Ion Transfer Rate Research Articles

Highly conductive dodecaborate/MXene composites for high performance supercapacitors

With the increasingly prominent energy and environmental issues, the supercapacitors, as a highly efficient and clean energy conversion and storage devices, meet the requirements well. However, it is still a challenge to enhance the capacitance and energy density of supercapacitors. A novel and highly conductive dodecaborate/MXene composites have been designed for high performance supercapacitors. The surface charge property of MXene was modified by a simple ultrasonic treatment with ammonium ion, and the dodecaborate ion can be inserted into the inner surface of MXene by electrostatic adsorption. Due to the unique icosahedral cage conjugate structure formed by the B-B bond and the highly delocalized three-dimensional π bond structure of the electrons, the negative charge is delocalied on the whole dodecaborate ion, which reduces the ability to bind to cations. Therefore, the cations can move easily, and the dodecaborate can act as a “lubricant” for ion diffusion between the MXene layers, which significantly improves the ion transfer rate of supercapacitors. The dodecaborate/MXene composites can achieve an extremely high specific capacitance of 366 F.g-1 at a scan rate of 2 mV.s-1, which is more than eight times higher than that of MXene (43 F1-) at the same scan rate. Our finding provides a novel route on the fabrication of the high performance supercapacitors.

YS2 monolayer as a high-efficient anode material for rechargeable Li-ion and Na-ion batteries

Here, we propose a new two-dimensional (2D) monolayer YS2 and investigate its properties as electrode material for Li-ion and Na-ion batteries (LIBs and NIBs), by combining the density functional theory (DFT) and the crystal structure prediction techniques. The results of phonon dispersion and ab-initio molecular dynamics (AIMD) simulations confirm that it is dynamically and thermally stable. The adsorption energies of Li and Na on YS2 are −2.038 and −1.939 eV, respectively. Besides, the superior electrical conductivity is maintained during the whole lithiation/sodiumation process. The calculated diffusion energy barriers for Li and Na on this YS2 monolayer are 0.33 and 0.24 eV, respectively, demonstrating a fast ion transfer rate. The theoretical specific capacities for Li and Na adatoms on YS2 are both 350 mA h g−1, comparable to the graphite electrode. These features indicate that 2D YS2 monolayer could be considered as a promising electrode material for both LIBs and NIBs.

Effect of Surface Modification of Heterogeneous Anion-Exchange Membranes on the Intensity of Electroconvection at Their Surfaces

Electroconvection is the principal mechanism that allows markedly increasing the rate of ion transfer through ion-exchange membranes in intensive current regimes. In this work, we investigated the possibility of intensifying electroconvection in solution near heterogeneous MA-41 anion-exchange membrane (Shchekinoazot production) by the modifying of its surface. The use of weakly crosslinked ion-exchange resin (MA-41P) in the course of the membrane manufacturing, with subsequent chemical modification of its surface (MA-41PM), is shown to make it possible to increase the limiting current density almost twice. The value of the reduced potential drop (after subtracting the ohmic contribution), at which significant generation of H+ and OH– ions begins, is shifted from 0.8 V in the case of MA-41 to 1.7 V in the case of MA-41PM. The current density related to the onset of water splitting is equal to 0.9$$i_{{{\text{lim}}}}^{{{\text{Lev}}}},$$ in the case of MA-41; 2$$i_{{{\text{lim}}}}^{{{\text{Lev}}}},$$ in the case of MA-41PM (where $$i_{{{\text{lim}}}}^{{{\text{Lev}}}}$$ is the theoretical value of the limiting current density). The special feature of the modified membrane behavior is the presence of a range of potential drop (between 50 and 80 mV in the reduced scale), in which the system with the MA-41PM has negative differential resistance: in this range, the potential drop decreases when the current density increases. This behavior occurs when measuring quasi-stationary I–V curves; correspondingly, in the chronopotentiogram there is a time interval, where the potential drop decreases with time. The electroconvection is intensified near a modified membrane due to a higher fraction of conductive areas on the surface of the modified membrane and the redistribution of these areas via formation of their agglomerates in the centers of the cells formed by the reinforcing mesh. Mathematical modeling shows the concentration polarization of the modified membrane being less than that of the pristine one. Meanwhile, the structure of electroconvective vortices is optimized: the vortices near the modified membrane are larger; they do not extinguish each other, unlike the case of MA-41.

