Chlorination Design for Highly Stable Electrolyte toward High Mass Loading and Long Cycle Life Sodium-Based Dual-Ion Battery.

Sodium-based dual ion batteries (SDIBs) have garnered significant attention as novel energy storage devices offering the advantages of high-voltage and low-cost. Nonetheless, conventional electrolytes exhibit low resistance to oxidation and poor compatibility with electrode materials, resulting in rapid battery failure. In this study, for the first time, a chlorination design of electrolytes for SDIB, is proposed. Using ethyl methyl carbonate (EMC) as a representative, chlorine (Cl)-substituted EMC not only demonstrates increased oxidative stability ascribed to the electron-withdrawing characteristics of chlorine atom, electrolyte compatibility with both the cathode and anode is also greatly improved by forming Cl-containing interface layers. Consequently, a discharge capacity of 104.6 mAh g-1 within a voltage range of 3.0-5.0 V is achieved for Na||graphite SDIB that employs a high graphite cathode mass loading of 5.0 mg cm-2, along with almost no capacity decay after 900 cycles. Notably, the Na||graphite SDIB can be revived for an additional 900 cycles through the replacement of a fresh Na anode. As the mass loading of graphite cathode increased to 10 mg cm-2, Na||graphite SDIB is still capable of sustaining over 700 times with ≈100% capacity retention. These results mark the best outcome among reported SDIBs. This study corroborates the effectiveness of chlorination design in developing high-voltage electrolytes and attaining enduring cycle stability of Na-based energy storage devices.

Solvation Structure Modulation of High-Voltage Electrolyte for High-Performance K-Based Dual-Graphite Battery.

Due to the advantages of dual-ion batteries (DIBs) and abundant resources, potassium-based dual-carbon batteries (K-DCBs) have wide application prospects. However, conventional carbonate ester-based electrolyte systems have obvious drawbacks such as poor oxidation resistance and difficulty in sustaining the anion intercalation process at high voltages, which seriously affect the capacity and cycle performance of K-DCBs. Therefore, a rational design of more efficient novel electrolyte systems is urgently required to realize high-performance K-DCBs. Herein, a solvation structure modulation strategy for the K-DCB electrolyte systems is reported. Consequently, substantial K+ ion storage improvement at the graphite anode and enhanced bis(fluorosulfonyl)imide anion (FSI- ) intercalation capacity at the graphite cathode are successfully realized simultaneously. As a proof-of-concept, the assembled K-DCB exhibited a discharge capacity of 103.4 mAh g-1 , and after 400 cycles, ≈90% capacity retention is observed. Moreover, the energy density of the K-DCB full cell reached 157.6 Wh kg-1 , which is the best performance in reported K-DCBs till date. This study demonstrates the effectiveness of solvation modulation in improving the performance of K-DCBs.

Concentrated Electrolyte for High-Performance Ca-Ion Battery Based on Organic Anode and Graphite Cathode.

Due to the large abundance, low redox potential, and multivalent properties of calcium (Ca), Ca-ion batteries (CIBs) show promising prospects for energy storage applications. However, current research on CIBs faces the challenges of unsatisfactory cycling stability and capacity, mainly restricted by the lack of suitable electrolytes and electrode materials. Herein, we firstly developed a 3.5 m concentrated electrolyte with a calcium bis(fluorosulfonyl)imide (Ca(FSI)2 ) salt dissolved in carbonate solvents. This electrolyte significantly improved the intercalation capacity for anions in the graphite cathode and contributed to the reversible insertion of Ca2+ in the organic anode. By combining this concentrated electrolyte with the low-cost and environmentally friendly graphite cathode and organic anode, the assembled Ca-based dual-ion battery (Ca-DIB) exhibits 75.4 mAh g-1 specific discharge capacity at 100 mA g-1 and 84.7 % capacity retention over 350 cycles, among the best results known for CIBs.

Molecular Coupling and Self-Assembly Strategy toward WSe2 /Carbon Micro-Nano Hierarchical Structure for Elevated Sodium-Ion Storage.

