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Interconnected carbon encapsulated silicon nanoparticles were fabricated via molecular self-assembly and in situ carbonization strategies, which can effectively dissipate expansion strain and enhance the lithium-ion reaction kinetics.

A nest-like N-doped hierarchical porous active carbon (N-HAC) with higher specific surface area is achieved successfully by melamine cyanurate (MCA) sacrifice template molding and assistant chemical activation of NH4Cl and CaCl2. The results testify that the nest-like hierarchical porous character of N-HAC can form an interconnecting structure and introduce plenty of supercapacitor reactive sites, which can facilitate the ion diffusion/migration among the material, while nitrogen doping in N-HAC can change the charge distribution of N-HAC and then promote the electron transfer. Attributing to the synergistic effect of the hierarchical porous structure and nitrogen doping, N-HAC800 displays the highest specific capacitance (284.6 F g−1 at 1 A g−1 and 183.1 F g−1 even at 20 A g−1) and superior cycle stability (80.4% after 5000 cycles at 100 mV s−1). This study results prove that the N-doped hierarchical porous active carbon is a promising material for energy storage application in the future.

Transition metal phosphides/phosphates (TMPs) are considered appealing electrode materials in energy-related fields, especially in supercapacitors. However, the dilemma of inadequate electrode kinetics and dimensional unreliability evoked by a huge volume variation during cycling significantly plagues their progress. To mitigate this issue, in this work, a 3D cross-network in situ assembled via self-derived N-doped carbon hybrid Ni-Co-P/POx 2D sheets is fabricated. Particularly, high-Fermi-level N-doped carbon well confines Ni-Co-P/POx nanoparticles at the molecular level, and N-doping leads to redistribution of spin/electron density in the carbon skeleton, effectively regulating the electron environment of nearby Ni-Co-based moieties, resulting in a relatively lower surface work function, as known via experimental and Kelvin probe force microscopy (KPFM) results, which favors electron flee from the electrode surface and facilitates electron transport toward a rapid supercapacitor response. Moreover, the well-defined 3D cross-network architectures featured with in-plane pores and interconnected with each other can provide more ion/electron transfer pathways and 2D sheets with excellent surface chemistry available for sustainable ion/electron mobility, synergistically affording the favorable electrode kinetics. Accordingly, the resultant Ni-Co-P/POx@NC electrode shows admirable specific capacitance, excellent rate survivability, and long-term cyclability. The as-assembled asymmetric device exhibits remarkable energy and power outputs (48.5 Wh kg

NiCo2O4 has been proved to be a promising electrochemical material. In this work, a hollow NiCo2O4 material with an urchin-like structure is constructed by a self-template of Ni–Co complex compound based on the Ostwald ripening mechanism. During the thermal annealing in air, the residual carbon from the organic ligands of the Ni–Co complex compound leads to the generation of oxygen vacancies in urchin-like NiCo2O4. The as-obtained hollow NiCo2O4 (H-NiCo2O4-12) material manifests admirable specific capacitance of 886 F g−1 at 1 A g−1 and preserves 80% of capacitance retention after 5000 cycles. Moreover, the assembled asymmetric supercapacitor of H-NiCo2O4-12//AC delivers an energy density of 33.4 W h kg−1 at 747.7 W kg−1 and a high coulombic efficiency of about 95%. The improved supercapacitive performance of H-NiCo2O4-12 is attributed to its synergistic effect of hollow structure and oxygen vacancies. The hollow urchin-like structure of NiCo2O4 can buffer the volume expansion, expose abundant reaction active sites, and favor the transport of ions; while oxygen vacancies can adjust the electronic structure of NiCo2O4, which can favor the transfer of electrons. This work may throw some light on designing a virtue supercapacitor electrode material with hollow nanoporous morphology.

A 3D cross-network Co3O4–xNx nanostructure is obtained from the cobalt–alanine complex (Co(C3H5O2N)2) by amino acid self-assembly and subsequent pyrolysis. In this process, Co(C3H5O2N)2 forms a 2D ...

