Ion Transport In Electrolytes Research Articles

Dielectric Modulus and Conductivity Scaling Approach to the Analysis of Ion Transport in Solid Polymer Electrolytes

Solid polymer electrolyte films (SPEs) based on poly(methyl methacrylate) are prepared using a solution cast technique. The temperature‐dependent behavior of dielectric, modulus spectra and ac conductivity has been investigated. The long tail of the real part of modulus (M′) in the low frequency indicates the capacitive nature of the samples. The frequency dependence of imaginary part of modulus (M″) shows a non‐Debye relaxation that has been explained using the Kohlrausch–Williams–Watts stretched exponential function. The activation energy for the relaxation is almost same as the activation energy for the conduction. The relaxation time obtained from the tangent loss graph (τδ) is about two orders of magnitude larger than that obtained from the imaginary part of modulus graph (τm). The ac conductivity has been found to obey Jonscher's universal power law. Transport parameters show that addition of filler creates additional hopping sites for the charge carriers and also increases the charge carrier density. It is also observed that the higher ionic conductivity at higher temperature is due to increased thermally activated hopping rates accompanied by a significant increase in carrier concentration. The contribution of carrier concentration to the total conductivity is also confirmed from Summerfield scaling. POLYM. ENG. SCI., 60: 297–305, 2019. © 2019 Society of Plastics Engineers

High-capacity K-storage operational to −40 °C by using RGO as a model anode material

Low operating temperatures (< −30 °C) result in slow ion transport (e. g. Li+, Na+, K+) in electrolytes and electrodes and severe decay in capacity of batteries. Here, we demonstrate a capacity of 171 mAh g−1 at −40 °C with a high capacity retention rate of 58% compared to room temperature by using the reduced graphene oxide (RGO) as the anode for potassium ion batteries (PIB). This high retention at low operational temperatures is one of the record high low-temperature capacities among all the carbon anode materials. RGO was prepared and optimized as a representative defective nanocarbon material with a moderate amount of surface defects, large interlayer spacing, and relatively high electrical conductivity. These factors are known to shorten the ions’ transport distance for storage for the ions and reduce the difficulty of ion intercalation, especially at low temperatures. For these reasons, the capacity of RGO at −40 °C is ten times that of graphite, hard carbon and GO. In addition, optimized RGO shows much higher K-storage capacity (171 mAh g−1) than Li and Na-storage capacities (68 and 56 mAh g−1, respectively), which reveals the importance of the coordination between the high mobility of K+ ion in the electrolyte and the surface-dominated ion storage mechanism on the energy storage performance at low temperatures.

Scalable and green production of porous graphene nanosheets for flexible supercapacitors

Graphene has showed great promise as supercapacitor electrode materials due to the fast charging/discharging rate, high theoretical surface area and excellent cyclic stability. However, scalable synthesis of graphene, especially with pore architectures, is still a challenge. Herein, we developed a scalable and green method to synthesize the porous graphene nanosheets (PGSs), which involved a liquid-phase exfoliation process carried out in water. PGSs and single-wall carbon nanotubes (SWCNTs) mixed suspension was vacuum filtered to fabricate the PGS@SWCNT composite film (GNCF) for electrode materials. The as-prepared free-standing GNCF electrode exhibits a desirable electrochemical energy storage capability due to the synergistic effect between PGSs and SWCNTs. In addition, the holes out of nanosheets and the pores in plane provide more electrolyte ion transport channels. To assess the practical applications of the GNCF, a flexible symmetric all-solid-state supercapacitor was fabricated. Such a supercapacitor device delivers a specific capacitance of 202.5 F/g at 10 mV/s, along with an outstanding cyclic stability with the capacitance retention of 91.2% over 10,000 cycles. These performances are comparable with those of the graphene-based electrodes prepared by CVD or Hummers’ method. These findings provide a cost-effective, environmentally friendly and scalable method for the production of porous graphene nanosheets which can be used in supercapacitors.

