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Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 15 of 32

ADVANCEMENTS IN LI METAL ANODE PROTECTION
Anode structural design
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Inhomogeneous Li nucleation behavior due to uneven distribution of electrons and Li ions can trigger fatal
Li dendrite growth. On this basis, various host strategies have been reported to homogenize the interface
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electric field, distribute the local current density and uniformize the Li flux. Furthermore, a stable host
minimizes the volume variation to avoid stress fluctuations. The hosts usually fall into three categories:
electron-conductive (EC) frameworks; ionic-conductive (IC) frameworks; mixed ion and electron-
conductive (MIEC) frameworks.

3D current collectors (i.e., EC frameworks) with high specific surface areas (SSAs) are conducive to
mitigating huge volume changes and reducing the local current density. For instance, 3D Cu current
collectors with submicron pores have been reported to solve the dendrite issue directly due to the large pore
[110]
volumes and high SSAs . Furthermore, the tip effect of the porous electrode induced the initial deposition
of Li metal on the microchannel walls, which constrained dendrite growth. Because the initial Li nucleation
sites evolve into bumps distributed on the current collector, whose edges are unable to capture Li ions under
the electric field, insufficient Li ions preferentially tend to gather on the tip, which is known as the so-called
“tip effect”, leading to dendritic deposition. Yan et al. explored the Li nucleation pattern on a variety of
[111]
metal substrates and revealed a special growth phenomenon that depends on the substrates [Figure 6A] .
The nucleation overpotentials of Li metal on Au, Ag, Zn and Mg substrates are almost zero, which can be
explained by the definite solubility of these metals in Li metal, leading to solid solution buffer layers before
the formation of Li metal. Carbon-based host materials can promote the energy density of batteries due to
low density. For example, Zuo et al. used a graphitized carbon fiber host for Li anodes, which guarantees
high areal capacity and metal plating/stripping efficiency, low voltage hysteresis and long lifespans .
[112]
Jin et al. reported a Li restoration method via a biochar capsule to host iodine that can effectively rejuvenate
electrochemically inactive Li based on an iodine redox chemistry .
[113]
To effectively achieve the inhibition of dendrites, it is critical to adjust the charge transfer process and
regulate the initial nucleation sites. Heteroatomic doping in a carbon matrix is one of the most effective
methods to guide the initial Li nucleation process. Specifically, 20 carbonaceous-based species (pristine and
[114]
heteroatom doped) were modeled with various dopants . As shown in Figure 6B, among the various
single-doped carbon matrices, aO-carbon matrices exhibit the largest binding energy toward Li atoms, thus
significantly decreasing the Li nucleation overpotential. In addition to heteroatom doping, a multivacancy
defect-enriched carbon matrix was presented as the anode current collector [Figure 6C] . Furthermore,
[115]
the modification of metallic oxides, such as ZnO, Co O , Cu O, TiO and other polar particles, on carbon
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matrices is also an effective strategy to induce the migration and diffusion of Li ions to modify the
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[116]
nucleation and deposition of Li metal . Zhang et al. demonstrated a promising strategy based on a
lithiophilic-lithiophobic gradient layer in which the bottom lithiophilic ZnO layer tightly anchors the whole
layer onto the Li foil to facilitate a stable SEI formation and the top lithiophobic carbon nanotube sublayer
[117]
can effectively suppress Li corrosion [Figure 6D] .
It is notable that in a matrix host material with high electronic conductivity, Li deposition could undesirably
take place outside the host, especially at high current densities. To resolve the problem, Yang et al. proposed
a 3D garnet-type Li La Ca -Zr Nb O framework as an IC Li host to achieve bottom-up
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2.75
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0.25
7
[118]
deposition . The structure with a planar Cu current collector guarantees the bottom plating of Li deposits,
while the dense garnet-type layer in the middle blocks potential dendritic growth, which prevents safety
risks caused by short circuit. It is noted that the features of high ionic conductivity (3 × 10 S cm ) and
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single-ion conductive feature of the garnet electrode also favors uniform and stable Li deposition. The