Single-atom Ag-decorated MnO2 hollow microspheres (Ag-MnO2 HMSs) with oxygen vacancies are successfully fabricated via a self-template strategy and redox etching method. The transmission electron m...

Three-dimensional N/S co-doped succulent-like hierarchical carbon (3D NS-SHC) is synthesized by carbonization of a supramolecular cluster. In this supramolecular process, potassium citrate can act as a reliable carbon source, while the thiourea as a N/S source and then, two molecules gradually tend to generate a giant “all-in-one” precursor via hydrogen bonding verified by Independent Gradient Model (IGM) calculation. TEM and SEM images show the N/S co-doped carbon holds a novel 3D succulent-like hierarchical structure. For NS-SHC-8:8 sample, element mapping images display the uniform N/S atoms distribution. These distinct features can be related to the supramolecular polymerization which promotes in-situ N/S co-doping homogenously and conduces to build 3D structure. Typically, NS-SHC-8:8 electrode exhibits prominent specific capacitance (258.5 F g−1 at 0.5 A g−1) and excellent cycle stability (94.4% after 20000 cycles) in three-electrode system. Furthermore, an assembled quasi-solid state symmetric supercapacitor delivers good energy density (10.2 Wh kg−1 at 250 W kg−1) and steady cycle endurance (85.1% after 10000 cycles). These eximious behaviors of NS-SHC-8:8 are mainly attributed to (1) the uniform N/S atoms distribution and their synergistic effect, which brings extra Faradaic reaction to higher specific capacitance, (2) the charming 3D succulent-like hierarchical structure, which serves as a multifunctional reservoir that can accommodate the ion/charge and facilitate their migration to further promote the electrochemical performance. Above mentions suggest that this uniformly heteroatom-modified carbon material produced by supramolecular technique is promising for high-performance energy storage devices.

Inspired by supermolecular self-assembly strategy, the N, P co-doped reduced graphene oxide (NP-rGO) material is fabricated by heat treatment from the supermolecular system of GO/MP (melamine and p...

Inspired by the supramolcular chemistry assembly theory, carbon-coated MnO2 nanospheres (C@MnO2) were successfully synthesized by a two-step method. First, the supramolecular solution was prepared by β-cyclodextrin inclusion of manganese acetate, which was then treated by a hydrothermal reaction and the calcination in the following. The scan electron microscopy (SEM) and transmission electron microscopy (TEM) images present that the C@MnO2 composites have the nanosphere morphology, in which the carbon layer originated from the carbonization of β-cyclodextrin was uniformly coated on the MnO2 nanoparticles. The electrochemical characteristics were examined by galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results demonstrated that C@MnO2 exhibited high specific capacitance (396.4 F g−1 at 2 A g−1) and good rate capability. In addition, 84.1% of the initial capacitance was maintained after 10000 cycles, indicating the excellent electrochemical stability. The superior electrochemical performance of the C@MnO2 was attributed to the outstanding uniform nanospheres structure that facilitates ion mobility and the synergistic effect between conductive carbon and pseudocapacitive MnO2. Hence, C@MnO2 can be a potential candidate material for the applications in supercapacitors.

A new type of sandwiched nanosheets 3D MoS2/C@RGO is synthesized by supramolecular self-assembly methods, as a novel and excellence electrode material. The surface characterization techniques show that the sandwiched 3D MoS2/C@RGO possesses rich porosity and large specific area, which is convenient for electrolyte ion transportation and beneficial for the improved electrical conductivity. An outstanding specific capacitance of the prepared MoS2/C@RGO of 340.0 F/g at 1 A g−1 is achieved with an excellent cycle stability of 90% after 1000 cycles at the scanning rate of 40 mV/s. The sandwiched 3D MoS2/C@RGO is suggested to be an ideal candidate for supercapacitors.

