Wood-Derived High-Mass-Loading MnO2 Composite Carbon Electrode Enabling High Energy Density and High-Rate Supercapacitor

Construction of ion/electron transfer multi-channels for the composite film electrode from GO and cellulose derived porous carbon in supercapacitor

Due to excellent flexibility and dispersibility, 2D graphene oxide (GO) is regarded as one of the prospective materials for preparing self-supporting electrode material. Nevertheless, the self-stacking characteristic of GO significantly restricts the ion transmission and accessibility in GO-based electrodes, especially in the direction perpendicular to the electrode surface. Herein, a novel composite film was fabricated from GO and 3D porous carbon (PC) through vacuum filtration combined with thermal reduction strategy. The combination of GO and PC not only avoids the self-stacking of GO, but also exposes more active sites for ions in the inner. A massive released nitrogen and oxygen-containing gases during the thermal reduction endows the reduced graphene oxide (RGO) with abundant porous and CC, which contributes to the energy storage in the direction perpendicular to the electrode surface. Besides, the high specific surface area of the prepared composite film is favorable for the storage and release of charge on the electrode surface. Benefiting from the above characteristics, the electrode assembled by the as-prepared film exhibits ultrahigh areal/volumetric specific capacitance in supercapacitor and ZIHCs (Zinc ion hybrid capacitors). This work provides a promising approach for the development of advanced self-supported electrode materials with desirable electrochemical properties.

High-mass loaded redox-active lignin functionalized carbonized wood collector to construct sustainable and high-performance supercapacitors

Traditional electrode materials for supercapacitors often face issues like high toxicity, cost, and non-renewability. To address these drawbacks, biomass-based alternatives are being explored, aligning with green development trends. Herein, carbonized wood (CW) with rich pore structure and redox-active lignin are combined to fabricate an all-wood-based sustainable supercapacitor electrode material. Due to its inherent porous structure, CW provides a larger surface area for accommodating active materials ion, enabling the electrode to achieve a higher lignin loading capacity of 2.82-11.68 mg/cm2. Furthermore, the utilization of lignin as a substitute for conventional transition metal-based pseudocapacitor material functionalized CW endows the electrode with exemplary electrochemical performance while guaranteeing the comprehensive sustainability of the electrode. This synergy confers the electrode with exceptional electrical performance, yielding an areal capacitance of 960.7 mF/cm2 at a current density of 1 mA/cm2. The symmetric supercapacitors (SSC) manufactured by this composite electrode can achieve a notable areal energy density of 0.14 mWh/cm2 and a power density of 15.98 mW/cm2, while maintaining an outstanding capacitance retention rate of 81 % after 50,000 cycles at 20 mA/cm2. The manufacture of CW-lignin electrode underscores the potential of utilizing renewable biomass resources as alternatives for developing high-performance energy storage applications, thereby reducing negative environmental impacts.

High performance supercapacitors deploying cube-templated tracheid cavities of wood-derived carbon

Wood-derived carbon, with its strong tracheid array structure, is an ideal material for use as a self-supporting electrode in supercapacitors. By leveraging the inherent through pore structure and surface affinity found in wood tracheids, we successfully engineered a highly spatially efficient cube-templated porous carbon framework inside carbonized wood tracheid cavities through precise control over precursor crystallization temperatures. This innovative cubic channel architecture effectively maximizes up to (79 ± 1)% of the cavity volume in wood-derived carbon while demonstrating exceptional hydrophilicity and high conductivity properties, facilitating the development of supercapacitors with enhanced areal/volumetric capacitances (2.65F cm-2/53.0F cm-3 at 5.0 mA cm-2) as well as superior areal/volumetric energy densities (0.37 mWh cm-2/7.36 mWh cm-3 at 2.5 mW cm-2). The fabrication of these cube-templated channels with high cube filling content is not only simple and precisely controllable, but also environmentally friendly. The proposed method eliminates the conventional acid-base treatment process for pore formation, facilitating the rapid development and practical implementation of thick electrodes with superior performance in supercapacitors. Moreover, it offers a universal research approach for the commercialization of wood-derived thick electrodes.