Sodium (Na) ion-based dual-ion batteries (Na-DIBs) have attracted great attention, owing to their benefits of low cost, high working voltage, and environmental friendliness. However, the limited capacity and low tap density of currently reported anode materials restrict the further improvement of Na-DIBs. Herein, a micro-nano structure with vertically aligned WSe2 nanoflakes anchored tightly on a micron-sized carbon sphere (WSe2 /CS) is successfully constructed via combining the molecular coupling and self-assembly strategy. Within this hierarchical structure, the WSe2 nanoflakes can shorten the diffusion path for Na+ ions and alleviate structural deformation during the charge/discharge process; meanwhile, the micron-sized carbon core provides conductive support and helps improve the total tap density of the anode electrode. As a result, this micron-sized WSe2 /CS displays a high specific capacity of ≈252.8 mAh g-1 and good cycling performance with ≈92% capacity retention after 1200 cycles. Moreover, by pairing this WSe2 /CS anode with environmental friendly graphite as cathode, a proof-of-concept Na-DIB shows 85.6% capacity retention after 1000 cycles, which is among the best performances of previously reported Na-DIBs.

Highly Concentrated Electrolyte towards Enhanced Energy Density and Cycling Life of Dual-Ion Battery.

Dual-ion batteries (DIBs) have attracted much attention owing to their low cost, high voltage, and environmental friendliness. As the source of active ions during the charging/discharging process, the electrolyte plays a critical role in the performance of DIBs, including capacity, energy density, and cycling life. However, most used electrolyte systems based on the LiPF6 salt demonstrate unsatisfactory performance in DIBs. We have successfully developed a 7.5 mol kg-1 lithium bis(fluorosulfonyl)imide (LiFSI) in a carbonate electrolyte system. Compared with diluted electrolytes, this highly concentrated electrolyte exhibits several advantages: 1) enhanced intercalation capacity and cycling stability of the graphite cathode, 2) optimized structural stability of the Al anode, and 3) significantly increased battery energy density. A proof-of-concept DIB based on this concentrated electrolyte exhibits a discharge capacity of 94.0 mAh g-1 at 200 mA g-1 and 96.8 % capacity retention after 500 cycles. By counting both the electrode materials and electrolyte, the energy density of this DIB reaches up to ≈180 Wh kg-1 , which is among the best performances of DIBs reported to date.

A Flexible Potassium-Ion Hybrid Capacitor with Superior Rate Performance and Long Cycling Life.

Potassium-ion batteries are promising candidates for large-scale energy storage applications owing to their merits of abundant resources, low cost, and high working voltage. However, the unsatisfying rate performance and cycling stability caused by sluggish K+ diffusion kinetics and dramatic volume expansion hinder the development of potassium-ion batteries. In this study, we design a flexible potassium-ion hybrid capacitor (PIHC) by combining the K-Sn alloying mechanism on the Sn anode and the fast capacitive behavior on the AC cathode with high surface area and mesoporous structure. After optimization, the fabricated Sn||AC PIHC achieves both a high energy density of 120 W h kg-1 and high power density of 2850 W kg-1, much better than other similar hybrid devices. Moreover, a gel polymer electrolyte with a 3D porous structure and high ionic conductivity was employed to improve the structural stability of the Sn anode, which not only realizes good flexibility but also achieves long cycling stability with a capacity retention of nearly 100% for 2000 cycles at a high current density of 3.0 A g-1.

Confinement-Enhanced Rapid Interlayer Diffusion within Graphene-Supported Anisotropic ReSe2 Electrodes.

To enhance interlayer lithium diffusion, we engineer electrodes consisting of epitaxially grown ReSe2 nanosheets by chemical vapor deposition, supported on three-dimensional (3D) graphene foam, taking advantage of its weak van der Waals coupling and anisotropic crystal structure. We further demonstrate its excellent performance as the anode for lithium-ion battery and catalyst for hydrogen evolution reaction (HER). Density functional theory calculation reveals that ReSe2 exhibits a low energy barrier for lithium (Li) interlayer diffusion because of negligible interlayer coupling and anisotropic structure with low symmetry that creates additional adsorption sites and leads to a reduced diffusion barrier. Benefitting from these properties, the 3D ReSe2/graphene foam electrode displays excellent cycling and rate performance with 99.6% capacity retention after 350 cycles and a capacity of 327 mA h g-1 at the current density of 1000 mA g-1. Additionally, it has exhibited a high activity for HER, in which an exchange current density of 277.8 µA cm-2 is obtained and only an overpotential of 106 mV is required to achieve a current density of -10 mA cm-2. Our work provides a fundamental understanding of the interlayer diffusion of Li in transition-metal dichalcogenide (TMD) materials and acts as a new tool for designing a TMD-based catalyst.

