Eutectic Li21Si5 Alloy As a Lithium-Containing Negative Electrode Material

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.