In-situ Preparation Of Polymer Electrolytes And The Application In Lithium Metal Batteries

The development of the current lithium-ion batteries（LIBs）is restricted by theoretical energy densities and safety.Lithium metal batteries（LMBs）employing solid electrolyts can circumvent some of the drawbacks of the traditional LIBs.Inorganic solid electrolytes（ISEs）show superior bulk conductivities but suffer from poor interfacial contacts with electrodes and difficulty in processing.In comparison with ISEs,polymer electrolytes（PEs）show greater flexibility and higher processability and have been widely researched.However,the solution-casting method（ex-situ polymerization method）is usually employed to prepare PEs,which requires a lot of toxic organic solvents.In addition,the irreversible side reaction between residual solvent and electrodes may worsen the electrode/electrolyte interface and furtherly leads to the poor electrochemical properties of batteries.The process of preparing PEs inside the batteries is usually referred to as the in-situ polymerization,which not only effectively solves the problems of ex-situ polymerization method but also decreases the interfacial resistance.What’s more,the in-situ preparation approach is compatible with the manufacturing process of commercial LIBs.Nevertheless,the reported in-situ methods of preparing PEs possess yet intrinsic shortcomings,for instance,irreversibly parasitic reactions between additional initiators and electrodes,the poor compatibility of PEs with a high-voltage cathode and harsh conditions of the polymerization reaction.The systematic researches aiming at the above problems are carried out in this thesis.The main contents are as follows:（1）A method of in-situ preparing the poly（ε-caprolactone）-based polymer electrolyte（PCL-PE）inside the battery is proposed,employing stannous octoate（Sn（Oct）₂）as a catalyst.The addition of propylene carbonate further improves ionic conductivity of PCL-PE.Specially,a Li Sn alloy layer is in-situ formd on the surface of lithium metal（Li）anode through the reaction of Sn（Oct）₂with Li,resulting in the uniform lithium plating.Thanks to the the conformal electrode/electrolyte interface formed by the in-situ solidification approach,the semisolid-state Li/Li Fe PO₄（LFP）cells deliver a discharge capacity retention of 72%over 150 cycles at 60°C.What is more,the in-situ assembled Li/LFP cells also sustains 150 m Ah g^-1 after 100 cycles under 30°C.（2）A method of in-situ preparing PCL-Li_5.5PS_4.5Cl_1.5(LPSCl_1.5)composite electrolytes is proposed,which effectively improves the the ion conductivity of PCL-PE.Specially,LPSCl_1.5 is able to induce the polymerization ofε-caprolactone（CL）-based precursor solution.In addition,a lithium ion（Li⁺）-conductive PCL interlayer is in-situ formed on the surface of LPSCl_1.5 based on the above polymerization mechanism.PCL interlayer improves the electrochemical stability of LPSCl_1.5 and Li metal.（3）A new“polymer-in-salt”（PISE）system based on poly（ε-caprolactone and DL-lactide）is in-situ prepared.The strategy effectively improves the ion conductivity of PCL-PE and compatibility with 4 V cathode.The ionic conductivity of PISE is 220 times higher than PCL-PE and the electrochemical window is about 5 V.The in-situ assembled Li/PISE/NCM622 batteries（4.3 V,vs.Li⁺/Li）display impressive cyclability with capacity retention of 92.5%over 160 cycles at 30°C.（4）An initiator-free,simple and efficient method of in-situ preparing PE—electronic beam（e-beam）irradiation—is developed.The in-situ assembled solid-state Li/LFP cells deliver a high discharge capacity retention of 85.0%over 270 cycles.In addition,e-beam can penetrate through aluminum-plastic film to induce polymerization,which directly leads to the transformation from the liquid-state pouch cells to the solid-state.The solid-state Li/LFP pouch cells exhibit excellent cycling performance and superior safety.