Hollow Carbon Nanobelts Codoped with Nitrogen and Sulfur via a Self-Templated Method for a High-Performance Sodium-Ion Capacitor.

Sodium-ion capacitors (SICs) have attracted enormous attention due to their high energy density and high power density. In this work, N and S codoped hollow carbon nanobelts (N/S-HCNs) are synthesized by a self-templated method. The as-synthesized carbon nanobelts exhibit excellent performance in pseudocapacitance and electric double layer anions adsorption. After pairing the N/S-HCNs cathode with a tin foil anode in a carbonate electrolyte, the obtained SIC achieves a high specific capacity of 400 mAh g-1 at 1 A g-1 (based on the mass of cathode material) and energy density of 250.35 Wh kg-1 at 676 W kg-1 (based on the total mass of cathode and anode materials). Besides, the presented SIC also demonstrates high cycling stability with almost 100% capacity retention after 10 000 cycles, which is among the best results of the reported SICs, suggesting the potential for high-performance energy storage applications.

Selenium Edge as a Selective Anchoring Site for Lithium-Sulfur Batteries with MoSe2/Graphene-Based Cathodes.

For lithium-sulfur batteries (LSBs), the dissolution of lithium polysulfide and the consequent "shuttle effect" remain major obstacles for their practical applications. In this study, we designed a new cathode material comprising MoSe2/graphene to selectively adsorb polysulfides on the selenium edges and thus to mitigate their dissolution. More specifically, few-layered MoSe2 was first grown on nitrogen-doped reduced graphene oxide (N-rGO) using the chemical vapor deposition method and then infiltrated with sulfur as the cathode for LSBs. An initial capacity of 1028 mA h g-1 was achieved for S/MoSe2/N-rGO at 0.2 C, higher than 981 and 405.1 mA h g-1 for pure graphene and sulfur, respectively, along with enhanced cycling durability and rate capability. Moreover, the density functional theory simulation, in addition to the experimental adsorption test, X-ray photoelectron spectroscopy analysis, and transmission electron microscopy technique, reveals the dual roles that MoSe2 plays in improving the performance of LSBs by functioning as the binding sites for lithium polysulfides and as the platform that enables fast Li-ion diffusion by reducing its diffusion barrier. The reported finding suggests that the transition-metal selenides could be an efficient alternative material as the cathode for LSBs.

Methacrylated gelatin-embedded fabrication of 3D graphene-supported Co3O4 nanoparticles for water splitting.

We developed a general platform for the fabrication of transition metal oxide nanoparticles supported by a graphene foam (GF) by first coating it with a methacrylated gelatin (GelMA) hydrogel, which served as a 3D matrix for nanoparticle dispersion. The engineered GelMA/GF matrix was hydrophilic with good mechanical strength and high conductivity, therefore providing a good platform for the dispersion of a variety of metal/oxide precursors. Due to this platform, well-dispersed Co3O4 nanoparticles with the smallest size of 3 nm assembled on the nitrogen-doped graphene foam (Co3O4/NGF). The crystalline transformation from a CoCl2[H2O]2 precursor to Co3O4 was revealed by in operando X-ray diffraction and absorption techniques. After applying Co3O4/NGF as a free-standing electrocatalyst for water splitting, the nanoparticles of size 3 nm exhibited optimal catalytic activity in alkaline media; the corresponding cell could promote water splitting at a current density of 10 mA cm-2 with only 1.63 V and exhibited excellent stability in a 25 h long-term operation. Our results demonstrate that the GelMA hydrogel-coated 3D graphene foam can be a promising platform for the design and fabrication of graphene-based multifunctional materials.