A high energy storage supercapacitor based on nanoporous activated carbon electrode made from Argan shells with excellent ion transport in aqueous and non-aqueous electrolytes

Abstract A high-performance supercapacitor electrode was obtained from waste biomass, Argan shells. This electrode was fabricated using a simple activation technique; demonstrating an industry-friendly process, which is capable of having precise control over pore size distribution of the activated carbon derived from a natural resource. The obtained samples exhibited different pore size distributions, based on different activating agents and different activation routes used in the process. Two distinctly different pore size distributions were obtained from a single source and same synthesis routes, only by exploiting the size differences of the activating agents, which are potassium (KOH) and sodium (NaOH) ions respectively. This process enabled a significant enhancement of specific energy, with no or very negligible change in specific power. Thanks to the fact that there was almost no loss in ion diffusion properties within the porous network of the electrodes even after a change in the electrolytes, to the more viscous ionic liquid (Ethyl-Methyl Imidazolium Tetrafluoroborate), from comparatively less viscous aqueous and organic-based electrolytes. This size-controlled activation enabled us obtaining appropriate pore size distribution, which is a combination of a large number of micropores and a sufficient number of mesopores. This provided a very high specific surface area of 2251 m2/g with a total pore volume of 1.04 cm3/g. Very high specific capacitance of ∼179 F/g in aqueous electrolyte (0.5 M K2SO4) was obtained. The excellent porosity resulted in very high specific energy of ∼65 Wh/kg and high specific power of ∼2 kW/kg in ionic liquid electrolyte for 3.5 V operating voltage.

Pore Size‐Engineered Three‐Dimensional Ordered Mesoporous Carbons with Improved Electrochemical Performance for Supercapacitor and Lithium‐ion Battery Applications

AbstractThree‐dimensional ordered mesoporous carbons (OMCs) are desirable for high performing energy storage devices because they provide a continuous electron pathway to ensure good electrical contact and also facilitate electrolyte ion transport by reducing diffusion lengths during charge‐discharge process. Here, we report the synthesis of three dimensional mesoporous carbon (CMK‐8) using KIT‐6 and sucrose as silica template and carbon source, respectively. Initially, KIT‐6, synthesized at different hydrothermal temperature (100, 130 and 150 °C) is used as silica template to prepare OMCs with different pore diameter. The resulting OMCs were extensively characterized by SAXS, FE SEM, HR‐TEM, BET and Raman techniques. The SAXS pattern shows distinct reflections at low 2θ range corresponds to ordered mesoporous structure of cubic Ia3d space group. The OMC possesses a specific surface area of 1017 m2/g with a mean pore size of 4.1 nm and large pore volume of 1.14 cm3 g−1. As electrode material for supercapacitor application, the resulting mesoporous carbon delivers a high capacitance of 252 F/g @ 0.5 A/g and shows good rate performance (75% retention in capacitance at high current rates) and outstanding cyclic stability (91% retention after 30,000 cycles). Further, as anode for Li‐ion battery application, it also exhibits promising specific capacity (856 mAh/g) with good cyclic and rate capability. The excellent electrochemical properties of the mesoporous carbon material are attributed to its three dimensional porous structure and high surface area with interconnected mesopores that provides rapid ion and electron transport during electrochemical process.

Hydrothermal Synthesis of NiCo2O4/CoMoO4 Nanocomposite as a High‐Performance Electrode Material for Hybrid Supercapacitors

AbstractThe hierarchical hollow NiCo2O4/CoMoO4 hybrid composite was directly grown on Ni foam (NF) under hydrothermal conditions at 110–140 °C for different growth times. This robust hollow structure benefits from a weakly alkaline environment provided by tetramethyl ammonium hydroxide and can provide a large surface area, low resistance, and many active sites to facilitate electrolyte ion transport and fast redox reactions. The as‐prepared NiCo2O4/CoMoO4 hybrid with synergetic effects shows improved electrochemical properties with an excellent capacitance (2080 F/g at 1 A/g) and long‐term cycling reliability (70 % retention undergoing 3000 cycles at 10 A/g). An assembled hybrid supercapacitor based on the as‐prepared NiCo2O4/CoMoO4 hybrid electrode delivers a high energy density of 35.63 Wh/kg at a power density of 845 W/kg and a long‐term cycling life (capacitance retention of 71.4 % over 9000 cycles). Thus, the as‐prepared NiCo2O4/CoMoO4 nanocomposite has a definite advantage for supercapacitor electrode materials.

Exceptionally Reversible Li-/Na-Ion Storage and Ultrastable Solid-Electrolyte Interphase in Layered GeP5 Anode.