Oxygen vacancies enhancing hierarchical NiCo2S4@MnO2 electrode for flexible asymmetric supercapacitors

The limited energy density of supercapacitors hampers their widespread application in electronic devices. Metal oxides, employed as electrode materials, suffer from low conductivity and stability, prompting extensive research in recent years to enhance their electrochemical properties. Among these efforts, the construction of core-shell heterostructures and the utilization of oxygen vacancy (VO) engineering have emerged as pivotal strategies for improving material stability and ion diffusion rates. Herein, core-shell composites comprising NiCo2S4 nanospheres and MnO2 nanosheets are grown in situ on carbon cloth (CC), forming nanoflower clusters while introducing VO defects through a chemical reduction method. Density functional theory (DFT) results proves that the existence of VO effectively enhances electronic and structural properties of MnO2, thereby enhancing capacitive properties. The electrochemical test results show that NiCo2S4@MnO2-V3 exhibits excellent 1376 F g-1 mass capacitance and 2.06 F cm-2 area capacitance at 1 A g-1. Moreover, NiCo2S4@MnO2-V3//activated carbon (AC) asymmetric supercapacitor (ASC) can achieve an energy density of 39.7 Wh kg-1 at a power density of 775 W kg-1, and maintains 15.5 Wh kg-1 even at 7749.77 W kg-1. Capacitance retention is 73.1 % after 10,000 cycles at 5 A g-1, and coulombic efficiency reaches 100 %, demonstrating satisfactory cycle stability. In addition, the device's excellent flexibility offers broad application prospects in wearable electronic applications.

Shielding Mn3+ Disproportionation with Graphitic Carbon-Interlayered Manganese Oxide Cathodes for Enhanced Aqueous Energy Storage System

Manganese dioxide (MnO2) materials have recently garnered attention as prospective high-capacity cathodes, owing to their theoretical two-electron redox reaction in charge storage processes. However, their practical application in aqueous energy storage systems faces a formidable challenge: the disproportionation of Mn3+ ions, leading to a significant reduction in their capacity. To address this limitation, the study presents a novel graphitic carbon interlayer-engineered manganese oxide (CI-MnOx) characterized by an open structure and abundant defects. This innovative material serves several essential functions for efficient aqueous energy storage. First, a graphitic carbon layer coats the MnOx molecular interlayer, effectively inhibiting Mn3+ disproportionation and substantially enhancing electrode conductivity. Second, the phase variation within MnOx generates numerous crystal defects, vacancies, and active sites, optimizing electron-transfer capability. Third, the flexible carbon layer acts as a buffer, mitigating the volume expansion of MnOx during extended cycling. The synergistic effects of these features result in the CI-MnOx exhibiting an impressive high capacity of 272 mAh g-1 (1224 F g-1) at 0.25 A g-1. Notably, the CI-MnOx demonstrates zero capacity loss after 90 000 cycles (≈3011 h), an uncommon longevity for manganese oxide materials. Spectral characterizations reveal reversible cation intercalation and conversion reactions with multielectron transfer in a LiCl electrolyte.

A comprehensive review on preparation and functional application of the wood aerogel with natural cellulose framework

As the traditional aerogel has defects such as poor mechanical properties, complicated preparation process, high energy consumption and non-renewable, wood aerogel as a new generation of aerogel shows unique advantages. With a natural cellulose framework, wood aerogel is a novel nano-porous material exhibiting exceptional properties such as light weight, high porosity, large specific surface area, and low thermal conductivity. Furthermore, its adaptability to further functionalization enables versatile applications across diverse fields. Driven by the imperative for sustainable development, wood aerogel as a renewable and eco-friendly material, has garnered significant attention from researchers. This review introduces preparation methods of wood aerogel based on the top-down strategy and analyzes the factors influencing their key properties intending to obtain wood aerogels with desirable properties. Avenues for realizing its functionality are also explored, and research progress across various domains are surveyed, including oil-water separation, conductivity and energy storage, as well as photothermal conversion. Finally, potential challenges associated with wood aerogel exploitation and utilization are addressed, alongside discussions on future prospects and research directions. The results emphasize the broad research value and future prospects of wood aerogels, which are poised to drive high-value utilization of wood and foster the development of green multifunctional aerogels.

