Elsevier

Electrochimica Acta

Volume 247, 1 September 2017, Pages 983-993
Electrochimica Acta

Research Paper
Properties of High Na-Ion Content N-Propyl-N-Methylpyrrolidinium Bis(Fluorosulfonyl)Imide -Ethylene Carbonate Electrolytes

https://doi.org/10.1016/j.electacta.2017.07.017Get rights and content

Abstract

Sodium-based batteries have emerged as promising alternatives to Li-based batteries for future safe, high energy-density energy storage. They are expected to be cheaper, due to the greater abundance of Na and likely reduction in raw material costs. In this study, we investigate the properties of superconcentrated sodium bis(fluorosulfonyl)amide (NaFSI) mixtures with the ionic liquid (IL) methylpropylpyrrolinium (C3mpyr) FSI in the presence of ethylene carbonate (EC) in the liquid and gel states. Ionic conductivity and thermal stability are evaluated through electrochemical impedance spectroscopy (EIS) and differential scanning calorimetry (DSC), respectively. NaFSI is soluble in the IL up to 55 mol% Na; adding EC (30 wt.%) to the IL almost doubles the ionic conductivity at ambient temperature. The temperature dependence of conductivity is well described by the Vogel-Tamman-Fulcher equation. NMR spectroscopy and Pulse Field Gradient NMR diffusion were employed to investigate transport in these electrolyte systems, while the chemical interactions were also studied using ATR-FTIR. Stable plasticized gel electrolytes were observed, even at 30 wt. % EC; the formation of the gel does not significantly affect the liquid-like ion dynamics in these materials, as shown by DSC and FTIR analysis. The Na+ transference number of Na0.55[C3mpyr]0.45[FSI] + 30 wt.% EC was up to 0.32, and deposition and dissolution of sodium metal were observed in cyclic voltammetry around 0 V vs. Na/Na+. Moreover, the suitability of the prepared electrolyte is preliminarily verified in half-cells at room temperature using Na3V2(PO4)3 as a cathode. The cells delivered capacity of 52.4 mAhg−1 at C/20.

Introduction

The growing use of portable electronic devices such as cell phones, laptops and tablets, as well as hybrid electric cars, is driving the development of the next generation of rechargeable batteries. Extensive research has been done on battery materials that produce better performance such as high energy density, benign environmental impact and long cycle life [1]. While LiB are currently further advanced and more widely implemented, the room temperature sodium battery has recently been investigated by many researchers seeking an alternative to lithium, because of the greater abundance of sodium, which should lead to cheaper batteries, especially for larger scale applications such as grid storage [2]. Although the sodium ion is slightly bigger and heavier, its diffusivity in organic ionic solvents is higher than that of the lithium ion [3].

Ionic liquids (ILs) have attracted great interest in recent years as a new class of electrolyte materials, and have been incorporated into a wide variety of electrochemical devices, where they provide a wide electrochemical window ( > 5 V) and high thermal stability (up to 300 °C) [4], [5], [6]. IL-based electrolytes have also been investigated for sodium batteries to improve safety and performance [7], [8], [9]. Of the various ILs studied, those which include the bis(fluorosulfonyl)imide (FSI) anion have gained significant attention for their relatively low viscosity and high conductivity [10], [11]. This anion was first introduced by Armand et al., and organic liquids containing LiFSI were successfully used as lithium battery electrolytes [12], [13].

Yoon et al. used FSI-based ILs in their lithium-conducting electrolytes, and found that mixtures containing Li salt concentrations up to 3.2 mol/kg can successfully cycle Li/LiCoO2 cells with fast charging and discharging capability [14]. In addition, Ding et al. also reported good sodium battery performance with a 0.8 mol.kg−1 NaFSI in C3mpyrFSI electrolyte (approximately 20 mol% NaFSI) and they subsequently also demonstrated a wider concentration range of NaFSI mixtures with C3mpyrFSI. In general, superconcentrated Na salt − IL mixtures are found to have elevated viscosity, which is usually accompanied by a decrease in the ionic conductivity at low or moderate temperatures. The presence of a diluent or plasticizer (an organic solvent such as ethylene carbonate (EC) or propylene carbonate (PC)) is well known to increase the ionic conductivity [15], [16], [17]; this approach is widely used in polymer electrolytes [18], [19], [20]. In the present work we investigate the use of EC as a diluent in NaFSI − C3mpyrFSI mixtures.

Another main challenge in the electrolytes area is the lack of safe and conductive solid-state Na-ion electrolytes to overcome the problems of leakage and ease of handling in cell design for electrochemical devices. We have prepared gelled electrolytes by physical gelation of the superconcentrated IL electrolytes with nano-size fumed silica, as reported in our previous study [21]. In the present study we investigated the effects of adding ethylene carbonate on the chemical interactions and electrochemical properties of both the superconcentrated liquid IL and gel electrolytes. We used NMR spectroscopy and Pulsed Field Gradient NMR (pfg NMR) to investigate transport in these electrolyte systems.

Section snippets

Preparation

C3mpyrFSI (99%) and NaFSI (99.9%) from Solvionic; silica, fumed powder, (≈ 0.007 μm) from Sigma Aldrich; and dichloromethane (DCM) from Systerm Chem AR were employed in this research without further purification. The sodium salt was added to the IL at various concentrations (20 to 55 mol%, as detailed in Table S1). The concentrations are denoted as Nax[C3mpyr]1-x [FSI] where x indicates the mol fraction of NaFSI. All solutions were stirred at room temperature until the salt had fully dissolved

Chemical Interactions via Fourier Transform Infra-Red (FTIR)

To understand the nature of the interactions in the various IL and IL-gel systems, we investigated their spectroscopic features. The characteristic FTIR bands of the FSI anion and the ether groups in EC are the main focus for detection of ionic interactions. These regions show interesting changes when the cations are coordinated with the FSI ion or the ether oxygen. Fig. 1 compares the infrared spectra of pure C3mpyrFSI, pure EC, IL:EC, Na0.55[C3mpyr]0.45[FSI], Na0.55[C3mpyr]0.45[FSI] + 30 wt.%

Conclusion

The effect of EC as a diluent in superconcentrated Na0.55[C3mpyr]0.45FSI has been studied. FTIR analysis showed that the Na+ ion coordinates with the FSI anions. In addition, interactions between cations (C3mpyr and/or Na+) and the oxygen atoms of EC can be observed in the C-O-C and Cdouble bondO regions, which are ascribed to ion-pairing/clustering. The Tg decreases from −80 °C to −84 °C in the presence of EC and the ionic conductivity improves for both liquid and gel states; this effect is more pronounced

Acknowledgements

The authors are grateful to the Malaysian Ministry of Higher Education for funding via FRGS/1/2015/SG06/UPNM/03/2. We also acknowledge the Australian Research Council for support of the NMR facility through the grant LE110100141. We also would like to thank Mr. Mohamad Firdaus b. Rosle from SIRIM for providing the cathode for charge/discharge analysis.

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