A TiN Nanorod Array 3D Hierarchical Composite Electrode for Ultrahigh-Power-Density Bromine-Based Flow Batteries.

Bromine-based flow batteries are well suited for stationary energy storage due to attractive features of high energy density and low cost. However, the bromine-based flow battery suffers from low power density and large materials consumption due to the relatively high polarization of the Br2 /Br- couple on the electrodes. Herein, a self-supporting 3D hierarchical composite electrode based on a TiN nanorod array is designed to improve the activity of the Br2 /Br- couple and increase the power density of the bromine-based flow battery. In this design, a carbon felt provides a composite electrode with a 3D electron conductive framework to guarantee high electronic conductivity, while the TiN nanorods possess excellent catalytic activity for the Br2 /Br- electrochemical reaction to reduce the electrochemical polarization. Moreover, the 3D micro-nano hierarchical nanorod-array alignment structure contributes to a high electrolyte penetration and a high ion-transfer rate to reduce diffusion polarization. As a result, a zinc-bromine flow battery with the designed composite electrode can be operated at a current density of up to 160 mA cm-2 , which is the highest current density ever reported. These results exhibit a promising strategy to fabricate electrodes for ultrahigh-power-density bromine-based flow batteries and accelerate the development of bromine-based flow batteries.

Constituting the NASICON type solid electrolyte coated material forming anti-high voltage system to enhance the high cut-off voltage performance of LiNi0.6Co0.2Mn0.2O2 via charge attracts electrostatic assembly

High cut-off voltage in lithium ion batteries (LIBs), now, is the vital threshold for further development of high energy density LIBs. In this work, the NASICON type solid electrolyte as high working voltage and unique ionic transfer rate material is introduced to facilitate the high cut-off performance of Ni-rich cathode material. Herein, considering the uniformity of coating outer layer, the charge attraction is employed as a coating method to form a homogeneous coating layer. The coating properties of LiTi2(PO4)3 coated LiNi0.6Co0.2Mn0.2O2 probed via X-ray diffraction, scanning electron microscope, transmission electron microscope, X-ray photoelectron spectroscopy. Typically, electrochemical tests show that LiTi2(PO4)3 was effectively promoting the electrochemical characters. The modified material preserves 94% after 100 cycles at 1C in voltage of 2.7–4.3 V. Especially, it maintains 92.3%/83.6% in high cut-off voltage of 2.7–4.5 V/2.7–4.8 V for 100 cycles, and keeps 117.4 mA h g−1/101.2 mA h g−1 after 500 long cycles.

Charge storage characteristics of mesoporous strontium titanate perovskite aqueous as well as flexible solid-state supercapacitor cell

Perovskite oxides with high specific surface area can be a pertinent approach to boost up the performance for supercapacitors. In the present work, we demonstrate the outstanding charge storage properties of strontium titanate with cubic structure using sol-gel synthesis technique. Cubic structure of perovskite is undeniably useful for supercapacitors owing to its three-dimensional diffusion channels for oxygen anion diffusion. Strontium titanate electrode in ultrathin films displays high capacitance of 592 F g−1 at 5 mV s−1 due to its large specific surface area, mesoporous structure and excellent mass transfer rate of electrolytic ions. The symmetric supercapacitor cell in aqueous media demonstrates high specific capacitance (212.5 F g−1 at 0.63 A g−1), good capacitance retention (74.5% at 2.5 A g−1), excellent cycling stability (99% after 5000 cycles), high energy density (27.8 Wh kg−1) and good power density (1921 W kg−1). Upon charging for ∼ 60 s, the cell illustrates high energy density of 17.2 Wh kg−1 (at 1104 W kg−1). Flexible and solid-state symmetric cell exhibits good energy/power density (3.62 Wh kg−1 and 965 W kg−1) without any support or hybrid cell design. Therefore, all these electrochemical attributes signify the promising potential of STO in energy storage devices.

