Versatile separators toward advanced lithium-sulfur batteries: status, recent progress, challenges and perspective.

Lithium-sulfur batteries (LSBs) have recently gained extensive attention due to their high energy density, low cost, and environmental friendliness. However, serious shuttle effect and uncontrolled growth of lithium dendrites restrict them from further commercial applications. As "the third electrode", functional separators are of equal significance as both anodes and cathodes in LSBs. The challenges mentioned above are effectively addressed with rational design and optimization in separators, thereby enhancing their reversible capacities and cycle stability. The review discusses the status/operation mechanism of functional separators, then primarily focuses on recent research progress in versatile separators with purposeful modifications for LSBs, and summarizes the methods and characteristics of separator modification, including heterojunction engineering, single atoms, quantum dots, and defect engineering. From the perspective of the anodes, distinct methods to inhibit the growth of lithium dendrites by modifying the separator are discussed. Modifying the separators with flame retardant materials or choosing a solid electrolyte is expected to improve the safety of LSBs. Besides, in-situ techniques and theoretical simulation calculations are proposed to advance LSBs. Finally, future challenges and prospects of separator modifications for next-generation LSBs are highlighted. We believe that the review will be enormously essential to the practical development of advanced LSBs.

Multifunctional Vanadium Nitride-Modified Separator for High-Performance Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) are recognized as among the best potential alternative battery systems to lithium-ion batteries and have been widely investigated. However, the shuttle effect has severely restricted the advancement in their practical applications. Here, we prepare vanadium nitride (VN) nanoparticles grown in situ on a nitrogen-doped carbon skeleton (denoted as VN@NC) derived from the MAX phase and use it as separator modification materials for LSBs to suppress the shuttle effect and optimize electrochemical performance. Thanks to the outstanding catalytic performance of VN and the superior electrical conductivity of carbon skeleton derived from MAX, the synergistic effect between the two accelerates the kinetics of both lithium polysulfides (LiPSs) to Li2S and the reverse reaction, effectively suppresses the shuttle effect, and increases cathode sulfur availability, significantly enhancing the electrochemical performance of LSBs. LSBs constructed with VN@NC-modified separators achieve outstanding rate performance and cycle stability. With a capacity of 560 mAh g-1 at 4 C, it exhibits enhanced structural and chemical stability. At 1 C, the device has an incipient capacity of 1052.4 mAh g-1, and the degradation rate averaged only 0.085% over 400cycles. Meanwhile, the LSBs also show larger capacities and good cycling stability at a low electrolyte/sulfur ratio and high surface-loaded sulfur conditions. Thus, a facile and efficient way of preparing modified materials for separators is provided to realize high-performance LSBs.

Synchronous embedded growth of Mo2C nanodisk arrays immobilized on porous carbon nanosheets for ultra-stable sodium storage.

Sodium ion capacitors (SICs) that combine the merits of both rechargeable batteries and supercapacitors have gained widespread recognition for their high energy density and extended cycle life as new energy storage devices. However, the purposeful design of advanced battery-type anodes has become an urgent need to remedy the dynamics mismatch with the capacitive cathode. Herein, we propose a simple but efficient bottom-up approach to build three-dimensional Mo2C/C hybrid architectures in situ as anodes for SICs. By finely regulating the ratio of carbon and molybdenum sources, the optimized Mo2C/C, where even thinner subunit assembled Mo2C nanodisk (∼47.1 nm in thickness) arrays are immobilized on carbon nanosheet substrate via the synchronous embedded growth, rapid electron and ion diffusion/transport expressways, abundant active sites and robust structural stability were achieved for efficient sodium storage. Benefiting from the synergistic contributions of the components, the optimum Mo2C/C anode displays an outstanding rate and long-cycle properties as a competitive anode. Moreover, the constructed Mo2C/C-based SICs exhibited an energy density of ∼16.7 W h kg-1 at 10 kW h kg-1, along with ∼22.5% capacitance degradation over 4000 cycles at 1 A g-1. This contribution will guide the precise synthesis of other versatile Mo2C-based hybrids towards energy-related applications and beyond.

