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Journal of Materials Chemistry A Paper cathodic directions, respectively. On the other hand, the CV curve within a potential window of 0–1.4 V vs. SHE (2.7–4.1 V vs. Na+/Na, green curve in Fig. 2b) reveals signicantly larger anodic currents and an additional reduction peak (at approxi- mately 1.15 V vs. SHE) in the cathodic scan. The increased current appears to be a result of the CER32 in addition to the OER, and the reduction peak corresponds to the CRR.31,33 Based on the CV results and the galvanostatic charge–discharge voltage prole, we consider the saltwater battery charges mainly by the OER due to the electrolysis of the saltwater at the cathode side, although the CER cannot be completely ruled out. The dominance of the OER is also corroborated by the CV result of 1 M Na2SO4 (Fig. 2b, dashed line), which is very close to that of 1 M NaCl. Parallel to the oxidation of saltwater by the OER, Na+ ions dissolved in the catholyte are transferred to the anode side through NASICON and then reduced to Na metal. Aer charging for 200 h at a current rate of 0.1 mA cm2, the anode showed pure Na metal extracted from saltwater (dark areas in Fig. 2c, for EDS results see Fig. S4, ESI†), demonstrating the usability of saltwater as a Na+ containing active material. Next, we examined the effect of salt concentration on the electrochemical properties of saltwater batteries. Fig. 2d displays the polarisation graphs of saltwater batteries with 0.4, 1, 3, and 5 M NaCl. The voltage points were obtained by aver- aging the voltage prole measured at various current densities for 1 h. At all concentrations, the charge voltage increased while the discharge voltage decreased with increasing current density, thus widening the charge–discharge voltage gap. Increasing the concentration of NaCl caused a larger decrease in the charge voltage, whereas the discharge voltage was rather decreased (refer to comparison between the 1 M and 5 M results in Fig. 2a). We measured the ionic conductivity of the saltwater at the corresponding concentrations (Fig. 2a, inset). Generally, the high ionic conductivity of the electrolyte in batteries enhances kinetics of the redox reaction and thereby enables a decrease in the charge voltage, while an increase in the discharge voltage, as a result of reduced ohmic resistance. The increase of ionic conductivity by increasing the salt concentration (within the range of 0.4–5 M), however, had a negligible effect on the charge and discharge voltages. Instead, the theoretical redox potential (Fig. 2d, inset) is the main factor in determining the charge/ discharge voltages of saltwater batteries. Electrochemical performances of the saltwater battery The electrochemical performance of the Na-metal-free saltwater battery is rst examined through half-cells: an anode half-cell with a hard carbon electrode and a cathode half-cell with salt- water. As a representative sodium-intercalation material, hard carbon exhibits a high reversible capacity (250–300 mA h g1) and a low sodium-ion-insertion potential (<1.0 V vs. Na+/Na).34,35 Hard carbon as the negative electrode can thus avoid poor reversibility (cyclability) stemming from the dendritic growth of Na metal during cell operation. Theoretically, the energy density of saltwater batteries is dependent on the concentration of NaCl (catholyte) and the capacity of the anode. Considering the maximum solubility of NaCl in water (6 M at 25 C) and the stability of the NaCl solution at room temperature, we assembled a half-cell saltwater battery with a 5 M saltwater catholyte (Na|5 M saltwater), whose theoretical energy density was calculated to be 368 W h kg1 (for details see the ESI†). For the anode, we eval- uated the electrochemical properties of a hard carbon electrode using a 2032 coin-type half-cell (Na|hard carbon). The galvano- static charge–discharge proles at a current rate of 0.025 mA cm2 (approximately 26 mA ghard carbon1) are shown in Fig. S5 (ESI†). The hard carbon electrode delivered initial discharge (sodiation) and charge (desodiation) capacities of 557 and 370 mA h g1, respectively, with an irreversible capacity decay of 187 mA h g1 during the rst cycle. The low initial coulombic efficiency (66%) could be ascribed to the formation of the solid- electrolyte interphase (SEI) on the hard carbon surface and the trapping of a small amount of Na+ in the hard carbon.36 Aer ve cycles, the hard carbon electrode displayed a reversible capacity of350mAhg1. Fig. 3a exhibits the galvanostatic charge and discharge curves of the cathode and anode half-cells. The cathode half-cell (Na|5 M saltwater) showed charge and discharge voltage plateaus at 3.76 V and 2.56 V, respectively. In the case of the anode half-cell, the charge–discharge proles were based on the 5th charge–discharge proles of the coin-type half-cell (Na|hard carbon). Next, we constructed the full-cell saltwater battery (hard carbon|5 M saltwater) and investigated its performance. The full-cell saltwater battery was tested at a current rate of 0.025 mA cm2 with a capacity cut-off condition of 300 mA h ghard carbon1 upon charging, and a voltage cut-off condition of 0.5 V upon discharging (Fig. S6, ESI†). At the rst cycle, the saltwater battery displayed a considerably low discharge capacity of 46 mA h g1 (initial coulombic efficiency 15%), which was attributed mainly to the formation of the SEI layer during charging (sodiation). It was assumed that during the rst charging, most of the input charge (300 mA h ghard carbon1) was consumed to form the SEI on the surface of the hard carbon anode. Indeed, the rst charge voltage prole of the full-cell corresponded to the sloping region (1.2–0.1 V) of the initial discharge voltage prole of the anode half-cell (Na|hard carbon) (Fig. S5, ESI†). Such a sloping region has been recognised as caused by SEI formation, as well as Na+-insertion within the graphene layers of hard carbon.36 As the cycling continued, the discharge capacity steadily increased and the charge–discharge voltage proles became saturated at the 7th cycle. Fig. 3b exhibits the 7th charge–discharge voltage prole, which was close to the voltage proles predicted from the two half-cell voltage proles in Fig. 3a (orange curves). The average charge and discharge voltages of the full-cell were 2.78 V and 2.45 V, respectively. To examine the Na+ insertion from the saltwater catholyte into the hard carbon anode, the anode surface of the full-cell disassembled aer the 8th charge process was examined by SEM with energy-dispersive X-ray spectroscopy (EDS). The EDS mapping data (Fig. 3c) revealed an increased amount of Na element with uniform distribution aer cycling compared to the pristine electrode, which only showed a weak Na signal from the remnant NaCF3SO3 salt. Fig. 3d shows the cycling perfor- mance of the full-cell saltwater battery at a current density of 7210 | J. Mater. Chem. A, 2016, 4, 7207–7213 This journal is © The Royal Society of Chemistry 2016PDF Image | Saltwater as the energy source for low-cost batteries
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