer resistance than the normal soaked SEI.This illustrates that not only the Li+transport through the thin SEI layer,but also the electron transfer across the Li-SEI interface,are significantly enhanced in the SEI formed on the fresh and flat Li surfaces.The USUT SEI bear all features for an ideal SEI,which can significantly suppress dendrite growth and thus improve the performances of Li metal anodes.For Na and K metals,the obtained USUT SEIs are also molecular-scale though do not contain organic-rich moieties.Importantly,however,since the primary problems for Na and K anodes arise from the poor quality of SEIs which then induces dendrite growth and causes excessive electrolyte consumption,the compact and stable Na+-rich and K+-rich SEIs on smooth Na and K surfaces may be adequate to circumvent the problems.3.In Li-O2 and Li-S batteries,Li metal anodes face with greater complicated issues associated with the cathodes side reaction,such as O2 corrosion and polysulfide shuttle effect,which even aggravates dendrite growth.We tackle the problems by creating a functional USUT SEI on Li metal surface through electrochemical polishing in the LiTFSI/DME-DOL with LiNO3 as additives,which is denoted as USUT-A SEI.Such SEI couples components reduced from LiNO3 and the multi-layered structure.We tested the resistance of USUT-A SEI on the Li metal surface to O2 corrosion and polysulfide shuttle effect by optical and electrochemical methods.The obtained results show that USUT-A SEI can act as an effective protective layer that prevent not only dendrite growth but also sever reactions between Li and O2/polysulfide.To further probe the potential application of USUT-A SEI,Li-O2 and Li-S batteries using electrolyte without any additives were conducted.The cell using Li metal anodes with USUT-A SEI exhibits a promising long-term stability.The detailed mechanism of corrosion resistance for USUT-A SEI and its stability under air atmosphere need to be further studied and improved.4.To improve the surface lithiophilicity of Cu current collector(CC),an atomic-scale design based on surface lattice matching of the bcc Li(110)and fec Cu(100)faces is developed.A facile electrochemical method is then established that involves potentiostatic polishing in concentrated phosphoric acid solution followed by deposition of Cu in chloride containing sulfuric acid solution to facet Cu CC for(100)dominated faces.The synergetic effect of strong adsorption of SO42-on the(111)face and the preferred adsorption of Cl-on the(100)face facilitates the growth of(100)facets.Such electrochemical faceting strategy can be implanted into Cu CCs ranging from 2D foil to 3D foam.The lithiophilic(100)-preferred orientations enables smooth Li electrodeposition on Cu CCs in any configuration.Further,a purposely designed USUT SEI is created for Li anodes prepared on CCs.The combined advantages of the surface lithiophilicity and the USUT SEI facilitate a high utilization of not only the surface but also the cavities of 3D Cu CCs.With these merits,the faceted Cu CCs coated with USUT SEI allow stable Li plating and stripping and improved CE under 0.5?4mA cm-2(1m Ah cm-2).A smooth Li thin film of up to 25 ?m has also be prepared to probe the potential application.The Li LiFePO4 full cells using such Li thin film anodes exhibits a long lifespan of 1000 cycles.5.We develop three plasmon-enhanced strategies based on different plasmonic sources,including local surface plasmon resonance(LSPR)from Li itself or otherSERS-active metals,for EC-SERS study on the Li/electrolyte interfaces.(1)We attempt to create LSPR from Li itself by electrochemical oxidation and reduction cycle(ORC)method,which can be directly used to detect the interface processes.However,ORC method cannot prepare nano-structured Li and even promote dendrite growth,thus resulting in the unavailability of creating LSPR from Li itself.(2)By means of the LSPR effect of Ag,which is one of the high SERS-active metal,in situ EC-SERS studies on the formation process of SEI film was performed before and after Li deposition on the Ag nanoparticles(~110 nm)-modified Ag electrode surface in LiPF6 containing carbonate ester electrolyte.The identification of the multiphase SEI compositions can be easily done before the Li deposition on Ag surface,and the characteristic bands of ROCO2Li?LiF?LiOH?Li2CO3 can be obviously detected upon extend of plating time.Nevertheless,once the Li deposition happened and Li-Ag alloys were formed,the characterized peaks of SEI film disappeared immediately and the evolution of SEI after Li deposition could not be detected,which is ascribe to the dielectric constant change of Ag after the lithiation.(3)Since not subject to the substrate generality,SHINERS is a more effective technique for SEI investigation.The SERS-active Ag nanoparticles(~100 nm)were coated with a uniform and ultra-thin shell inert Al2O3(-2 nm),and spread them over a Li anodes.The Al2O3 shell layers prevent the core from interacting with the electrolyte and also avoid the formation of Li-Ag alloys.The spontaneously formed SEI on Li anode,which was formed by soaking in electrolyte for different periods of time,was detected by EC-SHINERS.It is found that ROLi,LiOH,Li2O and Li3 N are the main components of the soaked SEI film.The SEI formed during the Li plating process was also studied by EC-SHINERS.Compared to soaked SEI,the compositions of SEI formed during Li plating are more complex and ever changing.In brief,by utilizing this method,SEI compositions on Li anode can be identified.This work demonstrates that EC-SHINERS can serve as a powerful means for in-situ investigation of Li/electrolyte interface.