Fabrication And Performance Improvement Strategies Of Solid-State Lithium-Air Batteries With Integrated Electrolyte And Cathode Structure

In recent years,lithium-air batteries have attracted extensive attention from academia and industry due to their high theoretical energy density.Despite the attractive energy density,lithium-air batteries are still in the early stages of research,and some key issues need to be resolved before practical application.One of the main problems comes from the use of liquid electrolyte.Liquid electrolytes suffer from volatilization,leakage,low oxygen solubility/diffusion rate,flammability,and chemical/electrochemical instability.These problems can be avoided by developing solid-state electrolytes.In addition,the solid electrolyte can protect the lithium anode from the erosion of oxygen,carbon dioxide,and moisture,and improve the long-term cycle stability of lithium-air battery.At the same time,the solid electrolyte can act as a physical barrier to prevent short circuits caused by lithium dendrites.At present,the development of solid-state lithium-air batteries mainly faces the following four problems:（1）The solid-state electrolyte is prepared separately,which requires sufficient thickness to ensure mechanical strength,coupled with poor ionic conductivity,resulting in excessive battery internal resistance;（2）The three-phase reaction interface of oxygen,electrons,and lithium ions is limited to the two-dimensional interface between the solid electrolyte and the cathode;（3）The solid electrolyte is in direct contact with the lithium anode and cathode,and this interface contact method causes excessive interface resistance;（4）The slow kinetics of lithium-air batteries lead to high charge-discharge overpotentials,which reduces the energy efficiency and requires the development of high-efficiency catalysts.In response to the above problems,we carried out research on solid-state lithium-air batteries with integrated electrolyte and positive electrode based on Li_1.3Al_0.3Ti_1.7（PO₄）₃（LATP）solid electrolyte and polymer-based solid electrolyte,and at the same time carried out research on high-efficiency catalysts.The main work carried out is as follows:An integrated solid-state lithium-air battery with an integrated cathode and electrolyte structure was designed and prepared based on LATP inorganic solid-state electrolyte.The bilayer structured LATP contains a dense electrolyte layer and a porous cathode support.The thickness of the electrolyte layer was reduced from 600μm to 58μm.After the conductive carbon layer was deposited on the porous layer,the three-phase reaction interface expanded from the two-dimensional interface between the traditional electrolyte and the cathode to the entire cathode.The in-situ solidified hybrid polymer electrolyte（HPE）is connected between the lithium anode and the integrated cathode.The HPE also serves as an interfacial buffer layer,which reduces the interfacial resistance and avoids side reactions between LATP and lithium metal.The integrated solid-state lithium-air batteries achieve enhanced electrochemical performance.Moreover,based on the integrated cathode,we adjusted the electronic conductance of the three-phase interfaces to explore the influence of the rate limiting factors on the reaction kinetics of solid-state lithium-air batteries.Graphene quantum dots（GQDs）were used to functionalize Ni Co₂O₄（NCO）catalyst to compensate the electronic conductance and stimulate its potential catalytic activity.Both GQDs and NCO nanosheet arrays were prepared by a hydrothermal method.We investigated the effect of this modification scheme on the catalytic activity of NCO and the reaction kinetics of Li-air batteries in electrolyte systems.The test substrate is three-dimensional porous graphene（GF）.The GQDs-modified NCO was directly grown on the substrate by a hydrothermal method to prepare a self-supporting ultralight composite cathode.The results show that both the ohmic resistance and the charge transfer resistance of the cathode decrease significantly after GQDs modification.The GQDs@NCO@GF cathode exhibits enhanced ORR and OER catalytic activities.The three-phase reaction interface expands from the NCO/GF boundary region to the entire surface of NCO.The GQDs@NCO@GF cathode exhibits an excellent discharge capacity of 7672 m Ah g_cathode^-1 at a current density of 0.1 m A cm^-2 with a lower charge-discharge overpotential.The results show that the GQDs modification strategy can effectively solve the bottleneck problem of limited kinetic activity of metal oxide catalysts due to insufficient electronic conductivity,which provides a new idea for the efficient utilization of metal oxide catalysts in the field of electrochemistry.Moreover,we applied the GQDs-modified NCO catalyst to the integrated solid-state Li-air battery with an integrated cathode-electrolyte structure.The vertical nanosheet-structured GQDs@NCO catalyst further expands the three-phase reaction interface of the integrated solid-state Li-air battery,while enhancing the OER catalytic activity at the interface.The GQDs@NCO catalyst improves the capacity,energy efficiency,and cycle stability of the integrated solid-state lithium-air battery.A new strategy was adopted to design and prepare HPEs with high ionic conductivity and high Li-ion transfer number at room temperature.The HPE consists of PVDF-HFP,Li TFSI,and TMP,and is in-situ prepared through weak hydrogen-bonding interactions.In-situ solidification mechanism and electrochemical performance of hybrid polymer electrolytes were studied in depth.The in-situ prepared HPE has a lithium ion migration number as high as 0.73,and an ionic conductivity as high as 1.08×10^-3 S cm^-1 at room temperature.Based on the HPE,we designed and prepared a novel integrated flexible solid-state lithium-air battery.The integrated cathode constructs a three-dimensional hybrid electronic and ion conductor,extending the three-phase reaction zone from the conventional interface between the electrolyte membrane and the cathode to the entire cathode.This interface engineering can significantly reduce interfacial resistance and accelerate reaction kinetics.The battery exhibits high safety,flexibility,and high electrochemical performance,including excellent mass/volume energy density,rate performance,and good cycle stability.