Investigating non-fluorinated anions for sodium battery electrolytes based on ionic liquids
Graphical abstract
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).
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