Research On Key Issues Of Interfaces In Solid-State Lithium Metal Batteries

In order to develop the next generation of safer,higher energy-density battery systems,solid-state lithium metal batteries are the mainstream of research.The non-flammable solid electrolyte is gradually used to replace the electrolyte used in the traditional liquid lithium ion battery,which can improve the safety of the battery to some extent.At the same time,the introduction of solid electrolytes has revived the possibility of using lithium metal as the anode for batteries.However,the introduction of solid electrolyte and lithium metal has brought many new challenges to the effective operation of solid-state batteries.Based on this,this dissertation focus on the problems caused by the introduction of lithium metal anode and solid electrolyte respectively.Specifically,it is mainly divided into the following three parts:Firstly,we took copper foam as the research object and carried out unilateral gold spraying treatment on it to construct 3D Janus structure.Then,copper foil,untreated copper foam and single-side gold spraying copper foam were respectively used to assemble different lithium copper half batteries matched with metal lithium sheets.When single-side gold spraying copper foam was used to assemble the half battery,the spraying side was placed away from the separator.Electrochemical data show that at the current density of 0.5 m A/cm²,the nucleation overpotential of lithium metal deposited on copper foil,untreated copper foam and single-side gold sprayed copper foam is 70 m V,30 m V and 5 m V,respectively.When the current density of 0.5m A/cm² and the capacity of 1 m Ah/cm² were tested,the results of 250 cycles showed that the single-side spray copper-foam battery had the best cycle stability and the average coulomb efficiency was 97.4%.The average coulombic efficiency of copper foam is 93.6%.Copper foil was the worst,with an average coulomb efficiency of86.4%.Further,we designed in-situ optical experiments to study the deposition behavior of lithium metal on untreated copper foam and single-side gold sprayed copper foam.At the current density of 1m A/cm²,the initial state,20 minutes,40minutes and 60 minutes of deposition were selected for recording.The experimental results show that the distribution of lithium metal on the untreated copper foam is uneven,and most of them are dendritic.When the lithium metal is deposited on the copper foam on the side of the gold spraying,the deposit is preferred on the copper foam on the side of the gold spraying,and the distribution is uniform,and no obvious dendrite exists.Density functional theory calculations show that the adsorption energy of single lithium atom on Au（100）,（110）and（111）crystal planes is lower than that on Cu（100）,（110）and（111）crystal planes.Secondly,we prepared NASICON solid electrolyte Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）powder by solid-phase sintering method,and further pressed and calcined it to prepare LAGP ceramic plates with a diameter of about 11 mm and a thickness of about 0.12mm,which were used as the object of subsequent research.Based on the CR2032battery case,we designed the in-situ battery case and carried out the in-situ optical microscope observation experiment.At the current density of 1 m A/cm²,the initial discharge state,discharge 10 min,discharge 20 min,discharge 30 min,discharge 40min and discharge 50 min states were selected for recording.The experimental results show that the deposition of lithium metal on the surface of LAGP ceramic surface is not uniform,and most of the lithium metal nucleates and grows at the defects,and finally develops into needle-like dendrites.Furthermore,we designed an in-situ atomic force（AFM）microscope experiment to study the deposition behavior of lithium metal on the surface of LAGP ceramic sheets at micro/nano scale.Under the Peak Force Tuna（PF-TUNA）mode of AFM,a higher contact current was measured between the particles on the surface of LAGP ceramic plates.AFM was used to apply different bias voltages to the in-situ battery,and it could be seen that lithium metal would nucleate out between LAGP particles in preference.With the increase of deposition capacity,lithium metal further grew.Under the reverse bias pressure,part of lithium metal dissolved and part of lithium metal became"dead lithium".Density functional theory（DFT）calculations show that the surface of LAGP may have higher electron conductance than its bulk phase.Finally,we prepared composite cathode electrodes by in-situ polymerization coating of artificial cathode electrolyte interphase（CEI）on porous Li Co O₂ electrode plates with in-situ solidification method.The precursor solution was prepared by dissolving 1 mole of lithium difluoro（oxalato）borate（Li DFOB）in vinylene carbonate（VC）and adding 0.1%azobisisobutyronitrile（AIBN）as initiator.In the glove box,1μL of the precursor solution was added onto an 8 mm diameter lithium cobalt oxide electrode plate and left for 15 min.Then,the precursor solution was heated to 60℃and left for 30 min to fully polymerize.The results of scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FTIR）and transmission electron microscopy（TEM）show that we have successfully coated the electrode plates and particles of Li Co O₂ with an artificial CEI.A solid lithium metal battery was assembled by matching the composite Li Co O₂electrode plate with a poly（ethylene oxide）-based solid polymer electrolyte and lithium metal.Electrochemical test results show that the capacity retention ratio of the battery is 71.5%after 500 cycles in the voltage range of 3.0～4.2 V.However,the capacity of the solid-state battery assembled with unmodified Li Co O₂ electrode plate decreases rapidly and only maintains the discharge specific capacity of 34.1 m Ah/g after 200 cycles.In addition,the adiabatic accelerating rate calorimeter test results show that the PEO-based solid-state lithium metal battery prepared by in-situ CEI coated Li Co O₂ plate exhibit excellent thermal safety,and does not occur thermal runaway when heated to 350℃.