Construction of Ultrastable Ultrathin Black Phosphorus Nanodisks Hybridized with Fe3 O4 Nanoclusters and Iron (V)-Oxo Complex for Efficient Potassium Storage.

The practical application of metalloid black phosphorus (BP) based anodes for potassium ion batteries is mainly impeded by its instability in air and irreversible/sluggish potassium storage behaviors. Herein, a 2D composite is purposefully conceptualized, where ultrathin BP nanodisks with Fe3 O4 nanoclusters are hybridized with Lewis acid iron (V)-oxo complex (FC) nanosheets (denoted as BP@Fe3 O4 -NCs@FC). The introduced electron coordinate bridge between FC and BP, and hydrophobic surface of FC synergistically assure that BP@Fe3 O4 -NCs@FC is ultrastable in humid air. With the purposeful structural and componential design, the resultant BP@Fe3 O4 -NCs@FC anode is endowed with appealing electrochemical performance in terms of reversible capacity, rate behavior, and long-duration cycling stability in both half and full cells. Furthermore, the underlying formation and potassium-storage mechanisms of BP@Fe3 O4 -NCs@FC are tentatively proposed. The in-depth insights here will provide a crucial understanding in rational exploration of advanced anodes for next-generation PIBs.

Facile Construction of Porous ZnMn2O4 Hollow Micro-Rods as Advanced Anode Material for Lithium Ion Batteries.

Spinel ZnMn2O4 is considered a promising anode material for high-capacity Li-ion batteries due to their higher theoretical capacity than commercial graphite anode. However, the insufficient cycling and rate properties seriously limit its practical application. In this work, porous ZnMn2O4 hollow micro-rods (ZMO HMRs) are synthesized by a facile co-precipitation method coupled with annealing treatment. On the basis of electrochemical analyses, the as-obtained samples are first characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy techniques. The influences of different polyethylene glycol 400 (PEG 400) additions on the formation of the hollow rod structure are also discussed. The abundant multi-level pore structure and hollow feature of ZMO HMRs effectively alleviate the volume expansion issue, rendering abundant electroactive sites and thereby guaranteeing convenient Li+ diffusion. Thanks to these striking merits, the ZMO HMRs anode exhibits excellent electrochemical lithium storage performance with a reversible specific capacity of 761 mAh g-1 at a current density of 0.1 A g-1, and a long-cycle specific capacity of 529 mAh g-1 after 1000 cycles at 2.0 A g-1 and keep a remarkable rate capability. In addition, the assembled ZMO HMRs-based full cells deliver an excellent rate capacity, and when the current density returns to 0.05 A g-1, the specific capacity can still reach 105 mAh g-1 and remains at 101 mAh g-1 after 70 cycles, maintaining a material-level energy density of approximately 273 Wh kg-1. More significantly, such striking electrochemical performance highlights that porous ZMO HMRs could be a promising anode candidate material for LIBs.

Chemomechanically Stable Small Single-crystal Mo-doped LiNi0.6 Co0.2 Mn0.2 O2 Cathodes for Practical 4.5†V-class Pouch-type Li-ion Batteries.

High voltage can cost-effectively boost energy density of Ni-rich cathodes based Li-ion batteries (LIBs), but compromises their mechanical, electrochemical and thermal-driven stability. Herein, a collaborative strategy (i.e., small single-crystal design and hetero-atom doping) is devised to construct a chemomechanically reliable small single-crystal Mo-doped LiNi0.6 Co0.2 Mn0.2 O2 (SS-MN6) operating stably under high voltage (≥4.5 V vs. Li/Li+ ). The substantially reduced particle size combined with Mo6+ doping absorbs accumulated localized stress to eradicate cracks formation, subdues the surface side reactions and lattice oxygen missing meanwhile, and improves thermal tolerance at highly delithiated state. Consequently, the SS-MN6 based pouch cells are endowed with striking deep cycling stability and wide-temperature-tolerance capability. The contribution here provides a promising way to construct advanced cathodes with superb chemomechanical stability for next-generation LIBs.

