3.1 | DIRECTLY USING LITHIUM ALLOYS TO REPLACE
METALLIC LITHIUM AS
ANODE
The lithium-free alloy anodes, such as Sn-Sb, Sn-Co, Ni-Sn alloy, etc.,
without pre-stored lithium, the overall energy density is limited by the
low-capacity lithium metal oxide cathodes, while the pure lithium metal
anode face its high reactivity and uncontrolled dendrite growth[39].
Li-containing alloy anodes inheriting the desirable properties of alloy
anodes and pure Li metal anodes. Lithium alloy anodes for rechargeable
ambient temperature lithium batteries have been studied since the early
1970[40, 41]. During the past 40 years, great deals of literatures
have been reported using the lithium-containing alloys as the anode
materials for lithium ion batteries[18, 30, 32-34, 38, 42-44]. They
can effectively reduce Li nucleation overpotential and decrease
interfacial resistance, guiding the formation and growth of
non-dendritic Li[25]. Among these lithium alloys anode, they can
mainly divided into two categories: binary lithium alloys and ternary
lithium alloys. And different lithium alloys anodes have their own
advantages and disadvantages.
For example, Li-Si alloy anodes exhibit multiple attractive properties:
i) fully lithiated Lix Si alloy has a sufficiently
low potential of around 10 mV versus Li/Li+ to
prelithiate all types of anodes including graphite, Si, Ge and
Sn[45]; ii) due to the super-high capacity of Si (4200
mA·h·g−1), Lix Si alloy anode
could also illustrate high specific capacity even a small percentage of
pre-storing lithium, i.e., Li4.4Si shows a capacity of
2000 mA·h·g−1 [45]. Most of the Li-Si electrodes
were obtained by electrochemical lithiation of Si-based
electrodes[46-48], which is very difficult to employ for practical
application[47]. But recently, there have been reported other two
methods to synthesis the Li-Si alloy: one is pressing plus
heat-treatment process as shown in Figure 1a[47], another one is
ball milling method (Figure 1b) at argon atmosphere[49-51].
Except the Lix Si alloy, the lithium alloy anodes
of other IVA group elements has also been widely reported, such as
Li-Sn
and Li-Ge alloy anodes[48, 52-58]. Similar as
Lix Si alloy, the Li-Sn and Li-Ge alloy anodes
also exhibit relative high specific capacity[48, 57]. While the
Li-Sn alloy anode also shows its unique merits, including the fast
interdiffusion of Li in Sn and the < 500 mV separation between
Li-Sn alloy formation and Li plating. Archer’s group reported a Li-Sn
hybrid battery anodes created by depositing an electrochemically active
Sn on a reactive Li metal electrode by a facile ion-exchange chemistry
as shown in Figure 1c, leading to very high exchange currents and stable
long-term
performance[59].
The Li-Sn anodes were shown to be stable at 3 mA·cm−2and 3 mA·h·cm−2. While in contrast to Si, Ge has the
benefit of forming a minimal amount of native oxide in its outermost
layer and the diffusivity of lithium in Ge is 400 times greater than
that of lithium in Si at room temperature, but as the high cost of Ge,
the Li-Ge alloy anodes have not gained much
attention[34].
The Li-Al alloy anode showed
higher
stability in the air, carbonate-based electrolyte and the electrolyte
with LiNO3 additive[60, 61]. Also, Al alloying with
Li exhibits much smaller volume change (≈96%) compared with other alloy
anodes, such as Li-Si (320%) and Li-Sn (260%) alloys anode[62]. In
addition, Li diffusion coefficient in Li-Al alloy
(6.0×10-10cm2·s−1)
exceeds that in bulk Li metal (5.69×10-11cm2·s−1)[63]. LiB alloy is
widely used as anode in thermal battery, which can be regarded as free
metal lithium metal filled in the fibrillar network framework of Li/B
compound (Li7B6)[64, 65], such a
porous structure can increase the specific surface area and adjust Li
ions even distribution. As the discharge potential of
Li7B6 is over 0.4 V (vs.