Size/Shape-Controllable Carbonized Wood Electrodes Enabled by an MXene Shell with Spatial Confinement and a Traction Effect on the Wood Cell Wall for Shape-Customizable Energy Storage Devices

The shrinkage and collapse of wood cell walls during carbonization make it challenging to control the size and shape of carbonized wood (CW) through pre- or postprocessing (e.g., sawing, cutting, and milling). Herein, a shape-adaptive MXene shell (MS) is created on the surface of the wood cell walls. The MS limits the deformation of wood cell walls by spatial confinement and traction effects, which is supported by the inherent dimensional stability of the MS and the formation of new C-O-Ti covalent bonds between the wood cell wall and MS. Consequently, the volumetric shrinkage ratio of CW encapsulated by the MS (CW-MS) is significantly reduced from 54.8% for CW to 2.6% for CW-MS even at 800 °C. The harnessing of this collapse enables the production of CW-MS with prolonged stability and high electric conductivity (384 S m-1). These properties make CW-MS suitable for energy storage devices with various designed shapes, matching the increasingly compact and complex structures of electronic devices.

Advancements in Manganese-Based Cathode for Sustainable Energy Utilization

Manganese-based compounds, especially manganese oxides, are one of the most exceptional electrode materials. Specifically, manganese oxides have gained significant interest owing to their unique crystal structures, high theoretical capacity, abundant natural availability and eco-friendly nature. However, as transition metal semiconductors, manganese oxide possess low electrical conductivity, limited rate capacity, and suboptical cycle stability. Thus, combining manganese oxides with carbon or other metallic materials can significantly improve their electrochemical performance. These composites increase active sites and conductivity, thereby improving electrode reaction kinetics, cycle stability, and lifespan of supercapacitors (SCs) and batteries. This paper reviews the latest applications of Mn-based cathodes in SCs and advanced batteries. Moreover, the energy storage mechanisms were also proposed. In this review, the development prospects and challenges for advanced energy storage applications of Mn-based cathodes are summarized.

In Situ Self-Assembly of Nitrogen-Doped 3D Flower-like Hierarchical Porous Carbon and Its Application for Supercapacitors

The hierarchical porous carbon-based materials derived from biomass are beneficial for the enhancement of electrochemical performances in supercapacitors. Herein, we report the fabrication of nitrogen-doped 3D flower-like hierarchical porous carbon (NPC) assembled by nanosheets using a mixture of urea, ZnCl2, and starch via a low-temperature hydrothermal reaction and high-temperature carbonization process. As a consequence, the optimized mass ratio for the mixture is 2:2:2 and the temperature is 700 °C. The NPC structures are capable of electron transport and ion diffusion owing to their high specific surface area (1498.4 m2 g-1) and rich heteroatoms. Thereby, the resultant NPC electrodes display excellent capacitive performance, with a high specific capacitance of 249.7 F g-1 at 1.0 A g-1 and good cycling stability. Remarkably, this implies a superior energy density of 42.98 Wh kg-1 with a power density of 7500 W kg-1 in organic electrolyte for the symmetrical supercapacitor. This result verifies the good performance of as-synthesized carbon materials in capacitive energy storage applications, which is inseparable from the hierarchical porous features of the materials.

Gas-Phase-Induced Engineering for Fabrication of 3D Hierarchical Porous Nickel and Its Application toward High-Performance Supercapacitors

