The Property Research Of Novel Sulfide Electrolytes For All-solid Lithium Batteries By Low-cost Preparation

For safety and energy density,all-solid-state lithium-ion batteries have received more and more attention.On one hand,all-solid-state lithium-ion batteries can fundamentally solve the problem that liquid lithium-ion batteries cannot be avoided by containing organic electrolytes.On the other hand,the problem that traditional liquid lithium-ion batteries cannot use lithium as cathode material is solved.The dense fast ion conductor electrolyte can prevent the piercing of lithium dendrite,so as to avoid causing the positive and negative pole short circuit problem,and the application of lithium metal can greatly improve the energy density of lithium-ion batteries.In addition,solid-state lithium batteries have advantages over liquid electrolytes in terms of high and low temperature stability and electrochemical windows.The core technology and research of all-solid-state lithium-ion battery focus on the fabrication of a fast lithium ion conductor electrolyte material with lithium ion conductivity comparable to liquid electrolyte.For example,the Li10oGeP2S12 has very high lithium ion conductivity.However,the sulfide materials system still have many problems unsolved.For example,Li6PS5Cl system has relatively low lithium ion conductivity,expensive raw materials,harsh preparation process,poor compatibility with positive and negative materials,serious interface reaction,and poor Stability.Although the LGPS system and its improved type(such as Si and Sn instead of Ge)have the advantages of high lithium ion conductivity,Environmentally friendly,they also have the problems of high raw materials cost,harsh synthesis process and poor interface stability.In view of this,the research work of this thesis mainly focuses on the theoretical design and experimental verification of the further improvement of lithium ion conductivity of Li6PS5CI system by materials genomic engineering method,low-cost preparation of amorphous sulfide electrolyte,the preparation of improved crystal sulfide of LGPS system,all-solid-state lithium-ion battery and interface modification and their basic scientific research and process improvement practices,including the following:For the problem of poor lithium ion conductivity of Li6PS5Cl sulfide electrolyte,two key ways to further improve the lithium ion conductivity of Li6PS5Cl sulfide electrolyte were discussed by using materials genomic engineering analysis and design method.(1)Lithium-rich treatment by non-stoichiometric ratio method by reducing Cl content,(2)weakening of the binding effect of anions on lithium ions by Te element doping,so as to soften the lattice and expand the lithium ion diffusion channel.2.According to the results of first-principles calculation and design,experiments were carried out and sulfides with high lithium ion conductivity were successfully prepared:Li6.25PS5.25Cl0.75 and Li6.25PTe0.125S5.125Cl0.75.(1)By XRD analysis,it was found that the lattice constant of the material increased significantly after non-stoichiometric treatment and doping a small amount of Te element.(2)By Raman spectroscopy analysis,the PS43+ peak shifts to the left obviously,which also verifies the existence of the lattice softening phenomenon.(3)Through the electrochemical impedance spectroscopy(EIS)test,the room temperature lithium ion conductivity of the compound has been significantly improved,that is,Li6PS5Cl(0.28 S/cm),Li6.25PS5.25Cl0.75(1.03 mS/cm),Li6.25PTe0.125S5.125Cl0.75(4.5 mS/cm),of which Li6.25PTe0.125S 5.125Cl0.75 can be as high as 18 mS/cm at 120,and the lithium ion conductivity can also be maintained at 1.6 mS/cm at-20?.In addition,the activation energy Ea of Li6.25PTeo.125S 5.125Cl0.75 is only 0.168 ev by the variable temperature experiment,which is obviously lower than the activation energy of the unmodified original Li6PS5Cl system(0.52 ev).The improved electrolyte material is less sensitive to temperature changes and lays the foundation for the preparation of all solid state batteries using the sulfide electrolyte system at low temperatures.(4)By conducting CV test on Li6.25PTe0.125S 5.125Cl0.75,the results show that the electrochemical window of the improved electrolyte reach above 7V,indicating that the sulfide electrolyte of the system has higher electrochemical window.The lithium symmetrical battery prepared by Li6.25PTe0.125S 5.125Cl0.75 and all-solid-state lithium battery prepared by lithium iron phosphate cathode and lithium as negative electrode were tested.It was found that the electrolyte in this system was relatively stable to lithium and had good cycle performance.