Researchers from the University of Wollongong, Australia and the Ulsan National Institute of Science and Technology (UNIST), S. Korea, have developed a germanium oxide/germanium nanocomposite (GeO2/Ge/C) anode material for Li-ion batteries that shows a high capacity of up to 1860 mAh/g at 1 C (2.1 A/g) rate and 1680 mAh/g at 10 C rate. A paper on their work is published in the ACS journal Nano Letters.
They attributed the good electrochemical performance to the increase in reversibility of the conversion reaction of GeO2 by the presence of the elemental germanium nanoparticles present in the composite.
Graphite has been used as the commercial anode material since the introduction of LIBs in the 1990s, although it has a quite low theoretical capacity of 372 mAh/g. Thus, much research has been focused on high capacity materials such as silicon (4200 mAh/g), germanium (1623 mAh/g), and tin (993 mAh/g) to replace the graphite anode. The oxides of these metals (SiO, GeO2, SnO, SnO2) are another group of materials which can provide high lithium storage capacity. In addition, there is widespread belief that, during the first lithiation, Li2O is irreversibly formed. If the Li2O component could be reversibly formed during cycling, these oxide materials could theoretically store up to 8.4 Li+. This would make them attractive alternatives as high capacity anode materials.
Germanium dioxide nanoparticles have been previously studied as anode material for LIBs and were reported to react with up to 9 Li+ during the first discharge cycle. The lithium storage mechanism is described by an initial conversion reaction followed by an alloying reaction. The lithium ions, however, could not be fully removed in the subsequent charging cycle due to the irreversible Li2O formation. This limits the theoretical lithium storage to 4.4 Li+ per GeO2 (1126 mAh/g) compared to 8.4 Li+ (2152 mAh/ g).
Recently, Kim et al. published a report on MGeO3 (M = Cu, Fe, and Co), in which the reversible formation of Ge−O bonds during lithium insertion and extraction was studied using X-ray absorption spectroscopy. One of the key factors contributing to the reoxidation of metallic germanium during delithiation is the presence of the transition metal nanoparticles. The metallic nanoparticles play a catalytic role in the decomposition of Li2O and also form a conductive network between germanium and Li2O to facilitate the reoxidation of germanium...In this work, the catalyst role of Ge in GeO2/Ge/C nanocomposite anode is investigated using partial reduction of the GeO2 in conjunction with the carbon coating process. The anode shows a higher capacity and better rate capability than the GeO2/C nanocomposite.—Seng et al.
The team synthesized the GeO2/Ge/C anode material using a simple method involving simultaneous carbon coating and reduction by acetylene gas. The nanosized GeO2/Ge particles are coated by a thin layer of carbon, which is also interconnected between neighboring particles to form clusters of up to 30 μm.
Lithium storage performances of GeO2/Ge/C, GeO2/C and GeO2 bulk and nanoparticles were tested in a coin type half-cell.
Based on their testing, the team determined that:
The reversibility of the conversion reaction of GeO2 is related to the carbon coating (GeO2/C) and the introduction of metallic germanium (GeO2/Ge/C).
The nanosize particles are very important to enable the reversible conversion reaction. Nanoparticles have a larger surface area, which promotes the reaction kinetics. In addition, nanoparticles are known to show a smaller absolute volume change, which could keep the Li2O and Ge in close proximity for the decomposition of lithia and simultaneous oxidation of germanium, they suggested.
The carbon coating is an important factor in increasing the reversibility of the conversion reaction. As carbon is highly conductive, the interconnected carbon shells of the GeO2/C and GeO2/Ge/C samples would provide an efficient network for electron transfer, which in turn increases the kinetics of the lithium reactions. The carbon shells also can act as a buffer matrix to limit the volume variation during charge and discharge cycles.
They also confirmed the catalytic effect of germanium in the decomposition of Li2O.
We found that the nanosized particles, carbon coating, and the elemental germanium in the composite play a crucial role in activating and improving the kinetics of the conversion reaction.—Seng et al.
Kuok Hau Seng, Mi-hee Park, Zai Ping Guo, Hua Kun Liu, and Jaephil Cho (2013) Catalytic Role of Ge in Highly Reversible GeO2/Ge/C Nanocomposite Anode Material for Lithium Batteries. Nano Letters doi: 10.1021/nl304716e