Lithium storage alloys f. ultrahigh energy density batteries

Lithium storage alloys are characterized by enormous theoretical specific charges and charge densities, which makes them an interesting alternative to carbonaceous anode materials, which are presently prevailing in Li-ion batteries. So, for example, 1 mol Sn can store up to 4.4 mol Li, which corresponds to 994 Ah kg-1 (with respect to the mass of the unlithiated material) or 2023 Ah L-1 (with respect to the volume of the lithiated material). For comparison, 1 mol graphite can store only 0.17 mol Li, which corresponds to 372 Ah kg-1 or 760 Ah L-1 . The major obstacle for a practical use of these materials are, however, the big volume changes, which accompany the uptake and release of such large amounts of Li (from Sn to Li 4.4 Sn: ~ 260 % volume increase, for comparison: from C6 to LiC6 : ~ 10 %). A result of this lack of dimensional stability is that in conventional coarse-grained materials the mechanical strain within the material gets so big that loss of contact or even full mechanical disintegration of the electrode occur, and that a repeated discharge and recharge is not possible. It was aim of project P12768 to get these volume changes under control by adopting suitable strategies. These include the control of the morphology of the Li-alloy (especially a decrease of the particle size to the nanometre range) as well as the use of multi-component and multi-phase materials. The effect of the morphology is not very surprising, as, though the relative volume changes are equal for coarse and fine-grained materials, the absolute volume changes decrease with decreasing particle size. I.e., the finer the material, the smaller the mechanical strain. The positive effect of the multi-phase composition is attributed to the fact, that the single phases react at different potentials during charge and discharge, i.e. one after each other. Hence, the non-reacting phases can buffer the volume changes of the reacting phases. Whereas the mentioned principles had already been roughly tested with electro-deposited alloy-films, the present project focussed on gaining a better understanding of the interaction between composition / morphology and performance, and above all on the development of new materials in the form of ultra-fine alloy-powders. These powders should then be mixed with binder materials and additives to increase the electronic conductivity, and be processed further to composite electrodes by casting/coating processes. The great advantage of the powder- technology is, that it resembles the state-of-the-art of electrode manufacturing, as it is used, e.g. for carbon-based anodes and oxide-based cathodes. It is thus an important requirement for the Li-alloys to make the jump from the laboratory to industrial production and finally onto the market. Beside the powder-based composite electrodes, electro-deposited materials have been used in some cases. Due to the absence of the additional components in composite electrodes (binder and conductive additives) they constitute simple model systems, e.g. for the investigation of the reaction mechanisms by X-ray diffraction. The metallic and intermetallic systems, which have been investigated, were multi-component and multi-phase materials from the systems Sn-Sb, Sn-Ag, Sb-Ag, and Sn-Sb-Ag, as well as for comparison the single phase materials Sn, Sb und Ag. The nano-crystalline powders have been obtained by reductive precipitation with NaBH4 of the respective metal precursors from aqueous solution. The addition of complexing agents (especially citrates) to the starting solution allowed to control the particle size. The standard binder and conductive additives were poly(vinylidene fluoride) (PVdF) and nano-crystalline Ni-powder, respecitvely. The alloy powders and the composite electrodes were characterized by a variety of analytical methods. Most important was the electrochemical characterization (constant current cycling, cyclic voltammetry, impedance spectroscopy, ), as it was the basis for an assessment of the actual performance (capacities, cycling-stability, electrode resistances, ) of the materials under investigations. Especially worth mentioning are also ex-situ and in-situ X-ray diffraction measurements, which served to identify the mechanisms of the reactions during charge and discharge. For the phase SnSb, for example, it could be demonstrated that during lithiation phase separation occurs and that the two binary phases Li x Sb and Li x Sn are formed rather than a stable ternary phase LixSnSb. This reaction mechanism is reversible and leads to a further nano-structuring during Li-uptake, which apparently helps to release mechanical strain and may be a possible explanation for the excellent cycling stabiliy of SnSb. Parallel to the optimiziation of the active material the optimization of the composite electrode occurred, both of its composition (active material : conductive additive : binder) and the preparation process (solvens for the electrode- slurry, pressure, etc.). Surprisingly grave was the influence of the binder. Only with an appropriate processing procedure and the resulting binder morphology and distribution was it possible to maintain the integrity of the composite electrode for a longer period and to prevent extensive swelling of the binder in the electrolyte. Unlike for almost dimensionally stable carbonaceous anode materials and oxidic cathode materials, also here the enormous volume changes are a great technological challenge. Therefore, at a later stage of the project, various alternative binders were tested. These include harder binder, which have been obtained, e.g. by cross-linking of suitable starting polymers. So far, however, the best results have been obtained with the original polymer (PVdF). With Sn/SnSb there is now, at the end of this project, a material available which shows capacities of 500 - 600 Ah kg-1 for more than 50 cycles. It therefore ranks among the best alloy systmes which have been proposed in the literature. Nevertheless, there is a number of problems, which still have to be resolved, and which constitute starting-points for a possible follow-up project: On the one hand the extension of the investigations to other metallic / intermetallic system (e.g. including Bi) seems reasonable. On the other hand, the synthesis of the alloy powders has to be further improved to further minimize the amount of oxide impurities and the high irreversible capacities in the first charge/discharge cycle. Finally, a continuation of the optimization of the design of the composite electrode and of the testing of new binder materials might result in a further increase the long-term cycling stability. For the project there have been many points of contact with project F911 from the special research programme (SFB) "Electroactive Materials", which is devoted to the interface between electrode and electrolyte in Li batteries. Furthermore, in cooperation with SFB projects F915 and F923 the kinetics of the Li uptake and release and (with the use of electron microscopy) the morphology of the powders and the composite electrodes have been investigated. The investigation of new binder materials proceeded in cooperation with SFB project F903. Special interest in this project was shown by Treibacher Chemische Werke (Austria) and by Mitsubishi Chemical Corporation (Japan), which lead to three patent applications (Tokuganhei) for Japan. In conclusion it can be said, that the major part of the original research objectives could be realized, and that therefore from the point of view of the project leaders and co-workers the project was a very successful one. This may also be seen from the large number of scientific publications. The obtained results and the new questions which have arisen from these results are a valuable starting point for further investigations on this topic.