Flexible, High-Wettability and Fire-Resistant Separators Based on Hydroxyapatite Nanowires for Advanced Lithium-Ion Batteries

Thermal-Responsive and Fire-Resistant Materials for High-Safety Lithium-Ion Batteries

As one of the most efficient electrochemical energy storage devices, the energy density of lithium-ion batteries (LIBs) has been extensively improved in the past several decades. However, with increased energy density, the safety risk of LIBs becomes higher too. The frequently occurred battery accidents worldwide remind us that safeness is a crucial requirement for LIBs, especially in environments with high safety concerns like airplanes and military platforms. It is generally recognized that the catastrophic thermal runaway (TR) event is the major cause of LIBs related accidents. Tremendous efforts have been devoted to coping with the TR concerns in LIBs, and thus enhance battery safety. This review first gives an introduction to the fundamentals of LIBs and the origins of safety issues. Then, the authors summarize the recent advances to improve the safety of LIBs with a unique focus on thermal-responsive and fire-resistant materials. Finally, a perspective is proposed to guide future research directions in this field. It is anticipated this review will stimulate inspiration and arouse extensive studies on further improvement in battery safety.

Gifts from Nature: Bio-Inspired Materials for Rechargeable Secondary Batteries

Materials in nature have evolved to the most efficient forms and have adapted to various environmental conditions over tens of thousands of years. Because of their versatile functionalities and environmental friendliness, numerous attempts have been made to use bio-inspired materials for industrial applications, establishing the importance of biomimetics. Biomimetics have become pivotal to the search for technological breakthroughs in the area of rechargeable secondary batteries. Here, the characteristics of bio-inspired materials that are useful for secondary batteries as well as their benefits for application as the main components of batteries (e.g., electrodes, separators, and binders) are discussed. The use of bio-inspired materials for the synthesis of nanomaterials with complex structures, low-cost electrode materials prepared from biomass, and biomolecular organic electrodes for lithium-ion batteries are also introduced. In addition, nature-derived separators and binders are discussed, including their effects on enhancing battery performance and safety. Recent developments toward next-generation secondary batteries including sodium-ion batteries, zinc-ion batteries, and flexible batteries are also mentioned to understand the feasibility of using bio-inspired materials in these new battery systems. Finally, current research trends are covered and future directions are proposed to provide important insights into scientific and practical issues in the development of biomimetics technologies for secondary batteries.

A Gas-Steamed MOF Route to P-Doped Open Carbon Cages with Enhanced Zn-Ion Energy Storage Capability and Ultrastability

Carbon micro/nanocages have received great attention, especially in electrochemical energy-storage systems. Herein, as a proof-of-concept, a solid-state gas-steamed metal-organic-framework approach is designed to fabricate carbon cages with controlled openings on walls, and N, P dopants. Taking advantage of the fabricated carbon cages with large openings on their walls for enhanced kinetics of mass transport and N, P dopants within the carbon matrix for favoring chemical adsorption of Zn ions, when used as carbon cathodes for advanced aqueous Zn-ion hybrid supercapacitors (ZHSCs), such open carbon cages (OCCs) display a wide operation voltage of 2.0 V and an enhanced capacity of 225 mAh g^-1 at 0.1 A g^-1 . Also, they exhibit an ultralong cycling lifespan of up to 300 000 cycles with 96.5% capacity retention. Particularly, such OCCs as electrode materials lead to a soft-pack ZHSC device, delivering a high energy density of 97 Wh kg^-1 and a superb power density of 6.5 kW kg^-1 . Further, the device can operate in a wide temperature range from -25 to + 40 °C, covering the temperatures for practical applications in daily life.

