Composite Strategy And Mechanism Study Of Lithium Lanthanum Titanates Solid-state Electrolyte Based On Synergetic Effect

The Lithium-ion batteries,as one of the representatives of today's green energy storage,have been widely used in life.Nevertheless,some problems have also been exposed,which are mainly ascribed to the flammable and explosive organic electrolyte.The utilization of lithium batteries is becoming increasingly important owing to the frequent occurrence of security incidents.Therefore,the electrolyte of lithium-ion batteries becomes the key of safety precaution.The emergence of fast ionic conductors?solid electrolytes?seems to be a solution to this problem.In 1835,Faraday discovered the ionic transmissions of AgS and PbF.In 1978,Hong et al found that the LISICON type fast ionic conductor has 0.13 S/cm conductivity at 300 degrees centigrade.Nowadays,there are 10^-2 S/cm sulfide electrolyte LGPS fast ion conductors at room temperature,and the development of solid ions has a qualitative leap.However,the preparation of the high ion conductivity and low interface impedance material in the atmosphere are still great challenges.The ionic conductivity of Lithium Lanthanum Titanates of perovskite structure at room temperature can reach to as high as 10^-3 S/cm,and it is a promising material for environmental stability.But there are two main shortcomings.First,the ionic conductivity of grain boundary is quite low.Second,It is easy to reduce the potential of titanium by matching the negative anode,such as lithium negative electrode,and so on.In view of that,this paper mainly studies the LLZO materials according to the above two shortcomings.The main idea is to optimize the ionic conductance of LLTO grain boundary by using two kinds of lithium rich materials,the stable fast ionic conductor LLZO and Li₃OCl with similar structure of LLZO at room temperature.The solid electrolyte is used to assemble solid-state batteries and a lithium salt-loaded Celgard separator is used as a modified layer between the negative electrode and solid electrolyte to optimize the interface for the restraint of Ti⁴⁺reduction.X-Ray Diffraction?XRD?,X-ray Photoelectron Spectroscopy?XPS?,Raman Spectra?Raman?,Scanning Electron Microscope?SEM?,Energy Dispersive Spectrum?EDS?,Transmission Electron Microscope?TEM?and other measures were employed to characterize the phase,structure and morphology of the composite.The galvanostatic charge-discharge and rate-charge-discharge test of the prepared solid-state battery were recorded by electrochemical workstation and charge and discharge test system;The main conclusions are as follows:1.LLZO is introduced into LLTO by solid-phase method to investigate the effect of different LLZO content on ionic conductivity of LLTO system.The experimental results shows that the increase of LLTO ion conductance is mainly attributed to the increase of grain boundary ion conductance.After high temperature calcination,LLZO partially decomposes,disperses in LLTO grain boundary,forms a complex grain boundary,and improves grain boundary space charge layer.The composite electrolyte LLTO-10wt%LLZO obtained by keeping at 1350°C for 10 h has the highest ionic conductivity of 3.03×10^-4?S/cm?at room temperature,which is about 5 times that of the pure LLTO ion conductivity and grain boundary conductance of 3.41×10^-4?S/cm?,which is the highest grain boundary electrical conductivity in the LLTO-based composite electrolyte.In addition,the grain size and density are also factors that affect the grain boundary conductivity.2.The solid CR2025 button cell assembled by the LLTO-10wt%LLZO powder possesses the first discharge specific capacity of 147 mAh/g at a rate of 0.05 C and an approximately 91.8%specific capacity retention rate after 40 cycles.The first discharge specific capacity is 118 mAh/g at a rate of 0.5 C,and the specific capacity retention rate of the battery is about 90%after 200 cycles.Combined with the battery's rate discharge performance,this battery has excellent stability and reversible performance.3.The effect of different Li₃OCl content on ionic conductivity of LLTO system is investigated by solid-phase method.The addition of Li₃OCl makes up for the volatilization of the lithium source at high temperatures,increases the concentration of carriers?or holes?in the grain boundary and in the crystal,reduces the activation energy of the system,and makes the ionic conductivity of the LLTO effectively increase.At room temperature,The ion conductivity of the LLTO-2wt%Li₃OCl composite electrolyte obtained by incubating at 1250°C for 10h in an argon atmosphere is1.43×10^-4?S/cm?,which was 8.5 times that of the pure LLTO ion.4.Similarly,the solid-state CR2025 button cell,which uses LLTO-2wt%Li₃OCl powder with the highest ionic conductivity as the electrolyte,achieves a first charge capacity of 145 mAh/g at a rate of 0.05 C and retains a 92%specific capacity after 40cycles.