Anionic redox reactions would achieve a higher capacity than typical transition-metal-oxide cathodes, offering low-cost chemistry for advanced lithium-ion batteries. Li-Cl 2 chemistry using anionic redox reactions of Cl 0/−1 shows superior operation voltage (∼3.8 V) and capacity (756 mAh g −1 ). However, a redox-active and reversible chlorine cathode has not been developed in organic electrolyte-based lithium-ion batteries. Chlorine ions bonded by ionic bonding hardly dissolve in organic electrolytes, imposing a thermodynamic barrier for redox reactions. Meanwhile, chlorine gas is easily formed during oxidation. Herein, we report an interhalogen compound, iodine trichloride (ICl 3 ), as the cathode to address these two issues. In situ and ex situ spectroscopy data and calculations reveal that reduced Cl − ions are partially dissolved in the electrolyte, and oxidized Cl 0 is anchored by forming interhalogen bonds. A reversible Li-Cl 2 delivers a specific capacity of 302 mAh g −1 at 425 mA g −1 and a 73.8% capacity retention at 1,250 mA g −1 .
The chlorine-based redox reaction (ClRR) could be exploited to produce secondary high-energy aqueous batteries. However, efficient and reversible ClRR is challenging, and it is affected by parasitic reactions such as Cl 2 gas evolution and electrolyte decomposition. Here, to circumvent these issues, we use iodine as positive electrode active material in a battery system comprising a Zn metal negative electrode and a concentrated (e.g., 30 molal) ZnCl 2 aqueous electrolyte solution. During cell discharge, the iodine at the positive electrode interacts with the chloride ions from the electrolyte to enable interhalogen coordinating chemistry and forming ICl 3 - . In this way, the redox-active halogen atoms allow a reversible three-electrons transfer reaction which, at the lab-scale cell level, translates into an initial specific discharge capacity of 612.5 mAh g I2 −1 at 0.5 A g I2 −1 and 25 °C (corresponding to a calculated specific energy of 905 Wh kg I2 −1 ). We also report the assembly and testing of a Zn | |Cl-I pouch cell prototype demonstrating a discharge capacity retention of about 74% after 300 cycles at 200 mA and 25 °C (final discharge capacity of about 92 mAh).
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