Supramolecular in-situ self-assembly is achieved to prepare carbon nanodots decorated graphene/polyaniline composites by adding β-cyclodextrin (β-CD) as the bonding agent in this work. With its unique structure, β-CD can form a supramolecular system with graphene oxide layers, generating the inclusion compound with aniline monomers, which makes the in-situ polymerization of polyaniline on the surface of graphene oxide. In the hydrothermal process, β-CD is carbonized to carbon nanodots anchored on the surface of reduced graphene oxide. The scanning electron microscopy (SEM) images show that the prepared reduced graphene oxide/carbon nanodots/polyaniline (RGO@CN/PANI) nanocomposites exhibit the micro morphology of a regular polyaniline particle array on the surface of graphene layers. The electrochemical tests demonstrate that the RGO@CN/PANI nanocomposites exhibit high specific capacitance (up to 871.8 F g−1 at 0.2A g−1), low charge transfer resistance (Rct), and presentable cycling performance (72% capacitance retention after 10000 cycles), indicating the satisfied performance for supercapacitors. The promising performance of RGO@CN/PANI can be attributed to the synergistic effect of RGO, carbon nanodots and PANI, as well as the regular microstructure in which the effective reaction area of polyaniline is increased and the rapid ion transport channel is provided.

In this work, carbon nanoparticle-anchored MoS2 nanosheets are synthesized by a supramolecular self-assembly method, through which β-cyclodextrins (β-CDs) are combined with l -cysteine by the hydrogen bond and the electrostatic interaction in the aqueous solution, which are then carbonized to carbon nanoparticles and anchored on the surface of MoS2 nanosheets during the hydrothermal process. The carbon nanoparticles anchored on MoS2 nanosheets effectively inhibit the aggregation and restacking of MoS2, and then enhance the electrical conductivity of the composite. The superior electrochemical performance is exhibited through cyclic voltammetry and galvanostatic charge-discharge, including a high specific capacitance of 394.2 F g−1 at the scan rate of 5 mV s−1 and a high rate retention ratio of 63.3% when the scan rate increases from 5 to 200 mV s−1. The remarkable performance is mainly attributed to the better electrical conductivity and the higher rates of electron migration and ion transport, making the carbon nanoparticle-anchored MoS2 nanosheets a potential candidate material for advanced energy storage systems.

NiCo2O4 has been proved to have promising electrochemical performance. In this work, a new type of hollow urchin-like NiCo2O4 material is constructed by a self-template of Ni-Co complex compound based on the Ostwald ripening mechanism. During the thermal annealing in air, the residual carbon from the organic ligands of Ni-Co complex compound leads to the generation of oxygen vacancies in urchin-like NiCo2O4. The as-obtained hollow NiCo2O4 (H-NiCo2O4-12) material manifests admirable specific capacitance of 886 Fg-1 at 1Ag-1, and preserves 80% of capacitance retention after 5000 cycles. Meanwhile, the assembled asymmetric supercapacitor of H-NiCo2O4-12//AC delivers an energy density of 33.4 Wh g-1 at 747.7 W kg-1. The improved supercapacitive performance of H-NiCo2O4-12 should attribute to its synergistic effect of hollow structure and oxygen vacancies. The hollow urchin-like structure of NiCo2O4 can buffer the volume expansion, expose abundant reaction active sites and favor the transport of ions; while oxygen vacancies can adjust the electronic structure of NiCo2O4 which can favor the transfer of electrons. This work may throw some light on designing a virtue supercapacitor electrode material with hollow nanoporous morphology.