Commercial nickel foam (NF), which is composed of numerous interconnected ligaments and hundred-micron pores, is widely acknowledged as a current collector/electrode material for catalysis, sensing, and energy storage applications. However, the commonly used NF often does not work satisfactorily due to its smooth surface and hollow structure of the ligaments. Herein, a gas-phase-induced engineering, two-step gaseous oxidation-reduction (GOR) is presented to directly transform the thin-walled hollow ligament of NF into a three-dimensional (3D) nanoporous prism structure, resulting in the fabrication of a unique hierarchical porous nickel foam (HPNF). This 3D nanoporous architecture is achieved by utilizing the spontaneous reconstruction of nickel atoms during volume expansion and contraction in the GOR process. The process avoids the involution of acid-base corrosion and sacrificial components, which are facile, environmentally friendly, and suitable for large-scale fabrication. Furthermore, MnO2 is electrochemically deposited on the HPNF to form a supercapacitor electrode (HPNF/MnO2). Because of the fully open structure for ion transport, superhydrophilic properties, and the increased contact area between MnO2 and the current collector, the HPNF/MnO2 electrode exhibits a high specific capacitance of 997.5 F g-1 at 3 A g-1 and remarkable cycling stability with 99.6% capacitance retention after 20000 cycles in 0.1 M Na2SO4 electrolyte, outperforming most MnO2-based supercapacitor electrodes.

Intercalation chemistry engineering strategy enabled high mass loading and ultrastable electrodes for High-Performance aqueous electrochemical energy storage devices

Aqueous electrochemical energy storage devices (AEESDs) are considered one of the most promising candidates for large-scale energy storage infrastructure due to their high affordability and safety. Developing electrodes with the merits of high energy density and long lifespan remains a challenging issue toward the practical application of AEESDs. Research attempts at electrode materials, nanostructure configuration, and electronic engineering show the limitations due to the inherent contradictions associated with thicker electrodes and ion-accessible kinetics. Herein, we propose an intercalation chemistry engineering strategy to enhance the electrolyte ion (de)intercalation behaviors during the electrochemical charge-discharge. To validate this strategy, the prototypical model of a high-mass-loading MnO2-based electrode is used with controlled intercalation of Na+ and H2O. Theoretical and experimental results reveal that an optimal content of Na+ and H2O on the MnO2-based electrode exhibits superior electrochemical performance. Typically, the resultant electrode exhibits an impressive areal capacitance of 1551 mF/cm2 with a mass loading of 9.7 mg/cm2 (at 1 mA/cm2). Furthermore, the assembled full-cell with obtained MnO2-based electrode delivers a high energy density of 0.12 mWh/cm2 (at 20.02 mW/cm2) and ultra-high cycling stability with a capacitance retention percentage of 89.63 % (345 mF/cm2) even after 100,000 cycles (tested over 72 days).

Hydrothermal regulation of MnO2 on a wood-based RGO composite for achieving wide voltage windows and high energy density supercapacitors

The increasing need for improved energy storage devices renders it particularly important that inexpensive electrodes with high capacitance, excellent cycling stability, and environment-friendly characteristics are developed. In this study, a wood-derived carbon@reduced graphene (WRG) conductive precursor with an average conductivity of 15.38 S/m was firstly synthesized. The binder-free WRG-MnO2 electrode was successfully constructed by growing MnO2 onto a WRG under hydrothermal conditions. The asymmetric supercapacitor assembled with the WRG-20MnO2 cathode exhibited excellent electrochemical capacitive behavior with a voltage window of 0-2 V, maximum energy density of 52.3 Wh kg-1, and maximum power density of 1642.7 W kg-1, which is mainly due to the distinctive icicle-shaped structure of the MnO2. Thus, a facile strategy for developing high-performance hierarchical porous carbon electrodes that can be used in supercapacitors was developed herein, which may provide new opportunities to improve the high added value of poplar wood.

MnO2 porous carbon composite from cellulose enabling high gravimetric/volumetric performance for supercapacitor

Preparing electrode material integrated with high gravimetric/volumetric capacitance and fast electron/ion transfer is crucial for the practical application. Owing to the structural contradiction, it is a big challenge to construct electrode material with high packing density, sufficient ion transport channels, and fast electronic transfer pathways. Herein, MnO2 porous carbon composite with abundant porous structure and 3D carbon skeleton was facilely fabricated from Linum usitatissimum. L stems via NaOH activation and MnO2 introduction. The in-situ introduced MnO2 not only increases the packing density and the electrical conductivity of the porous carbon but also provides more active sites for oxidation reactions. These unique characteristics endow the resultant MnO2 porous carbon composite with remarkable gravimetric capacitance of 549 F g-1, volumetric capacitance of 378 F cm-3, and capacitance retention of 54.9 %. Giving the simple process and low cost, this work might offer a new approach for structural design and the practical application of high-performance electrode materials.

