14-Electron Redox Chemistry Enabled by Salen-Based π-Conjugated Framework Polymer Boosting High-Performance Lithium-Ion Storage

A paucity of redox centers, poor charge transport properties, and low structural stability of organic materials obstruct their use in practical applications. Herein, these issues have been addressed through the use of a redox-active salen-based framework polymer (RSFP) containing multiple redox-active centers in π-conjugated configuration for applications in lithium-ion batteries (LIBs). Based on its unique architecture, RSFP exhibits a superior reversible capacity of 671.8 mAh g-1 at 0.05 A g-1 after 168 charge-discharge cycles. Importantly, the lithiation/de-lithiation performance is enhanced during operation, leading to an unprecedented reversible capacity of 946.2 mAh g-1 after 3500 cycles at 2 A g-1. The structural evolution of RSFP is studied ex situ using X-ray photoelectron spectroscopy, revealing multiple active C═N, C─O, and C═O sites and aromatic sites such as benzene rings. Remarkably, the emergence of C═O originated from C─O is triggered by an electrochemical process, which is beneficial for improving reversible lithiation/delithiation behavior. Furthermore, the respective strong and weak binding interactions between redox centers and lithium ions, corresponding to theoretical capacities of 670.1 and 938.2 mAh g-1, have been identified by density functional theory calculations manifesting 14-electron redox reactions. This work sheds new light on routes for the development of redox-active organic materials for energy storage applications.

Mixed-Dimensional (2D/3D/3D) Heterostructured Vanadium Oxide with Rich Oxygen Vacancies for Aqueous Zinc Ion Batteries with High Capacity and Long Cycling Life

Heterostructure engineering and oxygen vacancy engineering are the most promising modification strategies to reinforce the Zn2+ ion storage of vanadium oxides. Herein, a rare mixed-dimensional material (VOx), composed of V2O5 (2D), V3O7 (3D), and V6O13 (3D) heterostructures, rich in oxygen vacancies, was synthesized via thermal decomposition of layered ammonium vanadate. The VOx cathode provides an exceptional discharge capacity (411 mA h g-1 at 0.1 A g-1) and superior cycling stability (the capacity retention remains close to 100% after 800 cycles at 2 A g-1) for aqueous zinc-ion batteries (AZIBs). Ex situ characterizations confirm that the byproduct Zn3V2O7(OH)2·nH2O is generated/decomposed during discharge/charge processes. Furthermore, VOx demonstrates reversible intercalation/deintercalation of H+/Zn2+ ions, enabling efficient energy storage. Remarkably, a reversible crystal-to-amorphous transformation in the V2O5 phase of VOx during charge-discharge was observed. This investigation reveals that mixed-dimensional heterostructured vanadium oxide, with abundant oxygen vacancies, serves as a highly promising electrode material for AZIBs, further advancing the comprehension of the storage mechanism within vanadium-based cathode materials.

Experimental Study and Mechanism Analysis of Paraffin/Sisal Composite Phase Change Energy Storage Fiber Prepared by Vacuum Adsorption Method

Sisal fiber exhibits a fibrous and porous structure with significant surface roughness, making it highly suitable for storing phase change materials (PCMs). Its intricate morphology further aids in mitigating the risk of PCM leakage. This research successfully employs vacuum adsorption to encapsulate paraffin within sisal fiber, yielding a potentially cost-effective, durable, and environmentally friendly phase change energy storage medium. A systematic investigation was carried out to evaluate the effects of sisal-to-paraffin mass ratio, fiber length, vacuum level, and negative pressure duration on the loading rate of paraffin. The experimental results demonstrate that a paraffin loading rate of 8 wt% can be achieved by subjecting a 3 mm sisal fiber to vacuum adsorption with 16 wt% paraffin for 1 h at -0.1 MPa. Through the utilization of nano-CT imaging enhancement technology, along with petrographic microscopy, this study elucidates the mechanism underlying paraffin storage within sisal fiber during vacuum adsorption. The observations reveal that a substantial portion of paraffin is primarily stored within the pores of the fiber, while a smaller quantity is firmly adsorbed onto its surface, thus yielding a durable phase change energy storage medium. The research findings contribute to both the theoretical foundations and the available practical guidance for the fabrication and implementation of paraffin/sisal fiber composite phase change energy storage mediums.

