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Graphene oxide is highly desired for printing electronics, catalysis, energy storage, separation membranes, biomedicine, and composites. However, the present synthesis methods depend on the reactions of graphite with mixed strong oxidants, which suffer from explosion risk, serious environmental pollution, and long-reaction time up to hundreds of hours. Here, we report a scalable, safe and green method to synthesize graphene oxide with a high yield based on water electrolytic oxidation of graphite. The graphite lattice is fully oxidized within a few seconds in our electrochemical oxidation reaction, and the graphene oxide obtained is similar to those achieved by the present methods. We also discuss the synthesis mechanism and demonstrate continuous and controlled synthesis of graphene oxide and its use for transparent conductive films, strong papers, and ultra-light elastic aerogels. Graphene oxide is a graphene derivative showing wide applications, but it suffers from harsh synthetic conditions and long reaction time. Pei et al. show a green electrochemical method to fully oxidize the graphite lattice in a few seconds, which is over 100 times faster than existing methods.

Calcium-ion batteries (CIBs) are attractive candidates for energy storage because Ca2+ has low polarization and a reduction potential (−2.87 V versus standard hydrogen electrode, SHE) close to that of Li+ (−3.04 V versus SHE), promising a wide voltage window for a full battery. However, their development is limited by difficulties such as the lack of proper cathode/anode materials for reversible Ca2+ intercalation/de-intercalation, low working voltages (<2 V), low cycling stability, and especially poor room-temperature performance. Here, we report a CIB that can work stably at room temperature in a new cell configuration using graphite as the cathode and tin foils as the anode as well as the current collector. This CIB operates on a highly reversible electrochemical reaction that combines hexafluorophosphate intercalation/de-intercalation at the cathode and a Ca-involved alloying/de-alloying reaction at the anode. An optimized CIB exhibits a working voltage of up to 4.45 V with capacity retention of 95% after 350 cycles.

Increasing fresh water demand for drinking and agriculture is one of the grand challenges of our age. Graphene oxide (GO) membranes have shown a great potential for desalination and water purification. However, it is challenging to further improve the water permeability without sacrificing the separation efficiency, and the GO membranes are easily delaminated in aqueous solutions within few hours. Here, we report a class of reduced GO membranes with enlarged interlayer distance fabricated by using theanine amino acid and tannic acid as reducing agent and cross-linker. Such membranes show water permeance over 10,000 L m −2 h −1 bar −1 , which is 10–1000 times higher than those of previously reported GO-based membranes and commercial membranes, and good separation efficiency, e.g., rhodamine B and methylene blue rejection of ~100%. Moreover, they show no damage or delamination in water, acid, and basic solutions even after months. Graphene oxide membranes show great potential for water filtering, but improving their performance and stability remains difficult. Here, the authors use theanine amino acid and tannic acid to reduce and cross-link graphene oxide membranes with remarkably high permeability and stability in aqueous solution.

Large-scale implementation of electrochemical hydrogen production requires several fundamental issues to be solved, including understanding the mechanism and developing inexpensive electrocatalysts that work well at high current densities. Here we address these challenges by exploring the roles of morphology and surface chemistry, and develop inexpensive and efficient electrocatalysts for hydrogen evolution. Three model electrocatalysts are flat platinum foil, molybdenum disulfide microspheres, and molybdenum disulfide microspheres modified by molybdenum carbide nanoparticles. The last catalyst is highly active for hydrogen evolution independent of pH, with low overpotentials of 227 mV in acidic medium and 220 mV in alkaline medium at a high current density of 1000 mA cm −2 , because of enhanced transfer of mass (reactants and hydrogen bubbles) and fast reaction kinetics due to surface oxygen groups formed on molybdenum carbide during hydrogen evolution. Our work may guide rational design of electrocatalysts that work well at high current densities. Hydrogen production from water provides one avenue toward harnessing renewable energy, although large-scale implementation remains a challenge. Here, authors explore roles of morphology and surface chemistry, and develop efficient catalysts for hydrogen evolution at high current densities.

