Research On Interfacial Stability And Electrochemical Performance Of Solid-state Lithium-metal Polymer Batteries

With the continuous development of lithium-ion batteries,the application scope of lithium-ion batteries has gradually extended from portable electronic products to new energy electric vehicles and grid energy storage.However,the traditional lithium-ion batteries still have potential safety issues because of the flammability of liquid carbonate-based electrolytes.In recent years,fire and explosion accidents have occurred frequently due to electrolyte leakage and internal short-circuit of lithium-ion batteries.Solid-state batteries are composed of solid materials（solid-state electrodes,solid-state electrolytes）with intrinsic safety,which also provides an opportunity for the application of lithium-metal anode,and therefore energy density of battery can be improved significantly.The electrolyte materials,the problems and corresponding modification of electrolyte/electrode interface of solid-state batteries were summarized and analyzed.In order to pursue solid-state lithium-metal polymer batteries with high stability and high voltage,the specific research works about electrode/electrolyte interface were carried out as follows:Based on organic-inorganic composite solid-state electrolytes（CSSEs）,the effect of heat treatment on the electrochemical performance of 3 V-class lithium-metal polymer battery Li Fe PO₄/CSSEs/Li and the corresponding mechanism were studied.The organic-inorganic composite solid-state electrolyte films with uniform filler distribution and uniform thickness were prepared by the combining of solution casting and hot pressing.The effect of inorganic fillers Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）in composite solid-state electrolyte on the stability of cathode/electrolyte interface was studied.The effect of heat treatment on interfacial stability and electrochemical performance of Li/CSSE@20/Li symmetric battery and LFP/CSSE@20/Li solid-state battery were studied by electrochemical tests（electrochemical impedance spectroscopy and charge discharge test）and physical characterization（scanning electron microscopy and EDS）.After heat treatment process,the interfacial resistance of the solid-state battery decreases significantly and the behavior of Li⁺plating/stripping on the surface of lithium metal anode is greatly enhanced.What’s more,the ion conductive network in composite cathode is effectively improved.The capacity retention of the LFP/CSSE@20/Li solid-state battery with heat treatment is over 94%after 600 cycles at 1C.The LFP/CSSE@20/Li pouch cell with heat treatment shows high safety and stability.The capacity degradation mechanism and improvement strategy of the 4 V-class lithium-metal polymer battery Li Ni_0.8Co_0.1Mn_0.1/CSSEs/Li were systematically studied.By the comparison of battery cycling performance between solid and liquid（1 mol/L Li PF₆ in EC/DMC=1/2 v/v）electrolytes and the analysis about the influence of charging cut-off potential and working temperature on the cycle stability of solid-state battery,we learn that the capacity degradation of 4 V-class lithium-metal polymer battery is caused by the stability failure of cathode/solid electrolyte interface.The effect of the type of working electrode on the real electrochemical stability of the composite solid-state electrolyte was studied systematically.Fourier transform infrared spectroscopy（FTIR）,in-situ differential electrochemical mass spectrometry（in-situ DEMS）and X-ray photoelectron spectroscopy（XPS）were used to characterize the oxidation decomposition behavior of PEO-Li TFSI polymer electrolyte under high voltage.The oxidation decomposition of lithium salt and polymer matrix at high voltage results in the ion conduction barrier and the decrease of interface stability,accompanied by the appearance of gas products.Finally,a variety of effective strategies for interface modification were proposed,including optimizing the proportion of filler in the composite solid-state electrolyte,adjusting the type of lithium salt in the composite cathode,and using oxidation-resistant positive electrode binder,which can effectively improve the cycling stability of solid-state battery.As a result,the capacity retention of the solid-state battery with interface modification reaches 88%after 200 cycles in the voltage range of 3.0-4.2V with a current density of 30 m A/g.A novel high voltage electrolyte（SN-DLi-FEC）,dual-lithium salt（Li TFSI+Li ODFB）coupled with fluoroethylene carbonate（FEC）,was designed and prepared.The chemical and electrochemical stability tests of six SN-based electrolytes confirmed that SN-DLI-FEC electrolyte exhibits the best interfacial stability to lithium-metal anode.XPS results demonstrated that SN-DLI FEC electrolyte can form organic-inorganic composite solid electrolyte interface（SEI）film rich in B-F bond and Li F on the surface of lithium-metal through spontaneous chemical reaction.Li Co O₂/SN-DLi-FEC/Li batteries show excellent room temperature and high temperature cycling performances,rate performance,and good thermal stability.At room temperature,the capacity retention of Li Co O₂/SN-DLi-FEC/Li battery is up to 88%after 300 cycles in the voltage range of 3.0-4.4 V at0.5C,and discharging capacity at 10C rate still exceeds 140 m Ah/g.Furthermore,at 60^oC,the capacity retention of Li Co O₂/SN-DLi-FEC/Li battery reaches 98%after 50 cycles at 0.5C,and the corresponding pouch cell shows no obvious swelling behavior after heating for 5 h at 120 ^oC.At last,SN-based composite solid-state electrolyte（SN-CSSE）was prepared by thermal polymerization method,with glass fiber（GF）membrane as physical framework and ethoxylated trimethylolpropane triacrylate（ETPTA）as polymerization monomer.The room ionic conductivity of SN-CSSE electrolyte reaches0.78 m S/cm and the oxidation potential is up to 5.2 V.The Li/SN-CSSE/Li symmetric battery can be stably cycled for 140 h at a current density of 0.05 m A/cm².The initial discharging capacity of Li Co O₂/SN-CSSE/Li battery at 0.5C is 145.6 m Ah/g,and the capacity retention is more than 99%after 80 cycles.