Rational Design And Intensive Study Of All-solid-state Lithium-air Batteries

In this decade,Li-air batteries have captured worldwide attention because of their extremely high theoretical energy density based on the mass of Li2O2.The cathode active materials oxygen can be attained from ambient air,which indicates the battery is light-weight and low-cost.As a result,Li-air batteries are considered to be one of the most promising next-generation energy storage systems.Despite these advantages,Li-air batteries still face many challenges.One of the main challenges arises from organic liquid electrolytes.The utilization of organic liquid electrolyte may result in volatilization,leakage,flammability and explosion.Besides,in Li metal batteries,lithium dendrites will continually grow and finally impale the electrolyte layer,resulting in short circuit.Especially,Li-air battery is an open system where the cathode is designed as a porous structure to accommodate the active gas oxygen.When the battery is used in ambient air,carbon dioxide and water may penetrate into the electrolyte and react with Li anode,which is detrimental to the battery performance.To offset these problems,substituting organic liquid electrolyte with inorganic solid electrolyte is a promising method.However,the development of all-solid-state Li-air batteries is at the initial stage.Various critical problems remain to be solved for the practical application of this system.One of the main challenges is the lack of suitable solid electrolytes.Besides,high discharge/charge overpotentials will result in low energy efficiency and poor cycle life,which requires more effective catalysts for ORR and OER progress.The interfacial impedance between solid electrolyte and Li anode should also be reduced by proper method.To deal with these critical challenges discussed above,we have devoted much time and effort in this work.The major innovations of this work can be summarized as follows:1.Fabrication and performance of all-solid-state Li-air battery with SWCNT/LAGP cathode:NASICON-type solid-electrolyte LAGP was fabricated via a solid-state reaction method.The as-prepared LAGP pellets has a lithium ionic conductivity over 10-4 S cm-1.In addition,we proposed an all-solid-state Li-air battery composed of a Li metal foil anode,a NASICON-type solid electrolyte LAGP,and SWCNTs/LAGP as air electrode.The good electronic properties,large surface area and better crystallinity of SWNTs benefit the electrochemical process.This all-solid-state Li-air battery showed a larger capacity of 2800 mAh g-1 for the first cycle discharge than that with MWCNTs/LAGP,which had a capacity of about 1700 mAh g’1.The cell exhibited a good cycling performance with a limited capacity of 1000 mAh g-1 at a current density of 400 mA·g-1.However,because of the influence of CO2 and H2O,the electrochemical process in air atmosphere may be largely different with that in pure oxygen atmosphere.Obviously,the air electrode also needs to be further optimized in order to construct a three dimensional network and more effective catalysts suitable for all-solid-state Li-air batteries are equally required.2.Intensive investigation on all-solid-state Li-air batteries with cathode catalysts of SWCNTs/RuO2:The all-solid-state Li-air batteries with a mixture composed of SWCNTs,RuO2 nanoparticles and LAGP particles as composite cathode were investigated in ambient air at room temperature.Charge overpotential was reduced because of the superior catalytic activities of RuO2 nanoparticles.Besides,SEM images showed the formation of chip-like products after discharge to 1000 mAh g-1 and wrinkles-like products after fully discharged to 2 V,respectively.FTIR and XPS results showed the discharge products were Li2CO3.Galvanostatic intermittent titration technique(GITT)demonstrated that the equilibrium voltage of all-solid-state Li-air batteries cycling in oxygen kept stable at 2.96 V while that cycling in ambient air maintained at 3.15 V.These results indicated that the electrochemical reactions of all-solid-state Li-air batteries can be ascribed to the formation and decomposition of lithium peroxide in oxygen and electrochemical reaction with moisture in air.3.Fabrication and performance of all-solid-state Li-CO2 batteries:We demonstrate that the all-solid-state Li-CO2 cell using SWCNTs and RuO2 as cathode catalyst,LAGP as solid electrolyte and Li metal as anode can be operated under pure CO2 atmosphere.This cell exhibited a discharge capacity of 1046 mAh·g-1 and a charge capacity of 1025 mAh·g-1 in the first cycle.Besides,the cell can sustain 10 cycles under a capacity limitation.The electrochemical performance of the cell was greatly improved by increase temperature.By means of FTIR and XPS tests,we confirmed the formation and decomposition of Li2CO3 during electrochemical process.Moreover,SEM images show the film-like structure of the discharge product Li2CO3 covering the cathode surface.In situ GC-MS measurements verified the decomposition of Li2CO3 and generation of CO2 on charge.4.Germanium thin film protected lithium aluminum germanium phosphate solid electrolyte for Solid-State Li-air Batteries:The stability of LAGP solid electrolyte with Li metal anode was improved by sputtering an amorphous Ge film.The Ge thin film can not only suppress the reduction from Ge4+ to Ge0 and Ge2+,but also produce an intimate contact between Li anode and solid electrolyte by forming an Li ionic conducting interlayer.The impedance plots and discharge/charge profiles show a greatly improved performance of Li/Ge/LAGP/Ge/Li symmetric cells compared with the cells without Ge film.Further investigation of SEM and XPS characterization indicated Ge film has a positive influence on the stability of the interface.A quasi-solid-state Li-air was assembled using the Ge film coating LAGP pellet.Superior cycling behavior was obtained for 30 cycles at room temperature.This work shows the introduction of a stable and ionic conducting interlayer is crucial to improving the performance of the solid-state Li batteries.All these results contribute to the development of all-solid-state Li-air batteries.This work indicated that the all-solid-state Li-air/O2/CO2 battery is very promising for future energy storage system,and further work focused on the design and fabrication of the solid-state battery and development of more effective catalyst in this system should be carries on.