Stable Hexaazatrinaphthalene-Based Planar Polymer Cathode Material for Organic Lithium-Ion Batteries

Organic materials have garnered intensive focus as a new group of electrodes for lithium-ion batteries (LIBs). However, many reported organic electrodes so far still exhibit unsatisfying cycling stability because of the dissolution in the electrolytes. Herein, a novel azo-linked hexaazatrianphthalene (HATN)-based polymer (AZO-HATN-AQ) is designed and fabricated by the polymerization of trinitrodiquinoxalino[2,3-a:2',3'-c]phenazine (HATNTN) and 2,6-diaminoanthraquinone (DAAQ). The abundant redox-active sites, extended π-conjugated planar conformation, and low energy gap endow the AZO-HATN-AQ electrode with high theoretical capacity, excellent solubility resistance, and fast Li-ion transport. In particular, the fully lithiated AZO-HATN-AQ still keeps the planar structure, contributing to the excellent cycling stability. As a result, AZO-HATN-AQ cathodes show high specific capacity (240 mAh g-1 at 0.05 A g-1), prominent rate capability (98 mAh g-1 at 8 A g-1), and outstanding cycling stability (120 mAh g-1 after 2000 cycles at 4 A g-1 with 85.7% capacity retention) simultaneously. This study demonstrates that rational structure design of the polymer electrodes is an effective approach to achieving excellent comprehensive electrochemical performance.

Unveiling the Nature of Ultrastable Potassium Storage in Bi0.48Sb1.52Se3 Composite

The conversion and alloying-type anodes for potassium-ion batteries (PIBs) have drawn attention. However, it is still a challenge to relieve the huge volume expansion/electrode pulverization. Herein, we synthesized a composite material comprising Bi_0.48Sb_1.52Se₃ nanoparticles uniformly dispersed in carbon nanofibers (Bi_0.48Sb_1.52Se₃@C). Benefiting from the synergistic effects of the high electronic conductivity of Bi_0.48Sb_1.52Se₃ and the mechanical confinement of the carbon fiber that buffers the large chemomechanical stress, the Bi_0.48Sb_1.52Se₃@C//K half cells deliver a high reversible capacity (491.4 mAh g^-1, 100 cycles at 100 mA g^-1) and an extraordinary cyclability (80% capacity retention, 1000 cycles at 1000 mA g^-1). Furthermore, the Bi_0.48Sb_1.52Se₃@C-based PIB full cells achieve a high energy density of 230 Wh kg^-1. In situ transmission electron microscopy (TEM) reveals an intercalation, conversion, and alloying three-step reaction mechanism and a reversible amorphous transient phase. More impressively, the nanofiber electrode can almost return to its original diameter after the potassiation and depotassiation reaction, indicating a highly reversible volume change process, which is distinct from the other conversion type electrodes. This work reveals the stable potassium storage mechanisms of Bi_0.48Sb_1.52Se₃@C composite material, which provides an effective strategy to enable conversion/alloying-type anodes for high performance PIBs for energy storage applications.

High-Rate-Capacity Cathode Based on Zn-Doped and Carbonized Polyacrylonitrile-Coated Na4MnV(PO4)3 for Sodium-Ion Batteries

Na₄MnV(PO₄)₃ (NMVP) is a promising cathode material for sodium-ion batteries (SIBs) because of its extraordinary three-dimensional structure that provides plenty of channels for sodium-ion migration. However, the unsatisfied electrical conductivity of NMVP limits its utilization in SIBs. Herein, Zn-doped NMVP with a uniform carbonized polyacrylonitrile (PAN) coating layer, named NMZVP@cPAN, was synthesized via a sol-gel method, and carbonized PAN was uniformly distributed on the surface of NMVP. Therefore, the NMZVP@cPAN cathodes exhibited an outstanding discharge capacity of 70.6 mA·h·g^-1 at 30 C and remarkable cycling stability with an admirable retention of 89.64% after 1000 cycles at 5 C. Rietveld refinement and ex situ X-ray diffraction analyses were performed to determine the change in the crystal structure. Density functional theory calculations were performed to determine the effects of Zn doping on the density of states and the migration energy barriers. Finally, the NMZVP@cPAN cathodes were successfully modified and could be used in SIBs as NMVP cathodes.

