Electrospun Flexible Cellulose Acetate-Based Separators for Sodium-Ion Batteries with Ultralong Cycle Stability and Excellent Wettability: The Role of Interface Chemical Groups

Recent advances in cellulose-based separators for zinc ion batteries: A review

Aqueous zinc ion batteries (AZIBs) have attracted increasing attention because of their high energy density, excellent safety features, and environmentally friendly properties. The separator plays a crucial role in the battery, greatly influencing its performance and stability. Therefore, the separator for AZIBs has attracted increasing interest in recent years. Glass fiber (GF) is the predominant separator material, and exhibits favorable hydrophilicity and remarkable ionic conductivity. Nevertheless, its internal inhomogeneous pore structure makes it difficult to achieve uniform deposition of zinc ions, while the GF separator is easy to pierce due to its inadequate mechanical properties. In response to these issues, cellulose materials have garnered significant interest owing to their exceptional hydrophilicity, cost-effectiveness, and widespread availability. This review summarizes the potential of cellulose separators to substitute GF separators for AZIBs because of their high porosity, ion mobility number, electrolyte wetting rate, and liquid absorption rate. Different methods for preparing cellulose separators for AZIBs and various strategies to enhance their performance are summarized. The future outlook of cellulose-based separators for ZIBs is also prospected.

Cellulose-Derived Battery Separators: A Minireview on Advances Towards Environmental Sustainability

Cellulose-derived battery separators have emerged as a viable sustainable alternative to conventional synthetic materials like polypropylene and polyethylene. Sourced from renewable and biodegradable materials, cellulose derivatives-such as nanofibers, nanocrystals, cellulose acetate, bacterial cellulose, and regenerated cellulose-exhibit a reduced environmental footprint while enhancing battery safety and performance. One of the key advantages of cellulose is its ability to act as a hybrid separator, using its unique properties to improve the performance and durability of battery systems. These separators can consist of cellulose particles combined with supporting polymers, or even a pure cellulose membrane enhanced by the incorporation of additives. Nevertheless, the manufacturing of cellulose separators encounters obstacles due to the constraints of existing production techniques, including electrospinning, vacuum filtration, and phase inversion. Although these methods are effective, they pose challenges for large-scale industrial application. This review examines the characteristics of cellulose and its derivatives, alongside various processing techniques for fabricating separators and assessing their efficacy in battery applications. Additionally, it will consider the environmental implications and the primary challenges and opportunities associated with the use of cellulose separators in energy storage systems. Ultimately, the review underscores the significance of cellulose-based battery separators as a promising approach that aligns with the increasing demand for sustainable technologies in the energy storage domain.

An anti-corrosive cellulose nanocrystal/carbon nanotube derived Zn anode interface for dendrite-free aqueous Zn-ion batteries

Notorious zinc dendrite growth and hydrogen precipitation reactions disrupt the galvanic/stripping process at the electrolyte/electrode interface, which seriously affects the cycling stability of zinc anodes in aqueous zinc ion batteries. To improve the stability and reversibility of zinc anodes, we report an artificial SEI consisting of hydrophobic carbon nanocrystals and highly conductive carbon nanotube networks. This interfacial hydrophobicity effectively excludes free water from the surface of the zinc anode, which prevents water erosion and reduces the interfacial side reactions, resulting in a significant improvement in the cycling stability and coulombic efficiency of Zn plating/stripping. Benefiting from the reversible proton storage and fast desolvation kinetic behavior of the CNC/CNT interlayer, the stable cycling time of Zn/Zn symmetric batteries exceeds 700 h even at a high current density of 5 mA cm-2. The assembled CNC/CNT@Zn‖V2O5 full cell maintains a high capacity of 101.1 mA h g-1 after 5000 cycles (1.0 mA g-1). This study opens up a new area for expanding the use of organic compounds in zinc anode protection and offers a promising strategy for accelerating the development of aqueous zinc-ion batteries.

