The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Owing to high specific energy, low cost, and environmental friendliness, lithium–sulfur (Li–S) batteries hold great promise to meet the increasing demand for advanced energy storage beyond portable electronics, and to mitigate environmental problems. However, the application of Li–S batteries is challenged by several obstacles, including their short life and low sulfur utilization, which become more serious when sulfur loading is increased to the practically accepted level above 3–5 mg cm−2. More and more efforts have been made recently to overcome the barriers toward commercially viable Li–S batteries with a high sulfur loading. This review highlights the recent progress in high-sulfur-loading Li–S batteries enabled by hierarchical design principles at multiscale. Particularly, basic insights into the interfacial reactions, strategies for mesoscale assembly, unique architectures, and configurational innovation in the cathode, anode, and separator are under specific concerns. Hierarchy in the multiscale design is proposed to guide the future development of high-sulfur-loading Li–S batteries.

Review on High-Loading and High-Energy Lithium–Sulfur Batteries

As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium-sulfur (Li-S) batteries. In this paper, a mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite is reported as a novel cathode for advanced Li-S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verifi ed by X-ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC-sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm −2 with a high sulfur loading (4.2 mg S cm −2 ) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm −2 ) is demonstrated by using the novel cathode, which is crucial for the practical application of Li-S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon-sulfur composite cathodes for Li-S batteries.

Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries

Silicon oxides have been recognized as a promising family of anode materials for high-energy lithium-ion batteries (LIBs) owing to their abundant reserve, low cost, environmental friendliness, easy synthesis, and high theoretical capacity. However, the extended application of silicon oxides is severely hampered by the intrinsically low conductivity, large volume change, and low initial coulombic efficiency. Significant efforts have been dedicated to tackling these challenges towards practical applications. This Review focuses on the recent advances in the synthesis and lithium storage properties of silicon oxide-based anode materials. To present the progress in a systematic manner, this review is categorized as follows: (i) SiO-based anode materials, (ii) SiO2-based anode materials, (iii) non-stoichiometric SiOx-based anode materials, and (iv) Si-O-C-based anode materials. Finally, future outlook and our personal perspectives on silicon oxide-based anode materials are presented.

Silicon oxides: a promising family of anode materials for lithium-ion batteries

Electrochemical energy storage devices are becoming increasingly more important for reducing fossil fuel energy consumption in transportation and for the widespread deployment of intermittent renewable energy. The applications of different energy storage devices in specific situations are all primarily reliant on the electrode materials, especially carbon materials. Biomass-derived carbon materials are receiving extensive attention as electrode materials for energy storage devices because of their tunable physical/chemical properties, environmental concern, and economic value. In this review, recent developments in the biomass-derived carbon materials and the properties controlling the mechanism behind their operation are presented and discussed. Moreover, progress on the applications of biomass-derived carbon materials as electrodes for energy storage devices is summarized, including electrochemical capacitors, lithium–sulfur batteries, lithium-ion batteries, and sodium-ion batteries. The effects of the pore structure, surface properties, and graphitic degree on the electrochemical performance are discussed in detail, which will guide further rational design of the biomass-derived carbon materials for energy storage devices.

Biomass derived carbon for energy storage devices

Marcasite (m-FeS2) exhibits higher electronic conductivity than that of pyrite (p-FeS2) because of its lower semiconducting gap (0.4 vs 0.7 eV). Meanwhile, as demonstrates stronger Fe–S bonds and less S–S interactions, the m-FeS2 seems to be a better choice for electrode materials compared to p-FeS2. However, the m-FeS2 has been seldom studied due to its sophisticated synthetic methods until now. Herein, a hierarchical m-FeS2 and carbon nanofibers composite (m-FeS2/CNFs) with grape-cluster structure was designed and successfully prepared by a straightforward hydrothermal method. When evaluated as an electrode material for lithium ion batteries, the m-FeS2/CNFs exhibited superior lithium storage properties with a high reversible capacity of 1399.5 mAh g–1 after 100 cycles at 100 mA g–1 and good rate capability of 782.2 mAh g–1 up to 10 A g–1. The Li-storage mechanism for the lithiation/delithiation processes of m-FeS2/CNFs was systematically investigated by ex situ powder X-ray diffraction patterns and sca...