Chitosan modified graphene oxide with MnO2 deposition for high energy density flexible supercapacitors

To obtain a flexible composite electrode material with excellent electrochemical performance, chitosan (CS)/graphene oxide (GO) composite pretreated from microwave hydrothermal is adopted as the carbon substrate, and MnO2 active material is uniformly deposited on their surface through anodic electrodeposition. In this composite system, CS penetrates into graphene sheets as small molecule units, forming NH-C=O groups with GO via dehydration condensation, which effectively inhibits the stacking of GO and improves the specific surface area, conductivity, as well as the wettability of the carbon support. MnO2 bonding with heteroatom N from CS enables high active material loadings and forms stable three-dimensional network structure, facilitating the enhanced electrochemical performance. Results indicate that increasing depositing MnO2 amount leads to more defective structures of the composite, which promotes their electrochemical performance when used as electrode material. The area specific capacitance of the optimal composite reaches 3553.74 mF/cm2 at 5 mA/cm2 in 1 M Na2SO4 electrolyte. Kinetic analysis shows the energy storage process is capacitance-dominated, with the redox reactions of MnO2 being the main contributor. The prepared asymmetric solid supercapacitor delivers an energy density high up to 0.585 mWh/cm2 at power density of 3000 mW/cm2, and their excellent flexibility makes them promising candidates as flexible sensor.

A high mass loading flexible electrode with a sheet-like Mn3O4/NiMoO4@NiCo LDH on a carbon cloth for supercapacitors

Mass loading is an important parameter to evaluate the application potential of active materials in high-capacity supercapacitors. Synthesizing active materials with high mass loading is a promising strategy to improve high performance energy storage devices. Preparing electrode materials with a porous structure is of significance to overcome the disadvantages brought by high mass loading. In this work, a Mn3O4/NiMoO4@NiCo layered double hydroxide (MO/NMO/NiCo LDH) positive electrode is fabricated on a carbon cloth with a high mass loading of 20.4 mg cm-2. The MO/NMO/NiCo LDH presents as a special three-dimensional porous nanostructure and exhibits a high specific capacitance of 815 F g-1 at 1 A g-1. Impressively, the flexible supercapacitor based on the MO/NMO/NiCo LDH positive electrode and an AC negative electrode delivers a maximum energy density of 22.5 W h kg-1 and a power density of 8730 W kg-1. It also retains 60.84% of the original specific capacitance after bending to 180° 600 times. Moreover, it exhibits 76.92% capacitance retention after 15 000 charge/discharge cycles. These results make MO/NMO/NiCo LDH one of the most attractive candidates of positive electrode materials for high-performance flexible supercapacitors.

Insulative Biobased Glaze from Wood Laminates Obtained by Self-Adhesion

The combination of optical transparency and mechanical strength is a highly desirable attribute of wood-based glazing materials. However, such properties are typically obtained by impregnation of the highly anisotropic wood with index-matching fossil-based polymers. In addition, the presence of hydrophilic cellulose leads to a limited water resistance. Herein, this work reports on an adhesive-free lamination that uses oxidation and densification to produce transparent all-biobased glazes. The latter are produced from multilayered structures, free of adhesives or filling polymers, simultaneously displaying high optical clarity and mechanical strength, in both dry and wet conditions. Specifically, high values of optical transmittance (≈85.4%), clarity (≈20% with low haze) at a thickness of ≈0.3 mm, and highly isotropic mechanical strength and water resistance (wet strength of ≈128.25 MPa) are obtained for insulative glazes exhibiting low thermal conductivity (0.27 W m-1 K-1 , almost four times lower than glass). The proposed strategy results in materials that are systematically tested, with the leading effects of self-adhesion induced by oxidation rationalized by ab initio molecular dynamics simulation. Overall, this work demonstrates wood-derived materials as promising solutions for energy-efficient and sustainable glazing applications.