Enhancing CA-based separators with thermo-responsive ionic liquids: A path to eco-friendly membrane production and multifaceted applications

We synthesized a temperature-responsive ionic liquid, [N4444][SS], and incorporated it into an environmentally friendly cellulose acetate (CA)-based battery separator. A pore was formed in the battery separator by [N4444][SS], which pierced a plasticized part due to water pressure. Varying drying temperatures during membrane fabrication revealed that the CA/[N4444][SS] membrane dried at 50 °C exhibited greater thickness and a smaller average pore size, resulting in an asymmetric internal structure. Despite the asymmetry, this membrane demonstrated significantly higher water flux and a lower Gurley value compared to the membrane dried at 25 °C, indicating minimal tortuosity and low resistance within the internal pores. Thermal behavior analysis through TGA and DSC, as well as FT-IR spectroscopy, confirmed that [N4444][SS] remains within the CA matrix, forming coordinative bonds. The findings suggest that the CA/[N4444][SS] membrane, when used as a Li-ion battery separator, could enhance Li-ion transport properties and conductivity. Moreover, the recyclability of the IL in the membrane fabrication process contributes to a more environmentally friendly approach.

Facile Al2O3 coating suppress dissolution of Mn2+ in Mn-substituted Na3V2(PO4)3 with outstanding electrochemical performance for full sodium ion batteries

Na3V2(PO4)3 (NVP) encounters significant obstacles, including limited intrinsic electronic and ionic conductivities, which hinder its potential for commercial feasibility. Currently, the substitution of V3+ with Mn2+ is proposed to introduce favorable carriers, enhancing the electronic conductivity of the NVP system while providing structural support and stabilizing the NASICON framework. This substitution also widens the Na+ migration pathways, accelerating ion transport. Furthermore, to bolster stability, Al2O3 coating is applied to suppress the dissolution of transition metal Mn in the electrolyte. Notably, the Al2O3 coating serves a triple role in reducing HClO4 concentration in the electrolyte, inhibiting Mn dissolution, and functioning as the ion-conducting phase. Likewise, carbon nanotubes (CNTs) effectively hinder the agglomeration of active particles during high-temperature sintering, thereby optimizing the conductivity of NVP system. In addition, the excellent structural stability is investigated by in situ XRD measurement, effectively improving the volume collapse during Na+ de-embedding. Moreover, the Na3V5.92/3Mn0.04(PO4)3/C@CNTs@1wt.%Al2O3 (NVMP@CNTs@1wt.%Al2O3) possesses unique porous structure, promoting rapid Na+ transport and increasing the interface area between the electrolyte and the cathode material. Comprehensively, the NVMP@CNTs@1wt.%Al2O3 sample demonstrates a remarkable reversible specific capacity of 122.6 mAh/g at 0.1 C. Moreover, it maintains a capacity of 115.9 mAh/g at 1 C with a capacity retention of 90.2 mAh/g after 1000 cycles. Even at 30 C, it achieves a capacity of 87.9 mAh/g, with a capacity retention rate of 84.87 % after 6000 cycles. Moreover, the NVMP@CNTs@1wt.%Al2O3//CHC full cell can deliver a high reversible capacity of 205.5 mAh/g at 0.1 C, further indicating the superior application potential in commercial utilization.

Woven fabric-based separators with low tortuosity for sodium-ion batteries

In order to achieve high-performance and stable sodium-ion batteries, numerous attempts have been made to construct continuous ion transport pathways, in which a separator is one of the key components that affects the battery performance. In this study, a novel low-tortuosity woven fabric separator is fabricated by combining a weaving technique with a cellulose-solution method, followed by an infusion of a TEMPO-oxidized bacterial cellulose slurry into woven fabric substrates. The macropores in the fabric combine with the micropores in the oxidized bacterial cellulose to form a separator with a suitable pore structure and low tortuosity, forming a continuous sodium ion transport channel within the sodium-ion battery and effectively enhancing ion transport dynamics. The results show that, compared with a commercial polypropylene separator, the TEMPO-oxidized bacterial cellulose-woven fabric separator has a special weaving structure and lower tortuosity (0.77), as well as significant advantages in tensile strength (3.07 MPa), ionic conductivity (1.15 mS c), ionic transfer number (0.75), thermal stability, and electrochemical stability. This novel and simple preparation method provides new possibilities for achieving high-performance separators of sodium-ion batteries through rational structural design by textile technology.

