4) Galvanic corrosion
In addition to Li loss from direct contact between the electrolyte and
Li, galvanic corrosion of Li occurs during the rest period because of
spontaneous electron flow due to the difference in the redox potential
(Figure 1F).[72] During the initial charge process
of AFLMBs, Cu substrates are easily exposed to electrolytes. As the
porous, organic-rich SEI formed on the surface of Cu CCs cannot entirely
prevent electron tunneling, the exposed Cu surface becomes a pathway for
electron transfer. The electrons from the oxidation of Li are
transported to the electrolyte through the exposed Cu, causing the loss
of Li with a pit shape and decomposition of the electrolyte
simultaneously. Eventually, dendrites were induced during the charging
process. Galvanic corrosion, which can significantly reduce the
reversibility of Li during the resting period, has not yet been taken
seriously; however, considering the actual operation of AFLMBs, much
attention is needed.
Despite the complicated challenges to overcome, the potential benefits
of AFLMBs must be apparent. With tremendous lessons learned from both
LIBs and LMBs, there have been rapid developments in extending the
limited cyclability of AFLMBs. In this review, we discuss CCs
modification, electrolyte optimization, and regulation of cycling
parameters in detail to provide an understanding of its impacts and
obtain insights. Finally, we provide perspectives on various strategies
emphasizing the practicality of AFLMBs.
2. Current Collector Engineering
2.1. The significance of current collector modification
As stated above, the behavior and morphology of Li plating are
significantly different from those of LMBs (Figure
2A).[73] Recently, Lin et al. measured the
pressure and thickness change of AFLMBs and LMBs during electrochemical
cycling using in situ pressure sensors. The pressure and thickness
change were 423 N and 86.33 μm in LMBs, which were much higher than
those of AFLMBs (291 N and 59.39 μm, Figure 2B). These results imply
that the electrodeposited Li on the Cu CC had a relatively dense and
chunky morphology compared to Li electrodeposited on Li metal. Optical
microscopy and SEM also confirmed that Li deposition on Cu CCs was more
favorable and non-dendritic than Li deposition on Li anodes (Figure 2C).
The dominant facet of commercial Li foils is the (200) surface, but the
(110) facet of Li plating renders a much higher electrochemical
reversibility than the (200) surface and occurs
predominantly.[74] The facet mismatch between the
substrate and electrodeposited Li causes unfavorable dendritic Li
growth; hence, Cu can be a better substrate for Li plating than Li. In
other words, the authors argued that AFLMB is a much better system in
terms of Ev because Li plating occurs with less porosity
than LMB. However, AFLMB exhibits severely inferior cyclability compared
to LMBs owing to the lack of excess Li in the anode; therefore, they
adopted an overlithiated cathode
(Li2Ni0.8Co0.1Mn0.1O2)
to replenish the irreversible Li loss. The Cu||Li-rich
NCM pouch cell achieved 976 Wh L-1, which is much
higher than that of Li||NCM (846 Wh
L-1), confirming that AFLMB is a superior system in
terms of the Ev in a practical, high-energy pouch cell,
and the swelling rate of the AFLMB pouch cell was also much better than
that of LMB. As such, the characteristics of the CC greatly affect not
only the cyclability of the AFLMB but also the Ev. As a
result, controlling the morphology of electrodeposited Li through CC
modification has attracted significant attention in the research field
of AFLMB.[75,76,77] We will categorize the
currently published articles on CC engineering into two categories:
artificial coatings and lithiophilic CCs, and we will describe each in
detail.
Overpotential is an important factor for the non-dendritic Li growth.
Pei et al. revealed the correlation between the overpotential (η) and
the critical radius of Li nuclei and areal nuclei density through
classical nucleation theory and electrochemical experiments (Figure
3A).[78] The nucleation barrier for
electrodeposition can be effectively changed by adjusting the
electrochemical supersaturation at the working electrode and the
overpotential of the reduction reaction. During electrodeposition, two
major overpotentials were observed: nucleation overpotential
(ηn) and plateau overpotential (ηp,
Figure 3B). The nucleation overpotential indicates that the potential of
the working electrode (Cu) drops sufficiently to drive the nucleation of
Li embryos, and the plateau overpotential represents the continuous
growth of Li embryos. The formation of a stable embryo of Li atoms is
less favorable and has a higher energy barrier than the addition of a Li
adatom to existing Li nuclei; thus, - ηp is always
smaller than – ηn. According to the classical
nucleation theory, the critical radius of nuclei (rcrit)
is inversely proportional to nucleation overpotential
(rcrit =
2ϒVm/F|ηn|, where ϒ =
surface energy of the Li-electrolyte interface, F = Faraday constant,
and Vm = molar volume of Li). However, the nucleation
and plateau overpotentials could be adjusted by adjusting the current
density in the Li||Cu half-cell configuration because
both overpotentials increased significantly as the current density
increased (Figure 3C). The authors demonstrated from SEM images after
electrodeposition of Li nuclei under different current densities that
the radius of the electrodeposited Li nuclei is inversely proportional
to an increase in current density, that is, an increase in the plateau
overpotential (Figure 3D). In order to achieve non-dendritic Li growth,
larger and fewer Li nucleations are more favorable than smaller and
denser Li nucleation, as shown in the left illustration in Figure 3E. As
a result, current collector engineering for reducing the overpotential
(both ηn and ηp) is considered to be one
of the key strategies for inducing Li plating with a large-grain and
chunky morphology.
A representative strategy to lower the nucleation overpotential and
induce lateral Li growth is to introduce elements capable of forming
alloys with Li, such as Au,[79,80]Ag,[81,82] Sn,[83,84]Zn,[85,86] and Mg.[87,88]Yan et al. investigated the nucleation overpotential of 11 elements (Au,
Ag, Zn, Mg, Al, Pt, Si, Sn, C, Cu, and Ni). In the case of Au, Ag, Zn,
and Mg, the nucleation overpotential is nearly zero because they can
form a solid solution at room temperature, which was confirmed by the
phase diagram of Li–Au. In the case of Pt and Al, the nucleation
overpotentials were observed to be 8 mV and 5 mV at 10 μA
cm-2, respectively, because both elements have
relatively low solubility in Li metal. In contrast, Cu and Ni, which
cannot form an alloy with Li, show a clear overpotential of
approximately 30 mV. C, Sn, and Si can form an alloy with Li metal but
cannot be soluble in Li metal, resulting in a nucleation overpotential
of ~ 15 mV at the same current density. In this context,
researchers have attempted to modify Cu CCs by introducing lithiophilic
elements such as Au,[89]Ag,[90,91,92,93,94]Sn,[47,95,96] Zn,[97]SiOx,[58]Te,[98] and GaInSn[99] to
enhance the electrochemical performance and Li plating morphology of
AFLMBs.
