Introduction
The metallic lithium as an anode in a rechargeable battery was first
explored by Whittingham in 1970s at Exxon and realized commercialization
by Moli Energy in the late 1980s[1-3], but frequent accidents,
including fires caused by dendrite formation, brought serious safety
issues to public attention, which leaded to the Moli Energy company
ultimately recall all the cells. The first commercialization of
lithium-metal anode rechargeable batteries ended in failure. Then Sony
company developed the graphite anodes to replace metallic Li anode,
matched with the LiCoO2 cathode, successfully built
reliable Li-ion cells that have been widely used until now[2, 4-6].
However, with the booming growth in consumer electronic devices and
electric vehicles, even the state-of-the-art lithium ion batteries
(LIBs) using graphite anodes (with a theoretical specific capacity of
372 mA·h·g–1) have almost reached their theoretical
energy density (350 Wh·kg–1), cannot provide the high
energy density required for their demands[7-10].
Therefore, recently, particularly in recent 10 years, the rechargeable
lithium metal batteries using the Li anode has acquired unprecedented
attentions as it has the unsurpassably highest theoretical capacity
(3860
mA·h·g−1,
or 2061 mA·h·cm–3) and lowest electrochemical
potential (–3.04 V versus the standard hydrogen electrode)[2,
11-15]. Additionally, with rapid development of cathodes alternative
to conventional intercalation cathodes used in state-of-the-art Li-ion
batteries, i.e., the sulfur (S) cathode (with energy density of ≈2600
Wh·kg–1)
for the Li-S battery and the oxygen cathode (with energy density of
≈11140 Wh·kg–1) for the Li-air battery[16, 17],
have undoubtedly hastened the arrival of rechargeable Li metal
batteries.
However, the safety issues by using the Li anode as well as its poor
cycle stability and low Coulombic efficiency are still not overcome and
hinds its practical applications. Tremendous efforts have been devoted
to solving the notorious lithium dendrite problem by employing advanced
electrolytes, separators, and novel electrode materials/structures. In
the aspect of electrolyte, increasing the concentrations of lithium
salts and adding inorganic or organic additives in electrolyte could not
only benefit to stabilize the spontaneous solid electrolyte interphase
(SEI) films to reduce the side reactions happening, but also control the
nucleation and growth of metallic lithium, thus enhancing the
stabilities of lithium anodes during the stripping and plating
processes[18-22]. Recently, our group employed octaphenyl
polyoxyethylene as an electrolyte additive to enable a stable complex
layer on the surface of the lithium anode. This surface layer not only
promoted uniform lithium deposition, but also facilitated the formation
of a robust SEI film[23]. While developed new
type of modified separators are
expected to physically suppress the growth of lithium dendrites. For
instance, in situ fabricating a stable tissue-directed/reinforced
bifunctional separator/protection film (TBF) on the surface of the Li
anode effectively protected lithium from the corrosion of
O2, discharge intermediates, H2O and
electrolyte, and reduce morphology change of the surface of lithium
anode[24]. In terms of novel electrode materials/structures,
infusion the Li metal into a carbon framework or synthesis of
lithium-based composites[25-27], enables to control the growth of
lithium dendrites, but also solves the infinite volume change issue of
lithium-based electrodes[18, 28]. For instance, Koratkar and
co-workers described defect-induced plating of metallic lithium within
the interior of a porous graphene network[27]. The network acted as
seed points that initiated plating of lithium metal and prevented
dendritic growth.
Based on the aforementioned considerations, stable operation of Li metal
anodes, no matter inhibit the lithium dendrite growth or controllable
side reaction and volume change, is critical for next-generation battery
technologies. Thus, in this short perspective article, we briefly review
the Li-containing alloys reported in the literatures to stabilize Li
metal anodes and propose a few new suggestions for protecting the Li
metal by reasonably combining the lithium-containing alloys with other
strategies.