The recycling of spent lithium-ion batteries is an effective approach to alleviating environmental concerns and promoting resource conservation. LiFePO 4 batteries have been widely used in electric vehicles and energy storage stations. Currently, lithium loss, resulting in formation of Fe(III) phase, is mainly responsible for the capacity fade of LiFePO 4 cathode. Another factor is poor electrical conductivity that limits its rate capability. Here, we report the use of a multifunctional organic lithium salt (3,4-dihydroxybenzonitrile dilithium) to restore spent LiFePO 4 cathode by direct regeneration. The degraded LiFePO 4 particles are well coupled with the functional groups of the organic lithium salt, so that lithium fills vacancies and cyano groups create a reductive atmosphere to inhibit Fe(III) phase. At the same time, pyrolysis of the salt produces an amorphous conductive carbon layer that coats the LiFePO 4 particles, which improves Li-ion and electron transfer kinetics. The restored LiFePO 4 cathode shows good cycling stability and rate performance (a high capacity retention of 88% after 400 cycles at 5 C). This lithium salt can also be used to recover degraded transition metal oxide-based cathodes. A techno-economic analysis suggests that this strategy has higher environmental and economic benefits, compared with the traditional recycling methods. Sustainable recycle of spent Li ion batteries is an effective strategy to alleviate environmental concerns and support resource conservation. Here, authors report the direct regeneration of LiFePO 4 cathode using multifunctional organic lithium salts, leading to high environmental and economic benefits.

The search for new two-dimensional monolayers with diverse electronic properties has attracted growing interest in recent years. Here, we present an approach to construct MA 2 Z 4 monolayers with a septuple-atomic-layer structure, that is, intercalating a MoS 2 -type monolayer MZ 2 into an InSe-type monolayer A 2 Z 2 . We illustrate this unique strategy by means of first-principles calculations, which not only reproduce the structures of MoSi 2 N 4 and MnBi 2 Te 4 that were already experimentally synthesized, but also predict 72 compounds that are thermodynamically and dynamically stable. Such an intercalated architecture significantly reconstructs the band structures of the constituents MZ 2 and A 2 Z 2 , leading to diverse electronic properties for MA 2 Z 4 , which can be classified according to the total number of valence electrons. The systems with 32 and 34 valence electrons are mostly semiconductors. Whereas, those with 33 valence electrons can be nonmagnetic metals or ferromagnetic semiconductors. In particular, we find that, among the predicted compounds, (Ca,Sr)Ga 2 Te 4 are topologically nontrivial by both the standard density functional theory and hybrid functional calculations. While VSi 2 P 4 is a ferromagnetic semiconductor and TaSi 2 N 4 is a type-I Ising superconductor. Moreover, WSi 2 P 4 is a direct gap semiconductor with peculiar spin-valley properties, which are robust against interlayer interactions. Our study thus provides an effective way of designing septuple-atomic-layer MA 2 Z 4 with unusual electronic properties to draw immediate experimental interest. The discovery of a new two-dimensional van der Waals layered MoSi 2 N 4 material inspires many attentions. Here, the authors report intercalation strategies to explore a much wider range of MA 2 Z 4 family and predict amount of materials accessible to experimental verifications with emergent topological, magnetic or Ising superconductivity properties.

It is crucial to align two-dimensional nanosheets to form a highly compact layered structure for many applications, such as electronics, optoelectronics, thermal management, energy storage, separation membranes, and composites. Here we show that continuous centrifugal casting is a universal, scalable and efficient method to produce highly aligned and compact two-dimensional nanosheets films with record performances. The synthesis  mechanism, structure  control and property  dependence of alignment and compaction of the films are discussed. Significantly, 10-μm-thick graphene oxide films can be synthesized within 1 min, and scalable synthesis of meter-scale films is demonstrated. The reduced graphene oxide films show super-high strength (~660 MPa) and conductivity (~650 S cm −1 ). The reduced graphene oxide/carbon nanotube hybrid-film-based all-solid-state flexible supercapacitors exhibit ultrahigh volumetric capacitance (407 F cm −3 ) and energy density (~10 mWh cm −3 ) comparable to that of thin-film lithium batteries. We also demonstrate the production of highly anisotropic graphene nanocomposites as well as aligned, compact films and vertical heterostructures of various nanosheets. Aligning 2D nanosheets to form a compact layered structure can maximize the in-plane properties. Here the authors report an efficient and scalable continuous centrifugal casting method to produce highly compact and well-aligned films of GO nanosheets that show record performances in some applications.

