Yijun Yang

Amorphous Phosphorus/Nitrogen-Doped Graphene Paper for Ultrastable Sodium-Ion Batteries

As the most promising anode material for sodium-ion batteries (SIBs), elemental phosphorus (P) has recently gained a lot of interest due to its extraordinary theoretical capacity of 2596 mAh/g. The main drawback of a P anode is its low conductivity and rapid structural degradation caused by the enormous volume expansion (>490%) during cycling. Here, we redesigned the anode structure by using an innovative methodology to fabricate flexible paper made of nitrogen-doped graphene and amorphous phosphorus that effectively tackles this problem. The restructured anode exhibits an ultrastable cyclic performance and excellent rate capability (809 mAh/g at 1500 mA/g). The excellent structural integrity of the novel anode was further visualized during cycling by using in situ experiments inside a high-resolution transmission electron microscope (HRTEM), and the associated sodiation/desodiation mechanism was also thoroughly investigated. Finally, density functional theory (DFT) calculations confirmed that the N-doped graphene not only contributes to an increase in capacity for sodium storage but also is beneficial in regards to improved rate performance of the anode.

“Protrusions” or “holes” in graphene: which is the better choice for sodium ion storage?

The main challenge associated with sodium-ion battery (SIB) anodes is a search for novel candidate materials with high capacity and excellent rate capability. The most commonly used and effective route for graphene-based anode design is the introduction of in-plane “hole” defects via nitrogen-doping; this creates a spacious reservoir for storing more energy. Inspired by mountains in nature, herein, we propose another way – the introduction of blistering in graphene instead of making “holes”; this facilitates adsorbing/inserting more Na+ ions. In order to properly answer the key question: ““protrusions” or “holes” in graphene, which is better for sodium ion storage?”, two types of anode materials with a similar doping level were designed: a phosphorus-doped graphene (GP, with protrusions) and a nitrogen-doped graphene (GN, with holes). As compared with GN, the GP anode perfectly satisfies all the desired criteria: it reveals an ultrahigh capacity (374 mA h g−1 after 120 cycles at 25 mA g−1) comparable to the best graphite anodes in a standard Li-ion battery (∼372 mA h g−1), and exhibits an excellent rate capability (210 mA h g−1 at 500 mA g−1). In situ transmission electron microscopy (TEM) experiments and density functional theory (DFT) calculations were utilized to uncover the origin of the enhanced electrochemical activity of “protrusions” compared to “holes” in SIBs, down to the atomic scale. The introduction of protrusions through P-doping into graphene is envisaged to be a novel effective way to enhance the capacity and rate performance of SIBs.

In Situ Electrochemistry of Rechargeable Battery Materials: Status Report and Perspectives

The development of rechargeable batteries with high performance is considered to be a feasible way to satisfy the increasing needs of electric vehicles and portable devices. It is of vital importance to design electrodes with high electrochemical performance and to understand the nature of the electrode/electrolyte interfaces during battery operation, which allows a direct observation of the complicated chemical and physical processes within the electrodes and electrolyte, and thus provides real-time information for further design and optimization of the battery performance. Here, the recent progress in in situ techniques employed for the investigations of material structural evolutions is described, including characterization using neutrons, X-ray diffraction, and nuclear magnetic resonance. In situ techniques utilized for in-depth uncovering the electrode/electrolyte phase/interface change mechanisms are then highlighted, including transmission electron microscopy, atomic force microscopy, X-ray spectroscopy, and Raman spectroscopy. The real-time monitoring of lithium dendrite growth and in situ detection of gas evolution during charge/discharge processes are also discussed. Finally, the major challenges and opportunities of in situ characterization techniques are outlined toward new developments of rechargeable batteries, including innovation in the design of compatible in situ cells, applications of dynamic analysis, and in situ electrochemistry under multi-stimuli. A clear and in-depth understanding of in situ technique applications and the mechanisms of structural evolutions, surface/interface changes, and gas generations within rechargeable batteries is given here.

Tuning of the Optical, Electronic, and Magnetic Properties of Boron Nitride Nanosheets with Oxygen Doping and Functionalization

Engineering of the optical, electronic, and magnetic properties of hexagonal boron nitride (h-BN) nanomaterials via oxygen doping and functionalization has been envisaged in theory. However, it is still unclear as to what extent these properties can be altered using such methodology because of the lack of significant experimental progress and systematic theoretical investigations. Therefore, here, comprehensive theoretical predictions verified by solid experimental confirmations are provided, which unambiguously answer this long-standing question. Narrowing of the optical bandgap in h-BN nanosheets (from ≈5.5 eV down to 2.1 eV) and the appearance of paramagnetism and photoluminescence (of both Stokes and anti-Stokes types) in them after oxygen doping and functionalization are discussed. These results are highly valuable for further advances in semiconducting nanoscale electronics, optoelectronics, and spintronics.

