Cathode Porosity Research Articles

Design optimization of LiNi0.6Co0.2Mn0.2O2/graphite lithium-ion cells based on simulation and experimental data

LiNi0.6Co0.2Mn0.2O2 cathodes of different thicknesses and porosities are prepared and tested, in order to optimize the design of lithium-ion cells. A mathematical model for simulating multiple types of particles with different contact resistances in a single electrode is adopted to study the effects of the different cathode thicknesses and porosities on lithium-ion transport using the nonlinear least squares technique. The model is used to optimize the design of LiNi0.6Co0.2Mn0.2O2/graphite lithium-ion cells by employing it to generate a number of Ragone plots. The cells are optimized for cathode porosity and thickness, while the anode porosity, anode-to-cathode capacity ratio, thickness and porosity of separator, and electrolyte salt concentration are held constant. Optimization is performed for discharge times ranging from 10 h to 5 min. Using the Levenberg-Marquardt method as a fitting technique, accounting for multiple particles with different contact resistances, and employing a rate-dependent solid-phase diffusion coefficient results in there being good agreement between the simulated and experimentally determined discharge curves. The optimized parameters obtained from this study should serve as a guide for the battery industry as well as for researchers for determining the optimal cell design for different applications.

Systematic Variation of Sulfur Cathode Chemistry and Its Effects on Cell Performance

In recent years, tremendous effort has been invested into researching energy storage chemistries that will replace lithium ion. New strategies with higher capacities must be considered, such as a chemical conversion cathode. The lithium sulfur chemistry is a promising candidate as it uses earth abundant, inexpensive, high capacity sulfur as its cathode. Conversion cathodes like sulfur are much more challenging as a secondary battery, and behave differently than intercalation cathodes. Because of this, different design criteria must be applied to conversion cathodes to maximize power and capacity. The interplay of conductive network, insulating active cathode, and binder all give rise to differences in performance for the dissolution and precipitation of the cathode material. This work attempts to identify key design criteria for sulfur cathodes by systemically varying the basic components of cathode chemistry and structure. A matrix of ~110 different cathode formulations were fabricated by varying conducting carbon, % active material, binder system, and sulfur loading. Performance of these cathodes was evaluated by means of 2032 coin cell tests under constant current and pulsed loads. First discharge capacity, capacity retention, coulombic efficiency, impedance, cathode surface area, and cathode porosity are utilized to gauge the effectiveness of the cathode and explain trends observed in cycling tests. The effects of cathode rheology on porosity surface area and overall cathode utilization was also investigated. Observed differences in performance between different electronically conductive elements and different cathode support elements were observed, with nuanced changes in morphology resulting in significant performance changes. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Figure 1

Development of Medium-Temperature Solid Oxide Fuel Cells and CO2 and H2O Co-Electrolysis Cells in Estonia

Estonian solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) programs started in 2001 and 2012, respectively. Solid oxide fuel and co-electrolysis cell single cells based on Ni-Ce0.9Gd0.1O2- d or Ni-Zr0.92Y0.08O2- d supporting anodes/cathodes, bi-layered Zr0.92Y0.08O2- d | Ce0.9Gd0.1O2- d or Zr0.94Sc0.06O2- d | Ce0.9Gd0.1O2- d electrolytes and micro-meso-porous La0.6Sr0.4CoO3- d, Gd0.6Sr0.4CoO3- d or Pr0.6Sr0.4CoO3- d cathodes/anodes, respectively, prepared from nano-microporous powders at different sintering temperatures and pore former additions in the raw cathode and anode pastes have been studied. Ni-free anodes/cathodes have been also synthesized and tested at temperatures from 550 to 800 °C. Different physical (FIB-TOF-SIMS, XRD, high-temperature XRD under electrochemical working conditions, FIB-SEM-EDX, AFM, gas chromatography-mass spectrometry, BET) and electrochemical (cyclic voltammetry, chronoamperometry and electrochemical impedance) methods have been applied. Influence of the anode and cathode porosity, depending on the electrode preparation methods, and also of the bi-layered electrolyte properties (chemical composition, thickness, etc.) on the power density of the SOFC and SOEC single cells has been shown and discussed. The synchrotron radiation based X-ray absorption spectroscopy under electrochemical polarization and different fuel feeding and temperature cyclation conditions has been applied for chemical analysis of surface composition of the SOFC anodes.