In this study, we synthesize two layered and amorphous structures of germanium phosphide (GeP5) and compare their electrochemical performances to better understand the role of layered, crystalline structures and their ability to control large volume expansions. We compare the results obtained with those of previous, conventional viewpoints addressing the effectiveness of amorphous phases in traditional anodes (Si, Ge, and Sn) to hinder electrode pulverization. By means of both comprehensive experimental characterizations and density functional theory calculations, we demonstrate that layered, crystalline GeP5 in a hybrid structure with multiwalled carbon nanotubes exhibits exceptionally good transport of electrons and electrolyte ions and tolerance to extensive volume changes and provides abundant reaction sites relative to an amorphous structure, resulting in a superior solid-electrolyte interphase layer and unprecedented initial Coulombic efficiencies in both Li-ion and Na-ion batteries. Moreover, the hybrid delivers excellent rate-capability (symmetric and asymmetric) performance and remarkable reversible discharge capacities, even at high current rates, realizing ultradurable cycles in both applications. The findings of this investigation are expected to offer insights into the design and application of layered materials in various devices.

Novel Ni6MnO8/NiMnO3 composite as a highly stable electrode material for supercapacitors

Abstract In our work, a Ni6MnO8/NiMnO3 composite, which are porous cuboids coated with porous spheres to form a 3D hierarchical structure, is successfully fabricated using a one-step solvothermal method. As an electrode material, this composite possesses high specific capacitance (291.9F/g at 1 A/g) and excellent rate capability (77.2% of specific capacitance maintained at 10 A/g). The specific capacitance retention is 83.4% after 5000 charge/discharge cycles at a current density of 3 A/g. The enhanced performance is mainly attributed to the synergistic effect of Ni6MnO8 and NiMnO3, which can provide more active sites for redox reaction and contribute to electrolyte ion transport. Results demostrate that the Ni6MnO8/NiMnO3 composite as an electrode material in supercapacitors exhibits outstanding electrochemical performance and potential for various applications.

Flexible carbon cloth based solid-state supercapacitor from hierarchical holothurian-morphological NiCo2O4@NiMoO4/PANI

Based on carbon cloth, flexible solid-state supercapacitors from bimetal oxide arrays of NiCo2O4@NiMoO4 deposited by conductive polymers of polyaniline (PANI) are prepared employing a two-step hydrothermal reaction, followed by high-temperature annealing and an in-situ polymerization process. The presence of NiMoO4 nanoplates prepared on NiCo2O4 nanowhires leads to the hierarchical flower-like nanostructures. In order to further enhance the conductivity and wettability, NiCo2O4@NiMoO4 is coated by the three-dimensional networks of PANI nanorods by means of the in-situ polymerization of aniline, the obtained NiCo2O4@NiMoO4/PANI nanocomposites illustrate a specific hierarchical holothurian-like morphology, which facilitates the adsorption and penetration as well as the rapid transportation of electrolyte ions within the nanopores in the active electrode materials, resulting in excellent electrochemical performances. The specific areal capacitance of the flexible carbon cloth-based electrodes reaches 2.38 F cm−2 at current density 1.0 mA cm−2, and the capacitance retention is 92.36% of the initial value after 5000 cycles, which finds applications in flexible and wearable devices.

Decoupling and correlating the ion transport by engineering 2D carbon nanosheets for enhanced charge storage

The high aspect ratio of two-dimensional (2D) carbon materials feature unique superiorities and great potential in supercapacitors. The transport of electrolyte ions inside the 2D carbon electrode that can directly affect the overall electrochemical performance, is a key issue that need to be concerned. Nevertheless, the research regarding this process always lags far behind the material concerns and the intrinsic factors to affect ion transport remain obscure. Here we for the first time focus on and correlate the potential and possible factors for addressing this issue using 2D porous carbon nanosheets with tunable thickness as representative demos. A compromise balance between the thickness and ion transport behavior is presented and investigated. The optimal thickness of nanosheets can effectively avoid curling and stacking of ultrathin nanosheets, shorten the distance of ion transport and accelerate the ion diffusion, which render the material with the low resistance corresponding to minimum Warburg coefficient and maximum ion diffusion coefficient for fast transport of electrolyte. A superior capacitance value of 280 F g−1 can be obtained at 0.5 A g−1, with a retention rate of 81% even at a large current density of 100 A g−1. Besides, the symmetric supercapacitor with 1-ethyl-3-methylimidazolium tetrafluoroborate electrolyte (EMIMBF4) delivers an energy density of 94 Wh kg−1 at 1.8 kW kg−1. This work illustrates the importance for transferring the focus from carbon materials to electrolyte ion transport behavior within electrode and provides a guidance for regulating the ion transport by engineering the microscopic thickness of advanced materials in energy-related fields.

