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We synthesize the size-tailored Ce:Li4Ti5O12 via a facile solid-state method. During the synthesis, a carbon-coating forms in-situ on the Ce:Li4Ti5O12 particles to limit their grain growth. The coating is later removed to facilitate the Li+ diffusion during charge/discharge. The composition, structure, and morphology of the obtained samples are characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, inductively coupled plasma elemental analysis, thermal gravimetric analysis, scanning electron microscopy, and transmission electron microscopy. The results confirm the particle-size tailoring effect induced by the carbon-coating. The size-tailored Ce:Li4Ti5O12 electrode demonstrates high reversible capacity of 164, 161, 158, 149, 136, and 116 mAh g−1 at 0.5C, 1C, 2C, 5C, 10C, and 20C, respectively, which are significantly improved from the pristine Li4Ti5O12. Moreover, it is also capable of maintaining 96% and 92% of the initial capacity at the high rate of 10C and 20C after 1000 cycles, exhibiting excellent cycling stability. Finally, we fabricate a full cell with LiFePO4 cathode, which delivers reversible capacity of 110 mAh g−1 at 5C with little capacity fading after 1200 cycles, confirming the outstanding electrochemical performance of size-tailored Ce:Li4Ti5O12 anode.

We report a facile synthetic method to produce the interconnected hollow carbon spheres with various wall-thickness (as low as 9 nm). Instead of synthesizing and dispersing the silica hard templates in advance, we produce the templates in-situ to form the interconnected hollow-sphere structure. This interconnected structure provides a fast and efficient pathway for electron transfer. Meanwhile, decreasing the wall-thickness effectively shortens the distance of ion diffusion, which further promotes the high-rate performances of the interconnected hollow carbon spheres. For supercapacitor applications, at 1 A g−1, the MF-1.1-20 (with 9-nm wall-thickness) electrode demonstrates a high specific capacitance of 208 F g−1 and retains 93% of its initial capacity after 10000 cycles. Under a large current density of 20 A g−1, MF-1.1-20's specific capacity remains 153 F g−1, demonstrating excellent high-rate performances. In another aspect, as the electrode for lithium-sulfur battery, MF-1.1-20/S composite electrode demonstrates high specific capacity of 996 mAh g−1 and 465 mAh g−1 at 0.2C and 10C, respectively. The excellent electrochemical behavior of the interconnected hollow carbon spheres demonstrates their potential for applications in high-rate energy storage devices.

We use a redox mediator pair - hydroquinone (for positive electrode) and sodium anthraquinone-2-sulfonate (for negative electrode) - to treat the activated carbon electrodes for fabricating asymmetric supercapacitors. This simple adsorption-based treatment improves the specific capacity of the activated carbon electrode from 69.3 mAh g−1 to 300.4 mAh g−1 (negative electrode) and 63.5 mAh g−1 to 301.1 mAh g−1 (positive electrode). The charge balance between the two electrodes is achieved by synchronous potential measurements on redox reaction of hydroquinone and anthraquinone-2-sulfonate. Under 5 A g−1, the asymmetric supercapacitor delivers a high energy density of 37.1 Wh kg−1 with the power density of 2087 W kg−1. Meanwhile, the asymmetric supercapacitor demonstrates excellent cycling stability, capable of retaining 81.8% of its capacity after 20000 cycles at 10 A g−1. A flexible solid-state asymmetric supercapacitor is fabricated with the treated electrode-pair and exhibits excellent flexing stability, high volumetric energy density (2.20 mWh cm−3 at 0.017 W cm−3), and good cycling stability (>91% capacity retention after 3000 cycles), which holds great potential for application in wearable electronics.

Combining graphene with iron oxide has shown great promise in mitigating the pulverization of iron-oxide anode during fast charge/discharge in lithium-ion batteries. However, the attachment between hydrophilic iron oxide and hydrophobic graphene sheets could potentially compromise during cycling, affecting its cycling stability. By exploiting the polymerization characteristics of pyrrole, we anchor and subsequently wrap the Fe(OH)3 colloidal nanoparticles onto graphene oxide surface with polypyrrole. The composite is further annealed to give polypyrrole-derived-carbon (PPy-C) wrapped Fe3O4@graphene (Fe3O4@graphene@PPy-C). PPy-C strengthens the interaction between the Fe3O4 and the graphene and helps preserving the structural integrity during cycling. As an anode material, Fe3O4@graphene@PPy-C composite exhibits a high reversible capacity of 721 mA h g−1 over 320 cycles at a current density of 0.2 A g−1. Even at a high current density of 2 A g−1, the electrode still achieves a high capacity of 406 mA h g−1. This strategy provides a new alternative to improve the cycling stability of iron-oxide/carbon composite anodes.

