Advanced Lithium Anode And Electrolyte For High-stability Lithium-oxygen Batteries

With the continuous improvement of people's quality of life and the continuous development of modern technology,energy and environmental problems have arisen at the same time.The strategy of using traditional fossil fuels as the main energy source does not meet the demand of the rule of current“clean and pollution-free”.Therefore,searching for new energy system as well as the designing reasonable storage methods are necessary to meet the demand of sustainable development.Among all the secondary batteries,lithium-oxygen battery has a higher energy density than other batteries,which has attracted many attentions among researches.Great progress has been made for the development of lithium-oxygen batteries in recent years,which has been a unique school in the new energy field.However,there still have a long distance in realizing the practical application of LOBs as many obstacles existed.It mainly including slow reaction kinetics,higher polarization,lower coulombic efficiency,poor stability and severe side reactions.Therefore,in view of the above aspects,the research contents of this paper mainly include the following aspects:?1?Lithium anode suffer from severe dendrites growth during deposition and dissolution process due to the nonuniform current density distribution.Moreover,LOBs are operated in open oxygen atmosphere.In such case,any exposed fresh Li surface would quickly react with O₂,H₂O,CO₂,N₂ dissolved in electrolyte,which lead to the consumption of active Li.In this,we fabricate a layer of uniform LiF nanoparticles on Li surface by using PFOSF as perfluoro reagent to protect Li metal anode in Li-O₂ battery.We carefuly study the stability of Li pellet in oxygen environment.As a result,the stability of Li metal is improved after PFOSF treatment.SEM and XRD analysis demonstrate that LiF could be formed from the reaction of PFOSF and LiOH,which is pre-deposited on Li surface with Li soaked in DME solvent with trace of H₂O contanmination.By comparing the cycling performance of Li-Li sysmetric cell and LOBs for prisitine Li and Li pellet with PFOSF treatment,we found that the cycle life can improve to 900 h with capacity limited at 0.4 mAh cm^-2 at a 0.2 mAcm^-2 current density in Li-Li symmetric cell.In contrast,a larger polarization is occurred for the cell without PFOSF treatment which can only cycling for 300 h.In addition,the Li-O₂ battery could be cycling for 135 rounds with capacity limited at 1000 mAh g^-1 at a 300 mA g^-1 current density,the cycle life is fourfold for the cell with pristine Li as anode.Therefore,we draw a conclusion that the stability of Li is greatly improved and the cycling performance of the Li-O₂ battery is enhanced after PFOSF treatment,which can effectively avoid the corrosion of O₂,H₂O and electrolyte.?2?We further proposed that boric acid can sever as a new SEI film-forming additive in lithium-oxygen battery.A covalently bonded SEI film is formed via the reaction of BA with the oxides and peroxides on the Li surface.Such SEI film is ionically conductive which can effectively prevent the contamination of oxygen,moisture and electrolyte.Li pellet can kept shining after exposing in air?38%relative humidity?for 5h with BA coated on the surface of Li,and it can keep stable after soaking in DMSO solvent with BA added.The Li-Li symmetrical cell with 20 mM BA added can run for 860 h in DMSO electrolyte at a current density of 0.25 mA cm^-2 with charge and discharge time terminated at 2 h,while the cell without BA can only cycle for 110 h.The lithium-oxygen battery with 20 mM BA added can run up to 146 cycles,while the cell without BA added can only cyclling for 23 cycles.TEM and XPS analysis indicated that the formed amorphous SEI film mainly consists of nano-crystalline lithium borates connected with amorphous borates,carbonates,fluorides and some organic compounds.The B-O bond was confirmed by FTIR analysis which can improve the stability of Li anode.?3?Lithium peroxide?Li₂O₂?as the main discharge product usually accumulated on the porous cathode after discharge in LOBs.However,the isolating and insoluble property of Li₂O₂ in electrolytes leading to a higher polarization in charge process.Moreover,it usually has a lower Li₂O₂ decomposition efficiency for solid catalysts because of a limited contact area between solid catalyst and Li₂O₂ particles.Instead,the kinetics reaction will accelerate with soluble catalyst as the homogeneous reaction of Li₂O₂ and catalyst.In this text,we propose bis?tetramethylcyclopentadienyl?iron?8MeFc?as a new redox mediator for LOBs.By comparing of the reaction rate of Li₂O₂ with 8MeFc⁺in TEGDME,DMSO and DMA solvent,we found that DMA is the optimal solvent because of the highest reversibility of 8MeFc and the highest oxidation rate of Li₂O₂.The generated Li₂O₂ is completely oxidized with charge platform at 3.25 V when 50 mM 8MeFc was added to1M LiNO₃/DMA electrolyte.However,the charge voltage is gradually increased as the cycling proceed,because the oxidazied redox mediator diffuses to the side of Li anode and reduced by Li,which leads to a lower round-trip efficiency.Therefore,a piece of PP separator with Super P coating was inserted between Li anode and membrane,20mM BA as added into electrolyte as additive to form a stable SEI film on the Li surface.Such strategy can suppress the diffusion of 8MeFc to Li anode.As a result,the cycle life is enhanced to 113 cycles.In addition,differential electrochemical mass spectrometry was used to detect the evolution rate of gas in charge.For the cell without redox mediator,larger amount of CO₂ is detected in the later charge process with charge voltage climb up to 4.25 V.While for the cell with 8MeFc added in electrolyte,no CO₂ appeared in charge process and the charge platform kept at 3.4 V.The above results indicate that 8MeFc is a suitable soluble catalyst for LOBs,which not only decreases the charge polarization but also suppresses the side reactions initiated by the higher charge voltage.It provides a better strategy for the effective decomposition of Li₂O₂.?4?Ether based electrolyte are prone to be attacked by the superoxide radicals via the hydrogen abstraction reaction in LOBs,which lead to the severe decomposition of electrolyte.In order to improve the stability of ether-based electrolyte in LOBs,we synthesized a fully methylated cyclic ether named 2,2,4,4,5,5-hexamethyl-1,3-dioxolane?HMD?to be used in LOBs.The hydrogen abstraction reaction is totally prevented as all the active?-H of HMD solvent are substituted by methyl groups.Almost no side reaction is detected after reacting with KO₂ or ¹O₂.In contrast,severe reactions derived from the composition of electrolyte are occurred in DME or DOL solvent with some lithium formate and lithium carbonates formed.With HMD as electrolyte solvent in LOBs,some spherical Li₂O₂ particles were uniformly grown on the carbon tube via surface nucleation mechanism.The larger contact area is benefit for the electron transfer in charge.As a result,the charge voltage is decreased to 3.8 V.The assembled Li-O₂ battery with HMD as electrolyte can cycling for up to 74 rounds with capacity limited at 1000 mAh g^-1 at a 300mA g^-11 of current density.The cycling life was further elongated to 157 cycles when 20mM boric acid added to the HMD based electrolyte to alleviate the corrosion of O₂-dissolved electrolyte.While the cell with DME or DOL based electrolyte only cycling for 42 rounds with BA additive.By SEM and FTIR analysis,many side products including lithium formate or lithium carbonates were found on the surface of cathode which may derived from the decomposition of ether based electrolyte.The above results demonstrate that fully methylated cyclic is more stable in presence of superoxide radicals.Such strategy is feasible to improve the stability of electrolyte which further promote the development of Li-O₂ battery.