Cathode-electrolyte interphases (CEIs) are determinative to the electrochemical performance of lithium-ion batteries, especially those with Ni-rich cathodes. Herein, we employed the simple electrostatic adsorption (SEA) method to generate a homogenous polydimethyldiallyl ammonium chloride/ graphene oxide (PDDA/GO) dual-layer coating on the surface of a model single-crystalline Ni-rich cathode, LiNi0.8Co0.1Mn0.1O2 (NCM811), and thus modified the CEIs in both compositions and microstructures. The PDDA/GO coating depresses the LiF component, enables a homogeneous organic-enriched CEI film with fast Li diffusion and effective corrosion protection on cathodes. Together with the mechanical confinement on cathodes, these characters of modified CEIs benefit the electrochemical stability effectively. For instance, NCM811-PDDA/GO cathodes exhibit a 77% capacity retention after 500 cycles at the current density of 50 mA g-1, against 28% for pristine NCM811. This study provides new insights in correlating the coating and CEIs, which effectively guide the development of Ni-rich cathodes for high performance. Such a simple strategy to form a homogeneous coating can also be extended into other electrode materials easily.

Transition metal phosphides (TMPs) have attracted great interest owing to the metallic properties and high specific capacities. Here, we designed hollow NiCoP nanocubes with increased specific surface area using a Ni-Co Prussian blue analogue as a self-template and NH3·H2O as an etching agent. During the synthesis, both carbonization and phosphorization are completed in one step. The obtained hollow structure alleviates the volume variation of electrode material during reversible electrochemical reaction. Meanwhile, the residual carbon distributed uniformly in NiCoP at the molecular level, resulting in a high conductivity. DFT calculations further reveal that the electrical conductivity of NiCoP is superior to those of monometallic phosphide and metal oxide. Therefore, the optimized NiCoP-4–500 displays a high specific capacity (1590 F g−1 at 1 A g−1) and outstanding cycling stability (78.2% retention after 12,000 cycles). Moreover, a prepared hybrid supercapacitor device delivers an energy density of 38.4 W h kg−1 with a power density of 799.9 W kg−1 at 1 A g−1. The results indicate that the obtained high-performance TMPs with hollow structures have an application potential for energy storage devices.

We report a supramolecular strategy to fabricate high-doping-leveled (10.5 at.%) porous nitrogen-doped reduced graphene oxide (P-NrGO) with outstanding electrochemical properties as electrode material for supercapacitors. The introduced supramolecular polymers melamine cyanurate (MC) acts a barrier to prevent the graphene sheets from restacking, and meanwhile functions as a soft template to create porous structure as well as nitrogen source for in-situ N-doping. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images show that the P-NrGO exhibits wrinkled and loose-packed thin layer structure with a large number of pores. X-ray photoelectron spectrometer (XPS) spectra demonstrate that pyridine and pyrrole-N was the main nitrogen bonding states, which were favorable for the improvements of supercapacitive performance. Attributed to the synergistic effects of the porous structure and the N-doping, the P-NrGO1 exhibits an increase of 56.5% in the specific capacitance (335 F g−1) when compared with the rGO (214 F g−1). In addition, the capacitance retention ratio reaches 94.3% after 10000 cycles, indicating the excellent electrochemical stability. The present work benefits the large-scale production of N-doped graphene oxide for high-performance supercapacitors.

Li4Ti5O12 (LTO)-carbon (C)-reduced graphene oxide (rGO) microspheres are synthesized via in situ supramolecular self-assembly, combined with spray drying and high-temperature calcination. Dopamine can polymerize on the surface of Ti(OH)4 and form polydopamine, through which graphene oxide(GO) connects with Ti(OH)4 uniformly and tightly to form a homogeneous supramolecular sol system. During the high-temperature calcination, polydopamine is carbonized to carbon to connect LTO with rGO, so that the aggregation of rGO is inhibited, and small-sized LTO particles are obtained. The scanning electron microscopy (SEM) image shows that in the as-prepared LTO-C-rGO microspheres, LTO particles with diameters of ∼50 nm remained homogeneous and wrapped in a three-dimensional network built by the rGO nanosheets. Electrochemical measurements show that the LTO-C-rGO anode exhibits a high reversible capacity of 184 mAh g–1 at 1 C and a high capacity retention of 94.5% after 500 cycles at 20 C. The above excellent electroc...