Exploration and Size Engineering from Natural Chalcopyrite to High-Performance Electrode Materials for Lithium-Ion Batteries.

Compared to chemosynthetic CuFeS2, natural chalcopyrite (CuFeS2) can be regarded as a promising anode material for exploring ultrafast and stable Li-ion batteries benefiting from it being firsthand, eco-friendly, and resource-rich. Considering the nonuniform size distribution in it and the fact that homogeneous grain distributions can effectively restrain the aggregation of active materials, the engineering of size is deemed an effective strategy to achieve excellent Li-storage performances. Herein, varisized natural CuFeS2 are obtained by facial mineral processing technology and outstanding Li-storage performances are exhibited. Along with the decreasing of size, the contribution of pseudocapacitive as well as the ion transfer rates are significantly boosted. As expected, even at 1 A g-1, a remarkable capacity of 1009.7 mA h g-1 is displayed by the sample with the smallest size and most uniform distributions even after 500 cycles. Furthermore, supported by the detailed analysis of in situ X-ray diffraction and kinetic features, a hybrid of multiple lithium-metal sulfur systems and the major origin of the enhanced capacity upon long cycles are confirmed. Remarkably, this work is expected to increase the far-ranging applications of natural chalcopyrite as a firsthand anode material for lithium-ion batteries (LIBs) and inform the readers about the effects of particle size on Li-storage performances.

Application of laser Scribed method to fabricate graphene/graphene oxide multilayer

Graphene is a flat layer of carbon atom, and is a layer of graphite with a thickness of a few tenths of a nanometer that due to its porous structure and high ionic transfer rate, it has been considered in electronic applications, such as cloud storage capacitors with high energy. In this research work, laser scribed technique has been regarded to synthesize grapheme on the surface of a DVD and manufacture graphene and graphene composite super capacitors with Molybdenum disulfide. For this purpose, first, by Hummer’s method, graphite was converted to graphene oxide (GO) in an acidic environment containing Sodium nitrate, Potassium permanganate and sulfuric acid. Centrifuges and ultrasonic devices were utilized for the homogenization of graphene oxide solution. GO homogeneous solution was applied on the surface of specific DVDs and the set was dried at room temperature. For GO reduction and transform it into graphene, a suitable laser, with programming of super capacitor particular pattern was used. By applying energy with the amount of resonance frequency of graphene and oxygen bond, the laser broke the connection and the reduction action and reaching to graphene was done. Thus, the optimal wavelength of laser was determined to reduce the GO. In this study, the process of graphene synthesis and applying the super capacitor specific pattern were carried out in single step that is the biggest advantage of laser scribed graphene (LSG) method. In present study, TEM was utilized to examine the layered structure of GO, SEM was used for microstructural studies the XPS was used to investigate elements present in the layer applied on DVD, and the Raman spectroscopy was applied to investigate the quality of prepared graphene through studying G and D peaks., two tests of cyclic voltammetry (CV) and Galvano static charge/discharge (CC) were applied to study the performance and power of energy storage in super capacitors, Finally the long-term charge-discharge stability of the LSG was plotted which indicates that specific capacitance has decreased very slightly from its primary capacitance of ~ 10 F cm-3 and its cyclic stability is favorable over 1000 cycles.

One-step fabrication of nanosized LiFePO4/expanded graphite composites with a particle growth inhibitor and enhanced electrochemical performance of aqueous Li-ion capacitors.