Recent developments in natural mineral-based separators for lithium-ion batteries

Lithium-ion batteries (LIBs) are currently the most widely used portable energy storage devices due to their high energy density and long lifespan. The separator plays a key role in the battery, and its function is to prevent the two electrodes of the battery from contacting, causing the internal short circuit of the battery, and ensuring the lithium ions transportation. Currently, lithium ion battery separators widely used commercially are polyolefin separators, such as polyethylene (PE) and polypropylene (PP) based separators. However, polyolefin separators would shrink at high temperatures, causing battery safety issues, and also causing white pollution. To solve these issues, the use of natural minerals to prepare composite separators for LIBs has attracted widespread attention owing to their unique nano-porous structure, excellent thermal and mechanical stability and being environmentally friendly and low cost. In this review, we present recent application progress of natural minerals in separators for LIBs, including halloysite nanotubes, attapulgite, sepiolite, montmorillonite, zeolite and diatomite. Here, we also have a brief introduction to the basic requirements and properties of the separators in LIBs. Finally, a brief summary of recent developments in natural minerals in the separators is also discussed.

In-Situ Intermolecular Interaction in Composite Polymer Electrolyte for Ultralong Life Quasi-Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries built with composite polymer electrolytes using cubic garnets as active fillers are particularly attractive owing to their high energy density, easy manufacturing and inherent safety. However, the uncontrollable formation of intractable contaminant on garnet surface usually aggravates poor interfacial contact with polymer matrix and deteriorates Li⁺ pathways. Here we report a rational designed intermolecular interaction in composite electrolytes that utilizing contaminants as reaction initiator to generate Li⁺ conducting ether oligomers, which further emerge as molecular cross-linkers between inorganic fillers and polymer matrix, creating dense and homogeneous interfacial Li⁺ immigration channels in the composite electrolytes. The delicate design results in a remarkable ionic conductivity of 1.43×10^-3  S cm^-1 and an unprecedented 1000 cycles with 90 % capacity retention at room temperature is achieved for the assembled solid-state batteries.

Nanocellulose-Reinforced Hydroxyapatite Nanobelt Membrane as a Stem Cell Multi-Lineage Differentiation Platform for Biomimetic Construction of Bioactive 3D Osteoid Tissue In Vitro

Severe bone defects, especially accompanied by vascular and peripheral nerve injuries, remain a massive challenge. Most studies related to bone tissue engineering have focused on osteogenic differentiation of mesenchymal stem cells (MSCs), and ignored the formation of blood vessels and nerves in the newly generated bone owing to the lack of proper materials and methodology for tuning stem cells differentiated into osteogenic, neuronal, and endothelial cells (ECs) in the same scaffold system. Herein, a nanocellulose-reinforced hybrid membrane with good mechanical properties and control over biodegradation by assembling ultralong hydroxyapatite nanobelts in a bacterial nanocellulose hydrogel is designed and synthesized. Osteogenic, neuronal cells are successfully differentiated on this hybrid membrane. Based on the multi-lineage differentiation property of the membrane, a bioactive 3D osteoid tissue (osteogenic, neural, and ECs) is mimetically constructed in vitro using layer-by-layer culture and integration. The bone regeneration ability of the as-prepared bioactive osteoid tissue is assessed in vivo via heterotopic osteogenesis experiments for eight weeks. The rapid new bone growth and formation of blood capillaries and nerve fibers prove that the hybrid membrane can be universally applied as a stem cell multi-lineage differentiation platform, which has significant applications in bone tissue engineering.

Thermoregulating Separators Based on Phase-Change Materials for Safe Lithium-Ion Batteries

Safety issues in lithium-ion batteries (LIBs) have aroused great interest owing to their wide applications, from miniaturized devices to large-scale storage plants. Separators are a vital component to ensure the safety of LIBs; they prevent direct electric contact between the cathode and anode while allowing ion transport. In this study, the first design is reported for a thermoregulating separator that responds to heat stimuli. The separator with a phase-change material (PCM) of paraffin wax encapsulated in hollow polyacrylonitrile nanofibers renders a wide range of enthalpy (0-135.3 J g^-1 ), capable of alleviating the internal temperature rise of LIBs in a timely manner. Under abuse conditions, the generated heat in batteries stimulates the melting of the encapsulated PCM, which absorbs large amounts of heat without creating a significant rise in temperature. Experimental simulation of the inner short-circuit in prototype pouch cells through nail penetration demonstrates that the PCM-based separator can effectively suppress the temperature rise due to cell failure. Meanwhile, a cell penetrated by a nail promptly cools down to room temperature within 35 s, benefiting from the latent heat-storage of the unique PCM separator. The present design of separators featuring latent heat-storage provides effective strategies for overheat protection and enhanced safety of LIBs.