Construction of Polarized Carbon–Nickel Catalytic Surfaces for Potent, Durable, and Economic Hydrogen Evolution Reactions

Electrocatalytic hydrogen evolution reaction (HER) in alkaline solution is hindered by its sluggish kinetics toward water dissociation. Nickel-based catalysts, as low-cost and effective candidates, show great potentials to replace platinum (Pt)-based materials in the alkaline media. The main challenge regarding this type of catalysts is their relatively poor durability. In this work, we conceive and construct a charge-polarized carbon layer derived from carbon quantum dots (CQDs) on Ni3N nanostructure (Ni3N@CQDs) surfaces, which simultaneously exhibit durable and enhanced catalytic activity. The Ni3N@CQDs shows an overpotential of 69 mV at a current density of 10 mA cm-2 in a 1 M KOH aqueous solution, lower than that of Pt electrode (116 mV) at the same conditions. Density functional theory (DFT) simulations reveal that Ni3N and interfacial oxygen polarize charge distributions between originally equal C-C bonds in CQDs. The partially negatively charged C sites become effective catalytic centers for the key water dissociation step via the formation of new C-H bond (Volmer step) and thus boost the HER activity. Furthermore, the coated carbon is also found to protect interior Ni3N from oxidization/hydroxylation and therefore guarantees its durability. This work provides a practical design of robust and durable HER electrocatalysts based on nonprecious metals.

Biofabrication Strategy for Functional Fabrics

Functional fabrics with various unique properties are necessary for making fantastic superior costumes just like a superhero suit in Marvel Comics, which are not only dreams of boys but also emerging textiles to facilitate human life. On the basis of the inspiration of a phenomenon in an extracurricular experiment for kids, we develop a biofabrication strategy to endow silk textiles with various unique physical and chemical properties of functional nanomaterials, where the functional textiles are weaved using silk spun by silkworms that are fed with functional nanomaterials. To confirm the feasibility of this strategy, a photoluminescent plain weave was prepared successfully via feeding biocompatible luminescent nanoparticles to Bombyx mori silkworms. As the functional nanomaterials are enclosed in the silkfibers, the given special properties will be permanent for further application. Considering the wondrous diversity of properties that a variety of nanomaterials possesses may be given to silk fabric, it is promising to see various miraculous costumes in the coming future.

Curving effects of concave dodecahedral nanocarbons enable enhanced Li-ion storage

Dodecahedral nanocarbons with highly curving concave faces are synthesized via a one-step direct pyrolysis strategy. When tested as anodes in lithium-ion batteries (LIBs), the concave dodecahedral carbons (CDCs) show much enhanced capacity (∼2 times) and improved rate capability (603 mA h g−1 at 2000 mA g−1), when compared with the perfect ones without curved planes (PDCs, 280 mA h g−1). The curving effects of CDCs on Li ion storage are further investigated by constructing a CDC particle-based nanobattery device in an in situ transmission electron microscope (TEM). During the lithiation process of CDCs, the spacing in the curved parts expands much more than the perfect ones (from 0.5 nm to 0.7 nm), providing increasing active sites for Li ion storage. And the stable solid electrolyte interphase (SEI) layer and structural integrity for CDCs are also visualized during the discharge process, in good agreement with their ultralong cycle life. This design concept, enhancing curved parts in the structures, provides guidance in the development of next-generation advanced electrode materials.

Improved cycling stability of NiS2 cathodes through designing a “kiwano” hollow structure

As one of the promising cathode materials, NiS 2 delivers an ultrahigh theoretical capacity of 870 mA h g −1 . However, it suffers from huge capacity fading during multiple cycles because of the close formation enthalpy between NiS 2 and other nickel sulphides (Ni 3 S 4 , Ni 3 S 2 and NiS) and the large volumetric expansion during charging. To overcome these drawbacks, and being inspired by the morphology of “kiwano”, i.e. the African horned melon, with many protrusions, we design and synthesize a unique “kiwano”-like hollow structure via a facile approach, which is beneficial to shorten the diffusion lengths, buffer the volume expansion and especially control the evolution of intermediate phases. The fabricated electrodes present significantly improved capacity (681 mA h g −1 after 100 cycles at 50 mA g −1 ), superior cycling stability (580.6 mA h g −1 even after 400 cycles at 0.2C) and fast Li + storage properties (264 mA h g −1 at 2C). The single intermediate phase Ni 3 S 4 , rather than other nickel sulphides, was observed in real time by using in situ transmission electron microscopy (TEM) upon direct lithiation. This was one of the key factors for the outstanding cycling stability. As visualized by in situ TEM, the pores in the kiwano structure can effectively buffer the volume expansion. In addition, Li + ions prefer to insert into NiS 2 through the (111) facet owing to their low activation energy. Density of states (DOS) calculations reveal that NiS 2 and the intermediate phase Ni 3 S 4 present extremely high electronic conductivity, thus delivering high rate capacity. These promising findings can provide a new perspective in high-performance lithium-ion batteries. present significantly improved capacity (681 mAh/g after 100 cycles at 50 mA/g), superior cycling stability (580.6 mAh/g even after 400 cycles at 0.2 C) and fast Li+ storage properties (264 mAh/g at 2 C). The single intermediate phase Ni3S4, rather than other nickel sulphides, was observed in real time by using in situ transmission electron microscopy (TEM) upon direct lithiation. This was one of the key factors for the outstanding cycling stability. As visualized by in situ TEM, the pores in kiwano structure can effectively buffer the volume expansion. In addition, Li+ ions prefer to insert into NiS2 through the (111) facet owing to its low activation energy. Density of states (DOS) calculations reveal that NiS2 and intermediate phase Ni3S4 present extremely high electronic conductivity, thus delivering high rate capacity. These promising findings can enlighten a new perspective in high-performance lithium-ion batteries.