Modification Of Sulfide Based Solid Electrolyte And Its Application In All-Solid-State Batteries

The development of electric vehicles and next-generation electronic devices has put forward the higher requirements for the battery systems.In addition to high energy density and long life,safety performance is also one of the factors that cannot be ignored.However,the traditional lithium-ion batteries use flammable organic electrolyte,which suffer from some safety issues,such as flammability,leakage and so on.Replacing liquid organic electrolyte with non-flammable solid state electrolyte is repected to resolve the problem fundamentally,and solid-state electrolytes possess the high mechanical strength,which can suppress the lithium dendrites and make the application of the lithium mental possible.Therefore,all-solid-state batteries has become one of the promising competitors of the nest-generation energy storage devices.Sulfide solidstate electrolytes are of increasing interest as solid electrolytes for all-solid-state batteries due to their high ionic conductivity and suitable mechanical properties.In recent decades,much efforts have been made on the modification of sulfide-based solid electrolytes,such as enhanced air stability,increased ionic conductivity,and improved interfacial stability.However,sulfide-based solid electrolytes are still confronted with many challenges from laboratory to commercialization,including 1)narrow the voltage window,2)poor compatibility of electrode-electrolyte interface,and 3)less effective methods for large-scale fabrication.This thesis mainly aims at the problems of sulfide electrolyte,including low ionic conductivity,poor interfacial compatibility,difficult preparation of ultrathin electrolyte membrane and the incompatibility between electrolyte membrane and electrode.High performance sulfide solid electrolyte was prepared by coating solid electrolyte,modifying the preparation process,exploring the preparation method of electrolyte film and optimizing the interface structure.The main research contents are as follows.（1）Sulfide electrolyte Li₇P₃S₁₁（LPS）was prepared by liquid-phase synthesis,and during the preparation of LPS electrolyte,the sulfide electrolyte was modified by adding graphene oxide（GO）to obtain GO-coated LPS electrolyte.Due to the presence of GO protective layer,the interfacial reaction between LPS and lithium metal is restincted,and the growth of lithium dendrites inside the solid electrolyte is also inhibited.The assembled Li/1%GO@Li₇P₃S₁₁/Li symetrical batteries can cycle for more than 600 h at 0.1 mA cm^-2,which is much better than that of Li/LPS/LPS cells（40 h）.In addition,the assembled all-solid-state battery also exhibits excellent electrochemical performance.The assembled all-solid-state battery with graphite as the anode exhibits high initial reversible capacity and excellent cycling stability.After 50 cycles,it still maintains a specific capacity of 260 mAh g^-1 and a capacity retention of 75%.The assembled all-solid-state battery with LiCoO₂ as cathode remains a discharge capacity of 80 mAh g^-1 after 100 cycles at 0.1C,with a capacity retention of 72.7%.（2）Sulfide electrolytes with good interfacial stability and high ionic conductivity were prepared by a two-step sintering method.By controlling the temperature and time of secondary sintering,the desity of solide electrolyted under different conditions was explored and thesulfide electrolytes with ionic conductivity(8.4 mS cm^-1)was obtained.Thanks to the secondary sintering,the density of the solid electrolytes can be enhanced and the exsists of poor ionic conductors is reduced,and thus the ionic conductivity is significantly higher than that of the other sulfide electrolytes（Li₂S-P₂S₅ binary system）.Besides,the compatibility with lithium metal is obviously enhanced,and the Li-Li symmetric battery with the secondary-sintered LPS can operate stably for 1000 h at a current density of 0.175 mA cm^-2 at room temperature.The all-solid-state lithium battery has excellent cycling performance with a capacity retention of 90.4%after 100 cycles at 0.1 C at room temperature.（3）Taking advantage of the fibrillized characteristics of PTFE,thin sulfide solid electrolyte film(～40μm and 8.5 mS cm^-1 at 25℃)and cathode film（～60 μm）are prepared by a solvent-free method.Because the good flexibility of the thin film enhances the interface contact,and the fibrillized PTFE can connect all parts of the cathode together and maintain structural integrity during the charge-discharge process,the ASSBs assembled with the all-dry processed electrolyte film and cathode exhibit an initial discharge capacity of 160 and 110 mAh g^-1 at 0.1 and 1 C,respectively,with ultra-long cycle life with excellent capacity retentions of 91.4%and 86.4%after 100 and 1000 cycles,respectively.Furthermore,all-solid-state pouch cells show excellent cycling performance with a reversible capacity of 132.6 mAh g^-1 and can cycle stably over 100 cycles.（4）The homogeneous LiI layer was prepared as a protective layer on the surface of Li metal anode by a simple in-situ solid-gas reaction method.Lithium iodide（LiI）is an environmentally friendly and chemically stabile lithium ions conductor,and is highly compatible with both lithium metal and solid-state electrolytes.The lithium iodide layer can improve the lithium-ion transport efficiency at the interface of Li/sulfide electrolyte film.Meanwhile,it can effectively inhibit the reaction between lithium metal and electrolyte.The LiI@Li/LPSCl/LiI@Li symmetrical cells can operate stably over 150 h at 0.1 mA cm^-2.The ASSBs with the Lil interfacial layer exhibit an initial discharge capacity of 200 and 107 mAh g^-1 at 0.1 and 1 C,respectively,and display ultra-long cycle life with excellent capacity retentions of 84.8%and 76%after 100 and 800 cycles,respectively.DFT calculation further confirms that the LiI layer can block the lithium dendrite permeation and exhibit high compatibility with lithium metal and sulfide electrolyte.