Nankai University: Preparation of high-efficiency cathode materials from fallen leaves

Lithium Power Grid News: A few days ago, the research group of Professor Zhou Zhen from the School of Materials Science and Engineering of Nankai University found titanium dioxide/carbon nanotubes, a composite anode material with fast reaction kinetics, and used leaves fallen off the campus as raw materials. The preparation of high-efficiency cathode materials greatly improves the overall performance of sodium ion capacitors. The relevant results are published in 'Advanced Energy Materials'.

As a new type of energy storage device, sodium ion capacitor takes into account the advantages of high battery energy density and high power density of supercapacitors. It has received extensive attention in recent years, but there are still high costs and material utilization. Problems such as low rate, insufficient rate performance and cycle stability. In addition, the energy storage per unit weight of the current sodium ion capacitor cathode material is much smaller than that of the negative electrode. In order to make the two have the same storage capacity, a large and heavy positive electrode has to be made. This also limits the use of sodium ion capacitors. Wide application in the field of energy storage.

'As a negative electrode material, it is required to have a low potential, while the positive electrode needs to have a high potential, so that the voltage of the battery or capacitor can be high.' Zhou Zhen introduced. The team used local materials and used pear leaves from the Jinnan campus of Nankai University as raw materials to prepare a biomass carbon material with a high specific surface area as an adsorption-type positive electrode. Most of the existing sodium ion capacitors use commercial activated carbon as the positive electrode. Compared with commercial activated carbon, this biomass carbon material prepared from leaves exhibits rapid absorption and desorption of anions and outstanding cycle stability, which can increase the storage capacity. And charge and discharge speed.

Zhou Zhen’s research group introduced carbon nanotubes through electrospinning technology, and designed and prepared nanorods in which titanium dioxide and carbon nanotubes are uniformly dispersed in carbon materials as embedded anodes. 'We use titanium dioxide to build sodium ion capacitors. Titanium dioxide's lower sodium storage potential and inherent pseudocapacitance characteristics effectively enhance the energy density and rate performance of the hybrid device, and greatly increase the total amount of electricity storage and charging and discharging speed of the positive electrode.' Zhou Zhen introduced. The introduction of carbon nanotubes effectively promoted the transmission of ions and electrons, effectively increased the pseudocapacitance ratio of the material, and thus brought more prominent rate performance. In the half-cell test, the material showed excellent cycle stability.

Sodium ions have a larger ion radius, which is more inclined to the pseudocapacitance reaction on the surface. In order to give full play to this feature, the team assembled the above two materials into a sodium ion capacitor through reasonable matching. The device exhibits high energy density (81.2 W h kg-1), excellent power density (12400 W kg-1) and ultra-long cycle stability (5000 cycles under 1 A g-1 high current capacity retention The rate is 85.3%), which effectively solves the problem of the mismatch between the positive and negative electrochemical reaction kinetics of the existing capacitors, and provides a new idea for the research and development of sodium ion capacitors.

Finally, the research team has further proved that the sodium ion capacitor has a high voltage output through the experiment of successfully driving the mini fan, which provides the possibility for future practical application.

'In fact, not only leaves, but also straw, vegetable leaves, etc. can also be used to make capacitor anodes. Large-scale production will be beneficial to the development of environmental protection to a certain extent.' Zhou Zhen introduced.

This work was funded by the National Key Research and Development Program.