Germanium oxide nanocomposite as lithium battery anode

    Recently, a research team from the University of Wollongong in Australia and the Ulsan National University of Science and Technology in South Korea has developed a new lithium-ion battery anode material, which is a nanocomposite material composed of germanium and germanium oxide (GeO2/Ge/C), and has a capacity of 1860 mAh/g at 1 coulomb rate and 1680 mAh/g at 10 coulomb rate. At present, the research team has published its research results in the journal Nano Letters.

    The researchers said that since the anode material of this lithium-ion battery is a nanocomposite material composed of germanium and germanium oxide, it contains germanium nanoparticles. It is precisely because of the presence of germanium nanoparticles that the reversibility of the germanium oxide conversion reaction is greatly improved, thereby also improving the electrochemical performance of the electrode material.

    In the 1990s, graphite was introduced into the anode material library as a commercial anode material. Although it has a relatively low theoretical capacity (372 mAh/g), it is still widely used. Because graphite has a low theoretical capacity, a lot of research is currently focused on the research of high-capacity materials, such as silicon (4200 mAh/g), germanium (1623 mAh/g), tin (993 mAh/g), etc. At the same time, the oxides of these materials (SiO, GeO2, SnO, SnO2) have a relatively high lithium ion saturation capacity. In addition, it is generally believed that the formation of Li2O is irreversible during the first polarization process. If the formation of Li2O compounds is reversible during the polarization cycle, the lithium ion saturation capacity of these oxides will be as high as 8.4Li. That is to say, it is very likely to become the first choice for high-capacity anode materials.

    Previous studies have shown that GeO2 nanoparticles used as anode materials can reach a lithium ion saturation capacity of 9 Li in the first discharge cycle. The researchers explained the lithium ion storage mechanism by describing the initial conversion reaction and the subsequent alloying reaction. However, since the formation of Li2O is irreversible during the first polarization process, lithium ions cannot be completely converted in subsequent charge and discharge cycles, which also limits the size of the lithium ion saturation capacity, where the lithium ion saturation capacity ranges from 4.4 Li (1126 mAh/g) to 8.4 Li (2151 mAh/g).

    Recently, Kim and other research team members have developed a new type of anode material - MGeO3 (M = Cu, Fe and Co). In the forward and reverse processes of lithium polarization, the reversible formation process of Ge-O bonds was observed by analyzing the X-ray absorption spectrum. It is precisely because of the presence of transition metal nanoparticles of germanium that the metal germanium reoxidation reaction is enhanced during the reverse lithium polarization process. At the same time, this metal nanoparticle germanium plays a role as a catalyst in the decomposition of Li2O and the alloying reaction of germanium and Li2O. The catalytic effect of the germanium particles in this anode material (GeO2/Ge/C) mainly utilizes the partial reduction properties of GeO2 and carbon coating. Compared with GeO2/C nanocomposites, GeO2/Ge/C nanocomposites have higher capacity when used as lithium-ion battery anode materials.

    The research team synthesized GeO2/Ge/C composites by using acetylene gas for reduction reaction and simultaneous carbon coating operation. GeO2/Ge nanoparticles will be coated with a layer of carbon coating, and the maximum gap between particle clusters is 30 microns.

    The specific lithium ion saturation capacity of GeO2/Ge/C, GeO2/C and GeO2 will be known through experiments in coin-type half-cells.

    Through experiments, the research team came to the following conclusions:

    �= 1 * GB3 ①The reversibility of the GeO2 conversion reaction is related to the GeO2/C carbon coating and the germanium in GeO2/Ge/C.

    = 2 * GB3 ②Nanostructure is crucial for reversible chemical reactions because it has a larger surface area, which helps to speed up the reaction. In addition, the volume change of the nanostructure is relatively stable, so it can ensure that the volume of Li2O and Ge is relatively close during the decomposition of lithium oxide and germanium oxide.

    = 3 * GB3 ③ The carbon coating is an important factor affecting the reversibility of the conversion reaction. Since carbon has good electrical conductivity, the interconnected network structure of GeO2/C carbon coating and GeO2/Ge/C carbon coating provides an effective conductive network for the movement of electrons, which promotes the conversion reaction. In addition, the carbon coating network structure also plays a buffering role in the volume change during the charge and discharge cycle.

    �= 4 * GB3 ④ Ge also acts as a catalyst in the decomposition process of Li2O.

    The figure above is a schematic diagram of the lithiation reaction mechanism of different materials. The reversible reaction mechanism of GeO2 in GeO2-nano, GeO2/C and GeO2/Ge/C is shown in the figure. GeO2 nanostructure is a necessary condition for the conversion reaction, while carbon coating can improve the reversibility of the conversion reaction. The Ge element particles in GeO2/Ge/C play a vital role in improving the reversibility of GeO2 conversion reaction.

    Finally, the research team stated that nanostructures, carbon coatings, and Ge particles all played an important role in activating and increasing the conversion rate.