A 3D stacked corrugated pore structure composed of CoNiO2 and polyaniline within the tracheids of wood-derived carbon for high-performance asymmetric supercapacitors

A novel 3D stacked corrugated pore structure of polyaniline (PANI)/CoNiO₂@activated wood-derived carbon (AWC) has been successfully constructed to prepare high-performance electrode materials for supercapacitors. AWC functions as a supporting framework that provides ample attachment sites for the loaded active materials. The CoNiO₂ nanowire substrate, consisting of 3D stacked pores, not only serves as a template for subsequent PANI loading, but also acts as an effective buffer to mitigate the volume expansion of the PANI during ionic intercalation. The distinctive corrugated pore structure of PANI/CoNiO₂@AWC facilitates electrolyte contact and significantly enhances the electrode material properties. The PANI/CoNiO₂@AWC composite materials exhibit excellent performance (14.31F cm^-2 at 5 mA cm^-2) and superior capacitance retention (80% from 5 to 30 mA cm^-2), owing to the synergistic effect among their components. Finally, PANI/CoNiO₂@ AWC//reduced graphene oxide (rGO)@AWC asymmetric supercapacitor is assembled, which has a wide operating voltage (0 ∼ 1.8 V), high energy density (4.95 mWh cm^-3 at 26.44 mW cm^-3) and cycling stability (90.96% after 7000 cycles).

Co-Pyrolysis for Pine Sawdust with Potassium Chloride: Insight into Interactions and Assisting Biochar Graphitization

This effort aimed to explore the activation and catalytic graphitization mechanisms of non-toxic salts in converting biomass to biochar from the perspective of pyrolysis kinetics using renewable biomass as feedstock. Consequently, thermogravimetric analysis (TGA) was used to monitor the thermal behaviors of the pine sawdust (PS) and PS/KCl blends. The model-free integration methods and master plots were used to obtain the activation energy (E) values and reaction models, respectively. Further, the pre-exponential factor (A), enthalpy (ΔH), Gibbs free energy (ΔG), entropy (ΔS), and graphitization were evaluated. When the KCl content was above 50%, the presence of KCl decreased the resistance to biochar deposition. In addition, the differences in the dominant reaction mechanisms of the samples were not significant at low (α ≤ 0.5) and high (α ≥ 0.5) conversion rates. Interestingly, the lnA value showed a linearly positive correlation with the E values. The PS and PS/KCl blends possessed positive ΔG and ΔH values, and KCl was able to assist biochar graphitization. Encouragingly, the co-pyrolysis of the PS/KCl blends allows us to target-tune the yield of the three-phase product during biomass pyrolysis.

Inspired by Wood: Thick Electrodes for Supercapacitors

The emergence and development of thick electrodes provide an efficient way for the high-energy-density supercapacitor design. Wood is a kind of biomass material with porous hierarchical structure, which has the characteristics of a straight channel, uniform pore structure, good mechanical strength, and easy processing. The wood-inspired low-tortuosity and vertically aligned channel architecture are highly suitable for the construction of thick electrochemical supcapacitor electrodes with high energy densities. This review summarizes the design concepts and processing parameters of thick electrode supercapacitors inspired by natural woods, including wood-based pore structural design regulation, electric double layer capacitances (EDLCs)/pseudocapacitance construction, and electrical conductivity optimization. In addition, the optimization strategies for preparing thick electrodes with wood-like structures (e.g., 3D printing, freeze-drying, and aligned-low tortuosity channels) are also discussed in detail. Further, this review presents current challenges and future trends in the design of thick electrodes for supercapacitors with wood-inspired pore structures. As a guideline, the brilliant blueprint optimization will promote sustainable development of wood-inspired structure design for thick electrodes and broaden the application scopes.

