Tough Polymer Gel Electrolytes for Aluminum Secondary Batteries Based on Urea: AlCl3, Prepared by a New Solvent-Free and Scalable Procedure

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.

Understanding the Molecular Structure of the Elastic and Thermoreversible AlCl3 : Urea/Polyethylene Oxide Gel Electrolyte

“…The intensity of these bands is proportional to the concentration of PEO in the gel. [9] Figure 5 shows some resemblance between the spectra of PEO50-1/U150 and that of the PEO:urea IC, mainly the absence of the PEO backbone band at 1095 cm À 1 in both, and the existence in its place of several less intense bands at lower wavenumbers. The similarity between the FTIR spectra of the IC and the PGE suggests that the PEO chain in the gel is highly constrained as occurs in the IC.…”

“…Figure shows the FTIR spectra of the PEO50‐1/U150 gel (PGE), U150, PEO, and a PEO/urea 1 : 1 inclusion compound (IC). As discussed elsewhere, the most intense FTIR band of solid PEO is not seen in the PGE, namely the characteristic backbone band centered at 1095 cm −1 assigned to combinations of stretching C−C and C−O, and COC deformation modes, which are very conformationally‐sensitive . However, the CH 2 twisting bands at 1241 and 1278 cm −1 , which are far less intense than the 1095 cm −1 band, are hinted in the PGE's spectrum (red rectangle in Figure ).…”

“…The gel PEO50‐1/U150 is prepared by mixing the powdery PEO with U150 and stirring the mixture while increasing the temperature up to 70 °C. When the polymer melts, viscosity increases conspicuously because of the mixing of the long polymer chains with the deep eutectic solvent . This preparation process takes a few minutes and requires no other steps, contrary to procedures where auxiliary solvents are used.…”

“…The PEO : urea 1 : 1 inclusion compound (IC), used as a reference for the interpretation of the FTIR spectrum of PEO50-1/U150, was prepared as reported in the literature [9] by adding the desired quantity of urea to 100 mL of MilliQ water. Under vigorous stirring at 70°C, 0.5 g of PEO were added stepwise.…”

“…In a recent work, [9] we proposed a solvent-free strategy consisting of the melting while mixing of polyethylene oxide (PEO) in AlCl 3 :urea electrolyte to prepare PGEs. Deep eutectic solvents prepared with AlCl 3 and urea at molar ratio of 1.5 : 1 (called U150 hereafter), have proved to be an excellent medium for the electrodeposition of aluminum.…”