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Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 23 of 32

Figure 9. (A) Schematic illustration of Li deposition behavior with PP separator and LLZTO-coated separator. (B) Simulation of Li-ion
distribution through LLZTO-coated separator. Reprinted with permission from Ref. [180] . Copyright (2018) American Association for the
Advancement of Science. (C) Schematic illustration of magnetron sputtering system for preparation of Mg-coated separator. Reprinted
with permission from Ref. [181] . Copyright (2019) Elsevier. (D) Schematic illustration of MOFs@PP separator regulating the transport of
+
Li and anions in LMBs. Reprinted with permission from Ref. [182] . Copyright (2021) Wiley-VCH. (E) Fabrication of Zr-MOCN@PP
membrane via photopolymerization and corresponding SEM images. (F) Schematic illustration of Li deposition in cells with different
separators: (top) UiO-66@PP; (bottom) Zr-MOCN@PP. Reprinted with permission from Ref. [183] . Copyright (2022) Springer Nature.
(G) Schematic illustration of Li deposition with GO-g-PAM-modified separator. Reprinted with permission from Ref. [184] . Copyright
(2017) Springer Nature. (H) A blank cell with (top) pristine separator and (bottom) FNC cell using FNC-coated separator. Reprinted
with permission from Ref. [185] . Copyright (2014) Springer Nature. (I) Schematic diagrams of Li dendrite detection in (left) routine and
(right) dual-layer separator-based cell. Reprinted with permission from Ref. [146] . Copyright (2017) Wiley-VCH.

detect Li dendrites. Thus, a functional separator was designed with a sensing terminal to realize dendrite
detection [Figure 9I] . The separator evolved into a triple-layer configuration of polymer-metal-polymer.
[146]
The voltage gap between the anode and the sandwiched metal can be identified by the electrochemical
potential difference. During the charge process, once Li dendrites appeared and expanded until they
eventually reached the sensing metal layer, a sharp voltage drop can be effectively recorded to indicate
dendritic Li growth. Once this is detected, the working battery is terminated to avoid the final short circuit,
battery thermal runaway, possible fire circumstance and even potential explosion. Apart from the detection
of dendrites, the sensing layer can even eliminate Li dendrites via incorporation with the Li metal reactive
materials. It is reported that a nanosized Si layer sandwiched between two layers of conventional polyolefin
separators can ensure the high safety of LMBs .
[186]
These strategies to suppress dendrite growth in LMBs, such as anode structural design, artificial SEI layers
and electrolyte and separator modification, are summarized in Table 1. Each one of them seems weak for
practical applications when facing tough and indomitable Li metal anodes. Mutual combination and
integration will be strong to face the challenges associated with Li metal anodes. For example, structured Li
metal anodes have more advantages in suppressing dendrite growth in the nucleation stage. If this strategy
is matched with optimized electrolytes (or other artificial SEI layer), the composite anode can hopefully
reduce the side reactions of Li metal, leading to high Coulombic efficiency with a dendrite-free Li depositing
morphology. In addition, the anode matrix can be designed to tightly contact the solid-state electrolyte to
reduce the interfacial impedance. Therefore, safe and high rate LMBs can be obtained.