A universal strategy to improve interfacial kinetics of solid supercapacitors used in high temperature

The growing application domain of energy storage devices (ESDs) is leading research to temperature tolerant supercapacitors. To realize reliable and safe devices, high modulus solid electrolytes are favored by most researchers. However, the inferior infiltrating ability of such electrolytes usually results in poor electrochemical performances of the ESDs. Herein, we adopted a hierarchical optimization strategy to address the aforementioned interfacial issues. Continuous ionic percolation throughout the hierarchical pores of the 3D electrode was formed by in-situ introducing an ionogel buffer layer. Benefiting from this, the rate of ions diffusing within electrodes was increased by 5 times. Furthermore, the kinetics of ions entering into nanopores was improved via introducing small size ions into ionic liquids (ILs) and adjusting the solvated structures. Both the capacity and rate performance of the electrochemical double layer capacitors (EDLCs) were improved. Additionally, the buffer layer exhibited sufficient thermostability to cooperate with poly(ether ether ketone) (PEEK)-based solid electrolyte. Consequently, the EDLCs exhibited excellent cycling stability (79% capacitance retention after 5000 cycles) at 120 °C and delivered a maximum energy density of 46.9 Wh kg^-1 with a power density of 926.9 W kg^-1. Our strategy is believed to be effective to cooperate with various solid electrolyte systems and offer a general design principle for durable and high performance EDLCs.

Hydroxyapatite Nanowire-Reinforced Poly(ethylene oxide)-Based Polymer Solid Electrolyte for Application in High-Temperature Lithium Batteries

Hybrid polymer electrolytes with excellent performance at high temperatures are very promising for developing solid-state lithium batteries for high-temperature applications. Herein, we use a self-supporting hydroxyapatite (HAP) nanowire membrane as a filler to improve the performance of a poly(ethylene oxide) (PEO)-based solid-state electrolyte. The HAP membrane could comprehensively improve the properties of the hybrid polymer electrolyte, including the higher room-temperature ionic conductivity of 1.05 × 10^-5 S cm^-1, broad electrochemical windows of up to 5.9 V at 60 °C and 4.9 V at 160 °C, and a high lithium-ion migration of 0.69. In addition, the LiFePO₄//Li full battery with a solid electrolyte possesses good rate capability, cycling, and Coulomb efficiency at extreme high temperatures, that is, after 300 continuous charge and discharge cycles at 4 C rate, the discharge capacity retention rate is 77% and the Coulomb efficiency is 99%. The use of the flexible self-supporting HAP nanowire membrane to improve the PEO-based solid composite electrolyte provides new strategies and opportunities for developing rechargeable lithium batteries in extreme high-temperature applications.

PVDF-Modified TiO2 Nanowires Membrane with Underliquid Dual Superlyophobic Property for Switchable Separation of Oil-Water Emulsions

