Among so-called “next generation” battery technologies, lithium metal batteries (LMBs) enabled by solid-state electrolytes are considered key to achieve rechargeable batteries with higher energy density and safety than current lithium ion batteries (LIBs). This article briefly evaluates various aspects of polymer electrolytes from history, macromolecular architecture, material classification, and electrode optimization, with special emphasis on solid polymer electrolytes (SPEs) and single ion conducting polymeric electrolytes. Representative interfaces and interphases as well as corresponding engineering strategies adopted for the anticipated goals are briefly summarized, including various approaches adopted to mitigate the shortcomings at the interfaces. Significant weight should be given for research and development of SPEs, as they could be an enabler for solid-state LMBs with attractive performance and made by comparatively easy electrode and cell processing techniques.

Single ion conducting polymer electrolytes (SIPEs) comprised of homopolymers containing a polysulfonylamide segment in the polymer backbone are presented. The polymer structure contains –C(CF3)2 functional groups that due to better solubility allow for effective lithiation, yielding well-defined materials. An optimized polymer electrolyte membrane was fabricated as a 3 : 1 blend of single ion conducting polymer and PVdF-HFP, which exhibits a high ionic conductivity of 0.52 mS cm−1 and an impressive lithium ion transference number of 0.9, as well as a 7Li self-diffusion coefficient of 4.6 × 10−11 m2 s−1 at 20 °C. The presented polymer electrolyte has superior oxidative stability and long-term stability against lithium metal, thus facilitating operation in LiNi1/3Mn1/3CO1/3O2 (NMC111)/lithium metal cells at 20 °C and 60 °C, thereby clearly demonstrating the application potential of this class of materials.

To unravel mechanistic details of the ion transport in liquid electrolytes, blends of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr₁₄TFSI), ethylene carbonate (EC) and dimethyl carbonate (DMC) with the conducting salts lithium hexafluorophosphate (LiPF₆) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) were investigated as a function of the IL concentration. Electrochemical impedance, Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) and Raman spectroscopy supported by Molecular Dynamics (MD) simulations allowed the structural and dynamic correlations of the ion motions to be probed. Remarkably, we identified that though the individual correlations among different ion types exhibit a clear concentration dependence, their net effect is nearly constant throughout the entire concentration range, resulting in approximately equal transport and transference numbers, despite a monitored cross-over from carbonate-based lithium coordination to a TFSI-based ion coordination. In addition, though dynamical ion correlation could be found, the absolute values of the ionic conductivity are essentially determined by the overall viscosity of the electrolyte. The IL/carbonate blends with a Pyr₁₄TFSI fraction of ∼10 wt% are found to be promising electrolyte solvents, with ionic conductivities and lithium ion transference numbers comparable to those of standard carbonate-based electrolytes while the thermal and electrochemical stabilities are considerably improved. In contrast, the choice of the conducting salt only marginally affects the transport properties.