Transition metal compounds: From properties, applications to wettability regulation

Transition metal compounds (TMCs) have the advantages of abundant reserves, low cost, non-toxic and pollution-free, and have attracted wide attention in recent years. With the development of two-dimensional layered materials, a new two-dimensional transition metal carbonitride (MXene) has attracted extensive attention due to its excellent physicochemical properties such as gas selectivity, photocatalytic properties, electromagnetic interference shielding and photothermal properties. They are widely used in gas sensors, oil/water separation, wastewater and waste-oil treatment, cancer treatment, seawater desalination, strain sensors, medical materials and some energy storage materials. In this view, we aim to emphatically summarize MXene with their properties, applications and their wettability regulation in different applications. In addition, the properties of transition metal oxides (TMOs) and other TMCs and their wettability regulation applications are also discussed.

SIS-Based Electrostatic Spinning High-Safety Lithium-Ion Battery Separators

As an important component, the properties of separators directly affect the capacity, life, and safety performance of lithium-ion batteries (LIBs). The high thermal stability and safety application value of the thermoplastic elastomer poly(styrene-b-isoprene-b-styrene) block copolymer (SIS) with different block ratios were explored to enhance the thermal stability and mechanical strength of the cross-linked polyacrylonitrile (PAN) membranes by vulcanization cross-linking and heat treatment. Among these membranes, the sample named the S/PAN/SIS-4019 separator was confirmed to be a self-closing separator that can cope with the thermal runaway, attributing to the continued fusion of the SIS soft and hard segments in the cross-linked structure under high-temperature heat treatment. Moreover, the tensile strength of S/PAN/SIS-4019 separator increased to 17.49 MPa, which was better than that of Celgard 2400, PAN, and other inlay separators. Using S/PAN/SIS-4019 as a battery separator, lithium-ion batteries showed a superior electrochemical performance compared to the usage of Celgard 2400. Owing to the stable pore structure and thermally protected self-shutdown mechanism, the overall properties of the obtained cross-linked separator were improved in terms of higher thermal stability, high ionic conductivity, and electrochemical properties.

Cellulose acetate network via ion pre-anchored strategy for simultaneous regulation of uniform Mg2+ flux and ion conductivity

Hybrid LiMg batteries stand out by combining the rapid lithium diffusion kinetics and the advantages of magnesium. However, the uneven Mg deposits may result in continuous parasitic reactions and penetrate the separator. Herein, cellulose acetate (CA), with functional groups, was applied to engineer coordination with MOFs and construct the evenly-distributed and ample nucleation sites. Moreover, the hierarchical MOFs@CA network was fabricated via the metal ion pre-anchored strategy to regulate the uniform Mg²⁺ flux and improve ion-conductivity simultaneously. Furthermore, the hierarchical CA networks with well-ordered MOFs provided efficient MOF-to-MOF ion-transportation channels and served as ion sieves to inhibit anion transportation, thereby mitigating polarization. The super dendrite-inhibition and interfacial compatibility was confirmed and the assembled Mo₆S₈//Mg batteries exhibited high capacity about 105 mAh g^-1 and capacity decay of 4 % after 600 cycles at 30 C, which surpassed the state-of-the-art LMBs system employing Mo₆S₈ electrode. The fabricated GPE sheds fresh strategy for the design of CA-based GPEs and bright light on the promise of high-performance LMBs.

An Ultrathin Nonporous Polymer Separator Regulates Na Transfer Toward Dendrite-Free Sodium Storage Batteries

Sodium storage batteries are one of the ever-increasing next-generation large-scale energy storage systems owing to the abundant resources and low cost. However, their viability is severely hampered by dendrite-related hazards on anodes. Herein, a novel ultrathin (8 µm) exterior-nonporous separator composed of honeycomb-structured fibers is prepared for homogeneous Na deposition and suppressed dendrite penetration. The unhindered ion transmission greatly benefits from honeycomb-structured fibers with huge electrolyte uptake (376.7%) and the polymer's inherent transport ability. Additionally, polar polymer chains consisting of polyethersulfone and polyvinylidene customize the highly aggregated solvation structure of electrolytes via substantial solvent immobilization, facilitating ion-conductivity-enhanced inorganic-rich solid-electrolyte interphase with remarkable interface endurance. With the reliable mechanical strength of the separator, the assembled sodium-ion full cell delivers significantly improved energy density and high safety, enabling stable operation under cutting and rolling. The as-prepared separator can further be generalized to lithium-based batteries for which apparent dendrite inhibition and cyclability are accessible and demonstrates its potential for practical application.

