Towards All-Extruded Custom-Shape Batteries

Abstract

The rapid growth of flexible electronics for wearable devices, on-skin or environmental sensors, and flexible displays have resulted in intensive research in the field of free form-factor energy-storage systems. Since batteries must withstand bending, folding, or stretching while maintaining their electrochemical performance, the design and processing of battery components, must be reconsidered. Moreover, there is a clear tendency to shift battery technology to clean and cost-effective dry methods with no toxic solvents. Dry fabrication methods are environmentally friendly, involve lower energy consumption, shorter time and are cost-effective. Hot-pressing and melting-extrusion strategies are appropriate for the fabrication of flexible, solid-polymer electrolytes and electrodes with high contents of active materials and high mass loading.Here we present the progress towards the simple, one-step extrusion of a solid-state battery with a multi-shell and layered architecture. Such structures fundamentally minimize the ionic pathway for Li+. As in the conventional battery, high ionic conductivity of extruded electrolyte and compatibility with electrode materials, are the essential properties for enabling high performance of the battery. The electrolytes are composed of LiTFSI, polyethylene oxide (PEO), which is a known lithium-ion conductor, and thermoplastic recyclable and biodegradable polymer- polyurethane (TPU) - for enhanced mechanical properties and high-temperature durability. The extruded electrolytes of different PEO-to-TPU ratios and salt content were characterized by means of ESEM imaging, mass spectroscopy, differential-scanning calorimetry (DSC), electrochemical-impedance spectroscopy (EIS) and solid-state nuclear magnetic resonance (NMR) diffusion measurements. The structure and electrochemical properties of extruded electrolytes were compared with cast films. It was found that TPU, having in its structure, both soft segments from polyol regions and hard segments from isocyanate not only improves the mechanical properties of neat PEO-based polymer electrolyte, but also forms complexes with lithium salt and serves as a conducting medium. Bulk conductivity vs. temperature dependencies obey Arrhenius behavior. The highest conductivity values varying from 0.3 to 1mS/cm over a temperature range from RT to 90ºC were achieved for the extruded 1:15 (Li:EO) solid polymer electrolyte. The NMR tests show that lithium self-diffusion coefficients in extruded PE with alumina filler are close to that in cast PE, but the anion diffusion coefficients (accessible through 19F NMR) are lower in extruded filaments, indicating a correspondingly higher Li+ transference number induced by the extrusion. Lithium cells with extruded LFP cathode and solid extruded polymer electrolyte demonstrated 40 reversible cycles with close to 96% coulombic efficiency. The electrochemical performance of half-cells composed of extruded LFP-based cathode and extruded TPU-MWCNT-based current collector will be presented.