(?)In view of the high cost of raw materials for the synthesis of most amorphous sulphide electrolytes at present,mechanical milling is either required for a long time(more than 40 h)in the synthesis process,or the amorphous sulfide electrolyte needs to be quenched at least 700?.But the melting treatment at high temperature results in the loss of S element volatilization and the corrosion of crucible.In this paper,LiH is proposed as the source of Li.Amorphous sulphide electrolytes can be synthesized by the introduction of most of other elements only from their simple materials,by ball milling for a short period of time and without high temperature melting treatment The self-propagating alloying reaction took place during the synthesis of this kind of material,and the reactants had good repeatability and stability.The method was simple and low cost.XRD and Raman spectroscopy analysis showed that this kind of amorphous electrolyte had the general characteristics of amorphous substances and the classical PS4 group.XRD and Raman spectroscopy analysis showed that the amorphous electrolytes had the general characteristics of amorphous substances,and had classical PS4 groups.The obtained amorphous sulfide electrolytes have lithium ion conductivity of 0.5?1.5 mS/cm,and the electrochemical window is over 5V.4.In this paper,LiH is innovatively proposed as a lithium source,adding Si(Sn,Al),P2S5 and LiCl powder,after a short time of ball milling,self-propagating reaction,the amorphous sulfide intermediate is obtained.Then the intermediate is sealed into a quartz bottle of grinding mouth for heat treatment,and nanocrystalline electrolyte with LGPS phase structure is obtained.Lithium ionization with 10 mS/cm can be obtained.Electrolyte materials with electron conductivity(Li10.2Si1.5PS11.7Cl0.3).XRD and Raman spectroscopy analysis showed that the grain size of this kind of material was nanocrystalline.CV test shows that the material has an electrochemical window of more than 5V.5.Li-S all-solid-state battery,LiCoO2 positive electrode solid-state battery and lithium niobate-coated lithium cobaltate positive solid-state lithium battery were prepared by using the synthesized amorphous and crystalline sulfide electrolytes.It was found that both amorphous electrolyte and crystalline sulfide electrolyte could withstand 4.2V voltage of lithium cobalt oxide,and the charge-discharge cycle curve of the battery showed a standard LiCoO2/SE/Li battery charge-discharge voltage platform.However,it is easy to see that sulfide electrolyte and lithium cobalt oxide have obvious interfacial reaction through a long cycle.The Coulombic efficiency is below 75%in the first week,and the capacity decreases rapidly with the cycle going on.The interface between sulfide electrolyte and cathode materials was improved significantly and the capacity attenuation problem was greatly improved by assembling all-solid-state batteries after lithium cobalt oxide was coated with lithium niobate.Ac irmpedance test was performed on the internal resistance of solid-state batteries after they were circulated for a period of time at different rates.It is found that when the charge and discharge current of the battery increases,the internal resistance increases firstly and then stabilizes gradually.This indicates that the interface reaction at the beginning of charging and discharging of the battery leads to an increase in the interface impedance,but the reaction gradually decreases after a period of charging and discharging cycles.The attenuation of capacity is probably due to the large internal resistance and the serious polarization.This is also demonstrated by capacity restorability of the battery performance at different rates of charging and discharging.Therefore,it is necessary to further improve the lithium ion conductivity of sulfide electrolyte powder,and also to improve the density of pressed powder to improve the accessibility of lithium ion transmission channels.It is necessary to improve the ion conduction network and electronic conduction network on the cathode side,so that the ions can enter the anode(or vice versa)through a reasonable ion network channel after they break away from the cathode pole particles,so as to reduce the serious polarization problem during large current charging and discharging.