Lin et al. reported an epitaxially-induced plating current collector
(E-Cu) by coating Cu CCs with a liquid metal (Ga: In: Sn =
68.5:31.5:10).[99] All three metals can form an
alloy with Li metal, resulting in a lower nucleation overpotential.
Furthermore, the alloying potential during Li deposition is
approximately 0.75 V (vs. Li+/Li), resulting in a
large proportion of the LiF-rich SEI layer. The coating layer on E-Cu
also enabled the rapid surface diffusion and charge transfer of
Li+, which led to non-dendritic Li growth (Figure 3F).
As a result, the Li growth morphology and electrochemical performance of
E-Cu were highly enhanced compared to those of Cu. In particular, the
CEs, including initial CE (ICE) and average CE (aCE) were largely
increased in Li||Cu half-cell tests with practical
operating conditions (current density = 0.5 mA cm-2,
capacity = 5 mA h cm-2). To prove the practical
application of E-Cu, the authors fabricated a 120-mA h scale AFLMB pouch
cell. Likewise, the ICE and capacity retention after 50 cycles of E-Cu
were recorded as 85% and 84%, respectively, but that of bare Cu only
exhibited 78% and 66%, and the nucleation overpotential of practical
AFLMB full-cell was highly improved (Figure 3G). Benefiting from the low
mass loading (0.14 mg cm-2) of the liquid metal and
anode-free configuration, the energy density of multi-layer
E-Cu||NCM811 reached 420 Wh kg-1.
Zheng et al. recently introduced a chemically lithiated
Li4.4Sn lithiophilic layer for practical AFLMBs.
Initially, tin-plated copper foils were produced by conventional
electroless plating methods,[47] then Sn@Cu CCs
were dipped into a Li-biphenyl solution, which caused chemical
lithiation to form a Li-Sn alloy.[100] Because Sn
has high lithiophilicity and alloyability with Li, both the nucleation
and plateau overpotentials were improved. In addition,
Li4.4Sn@Cu delivered a much higher exchange current
density, which implied faster charge transfer at the interface between
the electrolyte and electrode than that of bare Cu. The lithiophilic and
fast charge transfer kinetics of the Li4.4Sn layer
induced non-dendritic and compact Li nucleation and growth, which was
confirmed by atomic force microscopy (AFM) and SEM images. Finally, the
authors applied Li4.4Sn@Cu electrodes in a practical
pouch cell, which delivered a capacity of 360 mA h and 355 Wh
kg-1 of energy density. During the
50th cycle,
Li4.4Sn@Cu||NCM811 maintained 85.5%
of its initial capacity, but Cu||NCM811 only recorded
56.3% because of the uniform and dense deposition of Li, which
decreased the side reactions with the electrolyte and thus increased the
number of Li ions that could return to the cathode. Moreover, the rate
capability of the Li4.4Sn@Cu||NCM811
full cell was greater than that of the Cu||NCM811 cell
because of the rapid interface charge transfer between the electrode and
electrolyte. Researchers from Samsung have achieved state-of-the-art
all-solid-state lithium metal batteries (ASSB) in an anode-free
configuration with an argyrodite-type sulfide solid electrolyte. The
authors introduced a thin Ag-C nanocomposite layer (5–10 μm) as the
anode instead of the Li metal. Generally, pristine ASSB pouch cells
suffer from nonhomogeneous Li growth and short circuits. In addition,
the current collector-SSE interface cannot maintain sufficient contact,
resulting in non-uniform Li deposition. In contrast, the Ag-C
nanocomposite layer lowered the nucleation barrier by generating a Li-Ag
alloy. Moreover, the majority of the Ag NPs were found at the bottom,
close to the SUS current collector, and the particle size was
significantly reduced from the initial size. This suggests that the Ag
in the Ag-C nanocomposite layer moved in the direction of the CC
continuously during each cycle and did not return to its initial
position. In addition, the Li9Ag4 phase
was observed by XRD after 0.1C charging, and it disappeared and
converted to peaks of Ag in the subsequent discharging, which indicated
the recrystallization of Ag NPs. Owing to the outstanding
lithiophilicity of the Ag-C layer and improvement in Li growth
morphology, the 0.6 Ah class prototype pouch cell delivered a
Ev of over 900 Wh L-1 and superior
cyclability of over 1000 times
2.2. Artificial coating on CCs
Because of the highly reactive nature of Li metal and the low lowest
unoccupied molecular orbital (LUMO) level of the electrolyte, the
electrolytes were chemically and electrochemically degraded to generate
a non-uniform SEI layer during the charging of the AFLMB. The in-situ
generated SEI layer has inhomogeneity in both chemical species and
physical properties; hence, it suffers from uncontrollable Li dendritic
growth and rapid cell failure.[35] On the other
hand, the ex-situ generated SEI layer, the so-called artificial SEI
layer, can sufficiently provide the properties that the SEI layer should
have, such as mechanical strength, dielectric constant, ionic
conductivity, spatial homogeneity, fast ionic conductivity, and low
interfacial resistance. Various materials have been adopted for
artificial coating for many different purposes; therefore, we will
discuss representative substances among them.
Assegie et al. introduced a polyethylene oxide (PEO) film onto Cu CCs
for AFLMB.[49] PEO has been widely adopted in the
research field of polymer electrolyte because of 1) electrochemical
stability, 2) chemical stability with Li metal, 3) flexibility, 4)
regulating Li ion diffusion, 5) electrically insulating nature, 6) wide
potential window, and 7) high dielectric constant for solvation Li ions.
The authors were motivated by the above merits of PEO, so they
fabricated a PEO film by spin-coating approximately 200 nm with a
uniform and defect-less morphology. The thickness of the PEO film was
controlled by the time required for spin-coating. Thicker and
non-uniform PEO coating layer delivered poor electrochemical performance
in the Li||Cu half-cells compared to a thickness of
200 nm, resulting from the inhomogeneous morphology and defects of the
thick PEO film. Active copper may be exposed at the PEO film defect
sites, causing severe Li deposition and dendrite growth. As a result,
the AFLMB full cells with bare Cu and PEO@Cu (200 nm) were tested with
LFP cathode, and ether-based electrolyte and PEO@Cu delivered 15%
higher capacity retention at 100th cycle. PVDF
(polyvinylidene difluoride) has also been widely studied because of its
high dielectric constant and compatibility with Li metal. Tamwattana et
al. introduced not only PVDF, but also LiF nanoparticles to obtain a
higher dielectric constant.[101] There are three
phases of PVDF: α, β, and γ, of which β-PVDF has the highest polarity
and dielectric constant owing to its structural configuration.