A manganese dioxide (MnO2) film is electrodeposited onto a gold-coated quartz crystal electrode by a galvanostatic method. The scanning electron microscopy (SEM) image shows that the electrodeposited MnO2 film consists of nanorods with diameters of 15–20 nm. The cyclic voltammetric (CV) and electrochemical quartz crystal microbalance (EQCM) results demonstrate that the MnO2 electrodes exhibit similar specific capacitance (SC) and mass-to-charge ratios (MCR) in those electrolytes that contain the same cation but different anions. However, the SC and MCR are different in those electrolytes that contain the same anion but different cations. The SC of MnO2 electrodes increases with a decrease of the cationic radius and an increase of the cationic charge. The smaller the cationic radius, the smaller the deviation of the MCR from the equivalent weight (EW) of the cation. The results illustrate that the pseudocapacitive charge storage mechanism of MnO2 involves the cation in the electrolyte and its intercalation/deintercalation, which are affected by the charge number and the cationic radius. This work offers a theoretical basis for selecting suitable electrolytes to couple MnO2-based electrodes in supercapacitors.

A commercially feasible sodium-ion pouch cell with a capacity of 650 mAh is fabricated using the ternary layered Na0.76Ni0.3Fe0.4Mn0.3O2 and the hard carbon as the cathode and anode materials, respectively. 1.0 M NaPF6 in the mixed carbonate solvents is served as the electrolyte. According to the calculation of the frontier molecular orbital energy, adiponitrile (ADN) has a stronger ability of electron donating than the carbonate solvents and thus is easier to be reduced on the surface of the cathode material to form the solid-electrolyte interphase (SEI) film, so ADN is selected as the electrolyte additive to improve the performance of sodium-ion battery for the first time. The transmission electron microscopy (TEM) images show that the addition of ADN enables the formation of more uniform and compact SEI film on the cathode material surface. The X-ray photoelectron spectroscopy (XPS) results indicate that ADN promotes the formation of NaF and result in the formation of NaCN as the effective components of the SEI film. The electrochemical test results demonstrate that ADN effectively improves the electrochemical performance at high -low-temperature and cycling stability of Na0.76Ni0.3Fe0.4Mn0.3O2 as the cathode material of sodium-ion batteries, that is because the adding of ADN in electrolyte result in the formation of more effective SEI film. In particular, the battery using the electrolyte with 3% ADN exhibits the most obvious improvement, which owe to the formation of most compact, stable and effective SEI film after adding 3% ADN into the electrolyte. At the operating temperature of 45 °C, −10 °C, and −20 °C, the discharge capacity increases by 10.5%, 8%, and 13%, respectively. For the cycle life, the capacity retention of the battery without addition of ADN drops rapidly to 75% after 40 cycles, while the capacity retention of the battery with 3% ADN still remains 78% after 220 cycles.

Delicately engineering nano-architecture and electronic structure can assist in constructing the high-performance electrode materials for high-energy supercapacitor. Herein, a hollow nanocage is in-situ introduced into bimetallic phosphide-based materials via a green self-template strategy at room temperature. The homogeneous hollow configuration that N-doped carbon confined NiCoP nanoparticles hybrid nanocages (NiCoP/NC) is harvested via elaborately manipulating the physicochemical properties at the molecular level. This well-defined hollow geometry ensures the convenient electrolyte access for electroactive sites, thus facilitating the electrochemical activities. X-ray photoelectron spectroscopy reveals the electronic interaction among Ni, Co and P elements, which can induce an appropriate electric-field environment favors for the charge-carriers immigration. Density-functional-theory simulations indicate the increased electronic states of d-orbital around the Fermi level, which benefit for lower OH– adsorption energies (-3.80 eV), therefore resulting the faster electrochemical dynamics. Accordingly, as-prepared NiCoP/NC electrode achieves superb rate-capability (1127 and 873F g−1 at 1 and 16 A g−1, respectively) and long-term durability (75.5% retention after 8000 cycles). Furthermore, a hybrid supercapacitor based on NiCoP/NC cathode demonstrates an ultra-high energy density (52.5 Wh kg−1), outperforming similar hybrid devices. Our studies successfully give an insight into the relationship between nano-morphology, electronic modulation and electrochemical performance, which can provide some guidance for garnering a desirable electrode material.

