Red Phosphorus As Negative Electrodes for Na-Ion Batteries: Ionic Liquid Vs. Carbonate Ester Electrolytes

Lithium-ion accumulators are used widely as power sources of the portable small-sized electronic equipment such as the mobile telephone and the notebook PC etc. In addition, recently it has been utilized as power sources for the electric vehicles. However, the abundance of lithium in the Earth is relatively low, which is classified as a minor metal.  Furthermore, lithium is produced in the limited area, mainly in South America. In contrast, sodium of which the resource quantity is abundant could be used as a substitute for lithium. In addition red phosphorus (P) material as negative electrodes for Na cells has been studied, which have a considerable advantage in terms of materials abundance and relatively small volume change by sodiation as Na binary compounds.^[1] The reversibility as the electrode materials is, however, limited in conventional aprotic solvents, such as PC.^[1]

    On the other hand, ionic liquids have attracted growing interest for application to electrochemical devices, especially for Li-ion cells, due to their flame retardancy (non-volatility), thermal stability, and wide electrochemical windows. The practical use of ionic liquids has been suggested after the discovery of such ideal characters as the battery electrolyte. Less-flammable liquid electrolytes using ionic liquids have been developed in the past few years, and the performance of cells using ionic liquids has been investigated. The use of mixed electrolytes of carbonates esters solvent and the ionic liquid has been also studied to improve the safety of sodium-ion cells that incorporate flammable carbonates esters solvents, such as EC, PC, and DEC.

    In this work, we will present the electrochemical performance of the red P electrodes in the pure ionic liquid, N-methyl-N-propylpyridinium bisfluorosulfonyl amide (MPP-FSA)/ 0.25 mol dm^-3 NaFSA and carbonate ester electrolytes (EC/PC/3DEC, PC/ 1.0 mol dm^-3 NaPF₆) at room temperature. Red P electrodes were examined using half cells with metallic Na. The electrode consisted of mixing the active material (commercial red P 46 wt%) with acetylene black (AB, 32 wt%) and sodium polyacrylate binders (PANa, 22 wt%). The slurry was coated onto an aluminum current collector. The 2032-type coin cells were assembled in an argon-filled glove box with a sheet of sodium foil as the counter electrode and a glass filter GB-100R as the separator.

    The galvanostatic charge/discharge condition used was C/20 (125 mA g^-1) in the range of 0 and 2 V vs. Na/Na⁺. The specific capacity of the red P electrode in the conventional electrolyte with FEC was 1650 mAh g⁻¹. The potential change for the half cell with MPP-FSA as electrolyte is almost the same as that of the carbonates esters electrolytes. In other words, the reversible sodiation/desodiation can occur with red P electrode in MPP-FSA/ 0.25 mol dm^-3 NaFSA as the pure ionic liquid at room temperature. As shown in Fig.1-A, no reduction in capacity was observed during the 40 cycles. However, capacity decays in the case of carbonates esters electrolytes EC/PC/3DEC/1.0 mol dm^-3 NaPF₆ (Fig. 1-B) and PC/1.0 mol dm^-3 NaPF₆. Additionally, not only electrochemical performance will be presented but also the formation of passivating layer and thermal stability of ionic liquid electrolyte in the red P electrode will be further discussed.

References

[1] N. Yabuuchi, S. Komaba et al, ChemElectroChem in-press, DOI: 10.1002/celc.201, (2013)