Polymer electrolytes may enable next generation batteries that contain lithium (Li) metal as anode, providing higher energy densities than state-of-the-art Li ion batteries. During charging and discharging, Li metal is often inhomogeneously deposited and dissolved, respectively, thereby promoting side reactions and losses of available Li inventory, eventually reducing the cycling life of the battery. Here, polymer electrolytes may contribute to safer and more durable Li metal batteries due to reduced volatility compared to solvents used in liquid electrolytes and tunable mechanical properties of the electrolyte membrane. Ion transport in the bulk electrolyte and at the electrode|electrolyte interfaces governs charging/discharging performances. Despite decades of research, the interdependencies of interfacial variation and key properties of the electrolytes are not yet fully understood. Also, the applicability of polymer-based Li metal batteries in modern technology (e.g., in electric vehicles) is often presumed, but seldomly considered in depth. Hence, polymer-based Li metal batteries were analyzed in this thesis on different levels, ranging from microscopic charge transfer to the evaluation of the application potential of such batteries in state-of-the-art electric vehicles. Various polymer electrolytes were characterized with electrochemical impedance spectroscopy and distribution of relaxation times (DRT) analysis. Characteristic time constants reflecting both charge transfer and interphase resistances could be identified, depending on the operation temperature and type of electrolyte (dry/gel/liquid). Also, combining DRT and X-ray microtomography, dynamic changes of occurring Li microstructures could be monitored and correlated with experimentally accessible observables, such as the interphase resistances and the associated time constants. Strongly decreasing resistances were assigned to a growth of microscopical surface area, while a long-term increase of interphase resistances was associated with the growth of globular Li metal deposits, that often nucleate at electrolyte impurities. In addition, in situ⁷Li NMR spectroscopy was employed to resolve Li microstructures evolving on pristine and modified Li metal electrodes, corroborating the beneficial impact of Si-based coatings on cell cycle life and Li reversibility by promoting the formation of inorganic-rich solid electrolyte interphases. Invoking custom-made micro electrodes, the charge transfer kinetics between Li metal anodes and polymer electrolytes was characterized based on cyclovoltammetry, explicitly determining the exchange current densities, a fundamental equilibrium current density that limits reaction rates. Similar to liquid electrolytes, the charge transfer kinetics against Li metal could be modeled based on a Marcus-Hush type theory of charge transfer, though yielding substantially lower exchange current densities compared to liquid electrolytes. Lastly, the corresponding application potential of polymer-based Li metal batteries was evaluated, using a custom-made software, that enables projection of energy densities of multi-layered pouch cells based on parameters available and accessible from common experimental cell setups. Here, high cathode mass loadings (> 1 mAh cm^-2), thin excess Li metal anodes (< 20 μm) and thin electrolyte membranes (< 30 μm) constitute key strategies to further promote applicability. When implemented in state-of-the-art battery packs for electric vehicles, energy densities of up to 422 Wh L^-1 could be projected, clearly highlighting the future potential of polymer-based Li metal batteries.