Weixin Song

A carbon quantum dot decorated RuO2 network: outstanding supercapacitances under ultrafast charge and discharge

Carbon quantum dots (CQDs) due to their unique properties have recently attracted extensive attention from researchers in many fields. In the present work, a new application in the form of a CQD-based hybrid as an excellent electrode material for supercapacitors is reported for the first time. The CQDs are fabricated by a facile chemical oxidation method following which they are thermally reduced, and further decorated with RuO2 to obtain the composites. The hybrid exhibits a specific capacitance of 460 F g−1 at an ultrahigh current density of 50 A g−1 (41.9 wt% Ru loading), and excellent rate capability (88.6, 84.2, and 77.4% of capacity retention rate at 10, 20, and 50 A g−1 compared with 1 A g−1, respectively). Surprisingly, the hybrid shows exceptional cycling stability with 96.9% capacity retention over 5000 cycles at 5 A g−1. Such remarkable electrochemical performances can be primarily ascribed to the significantly enhanced utilization of RuO2 achieved by the efficient dispersion of tiny reduced CQDs and the formation of a CQD-based hybrid network structure that can facilitate the fast charge transport and ionic motion during the charge–discharge process. Additionally, the contact resistance at the interface between active materials and current collectors is concluded to be a key factor in determining the performance of the hybrid. These results above demonstrate the great potential of CQD-based hybrid materials in the development of high-performance electrode materials for supercapacitors.

Porous NiCo2O4 spheres tuned through carbon quantum dots utilised as advanced materials for an asymmetric supercapacitor

Carbon quantum dots (CQDs) tuned porous NiCo2O4 sphere composites are prepared for the first time via a reflux synthesis route followed by a post annealing treatment. Benefiting from the advantages of the unique porous structure with a large specific surface area, high mesoporosity and superior electronic conductivity, the as-obtained CQDs/NiCo2O4 composite electrode exhibits high specific capacitance (856 F g−1 at 1 A g−1), excellent rate capability (83.9%, 72.5% and 60.8% capacity retention rate at 20, 50 and 100 A g−1, respectively) and exceptional cycling stability (98.75% of the initial capacity retention over 10000 cycles at 5 A g−1). Furthermore, the assembled AC//CQDs/NiCo2O4 asymmetric supercapacitor manifests a high energy density (27.8 W h kg−1) at a power density of 128 W kg−1 or a high power density (10.24 kW kg−1) at the reasonable energy density of 13.1 W h kg−1 and remarkable cycling stability (101.9% of the initial capacity retention over 5000 cycles at 3 A g−1). The results above suggest a great potential of the porous CQDs/NiCo2O4 composites in the development of high-performance electrochemical energy storage devices for practical applications.

Sodium/Lithium storage behavior of antimony hollow nanospheres for rechargeable batteries.

Sodium-ion batteries (SIBs) have come up as an alternative to lithium-ion batteries (LIBs) for large-scale applications because of abundant Na storage in the earths crust. Antimony (Sb) hollow nanospheres (HNSs) obtained by galvanic replacement were first applied as anode materials for sodium-ion batteries and exhibited superior electrochemical performances with high reversible capacity of 622.2 mAh g(-1) at a current density of 50 mA g(-1) after 50 cycles, close to the theoretical capacity (660 mAh g(-1)); even at high current density of 1600 mA g(-1), the reversible capacities can also reach 315 mAh g(-1). The benefits of this unique structure can also be extended to LIBs, resulting in reversible capacity of 627.3 mAh g(-1) at a current density of 100 mAh g(-1) after 50 cycles, and at high current density of 1600 mA g(-1), the reversible capacity is 435.6 mAhg(-1). Thus, these benefits from the Sb HNSs are able to provide a robust architecture for SIBs and LIBs anodes.

First exploration of Na-ion migration pathways in the NASICON structure Na3V2(PO4)3

Ion occupation and migration pathways are investigated to explore the ion-migration mechanism of Na3V2(PO4)3 with the help of first principles calculations. Na3V2(PO4)3 with a NASICON framework generates high performances as a cathode material in sodium-ion batteries.

Sb porous hollow microspheres as advanced anode materials for sodium-ion batteries

Sb porous hollow microspheres (PHMSs) were prepared by a replacement reaction employing Zn microspheres (MSs) as templates. The obtained Sb PHMSs were first applied as anode materials for sodium-ion batteries (SIBs) and showed a high reversible capacity of 617 mA h g−1 at a current density of 100 mA g−1 after 100 cycles, exhibiting a high capacity retention of 97.2%. Even at a high current density of 3200 mA g−1, the reversible capacity can also reach 312.9 mA h g−1. The superior electrochemical performance of Sb PHMSs can be attributed to the unique structural characteristic of Sb with porous and hollow structure, which can accommodate the volume change and facilitate the Na+ diffusion during the sodiation and desodiation process.

