Abstract
Gel electrolytes are prepared with Ultra High Molecular Weight (UHMW) polyethylene oxide (PEO) in a concentration ranging from 5 to 30 wt.% and Li- and Na-doped 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14-TFSI) by a simple procedure consisting of dissolving PEO by melting it directly in the liquid electrolyte while stirring the blend. This procedure is fast, reproducible and needs no auxiliary solvents, which makes it sustainable and potentially easy to scale up for mass production. The viability of the up-scaling by extrusion has been studied. Extrusion has been chosen because it is a processing method commonly employed in the plastics industry. The structure and morphology of the gel electrolytes prepared by both methods have been studied by DSC and FTIR, showing small differences among the two methods. Composite gels incorporation high concentrations of surface modified sepiolite fibers have been successfully prepared by extrusion. The rheological behavior and ionic conductivity of the gels have been characterized, and very similar performance of the extruded and manually mixed gels is detected. Ionic conductivity of all the gels, including the composites, are at or over 0.4 mS cm−1 at 25 °C, being at the same time thermoreversible and self-healing gels, tough, sticky, transparent and stretchable. This combination of properties, together with the viability of their industrial up-scaling, makes these gel electrolyte families very attractive for their application in energy storage devices.
Highlights
Solid electrolytes can make batteries much safer, because they prevent liquid electrolytes leaks which are potentially toxic or corrosive, but mainly because they mitigate or avoid dendrite growth and subsequent short-circuits
While in previous work [15,16] we explored melt compounding in the polymer concentration range from 50 down to 30 wt.% of compatible polymer electrolytes, in this work we explore the use of scalable processing procedures in the polymer concentration range from 5 to 30 wt.% for the solvent-free preparation of polymer gel electrolytes
Instead of gels, highly plasticized polyethylene oxide (PEO) electrolytes are prepared by extrusion, the first step is the mixture of the Li salt with PEO to ensure an intimate blending between the polymer and the salt which avoids crystallization of PEO, especially when the anion TFSI is present
Summary
Solid electrolytes can make batteries much safer, because they prevent liquid electrolytes leaks which are potentially toxic or corrosive, but mainly because they mitigate or avoid dendrite growth and subsequent short-circuits. Some authors suggested that dendrite mitigation in solid or quasi solid electrolytes is a mechanical mechanism in which the elastic modulus of the electrolyte plays a role [7,8], and as a matter of fact, in recent times, many strategies to avoid dendrites were largely based on this mechanism [9]. If elastic modulus is the key factor, inorganic solid electrolytes perform well against dendritic growth for they constitute a good mechanical barrier against dendritic growth, but they present poor solid—solid interfaces with the electrodes and are frequently mechanically fragile. Polymer-based electrolytes can be made to wet the electrodes excellently and are mechanically tough, but in homogeneous polymerbased electrolytes where ion diffusion occurs via a Brownian fluid motion, the higher the elastic modulus (or viscosity) of the polymer electrolyte, the lower the ion mobility and so electrochemical performance will worsen
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