Separation membranes with underliquid dual superlyophobicity have recently caused widespread concern due to their switchable separation of oil-water mixtures and emulsions. However, the fabrication of the reported underliquid dual superlyophobic membranes is difficult, and the design of the underliquid dual superlyophobic surface of these membranes is challenging because of their complex surface composition. Theoretically, underliquid dual superlyophobicity is an underliquid Cassie state attainable by the synergy of the underliquid dual lyophobic surface and the construction of a high-roughness surface. Herein, we fabricated an underliquid dual superlyophobic membrane by combining underliquid dual lyophobic polyvinylidene fluoride (PVDF) and TiO₂ nanowires. PVDF-modified TiO₂ nanowire membranes with underliquid dual superlyophobicity were prepared via a simple adsorption and filtration approach. PVDF was coated onto TiO₂ nanowires to form a PVDF layer with a thickness of 6 nm. The PVDF modification provided flexibility to the fragile TiO₂ nanowires membrane and changed its wettability from underwater superoleophobicity/underoil superhydrophilicity to underliquid dual superlyophobicity. The PVDF-modified TiO₂ nanowires membrane efficiently separated both oil-in-water and water-in-oil emulsions. The binary cooperative effect between the TiO₂ nanowires and the coated PVDF layer was responsible for the underliquid dual superlyophobicity.

Preparation and Properties of an Alginate-Based Fiber Separator for Lithium-Ion Batteries

The membrane is one of the key inner parts of lithium-ion batteries, which determines the interfacial structure and internal resistance, ultimately affecting the capacity, cycling, and safety performance of the cell. In this article, an alginate-based fiber composite membrane was successfully fabricated from cellulose and calcium alginate with flame-retardant properties via a traditional papermaking process. In the membrane, the calcium alginate plays a bridging role and the cellulose acts as a filler. After 100 cycles, lithium-ion batteries by the alginate-based fiber separator exhibited better capacity retention ratios (approximately 90%) compared with those of commercial PP separators. Furthermore, the alginate-based fiber separator demonstrated fine thermal stability and electrochemical properties, showing a stable charge-discharge capability and no hot melt shrinkage at higher temperatures, which is a breakthrough in improving the safety of the cell. This research affords a new way for the large-scale fabrication of safe lithium-ion battery separators.

Liquid-Phase Synthesis of Iron Oxide Nanostructured Materials and Their Applications

Owing to their high natural abundance, low cost, easy availability, and excellent magnetic properties, considerable interest has been devoted to the synthesis and applications of iron oxide nanostructured materials. Liquid-phase synthesis methods are economical and environmentally friendly with low energy consumption and volatile emissions, and as such have received much attention for the preparation of iron oxide nanostructured materials. Herein, the liquid-phase synthesis methods of iron oxide nanostructured materials including the co-precipitation method, microemulsion method, conventional hydrothermal and solvothermal methods, microwave-assisted heating method, sonolysis method, and other methods are summarized and reviewed. Many iron oxide nanostructured materials, self-assembled nanostructures, and nanocomposites have been successfully prepared, which are of great significance to enhance their structure-dependent properties and applications. The specific roles of liquid-phase chemical reaction parameters in regulating the chemical composition, structure, crystallinity, morphology, particle size, and dispersive behavior of the as-prepared iron oxide nanostructured materials are emphasized. The biomedical, environmental, and electrochemical energy storage applications of iron oxide nanostructured materials are discussed. Finally, challenges and perspectives are proposed for future investigations on the liquid-phase synthesis and applications of iron oxide nanostructured materials.

Pulsed laser deposition temperature effects on strontium-substituted hydroxyapatite thin films for biomedical implants

Substituting small molecule drugs with abundant and easily affordable ions may have positive effects on the way countless disease treatments are approached. The interest in strontium cation in bone therapies soared in the wake of the success of strontium ranelate in the treatment of osteoporosis. A new method for producing thin strontium-containing hydroxyapatite (Sr-HA, Ca₉Sr(PO₄)₆(OH)₂) films as coatings that render bioinert titanium implant bioactive is reported here. The method is based on the combination of a mechanochemical synthesis of Sr-HA targets and their deposition in form of thin films on top of titanium with the use of laser ablation at low pressure. The films were 1-2 μm in thickness and their formation was studied at different temperatures, including 25, 300, and 500 °C. Highly crystalline Sr-HA target transformed during pulsed laser deposition to a fully amorphous film, whose degree of long-range order recovered with temperature. Particle edges became somewhat sharper and surface roughness moderately increased with temperature, but the (Ca+Sr)/P atomic ratio, which increased 1.5 times during the film formation, remained approximately constant at different temperatures. Despite the mostly amorphous structure of the coatings, their affinity for capturing atmospheric carbon dioxide and accommodating it as carbonate ions that replace both phosphates and hydroxyls of HA was confirmed in an X-ray photoelectron spectroscopic analysis. As the film deposition temperature increased, the lattice voids got reduced in concentration and the structure gradually "closed," becoming more compact and entailing a linear increase in microhardness with temperature, by 0.03 GPa/°C for the entire 25-500 °C range. Biocompatibility and bioactivity of Sr-HA thin films deposited on titanium were confirmed in an interaction with dental pulp stem cells, suggesting that these coatings, regardless of the processing temperature, may be viable candidates for the surface components of metallic bone implants.