High-efficiency and simple synthesis of Ti3C2T x nanoscrolls by surface energy modulation and cryogenic freezing.

To alleviate the restacking issue of 2D Ti3C2T x itself, we purposefully explore a simple but efficient method for controllable construction of 1D Ti3C2T x nanoscrolls with a high efficiency of ∼90.5%, with detailed regulation of the feed concentrations and surface energy of Ti3C2T x . The involved transformation mechanism from 2D nanosheets to 1D nanoscrolls is reasonably proposed.

Recent Progress of Carbon-Based Anode Materials for Potassium Ion Batteries.

With the increasing demand for clean energy, rechargeable batteries with K+ as carriers have attracted wide attention due to their advantages of expandability and low cost. High-performance anode materials are the key to the development of potassium ion batteries (PIBs), improving their competitiveness and feasibility. Carbon materials have become promising anodes for PIBs due to their abundant resources, low cost, non-toxicity and electrochemical diversity. This article reviews the research progress of carbon based anode materials in recent years. Firstly, the unique characteristics of carbon as a competitive anode for advanced PIBs are discussed, which provides guidance for optimal design and exploration. Then, various carbon materials as the anodes towards PIBs are summarized in detail, and the involved problems and corresponding solutions are analyzed. Finally, the future development and perspective of advanced carbons for next-generation PIBs are proposed.

Single-Crystal Nano-Subunits Assembled Accordion-Shape WNb2 O8 Framework with High Ionic/Electronic Conductivities towards Li-Ion Capacitors.

Recently, Li-ion capacitors (LICs) have drawn tremendous attention due to their high energy/power density along with long cycle life. Nevertheless, the slow kinetics and stability of the involved anodes as bottleneck barriers always result in the modest properties of devices. The exploration of advanced anodes with both high ionic and electronic conductivities as well as structural stability thus becomes more significant for practical applications of LICs. Herein, a single-crystal nano-subunits assembled hierarchical accordion-shape WNb2 O8 micro-/nano framework is first designed via a one-step scalable strategy with the multi-layered Nb2 CTx as a precursor. The underlying solid solution Li-storage mechanism of the WNb2 O8 just with a volumetric expansion of ≈1.5% is proposed with in situ analysis. Benefiting from congenitally crystallographic merits, single-crystalline characteristic, and open accordion-like architecture, the resultant WNb2 O8 as a robust anode platform is endowed with fast electron/ion transport capability and multi-electron redox contributions from W/Nb, and accordingly, delivers a reversible capacity of ≈135.5 mAh g-1 at a high rate of 2.0 A g-1 . The WNb2 O8 assembled LICs exhibit an energy density of ≈33.0 Wh kg-1 at 9 kW kg-1 , coupled with remarkable electrochemical stability. The work provides meaningful insights into the rational design and construction of advanced bimetallic niobium oxides for next-generation LICs.

Ultrasonic-Assisted Synthesis of N-Doped, Multicolor Carbon Dots toward Fluorescent Inks, Fluorescence Sensors, and Logic Gate Operations.

Over past decades, the multicolor carbon dots (M-CDs) have attracted enormous attentions due to their tunable photoluminescence and versatile applications. Herein, the nitrogen-doped (N-doped) M-CDs including green, chartreuse, and pink emissive CDs are successfully synthesized by ultrasonic treatment of kiwifruit juice with different additive reagents such as ethanol, ethylenediamine, and acetone. Owing to their strong fluorescence upon irradiation with 365 nm UV light, the highly water-soluble M-CDs present great potential in the anticounterfeit field as fluorescent inks. Particularly, the resulting green emission CDs (G-CDs) with excellent fluorescence and stability are applied as a label-free probe model for "on-off" detection of Fe3+. The fluorescence of G-CDs is significantly quenched by Fe3+ through static quenching. The nanoprobe demonstrates good selectivity and sensitivity toward Fe3+ with a detection limit of ~0.11 µM. Besides, the quenched fluorescence of G-CDs by Fe3+ can be recovered by the addition of PO43- or ascorbic acid (AA) into the CDs/Fe3+ system to realize the "off-on" fluorescent process. Furthermore, NOT and IMPLICATION logic gates are constructed based on the selection of Fe3+ and PO43- or AA as the inputs, which makes the G-CD-based sensors utilized as various logic gates at molecular level. Therefore, the N-doped M-CDs hold promising prospects as competitive candidates in monitoring the trace species, applications in food chemistry, anticounterfeit uses, and beyond.