It is reported that olivine-type lithium iron phosphate (LFP) for Li-ion batteries is one of the most widely utilized cathode materials, but its high-power applications are limited due to its intrinsically poor ion transfer rate and conductivity. Therefore, it is highly desired to fabricate LFP Li-ion capacitors (LICs) with high power performance and excellent cyclic reversibility, especially in safe, low cost and environmentally friendly aqueous electrolytes. Here, we fabricate LFP/expanded graphite (EG) nanocomposites by a one-step process, in which polyethylene glycol (PEG) is used as the particle growth inhibitor combined with vacuum infiltration of the LFP precursor into EG as a conductive sub-phase, and further investigate their high-power performance in aqueous LICs. Embedding spherical LFP nanoparticles with well-controlled size and agglomeration into the pores of EG and wrapping LFP nanoparticles by EG films contribute to the rapid electron and ion diffusion in LFP/EG composites, resulting in excellent cyclic reversibility and rate performance of LFP/EG composites. The aqueous LFP/EG//active carbon (AC) LICs were assembled in LiNO3 electrolytes with LFP/EG composites and AC as the positive and negative electrodes, respectively. The optimal LIC shows a power density of 2367.9 W kg−1 at an energy density of 6.5 W h kg−1, dramatically favorable rate characteristics and excellent cycle life with 82.1% capacitance retention of its primary capacitance at 2 A g−1 after 6000 cycles, markedly higher than those of the commercial LFP LIC. The presented aqueous LFP/EG//AC LICs with excellent electrochemical performance are expected to have broad high-power appliances that are cost-sensitive and highly secure.

A modelling and simulation study of soluble lead redox flow battery: Effect of presence of free convection on the battery characteristics

In this paper, we develop a mathematical model for soluble lead redox flow battery. The model accounts for simultaneous effect of forced convection and induced free convection in electrolyte domain of the battery. It predicts existence of dominant free convection over the forced convection in vicinity of the electrodes. The predictions suggests that both, electrode kinetics and ion transfer assisted by free convection alone, controls charge-discharge characteristics of the battery. The free convection augments ion transfer rate to the electrodes. By virtue of this, limiting current density at the electrodes increases to twice the theoretical limit under forced convection. The predictions coherently explains Collins et al. [1]‘s observations of high charge efficiencies when charging currents are higher than the theoretical limit. Also, the model consistently explains Pletcher et al. [2]‘s observation of insensitivity of the battery characteristics to the electrolyte flow rate. The model predicts satisfactory battery performance even at microscopic (μL s−1) flow rates which opens up the possibility for significant reduction in electrolyte pumping cost.

Monitoring of the Penetration of Chloride Ions to the Reinforcement Surface Through a Concrete Coating During Liquid Corrosion

The article is devoted to the study of chloritization of cement concretes during corrosion in liquid aggressive media. With the help of mathematical models, chloride ion concentration profiles are constructed for the thickness of the concrete coating. The possibility of using mathematical models in the description of corrosion of the second type of cement concretes to determine the mass transfer indicators and calculate the corrosion rate of reinforcing steel in concrete is shown. The effect of hydrophobizing additives on the rate of chloride ion transfer through the concrete coating to the reinforcement surface is considered. A study was made of the chlorination of cement concretes taking into account the phenomenon of pore and capillary colmatation. Graphical dependences of the rate of chloride ions transport on the speed and degree of colmatation of pores and capillaries of concretes based on Portland cement during corrosion in liquid aggressive media are presented. Time intervals for the beginning of corrosion of steel reinforcement are established for the corrosion of reinforced concrete in liquid aggressive media containing chloride ions.

Ultrafast Sodium Full Batteries Derived from XFe (X = Co, Ni, Mn) Prussian Blue Analogs.

