Fundamental Study On Interfacial Electrochemistry In Lithium-Air Batteries

Energy is the primary impetus behind the sustainable development of human society.With ever-increasing energy demand of modern society,it has become imperative to develop efficient,economical,green,and safe electrochemical energy storage devices.As a prevailing energy storage technology,Lithium-ion batteries can no longer meet the needs of special applications(e.g.,long-endurance electric vehicle,large-capacity storage)because of inherent physico-chemical limits in charge storage.Over recent years,tantalizing aprotic lithium-air batteries,e.g.,lithium-oxygen(Li-O2)and lithium-carbon dioxide(Li-CO2)batteries,have attracted significant research interest due to their ultra-high theoretical specific energy density.However,the practical realization of lithium-air batteries is still confronted with many challenges,such as poor cycle life,large overpotential,severe parasitic reactions and so on.The mechanical understanding of electrode/electrolyte interface electrochemistry and further rational design of efficient battery materials,are essential to promote the practical application of lithium-air batteries.This dissertation embroidered on fundamental scientific issues of interface electrochemistry of lithium-air batteries,and obtained some main research results,which are summarized as follows:(1)Aprotic Li-O2 batteries have attracted intensive attention by virtue of ultra-high theoretical energy density.However,the "sudden death" phenomenon is frequently observed in oxygen reduction reaction(Li+-ORR)process of Li-O2 batteries,resulting in a limited discharge capacity far below its theoretical promise.The soluble catalysts(e.g.,reduction mediators)promoted solution-mediated Li+-ORR,that is discharge Li2O2 in electrolyte solution,represents an elegant solution.However,no direct molecular evidence is available,and its link to Li-O2 batteries performance remains hypothesis.Here,we present in-situ electrochemical Surface-enhanced Raman spectroscopy(EC-SERS),obtaining direct key spectroscopic intermediates evidence of solution-mediated Li+-ORR on a model anthraquinone(AQ)thin film-modified Au electrode surface.Coupled with density functional theory(DFT)calculations and gas analysis using Differential Electrochemical Mass Spectrometry(OEMS),it is found that AQ first coordinates with Li+at open circuit potential,which then is reduced to form LiAQ complex when Li+-ORR potential started.Subsequently,the LiAQ complex bonds O2 to form the LiAQO2 intermediate and eventually turns into solution phase-Li2O2 by the LiO2 intermediate.This study provides the intuitive understanding of the AQ catalytic solution-mediated Li+-ORR mechanism at molecular level,giving critical insights to optimize and design of soluble catalysts to advance the Li-O2 batteries.(2)Aprotic Li-CO2 batteries represent a promising technology with energy conversion and storage and CO2 recycling.However,cathode passivation and large overpotential are frequently observed for current Li-CO2 batteries because of the insolubility and nonconductivity of the discharge product of Li2CO3.Toward maximizing the energy capabilities of the Li-CO2 electrochemistry,it is crucially important to have a fundamental understanding of the Li2CO3 formation mechanism in Li-CO2 batteries.It is found that trace O2 plays a crucial role in the normal operation of Li-CO2 batteries.Using in situ EC-SERS,the trace-O2-assisted Li-CO2 battery was interrogated.In high-donor-number(DN)solvents,Li2CO3 formation proceeds primarily via an "electrochemical solution route",with peroxodicarbonate(C2O62-)as the key intermediate,whereas in low-DN solvents Li2CO3 forms via a chemical reaction of Li2O2 and CO2 on the cathode surface,namely,a "chemical surface route".It is notable that during discharge the trace-O2 acts as a "pseudo-catalyst" to activate CO2 in high-DN solvents but not in low-DN solvents.The mechanistic study presented here will assist us in tailor-designing better electrolyte systems and enable more energetic electrochemistry operation far away from the poison of Li2CO3.(3)In Li-CO2 batteries,the Li+-CO2 reduction reaction(Li+-CO2RR)mechanism is often not clearly defined,thus being acquiesced to proceed via 4Li+3CO2→2Li2CO3+C in many studies without considering the electrolytes and/or electrode materials.Herein,we highlight the effect of the near-Fermi-level d-orbital states of catalysts on Li+-CO2RR activity,through the systematic comparison of three well-defined model electrodes,i.e.,Au,Cu,and monolayer Cu modified Au(CuML@Au).Using the EC-SERS coupled with DFT calculation,we obtained direct spectroscopic evidence(e.g.,CO2-intermediate,CO and Li2CO3 of Li+-CO2RR,revealing the surface-mediated reaction pathway of 2Li++2CO2+2e-→CO+Li2CO3 at the molecular level.In addition,the deep reduction of CO2(e.g.,formation of C and Li2CO3)must undergo the dimerization of the two CO2 intermediate,which depends on its migration rate on the electrode surface and/or in the bulk electrolyte.Our work provides a direct insight into the Li+-CO2RR mechanism and theoretical guidance for the rationally design cathode catalysts for versatile Li-CO2 batteries.(4)Current Li-CO2 batteries suffer from the sluggish kinetics of Li+-CO2RR that often leads to high discharge overpotential,low energy efficiency,and low power densities.Toward unlocking the energy capabilities of Li-CO2 batteries,it is crucially important to have a fundamental understanding of the kinetics aspect of the Li-CO2 electrochemistry.Here we report a brief but comprehensive model to bridge the overall reaction kinetics and the elementary steps of Li+-CO2RR in Li-CO2 batteries.A critical kinetics descriptor,i.e.,the adsorption energy of the LiCO2 intermediate on the cathode surface,is proposed to reveal the interplay and competition between two different Li+-CO2RR mechanisms(i.e.,the solution-and the surface-mediated pathways)occurring in Li-CO2 batteries.Our model indicates that tuning the Li+-CO2RR toward the solution-mediated pathway can avoid cathode surface passivation and is favorable for high-capacity and high-rate discharging of Li-CO2 batteries.The model study reported here sheds light on the kinetic aspect of Li-CO2 electrochemistry and would be beneficial for the design of better Li-CO2 batteries.(5)The solid-electrolyte interphase(SEI)on the lithium anode dictates the cycle performance of lithium-air batteries.Herein,we present Li-O2 batteries as an example to discuss the impact of O2 on the SEI formation mechanism on model Cu/DMSO interface,using a combination of in situ EC-SERS and Fourier-transform infrared spectroscopy(FTIR)methods.It is revealed that oxygen can significantly alter the SEI formation route and resultant different interfacial properties.Compared with deoxygenated electrolyte,oxygen is proposed to inhibit the fission of the C-S bond of DMSO solvent and following formation of unstable C=C and C≡C SEI compounds(e.g.,Li2C2),which allows SEI to become more uniform and less porous and its consequences for a reduced loss of active lithium and improved Coulomb efficiency.Our work is a good example to comprehensively consider the effect of oxygen on interphase chemistry using in situ spectroscopy,and the mechanic insights of SEI formation will be helpful in engineering lithium/electrolyte interface for better Li-O2 batteries.