We prepare a novel redox-additive organic electrolyte containing phenylenediamine for a graphene-hydrogel supercapacitor. By adopting this electrolyte, significant capacity enhancement (47–596% increase) is achieved. At the concentration of 0.02 mol l−1, the specific capacity value of the graphene-hydrogel electrode reaches 171 mAh g−1, 287 mAh g−1, and 353 mAh g−1 at 1 A g−1 for o-phenylenediamine (OPD), m-phenylenediamine (MPD), and p-phenylenediamine (PPD), respectively. The enhancement by PPD is more significant than OPD and MPD at concentrations higher than 0.02 mol l−1. This behavior is likely caused by the para-amino groups, which exhibits less stereo-hindrance during the absorption of phenylenediamine onto the graphene surface. In the electrolyte containing 0.04 mol l−1 of PPD, specific capacity of 516 mAh g−1 at 1 A g−1 and energy density of 143 Wh kg−1 with power density of 1.11 kW kg−1 are achieved. Meanwhile, 93.8% of the electrode's initial capacity (433 mAh g−1) is retained after 5000 cycles at 2 A g−1, demonstrating its excellent cycling stability.

The application of hollow carbon spheres in energy storage devices is still hindered by their relatively low electron transmission and ion diffusion kinetics, as well as their insufficient storage sites. Herein, we prepare interconnected porous honeycomb hollow carbon spheres (IPHHCSs) using hard-templating route by increasing the concentration of template spheres and introducing cationic surfactants simultaneously. The as-obtained IPHHCSs show favorable features for ESD, such as large contacted areas between spheres, high specific surface areas (951 m2 g−1), hierarchical porous frameworks with high volume of mesopores, and rich N-doping (3.36%), achieving a lower interface-resistance between spheres, more ion-storage sites, and developed mesoporous channels for rapid ions diffusion. Thus, such a unique carbon architecture endows IPHHCSs a superior rate capability for supercapacitor and lithium-sulfur battery applications. The IPHHCSs supercapacitor’s electrode delivers a high specific capacitance of 295F g−1 at 1 A g−1, an outstanding rate capability (87% capacitance retention from 1 A g−1 to 20 A g−1), with a high cycling life stability (1.2% loss after 10,000 cycles). As for lithium-sulfur batteries, the IPHHCSs/S cathode with sulfur loading of 70% demonstrated an initial reversible capacity of 1160 mAh g−1 at 0.2C and an excellent rate performance with a capacity retention of 53% at 10C. More notably, IPHHCSs/S electrode could also enable a remarkable cyclic stability that possesses a reversible capacity of 285 mAh g−1 at 10C upon 1000 cycles with a low capacity degradation of 0.042% per cycle. Controllable design of interconnected porous HCSs structures could decrease interface-resistance and promote ion diffusion, which can be an effective way for developing high-rate supercapacitors and Li–S batteries.

Recently, polyaniline@polyvinyl alcohol (PANI@PVA) composite materials are favored by many researchers due to the synergistic properties of conductive PANI and flexible PVA. Although PANI@PVA composite materials have been studied widely as electrodes for flexible supercapacitors, it is still a challenge to prepared PANI@PVA hydrogel electrodes with long cycle life. In this work, vertically ordered PANI@PVA hydrogel electrodes with an excellent stability were fabricated via electropolymerizing PANI within a template of vertically ordered PVA hydrogel for the first time. In brief, vertically ordered PVA hydrogel coating on a graphite paper collector was prepared through a repeated freezing-thawing method. Then PANI grew within the template of the vertical PVA hydrogel by electro-polymerization. When using the vertically ordered PANI as electrodes, PVA not only worked as gel electrolyte, but also provided support for excellent mechanical flexibility and durability of the electrodes. The vertical PANI@PVA electrode performed a much longer cyclic life (80% retention after 15,000 cycles) than a pure PANI electrode (80% retention after 3,000) and a disordered PANI@PVA electrode (80% retention after 4,300 cycles), benefiting from the vertical structure supported by the PVA hydrogel. Furthermore, asymmetric flexible supercapacitors (AFSCs) had been fabricated with the vertical PANI@PVA hydrogel anodes and activated carbon cathodes. The AFSCs showed excellent flexibility, high area capacity of 244 mC cm−2 at 1 mV s−1 (with a specific capacitance of 244 mF cm−2 for a voltage of 1.0 V), and superior cyclic stability with a capacitance retention of 80% after 7,000 cycles. These results strongly prove that using the polyaniline electropolymerized within a template of vertically ordered PVA as electrodes of flexible supercapacitors has a great potential in actual application.