Exploring high-rate electrode materials with excellent kinetic properties is imperative for advanced sodium-storage systems. Herein, novel cubic-like XFe (X = Co, Ni, Mn) Prussian blue analogs (PBAs), as cathodes materials, are obtained through as-tuned ionic bonding, delivering improved crystallinity and homogeneous particles size. As expected, Ni-Fe PBAs show a capacity of 81 mAh g-1 at 1.0 A g-1 , mainly resulting from their physical-chemical stability, fast kinetics, and "zero-strain" insertion characteristics. Considering that the combination of elements incorporated with carbon may increase the rate of ion transfer and improve the lifetime of cycling stability, they are expected to derive binary metal-selenide/nitrogen-doped carbon as anodes. Among them, binary Ni0.67 Fe0.33 Se2 coming from Ni-Fe PBAs shows obvious core-shell structure in a dual-carbon matrix, leading to enhanced electron interactions, electrochemical activity, and "metal-like" conductivity, which could retain an ultralong-term stability of 375 mAh g-1 after 10 000 loops even at 10.0 A g-1 . The corresponding full-cell Ni-Fe PBAs versus Ni0.67 Fe0.33 Se2 deliver a remarkable Na-storage capacity of 302.2 mAh g-1 at 1.0 A g-1 . The rational strategy is anticipated to offer more possibilities for designing advanced electrode materials used in high-performance sodium-ion batteries.

A charge-transfer resistance model and Arrhenius activation analysis for hydrogen ion transmission across single-layer graphene

Transmission rates for protons and deuterons across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures are measured as a function of temperature in electrochemical hydrogen pump cells. Rates of ion transmission through graphene are obtained in the form of area-normalized ion-transfer resistances, and are interpreted in terms of ion-exchange current densities and standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step is then used to provide rate constants for the fundamental microscopic step of ion (proton or deuteron) transmission across graphene. Application of this rate model to interpret variable-temperature data on proton and deuteron transmission rates provides values for the activation energy and pre-exponential factor for the fundamental ion transmission step across graphene. Activation energies obtained from the Arrhenius plots for proton and deuteron transmission are as follows; for proton, Eact = 48 ± 2 kJ/mole (0.50 ± 0.02 eV) and for deuteron, Eact = 53 ± 5 kJ/mole (0.55 ± 0.05 eV). The difference between these two values of approximately 5 kJ/mole is in good agreement with the expected difference in vibrational zero-point energies for OH and O-D bonds, albeit with some uncertainty given the uncertainties in the activation energy values. Pre-exponential frequency factor values of 8.3 ± 0.4 × 1013 s−1 and is 4.7 ± 0.5 × 1013 s−1 were obtained for proton and deuteron transmission respectively across graphene. These pre-factor values are both quite large, on the order of the values predicted from the Eyring – Polanyi equation with a transmission coefficient near one. The ratio of 1.8 for the rate pre-factors (H/D) is in reasonable agreement with the value of 1.3 for the ratio of bond vibrational frequencies for OH and O-D stretching, respectively. Taken together, these data support a model in which proton and deuteron transmission across graphene are largely adiabatic processes for which the differences in transmission rate at room temperature are due largely to differences in activation energies.

Mechanistic insight into the influence of Al2O3 concentration on the electro-reduction of V2O3 to vanadium in molten Na3AlF6

The influence of Al2O3 concentration on the direct electrochemical reduction of porous vanadium sesquioxide precursors in molten Na3AlF6 was investigated by constant voltage electrolysis and cyclic voltammetry. The products obtained by electrolysis at various Al2O3 concentrations were examined by XRD and SEM. Specifically, the morphologies of the outer metal layer initially formed were characterized by SEM and EDS, also their effects on the electrolysis process were systematically studied. The results indicate that the Al2O3 concentration in molten Na3AlF6 has a significant impact on the reduction process. When 0.5 mass% Al2O3 is added into molten Na3AlF6, the reduction rate of V2O3 can be improved because of the appropriate increase of aluminium reductant and relatively high oxide ion transfer rate. However, a high Al2O3 concentration (1.0–3.0 mass%) causes the massive precipitation of aluminium at cathodic outer metal layer and further forms Al–V alloy, causing the densification of the outer metal layer and hampering the reduction reaction kinetics.