Furthermore, the addition of LiF nanoparticles induced dielectric
interactions between PVDF during the film-forming process on the Cu CCs,
resulting in improved β-PVDF yield. The 2.5 μm of LiF@PVDF layer lowered
both the nucleation and plateau overpotential during Li deposition,
indicating a lower nucleation barrier and faster charge transfer than
the α- and γ -phases of PVDF (Figure 4A). The high-dielectric layer
homogenizes the electric field at the interface and mitigates the local
hot spot, resulting in a low overpotential at the interface, which
induces nondendritic growth. As shown in Figure 4B, the high dielectric
layer homogenizes the electric field at the interface and reduces local
hot spots (high local current density), resulting in a low overpotential
at the interface and non-dendritic Li growth. Furthermore,
LiF@PVDF||LFP showed better electrochemical
performance at 0.5 C and 1 C with an ether-based electrolyte in an AFLMB
full cell.
2.3. Carbon
Carbonaceous materials have also been reported to improve the
electrochemical performance of AFLMB, benefiting from their mechanical
strength and flexibility, and Li ion diffusion ability. Assegie et al.
introduced multilayer-graphene (MLG) onto electropolished Cu CCs via
chemical vapor deposition.[50] The number of
layers was carefully controlled to 1–5 layers of graphene by varying
the heating temperature, gas flow rate
(CH4/H2), and growth time. Both the
nucleation and plateau overpotentials were enhanced owing to the
interfacial stability and ability to distribute the local current
density of the MLG. In addition, the cyclability of the MLG-protected
anode was improved compared to bare Cu and single-layer graphene in
Li||Cu half-cells and Cu||LFP
full-cells. In particular, the discharge capacity of MLG anodes at the
100th cycle was approximately 92.62 m Ah
g-1, which corresponds to 61.34% of initial discharge
capacity and ~99% of coulombic efficiency, whereas,
bare Cu only delivered 46% of initial capacity after 100 cycles. The
electrodeposited Li metal on the MLG anodes showed a non-dendritic,
chunky morphology, while bare Cu had highly dendritic, porous features.
Furthermore, spin coating,[48] capillary liquid
bridge,[102] and
micro-patterning[103] were also adopted to
introduce a graphene-like carbon layer for controlling the Li growth
morphology to achieve high-performance AFLMB.
2.3. Metal oxide
Meanwhile, a metal oxide nanofilm with high electrical resistance has
been proposed to control the Li metal deposition morphology (Figure 4C).
Oyakhire et al. introduced sub-10 nm thickness of SnO2,
ZnO, and Al2O3 on Cu CCs via atomic
layer deposition.[104] Among them,
Al2O3, which has the highest electrical
resistance (22650 Ω/square), operated for more than 300 cycles at 1 mA
cm-2 and 1 mA h cm-2 in the
Li||Cu half-cell, while bare Cu, SnO2,
and ZnO-coated Cu only operated under 100 cycles. Unlike the
aforementioned discussions about Li nucleation and growth mechanisms,
the Li growth morphology and electrochemical performance are greatly
improved despite having a very high nucleation overpotential of
approximately 1 V owing to the high electrical resistance of the
Al2O3 layer. The authors suggested that
the high electrical resistance of Al2O3decreases the number of sites for the nucleation of Li metal, inducing
sparse nucleation of Li deposits, and that radial diffusion of Li ions
towards the nucleated deposits favors lateral growth of Li, inducing
dense and low-surface-area Li deposits. To demonstrate this hypothesis,
they fabricated a patterned substrate with both an inactive surface (50
nm thick Al2O3) and an active surface
(25 μm sized Cu hole, Figure 4D). It was observed that Li morphologies
arose from the active surfaces and spread radially outward into flat,
planar, pancake-like deposits when Li was deposited on top of the
patterned substrate in Figure 4D. The authors also extended their
results to practical AFLMB pouch cells, including a state-of-art 1M
LiFSI/FDMB electrolyte and 8-nm-thick
Al2O3 coated Cu CC. Whereas the cell
with bare Cu retained only 40% of its initial discharge capacity of
19.93 mA h after 100 cycles, the
Al2O3-modified CCs retained 60% of
their initial discharge capacity of 14.72 mA h (Figure 4F, G).
3. Electrolyte engineering
3.1. The significance of electrolyte engineering
Modifying CCs using an artificial layer is a powerful strategy for the
uniform deposition and reversibility of
Li.[46,105] Although a suitable cycle life cannot
be retained by CCs modification alone, significant cycle improvement can
be obtained when synergized with electrolyte
engineering.[58,99] During battery operation,
electrolytes not only act as charge carriers but also decompose to form
an SEI layer, which prevents the decomposition of additional
electrolytes and protects Li.[106,107] Because no
excess Li metal exists in AFLMBs, a SEI layer is firstly formed in-situ
during initial charging process. In general, SEI qualities considerably
affect the reversibility of Li.[68] Formation of a
heterogeneous SEI layer causes non-uniform conduction of Li ions,
resulting in dendritic and porous
Li.[42,45,108,109] Consequently, rapid capacity
decay and severe safety issues arise. In addition, a large volume change
during the stripping/plating process leads to rupture of the SEI layer,
which causes the consumption of fresh Li and electrolyte depletion.
Therefore, it is important to form a robust, thin, uniform SEI layer. As
the composition and uniformity of the SEI layer significantly depend on
the electrolyte, the reversibility of the Li metal is greatly affected
by the composition of the electrolyte.[68]Therefore, electrolyte engineering is crucial in AFLMBs.
3.2. HCEs and LHCEs
Although conventional carbonate electrolytes have been widely used for
LIBs and LMBs because of their high conductivity, oxidative stability,
and cost effectiveness,[110] high reactivity to Li
metal and the formation of a poor SEI layer cause rapid degradation of
Li metal. Therefore, ether electrolytes, which are more compatible with
Li metal anodes, have been applied to LMBs and
AFLMBs.[111,112] However, ether is not suitable
for high-voltage cathodes such as NMC, lithium manganese nickel oxide
(LMNO), and lithium nickel oxide (LNO) because of oxidative
decomposition at ~4 V.[45,113]Therefore, various strategies have focused on simultaneously extending
the anodic stability of ethers and building a robust high-quality SEI
layer.[45] In Figure 5A, there are many free
solvents that do not solvate Li ions at normal electrolyte
concentrations (~1 M). These free solvents are
susceptible to oxidation at the cathode and to reduction at the anode.
However, as the salt concentration increased (>3 M), most
of the solvents were coordinated to Li ions, reducing the free solvents.
