Eutectic Li21Si5 Alloy As a Lithium-Containing Negative Electrode Material

Abstract

Towards the development of next-generation lithium-ion batteries (LIBs) having very high energy density, lithium-free positive electrode materials, such as sulfur and metal fluorides, are gathering great attention from their much higher theoretical capacities than conventional positive electrode materials containing lithium. Such new positive electrode materials require high-capacity lithium-containing negative electrode materials for their counter electrodes. In this work, the potential of eutectic Li-Si alloy (Li21Si5 crystal) as a lithium-containing negative electrode material is examined in detail. Li21Si5 alloy is prepared by solidification of a molten Li-Si mixture having the eutectic composition. Generally, working electrodes for electrochemical measurements are prepared by mixing active materials (powder form) with binder polymers and conductive additives. However, Li21Si5 alloy strongly reacts with binder polymers. Therefore, we have evaluated the basic charge/discharge behavior of eutectic Li21Si5 by using a specific electrochemical cell in which Li21Si5 is embedded in a porous Cu-disc electrode to minimize unexpected degradation of reactive Li21Si5. It is found that the decrease of particle size significantly improves the charge/discharge performance of Li21Si5. In the initial delithiation stage, Li21Si5 with the small particle size of 1~2 μm shows high capacity over 1000 mAh g–1, which is attractive enough to construct high-energy LIBs by the combination with the lithium-free positive electrode materials, such as sulfur and metal fluorides. One of the interesting advantages of Li21Si5 is its different profile of structure change from Si during charge/discharge cycles, and this fact results in the great difference in their cyclability. To elucidate such difference, charge/discharge of Si powder (particle size is 1~2 mm) is also characterized by the same manner which is used for Li21Si5. Note that the volume fraction of Si/Cu was adjusted to the same as that of Li21Si5/Cu when a working electrode (porous Cu-disc is used as a current collector) is prepared. Since Li21Si5 has been already lithiated, its volume does not expand and there is no mechanical stress in the electrode during cycling. Thus, the Cu-disc electrode is not damaged even after 10 charge/discharge cycles, as clearly found from the photographs in Figure 1a. By contrast, the structure change of Si begins from a severe volume expansion up to about 3–4 times larger than the original Si volume. Such volume expansion aggressively destroys the Cu disc, and therefore, the lithiated Si is electrically isolated and loses its capacity very quickly. As is found from the photographs in Figure 1b, the Cu-disc electrode indeed collapses into powder after 10 charge/discharge cycles. The structure change of Li21Si5 by repeating charge/discharge is further analyzed in detail. It is suggested that Li21Si5 crystal turns into porous and amorphous Si upon delithiation, and the amorphous Si turns into amorphous Li-Si alloy upon the subsequent lithiation. Interestingly, the original particle size of Li21Si5 is almost unchanged even after charge/discharge cycles. Moreover, we have successfully demonstrated charge/discharge of a prototype LIB cell including Li21Si5 as a lithium-containing negative electrode, together with a lithium-free positive electrode, and the cell capacity is much higher than another test LIB cell including lithiated graphite instead of Li21Si5. Though the Cu-disc electrode is too heavy to be used for practical LIBs, it is found that carbon-coating of Li21Si5 is effective to prevent the unexpected reactions of Li21Si5 with binder polymers, and a lighter working electrode can be prepared. The present results suggest a promising possibility of Li-Si alloy as a counter part of high-capacity lithium-free positive electrode materials, towards ultrahigh-energy LIBs.