Abstract
Polymer gel electrolytes have been prepared with polyethylene oxide (PEO) and the deep eutectic mixture of AlCl3: urea (uralumina), a liquid electrolyte which has proved to be an excellent medium for the electrodeposition of aluminum. The polymer gel electrolytes are prepared by mixing PEO in the liquid electrolyte at T > 65 °C, which is the melting point of PEO. This procedure takes a few minutes and requires no subsequent evaporation steps, being a solvent-free, and hence more sustainable procedure as compared to solvent-mediated ones. The absence of auxiliary solvents and evaporation steps makes their preparation highly reproducible and easy to scale up. PEO of increasing molecular weight (Mw = 1 × 105, 9 × 105, 50 × 105 and 80 × 105 g mol−1), including an ultra-high molecular weight (UHMW) polymer, has been used. Because of the strong interactions between the UHMW PEO and uralumina, self-standing gels can be produced with as little as 2.5 wt% PEO. These self-standing polymer gels maintain the ability to electrodeposit and strip aluminum, and are seen to retain a significant fraction of the current provided by the liquid electrolyte. Their gels’ rheology and electrochemistry are stable for months, if kept under inert atmosphere, and their sensitivity to humidity is significantly lower than that of liquid uralumina, improving their stability in the event of accidental exposure to air, and hence, their safety. These polymer gels are tough and thermoplastic, which enable their processing and molding into different shapes, and their recyclability and reprocessability. Their thermoplasticity also allows the preparation of concentrated batches (masterbatch) for a posteriori dilution or additive addition. They are elastomeric (rubbery) and very sticky, which make them very robust, easy to manipulate and self-healing.
Highlights
Energy storage systems like batteries have become a strategic market in the current wireless and hyper-connected society, and they are a pillar of the decarbonization process
The only method reported to date to prepare polymer gels with aluminum liquid electrolytes was in situ polymerization of acrylamide [12], it has been reported that polyethylene oxide (PEO) was moderately soluble in other deep eutectic solvents (DES) [23]
The nomenclature chosen is the following: PEOa-b/Uc, where a identifies the molecular weight of the polymer in g mol−1 divided by 105, b stands for the polymer weight fraction in the gel and c identifies the type of uralumina employed to prepare the gel
Summary
Energy storage systems like batteries have become a strategic market in the current wireless and hyper-connected society, and they are a pillar of the decarbonization process. Battery demand is calculated to double every five years. The energy demand of Li-ion batteries, the most mature technology, has risen from 22 GWh in 2010 to a total expected demand of 390 GWh in 2030 [1]. The massive production of Li batteries will presumably end up by producing a shortage of the raw materials required to produce these batteries. One of the alternatives to overcome such a scenario is the development of batteries employing more accessible raw materials. Aluminum secondary batteries can display a higher volumetric capacity than lithium (for instance in metal-air batteries ~2000 mAh cm−3 of lithium, and ~8000 mAh cm−3 of aluminum) and the raw Polymers 2020, 12, 1336; doi:10.3390/polym12061336 www.mdpi.com/journal/polymers
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