Lithium–sulfur (Li–S) batteries are highly attractive as energy storage devices due to their low cost and high energy density. The undesired capacity degradation caused by the polysulfide shuttle, however, has hindered their commercialization. Herein, a Co3O4 nanoneedle array on carbon cloth (CC@Co3O4) nanocomposite has been prepared and demonstrated for the first time as a multifunctional “super-reservoir” electrode to prolong the cycle life of Li–S batteries. Owing to the polar surface of the Co3O4 nanoneedle array, soluble lithium polysulfides (Li2Sn, 4 < n < 8) can be effectively absorbed and then transformed to insoluble Li2S2/Li2S which evenly covers the surface of the Co3O4 nanoneedle during the discharge process. Further, during the charge process, the Co3O4 nanoneedle can catalyze the electrochemical transformation of Li2S2/Li2S into soluble polysulfides. A high initial capacity of 1231 mA h g−1 at 0.5C and a slow capacity decay of 0.049%/cycle at 2.0C over 500 cycles were achieved; excellent rate performance was also obtained.

High energy density batteries and high power density supercapacitors have attracted much attention because they are crucial to the power supply of future portable electronic devices, electric automobiles, unmanned aerial vehicles, etc. The electrode materials are key components for batteries and supercapacitors, which influence the practical energy and power density. Metal-organic frameworks possessing unique morphology, high specific surface area, functional linkers, and metal sites are excellent electrode materials for electrochemical energy storage devices. Herein, we review and comment on recent progress in metal-organic framework-based lithium-ion batteries, sodium-ion batteries, lithium-air batteries, lithium-sulfur/selenium batteries, and supercapacitors. Future perspectives and directions of metal-organic framework-based electrochemical energy storage devices are put forward on the basis of theoretical knowledge from the reported literature and our experimental experience.

Two-dimensional (2D) carbon materials have attracted intense research interest for electrical double layer capacitors (EDLCs) due to their high aspect ratio and large surface area. Herein, we propose an exfoliation–chlorination route for preparing ultrathin carbon nanosheets by using ternary layered carbide Ti3AlC2 as the precursor. Due to the large intersheet space of exfoliated layered carbide (MXene), the as-prepared carbon nanosheets exhibit a thickness of 3–4 nm and a large specific surface area of 1766 m2 g−1 with hierarchical porosity. These features significantly improve the ion-accessible surface area for charge storage and shorten the ion transport length in the thin dimension. As a result, the carbon nanosheets show a high specific capacitance (220 F g−1 at 0.5 A g−1), remarkable high power capability (79% capacitance retention at 20 A g−1) when measured in a symmetrical two-electrode configuration in an aqueous electrolyte. The method described in this work provides a new route to prepare 2D electrode materials from a bulk precursor, thus exploiting their full potential for EDLCs.

•MXene/LDH was prepared by liquid phase deposition method.•LDH platelets homogeneously grown on e-MXene substrate forms a 3D porous structure.•3D porous structure facilitates active sites exposure and electrolyte penetration.•MXene/LDH shows excellent electrochemical properties for supercapacitors.In this work, an exfoliated MXene (e-MXene) nanosheets/nickel-aluminum layered double hydroxide (MXene/LDH) composite as supercapacitor electrode material is fabricated by in situ growth of LDH on e-MXene substrate. The LDH platelets homogeneously grown on the surface of the e-MXene sheets construct a three-dimensional (3D) porous structure, which not only leads to high active sites exposure of LDH and facile liquid electrolyte penetration, but also alleviates the volume change of LDH during the charge/discharge process. Meanwhile, the e -MXene substrate forms a conductive network to facilitate the electron transport of active material. The optimized MXene/LDH composite exhibits a high specific capacitance of 1061 F g−1 at a current density of 1 A g−1, excellent capacitance retention of 70% after 4000 cycle tests at a current density of 4 A g−1 and a good rate capability with 556 F g−1 retention at 10 A g−1.Three-dimensional porous MXene/nickel-aluminum layered double hydroxide (MXene/LDH) composite prepared through facile liquid deposition method with high electro-active surface and enhanced electrical conductivity, is demonstrated as promising electrode material for high performance supercapacitors.