Modeling of Structural Changes in Cathodes of Lithium Ion Batteries Depending on State of Charge

Electrodes are responsible for the processes in a battery. On one hand to make the electrochemical reaction happen and on the other hand provide ion and electron conductivity. To find a material which provides the perfect environment for those two processes is a challenge and there is still a great potential of optimization. In order to improve these two processes, a three dimensional model of an electrode is applied. The structure is designed from porosity measurements and microscopies. This study focuses on changes of the dimension of the model based on charged and discharged electrodes. Changes in pore and particle sizes have an influence on the diffusion coefficient, tortuosity and the Bruggeman factor. These parameters are influenced by the state of charge (SOC) of the battery. A three dimensional model is made from focused ion beam scanning electron microscopy (FIB-SEM) images provided by Hutzenlaub et al. [1] and will be compared with a rebuild model from spheres on the bases of diffusion coefficient, tortuosity and the Bruggeman factor. This model is used as the basis to define the three parameters for the rebuilt model from spheres. The porosity changes will be determined from Hg-porosimetry. From scanning electron microscopy (SEM) images, the tortuosity will be calculated and in addition, a calculated porosity can be compared as a reference with the measurements. Volume changes of the solid structure inside an electrode occur depending on the state of charge. The binder inside the electrode is considered to stay the same, as it is not an active participant of the electrochemical reaction. Diffusion coefficient, tortuosity and changes of the Bruggeman factor, depending on the SOC, will be determined and modeled. Porosity measurements showed that a change of SOC changes the porosity significantly. The cathode porosity of a charged battery is smaller than of a discharged, the rate of influence on the here mentioned parameters will be the focus in this study. Porosity measurements of charged and discharged Lix(NiyCo1-y)2-x02 cathodes showed comparable particle distribution with LixCoO2 up to 12 µm [2]. With these results, a comparison between the porous structure from Hutzenlaub et al. [1] and the porosity measurements can be conducted. The representative model size is dependent on the porosity and resolution related with the particle sizes [3, 4]. The results of this study provide a three dimensional simulation model for porous electrode applicable for various batteries, based on measurements of porosity and SEM to calculate porosity and tortuosity, from which the Bruggeman factor results. This study will provide the opportunity to model representative porous cathodes for lithium ion batteries without FIB-SEM at different SOC. [1] T. Hutzenlaub, S. Thiele, R. Zengerle and C. Ziegler, Electrochemical and Solid State Letters, 15 (3) A33-A36 (2012). [2] D.E. Stephenson, E.M. Hartman, J.N. Harb, D.R. Wheeler, Journal of the electrochemical society, 154 (12) A1146-A1155 (2007). [3] J. Joos, T. Carraro, A. Wber, E. Ivers-Tiffée, Journal of Power Sources, 196, 7302-7307 (2011). [4] B. Rüger, J. Joos, A. Weber, T. Carraro, E. Ivers-Tifée, ECS Transactions, 25, 1211-1220 (2009).

Novel microstructural strategies to enhance the electrochemical performance of La0.8Sr0.2MnO3-δ cathodes.

Novel strategies based on spray-pyrolysis deposition are proposed to increase the triple-phase boundary (TPB) of La0.8Sr0.2MnO3-δ (LSM) cathodes in contact with yttria-stabilized zirconia (YSZ) electrolyte: (i) nanocrystalline LSM films deposited on as-prepared YSZ surface; (ii) the addition of poly(methyl methacrylate) microspheres as pore formers to further increase the porosity of the film cathodes; and (iii) the deposition of LSM by spray pyrolysis on backbones of Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), and Bi1.5Y0.5O3-δ (BYO) previously fixed onto the YSZ. This last method is an alternative to the classical infiltration process with several advantages for large-scale manufacturing of planar solid oxide fuel cells (SOFCs), including easier industrial implementation, shorter preparation time, and low cost. The morphology and electrochemical performance of the electrodes are investigated by scanning electron microscopy and impedance spectroscopy. Very low values of area specific resistance are obtained, ranging from 1.4 Ω·cm(2) for LSM films deposited on as-prepared YSZ surface to 0.06 Ω-cm(2) for LSM deposited onto BYO backbone at a measured temperature of 650 °C. These electrodes exhibit high performance even after annealing at 950 °C, making them potentially suitable for applications in SOFCs at intermediate temperatures.