Therefore, high-concentration electrolytes (HCEs) with ether solvents
can endure oxidative environments (above 4
V).[44,109] In addition, the ratio of anions
participating Li ion solvation sheath increases, forming contact ion
pairs and aggregates for HCEs (Figure 5A). Because anions coordinated
with Li ions are more likely to decompose when Li ions are reduced, the
anion-derived SEI greatly improves the Li plating/stripping
efficiency.[42,114] Qian et al. proved the
feasibility of AFLMBs using ether based high concentration electrolyte,
4 M LiFSI DME[44] motivated by the previous study
of their group[110] showing that the reversibility
of Li||Cu half-cells was greatly improved by 4 M LiFSI
DME. As shown in Figure 5B, the capacity of Cu|| LFP
decreased significantly within a few cycles when conventional carbonate
electrolytes (1 M LiPF6 EC/DMC) were used.
Interestingly, 4 M LiFSI DME improved the CEs (>99%) and
achieved high capacity retention of 60% after 50 cycles. This is
attributed to the nodular and more compact morphology of plated Li metal
compared to carbonate electrolytes owing to the anion-derived SEI layer.
Beyene et al. reported the synergistic effect of 3 M LiFSI DOL/DME (1:1,
v/v) with a resting protocol.[66] By plating Li at
a low current rate and resting it for 24 h, they found that a uniform
LiF-rich robust SEI layer was formed (Figure 5C) during the rest period.
However, in the case of 1 M LiPF6 EC/DEC (1:1, v/v), a
thick organic SEI layer was generated during the rest period because of
decomposition of the free solvents. As shown in Figure 5D, the stability
of the cells with the resting protocol was increased owing to the
synergistic effect with HCEs when using 3 M LiFSI DOL/DME. In contrast,
the cells with 1 M LiPF6 EC/DEC exhibited a faster
capacity decay with the resting protocol. In addition,
Cu||LFP cells with 3 M LiFSI DOL/DME achieved 35%
capacity retention after 95 cycles under the condition of 1.0 mA
cm-2.
However, the high viscosity of the HCEs lowers their ionic conductivity
and causes electrode wetting.[111,115] In
addition, the use of many salts increases the
cost.[111] To solve these problems, the co-solvent
is mixed with HCEs. Co-solvents are miscible with the solvent, lowering
the viscosity but unable to dissociate the salts, therefore not
affecting the Li ion coordination structure (Figure 5A). This
electrolytes with unique solvation are called localized high
concentration electrolytes (LHCEs). In addition, the wettability and
non-flammability were improved by the addition of
diluents.[115] Despite the low compatibility of
carbonates with Li metal, Hagos et al. used LiPF6 salts
with carbonates considering the cost and anodic stability of
solvents.[108] The addition of FEC diluent lowered
the viscosity of 2 M LiPF6 EC/DEC (1:1, v/v) +50 % FEC
(E2) from 8.571 mPa s to 5.680 mPa s. Using Raman spectroscopy, they
found that the Li ion solvation structure was not affected by the
addition of FEC (Figure 6A). MD simulations confirmed that the ratio of
EC and PF6- in the solvation Li ion
sheath was increased by the FEC diluent. Thus, LiF increased in the SEI
layer by the reduction of PF6- anions
(Figure 6B). As a result,
Cu||LiNi1/3Mn1/3Co1/3O2cells with E2 electrolytes attained 40% capacity retention at the
50th cycle, showing a significant improvement in
carbonate electrolytes. However, carbonate-based LHCE electrolytes
exhibit limited cyclability compared to ether
electrolytes.[42] Ren et al. used
1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl (TTE) as a diluent in
LiFSI DME (LiFSI-1.2DME-3TTE in molar ratio). The addition of TTE
lowered the viscosity of the electrolytes to 1/10 (4.8 cP) and improved
their wettability and ionic conductivity. In addition,7Li NMR analysis showed that the addition of the
diluent did not affect the Li-ion solvation structure. Furthermore,
highly fluorinated TTE not only contributes to the thin inorganic
cathode electrolyte interphase (CEI) layer formation at the cathode
(Figure 6C), but also promotes further decomposition to form a uniform
anion-derived SEI at the anode through migration of the LUMO towards the
FSI- anions. As shown in Figure 6D, the LiF-rich SEI
layer promoted compact and low-surface Li metal deposition. Owing to
synergistic effect of TTE on both anode and cathode,
Cu||NMC811 cells with lean electrolytes (3 g
Ah-1) showed 77% capacity retention after 70 cycles
and attained high energy density of 412 Wh kg-1(Figure 6E).
3.2. Dual salts electrolytes
Compared to a single salt, the co-existence of anions changes the
quality of the SEI layer by physical properties and decomposition
chemistry of each salt, resulting in significant effects on Li-metal
protection. Motivated by the synergistic effect of dual salts, Yao et
al. mixed relatively inexpensive LiTFSI with LiFSI and dissolved them in
DOL/DME (1:1, v/v) to synthesize dual-salt electrolytes. They determined
that LiFSI is more susceptible to reduction than LiTFSI, which is
responsible for the development of ionic conductive inorganic species
(LiF and Li2O) through preferential FSI- decomposition.
In addition, LiTFSI improved ionic conductivity. Owing to the
synergistic effect of the binary salts LiFSI and LiTFSI, a compact and
robust SEI was formed, improving the reversibility of Li. Xu and
co-workers group reported a high concentration of LiFSI with LiTFSI
dissolved in DME (4.6 m LiFSI + 2.3 m LiTFSI in DME, BSEE) to form a
stable electrolyte even at high voltage (4.4 V) while stabilizing the
plating/stripping of Li metal.[109] It was found
that the conductivity was not significantly different from the reference
1.0 m LiPF6 EC/EMC (3:7, v/v), Gen 2, even when a high
concentration of dual salt were used. Furthermore, LiTFSI prevented the
precipitation of LiFSI. From the XPS spectra, they found that more
Li2O and Li2S appeared in the single
salt electrolyte, 4.6 m LiFSi-DME (SSEE) due to the decomposition of
LiFSI during the initial cycle and after 200 cycles. However, these
species appear less in BSEE because of the presence of LiTFSI.
Initially, LiTFSI was decomposed, forming a C-F moiety and further
reducing to LiF after cycling. Combined with computational simulations,
they found that TFSI- anions affect the decomposition
kinetics of FSI- anions, and a uniform and robust SEI
layer is formed compared to the fast FSI-decomposition. The oxidation stability of BSEE increased up to
approximately 5 V by LSV analysis and was even more stable than Gen 2.