Hierarchical three-dimensional graphene-based carbon material for high-performance lithium ion batteries

Hierarchical three-dimensional graphene-based carbon materials have been successfully prepared via a template-assisted approach. It is found that the structure, morphology, graphitization degree, surface area and pore volume for the obtained carbon materials could be easily controlled by adjusting the amount of reactant and template. The results show that the obtained hierarchical nanostructured carbon material possesses a very stable reversible capacity of 395 mAh g−1 at 1C and exhibits extremely high reversible capacity of 163 mAh g−1 at the high rate of 20C. The large macroporous network and small mesopores in the materials can facilitate electrolyte and lithium ion transportation. The hollow carbon nanoparticles composed of stacked graphene can favor the electron diffusion within the materials. The micropores in the carbon materials can also increase the reversible capacity.

Multifunctional surface-modified ultrathin graphene flakes for thermal and electrochemical energy storage application

This paper presents the preparation of ultrathin graphene nanoflakes (UGF) from microwave-irradiated expanded graphite, and delaminated via solution-phase exfoliation for thermal and electrochemical energy storage application. The surface of the UGF were modified with a carbon rich layer (UGF-C) to minimize restacking and prone to oxidizer-etching to create pores to support electrolyte ion transport. UGF-C exhibited specific capacitance of 83F/g at current density of 0.25 A/g with 97% capacitance retention for 1000 cycles. Also, UGF incorporated xylitol based phase change materials enhanced thermal conductivity by 40.1%.

Influence of Host Polarity on Correlating Salt Concentration, Molecular Weight, and Molar Conductivity in Polymer Electrolytes.

We use coarse-grained molecular dynamics simulations to study the effect of salt concentration and host polymer molecular weight on ion transport in polymer electrolytes. We find that increasing salt concentration or molecular weight similarly slows polymer dynamics across a wide range of host polarities, and that the resulting relaxation times display a correlation to the product of the salt concentration and polymer molecular weight. However, we find that molar conductivity only decreases with polymer dynamics at high polarities but is uncorrelated with the latter at low polarities. We attribute such differences to the variation in ionic aggregation between high and low polarity electrolytes. At low polarity, ionic dissociation significantly increases with molecular weight and salt concentration, offsetting the slowdown in polymer dynamics and yielding the observed insensitivity of molar conductivity. However, at high polarity, ions are mostly dissociated, independent of either molecular weight or salt concentration, thereby strongly coupling molar conductivity to polymer dynamics.

A facile synthesis of nitrogen-doped hierarchical porous carbon with hollow sphere structure for high-performance supercapacitors

In this work, nitrogen-doped carbon with hierarchical porous and hollow sphere structure has been synthesized through a simple and facile route of spray drying using glycine as the nitrogen-containing carbon source. After KOH activation, the prepared material (NHPCA) shows a large specific surface area of 962 m2 g−1 with moderate N-doping of 5.74% and exhibits a high specific capacitance of 271 F g−1 in 6 M KOH electrolyte at 1.0 A g−1, remarkable rate capability and particularly stable cycling performance with no significant specific capacitance drop after 10000 cycles at 1.0 A g−1. The excellent electrochemical properties come from the unique structure and the doping of nitrogen. The hierarchical pore structure improves the efficiency of electrolyte ions transport, and diffusion and the hollow sphere structure further facilitates mass transport. The doping of nitrogen increases the total capacitance by providing redox pseudo-capacitance. The results indicate the as-prepared nitrogen-doped carbon with hierarchical porous and hollow sphere structure can be used as a hopeful candidate for an efficient electrode of commercial supercapacitors devices.

High-Performance Micropore- and Mesopore-Rich Activated Carbon in Ionic-Liquid Electrolyte

Electrolyte optimization plays a key role in deciding energy density (ED) and power density (PD) of supercapacitors. Increasing cell voltage is an effective way to improve ED and PD. Therefore, an electrolyte with a large potential window is highly desired. Ionic liquid (IL) electrolytes, characterized by wide potential windows, excellent thermal stability, intrinsic ionic conductivity, non-volatility, non-flammability, and low environmental concerns, are potential electrolytes and are used for micropore-and mesopore-rich activated carbon (ACmicro and ACmeso) supercapacitors. IL electrolytes consisting of various cations of EMI+ (1-ethyl-3-methyl imidazolium), PMP+ (N-propyl-N-methyl pyrrolidinium), and BMP+ (N-butyl-N-methyl pyrrolidinium) and various anions of TFSI– (bis(trifluoromethyl sulfonyl) imide), BF4 – (tetrafluoroborate), and FSI– (bis(fluorosulfonyl)imide) are explored. The electrolyte viscosity and conductivity and ion transport properties at the ACmicro and ACmeso electrodes are studied. Also, the capacitance, cycling stability, and rate capability of the ACmicro and ACmeso electrodes are systematically investigated, and post-mortem material analyses are conducted. This study shows the influence of IL composition on charge–discharge capacitances of the ACmicro electrodes is more pronounced than that for the ACmeso electrodes. The FSI-based IL electrolytes, whose electrochemical properties are dependent on cation, are found to be highly promising. Uniting EMI+ with FSI– results in low viscosity of electrolyte and high ion transport, optimizing the electrode capacitance and rate capability. Interchanging EMI+ with PMP+ enhances the cell voltage (to 3.5 V) and maximum ED (to 42 Wh kg−1) of the ACmicro cell at the cost of cycling stability.