Fabrication of LSM-SDC composite cathodes for intermediate-temperature solid oxide fuel cells

Microstructure, interfacial resistance, and activation energy for composite cathodes consisting of 50 wt% (La0.85Sr0.15)0.9MnO3-δ (LSM) and 50 wt% Sm0.2Ce0.8O1.90 (SDC) were studied for intermediate-temperature solid oxide fuel cells based on SDC electrolytes. Microstructure and interfacial resistance were greatly influenced by the characteristics of starting powder and temperatures sintering the electrodes. Optimum sintering temperatures were 1100 and 950 °C, respectively, for electrodes with SDC prepared using oxalate coprecipitation technique (OCP) and glycine-nitrate process (GNP). Area-specific resistances determined using impedance spectroscopy were 0.47 and 0.92 Ω cm2 at 800 °C for LSM-SDC/OCP and LSM-SDC/GNP, respectively. The high electrochemical performance is attributed to small grain size, high porosity, and high in-plane electrical conductivity of composite cathode, demonstrating the dramatic effects of microstructure on electrode performance. To increase the electrode performance, it is critical to enhance the diffusion rate of oxygen species.

The Effect of cathode Porosity on Solid Oxide Fuel Cell Performance

In the present study, the effect of porosity on the cathode microstructure (50:50 wt. % LSM: YSZ) of a Solid Oxide Fuel Cell (SOFC) has been examined. A 3-D finite element method for Mixed Ionic and Electronic Conducting Cathodes (MIEC) is presented to study the effects of porosity on cell performance. Each microstructure was realized using the Monte Carlo approach with the isotropic type of growth rate. The effect of porosity on the cathode of a solid oxide fuel cell involving the Three Phase Boundary Length (TPBL), electric conductivity of LSM phase, ionic conductivity of YSZ, mechanical behavior and tortuosity of the pore phase were explored in the present work. The cathode having a porosity value between 31 and 34% revealed the maximum TPBL value as well as a high variation in the electrical conductivity of the LSM phase. Pore phase tortuosity was also decreased by increasing the porosity factor.

Electrodeposition Based Synthesis of Metal-Air Battery Cathodes

Metal air batteries offer the opportunity for unprecedented energy density improvements relative to existing battery technologies. Air cathodes are usually comprised of a catalyst, conductive additives, binder, and current collector. The current collector provides support and conductivity, the binder provides physical adhesion, and the catalyst facilitates the oxygen reduction process at the cathode. Thus, each component plays an important role in the function of the overall electrode. Additionally, access of the catalyst particle to oxygen is critical, so the cathode porosity must be appropriately tuned. Electrodes are often fabricated using physical mixing methods, where issues can include poor interparticle contact and limited access of the interior catalyst particles to oxygen. An electrodeposition based strategy for fabrication of a composite air cathode will be discussed. The first layer consists of a current collector substrate, which can be two dimensional (planar) or three dimensional in design. The second layer consists of a conformal conductive polymer coating. The third layer consists of catalyst particles. The second and third layers are each prepared using stepwise deposition processes.Advantages of the electrodeposition fabrication method will be described. Three-dimensionally structured composites can be easily generated via use of three dimensional substrates. All of the catalyst nanoparticles are positioned on the outer surface of the composite electrode, allowing facile access of oxygen gas molecules to the catalyst particle surfaces. Finally, the conductive polymer provides added conductivity and improved stability to the composite. Resulting impact on conductivity and oxygen reduction activity of the composite electrode will be discussed.

