Perspective on Li-containing alloys for high-safety and
high-energy density
batteries
The introduction of Li-containing alloys seems to be a fundamental tool
to resolve the safety hazards caused by lithium dendrites. But there are
still many unanswered questions, for instance, the protective mechanisms
for different alloys are not clearly understood[86]; the
Li-containing alloy materials can effectively improve the state of
first-layer Li deposition, however, the substrate will revert to pure Li
as deposition continues[54]; the Li-containing alloy anodes still
face the great volume change and serious side reaction during the
striping/plating process, etc. Therefore, these scientific problems are
still urgently awaiting our solutions.
Simply using lithium metal alloy anode/lithium alloy artificial SEI film
or using lithium alloy to modify the current collector, separator and
electrolytes cannot fully solve the challenges faced by lithium anode.
Only by combining multiple strategies can the problems caused by lithium
anode be better or completely solved. Here we proposed several
suggestions (as shown in Figure 11) for the future research of
lithium-containing alloys employed in Li metal batteries:
i) As the binary Li-containing alloy either has the high reaction
activity (i.e Li-Na), or the great volume change (i.e. Li-Si, Li-Sn), or
the low energy density (i.e. Li-Bi, Li-Zn), or the high cost (i.e.
Li-Ag, Li-Cu) each kind of shortcoming, it can take more consideration
into ternary/multicomponent lithium alloys, which can make up for each
other through multiple components. But the presence of additional metal
that are not directly involved in electrochemical reaction results in
additional weight and volume and thus cause the specific energy density
reducing compared to using the pure lithium or binary lithium alloy
anodes[129]. Another disadvantage is that there still exist a
substantial change in specific volume upon charging and discharging
alloy electrode reactants, can lead to loss of electrical contact, and
thus capacity loss[129]. In addition, the complexity and cost of
preparing ternary/ multicomponent lithium alloys also need to be
considered
ii) Considering the substantial volume change exactly exists in
lithium-containing alloys anodes, constructing nanostructures to host
lithium alloy or preparing lithium alloy-based composites is also a good
choice[130, 131]. By encapsulating the lithium alloy into special
nanostructures, i.e. 3D graphene[132], CNT[131], etc., or using
polymers coating[25], the volume change of lithium alloy has been
effectively eliminated, because of these nanostructured host or extra
components, Li-containing alloy anodes will be more stable in the
organic electrolyte and the lithium dendrite would also be effectively
inhibited.
iii) Constructing artificial protection/SEI layer on the lithium alloy
anode, i.e., recently Won II Cho et al., reported a Li-Al anode
protected by a Langmuir-Blodgett artificial SEI composed of
MoS2[133]. Such a MoS2 artificial
SEI layer exhibited a combination of a high Li binding energy, molecular
smoothness, and low barrier to Li adatom diffusion, which favors
efficient binding of Li and transport away from the
electrode/electrolyte interface as well as favors stable and reversible
Li migration of
the
MoS2 coating Li-Al anode. As a result, the
MoS2 coating Li-Al alloy anode exhibited high
reversibility stable Li migration during recharge of the cells compared
to the Li-Al alloy anode without the MoS2 coating.
iv) Developing the suitable electrolytes and separators for
lithium-containing
alloys anodes. Modification of electrolyte or separators with lithium
alloy will limit its application in lithium metal batteries, as
electrolyte components have significantly influences on the
electrochemical performances of electrodes, no matter anode or cathode.
For example, the absence of volatile or flammable compounds is expected
to make solid electrolytes safer than their liquid counterparts at
elevated temperatures[11]. Additives, can decompose, polymerize or
adsorb on the Li surface, modifying the physico-chemical properties of
the SEI and therefore regulating the current distribution during Li
deposition[19, 23, 134]. Solvents, i.e, the ionic liquid, shows an
exciting role in improving the low temperature performances of
batteries[135]. Lithium salts could not only benefit to stabilize
the spontaneous solid electrolyte interphase (SEI) films, but also
control the nucleation and growth of metallic lithium, thus enhancing
the stabilities of lithium anodes during the stripping and plating
processes[21, 136].
While separators serving as a physical barrier between electrodes,
traditional polyolefin-based separators easily suffer a “shut-down”
problem when penetrate through lithium dendrites or exposed to
overheating and/or overcharge[11]. A suitable separator for
lithium-containing alloys anodes should have good thermal stability and
function to suppress the lithium dendrites. Additionally, a suitable
separator may also have the function to inhibit the polysulfides
shuttling for Li-S batteries and for Li-air batteries. Therefore, a
better strategy is combing the lithium alloys anodes with various
functional electrolytes and separators rather than use it to modify the
electrolyte or separators.