Dual-functional Separators Regulating Ion Transport Enabled by 3D-Reinforced Polyimide Microspheres Protective Layer for Dendrite-Free and High-Temperature Lithium Metal Batteries

High-energy-density lithium metal batteries (LMBs) are confronted with crucial concerns of security and a short cycle lifespan caused by the uncontrollable formation of lithium (Li) dendrites. The poor thermal stability and heterogeneous Li deposition of conventional polyolefin separators often cause battery short circuiting and thermal runaway in LMBs. Herein, a novel dual-functional PE composite separator (PI-COOH/PE) coated by carboxyl polyimide (PI) microspheres is fabricated by an etching-acidification method. The three-dimensional (3D) high-temp PI microsphere with rich carboxyl groups on the surface improve the security of LMBs at extremely high temperatures and facilitate the formation of a stable and uniform SEI layer, which contributes to accelerating the Li+ transport and stabilizing the formation of the SEI layer. Consequently, the Li symmetric cell assembled with the (PI-COOH)/PE separator exhibits stable overpotential over 3000 h, and the corresponding Li//NCM811 full cells also show a high-level discharge capacity of 146.6 mAh g-1 at 5 C. Meanwhile, it also demonstrates outstanding cycling stability and thermal safety, which can survive continuously over 160 min at 140 °C (vs 21 min for PE). The above results indicate the (PI-COOH)/PE separator constructed by a low-cost and industrial-friendly strategy simultaneously addresses high-temperature stability and dendrite resistance.

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Recent Progress of Advanced Functional Separators in Lithium Metal Batteries

As a representative in the post-lithium-ion batteries (LIBs) landscape, lithium metal batteries (LMBs) exhibit high-energy densities but suffer from low coulombic efficiencies and short cycling lifetimes due to dendrite formation and complex side reactions. Separator modification holds the most promise in overcoming these challenges because it utilizes the original elements of LMBs. In this review, separators designed to address critical issues in LMBs that are fatal to their destiny according to the target electrodes are focused on. On the lithium anode side, functional separators reduce dendrite propagation with a conductive lithiophilic layer and a uniform Li-ion channel or form a stable solid electrolyte interphase layer through the continuous release of active agents. The classification of functional separators solving the degradation stemming from the cathodes, which has often been overlooked, is summarized. Structural deterioration and the resulting leakage from cathode materials are suppressed by acidic impurity scavenging, transition metal ion capture, and polysulfide shuttle effect inhibition from functional separators. Furthermore, flame-retardant separators for preventing LMB safety issues and multifunctional separators are discussed. Further expansion of functional separators can be effectively utilized in other types of batteries, indicating that intensive and extensive research on functional separators is expected to continue in LIBs.

Monodispersed MOF-Modified Nanofibers as Versatile Building Blocks for the Ion Regulations in Safe Lithium-Sulfur Batteries

Lithium-sulfur battery is the most promising candidate for the next generation of rechargeable batteries because of the high energy density. However, the severe shuttle effect of lithium polysulfides (LiPSs) and degradation of the lithium anode during cycling are significant issues that hinder the practical application of lithium-sulfur batteries. Herein, monodispersed metal-organic framework (MOF)-modified nanofibers are prepared as building blocks to construct both a separator and a composite polymer electrolyte in lithium-sulfur systems. This building block possesses the intrinsic advantages of good mechanical properties, thermal stability, and good electrolyte affinity. MOFs, grown continuously on the monodispersed nanofibers, can effectively adsorb LiPSs and play a key role in regulating the nucleation and stripping/plating process of the lithium anode. When assembled into the separator, the symmetric battery remains stable for 2500 h at a current density of 1 mA cm^-2, and the lithium-sulfur full cell shows improved electrochemical performance. In order to improve the safety property, the composite polymer electrolyte is prepared with the MOF-modified nanofiber as the filler. The quasi-solid-state symmetric battery remains stable for 3000 h at a current density of 0.1 mA cm^-2, and the corresponding lithium-sulfur cell can cycle 800 times at 1 C with a capacity decay rate of only 0.038% per cycle.

Bi-Morphological Form of SiO2 on a Separator for Modulating Li-Ion Solvation and Self-Scavenging of Li Dendrites in Li Metal Batteries

The lithium (Li) metal anode is highly desirable for high-energy density batteries. During prolonged Li plating-stripping, however, dendritic Li formation and growth are probabilistically high, allowing physical contact between the two electrodes, which results in a cell short-circuit. Engineering the separator is a promising and facile way to suppress dendritic growth. When a conventional coating approach is applied, it usually sacrifices the bare separator structure and severely increases the thickness, ultimately decreasing the volumetric density. Herein, we introduce dielectric silicon oxide with the feature of bi-morphological form, i.e., backbone-covered and backbone-anchored, onto the conventional polyethylene separator without any volumetric change. These functionally vary the Li⁺ transference number and the ionic conductivity so as to modulate Li-ion solvation and self-scavenging of Li dendrites. The proposed separator paves the way to maximizing the full cell performance of Li/NCM622 toward practical application.