Lithium–sulfur (Li–S) batteries as lithium secondary batteries have drawn tremendous interest due to their high theoretical specific capacity and energy density. However, the low practical specific capacity and poor cycling life keep them from large scale usage. Herein, a novel binder based on a mixture of polyacrylic acid (PAA) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is designed to significantly improve the specific capacity and cycling stability of Li–S batteries via the synergistic effect of the different functional groups. The conductive PEDOT:PSS successfully facilitates electron transfer and prevents polysulfide dissolution. PAA improves the solvent system for sulfur cathodes and promotes lithium-ion transfer. The sulfur cathode with PAA/PEDOT:PSS binder in a ratio of 2:3 exhibits an initial specific capacity of 1121 mA h g−1 and 830 mA h g−1 after 80 cycles at 0.5C. The electrochemical performance of the sulfur cathode with the composite binder is better than either of the single-component binders.

In this work, phytic acid is used as the protonic acid dopant and soft template to synthesize 3D polyaniline (PANI) nanofiber networks. Then, the PANI nanofiber networks are transformed to porous nitrogen and phosphorus co-doped carbon nanofibers (P-NP-CNFs) by the carbonization and chemical activation process. P-NP-CNFs have a high specific surface area of 2586 m2 g−1 and a large pore volume of 1.43 cm3 g−1, which are favorable for enhancing the electrochemical performance of electrical double layer capacitors. Moreover, nitrogen and phosphorus doping in the carbon materials can increase the specific capacitance by a pseudocapacitive redox process. At a current density of 1 A g−1, P-NP-CNFs show a large specific capacitance of 280 F g−1 and a high specific capacitance retention of 94% after 10000 cycles. In particular, phosphorus doping can broaden the electrochemical window to increase the energy density. Therefore, the energy density of symmetric capacitors based on P-NP-CNFs is up to 22.9 W h kg−1 at a power density of 325 W kg−1, demonstrating that P-NP-CNFs are superior electrode materials for electrical double layer capacitors.

Porous nitrogen-doped hollow carbon spheres (PNHCS) had been prepared by pyrolysis of hollow polyaniline spheres (HPS), which were synthesized by the use of sulfonated polystyrene spheres (SPS) as a hard template. PNHCS have a specific surface area of 213 m2 g−1 and a pore volume of 0.24 cm3 g−1. At a current density of 0.5 A g−1, the specific capacitance of the PNHCS prepared is ca. 213 F g−1. The capacity retention after 5000 charge/discharge cycles at a current density of 1 A g−1 is more than 91%. The enhanced electrochemical performance can be attributed to the unique carbon nanostructure and nitrogen-doping of the PNHCS electrodes. The hollow macro-structure plays the role of an “ion-buffering” reservoir. The micropores of the PNHCS enlarge the specific surface area, while the mesopores offer larger channels for liquid electrolyte penetration. Nitrogen groups in the PNHCS not only improve the wettability of the carbon surface, but also enhance the capacitance by addition of a pseudocapacitive redox process.

The flexible electrodes have important potential applications in energy storage of portable electronic devices for their powerful structural properties. In this work, unique flexible films with polypyrrole/carbon nanotube (PPy/CNT) composite homogeneously distributed between graphene (GN) sheets are successfully prepared by flow-assembly of the mixture dispersion of GN and PPy/CNT. In such layered structure, the coaxial PPy/CNT nanocables can not only enlarge the space between GN sheets but also provide pseudo-capacitance to enhance the total capacitance of electrodes. According to the galvanostatic charge/discharge analysis, the mass and volume specific capacitances of GN-PPy/CNT (52 wt% PPy/CNT) are 211 F g−1 and 122 F cm−3 at a current density of 0.2 A g−1, higher than those of the GN film (73 F g−1 and 79 F cm−3) and PPy/CNT (164 F g−1 and 67 F cm−3). Significantly, the GN-PPy/CNT electrode shows excellent cycling stability (5% capacity loss after 5000 cycles) due to the flexible GN layer and the rigid CNT core synergistical releasing the intrinsic differential strain of PPy chains during long-term charge/discharge cycles.Graphical abstractA flexible film with coaxial polypyrrole/carbon nanotube (PPy/CNT) nanocables uniformly distributed between graphene (GN) sheets was prepared by a flow-assembly method, in which flexible GN layer and rigid CNT core could synergistical release the intrinsic differential strain of PPy chains during long-term charge/discharge cycles.Highlights► A hierarchical flexible film composed of GN, PPy and CNT was fabricated. ► The coaxial PPy/CNT nanocables homogeneously insert into the GN sheets. ► The novel film has unique advantages to deliver good electrochemical performance. ► The mass and volume specific capacitances of GN-PPy/CNT reach 211 F g−1 and 122 F cm−3. ► The GN-PPy/CNT shows only 5% capacity loss after 5000 cycles.

