Percolation Effects in Supercapacitors with Thin, Transparent Carbon Nanotube Electrodes

There are many design tradeoffs in lithium-ion batteries, the most notable being the loss of energy density at high power densities. New design requirements, such as low battery cost, further exacerbate these classic tradeoffs because they require thick electrodes with high loading. The power density in these dense electrodes is typically limited by Li+ depletion in the cathode electrolyte, which shuts off the cell before the full capacity can be extracted. In most industrial processes, concentration depletion can be overcome by introducing advection, but the small-scale pores and solid current collectors in batteries prevent the electrolyte from sustaining any form of bulk motion. Strategies to improve power density in thick electrodes, therefore, include increasing the electrode porosity (which reduces energy density) or increasing the electrolyte transference number (which is a difficult materials challenge).This work uses finite-element simulations to understand how battery performance would be influenced by internal advection of liquid electrolyte. We also propose a mechanism to achieve this through electro-osmotic flow of the electrolyte through charged separators. Our simulations demonstrate that the addition of a channel that circulates the electrolyte between the two ends of the cell can improve the battery’s volumetric energy density by 69% when discharged at a 4 C rate. Performance gains can be further amplified to 104% if an advective current can be maintained at 1 um/s.Figures 1(a) and 1(b) depict the simulation layout, which includes a standard lithium-ion battery with an 80-um thick lithium cobalt oxide (LCO) cathode, 50 um separator region, and 152 um LiC6 anode with 1 M LiPF6 in 1:1 EC/DEC. A modification introduces a thin channel, 1.2X the width of the cell (~340 um), that allows for electrolyte flow between the current collector ends of the cathode and anode. The multiphysics model follows a Newman approach and captures Li+ diffusion, migration, and advection in the electrolyte. Lithium diffusion and electron conduction are modeled in the electrodes, and Butler-Volmer kinetics govern the reaction kinetics. In both geometries, the LCO cathode is discharged from 4.3 V to 2.5 V.We perform simulations of the following geometries to understand the isolated effects of electrolyte continuity and advection: (1) a standard cell with no channel, (2) a cell with a channel but no flow, and (3) a cell with bulk motion of electrolyte through a channel. Figure 1(c) shows the electrolyte salt concentration within each battery at the end of discharge. The standard cell geometry notably features depletion of lithium ions ~15 um into the cathode, rationalizing the sharp drop in the cell’s discharge curve at 2.85 V (Figure 2). The addition of a channel results in migration and diffusion of lithium ions along the concentration gradient between the two current collector ends, significantly delaying ion depletion in the cathode during discharge. Introducing bulk motion further minimizes the depletion of electrolyte in the cathode and, in the extreme case of 100 um/s flow, completely eliminates ion depletion limitations.Figure 2 shows the voltage vs. energy density graph of each design, demonstrating a 104% increase in energy density for a channel-added geometry with 1 um/s of continuous bulk motion. Higher flow rates have marginal performance benefits, as a two order of magnitude increase in velocity leads to a <1% increase in energy density. Figure 2 further illustrates that, without bulk motion, the introduction of a channel results in an energy density increase of 69% relative to a normal, channel-less cell. Because the channel’s contribution to the battery size varies depending on the design of the modified cell, the discharge curves in Figure 2 do not account for volumetric changes and thus represent the maximum performance benefit that can be realized. Taking into account the channel’s volume in this particular design, however, only drops the performance gains to 82% for 1 um/s flow and 52% without any flow, demonstrating significant improvements despite the lack of robust design optimization.This simulation shows that advective flow of liquid electrolyte can improve the high rate performance of classic lithium-ion battery designs, as well as future designs with thick and dense electrodes. We believe that this can be rationally done by using porous current collectors and allowing flow through individual cell layers until the electrolyte is recirculated at cylindrical cell walls. To drive the flow internally, a charged membrane can function as an electro-osmotic pump that harnesses the inherent electric field of the electrolyte, allowing for massive gains in energy density within compact energy storage systems. Figure 1

A hybrid PEMFC/supercapacitor device with high energy and power densities based on reduced graphene oxide/Nafion/Pt electrode

Utilization of spray pyrolyzed porous nickel cobaltite electrode as an advanced material for NiCo2O4@Graphite asymmetric supercapacitor device