Toward High-Performance Mg-S Batteries via a Copper Phosphide Modified Separator

Magnesium-sulfur (Mg-S) batteries are emerging as a promising alternative to lithium-ion batteries, due to their high energy density and low cost. Unfortunately, current Mg-S batteries typically suffer from the shuttle effect that originates from the dissolution of magnesium polysulfide intermediates, leading to several issues such as rapid capacity fading, large overcharge, severe self-discharge, and potential safety concern. To address these issues, here we harness a copper phosphide (Cu3P) modified separator to realize the adsorption of magnesium polysulfides and catalyzation of the conversion reaction of S and Mg2+ toward stable cycling of Mg-S cells. The bifunctional layer with Cu3P confined in a carbon matrix is coated on a commercial polypropylene membrane to form a porous membrane with high electrolyte wettability and good thermal stability. Density functional theory (DFT) calculations, polysulfide permeability tests, and post-mortem analysis reveal that the catalytic layer can adsorb polysulfides, effectively restraining the shuttle effect and facilitating the reversibility of the Mg-S cells. As a result, the Mg-S cells can achieve a high specific capacity, fast rates (449 mAh g-1 at 0.1 C and 249 mAh g-1 at 1.0 C), and a long cycle life (up to 500 cycles at 0.5 C) and operate even at elevated temperatures.

Layer-by-Layer Assembly of CeO2-x@C-rGO Nanocomposites and CNTs as a Multifunctional Separator Coating for Highly Stable Lithium-Sulfur Batteries

Commercialization of high-energy Li-S batteries is greatly restricted by their unsatisfactory cycle retention and poor cycling life originated from the notorious "shuttling effect" of lithium polysulfides. Modification of a commercial separator with a functional coating layer is a facile and efficient strategy beyond nanostructured composite cathodes for suppressing polysulfide shuttling. Herein, a multilayered functional CeO_2-x@C-rGO/CNT separator was successfully achieved by alternately depositing conductive carbon nanotubes (CNTs) and synthetic CeO_2-x@C-rGO onto the surface of the commercial separator. The cooperation of multiple components including Ce-MOF-derived CeO_2-x@C, rGO, and CNTs enables the as-built CeO_2-x@C-rGO/CNT separator to perform multifunctions from the separator surface: (i) to hinder the diffusion of polysulfide species through physical blocking or chemical adsorption, (ii) to accelerate the sluggish redox reactions of sulfur species, and (iii) to enhance the conductivity for sulfur re-activation and efficient utilization. Serving as a multilayer and powerful barrier, the CeO_2-x@C-rGO/CNT separator greatly constrains and reutilizes the polysulfide species. Thus, the Li-S battery assembled with the CeO_2-x@C-rGO/CNT separator demonstrates an excellent combination of capacity, rate capability, and cycling performances (an initial capacity of 1107 mA h g^-1 with a low decay rate of 0.060% per cycle over 500 cycles at 1 C, 651 mA h g^-1 at 5 C) together with remarkably mitigated self-discharge and anode corrosion. This work provides guidelines for functional separator design as well as rare-earth material applications for Li-S batteries and other energy storage systems.

Dual Role of Mo6 S8 in Polysulfide Conversion and Shuttle for Mg-S Batteries

Magnesium-Sulfur batteries are one of most appealing options among the post-lithium battery systems due to its potentially high energy density, safe and sustainable electrode materials. The major practical challenges are originated from the soluble magnesium polysulfide intermediates and their shuttling between the electrodes, which cause high overpotentials, low sulfur utilization, and poor Coulombic efficiency. Herein, a functional Mo₆ S₈ modified separator is designed to effectively address these issues. Both the experimental results and density functional theory calculations show that the electrochemically active Mo₆ S₈ layer has a superior adsorption capability of polysulfides and simultaneously acts as a mediator to accelerate the polysulfide conversion kinetics. Remarkably, the magnesium-sulfur cell assembled with the functional separator delivers a high specific energy density (942.9 mA h g^-1 in the 1st cycle) and can be cycled at 0.2 C for 200 cycles with a Coulombic efficiency of 96%. This work demonstrates a new design concept toward high-performance metal-sulfur batteries.