Mixed cellulose ester membrane as an ion redistributor to stabilize zinc anode in aqueous zinc ion batteries

Aqueous zinc-ion batteries (AZBs) with high energy density, low cost and environmental characteristics, have become the promising device for energy storage. However, uncontrolled zinc dendrite growth remains an impediment to the popularization of AZBs. The unrestricted two-dimensional (2D) ions diffusion is the main cause of the above defect. In this work, mixed cellulose ester (MCE) membrane is proposed as the separator. A dense homogeneous pore structure can achieve a physical shunting effect on ion diffusion, which can control and homogenize the ion motion. Further, the mechanism of this physical pore effect is confirmed by comparing the behavior of Zn deposition in MCE systems with different pore sizes but the same composition. As conjectured, a membrane with a smaller pore size is more favorable. In addition, the MCE contains many polar oxygen-containing functional groups that can facilitate and modulate ion diffusion through coordination. This chemical ion guiding effect, together with the above physical pore effect, gives the separator the ability to suppress dendrite formation. Zn/Zn symmetric cells with this membrane exhibit ultralong cycle life exceeding 1250 h at 0.5 mA cm^-2 and 1000 h at 5 mA cm^-2. And the Zn//MnO₂ battery presents excellent cycle stability for more than 500 cycles with a capacity retention of 90.67%. This work proposes MCE separators and reveals their coordinated regulation of physical and chemical effects on metal-based anodes. This will shed light on the development of high-performance separators and AZBs.

Hierarchical Nanocellulose-Based Gel Polymer Electrolytes for Stable Na Electrodeposition in Sodium Ion Batteries

Sodium ion batteries (NIBs) based on earth-abundant materials offer efficient, safe, and environmentally sustainable solutions for a decarbonized society. However, to compete with mature energy storage technologies such as lithium ion batteries, further progress is needed, particularly regarding the energy density and operational lifetime. Considering these aspects as well as a circular economy perspective, the authors use biodegradable cellulose nanoparticles for the preparation of a gel polymer electrolyte that offers a high liquid electrolyte uptake of 2985%, an ionic conductivity of 2.32 mS cm^-1 , and a Na⁺ transference number of 0.637. A balanced ratio of mechanically rigid cellulose nanocrystals and flexible cellulose nanofibers results in a mesoporous hierarchical structure that ensures close contact with metallic Na. This architecture offers stable Na plating/stripping at current densities up to ±500 µA cm^-2 , outperforming conventional fossil-based NIBs containing separator-liquid electrolytes. Paired with an environmentally sustainable and economically attractive Na₂ Fe₂ (SO₄ )₃ cathode, the battery reaches an energy density of 240 Wh kg^-1 , delivering 69.7 mAh g^-1 after 50 cycles at a rate of 1C. In comparison, Celgard in liquid electrolyte delivers only 0.6 mAh g^-1 at C/4. Such gel polymer electrolytes may open up new opportunities for sustainable energy storage systems beyond lithium ion batteries.

Cellulose: Characteristics and applications for rechargeable batteries

Cellulose, an abundant natural polymer, has promising potential to be used for energy storage systems because of its excellent mechanical, structural, and physical characteristics. This review discusses the structural features of cellulose and describes its potential application as an electrode, separator, and binder, in various types of high-performing batteries. Various surface and structural characteristics of cellulose (e.g., fiber size, surface functional groups, the hierarchy of pores, and porosity levels) that contribute to its electrochemical performance are discussed. Cellulose structure/property/processing/function relationships are further focused and elucidated in terms of the latest developments in the emerging field of sustainable materials in Li-Ion, Na-Ion, and LiS batteries.

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.

