Forschungsarbeiten zur Herstellung von Anodenmaterialien für Lithiumionenbatterien

The intermetallic anodes for recharchable Li-ion batteries are especially interesting from the point of view of their theoretical capacities, which are much higher than that of graphite (LiC₆: 372 mAh/g; LiAl: 993 mAh/g; Li₂₂Si₅: 4191 mAh/g). The most serious disadvantage of intermetallic alloys is their limited cycle stability. Substantial volume changes during insertion-deinsertion of lithium cause large mechanical strain, which in combination with the brittleness of these materials results in a disintegration of the electrode and therefore reduces the performance parameters and electrode lifetime. In order to overcome this challenge an approach based on the LiAl-LiZn continuous solid solution with NaTl-type structure is proposed. Since Al and Zn are practically immiscible at room temperature, the formation of a composite material with a local decomposition into Al and Zn is expected during electrochemical Li extraction. Only nanosized Al and Zn clusters are expected at least in the first cycle, because of a low mobility of Al and Zn atoms within the “Li_x(Al,Zn)” composite. The reversible Li-extraction and insertion switches between two states, one preferring a homogeneous mixture of Al and Zn in fully lithiated Li(Al,Zn) and the another one with a tendency to a separation into Al and Zn in delithiated “Al,Zn” composite. Such a competition between intermixing of Al and Zn on one hand and their separation on the other might result in a composite structure with annealing microstructure and enhanced cycling stability. This strategy can be applied for two elements, which both can form alloys with Li (as in the case for Al and Zn) and, therefore, with high theoretical capacities. This concept can be modified by replacing one of the elements by another one partially or completely, which does not form a binary intermetallic compound with Li. This will result in lower theoretical capacities, but could stabilize the composite structures by inactive matrix components.

The alloys are prepared from the elements in a Ta-crucibles. To avoid contamination, all preparation steps were performed in a glove box under controlled argon atmosphere. The Bühler MAM-1 arc furnace is located in the glove box for the sealing of the Ta-crucibles. The crucibles are heated to the required temperature in the Fa. Hüttinger induction furnace, that is also placed in the glovebox.