Engineering of Aerogel-Based Biomaterials for Biomedical Applications

Biomaterials with porous structure and high surface area attract growing interest in biomedical research and applications. Aerogel-based biomaterials, as highly porous materials that are made from different sources of macromolecules, inorganic materials, and composites, mimic the structures of the biological extracellular matrix (ECM), which is a three-dimensional network of natural macromolecules (e.g., collagen and glycoproteins), and provide structural support and exert biochemical effects to surrounding cells in tissues. In recent years, the higher requirements on biomaterials significantly promote the design and development of aerogel-based biomaterials with high biocompatibility and biological activity. These biomaterials with multilevel hierarchical structures display excellent biological functions by promoting cell adhesion, proliferation, and differentiation, which are critical for biomedical applications. This review highlights and discusses the recent progress in the preparation of aerogel-based biomaterials and their biomedical applications, including wound healing, bone regeneration, and drug delivery. Moreover, the current review provides different strategies for modulating the biological performance of aerogel-based biomaterials and further sheds light on the current status of these materials in biomedical research.

Phase-Inversion Polymer Composite Separators Based on Hexagonal Boron Nitride Nanosheets for High-Temperature Lithium-Ion Batteries

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

Enhancement of the electrochemical performance of lithium-ion batteries by SiO2@poly(2-acrylamido-2-methylpropanesulfonic acid) nanosphere addition into a polypropylene membrane

Employing electrostatic self-assembly and free radical polymerization, the surface of SiO₂ nanospheres was coated with poly(2-acrylamido-2-methylpropanesulfonic acid) (SiO₂@PAMPS) bearing strong electron withdrawing sulfonic and amide groups, enhancing the dissociation ability of the lithium salt of the liquid electrolyte and absorbing anions via hydrogen bonds. After SiO₂@PAMPS nanospheres were introduced into the polypropylene (PP) membrane (SiO₂@PAMPS/PP), the electrolyte affinity and electrolyte uptake of the composite separators were significantly improved. The ionic conductivity of SiO₂@PAMPS/PP-18% (where 18% represents the concentration of the solution used for coating) soaked in liquid electrolyte was even 0.728 mS cm^-1 at 30 °C, much higher than that of the pristine PP membrane. The LiFePO₄/Li half-cell with SiO₂@PAMPS/PP-18% had a discharge capacity of 148.10 mA h g^-1 and retained 98.67% of the original capacity even after 120 cycles at 0.5C. Even at a rate of 1.0C, the cell capacity could be maintained above 120 mA h g^-1. Therefore, a coating formula was developed that could considerably improve the cycling ability and high rate charge-discharge performance of lithium ion batteries.

Novel Polyimide Separator Prepared with Two Porogens for Safe Lithium-Ion Batteries

A porous polyimide (PI) membrane is successfully prepared via nonsolvent-induced phase separation with two porogens: dibutyl phthalate and glycerin. The as-prepared uniform porous PI membrane shows excellent separator properties for lithium-ion batteries (LIBs). Compared with the commercial polyethylene (PE) separator, the PI separator exhibits significant thermal stability, better ionic conductivity, and wettability both in carbonate and ether electrolytes for LIBs. The battery coin-cells assembled with the PI separator is more robust and still works even after heating at 140 °C for 1 h, while the cells with the commercial PE separator could not charge any more due to the shrinkage of the PE under the same condition.