Metastable Marcasite-FeS2 as a New Anode Material for Lithium Ion Batteries: CNFs-Improved Lithiation/Delithiation Reversibility and Li-Storage Properties

Mesoporous SiO2 nanospheres (MSNs) and carbon nanocomposite with dual-porosity structure (DMSNs/C) were synthesized via a straightforward approach. Both MSNs and DMSNs/C showed uniform pore size distribution, high specific surface area, and large pore volume. When evaluated as an anode material for lithium ion batteries (LIBs), the DMSNs/C nanocomposite not only delivered an impressive reversible capacity of 635.7 mAh g–1 (based on the weight of MSNs in the electrode material) over 200 cycles at 100 mA g–1 with Coulombic efficiency (CE) above 99% but also exhibited excellent rate capability. The significant improvement of the electrochemical performance was attributed to synergetic effects of the dual-mesoporous structure and carbon coating layer: (i) the dual-porosity structure could increase the contact area and facilitate Li+ diffusion at the interface between the electrolyte and active materials, as well as buffer the volume change of MSNs, and (ii) the homogeneous carbon coating represented an excell...

Dual-Porosity SiO2/C Nanocomposite with Enhanced Lithium Storage Performance

In recent years, metal-organic compounds have been considered as ideal sacrificial templates to obtain transition metal oxides for electrochemical applications due to their diverse structures and tunable properties. In this work, a new kind of cobalt-based metal organic compound with a layered structure was designed and prepared, which was then transformed into ultrafine cobalt oxide (Co3O4) nanocrystallites via a facile annealing treatment. The obtained Co3O4 nanocrystallites further assembled into a hierarchical shale-like structure, donating extremely short ion diffusion pathway and rich porosity to the materials. The special structure largely alleviated the problems of Co3O4 such as inferior intrinsic electrical conductivity, poor ion transport kinetics and large volume changes during the redox reactions. When evaluated as anode materials for lithium-ion batteries, the shale-like Co3O4 (S-Co3O4) exhibited superior lithium storage properties with a high capacity of 1045.3 mA h g−1 after 100 cycles at 200 mA g−1 and good rate capabilities up to 10 A g−1. Moreover, the S-Co3O4 showed decent electrochemical performance in sodium-ion batteries due to the above-mentioned comprehensive merits (380 and 153.8 mA h g−1 at 50 and 5000 mA g−1, respectively).

Shale-like Co3O4 for high performance lithium/sodium ion batteries

In this work, the lightweight and scalable organic macromolecule graphitic carbon nitride (g-C3N4) with enriched polysulfide adsorption sites of pyridinic-N was introduced to achieve the effective functionalization of separator at the molecular level. This simple method overcomes the difficulty of low doping content as well as the existence of an uncontrolled form of nitrogen heteroatom in the final product. Besides the conventional pyridinic–N-Li bond formed in the vacancies of g-C3N4, the C–S bond was interestingly observed between g-C3N4 and Li2S, which endowed g-C3N4 with an inherent adsorption capacity for polysulfides. In addition, the microsized g-C3N4 provided the coating layer with good mechanical strength to guarantee its restriction function for polysulfides during long cycling. As a result, an excellent reversible capacity of 840 mA h g–1 was retained at 0.5 C after 400 cycles for a pure sulfur electrode, much better than that of the cell with an innocent carbon-coated separator. Even at a cur...

The Effective Design of a Polysulfide-Trapped Separator at the Molecular Level for High Energy Density Li-S Batteries.

Aqueous zinc batteries, that demonstrate high safety and low cost, are considered promising candidates for large‐scale energy storage. However, Zn anodes suffer from rapid performance deterioration due to the severe Zn dendrite growth and side reactions. Herein, with a low‐cost ammonium acetate (NH4OAc) additive, a self‐regulated Zn/electrolyte interface is built to address these problems. The NH4+ induces a dynamic electrostatic shielding layer around the abrupt Zn protuberance to make the Zn deposition uniform, and the OAc− acts as an interfacial pH buffer to suppress the proton‐induced side reactions and the precipitation of insoluble by‐products. As a result, in the electrolyte with the NH4OAc additive, Zn anodes exhibit a long cycling stability of 3500 h at 1 mA cm−2, an impressive cumulative areal capacity of 5000 mAh cm−2 at 10 mA cm−2, and a high Coulombic efficiency of ≈99.7%. A prototype full cell coupled with a NH4V4O10 cathode performs much better in terms of capacity retention than the additive‐free case. The findings pave the way for developing practical Zn batteries.

Huan-Huan Li

Papers

Metastable Marcasite-FeS2 as a New Anode Material for Lithium Ion Batteries: CNFs-Improved Lithiation/Delithiation Reversibility and Li-Storage Properties

Dual-Porosity SiO2/C Nanocomposite with Enhanced Lithium Storage Performance

Shale-like Co3O4 for high performance lithium/sodium ion batteries

The Effective Design of a Polysulfide-Trapped Separator at the Molecular Level for High Energy Density Li-S Batteries.

A Self‐Regulated Interface toward Highly Reversible Aqueous Zinc Batteries