Elsevier

Electrochimica Acta

Volume 175, 1 September 2015, Pages 62-67
Electrochimica Acta

Sodium ion dynamics in a sulfonate based ionomer system studied by 23Na solid-state nuclear magnetic resonance and impedance spectroscopy

https://doi.org/10.1016/j.electacta.2015.03.131Get rights and content

Abstract

A poly(2-acrylamido-2-methyl-1-propane-sulphonate) (PAMPS) ionomer containing both sodium and quaternary ammonium cations functionalised with an ether group, has been characterised in terms of its thermal properties, ionic conductivity and sodium ion dynamics. The ether oxygen was incorporated to reduce the Na+ association with the anionic sulfonate groups tethered to the polymer backbone, thereby promoting ion dissociation and ultimately enhancing the ionic conductivity. This functionalised ammonium cation led to a significant reduction in the ionomer Tg compared to an analogue system without an ether group, resulting in an increase in ionic conductivity of approximately four orders of magnitude. The sodium ion dynamics were probed by 23Na solid-state NMR, which allowed the signals from the dissociated (mobile) and bound Na+ cations to be distinguished. This demonstrates the utility of 23Na solid-state NMR as a probe of sodium dynamics in ionomer systems.

Introduction

Presently, lithium ion devices are the largest commercially manufactured electrochemical storage medium [1], [2] and are used in a vast range of applications from small portable electronics to large aerospace electrical systems. The demand for energy storage is expected to increase in magnitude in the coming decades with suggested applications including widespread sustainable transportation for hybrid/electric automotive vehicles and large-scale stationary grid-storage for the integration of intermittent renewable energy technologies [2], [3]. It is now becoming increasingly recognised that lithium reserves may not be sufficient for expansion into such widespread or large-scale energy storage, so alternative and low-cost battery chemistries, including sodium ion based systems, are of significant interest.

Sodium is chemically similar to lithium and although its theoretical energy density is lower [4], it is one of the most abundant elements, hence is likely to be more cost effective. Furthermore the redox potential of sodium is only 0.3 V above that of lithium (E0 (Na+/Na) = −2.71 V vs. standard hydrogen electrode [5]) making it an excellent alternative electrochemistry candidate. Despite these promising characteristics, current sodium battery technologies including the commercially available sodium-sulfur (Na-S) [6], [7], and sodium-nickel chloride (Na-NiCl2) [8] ZEBRA (Zero-Emission Battery Research Activities) based cells, operate at extremely high temperatures and therefore can be challenging to implement in practical applications [9], [10]. These cells use a solid β-alumina ceramic electrolyte typically operating in the vicinity of 300 °C [9], [11]. At these temperatures the electrodes are molten and the β-alumina ceramic electrolyte acts as an electrically insulating separator, therefore reducing the risk of internal short circuits [5]. However, the energy density of these batteries is reduced as heaters and thermal insulation are required to maintain these high operating temperatures to ensure sufficient ionic conductivity of the sodium ions [12]. Alternative solid electrolyte materials, such as those based on polymers that can operate at ambient temperatures, could resolve these issues. However, compared to lithium-based materials there have been relatively few studies of sodium-based polymer electrolytes [13], [14], [15], [16], [17], [18], [19], [20].

To avoid the problem of cell polarisation which can result in device failure [21], solid polymer electrolytes in which only the Na+ or Li+ cation is the sole charge carrier can be manufactured by chemically binding the anion to the polymer backbone or side chain. Such systems are known as polyelectrolytes, or ionomers. However, these systems generate an additional problem as a result of this anion tethering, as interactions between the cation and the immobilised anion can reduce the cation mobility, typically leading to lower conductivities. Several potential approaches exist to reduce this association and to promote the mobility of the cation. One method is to design systems with anionic centres that have reduced affinity for ion association, e.g., the perfluoro-sulfonate group in Nafion, with its diffuse negative charge. Another way to reduce these interactions is to use a larger, more diffuse cation, and in this respect sodium may be advantageous over lithium. Finally, the incorporation of additives to solvate the cation can be used to promote the decoupling of the cation motion from the polymer and side chain dynamics, leading to high conductivities even below the glass transition temperature (Tg) of the polymer. Additionally, the Tg of the material will typically be reduced due to the weaker electrostatic interactions between the cation and tethered anion.