Identifying Limiting Mechanisms in the Li/O2 Battery through Models and Experiments

Li/O2 batteries are of contemporary interest because of their high theoretical gravimetric energy density of ~11 kWh/kg. However, several factors limit the cell’s discharge depth and rechargeability. Commonly cited factors include transport of oxygen to the cathode surface, pore clogging, and charge transport, but current literature lacks consensus on which of these are most significant. To gain insight on the relative importance of these mechanisms, we investigated the cell’s limiting behavior using a combination of theory and experiment.A nanoscale stochastic model was developed to simulate the growth of a single Li2O2 nucleation site during discharge. Growth progressed according to a kinetic Monte Carlo routine in which the probability of an incremental growth step is weighted according to the local Li+ and O2 concentrations. The predicted Li2O2 morphologies are consistent with SEM observations: Li2O2 deposits appear spherical in shape when the cell is discharged at low current density (~0.1 mA/cm2), but conform to the cathode surface when discharged at high current density (~1.0 mA/cm2). From this analysis, it is possible to determine that the change in morphology is due to the surface concentration of Li+ rather than the O2 concentration. Since Li2O2 is a bulk insulator, only the cathode surface experiences an increased Li+ concentration.Building on these observations, a continuum model was also developed to examine the limiting factors on a more macroscopic scale. This model included Li+ and O2 mass transport, electrolyte potential, Butler-Volmer kinetics, and changes in porosity and surface area from Li2O2 deposition. The time-dependent concentration and porosity profiles were computed using a finite difference method. Neither the oxygen concentration nor the cathode porosity of the model cell decreased an appreciable amount during discharge. Additionally, the model cell discharged indefinitely when the electrical resistivity of the Li2O2 layer was neglected. These results suggest that Li2O2 resistivity, rather than oxygen transport or pore clogging, is the primary limiting mechanism in the Li/O2 battery and likely plays a significant role in rechargeability and cell lifetime.Cell cycling experiments suggest that Li2O2 resistivity limits charging capabilities as well. For cells that were allowed to fully discharge to 2.35 V (an underpotential of 0.6 V), the capacity of the cell faded quickly within the first 10 cycles. However, if the discharge step was stopped prior to the drop in cell voltage, the capacity could be retained over an extended number of cycles. This finding is consistent with the growing number of reports in the field, which shows that extend cycle life of Li/O2 batteries can be achieved by enforcing strict limits on the depth of discharge. We contend that this feature is a natural consequence of the voltage drop due to ohmic resistance through the Li2O2 layer. At full discharge, the resistance becomes significant during the subsequent charge stage, preventing the Li2O2 furthest from the cathode surface from oxidizing. At partial discharge, however, the Li2O2 layer is below this critical thickness, allowing for a more complete recharge.Our observations point to direct approaches for enhancing lifetime and cycling stability of Li/O2 batteries, without the tradeoffs inherent in interrupted discharge experiments. The talk will also show that electrode and electrolyte designs that build rationally on these ideas lead to Li/O2 cell configurations that approach the potential of this battery platform. Through ex-situ and in-situ physical and electrochemical characterization, we also analyze the cathode surface and discuss how physical features of the battery cathode evolve during discharge and charge of the cell.Figure 1: Comparison of simulated Li2O2 morphologies and SEM images of discharged cathodes at current densities of 0.05 mA/cm2 (top) and 0.2 mA/cm2 (bottom).

Performance Analysis of Polymer Electrolyte Membrane Fuel Cell Depending on the Flow Direction of Cathode Reactant Gas, Relative Humidity, and Porosity

Performance Analysis of Polymer Electrolyte Membrane Fuel Cell Depending on the Flow Direction of Cathode Reactant Gas, Relative Humidity, and Porosity

Vapor-grown carbon fibers enhanced sulfur-multi walled carbon nanotubes composite cathode for lithium/sulfur batteries

Vapor-grown carbon fibers (VGCFs) were introduced as conductive additives for sulfur-multiwalled carbon nanotubes (S-MWCNTs) composite cathode of lithium-sulfur batteries. The performance of S-MWCNTs composite cathodes with carbon black and VGCFs as sole conductive additives was investigated using scanning electron microscopy (SEM), galvanostatic charge-discharge tests and electrochemical impedance spectroscopy (EIS). The results show that the S-MWCNTs composite cathode with VGCFs displays a network-like morphology and exhibits higher activity and better cycle durability compared with the composite cathode with carbon black, delivering an initial discharge capacity of 1254 mA·h/g and a capacity of 716 mA·h/g after 40 cycles at 335 mA/g. The interconnected VGCFs can provide a stable conductive network, suppress the aggregation of cathode materials and residual lithium sulfide and maintain the porosity of cathode, and therefore the electrochemical performance of S-MWCNTs composite cathode is enhanced.

