Figure 2 (a) Voltage profiles of various materials with some solubility in Li during Li deposition at a current density of 10 µA·cm−2. To enhance the comparison, the curves are shifted horizontally according to the onset of lithium nucleation and vertically with a constant shift of 0.05V. The scale of the y-axis is indicated by the bar in the bottom left. (b) Shifted voltage profiles of various materials with negligible solubility in Li during Li deposition at a current density of 10 µA·cm−2. The horizontal grey lines show 0 V versus Li in a and b. (c) Schematic of Li metal nanocapsules design. Au NPs are loaded inside hollow carbon spheres, where a large void space is reserved for Li metal. Li is expected to nucleate from the Au seed. Carbon shells provide both confinement and protection of the Li metal, as well as conduction channels for both electrons and Li metal. (Reproduced from ref.[84], with permission from Copyright © 2016 Macmillan Publishers Limited.)
Even the binary Li-containing alloy systems, such as Li-Al, Li-Si, Li-Sn, Li-Ge, Li-Sb, etc., possessed a particularly larger capacity than the commercialized graphite, but these binary lithium alloy electrodes usually suffered rather poor cycle performance as catastrophic structural changes with large volume expansion[87]. To overcome this poor reversibility in such intermetallic lithium alloy electrodes, the ternary Li-containing alloy compounds have been investigated. For example, Hashimoto et al. reported Li4.4Gex Si1-x alloys synthesized by mechanically milling process[88]. The Li4.4Gex Si1-x alloys formed a solid solution over the whole composition range of 0≤x ≤1. Among the obtained alloys, the Li4.4Ge0.67Si0.33alloy showed the largest specific capacity of 190 mA·h·g−1 and good charge-discharge reversibility. Yang et al. reported a ternary Li2.6BMg0.05alloy as an alternative anode to metallic lithium[89]. The pristine Li2.6BMg0.05 consists of rhombohedral Li5B4, cubic lithium and Li3Mg7. Therefore, it owns both the advantages of Li–Mg alloy and Li–B alloy, which Li-Mg binary alloy with good solid solution over a wide range of composition without phase transformation, provides high specific capacity and Li-B alloy with porous structure can increase the specific surface area and its very negative potential close to pure lithium (ca. 20 mV vs. Li/Li+) provides a basic condition for the high energy density. Pan et al, reported Li2MgSi as a novel anode for Li-ion batteries[90]. Directly using Li2MgSi as an anode material can prevent the dissociation of metallic Mg and/or Li-Mg alloy from Mg2Si. And the pre-lithiated Li2MgSi is likely to reduce the stress/strain during delithiation/lithiation. In addition, by constructing a ternary alloy that contain inactive materials, such as LiCuSn and LiCuSb alloy[87, 91], the volume variation of active elements could be buffered by inactive medium when active elements are dispersed uniformly in situ or ex situ into the matrix of inactive components at nanoscale, as in the cases of active/inactive composites and intermetallic compounds[92].
Compared to plenty of binary Li-containing alloys anodes, there are few reports on multi-component lithium alloy anodes[36, 87-91, 93-97]. One reason is the much lower specific capacity of the ternary lithium alloys, i.e, Li4.4Ge0.67Si0.33[88], Li0.25CuP[96], etc., compared to the binary lithium alloys; another most possible reason is rather difficult to prepare high-purity multi-component lithium alloy[90], as a result, the theoretical capacities are hard to calculate as well as the electrochemical mechanisms are difficult fully understood[87].
Table 1 The typical lithium alloys anodes employed in lithium ion, Li-S and Li-O2 batteries as well as their advantages and disadvantages.