This study describes a simple and effective ionic liquid (IL)-assisted mechanochemical route to prepare a set of nanostructured graphene nanosheet/polypyrrole (GNS/PPy) composites of with different PPy loading. The functionalized IL 1-butyl-3-methylimidazolium tetrachloroferrate (Bmim[FeCl4]) used here acts as not only the dispersant of GNS but also the catalyst and dopant in the synthesis of PPy. FTIR illustrates the presence of PPy in the composites. A comparative study performed on composites and pure PPy has led to two main conclusions: on one hand, the microstructure of the GNS/PPy composites is dependent on the loading of PPy. On the other hand, GNS/PPy composites show improved conductivity and thermal stability compared with pure PPy.Highlights► GNS/PPy composites with hierarchical architectures are prepared. ► The IL not only the dispersant of GNS but also the catalyst and dopant of PPy. ► The microstructure of the GNS/PPy composites is dependent on the loading of PPy. ► GNS/PPy composites show improved conductivity and thermal stability over pure PPy.

A film composed of graphene (GN) sheets, polyaniline (PANI) and carbon nanotubes (CNTs) has been fabricated by reducing a graphite oxide (GO)/PANI/CNT precursor prepared by flow-directed assembly from a complex dispersion of GO and PANI/CNT, followed by reoxidation and redoping of the reduced PANI in the composite to restore the conducting PANI structure. Scanning electron microscope images indicate that the ternary composite film is a layered structure with coaxial PANI/CNT nanocables uniformly sandwiched between the GN sheets. Such novel hierarchical structure with high electrical conductivity perfectly facilitates contact between electrolyte ions and PANI for faradaic energy storage and efficiently utilizes the double-layer capacitance at the electrode–electrolyte interfaces. The specific capacitance of the GN/PANI/CNT estimated by galvanostatic charge/discharge measurement is 569 F g−1 (or 188 F cm−3 for volumetric capacitance) at a current density of 0.1 A g−1. In addition, the GN/PANI/CNT exhibits good rate capability (60% capacity retention at 10 A g−1) and superior cycling stability (4% fade after 5000 continuous charge/discharge cycles).Graphical abstractA hierarchical film with coaxial polyaniline/carbon nanotube (PANI/CNT) nanocables uniformly sandwiched between graphene (GN) sheets was prepared by filtration of the complex dispersion of graphite oxide (GO) and PANI/CNT.Highlights► A film composed of GN sheets, PANI and CNTs was fabricated. ► The coaxial PANI/CNT nanocables uniformly sandwiched between the GN sheets. ► The unique structure facilitates contact between electrolyte and electrode materials. ► Each component provides unique function to achieve superior electrochemical properties.

A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flow-directed assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g−1 at 0.1 A g−1 and a good rate capability (49% capacity retention at 50 A g−1), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices.Highlights► A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film fabricated by flow-directed assembly and hydrazine to reduce. ► The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets. ► The freestanding GN/MWCNT film has a potential application in flexible energy storage devices.

