Investigation On The Stability Of Metallic Lithium Anode For Advanced Li-Air Batteries

Nowadays,Lithium-ion battery technology is hard to satisfy the demand of the future long-distance electric vehicles.Rechargeable Li-air batteries(LABs,or Li-oxygen batteries,LOBs)with extremely high theoretical energy density have been regarded as a promising next-generation energy storage technology.However,under such a half-open working condition,despite the dendritic problems,lithium metal anode suffers attacks from the electrolyte and air,which result in the limited cycle life,low practical energy density and safety issues,restricting the further development of lithium-air batteries.In this thesis,we introduce an in-situ high current pretreatment technology and a combination of redox mediator(RM)with protected lithium metal anodes to enhance the electrochemical performance of Li-air cells.The main content of this thesis is concluded as follows,(1)An in-situ pretreatment method at a high current density is reported to enhance the electrochemical performance of Li-air batteries with LiAlx alloy anode containing 0.2 wt%A1 element.During the high current pretreatment process,Al atoms accumulate toward the LiAlx anode surface and form the Al2O3-rich solid electrolyte interphase(SEI).This SEI protective film can effectively suppress the Li dendrite growth and side reactions between the LiAlx anode and the electrolyte.With optimized pretreating current density and pretreatment durations,the cycle stability of Li-air batteries with LiAlx anode is dramatically improved.Under a fixed capacity of 1000 mA h g-1 at 500 mA g-1,the service life of the modified Li-air cell delivers improved cycle life of 667 cycles,while pristine cell can only work 17 cycles.(2)The high current pretreatment method is introduced to stabilize the lithium metal anode of Li-air batteries with wet electrolytes.With the help of certain water contents from 100 to 2000 ppm in the electrolyte,dense,solid and adequate Li2O-contained SEI film forms on the lithium anode surface after the high current pretreatment,enabling the good stability of lithium metal anode during electrochemical cycles.After pretreatment,the cycling life of Li-air cells with the water content of 100?2000 ppm can be improved to?400 cycles,ten times higher than the pristine.However,an adverse effect is found in the dry electrolyte(?50 ppm H2O)or a high-water-content electrolyte(10000 ppm H2O)due to the deterioration of Li metal anode after the high current pretreatment.Overall,this work broadens the pathways for the practical development of Li-air batteries.(3)Our research demonstrates that superior electrochemical performance could be achieved in the Li-air battery with ex-situ Li3PO4-protected Li metal anode cooperating with a LiI redox mediator.The addition of LiI can change the nucleation and growth mechanisms of discharge products,increasing the coulombic efficiency of the Li-air batteries,while the accelerated failure of the lithium metal anode occurs simultaneously.In this work,the Li3PO4-protected Li anode attributes the uniform Li-ions stripping/plating on the Li metal surface and inhibits the corrosion from electrolyte,leading to excellent electrochemical stability of Li metal anode.The modified Li-air cell exhibits a prolonged cycling life of 152 cycles with a fixed capacity of 1000 mA h g-1 at 2 A g-1,enhanced energy efficiency and rate capability.(4)Li-air battery with FeClx anchored on the CNT surface shows enhanced cycling performance and OER activity stability.FeClx-CNT air electrode exhibits the analogous redox potentials as the FeCl3 redox mediator in the electrolyte.The formation mechanism of discharge products evolves from the simple "surface-mediated mechanism" into a combination of the "solution-mediated mechanism" and "surface-mediated mechanism" on the FeClx-CNT electrode.Similarly,the morphology of the discharge product transforms from film-like structure to the coexistence of small spherical particles and film-like structures.Fe ions are confined to the cathode side and are prevented from diffusing to the lithium metal anode,avoiding the "shuttle effect"of RM and alloying reaction with Li metal.Li-air batterie with FeClx-CNT electrode exhibits excellent electrochemical performance over 284 cycles,high energy efficiency,and long-term OER stability during the electrochemical cycling.