As a result of synergistic effect of dual salts,
Cu||NMC622 anode-free full cells operated at C/10 and
D/3 with BSEE had a high initial coulombic efficiency of as well as
capacity retention of 90.9 mA h g-1 and high aCEs of
98.6 % after 54 cycles. In particular, in tests with various
compositions, while maintaining a constant total salt concentration, it
was found that an appropriate ratio of LiFSI to LiTFSI severely affects
the reversibility of Li. In 2019, Dahns group performed impressive work
by combining dual salts in carbonates.[43] They
designed practical concentration of salts (0.6 M LiDFOB, 0.6 M
LiBF4) dissolved in FEC:DEC (1:2, v/v). Anode-free pouch
cells with 0.6 M LiDFOB 0.6 M LiBF4 delivered 80 %
initial capacity for 90 cycles, outperforming the single salt cells and
other combinations of salts. This outstanding improvement in the
capacity retention of the dual-salt electrolyte was attributed to the
formation of compact bulky Li metal. This is because not only LiF but
also organic fluorine components are found, unlike LiPF6in the F 1s spectra for dual-salt electrolytes. This difference in SEI
composition induces a large change in the deposition morphology of Li.
However, continuous decomposition of salts eventually causes cell
degradation. In 2020, Dahn’s group investigated the electrolyte
degradation of dual-salt electrolytes and developed optimized
electrolytes.[116] They found that a favorable
plated Li morphology can be ascribed to SEI layer formation by the
continuous decomposition of salts. During cycling, the oxidation of
LiDFOB in the presence of FEC solvents at the positive electrode builds
a CEI layer, and unreacted LiDFOB generates LiBF4 by
reacting with polymeric byproducts of LiDFOB, which is beneficial to the
Li anode.[117] In addition, reduction of FEC make
CO2 and LiF.[118,119] In order to
compensate for the consumption of salts, they used high concentration
dual salt electrolytes by combining 2 M LiDFOB and 1.4 M
LiBF4 in FEC/DEC (1:2, v/v). With hot formation cycles
and pressurized conditions,[55,120] anode-free
pouch cells with high concentration dual salt electrolytes delivered 80
% of initial capacity after 200 cycles.
3.4. Solvents modification
HCE and LHCE changed the Li-ion solvation structure, resulting in an
anion-derived SEI. This Li-ion-conductive SEI layer is conducive to
compact and uniform Li deposition. However, a large amount of salt usage
increases the cost, and the co-solvent, which takes a large portion of
electrolytes, increases the ionic resistance and often requires unstable
solvents for Li-ion solvation.[121,122] Therefore,
an advanced solvent contributing anion-rich SEI layer through
appropriate Li solvation with a low salt concentration while having
little reactivity with Li metal is required. Motivated by the high
(electro)chemical stability of alkyl chains and F groups, Yu et al.
rationally designed fluorinated solvents.[45]First, the alkyl chain of ether was lengthened to synthesize
1,4-dimethoxylbutane (DMB). They then fluorinated the extended alkyl
groups of DMB to produce 1,4-dimethoxybutane (FDMB). LSV analysis showed
that the oxidation stability of LiFSI/FDMB was significantly improved to
6.14 V compared to LiFSI/DME (3.9 V) and LiFSI/DMB (5.2 V). Furthermore,
MD simulations revealed that Li ion binds to O, and F weakly in FDMB.
Owing to the unique solvation structure of FDMB, the ratio of
FSI- to solvent in the first solvation shell of
Li+ was 3.29, which was significantly higher than that
of DME (2.31) and DMB (2.29). This leads to a thin (~ 6
nm) and homogeneous SEI, which benefits Li ion conduction. Owing to high
compatibility with Li metal and oxidation stability of FDMB,
Cu||NMC532 anode-free pouch cells delivered 80% of
capacity retention after 100 cycles (CE, 99.98 %). Cui’s group further
extended the fluorinated chains of FDMB to improve the stability of Li
metal and oxidation.[113] However, the low Li ion
solvating ability of solvents with more -CF2- groups
reduces the ionic conductivity. Because of the low Li ion solvation
power of FDMB analogs, DME was mixed to reduce the ionic resistance.
Considering the stability of the electrolytes and ionic conductivity,
1,6-dimethoxyhexane (FDMH) with two more -CF2- chains
than FDMB with DME (LiFSI/6FDMH-DME,
vFDMH:vDME = 6:1) was chosen (Figure
7A). The optimized LiFSI/6FDMH-DME showed a high oxidation stability of
over 6 V and low interfacial resistance owing to the synergistic effects
of FDMH and DME. MD simulations revealed that both FDMH and DME
participate in Li-ion solvation, which is different from the concept of
LHCEs in that FDMH participates in Li-ion solvation. As shown in Figure
7B, Li||NMC532 shows high rate performance with high
capacity retention (over 80 %) at 1 C and excellent reversibility as
well due to improved kinetics of Li deposition. In addition, anode free
Cu||NMC811 pouch cells achieved 75% capacity
retention after 120 cycles (Figure 7C).
3.4. Additives
Various additives have been added to the electrolytes of LIBs and LMBs
to form a stable interphase on the electrodes, enhance their physical
properties, and improve safety.[123,124,125] Among
them, vinylene carbonate (VC) is known for improving the stability of
the anode by forming a polymeric SEI, which suppresses the volume
expansion. Brown et al. investigated the effect of VC in anode-free
Cu||LFP cells by adding VC to 1.2 M
LiPF6 EC:EMC and substituting solvents to
VC.[126] As shown in Figure 7D, the main SEI
components changed from lithium ethyl dicarbonate,
Li2CO3, and LiF to poly(VC) when VC
additive was added. These polymeric SEI components improve the
reversibility of Li metal in carbonate electrolytes. Similarly,
LiNO3 is a widely used additive in LMBs because of the
high ionic conductivity of the N species in the SEI layer
(Li3N,
LixNOy).[127]Motivated by the beneficial effects of LiNO3, Brown et
al. dissolved LiNO3 in
carbonates.[128] To increase the solubility of
LiNO3 in carbonates, they added triethyl phosphates
(TEP) (1M LiDFOB + 0.2M LiNO3 TEP/EC/DMC (8.4:8.4:83.2,
v/v/v)). As shown in the N 1s spectra (Figure 7E) ,
NO3-,
NO2- and N3- species
were detected on the cycled Li metal by the decomposition of
LiNO3. As a result, compared to the baseline electrolyte
(1 M LiDFOB EC/DMC (16.8:83.2, v/v)), the modified electrolytes with
LiNO3 and TEP doubled the cycle stability of anode-free
cells by the favorable effects of N species stabilizing the Li metal.