The need for alternative high energy and power density energy sources has led scientists to study extensively the electrochemical energy conversion/storage devices such as electrochemical double layer capacitors (EDLC), batteries, fuel cells, etc. The values of power and energy densities of EDLC depend strongly on the properties of the electrode material and electrolyte used. EDLCs based on non-aqueous electrolytes or room temperature ionic liquids have a wider region of ideal polarizability (ΔE) compared to those based on traditional aqueous electrolytes 1-4. The wider ΔE in turn improves the energy and power density values of EDLCs. The specific adsorption of halide ions at single crystal electrodes from aqueous and non-aqueous electrolyte solutions has been demonstrated before 5,6. However, the specific adsorption of ions is a complicated process (depending on the electrode potential, chemical composition and concentration of halides etc.) and the kinetics of formation/rearrangement of electrical double layer between electrode and ionic liquid (IL) needs an extended examination. The main aim of this work was to characterize the adsorption of iodide ions from the mixture of two ionic liquids based on the same electrochemically stable 1-ethyl-3-methylimidazolium cation. Electrochemical impedance spectroscopy and cyclic voltammetry methods have been applied to study the electrochemical characteristics of the interface between pyrolytic graphite (PG) and mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium iodide (1-10 wt%). All measurements were carried out in a 3-electrode electrochemical cell using a PG as a working, Pt net as a counter, and Ag wire coated with AgCl as a reference electrodes. Current density (i) vs. electrode potential (E) dependencies (Fig. 1) show that two peaks in CV are caused by the addition of x wt% of EMImI into EMImBF4 (for PG | EMImBF4 interface there is no peaks within the potential region from ─1.5 V to +1.5 V (vs. Ag | AgCl)). Reduction and oxidation processes are more intensive in more concentrated solutions. A strong dependence of current density on the potential scanning rate (υ) for PG | EMImBF4+EMImI interface was observed in the region of peaks. Also the capacitance values calculated from CV data (Ccv= i / υ) show the strong dependence on υindicating a very slow adsorption process occurs. The shape of the complex impedance plane, i.e. Nyquist plots, total impedance and phase angle vs. frequency dependences depend strongly on the electrode potential and on the concentration of the I-ions in the solution. The values of series differential capacitance (C s) were calculated from Nyquist dependencies at different fixed ac frequencies (C s = (Z''i2πf)−1; where i = √−1and Z’’ is the imaginary part of impedance). C s depends strongly on ac frequency and electrode potential, increasing at lower ac frequencies and at anodic range of potentials. C s values depend also on the added amount of I-(1-10%) to the solution, increasing from 1 to 5wt% and diminish in more concentrated solutions. References L. Siinor, J. Poom, C. Siimenson, K. Lust, E. Lust, J. Electroanal. Chem., 719, 133 (2014).L. Siinor, C. Siimenson, K. Lust, E. Lust, Electrochem. Commun. 35, 5 (2013).R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochim. Acta, 85, 139 (2012).R. Palm, H. Kurig, K. Tõnurist, A. Jänes, E. Lust, Electrochem. Commun. 22, 203 (2012).L. Siinor, K. Lust, E. Lust, J. Electroanal. Chem., 601, 39 (2007).M. Väärtnõu, E. Lust, J. Electroanal. Chem., 565, 211 (2004). Acknowledgements Estonian Energy Technology Program project SLOKT10209T, Estonian Centers of Excellence in Science project: High-technology Materials for Sustainable Development TK117, Functional Micro/Mesoporous Nanomaterials for Novel Energy Conversion and Storage Systems IUT 20-13, Functionalization of carbon nanomaterials: synthesis, characterization and application SLOKT12180T.

Biomass-based carbonaceous materials derived from Musa Sapientum have gained much attention in recent years for their application in energy storage devices, especially supercapacitors. In the present work, we synthesized carbonaceous material from banana peel as the biomass precursor by using a pyrolysis method carried out at various temperatures (600, 800, and 1000 °C). The characterization of the prepared carbonaceous materials BP600, BP800 and BP1000 was done by using different characterization techniques such as FTIR, XRD, FE-SEM, and TEM, studies. The electrochemical study of the synthesized material was carried out by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) electrochemical impedance spectroscopy (EIS). The supercapacitive performance of the material was studied using a 3-electrode system with 3M KOH as an electrolyte. As a result, the BP600 exhibited a better specific capacitance with higher energy and power densities along with a maximum cyclic stability of 16,000 cycles. To show the practical applicability of the material BP 600, two electrode system studies were carried out as well, which showed preferentially good values for specific capacitance with appreciable power and energy density values. The study provides us with a green approach for the fabrication of non-toxic, low-cost, and environmentally friendly potential porous carbonaceous electrode materials by converting bio-waste into a clean and renewable source of energy.