The addition of ionic liquid (IL) salt constituents into polymer electrolytes has been shown to improve ionic conductivity and overcome issues pertaining to the use of low molecular weight solvents in solid polymer electrolytes [22]. Colby et al. [23], [24] have demonstrated the effects of various organic IL cations in sulfonated polyester ionomers and sulfonated polystyrene. They demonstrated that the partial substitution of bulky IL quaternary ammonium cations into polymeric materials resulted in a significant reduction in Tg and a subsequent improvement in ionic conductivities.

Several sulfonate-based single-ion conducting ionomer systems have been developed including those based on poly(2-acrylamido-2-methyl-1-propane-sulfonate) (PAMPS) [22], [25], copolymers of sulfonate polyesters [26], [27], sulfonated ethylene and styrene systems [28] and styrene-ethylene and butylene-styrene systems [29], [30]. Very recently, Noor et al. studied a series of PAMPS-based systems containing both Na+ and the methyltriethyl ammonium cation (N1222, where the subscript numbers indicate the number of carbon sites in each alkyl chain) [31], [32]. In a co-polymer system with poly(vinyl sulfonate) (PVS), impedance measurements showed that the conductivities increased with decreasing Na+ content, with the highest values measured for the [PAMPS-N1222]0.9[PVS-Na]0.1 composition [31]. PAMPS homo-polymers were also studied with different Na+ content and different quaternary ammonium cation sizes [32]. Crucially, in these studies significant conductivities were measured below Tg, indicating the decoupling of the cation mobility from the polymer dynamics. 23Na NMR was used to probe the dynamics of the Na+ cation but analysis was difficult due to a lack of variation in the line widths. Earlier studies by Every et al. [33] involving the characterisation of polyelectrolytes based on the PAMPS ionomer with lithium and sodium salts have also been reported.

Watanabe et al. [34] have recently demonstrated the existence of Na+ complex formation in a series of sodium salts with tetraglyme or pentaglyme (i.e., CH3-(OCH2CH2-)xOCH3, x = 4,5), suggesting that the incorporation of ether group containing additives could lead to higher ionic conductivities in solid ionomer systems. In this contribution, we have incorporated a quaternary ammonium cation functionalised with an ether group (dimethylbutylmethoxyethyl ammonium, N114(2O1)) into a sodium-containing PAMPS homopolymer. The structure of the PAMPS monomer and this ammonium cation are shown in Fig. 1. The hypothesis is that the ether group will partially solvate the Na+, promoting its dissociation from the sulfonate groups and thereby leading to enhanced conductivities in this system. The cation dynamics are probed using a combination of impedance spectroscopy and 23Na solid-state NMR experiments, the latter of which reveals two distinct sodium cation environments experiencing different dynamics.

Section snippets

Sample Preparation

An ionomer system based on the poly(2-acrylamido-2-methyl-1-propane-sulfonate) (PAMPS) monomer and containing Na+ and dimethylbutylmethoxyethyl ammonium (N114(2O1)) cations in a molar ratio of 2:8 was synthesised according to the literature [31], [32]. This particular composition was chosen for two reasons. First, Noor et al. have recently shown that PAMPS samples with mixed Na and tetraalkylammonium cations generally show higher overall conductivities at lower sodium concentrations [32].

Thermal analysis

The DSC trace of the [N114(2O1)]0.8[Na]0.2 PAMPS ionomer reveals a single, broad endothermic transition centred at 67 °C (Fig. 2). This corresponds to the mid-point of the glass transition temperature Tg, and the onset for this transition was estimated as 56 °C. This is significantly lower than the Tgs reported for a previous series of Na-containing PAMPS-based ionomers recently studied by our group (93 °C for a co-polymer of PAMPS and poly(vinyl sulfonate) with Na+ and N1222 in a 2:8 ratio [31],

Conclusions

The sodium cation dynamics in the mixed-cation PAMPS ionomer system [N114(2O1)]0.8[Na]0.2 have been studied using impedance spectroscopy and 23Na solid-state NMR experiments. The glass transition temperature was determined as 56 °C, which is comparable to the lithium-containing analogue but significantly lower than another sodium-containing PAMPS ionomer featuring a smaller ammonium cation [N1222]0.8[Na]0.2, and this resulted in much higher conductivities in the system reported here (around 10−5 

Acknowledgements

The authors are grateful to the Australian Research Council for funding this work through the Discovery Project and Australian Laureate Fellowship programs (MF and DRM). Dr Jiazeng Sun (Monash University) is thanked for synthesising the sample. The Australian Research Council is also acknowledged for funding Deakin University's Magnetic Resonance Facility through LIEF grant LE110100141.

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