An investigation on strontium doped Sm2NiO4+δ cathode for intermediate temperature solid oxide fuel cells

The Sm2−xSrxNiO4+δ when x = 0.4–1.0 solid solutions are synthesized by combustion followed by sintering at 1100 °C for 10 h, yield crystallite size in the range 73–82 nm. The single-phase Sm2−xSrxNiO4+δ are of orthorhombic and tetragonal crystal structures when x = 0.4–0.7 and x = 0.8–1.0, respectively. The maximum electronic conductivity (σ = 0.361 S cm−1 at 640 °C) for Sm1.5Sr0.5NiO4+δ is due to the extrinsic defects. The decrease in conductivity when x > 0.5 is attributed to the formation of electrically neutral clusters. The narrow particle size distribution with mean particle size 423 ± 11 nm, of Sm1.5Sr0.5NiO4+δ is obtained by ball milling the ink/slurry at 300 revolutions per minute (rpm) for 1 h. The minimum area specific resistance (ASR) of 2.7 Ω cm2 at 700 °C is obtained when x = 0.5, and is attributed to uniform superfine grains and optimum porosity of cathode.

Development of Medium-Temperature Solid Oxide Fuel Cell Materials and Single Cells in Estonia

Estonian solid oxide fuel cell program started in 2001. Solid oxide fuel cell single cells based on Ni-Ce0.9Gd0.1O2-δ or Ni-Zr0.92Y0.08O2-δ supporting anodes, bi-layered Zr0.92Y0.08O2-δ | Ce0.9Gd0.1O2-δ or Zr0.9Sc0.1O2-δ | Ce0.9Gd0.1O2-δ electrolytes and micro-meso-porous La0.6Sr0.4CoO3-δ, Gd0.6Sr0.4CoO3-δ or Pr0.6Sr0.4CoO3-δ cathodes prepared from nano/microporous powders at different sintering temperatures and pore former additions in the raw cathode and anode pastes have been studied. Different physical (XRD, high-temperature XRD under electrochemical working conditions, FIB-SEM, AFM, and BET) and electrochemical (cyclic voltammetry, chronoamperometry and electrochemical impedance) methods have been applied. Influence of the anode and cathode porosity, depending on the electrode preparation methods, and also of the bi-layered electrolyte properties (chemical composition, thickness, etc.) on the power density of the single cells has been shown and discussed. During last two years the synchrotron radiation based X-ray absorption spectroscopy under electrochemical polarization, gas feeding and temperature cycling conditions has been applied for analysis of anode surface composition.

Fabrication of Laminate-Type SOFC; Printed Fuel Cell

We have developed a novel laminate-type SOFC stacks that are composed of thin each layers of electrolyte, electrodes and separator since 2005. These layers are prepared by tape casting and screen printing methods and these stacks are monolithically fabricated by simultaneous sintering these layers. We designate these novel stacks or systems as “Printed Fuel Cell”. We successfully fabricated 5-cell stacks that had 1.0-1.5 mm in thick. The stack showed reasonable OCV of more than 5 V at 750-800oC. Power density of the stack was previously lower than expected, but an adequate porosity and gas channel design of cathode much improved the performance.

Effect of LiCoO2Cathode Density and Thickness on Electrochemical Performance of Lithium-Ion Batteries

The consequences of electrode density and thickness for electrochemical performance of lithium-ion cells are investigated using 2032-type coin half cells. While the cathode composition is maintained by 90:5:5 (wt.%) with active material, Super-P electric conductor and polyvinylidene fluoride polymeric binder, its density and thickness are independently controlled to 20, 35, 50 um and 1.5, 2.0, 2.5, 3.0, 3.5 g , respectively, which are based on commercial lithium-ion battery cathode system. As the cathode thickness is increased in all densities, the rate capability and cycle life of lithium-ion cells become significantly worse. On the other hand, even though the cathode density shows similar behavior, its effect is not as high as the thickness in our experimental range. This trend is also investigated by cross-sectional morphology, porosity and electric conductivity of cathodes with different densities and thicknesses. This work suggests that the electrode density and thickness should be chosen properly and mentioned in detail in any kinds of research works.