Why Cellulose-Based Electrochemical Energy Storage Devices?

Recent findings demonstrate that cellulose, a highly abundant, versatile, sustainable, and inexpensive material, can be used in the preparation of very stable and flexible electrochemical energy storage devices with high energy and power densities by using electrodes with high mass loadings, composed of conducting composites with high surface areas and thin layers of electroactive material, as well as cellulose-based current collectors and functional separators. Close attention should, however, be paid to the properties of the cellulose (e.g., porosity, pore distribution, pore-size distribution, and crystallinity). The manufacturing of cellulose-based electrodes and all-cellulose devices is also well-suited for large-scale production since it can be made using straightforward filtration-based techniques or paper-making approaches, as well as utilizing various printing techniques. Herein, the recent development and possibilities associated with the use of cellulose are discussed, regarding the manufacturing of electrochemical energy storage devices comprising electrodes with high energy and power densities and lightweight current collectors and functional separators.

Stable Na Electrodeposition Enabled by Agarose-Based Water-Soluble Sodium Ion Battery Separators

Developing efficient energy storage technologies is at the core of current strategies toward a decarbonized society. Energy storage systems based on renewable, nontoxic, and degradable materials represent a circular economy approach to address the environmental pollution issues associated with conventional batteries, that is, resource depletion and inadequate disposal. Here we tap into that prospect using a marine biopolymer together with a water-soluble polymer to develop sodium ion battery (NIB) separators. Mesoporous membranes comprising agarose, an algae-derived polysaccharide, and poly(vinyl alcohol) are synthesized via nonsolvent-induced phase separation. Obtained membranes outperform conventional nondegradable NIB separators in terms of thermal stability, electrolyte wettability, and Na+ conductivity. Thanks to the good interfacial adhesion with metallic Na promoted by the hydroxyl and ether functional groups of agarose, the separators enable a stable and homogeneous Na deposition with limited dendrite growth. As a result, membranes can operate at 200 μA cm-2, in contrast with Celgard and glass microfiber, which short circuit at 50 and 100 μA cm-2, respectively. When evaluated in Na3V2(PO4)3/Na half-cells, agarose-based separators deliver 108 mA h g-1 after 50 cycles at C/10, together with a remarkable rate capability. This work opens up new possibilities for the use of water-degradable separators, reducing the environmental burdens arising from the uncontrolled accumulation of electronic waste in marine or land environments.

Polysaccharides for sustainable energy storage - A review

The increasing amount of electric vehicles on our streets as well as the need to store surplus energy from renewable sources such as wind, solar and tidal parks, has brought small and large scale batteries into the focus of academic and industrial research. While there has been huge progress in performance and cost reduction in the past years, batteries and their components still face several environmental issues including safety, toxicity, recycling and sustainability. In this review, we address these challenges by showcasing the potential of polysaccharide-based compounds and materials used in batteries. This particularly involves their use as electrode binders, separators and gel/solid polymer electrolytes. The review contains a historical section on the different battery technologies, considerations about safety on batteries and requirements of polysaccharide components to be used in different types of battery technologies. The last sections cover opportunities for polysaccharides as well as obstacles that prevent their wider use in battery industry.

Raw cellulose/polyvinyl alcohol blending separators prepared by phase inversion for high-performance supercapacitors

