We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

Thanks to their remarkable mechanical, electrical, thermal, and barrier properties, graphene-based nanocomposites have been a hot area of research in the past decade. Because of their simple top-down synthesis, graphene oxide (GO) and reduced graphene oxide (rGO) have opened new possibilities for gas barrier, membrane separation, and stimuli-response characteristics in nanocomposites. Herein, we review the synthesis techniques most commonly used to produce these graphene derivatives, discuss how synthesis affects their key material properties, and highlight some examples of nanocomposites with unique and impressive properties. We specifically highlight their performances in separation applications, stimuli-responsive materials, anti-corrosion coatings, and energy storage. Finally, we discuss the outlook and remaining challenges in the field of practical industrial-scale production and use of graphene-derivative-based polymer nanocomposites.

Synthesis, properties, and applications of graphene oxide/reduced graphene oxide and their nanocomposites

Most bulk-scale graphene is produced by a top-down approach, exfoliating graphite, which often requires large amounts of solvent with high-energy mixing, shearing, sonication or electrochemical treatment1–3. Although chemical oxidation of graphite to graphene oxide promotes exfoliation, it requires harsh oxidants and leaves the graphene with a defective perforated structure after the subsequent reduction step3,4. Bottom-up synthesis of high-quality graphene is often restricted to ultrasmall amounts if performed by chemical vapour deposition or advanced synthetic organic methods, or it provides a defect-ridden structure if carried out in bulk solution4–6. Here we show that flash Joule heating of inexpensive carbon sources—such as coal, petroleum coke, biochar, carbon black, discarded food, rubber tyres and mixed plastic waste—can afford gram-scale quantities of graphene in less than one second. The product, named flash graphene (FG) after the process used to produce it, shows turbostratic arrangement (that is, little order) between the stacked graphene layers. FG synthesis uses no furnace and no solvents or reactive gases. Yields depend on the carbon content of the source; when using a high-carbon source, such as carbon black, anthracitic coal or calcined coke, yields can range from 80 to 90 per cent with carbon purity greater than 99 per cent. No purification steps are necessary. Raman spectroscopy analysis shows a low-intensity or absent D band for FG, indicating that FG has among the lowest defect concentrations reported so far for graphene, and confirms the turbostratic stacking of FG, which is clearly distinguished from turbostratic graphite. The disordered orientation of FG layers facilitates its rapid exfoliation upon mixing during composite formation. The electric energy cost for FG synthesis is only about 7.2 kilojoules per gram, which could render FG suitable for use in bulk composites of plastic, metals, plywood, concrete and other building materials. Flash Joule heating of inexpensive carbon sources is used to produce gram-scale quantities of high-quality graphene in under a second, without the need for a furnace, solvents or reactive gases.

Gram-scale bottom-up flash graphene synthesis

Lithium-sulfur batteries are a major focus of academic and industrial energy-storage research due to their high theoretical energy density and the use of low-cost materials. The high energy density results from the conversion mechanism that lithium-sulfur cells utilize. The sulfur cathode, being naturally abundant and environmentally friendly, makes lithium-sulfur batteries a potential next-generation energy-storage technology. The current state of the research indicates that lithium-sulfur cells are now at the point of transitioning from laboratory-scale devices to a more practical energy-storage application. Based on similar electrochemical conversion reactions, the low-cost sulfur cathode can be coupled with a wide range of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum. These new "metal-sulfur" systems exhibit great potential in either lowering the production cost or producing high energy density. Inspired by the rapid development of lithium-sulfur batteries and the prospect of metal-sulfur cells, here, over 450 research articles are summarized to analyze the research progress and explore the electrochemical characteristics, cell-assembly parameters, cell-testing conditions, and materials design. In addition to highlighting the current research progress, the possible future areas of research which are needed to bring conversion-type lithium-sulfur and other metal-sulfur batteries into the market are also discussed.

Current Status and Future Prospects of Metal–Sulfur Batteries

Thick Electrode Batteries: Principles, Opportunities, and Challenges

Rechargeable aqueous zinc-ion batteries (RZIBs) provide a promising complementarity to the existing lithium-ion batteries due to their low cost, non-toxicity and intrinsic safety. However, Zn anodes suffer from zinc dendrite growth and electrolyte corrosion, resulting in poor reversibility. Here, we develop an ultrathin, fluorinated two-dimensional porous covalent organic framework (FCOF) film as a protective layer on the Zn surface. The strong interaction between fluorine (F) in FCOF and Zn reduces the surface energy of the Zn (002) crystal plane, enabling the preferred growth of (002) planes during the electrodeposition process. As a result, Zn deposits show horizontally arranged platelet morphology with (002) orientations preferred. Furthermore, F-containing nanochannels facilitate ion transport and prevent electrolyte penetration for improving corrosion resistance. The FCOF@Zn symmetric cells achieve stability for over 750 h at an ultrahigh current density of 40 mA cm−2. The high-areal-capacity full cells demonstrate hundreds of cycles under high Zn utilization conditions. Rechargeable aqueous zinc-ion batteries are promising but the zinc anode suffers from dendrite growth and electrolyte corrosion. Here, the authors develop a fluorinated covalent organic framework film as a protective layer for aqueous zinc anode battery.

