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a, b Charge and discharge voltage profiles of various electrolytes at 400 mA g−1 along with UV–vis spectrum of ICl in these solutions recorded with time. c Cyclic voltammetry (CV) profiles of selected electrolytes with/without Cl− at 0.5 mV s−1.
Early works conducted by Kolthoff and Jordan have systematically studied the I−/I2 redox processes32,33, indicating that under specified conditions both iodide and iodine yield an anodic wave corresponding to the formation of positive iodine (I+). Further studies34,35 concluded that IO3− was the direct oxidation product of iodine in the absence of halide (Cl−, Br−) or cyanide (CN−) in an aqueous solution. This is due to the interaction between electrophilic I+ and nucleophilic species like halide36, cyanide32, and amines37 which afford to charge-transfer complexes38.
The UV-Vis absorption spectra of the less concentrated electrolytes (1-0-0, 10-0-0) resemble the tendency of I+ decomposition in the 1 m ZnSO4 solution (Fig.1b). In contrast, enhanced capability to stabilize ICl could be achieved in concentrated systems (19-19-8, 30-19-8, 30-0-0). Little intensity changes of the ICl band was observed in the whole duration without the appearance of the I2 signal.
a, b Charge and discharge voltage profiles of the 30-0-0 and 19-19-8 electrolyte from 1.25 to 1.8 V at 400 mA g−1, respectively. c, d Full-range (0.6–1.8 V) cycling performance of different systems at 400 mA g−1.
It is apparent that a suitable concentration is required to allow the normal operation of the battery to suppress the self-discharge reaction. We speculate the enhanced stability towards I+ species is correlated to the suppressed water activity and the sufficient chloride ions in the concentrated solution, which will be further discussed in the next section.
a, b Raman spectra of solutions of different concentrations. c, d Radical distribution function(RDF) of the solutions. The solid lines are the radial distribution functions, and dotted lines are the coordination numbers. e A snapshot of the MD simulation box for 19-19-8 system, along with the dominant clusters existing in such system (f). Clusters and their occurrence in the 19-19-8 system. Cyan-blue atom is Zn2+, green atom is Cl−, yellow atom is Li+, red atom is O, dark blue atom is N, brown atom is C, gray atom is Me, and white atom is H. g Summary of free water and Cl− content extracted from the MD simulation.
We conclude that in dilute ZnCl2 solutions (10-0-0, 20-0-0, and 19-5-8), high water activity rendering rapid hydrolysis of ICl, which results in low coulombic efficiency as discussed in Fig.2. Whereas in solutions with very high concentrations (30-0-0 and 30-19-8), the low Cl−activity and the substantial coordination of iodide with Zn2+ (Zn-I clusters, Supplementary Fig.5) is attributed to the rapid capacity decay. Only the 19-19-8 solution takes full advantages of low fraction of free water to suppress I+ hydrolysis and sufficient ionic conductivity with high Cl− activity to realize good performance.
a Ex situ UV vis spectrum of 19-19-8 system recorded at different charge-discharge stages. b Typical voltage-capacity profile of 19-19-8. c In situ Raman spectrum of 19-19-8 system during charge/discharge. d Diffusion coefficient versus specific capacity, calculated from GITT measurements. e CV curves with different scanning rates from 0.1 to 0.8 mV s−1. f The plots of the oxidation and reduction peak-current with the function of the root of scanning rates.
Based on the electrochemical analysis and spectral results, we depict a clear portrait of the reaction mechanism for the four-electron conversion:
The electrolyte formula (19-19-8) we proposed has the advantage of suppressed H2O activity and preserved free chloride ions, which facilitate the four-electron conversion of iodine with a high reversible capacity. While a practical battery relies on the robust anode as well, we tested the zinc anode in the 19-19-8 electrolyte to elucidate the electroplating behavior.
a, b SEM images of the Zn electrodes cycled for 1 h at 1 mA cm−2 in 1 m ZnSO4 and 19-19-8, respectively. c Coulombic efficiencies of zinc plating/stripping on Ti foil (2 mA cm−2, 1 h for the plating). d The plating/stripping in Zn||Zn symmetric cells at 1 mA cm−2 with a sweep duration of 10 min.
a Comparative voltage profiles at 400 mA g−1 for 200 cycles. b Rate test at different current densities. c Cycling performance at 800 mA g−1 for 1000 cycles. d The energy density, average voltage, and specific capacity of aqueous zinc batteries based on intercalation chemistry and conversion chemistry. For comparison, energy densities were converted with active materials only. The intercalation Zn-ion batteries were quoted from ref. 12. e Long-term cycling at a high rate of 2000 mA g−1 for over 6000 cycles.
Solution-adsorption method was used for the preparation of cathodes. In brief, 300 mg of I2 were mixed with 300 mg PAC, followed by adding 20 ml deionized water. The Iodine ratio in the I2/PAC composite was about 15–20 wt%, as calculated by deducting the mass of PAC from the final weighted composite. The PAC/I2, sodium carboxymethyl cellulose (CMC) and super P were mixed in deionized water with a mass ratio of 8:1:1. Then the slurry was cast on a titanium foil followed by drying for 12 h in the air at 60 °C. The electrodes were cut into disks with a diameter of 10 mm. The average areal loading of PAC-I2 composite was about 4–5 mg cm−2 in the electrode.
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