Prussian Blue Analogues with Optimized Crystal Plane Orientation and Low Crystal Defects toward 450 Wh kg-1 Sodium-Ion Batteries

Prussian blue analogues (PBAs) have been regarded as promising cathode materials for sodium-ion batteries (SIBs) owing to their high theoretical energy density and low-cost. However, high water and vacancies content of PBAs lower their energy density and bring safety issues, impeding their large-scale application in SIBs. Herein, a facile "potassium-ions assisted" strategy is proposed to synthesize highly crystallized PBAs. By manipulating the dominant crystal plane and suppressing vacancies, the as-prepared PBAs exhibits increased redox potential resulting in high energy density up to ~450 Wh kg-1, which is at the same level of the well-known LiFePO4 cathodes for lithium-ion batteries. Remarkably, unconventional highly-reversible phase evolution and redox-active pairs were identified by multiple in-situ techniques for the first time. The preferred guest-ion storage sites and migration mechanism were systematically analysed through theoretical calculations. We believe these results could inspire the designing of safe SIBs with high energy density.

Boosting the Initial Coulomb Efficiency of Sisal Fiber-Derived Carbon Anode for Sodium Ion Batteries by Microstructure Controlling

As anode material for sodium ion batteries (SIBs), biomass-derived hard carbon has attracted a great deal of attention from researchers because of its renewable nature and low cost. However, its application is greatly limited due to its low initial Coulomb efficiency (ICE). In this work, we employed a simple two-step method to prepare three different structures of hard carbon materials from sisal fibers and explored the structural effects on the ICE. It was determined that the obtained carbon material, with hollow and tubular structure (TSFC), exhibits the best electrochemical performance, with a high ICE of 76.7%, possessing a large layer spacing, a moderate specific surface area, and a hierarchical porous structure. In order to better understand the sodium storage behavior in this special structural material, exhaustive testing was performed. Combining the experimental and theoretical results, an "adsorption-intercalation" model for the sodium storage mechanism of the TSFC is proposed.

Challenges and Perspectives of Organic Multivalent Metal-Ion Batteries

Rechargeable organic multivalent metal-ion batteries (MMIBs) have attracted a surge of interest as promising alternatives for large-scale energy storage applications because they can combine the advantages of both organic electrodes and multivalent metal-ion batteries. However, the development of organic MMIBs is hampered by many factors, which mean they lag far behind organic alkali-metal- (e.g., Li-, Na-, and K-) ion batteries. Herein, the challenges that are specifically faced by organic MMIBs are analyzed and the strategies that can probably solve such challenges are then discussed. As a special challenge that organic MMIBs are facing, the charge-storage mechanism is particularly underlined to deeply understand the structure-property relationships for guiding the future design of high-performance organic electrodes for MMIBs. The perspectives are thereby elaborated in this review with the outlook of practical applications of organic MMIBs.

Pseudocapacitance-Enhanced Storage Kinetics of 3D Anhydrous Iron (III) Fluoride as a Cathode for Li/Na-Ion Batteries

Transition metal fluoride (TMF) conversion cathodes, with high energy density, are recognized as promising candidates for next-generation high-energy Li/Na-ion batteries (LIBs/SIBs). Unfortunately, the poor electronic conductivity and detrimental active material dissolution of TMFs seriously limit the performance of TMF-LIBs/SIBs. A variety of FeF₃-based composites are designed to improve their electrochemical characteristics. However, the storage mechanism of the conversion-type cathode for Li⁺ and Na⁺ co-storage is still unclear. Here, the storage mechanism of honeycomb iron (III) fluoride and carbon (FeF₃@C) as a general cathode for LIBs/SIBs is analyzed by kinetics. In addition, the FeF₃@C cathode shows high electrochemical performance in a full-cell system. The results show that the honeycomb FeF₃@C shows excellent long-term cycle stability in LIBs (208.3 mA h g^-1 at 1.0 C after 100 cycles with a capacity retention of 98.1%). As a cathode of SIBs, the rate performance is unexpectedly stable. The kinetic analysis reveals that the FeF₃@C cathode exhibit distinct ion-dependent charge storage mechanisms and exceptional long-durability cyclic performance in the storage of Li⁺/Na⁺, benefiting from the synergistic contribution of pseudocapacitive and reversible redox behavior. The work deepens the understanding of the conversion-type cathode in Li⁺/Na⁺ storage.