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Horizontally Arranged Zinc Platelet Electrodeposits Modulated by Fluorinated Covalent Organic Framework Film for High-Rate and Durable Aqueous Zinc Ion Batteries

It is difficult to achieve high efficiency production of hydrophobic graphene by liquid phase exfoliation due to its poor dispersibility and the tendency of graphene sheets to undergo π−π stacking. Here, we report a water-phase, non-dispersion exfoliation method to produce highly crystalline graphene flakes, which can be stored in the form of a concentrated slurry (50 mg mL−1) or filter cake for months without the risk of re-stacking. The as-exfoliated graphene slurry can be directly used for 3D printing, as well as fabricating conductive graphene aerogels and graphene−polymer composites, thus avoiding the use of copious quantities of organic solvents and lowering the manufacturing cost. This non-dispersion strategy paves the way for the cost-effective and environmentally friendly production of graphene-based materials.

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A non-dispersion strategy for large-scale production of ultra-high concentration graphene slurries in water

Low-cost, scalable preparation of high-quality graphene has been a critical challenge that hampers its large-scale application. We here propose a novel, scalable liquidphase exfoliation method in which the intercalation, expansion, and exfoliation of graphite are achieved all under ambient conditions, not involving any heating or high-temperature treatment. We demonstrate that such room-temperature liquid-phase intercalation and expansion allow graphite flakes to expand up to 1000 times. Significantly different from thermally expanded graphite, the resulting chemically expanded graphite (CEG) exhibits a uniform, open, porous structure with a specific surface area (847 m2/g) comparable to the theoretical value of three-layer graphene. The CEG obtained is able to be exfoliated under mild conditions to give high-quality graphene with a yield of 70% relative to the starting graphite. The exfoliated graphene sheets have very few defects, with an atomic ratio of carbon to oxygen (C/O ratio) of 28. The as-prepar...

Room-Temperature Intercalation and ∼1000-Fold Chemical Expansion for Scalable Preparation of High-Quality Graphene

Sandwich, Vertical‐Channeled Thick Electrodes with High Rate and Cycle Performance

DOI: 10.1002/aenm.201802431 and energy density (2600 Wh kg−1), as well as the natural abundance of sulfur.[1–5] The insulating nature of sulfur and its volume expansion during discharge/ charge have been resolved to a large extent in the past decade[6–9]; however, the dissolution of lithium polysulfides (Li2Sx, x ≥ 4, LiPSs) into electrolyte has been a crucial challenge that hinders the practical application, which results in the notorious “shuttle effect,” migration of LiPSs to the lithium metal anode and uncontrolled deposition.[10–12] These cause the irreversible loss of active sulfur, rapid capacity decay, low coulombic efficiency, poor rate performance, and short cycle life in Li–S batteries.[7,13–15] Also, the shuttle effect slows down the kinetics of polysulfide conversion along the solid (S8)– liquid (Li2Sx, x ≥ 4)–solid (Li2S2 or Li2S) route, which results in the accumulation of LiPSs in electrolyte and rapid diffusion to the lithium anode, further exacerbating the shuttle effect due to the formation of non-reusable solid Li2S2 or Li2S. Many efforts have been attempted to overcome the shuttle effect in Li–S batteries, which mostly employ physical barriers or chemical adsorption for LiPSs. Typically, the physical confinement is realized by using miscellaneous carbon materials with large surface area and high conductivity to load sulfur.[18–21] However, the nonpolar carbon materials can only delay rather than suppress the shuttle effect; the polar LiPSs can still easily detach from the carbon surface and reenter into the electrolyte.[22,23] The chemical adsorption can effectively suppress the dissolution of lithium polysulfides through the strong chemical bond between LiPSs and the polar materials such as heteroatom-doped carbon, transition metal oxides/sulfides/carbides/nitrides, and metal–organic framework materials.[24–28] Whether physical or chemical adsorption, however, the confined LiPSs are mostly hard to be re-used, forming the so-called “dead sulfur.” Moreover, the kinetics of polysulfide conversions is difficult to be improved just through such adsorption strategies, which cannot solve the problem at the root. It is a crucial step for overcoming the shuttle effect to simultaneously enhance the anchoring and redox kinetics of LiPSs. The electrocatalysts such as metals,[29,30] metallic compounds,[10,31,32] and metal-free polar materials[17,33] have The performance of lithium–sulfur (Li–S) batteries is largely hindered by the shuttle effect caused by the dissolution of lithium polysulfides (LiPSs) and the sluggish reaction kinetics of LiPSs. Here, it is demonstrated that the nickel– cobalt double hydroxide (NiCo-DH) shells that encapsulate sulfur nanoparticles can play multiple roles in suppressing the shuttle effect and accelerating the redox kinetics of LiPSs by combining with graphene and carbon nanotubes to construct the conductive networks. The NiCo-DH shell that intimately contacts with sulfur physically confines the loss of sulfur and promotes the charge transfer and ion diffusion. More importantly, it can react with LiPSs to produce the surface-bound intermediates, which are able to anchor the soluble LiPSs and accelerate the redox kinetics. Such composite electrodes can load high contents of sulfur (>85 wt%) and the resulting Li–S battery exhibits a superior capacity (1348.1 mAh g−1 at 0.1 C), ultrahigh rate performance (697.7 mAh g−1 at 5 C), and ultralong cycle life (1500 cycles) with a decay rate of 0.015% per cycle.

Hongbin Lu

Papers

Horizontally Arranged Zinc Platelet Electrodeposits Modulated by Fluorinated Covalent Organic Framework Film for High-Rate and Durable Aqueous Zinc Ion Batteries

A non-dispersion strategy for large-scale production of ultra-high concentration graphene slurries in water

Room-Temperature Intercalation and ∼1000-Fold Chemical Expansion for Scalable Preparation of High-Quality Graphene

Sandwich, Vertical‐Channeled Thick Electrodes with High Rate and Cycle Performance

Nickel–Cobalt Double Hydroxide as a Multifunctional Mediator for Ultrahigh‐Rate and Ultralong‐Life Li–S Batteries