Modular functionalization of crystalline graphene by recombinant proteins: a nanoplatform for probing biomolecules.

Graphene, as well as other two-dimensional materials, is a promising candidate for use in bioimaging, therapeutic drug delivery, and bio-sensing applications. Here, we developed a protocol to functionalize graphene with recombinant proteins using genetically encoded SpyTag-SpyCatcher chemistry. SpyTag forms a covalent isopeptide bond with its genetically encoded partner SpyCatcher through spontaneous amidation under physiological conditions. The functionalization protocol developed is based on the use of short proteins as a linker, where two graphene-binding-peptides (GBPs) are attached to both ends of SpyTag (referred to as GStG), followed by the covalent conjugation with SpyCatcher-fusion proteins. The proposed method enables the decoration of crystalline graphene with various proteins, such as fluorescent proteins and affibody molecules that bind to cancerous cells. This scheme, which takes advantage of the cleanness of single-crystal graphene and the robustness of SpyTag-SpyCatcher chemistry, provides a versatile platform on which to study the biomolecule-surface and cell-substrate interactions and, indeed, may lead to a new way of designing biomedical devices. The interaction between peptides and graphene was clearly shown using molecular dynamics simulation and proven using specially designed experiments.

Shuttle Suppression by Polymer-Sealed Graphene-Coated Polypropylene Separator.

"Shuttle effect" of lithium polysulfides (LiPS) leads to a poor performance and a short cycle life of the Li-S battery, thus limiting their practical application. We demonstrate here that after coating polypropylene (PP) separator with a continuous monolayer graphene, the shuttle effect can be significantly suppressed by limiting the passage of long-chain LiPS. The graphene/PP separator can be further modified by sealing the big holes or pores on graphene with in situ polymerized nylon-66 via an interfacial polymerization reaction between diamine and adipoyl chloride supplied by the aqueous and oil phase, respectively, from each side of the membrane. With this engineered membrane, an initial specific capacity of 1128.4 mAh g-1 at 0.05C is achieved after test in a coin cell, higher than that of 983.2 mAh g-1 with pristine PP, along with increased Coulombic efficiency from 96.0 to 99.9% and enhanced cycling durability. Molecular dynamics simulations attest that the nanopores with appropriate size and structure are effective in acting as a "sieve" to selectively allow only Li+ ions to pass through but prevent LiPS from migrating to the anode, consequently alleviating the shuttle effect. Our method provides a facile solution toward the mitigated shuttle effect and eventually contributes to the high performance of Li-S battery.

Recoil Effect and Photoemission Splitting of Trions in Monolayer MoS2.

The 2D geometry nature and low dielectric constant in transition-metal dichalcogenides lead to easily formed strongly bound excitons and trions. Here, we studied the photoluminescence of van der Waals heterostructures of monolayer MoS2 and graphene at room temperature and observed two photoluminescence peaks that are associated with trion emission. Further study of different heterostructure configurations confirms that these two peaks are intrinsic to MoS2 and originate from a bound state and Fermi level, respectively, of which both accept recoiled electrons from trion recombination. We demonstrate that the recoil effect allows us to electrically control the photon energy of trion emission by adjusting the gate voltage. In addition, significant thermal smearing at room temperature results in capture of recoil electrons by bound states, creating photoemission peak at low doping level whose photon energy is less sensitive to gate voltage tuning. This discovery reveals an unexpected role of bound states for photoemission, where binding of recoil electrons becomes important.

Concurrent fast growth of sub-centimeter single-crystal graphene with controlled nucleation density in a confined channel.

The synthesis of large-domain-sized graphene requires a low nucleation density, which inevitably leads to a reduced growth rate. To achieve both a large domain size and high growth rate, we designed a simple channel structure that allowed us to control the nucleation density by tuning the flow dynamics and by introducing an additional catalyst inside to control the growth kinetics at the same time. The designed channel structure plays three roles in the growth of graphene: (1) it retains oxygen to passivate the active nucleation sites; (2) it restricts the mass transfer of CH4 to control the supersaturation for nucleation; and (3) it provides additional catalytic sites for the decomposition of CH4 to boost the graphene growth rate. Our strategy allowed the successful preparation of sub-centimeter-domain-sized graphene in 1 h with an average growth rate of 70 µm min-1, and with a hole mobility of 5500 cm2 V-1 S-1, which is sufficient for practical applications. Our method paves the way for the large-scale production of single-crystal graphene or other 2D materials at a highly efficient level.