Recent Advances in Carbon Anodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have gained tremendous attention for large-scale energy storage applications due to the natural abundance, low cost, and even geographic distribution of sodium resources as well as a similar working mechanism to lithium-ion batteries (LIBs). One of the critical bottlenecks, however, is the design of high-performance and low-cost anode materials. Graphite anode that has dominated the market share of LIBs does not properly intercalate sodium ions. However, other carbonaceous materials are still considered as one of the most promising anode materials for SIBs in virtue of their high electronic conductivity, abundant active sites, hierarchical porosity, and excellent mechanical stability. In this review, we have tried to summarize the latest progresses made on the development of carbon-based negative electrodes (including hard carbons, soft carbons, and synthetic carbon allotropes) for SIBs. We also have provided a comprehensive understanding of their physical properties, the sodium ions storage mechanisms, and the improvement measures to cope with the current challenges. In addition, we have proposed future research directions for SIBs that will provide important insights into further development of carbon-based materials for SIBs.

The emerging aqueous ammonium-ion batteries

Aqueous ammonium‐ion (NH₄⁺) batteries (AAIB) are a recently emerging technology that utilize the abundant electrode resources and the fast diffusion kinetics of NH₄⁺ to deliver an excellent rate performance at a low cost. Although significant progress has been made on AAIBs, the technology is still limited by various challenges. In this Minireview, the most recent advances are comprehensively summarized and discussed, including cathode and anode materials as well as the electrolytes. Finally, a perspective on possible solutions for the current limitations of AAIBs is provided.

Sulfur-Rich (NH4)2Mo3S13 as a Highly Reversible Anode for Sodium/Potassium-Ion Batteries

Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have attracted much attention owing to the inexpensive Na/K metal and satisfactory performance. Currently, there are still difficulties in research anode materials that can insert/extract Na/K ions quickly and stably. Herein, the sulfur-rich (NH₄)₂Mo₃S₁₃ is proposed as the anode for SIBs/PIBs and is obtained by a hydrothermal method. The sulfur-rich (NH₄)₂Mo₃S₁₃ with a three-dimensional structure shows a high capacity and long lifespans for Na⁺ (at 10 A g^-1 the capacity of 165.2 mAh g^-1 after 1100 cycles) and K⁺ (120.7 mAh g^-1 at 1 A g^-1 retained after 500 cycles) storage. In addition, the (NH₄)₂Mo₃S₁₃ electrode exhibits excellent electrochemical performance at low temperatures (0 °C). The mechanism of Na⁺ storage in (NH₄)₂Mo₃S₁₃ can be innovatively revealed through the combined use of electrochemical kinetic analysis and a series of ex situ characterization tests. It is believed that the present work identifies (NH₄)₂Mo₃S₁₃ as a promising anode for the SIBs/PIBs and will be of broad interest in research on engineering sulfur-rich transition metal sulfide and on energy storage devices.

Rational Design of a Polyimide Cathode for a Stable and High-Rate Potassium-Ion Battery

Potassium has similar chemical characteristics compared with lithium while it is more abundant and of low cost, resulting in widespread research attention on potassium-ion batteries (PIBs). Developing organic polymer cathodes has garnered extensive attention because of their merits of environmental friendship and structure diversity, while confronted with inferior cycle stability and low rate performance. In this paper, we utilize the low-cost graphite nanosheets to stabilize polyimide (PI@G) for PIBs. Additionally, the potassium storage mechanism of PI@G was further evaluated; the highly reversible chemical bonds (C═O) of PI@G are responsible to its long-term stability. Consequently, the PI@G exhibits a maximal capacity of 142 mA h g^-1 at the current density of 100 mA g^-1 and maintains a capacity of 118 mA h g^-1 after 500 cycles (corresponding to a capacity fade of 0.034% per cycle). Moreover, the full battery based on the PI@G cathode also reveals promisingly electrochemical performance. This study may have great significance to the application prospect of the organic cathode for PIBs.