The majority of published R&D in supercapacitors relies on testing at a small coin or Swagelok cell size. This is often due to a lack of availability of equipment and knowledge required to move to a larger scale. Whilst comparisons between materials and other components can be made at the small cell level, the actual performance in larger devices does not necessarily follow to the same extent. In many cases, a scaling factor can be applied to the values of energy and power density for an active material to give the corresponding values in large devices but this is not always the case. For the extrapolation of performance from small to large cell to be valid, many factors must be considered and matched to real devices such as electrode thickness, relative electrolyte amount, current collector properties and separator type. Whilst allowing an estimate of large device properties in this way from small cell level is useful, this approach does not consider many of the problems encountered when scaling up that also differ between cell geometries (cylindrical, pouch or prismatic) can result in predicted performances never being achieved. In the current work, the development from coin cell to >800 F A5 sized pouch cell (the same geometric area as the batteries of the Nissan Leaf) is reported using commercially available activated carbon (YP50-F, Kuraray, Japan) as the active material and TEABF4in acetonitrile electrolyte, with comparisons made throughout between the small and large scale. The scale-up of mixing of electrode ink is demonstrated from the 50 g to 10 Kg scale, with discussion of the effects of various components on the solid content achievable at the viscosities required for coating. Various parameters in electrode production are discussed in relation to their effects on cell performance at the coin and large pouch level including electrode thickness/loading and drying temperature profiles, the later of which being surprisingly shown to be responsible for delamination of electrode from current collectors and complete failure in large cells over time (not observed in coin cells). The effect of calendaring of electrodes at different temperatures is shown to produce a range of electrode densities, with the effect on adhesion, specific capacitance and ESR investigated in cell. It is shown that whilst electrode capacitance is, to a broad degree, unaffected by calendering, much benefit can be gained in terms of ESR and adhesion but that over-calendering has a major detrimental effect. The production of A5 sized pouch cells from the electrodes produced is illustrated, with early cell testing results used to highlight the problems faced in supercapacitor pouch cells resulting from early capacitance fade, gas production and a lack of compression. These problems are addressed through the application of external compression (as is provided in module assemblies and by the spring in coin cells) and through the float ageing of cells to a stable capacitance and subsequent gas removal, with the differences in time and energy required at coin and pouch cell level to achieve this highlighted. In conclusion, the effects and contributions of electrode thickness, number of electrodes, electrolyte mass and “parasitic” components such as packaging in pouch cells are considered in terms of both volumetric and gravimetric energy and power densities, contrasting the real device values obtained in scale-up with those of the active materials often reported and those predicted from coin cell results. Figure 1

Increasing concerns about environmental and energy problems accelerate the demands for high-energy-density batteries. Lithium-ion batteries (LIB) have large energy densities and have been well applied on not only portable devices such as smart phones but also electronic vehicles (EV). However, the energy densities of present LIBs for EV-use seem not to satisfy for a long cruising distance. Present LIBs for EV-use possess large power densities by the sacrifice of energy densities in order to shorten the charging time. In the practical LIBs for portable devices, dense and thick composite electrodes are used. In this case, the ion transport of electrolyte solution through the pores in the composite electrodes becomes a slow process. In order to enhance the energy densities with remaining high power densities, the ion transport rate should be increased for the dense and thick composite electrodes. Unfortunately, to develop dense and thick electrodes with high power densities, fundamental understanding of the ion transport in composite electrodes has not been clear. In this study, ion transportation behavior was investigated with ac impedance spectroscopy. In a conventional use of ac impedance spectroscopy, a composite electrode is used as a working electrode of the measurement cell. In this case, the result of impedance spectroscopy contains all resistances and capacitances accompanying the whole of the charge and discharge reaction including ion transfer through electrolyte and active materials, ion diffusion within active materials, etc. To focus on the impedance of ion transportation of electrolyte, it must be discriminated from that derived from other reaction steps. In order to obtain impedance spectra derived from only ion transportation, composite electrodes without a current collector were used. As a result, we obtained semi-circles, and this semi-circle was assigned to the ion transport process through the graphite composite electrode. As the potential went down, two semi-circles were observed. The characteristic frequency and the resistance of the semi-circle at higher frequency almost did not change. However, at lower frequency, the characteristic frequency and resistance changed. Further, we used a model electrode to understand the semi-circles. The detail analysis will be presented at the meeting.

