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Page 22 of 32 Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60

[172]
Li . Stiff coating layers can effectively enhance the penetrating resistance of the separators. Liang et al.
reported that curved surfaces are more favorable for dendrite suppression due to the fact that they can resist
[173]
puncture from the sharp tips of Li dendrites by redistributing the interfacial stress . Based on this
perspective, they proposed a separator with the coating layer of a nano-shield, like SiO spheres, which can
2
minimize the piercing force of Li dendrites and significantly improve the cycling life of LMBs. The nano-
shield protection theory is applicable to polystyrene sphere-coated separators, which can extend the
discharging lifespan of Li/Li symmetric cells to 70 h. Based on the same principle, Al O 3 [174] - and AlN -
[175]
2
coated separators were further used to suppress dendrite growth by enhancing the mechanical barrier. In
addition to inorganic coating materials, integrated separators consisting of polymers with high strength are
another strategy for the suppression of dendritic growth. For example, Hao et al. prepared a
poly(p-phenylene benzobisoxazole) PBO nanofiber membrane-based separator with high strength via a
highly scalable blade casting process . These membranes possess a strength of 525 MPa, a Young’s
[176]
modulus of 20 GPa, a thermal stability of up to 600 °C and impressively low ionic resistance, thereby
enabling the inhibition of dendrites in electrochemical cells .
[176]
+
The irregular pores and skeletal structures of conventional separators lead to an inhomogeneous flux of Li
ions due to the crowded Li ions in the pores during transport. However, the Li-ion concentration in the
+
+
vicinity of the skeletons remains unchanged. Under this condition, while the Li ions reach the anode
surface and obtain electrons to be reduced, the nonuniform Li deposition and formation of dendritic Li
occurs. Therefore, the redistribution of Li-ion flux is crucial to inhibit the dendritic formation of LMBs. For
example, ceramic and gel electrolytes with rapid Li-ion conductivity are generally employed in functional
separators for balancing the Li-ion distribution [177,178] . Compared with separators with electron-conducting
coating layers, Li-ion conductor-modified separators can avoid Li reduction in the coating layer due to the
fact that Li-ion conductors are electrically insulated. Furthermore, these separators possess remarkable
flexibility in composition and structure for ion redistribution compared to commercial separators. Ma et al.
proposed a 3D-ordered microporous separator using regenerated eggshell . Cations can transfer in 3D,
[179]
leading to superior electrochemical performances compared to batteries with conventional separators.
Similarly, Zhao et al. proposed that LLZTO ceramic modulated PP separators can be used as rectifiers to
[180]
+
redistribute Li ions for dendrite-free LMBs [Figure 9A] . The separator with a LLZTO coating layer
achieved the uniformity of ion concentration [Figure 9B]. Liu et al. decorated conventional Celgard
separators with metallic Mg nanoparticles [Figure 9C] . Due to lithiophilic Mg nanoparticles, Li ions
+
[181]
prefer to homogenously electroplate on the separator surface, which faces the anode side. Hao et al.
developed MOFs coated on a functionalized PP separator to regulate ion transport [Figure 9D] . The well-
[182]
defined intrinsic nanochannels in MOFs and the negatively charged channels both restrict the free
migration of anions, contributing to a high Li transference number of 0.68. Sheng et al. reported the
+
effective suppression of electrolyte-Li metal reactivity through a nanoporous separator [Figure 9E] .
[183]
Calculations assisted by diversified characterization reveal that the separator partially desolvates Li in space
+
created by its uniform nanopores and deactivates solvents before Li deposition occurs. The surface contact
gaps between the Zr-MOCN@PP and Li electrode are significantly mitigated compared to those between
the UiO-66@PP separator and Li electrode [Figure 9F]. Li et al. reported a polyacrylamide-grafted graphene
[184]
oxide molecular brush-modified PP separator [Figure 9G] . Furthermore, Liu et al. developed an
approach to immobilize Li ions by coating the separator with functionalized nanocarbon (FNC), leading to
+
Li dendrites growing toward each other simultaneously from both the FNC layer on the separator and the Li
[185]
metal anode, which changes the Li growth direction [Figure 9H] .
Generally, it is difficult to detect Li dendrites before the eventual short circuit and battery failure. However,
before the battery short circuit, dendritic Li must first reach the separator, which can act as an alarm to