MOFs Derived Hetero-ZnO/Fe2O3 Nanoflowers with Enhanced Photocatalytic Performance towards Efficient Degradation of Organic Dyes.

It is still a challenge for wastewater treatment to develop efficient yet low-cost photocatalysts on a large scale. Herein, a facile yet efficient method was devised to successfully synthesize ZnO/Fe2O3 nanoflowers (NFs) by using metal organic framework ZIF-8 as the precursor. The photocatalytic activities of the as-prepared hetero-ZnO/Fe2O3 NFs are purposefully evaluated by photocatalytic degradation of methylene blue (MB) and methyl orange (MO) under UV light irradiation. The resulting ZnO/Fe2O3 NFs display even higher photocatalytic activities than those of single-phase ZnO and Fe2O3 as a photocatalyst for the degradation of both MB ad MO. Particularly, nearly 100% MB can be photocatalytically degraded in 90 min under UV light irradiation using the hetero-NFs photocatalyst. The enhanced photocatalytic properties are probably ascribed to the synergistic contributions from the suitable band alignment of ZnO and Fe2O3, large surface area, and strong light absorption property. Radical scavenger experiments prove that the photogenerated holes, ·OH and ·O2-, play key roles in photocatalytic degradation process of organic dyes. Accordingly, the photocatalytic degradation mechanism of hetero-ZnO/Fe2O3 NFs towards dyes is tentatively proposed. The work contributes an effective way to rationally design and fabricate advanced photocatalysts with heterojunction structures for photocatalytic applications.

Recent Progress on In Situ/Operando Characterization of Rechargeable Alkali Ion Batteries.

The specific chemical and physical evolutions of electrode materials under operating conditions should be understood to optimize their electrochemical performances. The in-situ/operando techniques including Raman spectrum, transmission electron microscope, X-ray diffraction, X-ray absorption spectrum, and magnetization are powerful tools, which can provide the real-time surficial/interfacial changes of electrodes, the transformation of crystal lattice structures, the adjustment of electronic states and even the influence of magnetic properties under operating conditions. In this Review, the advantages and limitations of these in-situ/operando techniques in investigating the inner energy storage mechanisms of various type electrode materials are analyzed. The representative research results such as the ion dependent storage mechanism, step-alloying processes and space charge storage theory are highlighted. In addition, the challenges and opportunities of in-situ/operando characterizations are proposed as well.

Organic-Inorganic Hybridization Engineering of Polyperylenediimide Cathodes for Efficient Potassium Storage.

Polyperylenediimide (PDI) is always subject to its modest conductivities, limited reversible active sites and inferior stability for potassium storage. To address these issues, herein, we firstly propose an organic-inorganic hybrid (PDI@Fe-Sn@N-Ti3 C2 Tx ), where Fe/Sn single atoms are bound to the N-doped MXenes (N-Ti3 C2 Tx ) via the unsaturated Fe/Sn-N3 bonds, and functionalized with PDI via d-π hybridization, forming a high conjugated δ skeleton. The resulted hybrid cathode endowed with enhanced electronic/ionic conductivities, lowered dissociation barriers of multiple redox centers and a stable cathode electrolyte interphase layer displays a 14-electron involved high-rate capacities and long cycle life. Moreover, it shows competitive performance in full cells even under different folding states and low operating temperatures.

Unveiling Intrinsic Potassium Storage Behaviors of Hierarchical Nano Bi@N-Doped Carbon Nanocages Framework via In Situ Characterizations.