A Heat-Resistant Poly(oxyphenylene benzimidazole)/Ethyl Cellulose Blended Polymer Membrane for Highly Safe Lithium-Ion Batteries

A blended membrane based on poly(oxyphenylene benzimidazole) (PBI) and ethyl cellulose (EC) exhibits heat resistance and good electrochemical performance. The prepared blended polymer gel membranes show no visible dimensional change after being held at 350 °C for 30 min, whereas the polyethylene (PE) separator almost completely melts. In addition to excellent thermal stability, the self-supporting blended membranes also exhibit a uniform thermal distribution during the heating process from 60 to 200 °C. Additionally, the ionic conductivities of the PBI/EC blended membranes with different ratios are 1.24 mS cm^-1 (1:1), 2.58 mS cm^-1 (1:2), and 1.68 mS cm^-1 (1:3), which are much higher than those of the PE separator (0.39 mS cm^-1). Compared to that of the PE separator (113 mAh g^-1), the cell with a separator of PBI/EC = 1:2 retained a discharge capacity of 131 mAh g^-1 after 150 cycles at 0.5C. Meanwhile, the rate performance of the cell was also better than that of the PE separator, especially at high currents (5C). All of the results indicate that this blended polymer gel membrane with good thermal stability is expected to be applied to high-performance lithium-ion batteries.

A method to visually observe the degradation-diffusion-reconstruction behavior of hydroxyapatite in the bone repair process

Nanostructured hydroxyapatite (HAp) has been applied widely as a scaffold material for bone tissue engineering for its good osteoinduction and biodegradability. However, the degradation process and the distribution of degraded HAp within the bone-defect cavity is still not clear. To visually study the behavior of HAp in bone repair process, a membrane of HAp/terbium (Tb)-HAp nanowires (NWs) was prepared with a concentric circle structure (CCS), of which the inner circle and the outer ring were constructed with Tb-HAp and HAp NWs, respectively. HAp/Tb-HAp CCS membrane possessed good osteogenic capacity and efficient fluorescence in the center for visualization. The in vitro experimental results proved that the Tb-HAp and HAp NWs membranes both presented high cytocompatibility and adequate efficiency to induce osteogenic differentiation of bone marrow stem cells (BMSCs). HAp/Tb-HAp CCS membranes were then implanted into a rat calvarial bone-defect model to study the behavior of HAp in bone repair process in vivo by tracking the fluorescence distribution. The results showed that the fluorescence of Tb-HAp diffused gradually from the inner circle to the outer ring, which suggested that the HAp was first degraded, and then the degraded product was diffused and finally reconstructed. Further, the histological results proved that the doping of Tb did not impair the promotive effect of HAp on bone repair process. Therefore, this study provided a visual method to observe the degradation-diffusion-reconstruction behavior of HAp nanomaterials in bone repair process. STATEMENT OF SIGNIFICANCE: The study of dynamic degradation process of implanted hydroxyapatite (HAp) materials in bone-defect cavity is of great significance to bone tissue engineering applications. Here, we designed a HAp/Tb-HAp nanowires (NWs) membrane with concentric circle structure (CCS) to visibly observe the behavior of HAp during bone repair process. HAp/Tb-HAp CCS membrane possessed both osteoinduction ability and fluorescence property. Calvarial bone-defect repair experiments in vivo showed that the fluorescence of Tb-HAp diffused gradually from inner circle to outer ring, which suggested that HAp was first degraded, then diffused and finally reconstructed. Therefore, this invention provides not only a visible method to observe the degradation-diffusion-reconstruction behavior of HAp-based biomaterials, but also a basic understanding of the dynamic change of HAp-based biomaterials.