Transition metal sulfides (TMS) are of great interest as promising battery-type electrode materials, however, the poor conductivity and sluggish reaction kinetics seriously limit their application. Here, we designed a hollow structured precursor of Ni-based metal-organic frameworks (Ni-MOFs) via Ostwald ripening mechanism. Based on this unique precursor, a hollow carbon-coated nickel sulfide nanocrystal (H-NiS1-X/C) with sulfur vacancies was further synthesized through an ion exchange strategy and thermal annealing. By optimizing the content of sulfur source, the sample with appropriate S-vacancies (H-NiS1-X/C-50) was developed. Benefiting from its hollow structure and S-vacancies, this H-NiS1-X/C-50 displayed a high reversible specific capacity (1728 F g−1, 1 A g−1), stable cycling (72% capacity retention over 8000 cycles) and superior rate capability. After assembling the asymmetric supercapacitor, a high energy density of 36.88 Wh kg−1 was achieved. Experimental results and DFT calculations demonstrate that introducing S-vacancies builds an embedded electric field and produces lattice distortions in H-NiS1-X/C, thus enhancing the conductivity of the material. Our strategy also provides a facile way to construct high-performance TMS with unique hollow structure and S-vacancies for developing advanced energy storage devices.

Nanotunnels inserted hierarchical-porous carbon (NTHPC) have been synthesized successfully via a MnO2-introduced-tunnels strategy using MnO2 nanorods as template, with agar and β-cyclodextrin serving as hybrid carbon precursors. The as-prepared NTHPC possesses a higher specific surface area of 1441 m2 g−1, a moderate pore volume of 1.23 cm3 g−1, and the hierarchical-porous structure with inserted nanotunnels. Transmission electron microscopy has demonstrated that the width of the nanotunnels is between 20 and 100 nm, and the length ranges from 0.2 to 2.0 μm. Tests in a three-electrode system showed that the NTHPC has high specific capacitance (253.1 F g−1, 5 mV s−1), as well as good rate capability (203.3 F g−1, 100 mV s−1) and excellent cycling stability. More importantly, an assembled symmetric supercapacitor with NTHPC electrodes delivered an outstanding energy density of up to 34.9 Wh kg−1 with power density of 755.2 W kg−1. The remarkable electrochemical performance of the NTHPC is ascribed to the nanotunnels, which act as ion reservoirs and liquid transfer channels that can increase the ion transport rate and shorten the ion transfer distance. This study provides a novel method for the preparation of high-performance hierarchical-porous carbon and guidance for its potential applications in supercapacitors.

A new structured Mn3O4@C composite is fabricated with carbon filled in the pores and coated on the surface of the porous Mn3O4. This anode material exhibits superior electrochemical properties including high initial coulombic efficiency, outstanding rate properties and cycle performance. The initial coulombic efficiency of this composite approach to 69.5%, and it delivers a capacity of nearly 420 mAh g−1 even at a discharge current of 1800 mA g−1. The outperforming results could be ascribed to the unique structure of the composite, which can provide meaningful guidance to the design of further anode materials.