Ultrathin Cobalt Phthalocyanine@Graphene Oxide Layer-Modified Separator for Stable Lithium-Sulfur Batteries

Rechargeable lithium-sulfur (Li-S) batteries have aroused great attention due to their high energy density and low cost. However, Li-S batteries suffer from rapid capacity decay owing to the shuttle effect of the intermediate polysulfides. To tackle this issue, functional separators with the ability to absorb polysulfides play a vital role to block them from passing through the separator. Herein, an ultrathin and lightweight layer of graphene oxide (GO) loaded with Co phthalocyanine (CoPc) is fabricated on a polypropylene (PP) separator. The thickness of CoPc@GO is about 200 nm with a low areal mass of 22 μg cm^-2. CoPc is uniformly dispersed on GO sheets through π-π interactions, which inhibits the shuttle effect and facilitates the conversion of the intermediate polysulfides. In consequence, the battery with a CoPc@GO-PP separator exhibits good cycling stability with a low-capacity decay rate of 0.076% per cycle at 1 C over 400 cycles and a high specific capacity of 919 mA h g^-1 after 250 cycles at 0.5 C.

Surface-Functionalized Separator for Stable and Reliable Lithium Metal Batteries: A Review

Metallic Li has caught the attention of researchers studying future anodes for next-generation batteries, owing to its attractive properties: high theoretical capacity, highly negative standard potential, and very low density. However, inevitable issues, such as inhomogeneous Li deposition/dissolution and poor Coulombic efficiency, hinder the pragmatic use of Li anodes for commercial rechargeable batteries. As one of viable strategies, the surface functionalization of polymer separators has recently drawn significant attention from industries and academics to tackle the inherent issues of metallic Li anodes. In this article, separator-coating materials are classified into five or six categories to give a general guideline for fabricating functional separators compatible with post-lithium-ion batteries. The overall research trends and outlook for surface-functionalized separators are reviewed.

Layered Double Hydroxide Quantum Dots for Use in a Bifunctional Separator of Lithium-Sulfur Batteries

Functional separators, which are chemically modified and coated with nanostructured materials, are considered an effective and economical approach to suppressing the shuttle effect of lithium polysulfide (LiPS) and promoting the conversion kinetics of sulfur cathodes. Herein, we report cobalt-aluminum-layered double hydroxide quantum dots (LDH-QDs) deposited with nitrogen-doped graphene (NG) as a bifunctional separator for lithium-sulfur batteries (LSBs). The mesoporous LDH-QDs/NG hybrids possess abundant active sites of Co²⁺ and hydroxide groups, which result in capturing LiPSs through strong chemical interactions and accelerating the redox kinetics of the conversion reaction, as confirmed through X-ray photoelectron spectroscopy, adsorption tests, Li₂S nucleation tests, and electrokinetic analyses of the LiPS conversion. The resulting LDH-QDs/NG hybrid-coated polypropylene (LDH-QDs/NG/PP) separator, with an average thickness of ∼17 μm, has a high ionic conductivity of 2.67 mS cm^-1. Consequently, the LSB cells with the LDH-QDs/NG/PP separator can deliver a high discharge capacity of 1227.48 mAh g^-1 at 0.1C along with a low capacity decay rate of 0.041% per cycle over 1200 cycles at 1.0C.

Electrospun Polymer Nanofibers with TiO2@NiCo-LDH as Efficient Polysulfide Barriers for Wide-Temperature-Range Li-S Batteries

Herein, we introduce polymer nanofibers of TiO₂@NiCo-LDH as interlayers into Li-S batteries. From 0 to 60 °C, the interlayers can deliver high sulfur utilization, an outstanding rate capability, and excellent cycling life. High-temperature excitation makes it easier for the valence band electrons of TiO₂ to transition to the conduction band. The electron-hole pairs formed on the surface combine with the ether group of 1,3-dioxolane in the electrolyte, which greatly reduces the decomposition and volatilization rates of the electrolyte, ensuring Li-S batteries with good cycle performance at high temperatures. The capacity can stabilize at 798.6 mAh g^-1 after 100 cycles at 60 °C and 1C, and the battery can provide a capacity higher than 323.2 mAh g^-1 at 0 °C. Simultaneously, the lithium metal symmetrical battery with a functional separator can be continuously cycled for 1800 and 750 h without a short circuit at the current densities of 0.65 and 1.63 mA cm^-2, respectively.

Rapid Gas-Engineering to the Manufacture of Graphene-Like Mesoporous Carbon Nanosheets with a Large Aspect Ratio

Porous carbon nanosheets (PCNs) with a large two-dimensional morphology and high porosity have emerged as an important class of 2D materials, while developing novel technology to manufacture high-quality PCNs in terms of convenience, high output, and economic benefit remains a challenge. Herein, a rapid gas-engineering technology is developed to fabricate graphene-like mesoporous carbon nanosheets (MCNs) with large aspect ratios (>2500, length/thickness). By easy carbonization of calcium gluconate under reduced pressure, MCNs with ultrathin (∼12 nm) thickness, ultralarge (>20 μm) lamella morphology, and high surface area (∼1155 m²/g) are fabricated in kilogram scale. Two-dimensional lamella morphology transformation, pore architectures, and calcium compounds transformation mechanisms are unraveled by in situ variable temperature X-ray diffraction (VT-XRD), high-resolution transmission electron microscopy (HRTEM), ex situ scanning electron microscopy (SEM), and atomic force microscopy (AFM). The key to the synthesis is the negative pressure operation, which triggers the rapid gas expansion in a gas-solid system. This design relied on the gas expansion mechanism has realized producing of high-quality MCNs via a rapid, high-throughput, and cost-effective way. Due to high surface utilization and low weight density, when served as a lightweight separator coating layer, MCNs exhibit impressive capture ability toward polysulfides and achieve a high-stability lithium-sulfur battery.

