Figure 6 Schematic of the Li plating process on the (a) 3D Cu
foil and (b) 3D Cu@Al foil. (Reproduced from ref.[104], with
permission from Copyright © 2019 Wiley-VCH.) (c) Schematic illustration
of the proposed lithium nucleation and deposition processes on a bare Cu
foil (left) and a Zn modified Cu foil (right), (d) Cycling performance
of Li-S full cells tested at 0.2 C for the first 3 cycles and at 0.5 C
for the subsequent cycles with Li pre-deposited on a pristine Cu foil or
a Zn coated Cu foil as the anode. (Reproduced from ref.[105], with
permission from Copyright © 2019 Tsinghua University Press and
Springer-Verlag GmbH Germany, part of Springer Nature.) (e) Schematic
illustration of the preparation process of PW@3D-Li, SEM images of (f)
Au-G/Cu foam, (g) 3D Li and (h) PW@3D-Li, Lower-left inserts in (f), (g)
and (h) represent optical photographs of Cu foam, 3D-Li and PW@3D-Li,
respectively. (Reproduced from
ref.[25], with permission from Copyright © 2019 Elsevier B.V.)
Trapping Li into three-dimensional (3D) conductive host to construct
3D-Li is an effective strategy to suppress the growth of Li dendrites.
However, the increased contact area between 3D-Li and electrolyte
unavoidably induces more side reactions to further deteriorate the
electrochemical performance of lithium metal batteries. In order to keep
the advantages of 3D lithium-alloys matrix as well as reduce the side
effect, recently, Qu’s group construct a paraffin wax (PW) coating
Au-graphene/Cu foam current collector[25]. Such a unique structure
was able to effectively avoid the growth of Li dendrites and formation
of “dead Li” during the Li plating process, which could be attributed
to these merits: i) Au could react with Li to form
Lix Au alloys that lowered Li nucleation
overpotential and interfacial energy to effectively inhibit the
formation of dendritic Li; ii) 3D lithiophilic graphene decreased the
local current density and enabled the homogeneous growth of 3D Li; iii)
Cu skeleton not only afforded interconnected pores to accommodate the
volume variation of 3D-Li, but also served as a robust support to avoid
the collapse of overall electrode especially during fast
plating/stripping processes; iv) the PW protection layer confined Li
during the plating and stripping to mitigate the corrosion of
electrolyte and depress the formation of Li dendrites and “dead Li”.
3.3 | Artificial protective layers for lithium
alloys
anodes
Even using the 3D matrix to host the Li-containing alloys,
unfortunately, these materials still suffer from poor SEI stability,
resulting in unsatisfied electrochemical performances[48].
Therefore, constructing an artificial protective layer for lithium
alloys anodes has been proposed. For example, Cui’s groups reported two
methods to construct artificial-SEI layer of LiF to protect
Lix Si alloy nanoparticles via reducing
1-fluorodecane and fluoropolymer CYTOP, respectively[107, 108]. Ci
et al., reported a Li-O2 coin cell with the
LiAlx anode experienced a high-current
pretreatment[109], as a result, the SEI film (including
Al2O3, LiF, ROCO2Li,
LiOH, and Li2CO3) formed after the
pretreatment process facilitated the uniform Li+shuttling during the following Li plating/stripping process and
stabilizes the LiAlx anode interface even after
hundreds of cycles. The
LiAlxanode in lithium oxygen batteries could increase cycling to 667 cycles
under a fixed capacity of 1000 mA·h·g−1 compared to 17
cycles of LiAlx anode without pre-treatment.
Recently, Zhang’s group used a Li-Na alloy and 1,3-dioxolane (DOL) as
anode and additive, respectively, to control dendrite growth and buffer
the volume expansion of the alloy anode[85]. The 1,3-dioxolane
additive could in situ react with Li-Na alloy to form a robust and
flexible passivation film that suppress dendrite growth, buffer alloy
anode volume expansion, prevent cracking. As shown in Figure 7a-7d, only
the Li-Na alloy electrode with DOL additive existence can effectively
suppress dendrite growth and wouldn’t crack after cycling.