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Novel alkoxy-ammonium based ionic liquids for reversible zinc electrochemistry
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posted on 22.02.2017by Kar, Mega
Modern large scale technology, such as electric vehicles, has put increasing pressure on the need to develop improved energy storage systems capable of sustaining these powerful devices. One efficient way to store electrical energy is using high energy density batteries such as those based on metal-air electrochemistry. Metal-air batteries such as zinc-air rely on oxygen being the cathode reactant. The oxygen can enter from the atmosphere rather than being stored within the battery, therefore allowing the active anode, i.e. zinc, to occupy more space in the device and increasing the storage capacity of the battery. In terms of metal selection for these devices, metals such as zinc are safer, more stable and cheaper, when compared to lithium and sodium.
While zinc-air batteries have the capacity to store a large amount of energy, they are disadvantaged by poor rechargeability. One of the main opportunities in metal-air battery research lies in the development of novel electrolyte systems that could improve the long-term cycling of the device. Such novel electrolytes are the topic of this thesis, focussing on solvents and ionic liquids containing ether based functional groups; the broad hypothesis of the work is that the ether functionality can provide some control over zinc coordination in an electrolyte and therefore facilitate zinc electrochemistry.
The physical and electrochemical properties of an ether based solvent, tetraglyme, containing increasing concentrations of zinc chloride, were characterised in Chapter 2. By allowing the zinc ions to coordinate with the ether-oxygen atoms of the tetraglyme molecule, it was hypothesized that the formation of any unfavourable by-products in the electrolyte, that result in less reactive zinc and a lower capacity for the battery, would be eliminated. It was demonstrated through the thermal and physical properties of the solutions that zinc ions do coordinate to tetraglyme. However, spectroscopic data indicated that this coordination competes with the formation of zinc-chloro species, implying that the concentration and the type of zinc salt used (i.e. the anion it carries) plays a significant role in the interactions taking place. Nevertheless facile zinc oxidation and reduction were observed from this ethereal solvent, which suggests that ether based systems such as tetraglyme are promising candidates for zinc batteries.
The concepts of Chapter 2 were then used as the basis for incorporating ether functional groups into an ionic liquid in order to improve the thermal stability, safety and the long-term cycling of zinc in the following chapters. Moreover the introduction of the electron-donating oxygen atom on one of the alkyl chains of the cation causes a weaker electrostatic interaction between the cation and the anion and reduces the viscosity of the ionic liquid. Much of the work presented in this thesis describes the synthesis and characterisation of alkoxy ammonium based ionic liquids that incorporate such ether based functional groups in the cation, as described in Chapter 3 and Chapter 4. Chapter 3 investigates the role of increasing the ether chain length, to optimise the number of oxygen atoms, and the optimum anion required to support reversible zinc electrochemistry. This work shows that ILs containing alkoxy-cations with shorter ether chains, such as [N222(20201)]+, with weakly coordinating anions such as [NTf2] produce positive electrochemical properties for rechargeable zinc battery applications. The use of longer ether chains in the alkoxy-cations of the IL, such as [N222(202020201)]+, was found to strongly chelate the Zn2+ ions, thus decreasing the ability of the metal ions to be reduced.
Chapter 4 then explores the role of ILs containing alkoxy-cations with multiple shorter ether chains with [NTf2]- based anions. It was proposed that, in addition to the mono-alkoxy cation [N222(20201)]+, the use of multiple short ether-based alkyl chains on the cation, such as the di-alkoxy [N22(20201)(20201)]+ and the tri-alkoxy [N2(20201)(20201)(20201)]+ based cations, could mitigate the tendency towards strong chelation observed with the single longer ether chain cations, such as [N222(202020201)]+.
Additionally, following recent findings in the literature on the role of additives in ionic liquid electrolytes for rechargeable zinc batteries, water was also added to the IL/Zn(NTf2)2 mixtures to further enhance the physical properties of the ILs synthesised in Chapter 4. The results revealed that all three RTILs acquire similar physical properties and zinc is successfully reduced and oxidized from all of the systems. However, the spectroscopic studies and physical property data suggest that water is acting mostly as a diluting agent, as no substantial changes could be observed in the 1H NMR and the FT-IR spectra.
A more extensive electrochemical study of Zn cycling in these electrolytes is described in Chapter 5 of this thesis. The morphology of the Zn deposits was observed using Scanning Electronic Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) was performed to confirm the composition of the deposits. Extended cycle tests showed that the tri-alkoxy based [NTf2]- IL electrolyte can undergo at least 750 charge and discharge cycles at 0.1 mA/cm2. It thus appears that these materials have potential as electrolytes for long-term cycling in secondary zinc batteries and other energy storage applications.