The ever increasing demand for energy warrants the development of long-lasting energy storage devices capable of delivering high energy densities. Researchers at Singapore University of Technology and Design, Singapore have demonstrated an aqueous rechargeable battery using chloride ions in an aqueous NaCl solution with BiOCl anode and silver cathode, for the first time.
The anode was prepared by coating graphite papers using a slurry consisting of a mixture of BiOCl, polyvinylidene difluoride (PVDF) and carbon black in a ratio of 8:4:4 with N-methylpyrrolidone (NMP) as the solvent (BiOCl electrode). Graphite papers coated with Ag paste was used as the cathode (Ag electrode). The cathode and anode were dried in a vacuum oven at 100 °C for two days before assembling them in the battery. 1M NaCl (pH adjusted to 8.0) was used as the electrolyte and it was purged with Argon gas for 15 min before battery assembling. GB100R glass fiber membranes were used as separators.
The mechanism of the aqueous rechargeable chloride ion battery during charging and discharging involves reversible transport of chloride ions through the electrolyte and its reaction with the electrodes via redox electrochemistry (Fig. 1). During charging (Fig. 1(a)), the chloride ions are deintercalated from the BiOCl electrode, transferred through the electrolyte and intercalated into the Ag electrode to form AgCl while the chloride ions will be inserted back into the anode with the recovery of BiOCl during discharging (Fig. 1(b)).
Fig. 1 Schematic illustration of the aqueous rechargeable chloride ion battery during: (a) charging; and (b) discharging process.
The charge and discharge curves of BiOCl-Silver system using 1 M NaCl as the electrolyte is shown in Fig. 2(a). In spite of the low coulombic efficiency during the initial cycles (37.2%), the efficiency is increased to ~99% during the subsequent cycling (Fig. 2(b)), suggesting the high reversibility of the BiOCl-Ag battery. The discharge capacity is decreased from 92.1 mAh/g to 24.9 mAh/g with an increase in current density from 400 mA/g to 1200 mA/g (Fig. 2(c)). However, its ability to restore back a stable capacity when the current density is reverted back from 1200 mA/g to 400 mA/g (Fig. 2(c)) indicates that the performance of the aqueous chloride ion battery is stable and reversible. Volume contractions or expansions during the electrode phase transformation still remains as a problem that need to be solved.
Fig. 2 (a, b) Charge-discharge curves; (c) cycling performance; and (d) rate capabilities, of the BiOCl/Ag system in 1 M aqueous NaCl electrolyte.
The aqueous rechargeable chloride ion battery exhibits a stable discharge capacity of 92.1 mAh/g at 400 mA/g with a coulombic efficiency of ~100% and maintains its stability for 45 cycles without decay in performance.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Fuming Chen et al., Energy Storage Materials 7 (2017) 189–194.
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