Lithium ion batteries (LIBs) are one of the most potential energy storage devices among various rechargeable batteries due to their high energy/power density, long cycle life, and low self‐discharge properties. However, current LIBs fail to meet the ever‐increasing safety and fast charge/discharge demands. As one of the main components in LIBs, separator is of paramount importance for safety and rate performance of LIBs. Among the various separators, composite separators have been widely investigated for improving their thermal stability, mechanical strength, electrolyte uptake, and ionic conductivity. Herein, the challenges and limitations of commercial separators for LIBs are reviewed, and a systematic overview of the state‐of‐the‐art research progress in composite separators is provided for safe and high rate LIBs. Various combination types of composite separators including blending, layer, core–shell, and grafting types are covered. In addition, models and simulations based on the various types of composite separators are discussed to comprehend the composite mechanism for robust performances. At the end, future directions and perspectives for further advances in composite separators are presented to boost safety and rate capacity of LIBs.

Both organic and inorganic ingredients in SEI undergo reversible conversions and contribute capacity enhancement with the catalysis of Fe3C.

ideally stores five times more energy per mass (1675 mAh g −1 ) than intercalationtype cathodes by multielectron reactions of sulfur, namely S 8 + 16 e − + 16 Li + ⇌ 8 Li 2 S, [8,9] and leads to extensive researches on Li-S batteries.Nevertheless, the development of Li-S batteries is plagued by three issues: (1) the sluggish electrical conductivities of sulfur (σ = 5 × 10 −30 S cm −1 ) and its end products (Li 2 S, σ = 1 × 10 −13 S cm −1 ) lead to slow conversion from soluble lithium polysulfides (LiPSs, namely Li 2 S x , 4 ≤ x ≤ 8) to solid Li 2 S 2 /Li 2 S; (2) the hydrophilic LiPS species are inclined to shuttle through porous separator and deposit at Li anode as Li 2 S which is difficult to be reused owing to the high activation energy [10][11][12][13] ; (3) the shuttling effect of polysulfides causes severe self-discharge, continuous energy loss and unsatisfactory energy density. [14][15][16] Reconstructing separators architecture is an effective strategy to ameliorate the aforementioned issues. [17][18][19] Carbon materials with a high specific surface area are introduced to the separator surface to physically block LiPS and accelerate its conversion due to the high conductivity (σ = 9 × 10 1 -5 × 10 3 S m −1 ), [20] but porous carbon shows limited capability to confine LiPS owing to the feeble van der Waals adsorption. [21,22] Polar compounds, such as metal compounds (MA, where M is metal, and A is oxygen, nitrogen, or sulfur), are promising materials to bond to LiPS through surface M-S or A-Li bonding, which prevents the shuttling effect and changes the reduction pathway of LiPS with decreased the redox energy barrier. [23,24] However, the strong A-Li bonding of ≈2 eV impedes Li + ion transport which delays the reaction kinetics of LiPSs. [25] In addition, most of them own complex design processes. [26] Recent studies show that the low cost and natural abundance lamellar clays own much lower Li-ion diffusion barrier (such as lithium-montmorillonite (0.15 eV), MA materials (ZnS (0.494 eV), MgO (0.45 eV), Al 2 O 3 (1.22 eV), and CeO 2 (0.66eV)), [27,28] which allows free lithium-ion diffusion in the sulfur cathode and gives rise to improved electrochemical performances of Li-S batteries. Unfortunately, these lamellar clays modified structures still show unsatisfactory rate performances for practical application. [29] Hence, it is necessary to further develop optimized lamellar clays structure for high-rate Li-S batteries.

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