Abstract
Rechargeable metal-halogen (chlorine, bromine, and iodine) batteries have been pursued as potentially effective, low-cost, and mass-producible alternatives to current transition-metal-based batteries due to highly reversible redox and abundant resources. The conversion-type halogen cathode based on anionic chemistry would achieve a higher capacity than typical intercalation-type cathodes due to the rich valence state and multiple charge transfer. Besides, its effective redox activity and flexible match with various metal anodes (Li, Zn, Na, K, Al, Mg, Ca, etc.) give metal-halogen batteries rich diversity. However, several challenges, such as thermodynamic instability of the cathode, low conductivity and shuttling effect, must be overcome before a real breakthrough and their widespread application.Rechargeable lithium-iodine (Li-I2) batteries are highly attractive energy storage systems featuring high energy density, superior power density, sustainability and affordability owing to the promising redox chemistries of iodine. However, severe thermodynamic instability and shuttling issues of the cathode have plagued the active iodine loading, capacity retention and cyclability. Here, we report the development of highly thermally and electrochemically stable I-/I3--bonded organic salts as cathode materials for Li-I2 batteries. The chemical bonding of iodine/polyiodide ions with organic groups realizes that up to 80 wt% iodine is effectively stabilized without sacrificing a fast and reversible redox reaction activity. Thus, the shuttle effect gets significantly inhibited, improving cathode capacity and restraining side-reaction on the Li anode. As a result, such cathodes afford Li-I2 batteries a specific capacity of 173.6 mAh g-1methylamine hydroiodide (MAI) (217 mAh g-1I) at 0.5 C, superior rate capability of 133.1 mAh g-1MAI at 50 C, and ultrahigh capacity retention rate of 98.3% over 10000 cycles (5 months). In-situ, ex-situ spectral characterizations and density functional theory calculations clarify the robust chemical interaction between iodides and organic groups. The cathode chemistries elucidated here propel the development of Li-I2 batteries and are expected to be extended to other metal-iodine battery technology.
Li-Cl2 chemistry using anionic redox reactions of Cl0/-1 shows superior operation voltage (~ 3.8 V) and capacity (756 mAh g-1). However, organic electrolyte-based lithium-ion batteries have not developed a redox-active and reversible chlorine cathode. 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 (ICl3), 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 Cl0 is anchored by forming interhalogen bonds with I. A reversible Li-Cl2 at room temperature is developed, which delivers a specific capacity of 302 mAh g-1 at 425 mA g-1, and a 73.8% capacity retention at 1250 mA g-1. The demonstration of reversible interhalogen bonds enabled rechargeable Li-Cl2 battery opens a new avenue to develop halogen compound cathodes.
In Zn-I2 batteries, electrolyte environments, including cations, anions, and solvents are critical for the performance delivery of cathodes. Most works focused on interactions between cations and cathode materials, in contrast, there is a lack of in-depth research on the correlation between anions and cathodes. We systematically investigated how anions manipulate the coulombic efficiency (CE) of cathodes of zinc batteries, taking conversion-type I2 cathodes as typical cases for profound studies. It was found that the electronic properties of anions, including charge density and its distribution, can tune conversion reactions, leading to significant CE differences. Using operando visual Raman microscopy and theoretical simulations, we confirm that competitive coordination between anions and I- can regulate CEs by modulating polyiodide diffusion rates in Zn-I2 cells. Conversion I2 cathode achieves a 99% CE with highly electron-donating anions. Understanding the mechanism of anion-governed CEs will help us evaluate the compatibility of electrolytes with electrodes, thus providing a guideline for anion selection and electrolyte design for high-energy, long-cycling zinc batteries.
In summary, diverse metal-halogen batteries, including Li-I2, Li-Cl2 and Zn-I2, with high reversibility, capacity and rate-performance are developed and in-depth mechanisms are studied by experimental characterizations and theoretical calculations in this thesis. To solve the issues that metal-halogen batteries face, we propose several strategies of the chemical bonding of iodine/polyiodide ions with organic groups, interhalogen bonds, and regulating the coordination environment of halogen ions toward advanced energy storage devices. It is believed that the successful use of these strategies in this thesis paves new ways to develop energy-dense and high-power halogen-based metal-cathode materials in the future.
| Date of Award | 17 Jun 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chunyi ZHI (Supervisor) |