In Situ Grown Mesoporous Structure of Fe-Dopant@NiCoOX@NF Nanoneedles as an Efficient Supercapacitor Electrode Material

In this study, we designed mixed metal oxides with doping compound nano-constructions as efficient electrode materials for supercapacitors (SCs). We successfully prepared the Fe-dopant with NiCoOx grown on nickel foam (Fe-dopant@NiCoOx@NF) through a simple hydrothermal route with annealing procedures. This method provides an easy route for the preparation of high activity SCs for energy storage. Obtained results revealed that the Fe dopant has successfully assisted NiCoOx lattices. The electrochemical properties were investigated in a three-electrode configuration. As a composite electrode for SC characteristics, the Fe-dopant@NiCoOx@NF exhibits notable electrochemical performances with very high specific capacitances of 1965 F g−1 at the current density of 0.5 A g−1, and even higher at 1296 F g−1 and 30 A g−1, respectively, which indicate eminent and greater potential for SCs. Moreover, the Fe-dopant@NiCoOx@NF nanoneedle composite obtains outstanding cycling performances of 95.9% retention over 4500 long cycles. The improved SC activities of Fe-dopant@NiCoOx@NF nanoneedles might be ascribed to the synergistic reactions of the ternary mixed metals, Fe-dopant, and the ordered nanosheets grown on NF. Thus, the Fe-dopant@NiCoOx@NF nanoneedle composite with unique properties could lead to promising SC performance.

Thermal-Stable Separators: Design Principles and Strategies Towards Safe Lithium-Ion Battery Operations

Lithium-ion batteries (LIBs) are momentous energy storage devices, which have been rapidly developed due to their high energy density, long lifetime, and low self-discharge rate. However, the frequent occurrence of fire accidents in laptops, electric vehicles, and mobile phones caused by thermal runaway of the inside batteries constantly reminds us of the urgency in pursuing high-safety LIBs with high performance. To this end, this Review surveyed the state-of-the-art developments of high-temperature-resistant separators for highly safe LIBs with excellent electrochemical performance. Firstly, the basic properties of separators (e. g., thickness, porosity, pore size, wettability, mechanical strength, and thermal stability) in constructing commercialized LIBs were introduced. Secondly, the working mechanisms of advanced separators with different melting points acting in the thermal runaway stage were discussed in terms of improving battery safety. Thirdly, rational design strategies for constructing high-temperature-resistant separators for LIBs with high safety were summarized and discussed, including graft modification, blend modification, and multilayer composite modification strategies. Finally, the current obstacles and future research directions in the field of high-temperature-resistant separators were highlighted. These design ideas are expected to be applied to other types of high-temperature-resistant energy storage systems working under extreme conditions.

Layer-by-Layer Heterostructure of MnO2@Reduced Graphene Oxide Composites as High-Performance Electrodes for Supercapacitors

In this paper, δ-MnO₂ with layered structure was prepared by a facile liquid phase method, and exfoliated MnO₂ nanosheet (e-MnO₂) was obtained by ultrasonic exfoliation, whose surface was negatively charged. Then, positive charges were grafted on the surface of MnO₂ nanosheets with a polycation electrolyte of polydiallyl dimethylammonium chloride (PDDA) in different concentrations. A series of e-MnO₂@reduced graphene oxide (rGO) composites were obtained by electrostatic self-assembly combined with hydrothermal chemical reduction. When PDDA was adjusted to 0.75 g/L, the thickness of e-MnO₂ was ~1.2 nm, and the nanosheets were uniformly adsorbed on the surface of graphene, which shows layer-by-layer morphology with a specific surface area of ~154 m²/g. On account of the unique heterostructure, the composite exhibits good electrochemical performance as supercapacitor electrodes. The specific capacitance of e-MnO₂-0.75@rGO can reach 456 F/g at a current density of 1 A/g in KOH electrolyte, which still remains 201 F/g at 10 A/g. In addition, the capacitance retention is 98.7% after 10000 charge-discharge cycles at 20 A/g. Furthermore, an asymmetric supercapacitor (ASC) device of e-MnO₂-0.75@rGO//graphene hydrogel (GH) was assembled, of which the specific capacitance achieves 94 F/g (1 A/g) and the cycle stability is excellent, with a retention rate of 99.3% over 10000 cycles (20 A/g).