The development of a biodegradable cellulose-based separator with excellent performance has been of great research significance and application potential for the green development of supercapacitors. Herein, the regenerated porous cellulose/Polyvinyl alcohol films (CP-10, CP-15, CP-20, CP-25) with different mass ratio were successfully fabricated by a simple blending and phase inversion process. Their electrochemical properties as separators in assembled supercapacitor were evaluated. Fourier transform infrared spectroscopy and x-ray diffraction analysis indicate that intermolecular and intramolecular hydrogen bonding existed between cellulose and polyvinyl alcohol of the CP films. Compared with other CP films, the CP-20 film shows higher mechanical strength (28.02 MPa), better wettability (79.06°), higher porosity (59.69%) and electrolyte uptake (281.26 wt%). These properties of CP-20 are expected to show better electrochemical performance as separator. Indeed, the electrochemical tests, including electrochemical impedance spectroscopy, cyclic voltammetry, galvanostatic charge discharge, demonstrate that the SC-20 capacitor (with CP-20 as separator) shows the lowest equivalent series resistance of 0.57 Ω, the highest areal capacitance of 1.98 F cm^-2 at 10 mV s^-1, specific capacitance of 134.41 F g^-1 and charge-discharge efficiency of 98.62% at 1 A g^-1 among the four capacitors with CP films as separators. Comparing the assembled SC-40 and SC-30 with two commercial separators (TF4040 and MPF30AC) and SC-PVA with Polyvinyl alcohol (PVA) separator, the CV and GCD curves of SC-20 maintain the quasi rectangular and symmetrical triangular profiles respectively at different scan rates in potential window of 0-1 V. SC-20 exhibits the highest value of 28.24 Wh kg^-1 at 0.5 A g^-1 with a power density of 0.26 kW kg^-1, and 13.41 Wh kg^-1 at 10 A g^-1 with a power density of 6.04 kW kg^-1. SC-20 also shows the lowest voltage drop and the highest areal and specific capacitance. Moreover, SC-20 maintains the highest value of 86.81% after 4000 cycles compared to 21.18% of SC-40, 75.07% of SC-30, and 6.66% of SC-PVA, showing a superior rate capability of a supercapacitor. These results indicate that CP films can be served as promising separators for supercapacitors.

A Sodium-Ion Battery Separator with Reversible Voltage Response Based on Water-Soluble Cellulose Derivatives

The development of efficient, safe, and environmentally friendly energy storage systems plays a pivotal role in moving toward a more sustainable society. Sodium-ion batteries (NIBs) have garnered considerable interest in grid energy storage applications because of the abundance of sodium, low cost, and suitable redox potential. However, NIB technology is still in its infancy, especially with regard to separators. Here we develop a novel separator based on renewable water-soluble cellulose derivatives. Carboxymethyl cellulose (CMC) and hydroxyethyl cellulose (HEC) are cross-linked to afford large-specific-surface-area membranes upon nonsolvent-induced phase separation (NIPS). Long-term galvanostatic cycling in a symmetric Na/Na cell configuration shows an impressive reversible voltage response with a square wave shape of the polarization even after 250 h of cycling, indicating remarkably stable Na plating and stripping with Na dendrite growth suppression. This novel membrane is evaluated as a separator in Na₃V₂(PO₄)₃/Na half-cells. After 10 cycles at C/10, the cellulosic separator delivers a capacity of 74 mA·h·g^-1 with a 100% Coulombic efficiency compared to that of 61 mA·h·g^-1 and 96% obtained for Whatman GF/D as a commercially available separator. Our work provides novel cues for the development of biomass-derived porous membranes to function as battery separators, surpassing the performance of commercially available separators based on fossil resources in terms of capacity retention, Coulombic efficiency, homogeneous plating/stripping of Na, and dendrite growth suppression. These separators, which may be extended to other battery systems, are expected to play a significant role in developing sustainable energy storage systems.

Nature-Derived Cellulose-Based Composite Separator for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are emerging power sources for the replacement of lithium-ion batteries. Recent studies have focused on the development of electrodes and electrolytes, with thick glass fiber separators (~380 μm) generally adopted. In this work, we introduce a new thin (~50 μm) cellulose–polyacrylonitrile–alumina composite as a separator for SIBs. The separator exhibits excellent thermal stability with no shrinkage up to 300°C and electrolyte uptake with a contact angle of 0°. The sodium ion transference number, tNa+, of the separator is measured to be 0.78, which is higher than that of bare cellulose (tNa+: 0.31). These outstanding physical properties of the separator enable the long-term operation of NaCrO₂ cathode/hard carbon anode full cells in a conventional carbonate electrolyte, with capacity retention of 82% for 500 cycles. Time-of-flight secondary-ion mass spectroscopy analysis reveals the additional role of the Al₂O₃ coating, which is transformed into AlF₃ upon long-term cycling owing to HF scavenging. Our findings will open the door to the use of cellulose-based functional separators for high-performance SIBs.

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.

