| Li metal is an ideal anode for lithium-air(Li-air)battery due to its ultra-high specific capacity(3860 m A h g-1),low density(0.59 g cm-3)and ultralow negative electrochemical potential(-3.04 V vs.the standard hydrogen electrode).However,there are still exsiting many challenges for the commercialization of Li metal anode,such as the formation of unstable soild electrolyte interface(SEI)between Li metal and electrolyte,the growth of Li dendrite and the formation of dead Li.Especially,when Li metal anode is applied to rechargeable Li-air battery,the above challenges shall become more severly.On one hand,O2 and trace of H2O dissolved in the electrolyte are fatal to Li anode.One the other hand,the side reaction between Li metal and electrolyte will continuously consume the solvent,resulting a high overpotential and poor cycle life for Li-air battery.To solve the above issues,we try to improve the stability of SEI on Li metal surface and enhance the cycle life of Li-air battery via optimizing electrolyte additive,constructing a stable solid state electrolyte(SSE)and introducing an oxygen selective membrance(OSM).The main contents and results are as follows:(1)Zirconium tetrafluoride(ZrF4)is introduced as a dual-function electrolyte additive in lithium-oxygen(Li-O2)cell.On one hand,considering the unstable SEI on Li metal surface,ZrF4 can react with Li metal and form a stable SEI film on the Li metal surface.The results from X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDX)and Raman specta(Raman)reveal that the SEI film mainly consists of Li F-rich,Zr O2,Zr-O-Fxand Li2CO3.The cycle life of Li|Li symmetric cell increases from 100 h to 580 h with the stable SEI formed on the Li surface.On the other hand,ZrF4 increases ion-solvent coordination in electrolyte,as well as improves the lithium ion transfer number and lithium ion conductivity.Moreover,ZrF4 is a lewis acid and can promote the solution growth of Li2O2.As a result,the discharge specific capacity of the Li-O2 cell can be enhanced from 7700 mAh g-1 to 11700 mAh g-1 and the cycle life of Li-O2 cell is substantially increased from 21 cycles to 111 cycles.(2)A new aluminates-based solid state electrolyte(named Li-Al-O SSE)is fabricated on Li metal surface via the chemical reaction between LiOH and triethylaluminum(Al(C2H5)3)at the presence of bis(trifluoromethane)sulfonamide lithium salt(Li TFSI)/1,2-dimethoxyethane(DME)electrolyte.The results from XPS,XRD,SEM,EDX and transmission electron microscope(TEM)demonstrate that the Li-Al-O SSE mainly consists of polycrystalline LiAlO2,Li3Al O3,Al2O3,Li2CO3,Li F and some organic compounds.The conductivity of the Li-Al-O SSE is measured via electrochemical impedance spectroscopy(EIS)and direct-current polarization.The results demonstrate that the ionic conductivity is as high as 1.42?10-4 S cm-1 and the electronic conductivity is as low as 5.9×10-9 S cm-1 for the Li-Al-O SSE.The low electronic conductivity can effectively suppress the growth of Li dendrite.The Li-Al-O SSE is in situ formed on the Li metal surface,resulting a dense interface between Li metal and the Li-Al-O SSE and greatly decreasing the interfacial resistance.Moreover,the Li-Al-O SSE is stable in air and oxygen-saturated tetraethylene glycol dimethyl ether(TEGDME),which can effectively protect Li anode from the corrosion of H2O,O2 and organic solvent.When the Li-Al-O protected Li foil is applied to assemble Li|Li symmetric cell,the cell is able to steadily cycle for more than 1400 hours with a current density of 0.2 m A cm-2 and charge/discharge depth of 0.4 mAh cm-2,which is much longer than the control cell(about600 hours).In addition,when the current density is increased to 1 m A cm-2 and charge/discharge depth increased to 2 mAh cm-2,the“Li|Li-Al-O|Li”symmetric cell can also steadily cycle for more than 1100 hours.For the Li-O2 cell,the cell with Li-Al-O protected Li anode is able to steadily run for 179 cycles at a current density of 300 m A g-1with a fixed specific capacity of 1000 mAh g-1.On the contrary,the control cell with bare Li anode only run for 31 cycles.(3)An oxygen selective membrance(OSM)based on poly(perfluoropropylene oxide-co-perfluoroformaldehyde)(perfluoropolyether,abbreviated as PFPE)is introduced between positive collector and cathode cap.The PFPE-based OSM is effective to suppress the side reaction of moisture with Li anode and Li2O2.The results demonstrate that the permeability of O2 and CO2 through the membrance is 59.41 and 187.37 Barrer,respectively.And the permeability H2O is as low as 38.90 g m-2 d-1.The high O2permeability and low H2O permeability can ensure the Li-air cell run in pratical air.Moreover,CO2 can diffuse to Li anode and react with LiOH or Li metal to form a stable Li2CO3-rich SEI,which further protects Li anode from the corrosion.In addition,PFPE is immiscible with DMSO and can effectively suppress the evaporation of electrolyte.With protection of the PFPE-based OSM,the Li-air cell is able to run 144 cycles in 30%relative humidity(RH)ambient air with a current density of 100 m A g-1 and a capacity limited at 500 mAh g-1,which is much longer than the control cell(only 5 cycles). |
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