Cover Image: Issue 4, Materials Today.The continued evolution of various high-tech gadgets dictates that it is necessary to reduce the size of chemical power sources whilst retaining their extraordinary electrochemical performance characteristics, such as their discharge capacity and power density. Cathode materials must intercalate and deintercalate lithium ions thousands of times and maintain high electronic conductivity between tiny particles of material undergoing huge stress relaxation. Obviously, a high surface area favors fast ion exchange between the cathode and the surrounding electrolyte, and a low particle size or thickness of the cathode material preserves the stability of the crystal structure.
Traditional 2D cells aren't able to support the energy and size requirements of microdevices such as “smart dust” or intelligent medical implants [1]. Therefore, researchers must look towards the third dimension, and have already proposed several types of 3D cell. Most of them consist of arrays of nanorods, interconnected networks, or nanoparticles with unusual, complex morphologies. Other examples of 3D cells consist of porous materials, providing better electrochemical performance because of the faster and “deeper” lithium intercalation [2].
The best porous materials for this type of application can be discovered amongst the family of aerogels based on vanadium oxide. It is a commonly held notion that aerogels are composites of “being and nothingness” [1] and [3]. Aerogels are mesoporous materials with a nano-scale inorganic framework providing a very high surface area, low density, and access for electrolytes to their interior space. This morphology facilitates fast ion transport and electrochemical intercalation/deintercalation reactions since the thickness of the walls is just several nanometers. Thus these aerogels meet most of the necessary requirements and are considered as one of the more suitable cathode materials for future electronic and medical gadgets.
Depending on the preparation conditions, different compositions of VOx survive with various ratios of V4 /V5 . However, to increase the discharge capacity, vanadium (IV) ions can be oxidized by annealing in air to recycle the V2O5 phase. Generally, annealing at 100 – 300 °C introduces no changes in the original morphology of the material, maintaining its very high porosity. However, higher temperatures liberate an excess of the free Gibbs' energy of the system and this starts irreversible morphology evolution.
Today, R&D in the field of Li-ion batteries is carried out in Lomonosov Moscow State University by several groups. Materials with different composition and morphology are studied using the facilities of the Inorganic Materials Laboratory (Department of Materials Science, Chemistry Department), and the Faculty of Materials Science is celebrating its 20th anniversary this year. The students of the Faculty have successfully investigated 1D systems with record aspect ratios (nanowires, whiskers), 2D layered structures (xerogels and hybrid materials), as well as 3D systems of aerogels and smart composites all based on mysterious, rich structures of vanadium oxide [4] and [5].
The use of the unique equipment available allows the synthesis of aerogels from a wide variety of experimental parameters. For example, the preparation is not limited to a pair of solvents and a simple isothermal heating zone: it is instead possible to vary the pumping and relief velocities of solving, or the sequence of exchanged solvents. It is also possible to control the application and combination of pumping regimes. Thus, aerogels with different morphologies have been formed under conditions controlled and selected by the researcher.
This month's cover image shows an SEM micrograph of an annealed vanadia aerogel. The image was captured using a field emission LEO Supra 50 VP scanning electron microscope. The real size of the image is 52.6 × 34.4 square microns. The sample was obtained by annealing of as-prepared aerogel based on vanadium oxide at 500 °C in air and consists of plate-like particles with boomerang and other complex shapes. We assume that these particles are formed because of a thermally triggered recrystallization and agglomeration processes. The length of the plates is approximately 2 – 10 μm, and the width is about 2 – 5 μm. Outwardly, the sample resembles a randomly scattered puzzle. Although this material has an interesting morphology, its discharge capacity is low due to the destruction of the initial porous structure.
Further Reading
[1] J.W. Long et al. Chem Rev, 104 (2004), p. 4463
[2] W. Dong et al. Science and Technology of Advanced Materials, 4 (2003), p. 3
[3] D.R. Rolison et al. J Mater Chem, 11 (2001), p. 963
[4] S.V. Balakhonov et al. Mendeleev Commun, 20 (2010), p. 153
[5] S.V. Balakhonov et al. Mendeleev Commun, 21 (2011), p. 315