Background:
Aqueous batteries have attracted a lot of attention due to their inherent safety, low cost, and eco-friendliness. Aqueous batteries based on metal carriers (e.g., Li+, Na+, K+, Ca2+, Zn2+, Al3+) have been widely reported. Compared with metal ions, non-metallic ammonium ions (NH4+) have the advantages of abundant resources, low molar mass (18 g mol−1), small size of hydrated ions, and fast diffusion kinetics. In addition, NH4+ has a tetrahedral structure and exhibits a unique intercalation chemistry by forming hydrogen bonds with the electrode material, which contributes to the achievement of excellent electrochemical properties. Therefore, aqueous ammonium-ion batteries are considered to be a promising energy storage device.
So far, various cathode materials, such as Prussian blue analogues (PBAs) and metal oxides, have been used in aqueous ammonium-ion batteries. However, intercalated cathode materials cannot meet the requirements of high-performance aqueous ammonium-ion batteries due to their limited capacity (typically <200 mAh g−1) and slow kinetics. Halogen-based conversion reactions (e.g., Br2/Br-) can provide high specific capacity (335 mAh g-1) and high redox potential (about 1.08 V vs. SHE). Therefore, replacing the intercalated cathode with a converted halogen cathode (such as Br2/Br-) can effectively improve the energy density and reaction kinetics, and avoid structural collapse and other problems. In addition, for the anode, low-cost organic materials can hold a large number of ammonium ions, which is a potential aqueous ammonium-ion battery anode material. However, irreversible NH4+ intercalation not only causes the electrode material to overexpand, but also continuously consumes the electrolyte, resulting in lower coulombic efficiency during cycling.
Outline of Research
基于此，青岛大学刘晓敏教授与唐啸教授团队在国际知名期刊Energy storage Materials上发表题为“Unlocking the potential of ionic liquid-functionalized aqueous electrolytes for aqueous ammonium-bromine/ion batteries”的研究文章。 该工作通过使用3,4,9,10-苝四羧酸二酐(PTCDA)有机材料为负极，Br2/Br−氧化还原电对为正极，离子液体基水系溶液(7 mol kg-1waterNH4Br and 1 mol kg-1water ionic liquid (TPABr 或BMIMBr))为电解液构建了水系铵-溴/离子电池(ABBs)。 实验研究和理论计算结果表明，离子液体功能化电解液中的有机阳离子(TPA+或BMIM+)与铵离子可以共嵌入PTCDA负极中，从而提高了NH4+存储的可逆性。 此外，有机阳离子不仅构建了一个疏水界面层，将水分子从阳极表面排开，而且还通过形成固体络合物(TPABr3)来稳定正极侧的活性物质。 因此，水系铵-溴/离子电池在室温和低温下都具有令人满意的电化学性能。
Research Highlights:
⭐ An aqueous ammonium-bromine/ion battery was constructed. Experiments and calculations show that organic cations and NH4+ can be co-embedded in the PTCDA anode, thereby improving the reversible storage of NH4+ in the PTCDA anode.
⭐ Organic cations can form a hydrophobic cation sieve on the surface of the negative electrode to protect the PTCDA anode. At the same time, on the cathode side, organic cations can form a solid complex (TPABr3) with the intermediate product (Br3-), thereby inhibiting the migration of active species and improving the cycling performance.
Illustrated reading
KEY 1. MD simulations and DFT calculations reveal solvation structures and complex formation in functionalized electrolytes
The solvation structure of 1 m NH4Br, 7 m NH4Br, 7 m NH4Br + 1 m BMIMBr and 7 m NH4Br + 1 m TPABr (expressed as E1, E2, FE1 and FE2, respectively) was investigated by molecular dynamics (MD) simulations. As the electrolyte concentration increases with the introduction of ionic liquids, the number of water molecules in the NH4+ solvated sheath decreases significantly, thereby inhibiting side reactions at the electrode/electrolyte interface (Fig. 1b-d). In addition, the formation of TPABr3 and BMIMBr3 complexes was investigated by density functional theory (DFT) calculations (Fig. 1e). The results show that organic cations, especially TPA+, can spontaneously convert polybrominates (e.g., Br3−) of the cathode into complexes. It is worth noting that TPABr3 is a condensed solid state, which can effectively inhibit the migration of active substances and slow down the capacity decay of aqueous ABBs.
Figure 1. Schematic diagram of the reaction mechanism and electrolyte structure analysis of aqueous ammonium-bromine/ion batteries
▲The solvation structure of 1 m NH4Br, 7 m NH4Br, 7 m NH4Br + 1 m BMIMBr and 7 m NH4Br + 1 m TPABr (expressed as E1, E2, FE1 and FE2) was studied by molecular dynamics (MD) simulations. As the electrolyte concentration increases with the introduction of ionic liquids, the number of water molecules in the NH4+ solvated sheath decreases significantly, thereby inhibiting side reactions at the electrode/electrolyte interface (Fig. 1b-d). In addition, the formation of TPABr3 and BMIMBr3 complexes was investigated by density functional theory (DFT) calculations (Fig. 1e). The results show that organic cations, especially TPA+, can spontaneously convert polybrominates (e.g., Br3−) of the cathode into complexes. It is worth noting that TPABr3 is a condensed solid state, which can effectively inhibit the migration of active substances and slow down the capacity decay of aqueous ABBs.
KEY 2. Organic cations in ionic liquids are embedded in the PTCDA anode to enhance the reversibility of NH4+ storage
Experimental studies and theoretical calculations show that TPA+ and BMIM+ can be embedded in the PTCDA anode as pillars, thereby causing the phase transformation of the anode material. As shown in Figure 2e-f, the BMIM+ cation is embedded in the PTCDA anode, while the TPA+ cation is embedded in the PTCDA negative electrode with one of the propyl chains, and the other three propyl chains remain on the electrode surface. The electrochemical behavior of organic macromolecules embedded in the organic anode of PTCDA was further analyzed by electrochemical quartz crystal microbalance (EQCM) test (Fig. 3), and it was found that the intercalation of organic macromolecules could effectively enhance the storage reversibility of NH4+ and improve the coulombic efficiency of the PTCDA anode. At the same time, EQCM and molecular dynamics simulations showed that the organic macromolecules formed a hydrophobic cation layer on the surface of the negative electrode, thereby reducing the number of water molecules near the electrode interface and effectively inhibiting the occurrence of harmful side reactions.
Figure 2. Electrochemical properties and structural evolution of PTCDA anode in pure electrolyte
Figure 3. Electrochemical properties and storage mechanism of PTCDA anode in aqueous functionalized electrolyte
KEY 3. Electrochemical performance at room temperature and low temperatures
The aqueous ammonium-bromine/ion battery based on PTCDA organic anode, Br2/Br− cathode, and ionic liquid functionalized aqueous electrolyte exhibits stable rate and cycling performance at room temperature, showing high specific capacity (118 mAh g-1 based on anode mass) and capacity retention rate (79.4% capacity retention rate after 2000 cycles). In addition, by using activated carbon with a high specific surface area as the cathode host material, the battery capacity retention rate was 98.8% after 2000 cycles at a low temperature of -20°C.
Figure 4. Electrochemical performance of the whole cell
Bibliographic information
Unlocking the potential of ionic liquid-functionalized aqueous electrolytes
for aqueous ammonium-bromine/ion batteries
https://doi.org/10.1016/j.ensm.2024.103553
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