Polymer-confined growth of perforated MoSe2 single-crystals on N-doped graphene toward enhanced hydrogen evolution.

The edge and corner atoms of 2D transition metal dichalcogenides (TMDCs) are the main electrocatalytically active sites for electrochemical reaction. Here, we demonstrate an approach to generate abundant edge/corner atoms in molybdenum diselenide (MoSe2) nanocrystals supported by nitrogen-doped graphene (NG) which consequently leads to significantly enhanced hydrogen evolution reaction (HER) activity. These structures were fabricated by firstly absorbing the Mo-containing precursor within polymer-functionalized graphene oxide, then selenized to obtain MoSe2 nanocrystals on the surface, and finally H2 etching was performed to form perforated structures. The use of a functional polymer as an absorption matrix efficiently mitigates aggregation which allows us to obtain MoSe2 single-crystals of ∼150 nm in lateral dimension, while maintaining high MoSe2 loading. We observed a remarkably enhanced electrocatalytic activity resulting from a significantly increased abundance of edge/corner atoms in hydrogen evolution measurements. Specifically, with this perforated MoSe2/NG-modified cathode, current densities of -1 and -10 mA cm-2 were realized with the overpotentials of only 30 and 106 mV, along with a small Tafel slope of 57 mV dec-1 and large exchange current density of 127.4 µA cm-2 in 0.5 M H2SO4. Such an efficient strategy also opens doors for the unparalleled design and fabrication of TMDC-based nanocomposites for electrochemical applications.

Polymer-Embedded Fabrication of Co2P Nanoparticles Encapsulated in N,P-Doped Graphene for Hydrogen Generation.

We developed a method to engineer well-distributed dicobalt phosphide (Co2P) nanoparticles encapsulated in N,P-doped graphene (Co2P@NPG) as electrocatalysts for hydrogen evolution reaction (HER). We fabricated such nanostructure by the absorption of initiator and functional monomers, including acrylamide and phytic acid on graphene oxides, followed by UV-initiated polymerization, then by adsorption of cobalt ions and finally calcination to form N,P-doped graphene structures. Our experimental results show significantly enhanced performance for such engineered nanostructures due to the synergistic effect from nanoparticles encapsulation and nitrogen and phosphorus doping on graphene structures. The obtained Co2P@NPG modified cathode exhibits small overpotentials of only -45 mV at 1 mA cm(-2), respectively, with a low Tafel slope of 58 mV dec(-1) and high exchange current density of 0.21 mA cm(-2) in 0.5 M H2SO4. In addition, encapsulation by N,P-doped graphene effectively prevent nanoparticle from corrosion, exhibiting nearly unfading catalytic performance after 30 h testing. This versatile method also opens a door for unprecedented design and fabrication of novel low-cost metal phosphide electrocatalysts encapsulated by graphene.

Grain size control in the fabrication of large single-crystal bilayer graphene structures.

Bilayer graphene (BLG) can provide a tunable band gap when exposed to a vertical electric field. We report here an approach to the synthesis of large single-crystal BLG structures with diameters up to 0.54 mm. We found that both absorption-diffusion and gas-phase penetration mechanisms contributed to the growth of the lower second layer and that the absorption-diffusion mechanism favors faster BLG growth. Our strategy was to suppress nucleation in the growth of the first layer using an established surface oxidation method to maintain a low coverage of graphene on Cu foil. We subsequently maximized the growth of the second layer by increasing the duration of absorption-diffusion. The chemical treatment used to polish the Cu surface to reduce the nucleation of growth in the monolayer increased the nucleation density during the growth of the second layer. Electron transport measurements on dual-gated field-effect transistors showed that the BLG fabricated was of high quality with a sizeable tunable band gap. Our approach may have broad applications for the controlled synthesis of bilayers in materials chemistry.