Study On Performance Improvement Of Solid State Lithium Metal Battery Based On LATP And LLZTO

With the increasing environmental damage brought about by the global economic development,people are increasingly realize the magnitude of ecological protection.As a result,clean and renewable solar,wind and tidal energy are being vigorously developed.The continuous improvement of energy structure has also promoted the research of high energy and high power storage devices.Lithium-ion batteries（LIBs）are used for portable electronic devices and automobiles.This is mainly attributed to its high operating voltage,high specific energy,long cycle life.However,the traditional LIBs are mostly organic electrolyte,which is flammable,explosive,easy to leak,easy to corrosion and other serious safety risks.In order to solve the safety hazard of liquid electrolyte of lithium ion battery,researchers began to study solid electrolyte.The application of solid electrolyte can not only solve the safety problem,but also further improve the energy density of the battery by applying well-matched lithium metal into the battery.At present,the research on solid electrolyte essential center at polymer solid electrolyte and inorganic solid electrolyte.Among other,the polymer solid electrolyte has good flexibility,which can realize excellent contact between electrolyte and lithium metal.However,due to the slow migration of lithium ions in the polymer,the polymer solid electrolyte has low ionic conductivity at room temperature.Inorganic solid electrolyte has a high conductivity,and the internal density,grain boundary blocking effect is greatly reduced,however,the contact between inorganic solid electrolyte and electrode is rigid solid-solid contact,there is a problem of poor contact,thus affecting the performance of the battery.Aiming at the above problems,this paper proposes a series of innovative strategies to improve the ionic conductivity of the electrolyte,improve the interface,and greatly improve the electrochemical performance of the solid-state lithium battery.The main research contents are as follows:（1）NASICON fast ion conductor Li_1.7Al_0.3Ti_1.7（PO₄）₃（LATP）was prepared by hydrothermal method.The composite solid electrolyte（CSE）was prepared by mixing polyvinylidene-hexafluoropropylene copolymer（PVDF-HFP）and LATP powder in a certain proportion.LATP itself can conduct lithium ions,and its addition greatly improves the lithium ion conductivity of PVDF-HFP polymer solid electrolyte.We found that the CSE prepared had the highest ionic conductivity(3.64×10^-3 S cm^-1 at room temperature)when the mass fraction of LATP was 15 wt.%.The electrochemical performance of the CSE prepared at optimum proportions was tested:maintaining a minimal overpositioning（18mV）cycle for over 3000 hours at 0.2 mA cm^-2 current.The assembled full battery（Li/LiFePO₄）maintained a capacity of 138.9 mAh g^-1 after 700 cycles at room temperature,with a capacity retention rate of 92.6%.（2）Compared with inorganic ceramic solid electrolyte,the lithium ion conductivity of complex solid electrolyte is still lower.Therefore,we focus on inorganic ceramic solid electrolyte.In order to improve the wettability of lithium metal on the surface of inorganic solid electrolyte,we sputtered a layer of aluminum-silicon alloy material that can react with lithium metal on the surface of solid electrolyte by magnetron sputtering method to modify the surface of solid electrolyte.Firstly,Li_6.75La₃Zr_1.75Ta_0.25O₁₂（LLZTO）powder was prepared by solid phase reaction at 900℃.The powder was then prepared into sheets with a diameter of 15 mm and a thickness of 1 mm under mechanical pressure,and sintered at 1000℃ for 2 hours to increase its densification.Then magnetron sputtering Al-Si alloy buffer layer（Al:Si=85:15,molar ratio）on the surface of LLZTO sheet.The solid state battery has the minimum interface impedance（15.0Ωcm²）when the thickness of Al-Si buffer layer is controlled to 10 nm by magnetron sputtering.The full battery with LFP as anode and lithium metal as cathode can stably cycle 600 cycles at room temperature and the capacity is maintained at 120.7 mAh g^-1,The loss rate of single cycle capacity is only 0.032%.（3）Although the magnetron sputtering method has achieved a good effect,but the operation of magnetron sputtering is too complicated,not suitable for large-scale application.A layer of ZnI₂ was constructed on the surface of LLZTO by solution impregnation method.ZnI₂ reacted with heated lithium metal to form a mixed conductive layer of LiI/ZnLi_x to reduce the interface impedance.The prepared LLZTO tablets are polished and soaked in anhydrous ethanol solution of the ZnI₂ for 10 seconds,then taken out and dried to obtain LLZTO tablets covered with the surface of the ZnI₂.After testing,when the concentration of the ZnI₂ solution was 1.0 mol/L,the interface impedance value was the lowest（10.3Ωcm²）.At the same time,the full battery assembled with LFP as anode was stably cycled 200times at room temperature,the capacity was maintained at 143.9 mAh g^-1,and the capacity loss was 0.021%per cycle.