Nonaqueous Sodium-Ion Full Cells: Status, Strategies, and Prospects

With ever-increasing efforts focused on basic research of sodium-ion batteries (SIBs) and growing energy demand, sodium-ion full cells (SIFCs), as unique bridging technology between sodium-ion half-cells (SIHCs) and commercial batteries, have attracted more and more interest and attention. To promote the development of SIFCs in a better way, it is essential to gain a systematic and profound insight into their key issues and research status. This Review mainly focuses on the interface issues, major challenges, and recent progresses in SIFCs based on diversified electrolytes (i.e., nonaqueous liquid electrolytes, quasi-solid-state electrolytes, and all-solid-state electrolytes) and summarizes the modification strategies to improve their electrochemical performance, including interface modification, cathode/anode matching, capacity ratio, electrolyte optimization, and sodium compensation. Outlooks and perspectives on the future research directions to build better SIFCs are also provided.

Marine-Biomass-Derived Porous Carbon Sheets with a Tunable N-Doping Content for Superior Sodium-Ion Storage

Synthesis of the electrode materials of sodium-ion storage devices from sustainable precursors via green methods is highly desirable. In this work, we fabricated a unique N, O dual-doped biocarbon nanosheet with hierarchical porosity by direct pyrolysis of low-cost cuttlebones and simple air oxidation activation (AOA) technique. With prolonging AOA time, thickness of the carbon sheets could be reduced controllably (from 35 to 5 nm), which may lead to tunable preparation of carbon nanosheets with a certain thickness. Besides, an unexpected increase in N-doping amount from 7.5 to 13.9 atom % was observed after AOA, demonstrating the unique role of AOA in tuning the doped heteroatoms of carbon matrix. This was also the first example of increasing N-doping content in carbons by treatment in air. More importantly, by optimizing the thickness of carbon sheets and heteroatom doping via AOA, superior sodium capacity-cycling retention-rate capability combinations were achieved. Specifically, a current state-of-the-art Na⁺ storage capacity of 640 mAh g^-1 was obtained, which was comparable with the lithium-ion storage in carbon materials. Even after charging/discharging at large current densities (2 and 10 A g^-1) for 10 000 cycles, the as-obtained samples still retained the capacities of 270 and 138 mAh g^-1, respectively, with more than 90% retention. The assembled sodium-ion capacitors also delivered a high integrated energy-power density (36 kW h kg^-1 at an ultrahigh power density of 53 000 W kg^-1) and good cycling stability (90.5% of capacitance retention after 8000 cycles at 5 A g^-1).

Hierarchical Carbon@SnS2 Aerogel with "Skeleton/Skin" Architectures as a High-Capacity, High-Rate Capability and Long Cycle Life Anode for Sodium Ion Storage

Developing high-performance electrode materials with high energy and long-term cycling stability is a hot topic and of great importance for sodium ion batteries (SIBs). In this work, a highly porous carbon/tin sulfide aerogel with a "skeleton/skin" morphology (SSC@SnS₂) has been developed and further used as a binder-free anode for SIBs. This SSC@SnS₂ electrode delivers a high specific capacity of 612 mA h g^-1 at 0.1 A g^-1, a good rate capability, and a long-term cycling stability up to 1000 times with an average Coulombic efficiency of ∼99.9%. Meanwhile, this SSC@SnS₂ aerogel also achieves a stable cycling performance even at a high current density up to 5.0 A g^-1. The fast-yet-stable sodium ion storage performance of the prepared SSC@SnS₂ aerogel can be ascribed to the reasons that (i) the carbon nanofiber/graphene skeleton provides unimpeded pathways for the rapid transfer of electrons; (ii) thin SnS₂ skin with nonaggregated morphology can provide a great number of active sites for sodium ion storage; (iii) the porous structure of the SSC@SnS₂ aerogel ensures a rapid penetration of electrolyte and can further accommodate the volume expansion of active SnS₂ nanoflakes; and (iv) the intermediate product of Na₁₅Sn₄ alloy contributes greatly to the sodium ion storage performance of the SSC@SnS₂ aerogel. The excellent electrochemical performances coupling with the unique structural features of this SSC@SnS₂ aerogel make it a promising anode candidate for SIBs.