A series of Zr4+ and/or Al3+ doped LiNi0.83Co0.11Mn0.06O2 are synthesized via a solid phase method. TEM shows that the co-doped specimen consists of a layered core and a coherent interface cation-mixed shell. XRD and XPS results clarify that Zr4+ doping increases the Li+/Ni2+ cation site exchange degree and the concentration of Ni2+. The morphology and structural integrity of the co-doped electrode maintains well, and it provides a higher cycle retention (89.7%) than blank (60.1%) after 150 cycles and a superior capacity of 152.8 mAh g−1 at 7C (only 57.7 mAh g−1 for blank) and an enhanced elevated temperature cycle retention (83.0%) than blank (33.7%) after 100 cycles at 55 °C. Generally, Zr4+ raises the concentration of Ni2+ to keep electric neutrality and then reconstruct a stable interfacial structure to inhibit side reactions, the structural stability could be also enhanced by the strong Zr–O and Al–O bonds. Meanwhile, Al3+ served as centers of positive charge can suppress the phase transformation. Benefiting from the synergistic effect of Zr4+/Al3+, as well as the induced coherent interface cation-mixed shell, co-doped specimen shows enhanced structural stability and the superior electrochemical performance. This study provides a route to prepare advanced layered cathodes for lithium-ion batteries.

Mn3O4 anchored nitrogen-doped reduced graphene oxide is fabricated via a facile hydrothermal route by using the Mn2+-GO supramolecular system as precursor and the ammonia as precipitant and nitrogen source. A combined characterization method including UV-vis absorption spectrum, fluorescence spectrum, and zeta potential and particle sizing measurement is adopted to study the interactions between Mn2+ and GO nanosheets. More importantly, the electrochemical activation process of the Mn3O4/N-rGO electrode in 1 mol L−1 Na2SO4 is investigated in detail. It is found that the activation process originates from the conversion of structure and morphology from spinel Mn3O4 to nano-structured birnessite MnO2. The activation process mainly occurs in the first 200 cycles, in which the specific capacitance increases by 88.2%. The electrochemical experimental results indicate that the Mn3O4/N-rGO electrode after the activation process exhibits improved capacitive performances with a specific capacitance of 141 F g−1 at 0.2 A g−1 and a superior cycle stability (90.3% capacitance retention after 5000 cycles). This study will give a further insight into the distinctive electrochemical activation process and the supercapacitive properties of Mn3O4-based materials.

Na-doped Ni-rich layered oxide LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode materials are synthesized via co-precipitation method in this work. The crystal structure, thermal stability as well as electrochemical performance are studied. Results show that when the Na-doping amount is less than and equal to 2%, the samples can exhibit better electrochemical performance than the parent compound. One reason is that the Na doping can reduce the cationic mixing in the compound, as confirmed by X-Ray powder diffraction (XRD). Beside, results from X-ray photoelectron spectroscopy (XPS) and galvanostatic intermittent titration technique (GITT) show that the introduced sodium can also impact the chemical environment of lithium, making the latter become more easily de-intercalated from the bulk material. However, as the Na-doping amount increases, the electrochemical properties of the samples were deteriorated. Furthermore, results from differential scanning calorimetry (DSC) measurement indicate that the added Na + is also beneficial for improving the thermal stability of the layered oxide.

Graphene oxide/CoAl-layered double hydroxide (GO/CoAl-LDHs) composites were successfully prepared by a face-to-face electrostatic self-assembly method. Positively charged colloidal CoAl-LDHs nanosheets (CoAl-LDH-NS) and negatively charged colloidal GO nanosheets were uniformly mixed; then CoAl-LDH-NS were assembled on GO nanosheets via electrostatic attraction to prepare GO/CoAl-LDHs composites. The GO/CoAl-LDHs composites were then transformed to the reduced graphene oxide/CoAl-layered double hydroxide (RGO/CoAl-LDHs) composites with a hydrothermal method. The results showed that the content of RGO has a remarkable influence on the capacitive properties of composites. Among the composites with different RGO content, the RGO/CoAl-LDHs composite containing 12.0 wt% RGO displayed a maximum specific capacitance of 825 F g −1 at low current density of 1 A g −1 , while that of pure CoAl-LDHs was 552 F g −1 . The capacitance retention of the RGO/CoAl-LDHs composite from 1 A g −1 to 8 A g −1 is found to be 62.3%, whereas that of pure CoAl-LDHs is only 31.9%. Face-to-face self-assembling between positively charged CoAl-LDH-NS and negatively charged GO nanosheets can effectively reduce the self-agglomeration of GO nanosheets and avoid CoAl-LDH-NS stacking together, which lead to improvement of the capacitive performance.

