Research On Preparation And Doping Modification Of Argyrodite-type Sulfide Electrolyte By Liquid Phase Method

With the upgrading of emerging electronic technology products such as new energy vehicles,higher requirements are placed on the safety and energy density of traditional lithium-ion batteries.Solid-state lithium-ion batteries can increase the energy density of the battery and greatly improve the safety performance of the battery.One of the core components are solid-state electrolytes.Among these solid-state electrolytes,argyrodite-type sulfide electrolyte（Li₆PS₅X（X=Cl,Br,I））has attracted much attention because of its ultra-high lithium-ion conductivity,excellent mechanical ductility and good interfacial contact with electrodes,etc.This kind of electrolyte is mostly prepared by the solid-phase reaction method and the obtained products have high ionic conductivity.But this synthesis process is complicated,time-consuming and energy-consuming.In comparison,the liquid-phase method has a shorter time and simpler process,and is considered to be a more efficient method for the production of sulfide solid electrolytes.However,there are still some problems in the liquid-phase method.For example,the ionic conductivity of the sulfide electrolyte prepared based on the dissolution precipitation method has dropped significantly.The multi-step liquid-phase method can be used to prepare sulfide electrolyte with higher ionic conductivity,but the process is complicated.At present,there are few attempts on simple one-step liquid phase method,and there are some shortcomings such as unstable solvent with raw materials,low ionic conductivity or long annealing time,high annealing temperature and unclear mechanism.In view of the above problems,we developed a simple one-step liquid-phase method to prepare argyrodite-type electrolyte with high ionic conductivity,and studied the reaction mechanism of the preparation process;and took Li₆PS₅I as an example to explore the feasibility of element doping by the one-step liquid-phase method.The details are as follows:（1）A one-step liquid-phase method with acetonitrile as solvent was developed to prepare the argyrodite-type solid electrolyte,which simplifies the preparation process,shortens the preparation time and reduces the annealing temperature.Li₂S,P₂S₅and lithium halide only need to be stirred once in liquid phase,and the dried precursors are annealed to achieve the successful preparation of Li₆PS₅Cl,Li₆PS₅Br and Li₆PS₅I,which shows the universality of this method.Among them,the prepared Li₆PS₅Cl has an ionic conductivity of 1.1×10^-3S cm^-1at room temperature and an activation energy of 0.27 e V,which is comparable to the ionic conductivity of the electrolyte prepared by the traditional solid-phase method.Through the XRD test characterization of the precursors after different annealing temperatures,the specific reaction mechanism was explored:first,Li₂S and P₂S₅reacted in acetonitrile to form Li₃PS₄·2ACN,during the annealing process Li₃PS₄·2ACN was converted into Li₃PS₄and reacted with the rest of Li₂S and lithium halide to form argyrodite-type sulfide electrolyte.In the control experiment,Li₃PS₄,Li₂S and Li Cl formed Li₆PS₅Cl after annealing,which well verified the importance of forming Li₃PS₄·2ACN in the liquid-phase preparation process.The first cycle discharge specific capacity of S/Li₆PS₅Cl-PTFE/Li-In button-type battery reached695.4 m A h g^-1at 60°C with the current density of 0.05C rate.（2）Aiming at the problem that the doping modification of sulfide electrolytes still relies on the traditional solid-phase method,based on the preparation mechanism of the previous chapter,an ion doping method of sulfide electrolyte based on liquid-phase method was developed.According to the hard-soft-acid-base theory,the soft acid metals cadmium（Cd）and stibium（Sb）were selected for P-site doping of Li₆PS₅I electrolyte to improve its ionic conductivity and air stability.The test results prove that Cd cannot be doped into Li₆PS₅I,while Sb can be successfully doped.By adjusting and optimizing the doping ratio of stibium,it is found that LPSI-10Sb with a doping ratio of 10%has the highest ionic conductivity and the lowest activation energy.The ionic conductivity is 3.2×10^-6S cm^-1at 25°C,and the activation energy is 0.40 e V.In addition,we also conducted the air stability test of the electrolyte,and it was found that the mass of LPSI-10Sb grows more slowly,indicating that the doping of stibium also improves the air stability of Li₆PS₅I electrolyte.