Abstract
Owing to the emergence of energy storage and electric vehicles, the
desire for safe high-energy-density energy storage devices has increased
research interest in anode-free
lithium metal batteries (AFLMBs). Unlike general LMBs, in which excess
Li exists to compensate for the irreversible loss of Li, only the
current collector is employed as an anode and paired with a lithiated
cathode in the fabrication of AFLMBs. Owing to their unique cell
configuration, AFLMBs have attractive characteristics, including the
highest energy density, safety, and cost-effectiveness. However,
developing AFLMBs with extended cyclability remains an issue for
practical applications because the high reactivity of Li with limited
inventory causes severely low Coulombic efficiency, poor cyclability,
and dendrite growth. To address these issues, tremendous effort has been
devoted to stabilize Li-metal anodes for AFLMBs. In this review,
we highlight the importance and
challenges of AFLMBs. Then, we thoroughly review diverse strategies,
such as modifying current collectors, the formation of robust interfaces
by engineering advanced electrolytes, and operation protocols. Finally,
a future perspective on the strategy is provided to insight into the
basis of future research. We hope that this review provides a
comprehensive understanding by reviewing previous research and arousing
more interest in this field.
1. Introduction
Since the first commercialization
of lithium-ion batteries (LIBs) by Sony Corp. in 1991, LIBs have been
successfully used in applications ranging from small portable devices to
grid energy storage systems.[1,2] In the
21st century, global environmental issues have driven
the development of electric vehicles (EVs) and renewable energy, which
require greater energy storage density. However, state-of-the-art LIBs
have almost reached the theoretical limit of energy density
(~ 300 Wh kg-1) because of the low
theoretical capacity of intercalation-type electrode materials (e.g.,
graphite and lithium metal oxides).[3,4,5]Therefore, next-generation anodes such as alkali metals (Li,
Na,[6,7,8,9,10]K,[11,12,13,14]), alkaline earth metals
(Mg,[15,16,17,18,19]Ca[20,21,22,23,24]), and multivalent metals
(Zn,[25,26,27,28,29]Al[30,31]) are in the spotlight to go beyond LIBs.
Among them, Li metal has gained the most attention as a next-generation
anode material to overcome the theoretical limitations of
intercalation-type anodes.[4,32] Li metal has very
promising characteristics for high energy density, namely the lowest
reduction potential (-3.04 V vs. standard hydrogen electrode, SHE), high
theoretical capacity (3,860 mA h g-1), and being
lightweight (0.53 g cm-3). However, thick Li metal
anodes (>250 μm) are frequently used in lithium metal
batteries (LMBs)[33], [34] to compensate for
the irreversible loss of Li formed by electrical
isolation[35,36] and solid-electrolyte interphase
(SEI) layer generation.[37,38,39] This
configuration extends the cyclability of the cells but significantly
lowers the energy density, which is often lower than that of
LIBs.[4,39] To overcome the above issue, thin Li
metal (20 ~ 50 μm) electrodes have been employed.
Recently, an anode composed of sole CCs without Li metal was used by
pairing with a lithiated cathode, where the negative-to-positive (N/P)
ratio was 0. With respect to energy density, this extreme system, called
anode-free lithium metal batteries (AFLMBs), is an optimum
circumstance.[40,41] Despite the considerable
advantages of the anode-less configuration, the cycle stability of
AFLMBs is too poor because the absence of a Li reservoir in the anode
causes rapid degradation of the cycle.[42,43]Nevertheless, AFLMBs have received significant attention since a
breakthrough by Quian et al. in 2016,[44] who
initiated an anode-less boom by proving the feasibility of AFLMBs.
Subsequently, various efforts have been made to enhance the cyclability
of AFLMBs.[43,45] In this review, we categorize
the various strategies for high performance AFLMBs into three types. The
first is the rational design of CCs to enhance the reversibility of Li
metal. The construction of elaborate 3D
structures,[46] diverse lithiophilic
sites[47], artificial layers on
CCs,[48,49] and carbon
hosts[50] has been explored to reduce the
nucleation energy barrier, lead to uniform deposition of Li, and
suppress dendrites. The next one is electrolyte engineering. The SEI
layer formed by electrolyte decomposition has the greatest effect on the
reversibility of Li metal.[43,45,51, 52] Various
studies have fabricated high-quality SEI layers, including by adjusting
salt concentration,[44] changing solvent
characteristics,[45,53] and introducing
additives.[52,54] The last category is operating
protocols. Various parameters such as temperature, external pressure,
current density, and cut-off voltages dramatically affect the cycle
stability.[55,56,57]
The initial cell configuration of AFLMBs is the same as that of general
LMBs, except that the anode is composed of sole CCs without Li metal
(Figure 1A). Therefore, lithiated cathode materials are necessary in
AFLMBs. Generally, intercalation materials such as lithium cobalt nickel
manganese oxide (NMC),[43] lithium iron phosphate
(LFP),[44,58] lithium nickel cobalt aluminum oxide
(NCA), or conversion-type (e.g., Li2S)
cathodes[59,60] are utilized as cathodes for
AFLMBs. During the initial charging process, Li ions are delithiated
from the cathode and reduced on the surface of the CCs in the anode. The
subsequent discharge and charge processes progressed similarly to the
LMBs.