Synthesis of a Conceptual New Single-Ion Conducting Polymer Electrolyte for All-Solid-State Batteries

Abstract

The increasing global population and the rapid change in climatic conditions strengthens the demand for a more efficient handling of energy consumption and energy storage technologies. In this context, all-solid-state batteries (ASSBs) as next-generation rechargeable lithium-ion batteries offer an improved energy and power density based on the integration of novel separators and electrode materials. In comparison to their liquid representatives, ASSBs provide a higher intrinsic safety via non-flammable components and in addition greater long-term durability. In relation to common electrolyte classes, solid polymers stand out due to their good mechanical flexibility, easy film-formation ability, good contact supply between cell components, and lower production costs, favoring the application as electrolytes, protective coatings, as well as additives in cathode composites. Among a variety of polymers, poly(ethylene oxide) (PEO) is a well-studied and established host polymer due to its great ability to dissolve lithium salts like LiBH4, LiPF6 or LiTFSI. Based on a low glass transition temperature (Tg) of about - 60 to - 50 °C, PEO shows a high degree of polymer chain flexibility which facilitates the migration of the lithium cations through the solid electrolyte. Nevertheless, its semi-crystalline character often leads to low conductivities below its melting point (Tm) of around 60 °C, making pure PEO unattractive as solid electrolyte. Copolymerization, the addition of a small amount of solvent to produce so called gel polymer electrolytes, the addition of some plasticizers, or the interplay of polymer and inorganic fillers try to counteract this trend. The synthetical concept behind this work is aiming to benefit from the flexible nature of PEO, and at the same time to improve the ionic conductivities and especially the lithium migration by structural realignment away from a dual-ion towards a single-ion conducting polymer electrolyte (SICPE). The polar PEO-backbone ensures polymer chain mobility, whereas a rigid aromatic structure unit as side chain bears the fixed anionic group. The immobilization of the anionic charges on the polyether backbone tends to guide the electrolyte to higher lithium transference numbers, fewer polarization effects, and the suppression of dendrite growth, resulting in an overall improved cell performance. Synthesis steps via the novel SICPE are characterized with the help of nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Thermal analyses of the homopolymer already show the positive influence of the flexible PEO-backbone by lowering the Tg from 100 to 150 °C for related aliphatic derivatives down to 60 to 80 °C in our case. On the basis of electrochemical impedance spectroscopy (EIS) measurements, the interplay of chemical, thermal, and electrochemical key properties is investigated. The gained results tend to open a discussion panel concerning occurring challenges for solid-state polymer electrolytes and the interdependencies between electrolyte constitution and polymer characteristics.