TiO2/GO-coated functional separator to suppress polysulfide migration in lithium-sulfur batteries

Lithium-sulfur batteries render a high energy density but suffer from poor cyclic performance due to the dissolution of intermediate polysulfides. Herein, a lightweight nanoporous TiO₂ and graphene oxide (GO) composite is prepared and utilized as an interlayer between a Li anode and a sulfur cathode to suppress the polysulfide migration and improve the electrochemical performance of Li/S batteries. The interlayer can capture the polysulfides due to the presence of oxygen functional groups and formation of chemical bonds. The hierarchically porous TiO₂ nanoparticles are tightly wrapped in GO sheets and facilitate the polysulfide storage and chemical absorption. The excellent adhesion between TiO₂ nanoparticles and GO sheets resulted in enhanced conductivity, which is highly desirable for an efficient electron transfer process. The Li/S battery with a TiO₂/GO-coated separator exhibited a high initial discharge capacity of 1102.8 mAh g^-1 and a 100th cycle capacity of 843.4 mAh g^-1, which corresponds to a capacity retention of 76.48% at a current rate of 0.2 C. Moreover, the Li/S battery with the TiO₂/GO-coated separator showed superior cyclic performance and excellent rate capability, which shows the promise of the TiO₂/GO composite in next-generation Li/S batteries.

Effective Trapping of Lithium Polysulfides Using a Functionalized Carbon Nanotube-Coated Separator for Lithium-Sulfur Cells with Enhanced Cycling Stability

The critical issues that hinder the practical applications of lithium-sulfur batteries, such as dissolution and migration of lithium polysulfides, poor electronic conductivity of sulfur and its discharge products, and low loading of sulfur, have been addressed by designing a functional separator modified using hydroxyl-functionalized carbon nanotubes (CNTOH). Density functional theory calculations and experimental results demonstrate that the hydroxyl groups in the CNTOH provoked strong interaction with lithium polysulfides and resulted in effective trapping of lithium polysulfides within the sulfur cathode side. The reduction in migration of lithium polysulfides to the lithium anode resulted in enhanced stability of the lithium electrode. The conductive nature of CNTOH also aided to efficiently reutilize the adsorbed reaction intermediates for subsequent cycling. As a result, the lithium-sulfur cell assembled with a functional separator exhibited a high initial discharge capacity of 1056 mAh g^-1 (corresponding to an areal capacity of 3.2 mAh cm^-2) with a capacity fading rate of 0.11% per cycle over 400 cycles at 0.5 C rate.

Sulfonic Groups Originated Dual-Functional Interlayer for High Performance Lithium-Sulfur Battery

The lithium-sulfur battery is one of the most prospective chemistries in secondary energy storage field due to its high energy density and high theoretical capacity. However, the dissolution of polysulfides in liquid electrolytes causes the shuttle effect, and the rapid decay of lithium sulfur battery has greatly hindered its practical application. Herein, combination of sulfonated reduced graphene oxide (SRGO) interlayer on the separator is adopted to suppress the shuttle effect. We speculate that this SRGO layer plays two roles: physically blocking the migration of polysulfide as ion selective layer and anchoring lithium polysulfide by the electronegative sulfonic group. Lewis acid-base theory and density functional theory (DFT) calculations indicate that sulfonic groups have a strong tendency to interact with lithium ions in the lithium polysulfide. Hence, the synergic effect involved by the sulfonic group contributes to the enhancement of the battery performance. Furthermore, the uniformly distributed sulfonic groups working as active sites which could induce the uniform distribution of sulfur, alleviating the excessive growth of sulfur and enhancing the utilization of active sulfur. With this interlayer, the prototype battery exhibits a high reversible discharge capacity of more than 1300 mAh g^-1 and good capacity retention of 802 mAh g^-1 after 250 cycles at 0.5 C rate. After 60 cycles at different rates from 0.2 to 4 C, the cell with this functional separator still recovered a high specific capacity of 1100 mAh g^-1 at 0.2 C. The results demonstrate a promising interlayer design toward high performance lithium-sulfur battery with longer cycling life, high specific capacity, and rate capability.