Modelling the micro–macro homogeneous cycling behaviour of a lithium–air battery

A micro–macro homogeneous mathematical model is developed for a rechargeable Li–air battery using a concentrated binary electrolyte theory. The dynamic behaviour of the porous cathode is determined by a numerical solution of the combined continuity, transport and kinetics equations. This model considers the time and space dependence of the battery system, including the mass transport along the depth of the cell, the local mass transfer between lithium peroxide (Li2O2) layer inside the cathode and active surface morphology changing with the Li2O2 growth. The model predicts that the battery capacity and discharge potential are sensitive not only to the solubility of oxygen, which is predominantly limited by depletion of the oxygen concentration, as well as to the cathode porosity, the cathode structure and kinetic parameters. In addition, the charging behaviour is simulated by modelling the dissolution of solid Li2O2 product. The model suggests that the charging voltage can be lowered depending on capability of electrolyte to dissolve the Li2O2 discharge products. As a result, a simulation of electrode behaviour is obtained.

A PRS model for accurate prediction of the optimal solid oxide cathode structure for the preparation of metals in molten chlorides

A theoretical model correlating the precursor porosity, P (in volume percentage), the metal-to-oxide molar volume ratio, R, and the cathode volume shrinkage S (in volume fraction, experimentally determined) (the PRS model) with the deoxidation speed of the solid metal oxide cathode in molten chlorides has been developed, allowing accurate prediction of the optimal cathode porosity by a simple equation, Popt=3R+S−13R×100%. Tests of this equation in practices suggest Popt to be an intrinsic parameter to the oxide. For example, the predicted values of Popt for the electrolysis of TiO2 and Ta2O5 cathodes are about 67% and 25% respectively, and a cathode porosity far away from Popt would suppress the deoxidation seriously. The established model has been well verified by the electrolysis of the oxides of Si, Ti and Ta.

An efficient decal transfer method using a roll-press to fabricate membrane electrode assemblies for direct methanol fuel cells

Abstract This study has focused on the development of a roll-press based decal transfer method to fabricate membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs). This method exhibits an outstanding transfer rate of catalyst layers from substrates to the membrane, despite hot-pressing at a considerably lower pressure and for a much shorter duration than the flat-press based conventional decal method. The MEA produced by a roll-press (R-MEA) delivers an excellent single-cell performance with power densities more than 30% higher than that fabricated using a flat-press (F-MEA). The new method considerably improves catalyst active sites in both electrodes and renders a high cathode porosity. The superior pore structure of the cathode makes the R-MEA more efficient in terms of performance and operation stability under lower air stoichiometries. Moreover, MEAs can be prepared in a continuous mode using this new method due to the unique design of the roll-press. All these advantages demonstrate the superiority of this method over the conventional flat-press decal method and make it suitable for use in the commercial manufacturing of MEAs for direct methanol fuel cells.

Optimizing LiNi1/3Mn1/3Co1/3O2 Capacity by Controlling Cathode Porosity through Calendaring

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A reduced temperature solid oxide fuel cell with three-dimensionally ordered macroporous cathode

Three-dimensionally ordered macroporous cathode was fabricated for a zirconia based micro-tubular solid oxide fuel cells (SOFCs). Three different cathodes (cathode A, no pore former; cathode B, with pore former (1.5 μm in diameter); cathode C, with pore former (0.8 μm in diameter)) were compared to investigate how the microstructure of it affected the cell performance at various operating temperatures. Micro-sized pores were well distributed within cathode B and C. The total porosity of cathode A is 35%, while it respectively reached 42 and 50% for cathodes B and C. At the same time, the specific surface area of them was 28.8 and 52.0% larger than that of the cathode A. As a result, the peak power density of the zirconia based cell, with cathode C, was 0.25 and 0.56 W cm−2 at 550 and 600 °C, while the respective value was just 0.11 and 0.30 W cm−2 for the cell with cathode A. Thus, optimizing microstructure of cathode should be one of the best approaches for lowering the operating temperature for SOFCs.