Li/Li+), thus when Li-B alloy used in metal lithium
battery, its free metal lithium participates in electrochemical reaction
preferentially[64]. Additionally,
Li7B6 has a good conductivity
(1.43×103Ω−1·cm−1) and a high Li ion
diffusion rate comparative to metallic lithium[64]. Thus, in 2013,
Yang’s group firstly investigated Li-B alloy as anode for lithium/sulfur
battery[64]. It is because of the above advantages, Li-B alloy has
better behaviors in restraining the formation of dendritic lithium,
reducing the interface impedance of electrode and improving the cycle
performance of the battery. For Li-In alloy electrode, Archer’s group
found the interfacial of the resultant Li-In alloy electrode was
significant lower than that of the pristine Li metal, which allowed Li
ions diffused along the surface to form uniform deposit on the hybrid
electrode[66]. As a result of the enhanced interfacial ion transport
mechanism, compact and uniform electrodeposition for the Li-In alloy
anode at long time scales has been realized. And the Li-In hybrid anodes
to full cells employing high-loading commercial cathodes (LTO and nickel
manganese cobalt oxide) showed that the electrodes can be cycled stably
for over 250 cycles with close to 90% capacity retention. Recently,
Adelhelm investigated the different In/Li ratio on the performances of
Li-In anode[67]. The right In/Li ratio, i.e. 1.27:1, enabled stable
lithium insertion/deinsertion in symmetrical cells for at least 100
cycles; while too much lithium in the electrode leaded to a drop in
redox potential combined with a rapid build-up of interface resistance.
Compared with the group IVA and group IIIA lithium alloys, even as early
as 1970s, the Li3Sb and Li3Bi alloy
anodes have also been investigated in metal lithium batteries[68,
69], group VA
lithium
alloys directly used as anodes in metal lithium batteries are not too
many[43, 70-72]. That mainly because the higher toxicity of some VA
elements, such as Sb and As, and the smaller gravimetric capacity of
Bi[34]. In addition, the synthesis conditions of VA lithium alloys
are higher demanding, taking the Li-Sb alloy, except complex
prelithiation with Sb, another method is prepared by electrolysis of the
molten LiCl-KCl eutectic mixture with a liquid antimony cathode at
high-temperature[71]. In contrast, the Bi/Sb-based nanocomposites
and Bi/Sb-based intermetallics could be easily and large-scale
production, as well as demonstrated good electrochemical performances
when used as anode materials for LIBs[34]. Therefore, during the
past 40 years, there has been no significant development of the group VA
lithium alloys as the anodes in metal lithium batteries.
Beside these three group elements, some other elements such as Na[73,
74], Mg[75-80], Zn[81, 82], In[66, 67], Ag[83, 84],
Au[84], etc., can alloy with Li as well. And recently, these
lithium-alloys used as the anode materials in lithium metal batteries
have gained increasing attentions.
Li-Na alloy can supply Li+ on stripping and thus
ensure the
electrostatic
shield effect of Li+ [85]. And Li-Na alloy would
not sacrifice the specific capacity of the anode because Li and Na
metals exhibit similar reaction activities as well as electrolyte
compatibility of Li+ and Na+
[85]. However, developing a Li-Na alloy anode might be difficult
because of volume expansion[73, 74, 85], which causes SEI damage,
large internal resistance and low Coulombic efficiency. Recently,
Zhang’s group reported a Li-Na alloy anode used in Li-O2batteries[85]. By optimizing the Na/Li value of the alloy, a
dendrite-suppressed, oxidation-resistant and crack-free Li-Na alloy
anode could be obtained[85], thus realizing an alloy anode with a
long cycle life.
The Li-Mg alloy is advantageous because of the generally lower
reactivity of Li (or relatively low Li activity), the large solid
solution range, the mechanical integrity of Mg framework and a
relatively large diffusion coefficient of Li in Mg
(∼10−7 cm2·s−1 for
the Li-Mg alloy produced by vapor deposition)[75, 77-79, 86]. Mg
alloying can increase lithium utilization, when no external pressure is
applied while pure lithium metal is superior for setups that allow stack
pressures in the MPa range[75]. And appropriate amount Mg, i.e, 10
at%, introducing into Li metal anode can also effectively prevent
contact loss[75]. Due to these various advantages, recently, Gao’s
group reported Li-Mg alloy as an anode for Li-S batteries[76].
Compared to the metallic Li anode, the Li-Mg alloy showed remarkable
improvement on stability at the surface and in the bulk during cycling
as shown in Figure 1d and 1e. And they also found after Li stripping, a
conducting Li-poor Li-Mg alloy matrix was formed, facilitating
subsequent plating and diffusion of Li ions.