Understanding the Behavior and Mechanism of Oxygen-Deficient Anatase TiO2 toward Sodium Storage

TiO₂ has drawn increasing research attention as negative electrode material in sodium ion battery because of its natural abundance, low cost, nontoxicity, and facile preparation. Despite tremendous studies carried out, the sodium storage mechanism is still under discussion, and the electronic and local structures of TiO₂ during sodiation/desodiation process are not well understood either. Herein, we reported a mechanism study of graphene-supported oxygen-deficient anatase TiO₂ nanotubes (nanowires) as the negative electrode material for sodium ion batteries. Different from the previous reports, the insertion/extraction of Na⁺ ions leads to almost no changes of titanium valence state but there is a charge redistribution of O 2p orbitals which alters the hybridization between O 2p and Ti 3d states, suggested by the combined electrochemical and X-ray spectroscopic study. Both the electronic and local structures of TiO₂ during the reversible sodiation/desodiation process are revealed from the Ti L-edge and O K-edge spectra. This detailed study would shed light on the material design and structural optimization of TiO₂ as energy storage material in different systems.

Polyanthraquinone-Triazine-A Promising Anode Material for High-Energy Lithium-Ion Batteries

A novel covalent organic framework polymer material that bears conjugated anthraquinone and triazine units in its skeleton has been prepared via a facile one-pot condensation reaction and employed as an anode material for Li-ion batteries. The conjugated units consist of C═N groups, C═O groups, and benzene groups, which enable a 17-electron redox reaction with Li per repeating unit and bring a theoretical specific capacity of 1450 mA h g^-1. The polymer also shows a large specific surface area and a hierarchically porous structure to trigger interfacial Li storage and contribute to an additional capacity. The highly conductive conjugated polymer skeleton enables fast electron transport to facilitate the Li storage. In this way, the polymer electrode shows a large specific capacity and favorable cycling and rate performance, making it an appealing anode choice for the next-generation high-energy batteries.

Advanced Nanostructured Anode Materials for Sodium-Ion Batteries

Sodium-ion batteries (NIBs), due to the advantages of low cost and relatively high safety, have attracted widespread attention all over the world, making them a promising candidate for large-scale energy storage systems. However, the inherent lower energy density to lithium-ion batteries is the issue that should be further investigated and optimized. Toward the grid-level energy storage applications, designing and discovering appropriate anode materials for NIBs are of great concern. Although many efforts on the improvements and innovations are achieved, several challenges still limit the current requirements of the large-scale application, including low energy/power densities, moderate cycle performance, and the low initial Coulombic efficiency. Advanced nanostructured strategies for anode materials can significantly improve ion or electron transport kinetic performance enhancing the electrochemical properties of battery systems. Herein, this Review intends to provide a comprehensive summary on the progress of nanostructured anode materials for NIBs, where representative examples and corresponding storage mechanisms are discussed. Meanwhile, the potential directions to obtain high-performance anode materials of NIBs are also proposed, which provide references for the further development of advanced anode materials for NIBs.

Electrochemical Performance and Storage Mechanism of Ag2 Mo2 O7 Micro-rods as the Anode Material for Lithium-Ion Batteries

Ag₂ Mo₂ O₇ micro-rods are prepared by one-step hydrothermal method and their lithium electrochemical properties, as the anode for lithium-ion batteries, are comprehensively studied in terms of galvanostatic charge-discharge cycling, cyclic voltammetry, and rate performance measurements. The electrode delivers a high reversible capacity of 825 mAh g^-1 at a current density of 100 mA g^-1 and a superior rate capability with a discharge capacity of 263 mAh g^-1 under the high current density of 2 Ag^-1 . The structural transition and phase evolution of Ag₂ Mo₂ O₇ were investigated by using ex situ XRD and TEM. The Ag₂ Mo₂ O₇ electrode is likely to be decomposed into amorphous molybdenum, Li₂ O, and metallic silver based on the conversion reaction. Silver nanoparticles are not involved in the subsequent electrochemical cycles to form a homogeneous conducting network. Such in situ decomposition behavior provides an insight into the mechanism of the electrochemical reaction for the anode materials and would contribute to the design of new electrode materials in future.