Toward Understanding the Different Influences of Grain Boundaries on Ion Transport in Sulfide and Oxide Solid Electrolytes

Solid electrolytes provide a route to the development of all-solid-state batteries that can potentially surpass the safety and performance of conventional liquid electrolyte-based devices. Sulfide ...

Role of Defects in Ion Transport in Block Copolymer Electrolytes.

Ion conducting block copolymers can overcome traditional limitations of homopolymer electrolytes by phase separating into nanoarchitectures that can be simultaneously optimized for two or more orthogonal material properties such as high ionic conductivity and mechanical stability. A key challenge in understanding the ion transport properties of these materials is the difficulty of extracting structure-function relationships without having complete knowledge of all nanoscale transport pathways in bulk samples. Here we demonstrate a method for deriving structure-transport relationships for ion conducting block copolymers using thin films and interdigitated electrodes. Well-defined and directly imaged structure in films of poly(styrene)-block-poly(2-vinylpyridine) is controlled using techniques of directed self-assembly then the poly(2-vinylpyridine) is selectively converted into an ion conductor. The ion conductivity is found to be directly proportional to the total number of connected paths between electrodes and the path length. A single defect such as a dislocation anywhere in the path of an ion conducting route disconnects and precludes that pathway from contributing to the conductivity and results in an increase in the dielectric parameter of the film. When all the ion conduction pathways are blocked between electrodes, the conductivity is negligible, 4 orders of magnitude lower compared to a completely connected morphology and the dielectric parameter increases by a factor of 50. These results have profound implications for the interpretation, design, and processing of block copolymer electrolytes for applications as ion conducting membranes.

Influence of morphology of colloidal nanoparticle gels on ion transport and rheology.

We develop a simple model to probe the ion transport and mechanical properties of low volume fraction colloidal nanoparticle gels. Specifically, we study the influence of the morphology of gels on ion diffusion and the corresponding roles of affinity to and enhanced ion transport along nanoparticle surfaces. We employ kinetic Monte Carlo simulations to simulate ion transport in the colloidal gels, and we perform nonequilibrium molecular dynamics to study their viscoelastic behavior. Our results indicate that in the presence of enhanced diffusion pathways for ions along the particle surface, morphology has a significant influence on the diffusivity of ions. We demonstrate that some gel morphologies can exhibit simultaneously enhanced ion transport and mechanical properties, thus illustrating a strategy to decouple ion transport and mechanical strength in electrolytes.

Freestanding, Three-Dimensional, and Conductive MoS2 Hydrogel via the Mediation of Surface Charges for High-Rate Supercapacitor

Conductive hydrogels with fluidic nanochannels represent one of the most promising capacitive electrodes due to their highly porous structure for the rapid kinetics of electrolyte ion transport. Re...

Specific Features of Ion Transport in New Nanocomposite Gel Electrolytes Based on Cross-Linked Polymers and Silica Nanoparicles

The spesific features of ion transport are studied in the nanocomposite system based on a network matrix synthesized by radical polymerization of polyethylene glycol diacrylate in the presence of liquid aprotic electrolyte containing 1 M LiBF4 in γ-butyrolactone and SiO2 nanopowder. The self-diffusion coefficients measured by 7Li NMR spectroscopy with pulsed field gradient have a maximum at 2 wt % SiO2 nanoparticles. Nanocomposites of the latter composition exhibit the highest cation transport numbers (0.49) and the maximum of conductivity in the temperature interval under study (from –70 to 100°С): 4 mS/cm at 20°С and 1 mS/cm at –70°С. The second conductivity maximum at 6 wt % is characterized merely by the low effective activation energy of conduction. Possible mechanisms are invoked to explain the increase in conductivity. The first mechanism is based on the increase in the number of mobile charge carriers which may be a result of salt dissociation to ions, the second mechanism is associated with the development of a large number of favorable pathways for ion transport.