For improving the energy density of carbon-based supercapacitors, the highly reversible redox mediators with giant pseudocapacitance have been studied extensively. In this work, the dual-redox combination of sodium anthraquinone-2-sulfonate (AQS) and potassium bromide (KBr) is introduced into the carbon-based supercapacitors to enhance the electrochemical performances. In the dual-redox enhanced supercapacitors (DRESc), the performances of negative and positive electrodes are improved by AQS and KBr, respectively. In order to use AQS and KBr rationally and effectively, two different approaches are studied: (I) redox mediators are adsorbed in the electrodes; (II) redox mediators are added into the electrolyte. The experimental results demonstrate that the combination of adsorbing AQS onto the negative electrode (the approach I) and adding KBr in the electrolyte (the approach II) is the best approach. Furthermore, electrochemical results prove that it is an optimal choice to adsorb 3 mg AQS for the negative electrode in this work. Meanwhile, to match the AQS-enhanced negative electrode, synchronous potential measurements indicate that the optimal choice for the positive electrode is adding 1.5 M KBr in the 1 M H2SO4 electrolyte. Finally, the DRESc achieved a maximum gravimetric energy density (EG) of 32.05 Wh kg−1 at 0.5 A g−1 and a maximum gravimetric power density (PG) of 2200 W kg−1 at 8 A g−1, implying that the AQS-Br dual-redox system is a promising candidate for energy storage and conversion.

Flexible solid-state supercapacitors provide a promising energy-storage alternative for the rapidly growing flexible and wearable electronic industry. Further improving device energy density and developing a cheap flexible current collector are two major challenges in pushing the technology forward. In this work, we synthesize a nitrogen-doped graphene/MnO2 nanosheet (NGMn) composite by a simple hydrothermal method. Nitrogen-doped graphene acts as a template to induce the growth of layered δ-MnO2 and improves the electronic conductivity of the composite. The NGMn composite exhibits a large specific capacitance of about 305 F g–1 at a scan rate of 5 mV s–1. We also create a cheap and highly conductive flexible current collector using Scotch tape. Flexible solid-state asymmetric supercapacitors are fabricated with NGMn cathode, activated carbon anode, and PVA–LiCl gel electrolyte. The device can achieve a high operation voltage of 1.8 V and exhibits a maximum energy density of 3.5 mWh cm–3 at a power densit...

Monitoring the environmental humidity are of great significance to human comfort and product quality in many fields. However, high performance humidity sensors are difficult to be widely used because of their cost and complex preparation process. It is also highly desirable to develop a facile fabrication method that is capable of producing humidity sensors of customized size, shape, and geometry. Here, using a low power DVD drive, we successfully fabricate a highly sensitivity and low-cost humidity sensor, consisting of laser-scribed graphene (LSG) electrode and graphite oxide (GO) sensing layer. The LSG-GO humidity sensor operates in the relative humidity (RH) range of 19 %–97.4 %, demonstrating high sensitivity (4770.14 pF/% RH), quick response/recovery, and excellent sensing stability. In addition, the possible humidity sensing mechanism is discussed with the help of impedance spectroscopy. These results indicate that the LSG/GO sensor, enabled by the facile fabrication process with low power DVD drives, demonstrates promising potential with broad application prospects.

Expanders are important additives for lead-acid battery, which reduce the degree of compaction within negative plates and provide more ionically transport channels within the plates during the charge/discharge process. Lignosulfonates (LS) with a 3D structure is one of the most often used expander, which ensures the low compaction of negative plate. However, the insulativity of LS increase cathodic polarization and retard the conversion of PbSO4 to Pb during charging progress, which intensify the PbSO4 accumulation in the negative plate and shorten the HRPSoC cycle life of the battery. Focusing on the problems, we used polyaniline-modified lignosulfonates (PANI/LS) composites as expanders. The composites inherit good electronic conductivity of PANI and ionic transport channels from the LS with a 3D structure. Furthermore, the PANI/LS composites havea higher hydrogen evolution over-potential than LS, which avoids the electrolyte dry-out during charging process. The battery tests indicate PANI/LS can refine the size of PbSO4 particles and promote the conversion of PbSO4 to Pb. The HRPSoC cycle life of battery with 2PANILS composite expander is 7.3 times longer than that of the battery with LS expander. The negative active material utilization (UNAM) of the battery containing 2PANILS composite expander is 2 times larger than the battery with LS expander. In addition, the preparation process for the negative plate with 2PANI-LS expander is facile and has potentials for large-scale industrial production.