Microwave-assisted synthesis of SnO2/mesoporous carbon core-satellite microspheres as anode material for high-rate lithium ion batteries

The highly ordered microspheres containing ultrafine SnO2 nanoparticles and mesoporous carbon is fabricated using microwave-assisted hydrothermal method for high-rate and durable Li-ion battery application. The uniformly carbon nanosphere with an average diameter of around 90 nm exhibits large specific surface area and good mechanical stability to minimize the volume expansion of SnO2 during intercalation-deintercalation. The homogeneous coverage with 3–6 nm SnO2 nanoparticle provides sufficient active sites to facilitate the rapid lithiation-delithiation process through the fast ion transfer rates. Remarkably, SnO2/OMCS at Sn/C ratio of 35% exhibits an excellent initial capacity of 1770 mAh g−1 at a current density of 35 mA g−1 and can maintain at a stable capacity of 400 mAh g−1 under an ultra-high current density of 3500 mA g−1 after 400 cycles. The intimate contact of SnO2 with ordered mesoporous carbon sphere shortens the electrons transport paths, resulting in the enhancement of stable lithium storage capacity at a high current density. Results obtained in this study clearly demonstrate that SnO2/OMCS nanocomposite is a promising material for lithium-ion battery and can provide an alternative to fabricate novel mesoporous carbon sphere-based nanocomposites for long cycle life and high-rate energy storage application.

Multi-scale impedance model for supercapacitor porous electrodes: Theoretical prediction and experimental validation

In this paper, a multi-scale impedance model is developed for evaluating the charge storage capacity and ion-transfer rate of supercapacitor porous electrodes. This model can be used to theoretically understand the contributions of the intra-particle pore at nano-scale, the inter-particle pore at micron-scale, and the porous electrode at millimeter-scale to the performance of supercapacitor. Then, impedance spectrum characteristics of porous electrodes are screened via numerical simulation based on the developed multi-scale impedance model, especially for different electrode thicknesses and various faradaic processes with different time constants. Subsequently, seven supercapacitor samples, four of them with electrode thicknesses of 60, 100, 180, and 370 μm respectively, and three of them with different types of current collectors (nickel, carbon-coated stainless-steel, and stainless-steel), are fabricated and characterized to validate the developed model. The fitting results and residuals of the measured impedance data validate the linear relationship of the scaled equivalent resistance of electrolyte v. s. electrode thickness and the scaled characteristic time constant v. s. the square of electrode thickness. The validated multi-scale impedance model might offer an effective method to rationally balance or optimize charge storage and charge transfer via morphology design for porous electrodes.

New Binder‐Free Metal Phosphide–Carbon Felt Composite Anodes for Sodium‐Ion Battery

AbstractMetal phosphides are promising anode candidates for sodium‐ion batteries (SIBs) due to their high specific capacity and low operating potential but suffer from poor cycling stability caused by huge volume expansion and poor solid‐state ion transfer rate. Herein, a new strategy to grow a new class of mesoporous metal phosphide nanoarrays on carbon felt (CF) as binder‐free anodes for SIBs is reported. The resultant integrated electrodes demonstrate excellent cycling life up to 1000 times (>90% retention rate) and high rate capability of 535 mAh g−1 at a current density of 4 A g−1. Detailed characterization reveals that the synergistic effect of unique mesoporous structure for accommodating huge volume expansion during sodiation/desodiation process, ultrasmall primary particle size (≈10 nm) for providing larger electrode/electrolyte contact area and shorter ion diffusion distance, and 3D conductive networks for facilitating the electrochemical reaction, leads to the extraordinary battery performance. Remarkably, a full SIB using the new CoP4/CF anode and a Na3V2(PO4)2F3 cathode delivers an average operating voltage of ≈3.0 V, a reversible capacity of 553 mAh g−1, and very high energy density of ≈280 Wh kg−1 for SIBs. A flexible SIB with outstanding mechanical strength based on this binder‐free new anode is also demonstrated.