Metallic bismuth has drawn attention as a promising alloying anode for advanced potassium ion batteries (PIBs). However, serious volume expansion/electrode pulverization and sluggish kinetics always lead to its inferior cycling and rate properties for practical applications. Therefore, advanced Bi-based anodes via structural/compositional optimization and sur-/interface design are needed. Herein, we develop a bottom-up avenue to fabricate nanoscale Bi encapsulated in a 3D N-doped carbon nanocages (Bi@N-CNCs) framework with a void space by using a novel Bi-based metal-organic framework as the precursor. With elaborate regulation in annealing temperatures, the optimized Bi@N-CNCs electrode exhibits large reversible capacities and long-duration cyclic stability at high rates when evaluated as competitive anodes for PIBs. Insights into the intrinsic K+ -storage processes of the Bi@N-CNCs anode are put forward from comprehensive in situ characterizations.

Recent Progress in "Water-in-Salt" Electrolytes Toward Non-lithium Based Rechargeable Batteries.

Aqueous non-lithium based rechargeable batteries are emerging as promising energy storage devices thanks to their attractive rate capacities, long-cycle life, high safety, low cost, environmental-friendliness, and easy assembly conditions. However, the aqueous electrolytes with high ionic conductivity are always restricted by their intrinsically narrow electrochemical window. Encouragingly, the highly concentrated "water-in-salt" (WIS) electrolytes can efficiently expand the stable operation window, which brings up a series of aqueous high-voltage rechargeable batteries. In the mini review, we summarize the latest progress and contributions of various aqueous electrolytes for non-lithium (Na+, K+, Zn2+, Mg2+, and Al3+) based rechargeable batteries, and give a brief exploration of the operating mechanisms of WIS electrolytes in expanding electrochemically stable windows. Challenges and prospects are also proposed for WIS electrolytes toward aqueous non-lithium rechargeable metal ion batteries.

Facile Solvothermal Synthesis of Hollow BiOBr Submicrospheres with Enhanced Visible-Light-Responsive Photocatalytic Performance.

In this work, hierarchical hollow BiOBr submicrospheres (HBSMs) were successfully prepared via a facile yet efficient solvothermal strategy. Remarkable effects of solvents upon the crystallinities, morphologies, and microstructures of the BiOBr products were systematically investigated, which revealed that the glycerol/isopropanol volumetric ratio played a significant role in the formation of hollow architecture. Accordingly, the underlying formation mechanism of the hollow submicrospheres was tentatively put forward here. Furthermore, the photocatalytic activities of the resulting HBSMs were evaluated in detail with photocatalytic degradation of the organic methyl orange under visible light irradiation. Encouragingly, the as-obtained HBSMs with striking recyclability demonstrated excellent visible-light-responsive photocatalytic performance, which benefits from their large surface area, effective visible light absorption, and unique hollow feature, highlighting their promising commercial application in waste water treatment.

Construction of a multi-dimensional flexible MnS based paper electrode with ultra-stable and high-rate capability towards efficient sodium storage.

Recently, there has been an urgent need for flexible and low cost rechargeable batteries for the emerging flexible and wearable electronic devices. Herein, MnS nanoparticles embedded in carbon nanowires/reduced graphene oxide (MnS@CNWs/rGO) composite paper were synthesized via a simple yet scalable strategy with Mn based coordination nanowires and graphene oxide as precursors. The combination of multi-dimensional subunits offers not only a robust structure but also abundant pathways for fast electron/ion diffusion. When directly used as a free-standing electrode for sodium ion batteries (SIBs), the ultra-flexible paper anode exhibits excellent mechanical and electrochemical performance, benefitting from the synergistic effects between nano-dimensional MnS encapsulated in CNWs and conductive rGO nanosheets. Remarkably, a high reversible gravimetric/volumetric capacity of ∼560 mA h g-1/∼362.3 mA h cm-3 is obtained using the self-supported flexible electrode at a current density of 0.1 A g-1, which is almost 92.4% of the theoretical capacity of MnS. More competitively, the flexible MnS@CNWs/rGO anode exhibits an unprecedented long cycle life with a high reversible capacity of ∼150 mA h g-1 at 1 A g-1 after 10, 000 cycles. This highly favours the promising application of MnS@CNWs/rGO paper in advanced flexible SIBs as an appealing anode.