The uneven deposition of Li by tip effects accelerates the dendritic
growth of Li.[129,130] Alkali metal
(Cs+, K+, Na+)
ions have been exploited as additives for
LMBs.[54,131] During Li deposition, alkali metal
ions are aggregated at the tip where the electric field is concentrated
and repel Li ions through electric repulsion preventing tip growth
(Figure 7F). Synergistically, anions decompose to form a Li
ion-conducting SEI layer. Sahalie et al. used KNO3 as an
additive in 1M LiPF6 EC/DEC.[132]With the advantages of shielding effect of K ions and formation of
favorable SEI from NO3- anions,
Cu||NMC cells achieved 42% of capacity retention
after 51 cycles. Furthermore, Hagos et al. used 2 wt%
KPF6 and 2 vol% tris(trimethylsilyl) phosphite (TMSP)
as dual additives to 1M LiPF6EC/DEC.[133] Like KNO3 additives,
K ions prevent the tip growth of Li, and
PF6- contributes to the Li-ion
conduction interphase. In addition, TMSP suppresses SEI degradation by
scavenging HF, which is generated by traces of water and
PF6-.[134] Owing
to the synergistic effect of dual additives, Cu||NMC
cells showed 48% capacity retention after 20 cycles. As stated for
electrolyte modification, tremendous efforts have been made to configure
the inorganic-rich SEI by modifying the Li ion solvation
structure.[42,45,109] In 2022, Kim et al.
dispersed Li2O nanoparticles to 1 M
LiPF6 in EC/DEC.[135] As shown in
Figure 5D, radial distribution functions (RDFs) revealed that the ratio
of fluorinated species (PF6- and FEC)
to non-fluorinated (DEC and EC) increases around Li2O.
Owing to the formation of an inorganic-rich SEI layer by the
decomposition of F-containing species in the Li ion solvation sheath,
Li2O suspension electrolytes stabilize the Li metal with
improved CE. With the Li2O suspension electrolytes, both
the initial and average CEs were improved for the
Cu||NMC811 cells. More importantly, the
Li2O suspension was applicable to recently reported
advanced electrolytes (LHCEs and fluorinated
solvents).[42,45] Although the improvement in CE
was small compared to that of carbonate electrolytes, the
Li2O suspension improved the CEs of the cells with
advanced electrolytes, suggesting the versatility of suspension
electrolytes. Dahn’s group explored the effects of conventionally used
various additives and co-solvents to their dual-salt
electrolytes,[43] 0.6 M LiDFOB 0.6 M
LiBF4 in FEC:DEC (1:2, v:v). They reported 65 different
electrolyte formulations and compared the total energy delivery over 140
cycles.[52] Of the 65 electrolyte additives, only
tris (2,2,2-trifluoroethyl) phosphate (TTFEP), p-toluene sulfonyl
isocyanate (PTSI), 1,5-dicyano pentane (DCP), and LiClO4showed positive effects, and the other additives deteriorated the
performance. A variety of electrolytes can be prepared by the
combination of additives and base electrolytes, but the appropriate
species and quantity of additives greatly affect the performance of the
cells. This shows that electrolyte optimization with additives is a
complex and time-consuming process, and the mechanism is not yet fully
understood.
4. Other system
4.1. Sulfur
Owing to the high price, toxicity, and limited specific capacity
(~ 200 mA h g-1) of intercalation-type
cathodes,[136] conversion-type cathodes have
attracted enormous attention.[137,138] Among them,
lithium sulfide (Li2S), which is a fully discharged
state of sulfur (S), is an attractive material with a high theoretical
specific capacity (1166 mA h g-1) and its
applicability as a cathode material for
AFLMBs.[59,139] In 2018, Nanda et al. reported
anode-free lithium-sulfur batteries (AFLSBs).[59]AFLSBs were composed of a Li2S/CNT cathode with a bare
Cu foil (Figure 8A). A significantly different capacity retention of
Li2S/CNT cathode was observed compared to that of the LFP cathode in
Figure 8B. Capacity of Cu||LFP cathode decreased
rapidly within a few cycles (2.2 % after 10 cycles). On the other hand,
Li2S/CNT cathode retained 51.5% of its initial capacity
even after 100 cycles. This excellent cycle performance of the AFLSBs is
attributed to the intrinsic properties of stabilizing the Li metal in
the (AF)LSBs.[140] During the charge and discharge
processes of (AF)LSBs, intermediate polysulfide
(Li2Sx, 2 < x ≤ 8) species
diffuse to the anode and reduce to ionic conductive Li2S
and Li2S2 by reacting with Li metal and
stabilizing Li deposition.[141] The use of
Li2S as an active material for anode-free systems is not
only advantageous in terms of capacity but also greatly stabilizes the
stripping and plating of Li metal. Although AFLSBs have shown higher
cyclability than AFLMBs with NMC and LFP cathodes, it is still difficult
to achieve more than 100 cycles without modification. To improve the
reversibility of Li metal, Manthiram’s group incorporated 10 wt% (vs.
Li2S) of Te as a cathode
additive.[11, 60] Anode-free
Ni||(Li2S+0.1 Te) full cells at C/5
attained 50 % capacity retention after 240 cycles (Figure 8C). On the
other hand, in the case of AFLSBs without Te, the capacity retention
dropped to 50 % after only 34 cycles. The improved cycle stability with
the Te additive is attributed to the formation of soluble
polytellurosulfides
(Li2TexSy), which are
derived from the reaction between Te and lithium polysulfides. As
illustrated in Figure 8D, these soluble
Li2TexSy diffused to the
anode and reacted with Li metal, forming
Li2TeS3 and Li2Te on the
Li metal anode. The covalence of the Te–S bonds increases the Li ion
diffusivity through the SEI layer. Te additives are advantageous for
improving the stability of Li metal by simply mixing Te with
Li2S without complex modifications of the anode or
cathode owing to the unique characteristics of LSBs. To realize
high-performance AFLSBs, not only the stability of the anode, but also
the problems of the cathode, such as the low electrical conductivity of
Li2S, low kinetic shuttling of lithium polysulfides, and
low utilization of Li2S must be solved
simultaneously.[142,143,144] In this regard, He et
al. synthesized a Li2S/electrocatalysts
(Li2S-Co/Co9S8)
nanoparticle composite via the carbothermal reduction reaction of
Li2SO4 and CoSO4 (Figure
8E).[145] Nano-sized Co-based catalysts in
intimate contact with Li2S accelerate the polysulfide
conversion reaction and suppress the shuttle effect, resulting in a high
Li2S utilization ratio. Finally, they introduced
Te[60] into the
Li2S−Co9S8/Co cathode to
stabilize the anode
(Li2S−Co9S8/Co−Te). With
high Li2S loading (4 mg cm-2) at 0.1
C, they compared the performance of
Li2S−Co9S8/Co−Te to
Li2S-C (without electrocatalyst and Te) and
Li2S−Co9S8/Co (without
Te) cathodes (Figure 8F). As expected,
Li2S−Co9S8/Co−Te cathode
delivered high initial capacity of 1025 mA h g-1 and
low capacity decay (84% capacity retention after 100 cycles).
