Discharge behaviour and interfacial properties of a magnesium battery incorporating trihexyl(tetradecyl)phosphonium based ionic liquid electrolytes
Introduction
Metal-air cells offer great possibilities for inexpensive, reliable energy storage with higher energy densities than other types of battery chemistries [1]. Electrochemical energy conversion based on such a system is of great practical importance since the air electrode provides the battery with a free and virtually inexhaustible cathode reactant that is contained outside the battery cell. The specific energy and energy density of such systems can be very high (due to larger amounts of anode that can be incorporated into the cell, since oxygen is harvested from the atmosphere) and, as a result of this performance potential, researchers have developed several novel reserve battery cells for long life isolated and undersea operations [2], [3].
Several metallic anodes have been investigated for metal-air batteries. Aluminium, lithium, calcium, magnesium and zinc all have attractive energy densities for this application [1], [4], [5], [6], [7]. Of these potential candidates, zinc has received the most attention because of its relative stability in aqueous and alkaline electrolytes. Commercial primary zinc/air batteries are used in long term, low current discharge applications in a range of technologies including hearing aids, pagers, remote railroad signals and communications [8], [9].
The electrochemical properties of magnesium give it the potential for nearly three times the energy density of metal air batteries based on Zn. Furthermore, the enhanced safety and lower price of magnesium compensates for its lower energy density compared to Li-based batteries, and its elemental abundance and the overall benign nature of its compounds is significant enough to make it attractive as the negative anode for primary batteries [10]. A highly negative standard potential (−2.37 vs. NHE), high theoretical specific charge capacity (2.2 Ah/g), [11] high theoretical energy density (3.8 Ah/cm3) [12] and low cost are important features of magnesium as a battery anode. Its high potential allows for fewer cells to be used to achieve a desired output potential, thus more active material can be incorporated into each cell, which results in larger capacities. Alternatively, a cell of the same capacity as a comparable zinc alkaline cell can be made in a smaller size. Other desirable features, including ease of handling (when compared to lithium) and low toxicity, make magnesium an attractive potential candidate as a high-energy storage electrode.
While magnesium has successfully been used in a Mg/MnO2 battery, and high capacity cathodes based on MgxMOy or MgxMSy, where M = transition metal, have been investigated [13], one of the more intriguing systems is the Mg/air battery. However, the continuous discharge life of magnesium-air systems is generally short as the deposition of magnesium hydroxide by-product degrades the porous carbon air cathode [6]. Recent developments have led to more stable cathodes [14], [15], though magnesium hydroxide build up on the anode surface continues to affect battery lifetime [2], [3], [16]. The short shelf life of magnesium/air cells is mainly due to the high rate of localised corrosion of the magnesium anode at open circuit, which is detrimental to battery efficiency and electrical performance.
Magnesium-air batteries are yet to be successfully commercialised and there are few developed magnesium-air systems. They are used as primary [9], [17], [18] and reserve batteries [19], particularly in seawater applications using dissolved oxygen found in the water as the cathodic reactant. Most of these use alkaline solutions as an electrolyte, as neutral [20] and acidic [21] solutions show low stability and output voltage of the battery. Selecting a suitable electrolyte is necessary in ensuring a long shelf life and high electrical performance.
Our previous work has shown that electrolytes based on an ionic liquid (IL), trihexyl(tetradecyl)phosphonium chloride ([P6,6,6,14][Cl]), were capable of forming an amorphous gel-like interface on Mg that led to a level of passivation when the cell was at open circuit. The addition of water to non-aqueous electrolytes for Mg batteries has previously been shown to improve battery performance by Abraham [22]. Indeed, we have previously shown that H2O played an important role in the formation of the protective film on the Mg surface and in the operation of the cell. A discharge rate of 1 mA/cm2 and cell voltage −1.6 V vs. Ag|AgCl was achieved using an 8 wt% H2O solution in the [P6,6,6,14][Cl] IL [23]. This paper will further explore the properties of the Mg-interface that develops in this IL, hereafter referred to as the [P6,6,6,14][Cl] electrolyte. We will also discuss the behaviour of the oxygen reduction reaction in this electrolyte and the effect of water on this.
Section snippets
Experimental
Trihexyl(tetradecyl)phosphonium chloride ionic liquid, [P6,6,6,14][Cl], was received from Cytec Inc. (assay shows HCl: 0.8 wt%, R3P.HCl: 1.0%, tetradecylchloride C14Cl: 1.2% trihexylphosphine: 2.7%). In order to remove the acidic and other impurities, the ionic liquids were passed through a column of basic alumina and activated charcoal. All ILs were first dissolved in HPLC acetone (Sigma), and then passed through a column containing coarse SiO2 sand, activated charcoal, basic activated alumina
Results and discussion
In our previous work we showed that the concentration of water in the [P6,6,6,14][Cl] ionic liquid was critical in achieving high current densities and stable galvanostatic discharge on a Mg anode [23]. Furthermore, replacing the chloride anion with a dicyanamide ion (i.e., [P6,6,6,14][dca] plus 3 wt% H2O) led to a rapid shutdown of the galvanostatic discharge processes. Fig. 1 presents a comparison of the galvanostatic discharge of Mg in these two electrolytes. The data were rationalised in
Conclusions
A stable interfacial anodic film develops during the discharge of a magnesium cell in an electrolyte based on trihexyl(tetradecyl)phosphonium chloride. This electrolyte was previously shown to be a promising candidate for magnesium batteries, as the film was capable of stabilising the magnesium surface during galvanostatic discharge. Here we provide further evidence of the gel-like structure of the surface film and show that it is a highly hydrated complex based on the phosphonium cation,
Acknowledgements
This research was supported by the Australian Research Council through the Centre of Excellence program. The authors thank Dr Marianne Seter (Deakin University) for her assistance with Mass Spectroscopy and Dr Tony Hollenkamp (CSIRO) for his support and guidance throughout this research. Ionic Liquid received from Cytec Inc. is gratefully acknowledged.
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