Electrochemical Research On Li Metal Anode Interface In Complex System

Li-ion batteries have been widely used in our daily life.Because of the relative low energy density,smart phone or electric automobile with Li-ion batteries as power sources usually present poor endurance.Developing new energy storage systems with higher energy density is a good choice to solve this problem.A commercial Li-ion battery usually uses the graphite anode,which has a specific capacity of about 370 mAh g-1.Comparing to the graphite,Li metal presents a much higher specific capacity of 3860 mAh g"1 and a lower potential,has been considered as an idea anode material.Batteries with Li metal anode can deliver much higher energy density than the State-of-the-art Li-ion batteries.However,Li metal anode exhibits poor reversibility upon cycling,thereby hindering its practical application.It is because Li metal can react with the electrolytes,the continued growth of Li dendrites during the discharge-charge process will expose numerous fresh Li metal to the electrolytes,leading to a rapid consumption of both Li metal and electrolytes.Consequently,in a typical Li metal battery,a large amount of excess Li and electrolyte are required,which will reduce the energy density of the battery and raise the costs and safety issues.Besides,the accumulated side products will gradually block the pathways for Li-ion,causing an increased impendence and reduced energy efficiency.In conclusion,achieving a stable Li metal anode with high cycling stability and Coulombic efficiency is the key point to develop a stable Li metal battery.In this paper,we study the interfacial electrochemistry of Li metal anode and develop some new methods to protect the Li metal anode.The major innovations can be summarized as follows:(1)The ethers-based electrolytes in the Li-O2 batteries present very poor stability to the Li metal anode.In this work,we provide a new strategy to overcome the problem.A porous anode electrode is used to enhance the diffusion of O2,O2 thus can be applied to protect the Li metal anode.Comparing to the Li salt and the solvent,O2 in the electrolyte will be preferentially reduced during the first discharge process.In situ electrochemical Raman spectrum is recorded to investigate the growth of the SEI layers.We find in the O2-added battery,O2 will quickly react with Li metal by forming Li2O-,Li2O2-,and LiOH-rich SEI layers.The SEI layers can suppress the growth of Li dendrites and protect the Li metal from further corrasion by the solvent and the Li salt.Therefore,the Coulombic efficiency and cycling stability of the Li metal anode are dramatically improved.When the battery operates at a capacity of 1.0 mAh cm-2,Li anode remains stable over 800 cycles with a high average Coulombic efficiency of 98.7%.(2)With a porous anode,CO2 can be used as an electrolyte additive to protect the Li metal anode and an ultra-high Coulombic efficiency is achieved.We find CO2 in the electrolyte will be preferentially reduced during the first discharge process.According to the Raman and XPS analysis,when CO2 is added,Li2CO3 becomes the dominate product of the SEI layers,and the decomposition of the solvent and the Li salt are efficiently suppressed.Specifically,under a high pressure of CO2 and a low current density,an ultra-high Coulombic efficiency of 99.9%was observed for the first time.Nevertheless,increasing the current density to 0.5 mA cm-2,or reducing the pressure of CO2 to 1 atm,the Coulombic efficiency of the Li anode quickly dropped to about 98%.We wonder it is because the replenish of CO2 cannot meet the requirement of forming stable SEI layers.(3)We report a concentrated ternary-salts electrolyte to protect the Li metal anodes.LiNO3 and LiFSI in the electrolyte contribute to the formation of stable Li2O and LiF,LiTFSI is used to avoid the polymerization of electrolyte.In the concentrated ternary-salts electrolyte,a different Li metal nucleation process can be observed,the growth of Li dendrites and the "edge effect" are also significaintly suppressed.Li metal in the electrolyte remains stable over 450 cycles,when 0.5× excess Li is used,the Coulombic efficiency reaches 99.4%.Considering the high stability of the Li metal anode in the ternary-salts electrolyte,a Li metal-LFP full-cell with only 0.44× excess Li is assembled,the battery remains stable over 70 cycles without evident capacity fading.(4)Using TiC-C composites as the cathode materials and porous Li metal anode to promote the cycling stability of the Li-O2 batteries.Ordered mesoporous TiC-C is prepared through solvent-evaporation-induced-self-assembly(EISA)and in-situ carbothermal reduction approaches,it is then applied as the cathode materials for Li-O2 batteries.The electrochemical performances of the Li-O2 batteries are significantly improved,the discharge capacity increases from 1820 mAh g-1 to 3460 mAh g-1,and the charge/discharge overpotential is 200 mV/60 mV reduced respectively.When the battery cycles under a limited capacity,a high stability of over 80 cycles is achieved.The charge process is further studied by in situ differential electrochemical mass spectrometry(DEMS)and O2 is the main product.According to our earlier studies,with a porous anode,O2 can be used to protect the Li metal anode.Here we design a Li-O2 batteries with a special structure that both anode and cathode are open to the outside.In this battery,a Li-O2 batteries with a 2× excess Li remains stable over 20 cycles.In this paper,we present some new methods to protect the Li metal batteries,we believe our works are inspirational to the development of Li-O2 batteries,Li-CO2 batteries and Li-metal battery with intercalation cathodes.