Stable zinc cycling in novel alkoxy-ammonium based ionic liquid electrolytes
Introduction
Room temperature ionic liquids (RTILs) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] have attracted great attention as potential electrolytes in the development of advanced battery technologies. Their characteristic properties of high conductivity, low volatility and low flammability would be ideal for reversible zinc-air batteries, where currently the aqueous alkaline conditions of the electrolyte are responsible for many of the failure mechanisms of the primary device. Zinc-air batteries consist of a zinc anode with the cathode reducing oxygen. The electrochemical reactions occurring in this battery are shown in Scheme 1 [11], [12], [13], [14], [15]. When the battery is discharged in the presence of an alkaline electrolyte, zinc hydroxide is produced, which is converted into insoluble zinc oxide (ZnO) when the solubility limit of the hydroxide species is reached. The formation of solid ZnO can be a difficult process to reverse during recharge, as it relies on the resolubilization of the Zn species back into the electrolyte prior to reduction [16]. In addition, CO2, which also enters from the atmosphere through the pores of the cathode, reacts with the aqueous electrolyte to form insoluble bicarbonate (HCO3) and carbonate (CO32) salts, blocking the electrode-electrolyte interface and preventing long-term cycling of the battery [17].
Recent literature has demonstrated that the electrochemical deposition of zinc from various RTILs is possible and offers advantages over organic and aqueous electrolytes, which suffer from issues such as volatility, instability and flammability [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. RTILs can help overcome the above limitations of zinc electrochemistry in a number of ways. Firstly, in the context of a Zn-air cell, the use of the non-volatile RTIL instead of the aqueous electrolyte eliminates the possibility of evaporation, preventing the cell from drying out [23]. Secondly, the structural properties (i.e. molecular interactions of the cations and anions) of RTILs are tuneable, whereby attaching certain functional groups can enhance coordination to the zinc ions in the electrolyte and hence avoid formation of oxide/hydroxides or other undesirable by-products. Manipulation of the donor atom/Zn2+ interactions has been shown to play a large role in determining whether Zn reduction produces a smooth, non-dendritic morphology [22]. This concept is well known and exploited in the traditional electroplating industry by the addition of organic and inorganic compounds (known as brighteners) to electroplating baths [28], [29]. The addition of solvents or additives can also enhance the mobility of zinc ions in an RTIL, resulting in higher ionic conductivity (and hence Zn deposition and stripping currents). Currently, the electrochemical performance of a rechargeable zinc-air battery in RTILs is being studied by a number of research groups [22], [23], [28], [29], [30], [31]. However, this work is still very much in progress.
It is commonly known that due to their Lewis basicity, the oxygen atoms in ethereal solvents such as tetraglyme, CH3O-(-CH2CH2O-)4-CH3, are able to strongly bind to Lewis acids such as Li+ and Zn2+ ions [32], [33], [34], [35], [36]. Recent advances have shown that incorporating such ethereal functional groups into the cation can improve the physical and electrochemical properties of RTILs [37], [38], [39], [40]. In our previous work on designing novel electrolytes for zinc-air batteries, the inclusion of various oligo-ether side chains into a quaternary ammonium cation, with bis (trifluoromethylsulfonyl) imide [NTf2] as the anion, was found to enhance the coordination and solubility of zinc ions and enhance the deposition and stripping reactions [40]. It was demonstrated that while quaternary cations with one long oligo-ether chain-length produced a lower viscosity than the shorter chain side groups, zinc electrochemistry was better facilitated in the latter RTIL. This suggests that, upon the addition of zinc, the RTIL with the longer ether chain complexes more strongly to the metal ion, increasing the overall viscosity and decreasing the ability of the Zn2+ ions to be easily reduced. Therefore to maintain a similar low viscosity, while also further increasing the mobility of the Zn2+ ions to better facilitate their electrochemistry, a related, novel family of quaternary ammonium RTIL cations with multiple oligo-ether based functionalised groups, such as the di-alkoxy [N22(20201)(20201)]+ and the tri-alkoxy [N2(20201)(20201)(20201)]+, in addition to the mono-alkoxy [N222(20201)]+ cation (See Scheme 1), were synthesized and studied in this current work. It is hypothesized that the use of multiple ether-based functional groups can eliminate the formation of any complex crown ether like cyclic structure that can arise from the use of a long oligo-ether type chains, and thus support the electrochemical reversibility of zinc [24]. The electrochemical behavior and long-term charge/discharge cycling of zinc in the three RTIL/Zn mixtures (Scheme 2) are also studied here. Given water will be present in a zinc-air battery from moisture, and is likely to improve the transport properties of the electrolyte, RTIL/Zn mixtures with added water were included in the studies.
Section snippets
Experimental
The synthesis of N,N,N-triethyl-2-(2-methoxyethoxy) ethanyl-ammonium bis(trifluoromethylsulfonyl) imide [N222(20201)] [NTf2], N,N-diethyl-2-(2-methoxyethoxy)-N-(2-(2-methoxyethoxy) ethyl) ethan-1-aminium bis(trifluoromethylsulfonyl) imide [N22(20201)(20201)] [NTf2] and N-ethyl-2-(2-methoxyethoxy)-N,N-bis(2-(2-methoxyethoxy) ethyl) ethan-1-aminium bis(trifluoromethylsulfonyl) imide, [N2(20201)(20201)(20201)] [NTf2] are described below:
Thermal Properties
The thermal properties are explored in Fig. 1, which shows the TGA curves for the pure ILs. The RTIL with only one oligo-ether side-chain is more stable than those with two or three oligo-ether side chains. [N222(20201)] [NTf2] starts to decompose from 350 °C while [N22(20201)(20201)] [NTf2] and [N2(20201)(20201)(20201)] [NTf2] start to decompose from approximately 320 °C and 300 °C respectively. These results are in agreement with Fang et. al.s work where they have shown the thermal stability
Conclusions
A new family of [NTf2] based RTILs, with cations [N222(20201)]+, [N22(20201)(20201)]+ and [N2(20201)(20201)(20201)]+ were synthesised and investigated as potential electrolytes for rechargeable zinc batteries. Increasing the number of ether functional groups on the cation from [N222(20201)]+ to [N2(20201)(20201)(20201)]+ predominantly results in a lower Tg and viscosity. However the latter RTIL exhibits a lower ionicity and therefore lower molar conductivity.
The addition of zinc to these RTILs
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2021, Journal of Molecular LiquidsCitation Excerpt :Recent developments have shown that integrating certain etheric functional groups into the cation will enhance the physical and electrochemical properties of RTILs [94]. In this background, a novel family of quaternary ammonium RTIL cations with multiple oligo-ether-based functionalized groups was studied to support zinc's electrochemical reversibility [79]. Interestingly, the application of different water concentrations does not move the 1H resonances back to the RTIL, which suggests that the water molecules do not appreciably interact with the zinc ion in these systems, according to the most sensitive spectroscopic techniques such as NMR (Fig. 2c).
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2021, Current Opinion in Green and Sustainable ChemistryCitation Excerpt :In some of these systems, the small amounts of additives have further enhanced mass transport and efficiency of metal batteries [53–55]. Owing to their compatibility with aqueous media, water as an additive in highly concentrated systems have especially been investigated in RZBs [15,56,57]. Ghazvini et al [58] reported the formation of electroactive zinc species occurring at high concentrations of zinc acetate (Zn(OAc)2) (>4M) in an RTIL, 1-ethyl-3-methylimidazolium acetate [C2mim][OAc], enabling Zn deposition/stripping.