Studies On Engineering And Interfacial Compatibility Of In-situ Gel Electrolytes For Lithium Batteries

Traditional commercial lithium-ion batteries with liquid electrolytes still suffer from safety hazards in the case of internal battery failure,package damage,abnormal charge and discharge,and elevated operating temperature because of their low thermal stability,volatile,flammable and explosive features.Replacing liquid electrolytes with solid electrolytes would overcome the shortcomings of conventional liquid electrolytes and greatly improve their safety.Gel polymer electrolytes（GPEs）possess the advantages of safety of all-solid electrolytes and high ion conductivity of liquid electrolytes which is a promising direction for industrialization.However,the development of quasi solid state lithium batteries based on gel electrolytes is limited by key issues including the incompatibility of production processes with existing battery manufacturing processes and poor electrolyte-electrode interface stability.From the perspective of skeleton design,the quasi-solid-state lithium battery with high energy density and long cycle life can be obtained by adjusting the polymer skeleton structure to improve the interfacial stability of electrolyte-anode and electrolyte-cathode.Main research contents and results are as follows:（1）The gel electrolyte with combined polymer skeleton of linear polyethylene glycol dimethacrylate（PEGDMA）monomer and polybranched pentaerythritol tetraacrylate（PETEA）monomer is applied to construct solid state lithium batteries.After optimizing the monomer ratio and polymer contents,the ionic conductivity at room temperature could reach 7.6 m S cm^-1.The in-situ GPE can construct a stable interface on the surface of Li metal anode and guarantee a long cycling life with capacity retention rate of 92.3%after 500 cycles at 0.5 C in the Li Fe PO₄|GPE|Li battery.（2）In the consideration of regulating the structure of electrolyte/anode interface,nonpolar triallyl cyanurate with ring-centre structure of-C=N coupling with pentaerythritol tetraacrylate as cross-linking agents are applied to construct in-situ synthesis of GPE.The micro-electric field created by TAC after polymerization provides uniform adsorption sites for Li⁺flow and induce its even distribution according to the corresponding adsorption model and charge distribution.This interface modeling path displays high ionic conductivity(7.93 m S cm^-1)at room temperature and ensures a long cycling life with capacity retention rate of 80.6%after 700cycles at 1 C in the Li Fe PO₄|GPE|Li battery.（3）In view of the limited stability of the gel electrolyte-negative electrode interface,polymer monomer containing-CF₃ terminal group trifluoroethyl methacrylate（TFMA）and PEGDMA are applied to construct gel electrolyte with consideration of regulating stability and reactivity of polymer molecules.The polymer-assisted SEI can remain stable during cycling,and the-CF₃ group with certain reactivity reacts with metallic lithium to form a Li F-rich SEI,which is helpful to improve the stable cycling of quasi-solid-state lithium metal batteries.The ionic conductivity of the polymerized gel electrolyte can reach up to 4.6 m S cm^-1 at room temperature,and ensures a long cycling life with capacity retention rate of90.0%after 1000 cycles at 0.5 C in the Li Fe PO₄|GPE|Li battery.The reactivity and stability of the polymer backbone were further adjusted by introducing F contained end groups,and the possible reaction mechanisms with lithium ions are calculated according to the reaction energy of the relevant reactions.The TTEMA GPE proposes a more stable cycling process with a capacity retention of 93.7%at 0.5C and 87.6%at 1C after1000 cycles with Li Fe PO₄ cathodes.（4）In order to improve the stability of the gel electrolyte in high-voltage battery systems,-CF₃ end groups contained polymer monomer TTEMA and multi-branched PETEA as cross-linking agents are applied to construct in-situ synthesis of GPE.The Li F-rich phase SEI assisted by this polymer on the lithium anode ensures the uniform deposition of lithium ions while maintaining stability.The organic CEI formed on the positive electrode side coats the positive electrode surface to prevent the corrosion of the positive electrode material from HF.The interaction between the polymer skeleton and lithium salt anions along with solvent molecules prevents the continuous decomposition reaction when the electrolyte contacts the Li Co O₂ cathodes at high voltage.The quasi-solid-state lithium battery assembled with the Li Co O₂ can achieve a capacity retention of 82.1%after 500 cycles at a voltage of 4.45V in 0.2 C.There are 95 figures,13 tables and 269 references in this dissertation.