Surprisingly, anode-free pouch cells modified with
Li2S−Co9S8/Co−Te under
practical conditions (Li2S, 4 mg cm-2and lean electrolyte 4.5 μL mg-1) achieved promising
results with a high energy density of 221 W h kg-1. To
enhance the cyclability and capacity of AFLSBs, it is necessary not only
to stabilize the anode, but also to prevent the dissolution of
polysulfide and Li2S occurring at the cathode at the
same time.
4.1. Li reservoir
Although irreversible Li loss caused by dead Li and SEI formation occurs
in the same manner for AFLMBs and LMBs, replenishment of Li loss does
not occur in AFLMBs, resulting in a significant difference in
cyclability between LMBs and AFLMBs. To compensate the initial
irreversible loss of Li, pre-lithiation strategies have been widely used
in LIBs[146] to compensate the Li loss during
initial charge. Similarly, Wang’s group demonstrated an in-situ-built Li
reservoir during the initial charge.[147] They
combined a Li-rich Li2CuO2 (LCO)
additive with NCM811, (80-x)NMC-xLCO. LCO delivered an irreversible
capacity of 321 mA h g-1 within a potential window of
3.2~4.2 V. As shown in Figure 9A, the Li reservoir was
successively built by incorporating LCO; the reservoir size increased as
the amount of LCO increased. With two formation cycles at 0.1 mA
cm-2 for formation of the Li reservoir, cycle
stability of the Cu/NMC-LCO cells was largely extended. In 2021,
motivated by the sacrificial Li compound strategy, Qiao et al.
incorporated lithium oxide (Li2O) as a sacrificial agent
in the cathode for AFLMBs.[148] It should be noted
that Li2O has been studied extensively in metal-air
batteries and as a sacrificial additive for LIBs because of its high
theoretical capacity (O2 + 4Li+ +
4e- Li2O, 1793 mA h
g-1).[149,150] By mixing 25 wt%
Li2O with NCM811, an irreversible capacity of 320 mA h
gNCM-1 attributed to the oxidation of
Li2O appeared during the initial charging process. TTE
additives were used to inhibit the oxygen evolution reaction. The
nucleophilic reaction of TTE and the superoxide anion
(O2-) prevents oxygen evolution and
produces a cathode electrolyte interface (CEI) layer composed of LiF,
which further increases the oxidative stability of the ether
electrolyte. When fabricated as anode-free pouch cells with
Li2O additive, not only a high energy density of 320 W h
kg-1 was achieved, but also no significant capacity
decay was observed over 200 cycles and 80 % of initial capacity was
maintained after 300 cycles. The Li donor reacts irreversibly during the
first charging process to form a Li reservoir, and it is very effective
in increasing the lifespan of AFLMBs by supplementing Li loss. Huang et
al. used lithium oxalate (LO) as an additive in the cathode as the
sacrificial Li source.[151] During initial
charging, LO irreversibly oxidizes at 4.7 V to form Li reservoir and
carbon dioxide (Figure 9B). As synthesized CO2 diffused
to the anode and formed an Li2CO3-rich SEI layer on the Li
metal.[152] Furthermore, Dong et al. reported
Li2CO3 additive in AFLMBs. In addition
to the formation of a Li reservoir from
Li2CO3 incorporated into the cathode
during the initial charge process,
Li2CO3 coated on the anode reacts with
PF5- resulting in a LiF-rich SEI
layer. The dual function of the additive resulted in anode-free
Cu@Li2CO3/NCM811@Li2CO3cells delivering high capacity retention (81.60%) at 1/3 C after 100
cycles.[153] However, the additive takes up
weight, which lowers the energy density after the first charging
reaction. Therefore, additional additives improve the cycle stability
while simultaneously sacrificing the energy density. In the absence of
additives, Lin et al. synthesized Li-rich Li2NCM811
(Li2[Ni0.8Co0.1Mn0.1]O2)
to build a Li reservoir in the anode.[62] When
additional Li ions are introduced into NCM811, they are partially stored
in tetrahedral sites and the NMC811 is transformed into P3m1 type
Li2NCM811 (Figure 9C). After delithiation, the structure
reversibly changed to R3m NCM811. With a partially over-lithiated
Li2NCM811 cathode, the pouch cell composed of
Li1.37[Ni0.8Co0.1Mn0.1]O2cathode enabled high capacity retention of 84% at
100th cycle and achieved high energy density (447 Wh
kg-1) in lean electrolyte (2 g [A
h]-1) and high loading system (25 mg
cm-2) in Figure 9D.
5. Protocols
AFLMBs suffer from drastic capacity decay upon
cycling.[44,57,154] Electrolytes and CCs
modifications have improved the CEs of AFLMBs. In addition to the
internal components of cells, various operational conditions of cells
significantly affect the electrochemical
performance.[155,156] Among these, the application
of external pressure is widely used for (AF)LMBs to reduce SEI cracking
and dendrite formation.[157] Therefore, most
anode-free pouch cells are tested under pressurized condition. Dahn et
al. investigated the effects of pressurized conditions on anode-free
pouch cells with two different electrolytes (1M LiPF6dissolved in FEC:DEC and FEC: bis(2,2,2-trifluoroethyl) carbonate,
TEFC).[56] Surprisingly, cyclability of cells
containing FEC:DEC were greatly improved to 100 cycles at 1725 kPa in
Figure 10A. However, a higher pressure (above 1725 kPa) did not further
enhance the cyclability because of electrode degradation by polarization
growth. In contrast, the cycle life of cells with FEC:TEFC was saturated
at a relatively low pressure of 795 kPa. In addition, as shown in Figure
10B, the morphology of the plated Li at high pressure (485 kPa) with
more fluorinated solvents (FEC:TFEC) was more compact than that with
FEC:DEC because of contribution of fluorinated solvents to form LiF-rich
SEI layer. Therefore, although higher pressure is beneficial to Li
deposition, proper pressurized condition should be set considering
electrolyte properties.
In addition, the current density seriously affects dendritic growth as
Li ions are depleted near surface of Li under a higher current density.
In 2020, Louli et al. investigated the effects of symmetric and
asymmetric charge/discharge rates.[120] They
showed that an asymmetric slow charge benefits cyclability more than an
asymmetric fast charge or a symmetric charge and discharge (Figure 10C).
This was because the slow charge induced a low concentration gradient of
Li ions near the electrode surface, leading to uniform Li growth.
Conversely, fast discharge is beneficial for preferential Li stripping
owing to the high localized current density of the
protrusions.[158,159] Therefore, a flat uniform
surface of Li metal was built by an asymmetric faster charge. However, a
slower charge results in dendritic and tortuous Li metal with high
Li-ion gradients on the electrode surface, which deteriorates the cycle
life. As shown in Figure 10D, a high-capacity throughput was delivered
by the asymmetric slower charge.