Two-Dimensional Core-Shell Structure of Cobalt-Doped@MnO2 Nanosheets Grown on Nickel Foam as a Binder-Free Battery-Type Electrode for Supercapacitor Application

Herein, we present an interfacial engineering strategy to construct an efficient hydrothermal approach by in situ growing cobalt-doped@MnO₂ nanocomposite on highly conductive nickel foam (Ni foam) for supercapacitors (SCs). The remarkably high specific surface area of Co dopant provides a larger contacting area for MnO₂. In the meantime, the excellent retentions of the hierarchical phase-based pore architecture of the cobalt-doped surface could beneficially condense the electron transportation pathways. In addition, the nickel foam (Ni foam) nanosheets provide charge-transport channels that lead to the outstanding improved electrochemical activities of cobalt-doped@MnO₂. The unique cobalt-doped@MnO₂ nanocomposite electrode facilitates stable electrochemical architecture, multi-active electrochemical sites, and rapid electro-transports channels; which act as a key factor in enhancing the specific capacitances, stability, and rate capacities. As a result, the cobalt-doped@MnO₂ nanocomposite electrode delivered superior electrochemical activities with a specific capacitance of 337.8 F g^-1 at 0.5 A g^-1; this is greater than pristine MnO₂ (277.9 F g^-1). The results demonstrate a worthy approach for the designing of high-performance SCs by the grouping of the nanostructured dopant material and metal oxides.

Binder-Free MnO2/MWCNT/Al Electrodes for Supercapacitors

Recently, significant progress has been made in the performance of supercapacitors through the development of composite electrodes that combine various charge storage mechanisms. A new method for preparing composite binder-free MnO2/MWCNT/Al electrodes for supercapacitors is proposed. The method is based on the original technique of direct growth of layers of multi-walled carbon nanotubes (MWCNTs) on aluminum foil by the catalytic pyrolysis of ethanol vapor. Binder-free MnO2/MWCNT/Al electrodes for electrochemical supercapacitors were obtained by simply treating MWCNT/Al samples with an aqueous solution of KMnO4 under mild conditions. The optimal conditions for the preparation of MnO2/MWCNT/Al electrodes were found. The treatment of MWCNT/Al samples in a 1% KMnO4 aqueous solution for 40 min increased the specific capacitance of the active material of the samples by a factor of 3, up to 100-120 F/g. At the same time, excellent adhesion and electrical contact of the working material to the aluminum substrate were maintained. The properties of the MnO2/MWCNT/Al samples were studied by electron probe microanalysis (EPMA), Raman spectroscopy, cyclic voltammetry (CV), and impedance spectroscopy. Excellent charge/discharge characteristics of composite electrodes were demonstrated. The obtained MnO2/MWCNT/Al electrodes maintained excellent stability to multiple charge-discharge cycles. After 60,000 CVs, the capacitance loss was less than 20%. Thus, this work opens up new possibilities for using the MWCNT/Al material obtained by direct deposition of carbon nanotubes on aluminum foil for the fabrication of composite binder-free electrodes of supercapacitors.

Facile Electrodeposition of NiCo2O4 Nanosheets on Porous Carbonized Wood for Wood-Derived Asymmetric Supercapacitors

Multichannel-porous carbon derived from wood can serve as a conductive substrate for fast charge transfer and ion diffusion, supporting the high-theory capacitance of pseudocapacitive materials. Herein, NiCo₂O₄ nanosheets, which are hierarchically porous, anchored on the surface of carbonized wood via electrodeposition for free-binder high-performance supercapacitor electrode materials, were proposed. Benefiting from the effectively alleviated NiCo₂O₄ nanosheets accumulation and sufficient active surface area for redox reaction, a N-doped wood-derived porous carbon-NiCo₂O₄ nanosheet hybrid material (NCNS–NCW) electrode exhibited a specific electric capacity of 1730 F g⁻¹ at 1 A g⁻¹ in 1 mol L⁻¹ KOH and splendid electrochemical firmness with 80% capacitance retention after cycles. Furthermore, an all-wood-based asymmetric supercapacitor based on NCNS–NCW//NCW was assembled and a high energy density of 56.1 Wh kg⁻¹ at a watt density of 349 W kg⁻¹ was achieved. Due to the great electrochemical performance of NCNS–NCW, we expect it to be used as an electrode material with great promise for energy storage equipment.