Abstract
Aqueous zinc-ion batteries (AZIBs) have attracted intensive research interests due to they use cheap, non-combustible water instead of flammable organic electrolytes as electrolytes, which can effectively eliminate safety hazards. However, the use of non-renewable inorganic element (e.g., Co, Ni, Mn) as the electrode is strictly constrained by the surge in raw material price. Hence, it is crucial to develop low-cost and high-security batteries that satisfy "carbon neutral"" environmental ideals.Compare with inorganic material, organic electrode material (OEMs) has received extensive attention during the past decade because of their environmental friendliness, sustainability, flexible structure designability, and abundance in the Earth’s crust. According to the free radical state of the redox reactions, organic electrode materials can be classified into three types: n-type, p-type, and bipolar type. In an electrochemical reaction, the n-type organic molecule typically delivered a high capacity, albeit with relatively low redox potential. Conversely, P-type organic materials boast higher redox potential, but exhibit lower capacity and battery stability. Therefore, appropriate strategies need to be proposed to address the low potential issue of n-type organic electrode and the low stability issue of p-type organic electrode to obtain high energy density and long-cycle safe aqueous batteries.
Firstly, the redox processes of p-type organics were analyzed and the relationship between energy change and battery voltage output. The organosulfur compounds, as typical p-type materials, were selected as cathode materials, which redox process based on a two-step. Step 1: the oxidation of p-type organics removes electrons from the p-conjugated structure (the electronic transitions of organic compounds are from an occupied orbital to an unoccupied orbital and the energy change in this process is represented by ∆G1). Step 2: the positive charge left on the organics is balanced by anions from the electrolyte (the de-solvation energy of anions is represented by ∆G2 and the coordination energy is represented by ∆G3). Based on the redox process, we proposed a dual-step strategy to effectively tune the energy change. Using thianthrene (TT) materials with a more uniform electron cloud distribution as the cathode may result in gaining and losing electrons more difficult (∆G1 increase), which improved the voltage from 0.8 V to 1.3 V. The solvents capable of forming tight solvent sheath structures with anions further increased the voltage from 1.3 V to 1.7 V (∆G2 increase). Finally, the optimized Zn||TT battery exhibits a high discharge voltage of 1.7 V and a remarkable capacity retention of 82% even after 8000 cycles in an acetonitrile-based electrolyte. Our results provide new insight into the voltage output of organic cathode-based batteries and pave the way toward developing stable organic cathodes with high potential.
Besides, to address the issue of unstable stability in p-type organic materials, two innovative solutions were proposed. We employed a two-electron redox p-type organic cathode, DMPZ (5,10-dihydro-5,10-dimethylphenazine), to investigate the underlying cause of capacity fade. Specifically, capacity fading is closely associated with the nucleophilic attack of the electrolyte on the oxidized organic electrode. Accordingly, two innovative solutions were proposed to address the aforementioned problems: (1) forming a highly concentrated electrolyte through the hybridization of weak Lewis basicity zinc salts; (2) regulating the temperature to reduce the Lewis basicity of electrolyte. The optimized Zn||DMPZ cell exhibited almost negligible capacity fading even after 2600 cycles, particularly when tested at -60 ℃. This work shed new light on optimizing the stability of p-type organic-based aqueous batteries.
Furthermore, to address the low voltage issue of n-type organic materials, we regulated protons to tailor the enol conversion reaction of quinone-based organic cathodes for high-performance aqueous batteries. We unveiled that the proton storage could significantly determine the reaction products of the enol conversion reaction and the electrochemical stability of the organic electrode. Specifically, the protons preferentially coordinated with the prototypical pyrene-4,5,9,10-tetraone (PTO) cathode, and increasing the proton concentration in the electrolyte could improve the working potentials and cycling stability through tailoring the enol conversion reaction. We also found exploiting Al2(SO4)3 as a pH buffer can increase the energy density of the Zn||PTO battery from 242.8 Wh kg-1 to 284.6 Wh kg-1. Our research has a guiding significance for emphasizing proton storage of organic electrodes based on enol conversion reactions and improving the electrochemical performance.
In summary, in response to the distinct redox mechanisms of n-type and p-type organic materials, innovative and effective strategies for electrochemistry performance enhancement have been developed in pursuit of organic-based aqueous battery with high energy density. The research and strategies presented in this thesis are anticipated to significantly transform the design of high-performance aqueous organic-based batteries, delivering an exceptional combination of high energy density and reliable energy output and is expected to apply to other organic material systems.
| Date of Award | 5 Aug 2024 |
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| Original language | English |
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| Supervisor | Chunyi ZHI (Supervisor) |