Also, adjusting the depth of discharge (DoD) affects the cycle stability
of the AFLMBs. The Li reservoir could be formed by controlling the
discharging cutoff voltage because Li was not completely removed from
the anode owing to the slow kinetics of the cathode during the discharge
process (Figure 10E).[120] In-situ built Li
reservoirs are favorable for extending cycle stability until depletion,
as in general Li excess LMBs or Li reservoir strategy of ALFMBs. Louli
et al. investigated the correlation between the lower cut-off voltage
and the cycle life of anode-free CuNMC532 cells. After fully charging to
4.5 V, various cut-off voltages were set to investigate the effects of
DoD (Figure 10E). As lower cut-off voltage is set from 3.0 V (90% of
DoD) to 4.05 V (23% of DoD), 1.7 ~ 11.5 μm of excess Li
reservoir is built. Anode-free pouch cells with the lowest DoD can
maintain capacity without decay for more than 1000 cycles because of the
continuous replenishment of Li, while other cells fail more quickly as
the DoDs deepen. Although a low DoD protocol is beneficial for extending
the cyclability and high capacity throughput, as shown in Figure 10F,
the discharge capacity is limited, requiring a frequent charging process
because the amount of Li used for charging and discharging is small.
Therefore, it is important to precisely set the cut-off voltage while
carefully considering the balance between cycle stability and discharge
capacity. Furthermore, temperature is an important parameter for
electrochemistry because it affects the diffusion, viscosity, and rate
of the decomposition reaction of the
electrolyte.[155,160] In particular, Li becomes
softer as the temperature increases, inducing lateral and compact
morphologies.[55] However, decomposition of salts
could be accelerated resulting in unsatisfactory cycle performance.
Genovese et al. used a hot formation protocol to preserve the
electrolyte and benefit from high
temperatures.[55] As shown in Figure 10G, the
capacity retention of the anode-free pouch cell improved when operated
at 40 °C; in contrast, a large capacity decay occurred for the cells
operated at 20 °C. However, the cycle stability of the cell operated at
low temperature was improved dramatically by two initial cycles at 40 °C
with asymmetric charge and discharge (C/10 and D/2) between
3.6~4.5 V. This significant improvement in the hot
formation protocol comes from the beneficial SEI layer through active
CO2 gas generation by the decomposition of LiDFOB.
Combined with high pressure, anode-free Cu||NMC532
batteries with hot formation protocols operated at low temperatures
achieved over 200 cycles with 80% capacity retention (Figure 10H).
6. Perspective
Despite numerous efforts dedicated to the research of AFLMB, there are
still many challenges to overcome, including its extremely low
cyclability. As a result, in this section, we focus on the broad issues
and prospects for future research on AFLMBs.
6. 1. Designing practical electrolytes
Commercial carbonate-based electrolytes for LIBs are not applicable to
practical AFLMB because of their higher reactivity with Li metal. Even
at high operating pressures, the discharge capacity of pouch cells
containing commercial carbonate-based electrolytes reached zero in
approximately 30 cycles. In this context, ether-based electrolytes have
attracted considerable attention owing to their compatibility with Li
metal. As mentioned earlier, the low oxidation stability, which is
considered a major disadvantage of ether-based electrolytes, was
alleviated by a HCEs, LHCEs, or fluorination of solvent molecules. In
particular, fluorination of solvent molecules not only improves their
oxidative stability but also increases the proportion of LiF in the SEI
layer, resulting in state-of-the-art AFLMB pouch-cell performance. On
the other hand, these strategies have a trade-off relationship between
electrochemical performance and high contents of fluorine elements in
electrolytes, which causes severe cost and environmental
issues[45,113]. As a result, fluorine-free or
low-fluorine electrolytes have gained immense attention in the research
field of alkali metal anodes.
6. 2. Practical test conditions
AFLMBs meet the social need for high energy density, which can maximize
the range of electric vehicles and expand their applications.
Accordingly, a low E/C ratio (amount of electrolyte to the capacity of
the cathode, under 3 g Ah-1) and high areal capacity
of the cathode (over 3.8 mA h cm-2) should be
satisfied to achieve a high energy density
AFLMB[161]. However, many papers have recently
reported fabrication of AFLMB full cells with low-loading cathodes and
flooded electrolytes. In such cases, assessments of the practical
application of the strategy may be doubted because the cyclability of
AFLMB under harsh conditions is completely different from that under
mild conditions. For instance, when 4 mA h cm-2 of the
cathode is operated at 0.25 C, a current density of 1 mA
cm-2 is applied to the anode (if the sizes of the
cathode and anode are equal), but in the case of 1 mA h
cm-2 an equal current density is applied at 1 C. In
addition, electrolyte drying due to the reaction with Li metal increases
dramatically with Li utilization. Consequently, the test protocol and
conditions should be carefully established to demonstrate the practical
application of this strategy.
6. 3. Introducing sacrificial cathode additives
The inherent limitations of the AFLMB are obvious. With zero excess Li,
even if a Coulombic efficiency of 99.9% is achieved, only 80% of the
initial capacity can be maintained for 223 cycles. Therefore,
sacrificial cathode additives, such as
Li2O[148] and
LCO[151] which provide excess Li at the cathode,
should be introduced to extend the cycle life of AFLMBs. Because an LMB
with a thin Li anode suffers from 1) high production cost, 2)
energy-intensive, repetitive rolling process, and 3) thickness control
limit, sacrificial cathode additive in AFLMB is considered to be a
better option for excess Li. Moreover, the introduction of sacrificial
additives slightly reduces the energy compared to the typical AFLMB, but
it is believed that a high energy density and cyclability can be
achieved simultaneously by investigating the optimal point for the
capacity of the sacrificial additive. Finally, it is anticipated that by
simultaneously implementing the various fundamental approaches outlined
above, many issues with AFLMBs can be resolved.
7. Conclusion
In conclusion, AFLMBs are regarded as promising systems because of their
unique cell configuration, in which the energy density is maximized.
Furthermore, the absence of highly reactive Li metal for initial cell
fabrication gives rise to significant advantages, such as easy
processability, reduced cost, and high safety. However, the rapid
capacity degradation and low cyclability by parasitic reaction of Li and
low reversibility are major concerns for AFLMBs. In this review,
fundamental issues and major challenges are presented, and recently
reported major strategies are summarized. To overcome challenging nature
of metallic Li, many efforts have been devoted to suppressing dendrites,
dead Li, electrolytes consumption by focusing on developing beneficial
layer through artificial coating on CCs and electrolytes modification.
In addition, various protocols are proven to be effective in extending
cycle life. To cope with the limitation of limited inventory of Li,
these strategies should be combined together to give synergistic effect
in enhancing reversible reaction of Li. Finally, as we provide in
perspective, practical electrolytes should be considered and tested with
reliable testing conditions and impact of various sacrificial agents
should be explored for future AFLMBs.