Full
WANG Xiaoliang (1980-), male, Ph.D., Professor. E-mail: ningke@163.com
Outline
Supercapacitors have attracted recent attention because of their high power density, fast charging and discharging capability, long cycle life, and good stability. However, the generally low energy density of supercapacitors restricts their applications. The electrode material is the most critical part in the supercapacitor and significantly affects the performance. In recent years, a rational design of electrode materials is proved to be an effective way to improve the capacitance performance of supercapacitors. Among many electrode materials, layered double hydroxide (i.e., hydrotalcite and hydrotalcite-like) has the advantages of adjustable morphology and composition, high theoretical specific capacitance, low cost, easy synthesis and considerable energy density and power density, etc., and emerges in the field of supercapacitor electrode materials. The unique two-dimensional structure of the layered double hydroxide (LDH) and its energy storage mode through electrolyte ions embedded and dislodged in the interlayer dictate that an interlayer spacing modulation is an effective method to improve its capacitance performance. Enhancing the utilization of electrochemically active sites between LDH layers and reducing the ion transport velocity resistance via regulating the layer spacing become a commonly used and effective structural modulation method for two-dimensional electrode materials such as LDH.
This review summarized the research progress on the application of LDH interlayer spacing regulation in the field of supercapacitors, introduced the structure of LDH, the energy storage mechanism, and the advantages of LDH as an electrode material, and described the influencing factors and the advantages and disadvantages about the interlayer spacing regulation from the two different perspectives of the ion-exchange method and the one-step synthesis method, respectively. The target anion intercalation in the two methods concerns the anion charge density and anion concentration in the reaction system. The interlayer spacing values of different guest intercalated LDH, such as inorganic anions, organic anions, and molecules, were discussed. The factors affecting interlayer spacing and capacitance performance were summarized. A small interlayer spacing significantly inhibits ion diffusion and charge transport, which is not conducive to achieving a high capacitance performance. Organic anion intercalation is more likely to achieve a larger interlayer spacing and a higher capacitance performance. Appropriate interlayer objects and interlayer spacing are crucial for achieving a high capacitance performance. The influence of metal cations ratio on the interlayer spacing and capacitance performance was evaluated, and three methods for regulating metal cations were summarized. The ratio of divalent and trivalent cations in the synthesis process can be adjusted to achieve the control of positive charge concentration in the basal layer and the control of interlayer force, thus controlling the interlayer spacing. The oxidation of divalent cations during or after synthesis is controlled to achieve a positive charge density. Selecting cations of different radii to synthesize LDH affects the distribution of basal layer charges, thereby affecting the interlayer spacing. This review elaborated on the method of adding intercalation agents during the process of regulating interlayer spacing, which can simultaneously increase the interlayer spacing and load mass, and is of great significance for obtaining high load and high capacitance electrodes. This review outlined common methods for improving capacitance performance by regulating interlayer spacing and the underlying mechanisms for improving the capacitance performance, playing an important role in promoting the in-depth research and application of LDH electrode materials.
Some prospects for future development were provided. In the future, with the continuous development of precise temperature control, uniform, and rapid heating synthesis technologies such as microwave synthesis, the combination of artificial intelligence control technology and synthesis equipment, the advancement of multi index observation in-situ experimental technology, and the precise observation and control of synthesis heating rate, temperature field, and concentration field are effective ways to achieve precise synthesis of hydrotalcite and precise control of interlayer spacing. And it is expected to achieve precise control of the types, quantities, and layout of interlayer objects according to demand. The influences of interlayer object types, quantities, arrangement attitudes, and pores on the electrolyte ion transport, and the related mechanism on the interlayer active site electrochemical reactions, and the related mechanism on the interlayer spacing and layer structure evolution can be investigated more accurately. The combination of multi-index observation in-situ synthesis experimental technology with the first-principles, molecular dynamics, and machine learning simulation methods can be ultilized to analyze the microstructure, electrochemical reactions, and other processes.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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