The high-throughput scalable production of cheap, efficient and durable electrocatalysts that work well at high current densities demanded by industry is a great challenge for the large-scale implementation of electrochemical technologies. Here we report the production of a two-dimensional molybdenum disulfide-based ink-type electrocatalyst by a scalable exfoliation technique followed by a thermal treatment. The catalyst delivers a high current density of 1000 mA cm −2 at an overpotential of 412 mV for the hydrogen evolution. Using the same method, we produce a cheap mineral-based catalyst possessing excellent performance for high-current-density hydrogen evolution. Noteworthy, production rate of this catalyst is one to two orders of magnitude higher than those previously reported, and price of the mineral is five orders of magnitude lower than commercial Pt electrocatalysts. These advantages indicate the huge potentials of this method and of mineral-based cheap and abundant natural resources as catalysts in the electrochemical industry. The large-scale implementation of electrochemical technologies will require the high-throughput production of high-performance, inexpensive catalysts. Here, authors demonstrate earth abundant molybdenite as raw materials to produce efficient MoS 2 catalysts for high current density H 2 evolution.

Recycling cathode materials from spent lithium‐ion batteries (LIBs) is critical to a sustainable society as it will relief valuable but scarce recourse crises and reduce environment burdens simultaneously. Different from conventional hydrometallurgical and pyrometallurgical recycling methods, direct regeneration relies on non‐destructive cathode‐to‐cathode mode, and therefore, more time and energy‐saving along with an increased economic return and reduced CO 2 footprint. This review retrospects the history of direct regeneration and discusses state‐of‐the‐art development. The reported methods, including high‐temperature solid‐state, hydrothermal/ionothermal, molten salt thermochemistry, and electrochemical method, are comparatively introduced, targeting at illustrating their underlying regeneration mechanism and applicability. Further, representative repairing and upcycling studies on wide‐applied cathodes, including LiCoO 2 (LCO), ternary oxides, LiFePO 4 (LFP), and LiMn 2 O 4 (LMO), are presented, with an emphasis on milestone cases. Despite these achievements, there remain several critical issues that shall be addressed before the commercialization of the mentioned direct regeneration methods.

Potassium-ion batteries are a compelling technology for large scale energy storage due to their low-cost and good rate performance. However, the development of potassium-ion batteries remains in its infancy, mainly hindered by the lack of suitable cathode materials. Here we show that a previously known frustrated magnet, KFeC 2 O 4 F, could serve as a stable cathode for potassium ion storage, delivering a discharge capacity of ~112 mAh g −1 at 0.2 A g −1 and 94% capacity retention after 2000 cycles. The unprecedented cycling stability is attributed to the rigid framework and the presence of three channels that allow for minimized volume fluctuation when Fe 2+ /Fe 3+ redox reaction occurs. Further, pairing this KFeC 2 O 4 F cathode with a soft carbon anode yields a potassium-ion full cell with an energy density of ~235 Wh kg −1 , impressive rate performance and negligible capacity decay within 200 cycles. This work sheds light on the development of low-cost and high-performance K-based energy storage devices. The abundance and low cost of potassium makes potassium batteries a promising technology for large scale energy storage. Here the authors apply a previously known frustrated magnet, KFeC 2 O 4 F, as the cathode in which the unique structure and Fe 2+ /Fe 3+ redox enable excellent cycling stability.