Ana López-Cudero

The incorporation of small amounts of polymers into liquid electrolytes to produce gel electrolytes has many benefits. Decreasing, or even avoiding flow reduces accidents caused by leaks, allows for simpler and more flexible geometries and configurations, stabilizes electrochemical cycling, and may even permit the separate recycling of each cell component. Direct mixing of polymers with liquid electrolytes is very frequently possible, and it is a sustainable and scalable procedure that avoids evaporation stages and produces thermoreversible materials. In this work, some of the polymers most commonly used for gel electrolyte formationpolyvinylidene fluoride (PVDF), polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyvinyl chloride (PVC), poly­(methyl methacrylate) (PMMA), and poly­(ethylene oxide) (PEO)are employed to produce gels with the electrolyte NaPF₆ 1.1 m in diglyme, and their behavior and performance are described. All these polymers are soluble in the electrolyte, and the ultrahigh molecular weight (UHMW) PEO and the halogenated polymers are able to form self-standing gels at concentrations ranging from 5 to 10 wt %. The fluorinated polymer gels left standing in an Ar glovebox progressively become colored, turning almost black after one month, showing a lack of long-term chemical stability. PVC 10 wt % and UHMW PEO 5 wt % gels, however, become hard gels with no macroscopic phase separation and remain stable for periods of months. Their electrochemical stability against Na electrodes was tested, showing that the PVC gel is not stable, and only the UHMW PEO 5 wt % gels were further tested with Prussian white (PW) cathodes. The PEO gel electrolyte, without any physical separator, showed equivalent performance in PW||Na half-cells as the baseline electrolyte with a glass fiber separator, while also being self-standing and solid-like in consistency. The PEO gel electrolytes also showed applicability to PW||HC full cells. Moreover, after electrochemical testing, the coin cells with PEO gels were easily disassembled, and their components were recovered.

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.

Currently, according to conventional charge–discharge profiles, energy consumed in charging Capacitive Deionization (CDI) systems is always a function of different parameters (current used for charging, capacitance and current employed for discharging) making it difficult to separate the effect of these parameters on CDI performance and energy efficiency. Thus, energy efficiencies are strongly influenced by the current in the preceding charge or discharge stage of the process. We find consistently that this phenomenon, which to our knowledge has not been addressed in previous CDI communications, is much more intense when different currents are applied for each of the charging and discharging cycles. The investigation reported here provides a mechanistic analysis of the operational aspects of CDI and develops a new procedure that allows for a precise evaluation of performance and energy efficiency. Furthermore, the model developed here allows one to separate charge and discharge cycles, and therefore contributes to the possibility of defining an operational mode for real-world devices in which effective separation of deionization and regeneration steps needs to be implemented. This method of analysis could be useful not only for CDI but also for other electrochemical systems such as in secondary batteries and supercapacitors where charge and discharge are typically employed.