Designing a Multifunctional Separator for High-Performance Li-S Batteries at Elevated Temperature

The practical applications of lithium-sulfur (Li-S) batteries are seriously limited by the undesirable polysulfide shuttling and lithium dendrite growth. Herein, a multifunctional membrane is designed and prepared by coating a lithiated Nafion (Li@Nafion) layer and an Al₂ O₃ layer on the two sides of a routine polymer membrane (polypropylene/polyethylene/polypropylene, PEP). The Li@Nafion layer faced to the sulfur cathode builds a "polysulfide-phobic" surface to restrain the shuttle effect via Coulomb repulsion, while the Al₂ O₃ layer with a uniform porous structure aids in regulating homogeneous Li⁺ fluxes to achieve stable Li electrodeposition. As a result, the Li//Li symmetric cell with a Li@Nafion/PEP/Al₂ O₃ (LNPA) separator realizes stable Li plating/striping even after 1000 h at a high current density (5 mA cm^-2 ). Moreover, the Li-S batteries incorporating LNPA separators not only can achieve excellent outstanding cyclic stability at an ultrahigh sulfur loading (7.6 mg cm^-2 ), but also exhibit impressive electrochemical performance at an elevated temperature (60 °C). The rational design of the LNPA separator presents new insights to develop high-performance Li-S batteries.

Bendable Network Built with Ultralong Silica Nanowires as a Stable Separator for High-Safety and High-Power Lithium-Metal Batteries

Separators are key safety components for electrochemical energy storage systems. However, the intrinsic poor wettability with electrolyte and low thermal stability of commercial polyolefin separators cannot meet the requirements of the ever-expanding market for high-power, high-energy, and high-safety power systems, such as lithium-metal, lithium-sulfur, and lithium-ion batteries. In this study, scalable bendable networks built with ultralong silica nanowires (SNs) are developed as stable separators for both high-safety and high-power lithium-metal batteries. The three-dimensional porous nature (porosity of 73%) and the polar surface of the obtained SNs separators endue a much better electrolyte wettability, larger electrolyte uptake ratio (325%), higher electrolyte retention ratio (63%), and ∼7 times higher ionic conductivity than that of commercial polypropylene (PP) separators. Moreover, the pore-rich structure of the SNs separator can aid in evenly distributing lithium and, in turn, suppress the uncontrollable growth of lithium dendrites to a certain degree. Furthermore, the pure inorganic structure endows the SNs separators with excellent chemical and electrochemical stabilities even at elevated temperatures, as well as excellent thermal stability up to 700 °C. This work underpins the utilization of SNs separators as a rational choice for developing high-performance batteries with a metallic lithium anode.

Electrospun Core-Shell Nanofiber as Separator for Lithium-Ion Batteries with High Performance and Improved Safety

Though the energy density of lithium-ion batteries continues to increase, safety issues related to the internal short circuit and the resulting combustion of highly flammable electrolytes impede the further development of lithium-ion batteries. It has been well-accepted that a thermal stable separator is important to postpone the entire battery short circuit and thermal runaway. Traditional methods to improve the thermal stability of separators include surface modification and/or developing alternate material systems for separators, which may affect the battery performance negatively. Herein, a thermostable and shrink-free separator with little compromise in battery performance was prepared by coaxial electrospinning and tested. The separator consisted of core-shell fiber networks where poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) layer served as shell and polyacrylonitrile (PAN) as the core. This core-shell fiber network exhibited little or even no shrinking/melting at elevated temperature over 250 °C. Meanwhile, it showed excellent electrolyte wettability and could take large amounts of liquid electrolyte, three times more than that of conventional Celgard 2400 separator. In addition, the half-cell using LiNi1/3Co1/3Mn1/3O2 as cathode and the aforementioned electrospun core-shell fiber network as separator demonstrated superior electrochemical behavior, stably cycling for 200 cycles at 1 C with a reversible capacity of 130 mA·h·g−1 and little capacity decay.