Cathode Construction And Reaction Mechanisms Of Solid-state Li-air And Li-ion Batteries Based On Garnet-type Electrolyte

Li-air battery has attracted a lot of reesarchers'attention for its highest theoretical specific capacity among all next-generation battery systems.The aprotic Li-air battery which uses traditional non-aqueous liquid organic electrolyte has been widely studied for its similarity to traditional Li-ion battery.However,the liquid organic electrolyte can lead to severe issues in Li-air battery due to its limitations,such as low energy efficiency,poor cycle performance,electrolyte volatiling in open system.The H₂O and CO₂ in ambient air can also diffuse through the liquid electrolyte and contaminates the Li anode,which restrain the operation in ambient air.Replacing the liquid electrolyte by solid-state electrolyte is a promising solution to above issues.Garnet type Li_6.4La₃Zr_1.4Ta_0.6O₁₂?LLZTO?is a promisiing solid-state electrolyte owing to its high ionic conductivity(higher than 10^-3 at room temperature),wide electrochemical window and stability when contacts with metallic Li.This thesis carried out the research on the solid-state Li-air and Li-ion batteries based on LLZTO solid-state electrolyte.The major results are as fellows;?1?A solid-state air cathode was constructed with Ketjen Black?KB?as electric conducting pathway,LLZTO particles as ionic conducting backbone and Poly?propylene carbonate?/Polyimide:Lithium Triflimide?PPC/PI:LiTFSI?polymer electrolyte as ionic conducting fillers.The solid-state Li-air battery with PPC:LiTFSI cathode can work in ambient air at 80°C.The first discharge specific capacity is as high as 20300 mAh g^-1_carbon(6430 mAh g^-1_cathode,20?A).The cells can cycle 50 times with a cut-off specific capacity of 1000 mAh g^-1_carbon.The main discharge product is characterized to be Li₂CO₃ and can decomposes upon charging at least partially.The cells with PI:LiTFSI cathode can work under 200°C in ambient.The first discharge specific capacity is 3224 mAh g^-1_carbon and the charge overpoential is 0.35 V.The cell can cycle 20 times with a large current density of 20 mA cm^-2?50 A/g?.?2?In order to eliminate the carbon componet in solid-state air cathode and study the decomposition products of Li₂CO₃ by DEMS,an thin-film Au air cathode was deposited on LLZTO by DC magnetic sputtering.A solid-state Li-air battery with stable non-carbon cathode was constructed.The main discharge product is characterized to be Li₂CO₃ after discharge in air and it decomposes after charge.The charge process was studied by in-situ DEMS by using Ar as carrier gas.With the non-carbon cathode,O₂ was detected during the charge process of Li₂CO₃ which provides the evidence that Li₂CO₃ can decompose through the reaction of Li₂CO₃?2Li+CO₂+1/2O₂.?3?Solid-solid interfaces are major components of the internal resistance of solid-state batteries.In order to study the interface resistances in solid-state lithium batteries,Li/LLZTO/Li and Cathode/LLZTO/Cathode symmetric batteries were constructed.The cathode contains LiFePO₄ as cathode material in Ar atmosphere.Electrochemical impedance spectroscopy?EIS?was carried out to study the interfaces in these batteries.The resistance of Cathode/LLZTO and Li/LLZTO interfaces has an order of 10⁴?and 10³?,respectively.The resistance of the cell could be dramatically reduced by improving temperature and the reduction can remain after cooling back.The activate energy of cathode/LLZTO?0.21 eV?is smaller than Li/LLZTO?0.26 eV?,which was determined by Arrhenius fitting.Polyvinylpyrrolidone?PVP?additive can reduce the resistance of cathode/LLZTO interfaces in solid-state battery and make the cell work at room temperature.