Design Of Organic Electrolyte For High-Performance Lithium-Air Batteries

Energy and environment are two crucial themes in the course of human societal development,with the breakthrough of conversion and utilization of clean and efficient energy being pivotal for achieving progress in human society.Lithium-air batteries（LABs）realize efficient energy conversion and storage based on lithium metal and oxygen in the air.With a theoretical energy density of up to 3500 Wh kg^-1,LABs are hailed as the"holy grail"of secondary batteries.However,the performance realization of lithium-air batteries faces numerous obstacles.On the anode side,the high reactivity of lithium metal and the uneven deposition/dissolution of lithium ions lead to continuous side reactions and lithium dendrite growth.On the cathode side,the high decomposition potential of the discharge product,lithium peroxide（Li₂O₂）,during the charging process results in a severe reduction in energy conversion efficiency.Additionally,LABs are typically studied in oxygen environments and correspondingly referred to as lithium-oxygen batteries（LOBs）.When used in real air environments,they face additional challenges such as moisture erosion and electrolyte volatilization,leading to obvious battery performance degradation.The electrolyte must exhibit electrochemical/chemical stability while optimizing the rate performance of batteries based on heterogeneous mass transport reactions and leveraging the high energy density characteristics of LABs.To solve the above problems,this thesis is based on the study of the intrinsic stability of the electrolyte in LABs,electrolyte-interface compatibility,the modulation of electrolyte on battery electrochemical reactions,and practical battery performance.By considering the design of high-performance electrolytes comprehensively,we have achieved the following results:1.Based on the differences in Hansen solubility parameters,we designed a dual electrolyte system for LOBs.Acetonitrile（AN）and sulfolane（TMS）were chosen as the catholyte and anolyte to establish a liquid-liquid dual-zone system,respectively.On the anode side,TMS effectively inhibits the diffusion of AN towards the lithium anode,while simultaneously achieving long-term protection of lithium metal through in situ film formation during battery cycling.On the cathode side,the regulation of the lithium superoxide（Li O₂）intermediate by AN facilitates the generation of low crystallinity discharge products lithium peroxide（Li₂O₂）at large current densities.The products can be effectively decomposed during the charging process,significantly reducing the overpotential of the battery at high rates.In terms of optimizing the stability of the electrolyte system,we effectively suppressed electrolyte volatility through salt concentration adjustment and conducted stability tests on the electrolyte regarding reactive oxygen species（ROS）.Furthermore,we thoroughly investigated the impact of different solvent characteristics on the critical current density for product nucleation,thereby providing further insights into the unified understanding of the solvent-current effects on the morphology of the products.We have achieved the long-cycle performance of LOBs based on nitrile electrolytes for the first time.Furthermore,the validation of Li-air pouch cells and optimization of the gelation process demonstrate the application potential of the dual-electrolyte system.2.We successfully introduced a multifunctional nitrile additive,2-methoxy benzonitrile（2-MBN）,into the LOBs by designing group.With regard to stability,the methoxy group effectively reduces the reactivity of anion radical polymerization with lithium metal,significantly enhancing the compatibility of 2-MBN with lithium metal.Simultaneously,the 2-MBN lacks unstableα-H,which makes it less vulnerable to be nucleophilic attacked by superoxide radical anions.On the anode,its competitive coordination with Li⁺effectively promotes rapid Li⁺transport and more uniform Li⁺deposition on the lithium metal surface.Throughout the battery cycling,it also modulates the internal components of the anode protection layer by increasing the formation of high-quality lithium nitride components.On the cathode,this additive alters the formation pathway of Li₂O₂ and induces the formation of more easily decomposable sheet-like discharge products.Benefiting from the comprehensive effects of this additive on both the cathode and anode,we realized the long-cycle performance of LOBs under high-rate and large-capacity conditions.3.We employed an in situ solidification progress and composite group design to prepare a high-performance gel polymer electrolyte（GPE）-based LABs.The aprotic electrolyte extensively studied in LOBs,such as dimethyl sulfoxide（DMSO）,faces challenges in dissociation of coordinated Li⁺ions due to its high donor number（DN）value.Meanwhile,there is limited stability of DMSO to lithium metal.Furthermore,DMSO is prone to volatilize and absorb water in an air environment,posing additional obstacles for practical application.To address these issues,we introduced polyethylene glycol（PVA）into the DMSO-based electrolyte and systematically incorporated carbamate and borate groups to fully consume hydroxyl groups through polymer group modification.The approach achieves comprehensive enhancements in the electrochemical oxidation stability,ion thermal stability,and inherent hydrophobicity of the electrolyte.Simultaneously,it optimizes the Li⁺transport and the formation of a high-quality solid electrolyte interphase（SEI）on the lithium anode.On the other hand,the in-situ solidification strategy improves the contact at the electrode/electrolyte interfaces.Combining this upgraded DMSO-based GPE,the Li-air battery can operate stable for 241 cycles at 500 m A g^-1 and1000 m Ah g^-1 in air.Furthermore,we optimized the cell structure to assemble lithium-air pouch cells and realized high-performance full cell with high energy density(757.5 Wh kg^-1)in the real air environment.