Understanding the crystal structure-dependent electrochemical capacitance of spinel and rock-salt Ni-Co oxides via density function theory calculations.

The spinel NiCo2O4 and rock-salt NiCoO2 have been well established as attractive electrodes for supercapacitors. However, what is the intrinsic role of the congenital aspect, i.e., crystal structure and the surface and/or near-surface controlled electrochemical redox behaviors, if the acquired features (i.e., morphology, specific surface area, pore structure, and so on) are wholly ignored? Herein, we purposefully elucidated the underlying influences of unique crystal structures of NiCo2O4 and NiCoO2 on their pseudocapacitance from mechanism analysis through the density function theory based first-principles calculations, along with the experimental validation. Systematic theoretical calculation and analysis revealed that more charge carriers near the Fermi-level, stronger affinity with OH- in the electrolyte, easier deprotonation process, and the site-enriched characteristic for low-index surfaces of NiCoO2 enable its faster redox reaction kinetics and greater charge transfer, when compared to the spinel NiCo2O4. The in-depth understanding of crystal structure-property relationship here will guide rational optimization and selection of appropriate electrodes for advanced supercapacitors.

Construction of Hierarchical Nanotubes Assembled from Ultrathin V3 S4 @C Nanosheets towards Alkali-Ion Batteries with Ion-Dependent Electrochemical Mechanisms.

Ultrathin core-shell V3 S4 @C nanosheets assembled into hierarchical nanotubes (V3 S4 @C NS-HNTs) are synthesized by a self-template strategy and evaluated as general anodes for alkali-ion batteries. Structural/physicochemical characterizations and DFT calculations bring insights into the intrinsic relationship between crystal structures and electrochemical mechanisms of the V3 S4 @C NS-HNTs electrode. The V3 S4 @C NS-HNTs are endowed with strong structural rigidness owing to the layered VS2 subunits and interlayer occupied V atoms, and efficient alkali-ion adsorption/diffusion thanks to the electroactive V3 S4 -C interfaces. The resulting V3 S4 @C NS-HNTs anode exhibit distinct alkali-ion-dependent charge storage mechanisms and exceptional long-durability cyclic performance in storage of K+ , benefiting from synergistic contributions of pseudocapacitive and reversible intercalation/de-intercalation behaviors superior to those of the conversion-reaction-based Li+ -/Na+ -storage counterparts.

Scalable Synthesis of One-Dimensional Mesoporous ZnMnO3 Nanorods with Ultra-Stable and High Rate Capability for Efficient Lithium Storage.

The cost-efficient ZnMnO3 has attracted increasing attention as a prospective anode candidate for advanced lithium-ion batteries (LIBs) owing to its resourceful abundance, large lithium storage capacity and low operating voltage. However, its practical application is still seriously limited by the modest cycling and rate performances. Herein, a facile design to scalable synthesize unique one-dimensional (1D) mesoporous ZnMnO3 nanorods (ZMO-NRs) composed of nanoscale particles (≈11 nm) is reported. The 1D mesoporous structure and nanoscale building blocks of the ZMO-NRs effectively promote the transport of ions/electrons, accommodate severe volume changes, and expose more active sites for lithium storage. Benefiting from these appealing structural merits, the obtained ZMO-NRs anode exhibits excellent rate behavior (≈454 mAh g-1 at 2 A g-1 ) and ultra-long term cyclic performance (≈949.7 mAh g-1 even over 500 cycles at 0.5 A g-1 ) for efficient lithium storage. Additionally, the LiNi0.8 Co0.1 Mn0.1 O2 //ZMO-NRs full cell presents a practical energy density (≈192.2 Wh kg-1 ) and impressive cyclability with approximately 91 % capacity retention over 110 cycles. This highlights that the ZMO-NRs product is a highly promising high-rate and stable electrode candidate towards advanced LIBs in electronic devices and sustainable energy storage applications.