Elsevier

Electrochemistry Communications

Volume 71, October 2016, Pages 48-51
Electrochemistry Communications

Investigating non-fluorinated anions for sodium battery electrolytes based on ionic liquids

https://doi.org/10.1016/j.elecom.2016.07.014Get rights and content

Highlights

  • Stable plating/stripping of Na from non-fluorinated dicyanamide electrolyte

  • Water content effects on Na electrochemistry in this pyrrolidinium ionic liquid

  • Sodium cyclic voltammetry at Ni electrode tuned via [Na+] and moisture content

Abstract

In order for sodium batteries to become a safe, lower cost option for large scale energy storage, minimising the price of all components is important. We report here on the application of a pyrrolidinium room temperature ionic liquid comprising the dicyanamide anion as a successful electrolyte system for sodium metal batteries that does not contain expensive fluorinated species. The effects of plating/stripping of sodium from Na metal electrodes has been investigated in a symmetrical Na | electrolyte | Na configuration at a current density of 10 μA cm 2. Comparisons are drawn to reference organic electrolytes comprising propylene carbonate-fluoroethylene carbonate. Residual water molecules in the ionic liquid electrolyte are observed to have a significant effect upon the surface film and subsequent favourable plating/stripping behaviour of symmetrical cells and this is explored in detail. An increase of the moisture content from 90 ppm to 400 ppm impedes both electrodeposition and electrodissolution of the Na+/Na. This is investigated at Ni electrodes using cyclic voltammetry at different Na+-salt concentrations to further understand the mechanism.

Introduction

The role of technologies such as batteries to enable the storage of energy from renewable sources is becoming crucial [1], [2]. Seamlessly incorporating renewable energy into an ever evolving smart grid requires an efficient and inexpensive, large scale rechargeable battery system. Room temperature sodium energy storage has recently been investigated in many laboratories due to the considerable price reduction and the greater abundance of sodium in the Earth's crust in comparison to lithium [3], [4]. In addition, a sodium technology may provide a specific energy of 150 Wh kg 1, comparable to current lithium-ion cells (typically 100 to 265 Wh kg 1). Thus sodium metal battery technologies are touted as being solutions to stationary energy storage systems for both domestic and industrial applications such as load levelling [4], [5].

The majority of current anode and cathode materials for Na batteries are based upon the dimensions, chemistry and behaviour of lithium ions, as are the multitude of electrolyte solutions commonly utilised. Though Li and Na have similarities, a holistic approach is necessary to determine the appropriate materials required to realise sodium secondary cells. This entails a paradigm shift from graphite/olivine electrodes paired with carbonate electrolytes, towards materials which are not only effective, but also provide safe performance and environmental friendliness.

High rate electrochemical cycling of symmetrical sodium cells has been shown recently using a pyrrolidinium cation based room temperature ionic liquid (RTIL) at ambient temperature [6]. Here we further investigate an analogous RTIL based on the dicyanamide anion; this avoids the use of fluorinated anions that otherwise add cost (and some safety issues) to electrolyte formulations. The behaviour of symmetrical cells is compared with a reference organic carbonate electrolyte which have been shown to provide good performance [7], but lack safety due to flammability.

Analogous cyano based ionic liquids have been implemented in lithium electrochemistry previously [8], and have highlighted the importance of controlled moisture content for the effective cycling performance of symmetrical and full cells. In that study cyclic voltammetry was used to determine the feasibility of electrodeposition/dissolution of Li+ metal as a function of water content, ranging from as low as 36 ppm up to 630 ppm. Lithium deposition and subsequent dissolution was not possible at low water content (36 ppm) and this was ascribed to a minimum moisture content required to provide a stable solid-electrolyte interphase (SEI). Cyclic voltammograms for electrolytes with water content of 97 ppm were optimal, while further increased water contents led to decreases in current density for plating and stripping. Yoon et al. concluded that increasing the moisture content is detrimental to the electrochemical kinetics and that optimum moisture contents contributed to higher current density and efficiencies [8].

Following on from these studies, we herein report a study of the electrochemical behaviour of two sodium ion containing ionic liquid electrolytes with differing moisture contents at a Ni electrode as well as Na metal electrode in order to ascertain whether moisture content plays a role and to what degree. We do so using the techniques of symmetrical cell cycling and cyclic voltammetry. In addition, these techniques are used to gain further insight into the processes which are occurring at the electrode during cycling, and compared with a reference organic carbonate electrolyte system [7].

Section snippets

Material and methods

N-butyl-N-methyl-pyrrolidinium dicyanamide (Merck, C4mpyr[dca], 40 ppm), sodium bis(trifluoromethanesulfonyl)imide (Merck, Na[NTf2]) and sodium dicyanamide (98%, Solvionic SA, Na[dca]) were dried in vacuo at 120 °C for 72 h [9]. Sodium foils were prepared from larger Na rods stored under paraffin oil (Merck Millipore) which were rolled flat and polished under hexane (Fluka, 99.9%) using a Nylon brush [10]. Calcium hydride (Sigma) was added to hexane and refluxed overnight, then distilled and kept

Results and discussion

Cell cycling shown in Fig. 1 shows the direct comparison of the C4mpyr[dca] electrolyte versus the organic electrolyte in Voltage-time plots which are adjusted to show cycle number. All cells displayed high DC impedance (over 10 kΩ) which is much larger than previously noted for comparable lithium cells which were typically in the order of 10 to 1000 Ω [8]. The cell constructed using organic electrolyte was cycled 50 times and at a current density of J = 0.04 mA cm 2. From an initial polarisation of

Conclusions

When the water content of an N-butyl-N-methyl-pyrrolidinium dicyanamide ionic liquid electrolyte including Na[dca] salt is maintained at 90 ppm the cycling at a sodium electrode is achieved. Repetitive stripping/plating cycling studies show that a Na | Na symmetrical cell can cycle effectively for > 100 cycles at a current density of 0.01 mA cm 2 without short-circuits, and with a V-t profile showing stability and labile processes at longer timescales. By maintaining a low but not zero water content

Acknowledgements

We acknowledge the ARC (Australian Research Council) for its financial support through Australian Laureate FellowshipFL110100013 (MF) and FL120100019 (DRM) and under Australian Discovery Projects (DP130101652 and DP160101178).

References (15)

  • B.L. Ellis et al.

    Sodium and sodium-ion energy storage batteries

    Curr. Opin. Solid State Mater. Sci.

    (2012)
  • A. Basile et al.

    Extensive charge–discharge cycling of lithium metal electrodes achieved using ionic liquid electrolytes

    Electrochem. Commun.

    (2013)
  • M. Kar et al.

    Ionic liquid electrolytes as a platform for rechargeable metal–air batteries: a perspective

    Phys. Chem. Chem. Phys.

    (2014)
  • M.S. Whittingham

    History, evolution, and future status of energy storage

    Proc. IEEE

    (2012)
  • S.K. Das et al.

    Sodium–oxygen batteries: a new class of metal–air batteries

    J. Mater. Chem. A

    (2014)
  • H. Pan et al.

    Room-temperature stationary sodium-ion batteries for large-scale electric energy storage

    Energy Environ. Sci.

    (2013)
  • M. Forsyth et al.

    Novel Na+ ion diffusion mechanism in mixed organic–inorganic ionic liquid electrolyte leading to high Na+ transference number and stable, high rate Electrochemical cycling of sodium cells

    J. Phys. Chem. C

    (2016)
There are more references available in the full text version of this article.

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