A porous graphene layers with a three-dimensional structure (3DG) was prepared via a gas foaming method based on a polymeric predecessor. This intimately interconnected 3DG structure not only significantly increases the specific surface area but also provides more channels to facilitate electron transport. In addition, 3D N-doped (3DNG) layers materials were synthesized using melamine as a nitrogen source. The nitrogen content in the 3DNG layers significantly influenced the electrochemical performance. The sample denoted as 3DNG-2 exhibited a specific capacitance of 335.2 F g −1 at a current density of 1 A g −1 in a three-electrode system. Additionally, 3DNG-2 exhibited excellent electrochemical performance in aqueous and organic electrolytes using a two-electrode symmetric cell. An energy density of 58.1 Wh kg −1 at a power density of 2500 W kg −1 was achieved, which is approximately 3 times that (19.6 Wh kg −1 ) in an aqueous electrolyte in a two-electrode system. After 1000 cycles, the capacity retention in aqueous electrolyte was more than 99.0%, and this retention in organic electrolytes was more than 89.4%, which demonstrated its excellent cycle stability. This performance makes 3DNG-2 a promising candidate as an electrode material in high-power and high-energy supercapacitor applications.

Delicate design of nanostructured electrode materials with built-in electric field effect can make them have satisfactory electrochemical performance through the synergistic effects. Here, we prepare an advanced electrode material, a hollow N-doped carbon@O-vacancies NiCo2O4 nanocage (HNC@NiCo2O4), in which NiCo2O4 nanosheets are homogeneously grown on the N-doped carbon frame. This unique hollow nanocage can increase the contact-area between the material and electrolyte, further improving the electrochemical activities. Density functional theory simulations reveal that O-vacancies in NiCo2O4 can narrow the bandgap, which may contribute to the charge transfer dynamic of HNC@NiCo2O4 series. Meanwhile, N-doping can change the carbon-skeleton states density, thereby improving the electron-accepting capabilities for nearby NiCo2O4, and thus elevating the capacitive behaviors. Accordingly, a built-in electric field is in-situ constructed via the O-vacancies and N-doping. Due to the synergistic effect of a hollow structure and built-in electric field, as-fabricated HNC@NiCo2O4-0.45 electrode delivers noticeable specific-capacitance (966.2 F g−1@1 A g−1) and long-term endurance. Impressively, the HNC@NiCo2O4-0.45 electrode is employed to assemble a hybrid supercapacitor with the active carbon anode, which behaves a high energy-density (41.9 Wh kg−1 at 750.4 W kg−1). Above results validate that the electric field modulation and nanostructure construction are important tactics to design a high-performance supercapacitor electrode.

Adding redox-active species into the electrolyte can supply extra capacitance to the carbon-based supercapacitors. However, the devices may suffer from serious self-discharge caused by the shuttling and intermixing of charged active species between the electrodes. Confining these active species in a solid-state electrode material is an effective strategy to avoid this. Inspired by this, we fabricate a three-dimensional reduced graphene oxide confined methylene blue composite via a supramolecular strategy, combined with a hydrothermal reduction and a freeze-drying process. Benefited from the porous and conductive structure of the product, the fast-reversible electrochemical reaction of methylene blue, and the spatial confinement effect of the methylene blue confined in reduced graphene oxide, the as-prepared product exhibits a high-rate capability (311 and 262 F g−1 at 1 and 20 A g−1, respectively) and an excellent cycle stability (96% capacitance retention after 10000 cycles) in a three-electrode system. Moreover, a solid-state symmetric supercapacitor device based on the as-prepared product delivers the maximum energy density of 8.2 Wh kg−1 and outstanding cycle stability. This supramolecular-induced confining active species in reduced graphene oxide strategy provides insight into the fabrication of other high-performance graphene-based pseudocapacitors.