Proton Transfer Can Occur at High Rates through Single-Layer Graphene in Nafion | Graphene | Nafion Sandwich Structures

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures. Single-layer graphene prepared by chemical vapor deposition onto copper foil was sandwiched between Nafion membranes by a series of hot-pressing steps coupled with chemical etching to remove the copper. Proton current densities in excess of 1 A / cm2 were observed for proton transfer across the graphene at cell bias voltages less than 200 mV. Confocal Raman spectroscopy measurements of the Nafion / graphene sandwich structures are consistent with single-layer graphene having very few structural defects embedded between Nafion membranes. Ion-transfer rates for lithium, sodium, potassium, and other cations were also measured and found to be much lower than rates for proton transfer, in most cases by many orders of magnitude. Two-layer and few-layer (3-5 layer) graphene sandwich structures were also studied and found to give proton currents much lower than single-layer graphene. Proton currents are described in terms of an area-normalized resistance following correction for all other contributions to series resistance in the cells. Resistances are then analyzed in terms of a rate constant for proton self-exchange across graphene. Figure 1

Enhanced electrochemical performances of heteroatom-enriched carbon with hierarchical pores prepared by trehalose as a pore-forming agent and a simple one-step carbonization/activation process for supercapacitors

It is highly desired to simultaneously introduce active heteroatoms and abundant hierarchical pore structures for enhanced electrochemical performances on carbon materials. Herein, trehalose as a pore-forming agent was added into polyvinylpyrrolidone/melamine formaldedyde resin mixture with high concentrations of nitrogen and oxygen. Then a simple one-step carbonization/activation process was adopted and heteroatom-enriched carbon with hierarchical pores (HPC) was fabricated successfully. HPC/HPC symmetric supercapacitors were assembled using KOH electrolyte. It is clearly demonstrated that due to the pore-forming action of trehalose HPC shows the porous honeycomb, interconnected and worm-like pore structure, which is favorable to enhance the double-layer capacitance. It is confirmed that in our system the three active species of pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5) and quinone type oxygen (O-I) are responsible for the pseudocapacitive behavior. Based on XPS, nitrogen adsorption/desorption isotherms and electrochemical impedance spectroscopy, it is deduced that the ratio-optimized HPC-T30 exhibits high concentration of three active species (8.17 at.%), increased specific area (351.26 m2 g−1) and tuned hierarchical pore structures with substantial micropores (micropore area of 321.68 m2 g−1) and a small amount of mesopores and macropores, which lead to decrease of charge transfer resistance, increase of transfer rate of electrolyte ions in the pores and excellent electrochemical performances. In cyclic voltammetry tests of three-electrode system and galvanostatic charge/discharge tests of two-electrode system, HPC-T30 displays high specific capacitance, 46% and 1.2-time enhancement compared to untreated HPC-T0, respectively. The optimized HPC-T30/HPC-T30 supercapacitor delivers the energy density of 6.69 W h kg−1 in 6 M KOH electrolyte. Furthermore, the supercapacitor shows a capacitance retention of 91.16% up to 6000 cycles and the coulombic efficiency reaches nearly 100% for each charged/discharge cycle, demonstrating its good cyclic stability.

Synthesis of Spherical Al-Doping LiMn2O4 via a High-Pressure Spray-Drying Method as Cathode Materials for Lithium-Ion Batteries

Uniform and spherical LiAl0.075Mn1.925O4 particles have been successfully synthesized by the high-pressure spray-drying method. The structures and electrochemical properties of the particles were characterized by various techniques. Benefiting from the sphere-like morphology and Al-doping, LiAl0.075Mn1.925O4 delivers a capacity retention of 81.6% after 1000 cycles at 2°C, while LiMn2O4 exhibits a capacity retention of only 32.2%. The rate capability and reversible cycling performance are also improved. Furthermore, this work significantly alleviates the dissolution of Mn in LiMn2O4 materials, and effectively improves the transfer rate of lithium ions at the electrode/electrolyte interface. The spherical LiAl0.075Mn1.925O4 prepared by a facile method shows great potential for practical application in low-cost and long-life lithium-ion batteries.