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.