Porous nitrogen-doped hollow carbon spheres (PNHCS) had been prepared by pyrolysis of hollow polyaniline spheres (HPS), which were synthesized by the use of sulfonated polystyrene spheres (SPS) as a hard template. PNHCS have a specific surface area of 213 m2 g−1 and a pore volume of 0.24 cm3 g−1. At a current density of 0.5 A g−1, the specific capacitance of the PNHCS prepared is ca. 213 F g−1. The capacity retention after 5000 charge/discharge cycles at a current density of 1 A g−1 is more than 91%. The enhanced electrochemical performance can be attributed to the unique carbon nanostructure and nitrogen-doping of the PNHCS electrodes. The hollow macro-structure plays the role of an “ion-buffering” reservoir. The micropores of the PNHCS enlarge the specific surface area, while the mesopores offer larger channels for liquid electrolyte penetration. Nitrogen groups in the PNHCS not only improve the wettability of the carbon surface, but also enhance the capacitance by addition of a pseudocapacitive redox process.

In this work, phytic acid is used as the protonic acid dopant and soft template to synthesize 3D polyaniline (PANI) nanofiber networks. Then, the PANI nanofiber networks are transformed to porous nitrogen and phosphorus co-doped carbon nanofibers (P-NP-CNFs) by the carbonization and chemical activation process. P-NP-CNFs have a high specific surface area of 2586 m2 g−1 and a large pore volume of 1.43 cm3 g−1, which are favorable for enhancing the electrochemical performance of electrical double layer capacitors. Moreover, nitrogen and phosphorus doping in the carbon materials can increase the specific capacitance by a pseudocapacitive redox process. At a current density of 1 A g−1, P-NP-CNFs show a large specific capacitance of 280 F g−1 and a high specific capacitance retention of 94% after 10000 cycles. In particular, phosphorus doping can broaden the electrochemical window to increase the energy density. Therefore, the energy density of symmetric capacitors based on P-NP-CNFs is up to 22.9 W h kg−1 at a power density of 325 W kg−1, demonstrating that P-NP-CNFs are superior electrode materials for electrical double layer capacitors.

Lithium–sulfur (Li–S) batteries are highly attractive as energy storage devices due to their low cost and high energy density. The undesired capacity degradation caused by the polysulfide shuttle, however, has hindered their commercialization. Herein, a Co3O4 nanoneedle array on carbon cloth (CC@Co3O4) nanocomposite has been prepared and demonstrated for the first time as a multifunctional “super-reservoir” electrode to prolong the cycle life of Li–S batteries. Owing to the polar surface of the Co3O4 nanoneedle array, soluble lithium polysulfides (Li2Sn, 4 < n < 8) can be effectively absorbed and then transformed to insoluble Li2S2/Li2S which evenly covers the surface of the Co3O4 nanoneedle during the discharge process. Further, during the charge process, the Co3O4 nanoneedle can catalyze the electrochemical transformation of Li2S2/Li2S into soluble polysulfides. A high initial capacity of 1231 mA h g−1 at 0.5C and a slow capacity decay of 0.049%/cycle at 2.0C over 500 cycles were achieved; excellent rate performance was also obtained.

Two-dimensional (2D) carbon nanosheets have emerged as an attractive candidate for electrical double layer capacitors (EDLCs) due to their large specific surface area, good electrical conductivity and high charge mobility. However, the easy aggregation nature of nanosheets hinders rapid transport of electrolyte ions, reducing the ion-accessible area and restricting the ion transportation. Herein, we propose a template strategy for preparing three-dimensional (3D) porous carbon nanosheets (PCNs) with an oriented and interconnected nanostructure. Zinc layered hydroxide nitrate is used as a layered template and provides a nanospace to confine the carbonization process of the organic carbon precursor (gallic acid). The unique nanostructure and large surface area of chemically activated PCNs (aPCNs) significantly shorten the ion transport length in low dimensions and improve the electrolyte wettability and ion accessible surface area for charge storage. The aPCNs exhibit excellent performance as demonstrated by their large specific capacitance (327 F g−1 at a current density of 0.5 A g−1), superior rate capability (retaining 60.2% at 20 A g−1) and stable cyclability. In particular, the assembled symmetric device based on aPCNs delivers an energy density as high as 10.2 W h kg−1 at a power density of 301 W kg−1.