Multifunctional dual Na3V2(PO4)2F3 cathode for both lithium-ion and sodium-ion batteries

Na3V2(PO4)2F3 with a NASICON-type structure is shown to be synthesised with the particle surface found to be coated with amorphous carbon with its thickness in the range of 25–32 nm. The crystallographic planes (hkl) are labelled according to Density Functional Theory (DFT) calculations towards the as-prepared Na3V2(PO4)2F3. The performances of Na3V2(PO4)2F3 have been investigated in lithium- and sodium-ion batteries, exhibiting a specific capacity of 147 mA h g−1 with an average discharge plateau around 4 V vs. Li+/Li, and 111.5 mA h g−1 with three discharge plateaus in sodium-ion batteries. A predominant Li ion insertion mechanism is verified by comparing the redox potentials from CV and charge/discharge curves. It is found that the main migration from/into the crystallographic sites of Na3V2(PO4)2F3 of Li ions is favoured to obtain satisfactory properties by a two-step process, while the Na ions are found to require three steps. The stable and three-dimensional open framework of Na3V2(PO4)2F3 is considered to be vital for the excellent C-rate and cycling performances, as well as the fast ion diffusion with a magnitude of 10−11 cm2 s−1, which could demonstrate that Na3V2(PO4)2F3 is a multifunctional dual cathode for both lithium and sodium ion batteries and capable to be a promising candidate in the construction of high-energy batteries.

Na2FePO4F cathode utilized in hybrid-ion batteries: a mechanistic exploration of ion migration and diffusion capability

Layered Na2FePO4F is utilized as a cathode in hybrid-ion batteries in order to explore the ion migration and diffusion capability. It is the first time that the ion migration mechanism and capability in a hybrid-ion battery is investigated by considering the activation energies of different migration ways. It is proposed that a rapid ion exchange of Na+ ions on the Na(2) site of the crystal structure with Li+ ions can take place to produce the NaLiFePO4F phase and is firstly confirmed by first principle calculations. Li+ ion conduction in NaLiFePO4F is prone to be two-dimensional (2D) in the interlayer plane with an essentially restricted migration along the [010] direction for interlayer transport due to the much higher energy value (4.53 eV for sodium ion and 1.63 eV for lithium ion). Additionally, the 2D ways which need lower activation energies along [100] and [001] directions and the small volume variation during redox cycling are responsible for the large diffusion capability with a maximum magnitude of 10−10 cm2 s−1.

Graphene ultracapacitors: structural impacts

The structural effects of graphene on the electrochemical properties of graphene-based ultracapacitors are investigated for the first time, where the competitive impacts resulting from the edge content, specific surface area, edge/basal defects, oxygen-containing groups and metal oxides/surfactant impurities are taken into consideration, demonstrating that not one element, but all are responsible for the final behavior of graphene-based ultracapacitors. This work will be of wide importance to research producing graphene-based energy storage/generation devices.

Investigation of the Sodium Ion Pathway and Cathode Behavior in Na3V2(PO4)2F3 Combined via a First Principles Calculation

The electrochemical properties of Na3V2(PO4)2F3 cathode utilized in the sodium ion battery are investigated, and the ion migration mechanisms are proposed as combined via the first principles calculations. Two different Na sites, namely, the Na(1) and Na(2) sites, could cause two sodium ions of Na3V2(PO4)2F3 to be extracted or inserted by a two-step electrochemical process accompanied by structural reorganization that could be responsible for the redox reaction of V(3+/4+). Because the calculated average voltage (V(avg)) of the second charging plateau is 4.04 V for the optimized system but 4.38 V for the unoptimized one, the reorganization of the cathode system can make a stable configuration and lower the extraction energy. Three designed pathways for sodium ions along the x, y, z directions in Na3V2(PO4)2F3, known as a 3D ions transport tunnel, have activation energies (Ea) of 0.449, 0.2, and 0.323 eV, respectively, by using DFT calculations, demonstrating the different feasibilities of the migration directions.

Recent development of LiNixCoyMnzO2: Impact of micro/nano structures for imparting improvements in lithium batteries

The recent advancement in the design, synthesis, and fabrication of micro/nano structured LiNixCoyMnzO2 with one-, two-, and three-dimensional morphologies was reviewed. The major goal is to highlight LiNixCoyMnzO2 materials, which have been utilized in lithium ion batteries with enhanced energy and power density, high energy efficiency, superior rate capability and excellent cycling stability resulting from the doping, surface coating, nanocomposites and nano-architecturing.