Study On The Li|LLZO Interface Based On Garnet-type Solid Electrolyte

It has become a global consensus to strive to achieve carbon neutralization by 2050.To achieve this goal,the large-scale development and utilization of clean energy,such as wind and solar,has become a major strategic support.Electrochemical energy storage technology,represented by the lithium battery,is the most critical part.With the wider application of lithium batteries in life and production,the demand for advanced batteries with higher energy density and higher safety is also increasing,and the global competition for the development of lithium batteries has become increasingly fierce.Solid-state lithium metal batteries(SSLBs)have been recognized as a new technology in the next generation of battery technology.The reason is that SSLBs are expected to achieve higher energy density when using high-voltage cathodes and lithium metal anode,and to solve the safety problems faced by current lithium ion batteries at the same time.Garnet-type solid electrolytes Li7La3Zr5O12(LLZO),as one of the most important components in SSLBs,have attracted much attention because of their excellent physicochemical properties,such as,the excellent chemical/electrochemical stability with lithium metal,high ionic conductivity,high mechanical strength,and good thermal stability.However,the current research of SSLBs based on LLZO solid electrolyte(SE)still faces multiple challenges,including low quality of ceramic electrolytes,poor airstability,unrepeatable interface construction strategies,large interfacial impedance,poor electrochemical-mechanical stability,and the most critical lithium dendrite issue.Among them,the complex solid|solid interface problem is particularly prominent,which is also considered to be the biggest bottleneck in the development of solid-state battery technology.Solving these key problems is of great significance for the development of solid-state ionics and the establishment of structural-performance relationship of solid-state battery systems.To solve these challenges,we took Li|LLZO interface as a research template,and systematic investigations were carried out.The main content of this dissertation includes:Aiming at the poor contact between LLZO and metallic lithium(Li),which is originated from the poor air-stability of LLZO materials.A simple "acid pickling"(AP)strategy was proposed to remove the surface contaminants and form 3D-like porous structure layer near surface of LLZO at the same time.As a result,the poor contact of Li|LLZO interface has been greatly improved,and the total interface resistance in symmetric cell after treatment is only about 10 Ω cm2 based on the electrochemical impedance spectroscopy(EIS)results.The Li|LLZO(AP)|Li symmetric cells attain a stable cycle life for over 3200 h under a current density of 0.2 mA cm-2 and 1200 h under a current density of 0.4 mA cm-2 at room temperature.Moreover,the feasibility of large-capacity cycle is also verified(0.3 mA cm-2,1 mAh cm-2 and 0.4 mA cm-2,2 mAh cm-2).In addition,quasi solid-state lithium batteries with excellent performance are successfully designed with high voltage LiNi0.5Mn1.5O4 and LiCoO2 cathodes,which show competitive performance than the liquid battery.Our strategy is of great importance for the final application of LLZO materials.To investigate the influence of defects on the electrochemical deposition behaviors,three LLZO ceramic electrolytes(S-84,S-91,S-99)with different relative densities were synthesized.The morphology,structure and basic electrochemical parameters of these materials were analyzed by XRD,SEM,transmission CT and electrochemical methods.The electrochemical performance show that the critical current density(CCD)of polycrystalline electrolyte is closely related to its internal defects.Furthermore,by comparing the deposition behaviors of LLZO at different relative densities with in-situ SEM,we systematically show some important information on both the spatial distribution and the distinct local morphology evolution of electrodeposited Li,as well as the relationship of polycrystalline defects with nucleation sites and local Li growth manner for the first time.The intrinsic local defects(such as pores,grain boundaries and impurities)of polycrystalline LLZO are found to be responsible for the different deposition kinetics and morphologies at the electrode|SE interface.In addition,modeling studies based on finite element analysis method show that these internal defects hinder the transport of lithium ions inside the electrolyte and affect the concentration distribution of lithium ions at the interface.Improve the quality of ceramic electrolyte can effectively promote the uniformity of Li ion flux and increase the numbers of deposition active site at the Li|SE interface,thus reducing the inhomogeneity of interface current density distribution and increasing the critical current density of the electrolytes.To reveal the influence of the physical and chemical properties of Li metal on the deposition process,an electrochemical-mechanical model at the Cu|LLZO interface was built.The model implies that the(local)current density and interfacial constraint conditions are the keys to achieve stable deposition.Hence,the effects of current density and electrode film thickness on deposition kinetics and Li deposition morphology at Cu|LLZO interface were investigated by electrochemical testing and in-situ SEM.These results show that with the increase of current density,the nucleation overpotential increases,while the critical nucleation radius and the size of Li whiskers decrease,but these tiny whiskers are more mechanically destructive.On the other hand,with the increase of Cu film thickness,the critical nucleation radius increases,while the nucleation overpotential and internal stress decrease.Next,the local stress evolution process of the deposition interface under different current densities were traced by the stylus profiler for the first time.We show that there is a competition-equilibrium relationship between the deposition rate of lithium metal and its diffusion creep rate.Under low current density,the deposition rate of Li metal is less than its diffusion creep rate.Therefore,the deposited Li tends to extend and grow along the deposition plane,so that the deposition stress is released.On the contrary,under high current density,the deposition rate is greater than the diffusion creep rate of Li metal,the local normal stress will increase continuously,which can lead to electrolyte rupture or lithium dendrite puncture.In addition,studies on the deposition behavior of the lithiophilic coatings(such as Au,Ag)confirm that these coatings can realize uniform nucleation,so as to reduce the local current density and alleviate the accumulation of compressive stress.This work reveals the stress evolution process of the deposition interface from a new perspective,which will help us to understand the electrochemical-mechanical interaction of the interface during deposition.