Although batteries have been successfully employed in light-duty vehicles (hybrid and fully electric cars), batteries still find it is hard to satisfy power-hungry platforms such as trucks, trams, and buses. Conventional lithium ion batteries (LIB) could provide power density less than 3 kW/kg with relatively moderate energy density. On the other side, supercapacitors (SCs) that are known to provide power density of more than 10 kW/kg have a very low energy density of less than 1 Wh/kg and thus, SCs could not be used solely as energy storage. However, employing anode from LIB and electrode from SC together in a single system would benefit both high energy and power density, which system is-so called lithium ion capacitor (LIC). To date, so many attempts have been done to investigate the best anode material combination with activated carbon (AC) to obtain high performance LIC. The anode materials that are commonly utilized in LIB such as graphite, hard carbon, Li4Ti5O12 (LTO), Fe3O4, SnO2, and silicon based material have been extensively studied for LIC. Graphite that is known as an ubiquitous anode material in LIBs provided relatively high energy density of c.a. 100 Wh/kg and maximum power density of less than 2 kW/kg in LIC. Among all, LTO application was found to be the least desirable material for LIC since it provided relatively low maximum energy and power density, which was merely less than 40 Wh/kg and 2 kW/kg, respectively, due to its low conductivity and high voltage plateau at around 1.5 V. Grphene has been used to reinforce rate capability of Fe3O4 and it demonstrated promising energy density as high as 86 Wh/kg. However, the maximum power density of the enhaced electrode material was only 2.6 kW/kg. Higher power LIC can be achieved by harnessing hard carbon anode material, which displayed maximum power density of about 50 kW/kg. However, the high power density of hard caron in LIC was compensated by extremely low energy density of less than 2 Wh/kg. Herein, we investigate the electrochemical performance of LIC by using silicon oxycarbide (SiOC) as the anode and the AC from biomass (Zalacca salacca) peels as the cathode to provide massive power while maintaining the energy density relatively high. SiOC showed an outstanding rate capability performance indicated by relatively high discharge capacity of over than 200 mAh/g at current density of 80 A/g. The excelent rate capability was attributed to the fast lithium diffusion in SiOC, which was represented by high lithium diffusion coefficient (DLi+) of 5.1 × 10-6 cm2/s. Meanwhile, the AC derived from biomass is promising since its surface area (c.a. 2600 m2/g) and capacitance (163.7 F/g) was significantly higher than that of commercial AC. Combination of SiOC and AC derived from Zalacca salacca in a single unit of LIC could be used to provide a massive power density of 155kW/kg while maintained relatively high energy density of 25 Wh/kg. Moreover, the resulting LIC demonstrated highly stable cycle retention at high C-rate. After reaching equilbrium state, the cycle retention of LIC at 50C was 99.4% after more than 40,000 cycles. At higher C-rate of 100C, the cycle retention of the LIC was 99% even after more than 75,000 cycles. The present study indicates that LIC design based on SiOC and AC from Zalacca salacca peels is suitable for energy storage in high-duty platforms such as trucks, trams, and buses.

In the past few years, considerable effort has been devoted to the study of graphene as energy storage material for supercapacitors. More recently other 2D materials have emerged such as MXenes and transition metal dichalcogenides. In this study, chemically exfoliated and restacked nanosheets of metallic 1T phase MoS2 has been introduced as a supercapacitor electrode and its electrochemical charge storage mechanism has been investigated in aqueous and organic electrolytes. High volumetric capacitance values (650F/cm3) have been obtained with 1T phase MoS2 even at high scan rates. High volumetric energy density (0.1Wh/cm3) and power density (51W/cm3) values were measured with organic electrolytes. During chemical exfoliation and restacking process, structural, chemical and electronic properties of MoS2 change via phase transformation from semiconducting 2H MoS2 to metallic 1T MoS2. High charge storage performances can be explained with the distinct structural and electronic properties of metallic 1T phase MoS2. The good electrical conductivity of the 1T phase enables thicker and binder free film fabrication. Interestingly the capacitance obtained from 1T MoS2 is surprisingly high comparing the modest surface area of MoS2. In order to understand the charge storage mechanism, ex-situ XRD measurements were conducted. Our results demonstrate that cations (Proton, Li+, Na+, K+, TEA+) intercalate to the two dimensional MoS2 flakes and enhance the accessible surfaces for the charge storage. Charge storage performance is stable over 5000cycles (97% capacitance retention). Our study provides insight into the electrochemical behavior of the phase engineered exfoliated and restacked nanosheets.

Functionalized interfacial cover design toward pure silicon anode for high power density lithium–ion capacitor

The ability to engineer electrode microstructures to increase power and energy densities is critical to the development of high-energy density lithium-ion batteries. Because high tortuosities in porous electrodes are linked to lower delivered energy and power densities, in this paper, we experimentally and computationally study tortuosity and consider possible approaches to decrease it. We investigate the effect of electrode processing on the tortuosity of in-house fabricated porous electrodes, using three-dimensionally reconstructed microstructures obtained by synchrotron x-ray tomography. Computer-generated electrodes are used to understand the experimental findings and assess the impact of particle size distribution and particle packing on tortuosity and reactive area density. We highlight the limitations and tradeoffs of reducing tortuosity and develop a practical set of guidelines for active material manufacture and electrode preparation.

Strontium phosphide-polyaniline composites for high performance supercapattery devices

Developing dielectric capacitors with both a high power density and a high energy density for application in power electronics has been a long-standing challenge. Glass-ceramics offer the potential of retaining the high relative permittivity of ceramics and at the same time of exhibiting the high dielectric breakdown strength and fast charge/discharge rate of glasses, thus producing concurrently high power and energy densities in a single material. In this work, glass-ceramics are fabricated to achieve simultaneously high power and energy densities, high efficiency, and thermal stability by tuning the glass crystallization process via a suitable nucleating agent and a high oxygen partial pressure. Under the same practical charge-discharge test conditions, the as-prepared glass-ceramics combine the high energy density of ceramics and ultrafast discharge rate of glasses, producing the highest power density among glass- and ceramic-based dielectric materials. This work demonstrates the significant potential of achieving both high power and energy densities in glass-ceramics by optimizing the glass crystallization process.

Li-ion batteries are commonly used in portable electronic devices due to their outstanding energy and power density. However, in order to reach the requirements of the automotive industry for next-generation electric vehicles regarding safety, life-time, energy density, and fast charging further developments are inevitable. Additionally, a reduction of material and production costs is needed to improve the price competitiveness. In this contribution we will present a study of different electrode design concepts with the goal to optimize energy and power density of Li-Ion battery electrodes and cells by microstructure resolved electrochemical simulations [1]. State-of-the-art NMC positive electrodes with different thickness and density were prepared and characterized electrochemically in collaboration with our partners [2]. In a next step reconstructions of the electrodes were created with the help of synchrotron tomography and a 3D stochastic microstructure generator [3], [4]. The resulting microstructures are then input to electrochemical simulations within our software BEST and good qualitative agreement between the simulations and experimental data can be reported. Especially, the pivotal role of inactive materials for battery performance could be demonstrated through microstructure-resolved impedance simulations. Based on these results different design concepts were evaluated regarding their energy and power density. An attractive strategy to decrease the share of inactive materials is to increase the mass loading of the electrodes [5], [6]. This concept provides a high theoretical capacity and energy density. However, it also shifts the transport limitations of shuttling lithium ions in the electrolyte to lower C-rates and reduces the rate capability and practical capacity of the cell [6], [7]. In order to enable a fast charging of the batteries structuring techniques are investigated. A promising concept is the laser perforation of the electrodes which creates a hierarchical pore network with macroscopic transport pathways between anode and cathode. Our simulations confirm the beneficial effects which are found in the experiments and we perform an extensive simulation study in order to investigate the effect of different hole geometries and configurations on battery performance and life time. Our study shows that this simulation-based approach is a powerful and efficient tool for the analysis and design of porous electrodes for Li-ion batteries. Acknowledgement This work has been funded by the ‘Bundesministerium für Bildung und Forschung’ within the project HighEnergy under the reference numbers 03XP0073D.

Our increasing energy demands have spurred a rigorous search for renewable energy sources to reduce our dependence on fossil fuels. However, efficient use of renewable energy is possible only with advances in both energy generation and storage. Today’s batteries and capacitors, which are the main energy storage devices, cannot meet the world’s demand for combined power and energy densities. To enhance the viability of such energy storing devices, the Clemson Nanomaterials Center (CNC) has developed a mix of scalable processes for carbon nanotube-based hybrid electrodes that show promise as a cost-effective alternative to standard activated carbon-based electrodes. Working together with industrial partners, CNC has fabricated supercapacitors with energy and power densities in the range of ~11–35 Wh/kg and ~1.2–9 W/kg, respectively. Although this research development is transformative, further studies to optimize the separator and electrolyte technologies are needed to maximize both the energy and power density in a single device.