Four positive electrodes are extracted from two high-power and two high-energy Li-ion battery consumer cells, and then analyzed by focused ion beam-scanning electron microscopy (FIB-SEM) tomography. The active material phase, carbon black-binder phase and pore phase are quantitatively evaluated using adequate 3D analysis techniques by their volume fractions, surface areas, particle size distributions and tortuosities. The active material phase is composed of LiCoO2 (Sanyo 1500 mA h), or LiNiCoAlO2 (Sony 2600 mA h) or one of two LiNiCoAl2-LiCoO2 blends (Kokam high-power 350 mA h and high-energy 560 mA h). Special emphasis was put on the three-dimensional distribution of the submicron-sized (yet highly agglomerated) carbon black-binder phase. Vacuum infiltration with a two-component silicone resin gave greyscale contrast between the carbon black-binder and pore phases for contour description at voxel sizes from 30 to 50 nm. This study elucidates the cathode microstructural features for high-energy and high-power application and provides a complete parameter set of commercial electrodes with differing chemical composition. This information can be used by the scientific community as input parameters in simulations and models, filling the gap between the experimental and the simulation fields.

In state-of-the-art, anode-supported, solid oxide fuel cells (SOFC) the fuel electrode is made of two different porous Ni/stabilized zirconia layers, both with specified phase compositions and microstructural characteristics. It is generally assumed that electro-oxidation of fuel takes place in the thinner anode functional layer (AFL) adjacent to the electrolyte, while gas diffusion takes place in the thicker anode substrate (AS). This assumption is not always applicable and established models fail. This paper introduces a generally valid equivalent circuit model by means of a three-channel transmission line model (3CTLM) that considers electro-oxidation, ionic conduction and gas diffusion. It is parameterized by (i) impedance measurements on model-electrodes, (ii) conductivity measurements, (iii) FIB/SEM-tomography and (iv) gas theory. The complex impedance curve is derived by a newly developed numerical routine that uses calculation steps from network analysis. A model validation is performed by comparing measured and simulated impedance spectra of anode-supported SOFCs varying in AFL thickness from 3 to 22 mu m. The 3CTLM is capable of predicting the impedance of any Ni/stabilized zirconia anode design. A model-based optimization is demonstrated, selecting different ionic conducting phases in AFL and AS for combinations of Ni/10Sc1CeSZ, Ni/8YSZ and Ni/3YSZ, across a temperature range of 600-900 degrees C.

Electrochemical Impedance Spectroscopy (EIS) is a suitable tool for identifying the performance-related polarization processes in a polymer electrolyte membrane fuel cell. A physically meaningful impedance model is needed when drawing conclusions about further cell improvement. This study focuses on, the characterization of the porous electrode structure by applying a transmission line model (TLM) to the measured spectra. The fitting procedure is supported by the distribution of relaxation times (DRT) method enabling a separation of loss processes by their individual time constants. We are able to separate and quantify (i) the gas diffusion in the porous media (2-10 Hz), (ii) the charge transfer resistance at the Pt catalyst (2-200 Hz), and (iii) the ionic transport resistance in the catalyst layer (300-30,000 Hz), across a broad range of operating conditions (current density, relative humidity, gas compositions). The TLM approach directly reveals the electrodes' transport and reaction properties, e.g. ionic conductivity and the Tafel slope. Under high electrical load the ionic transport losses in the catalyst layer contribute more to polarization than expected. Interestingly, the oxygen reduction reaction is found to be describable with a single, current-independent Tafel slope.

In this study, we examined the properties of model heterointerfaces created with yttrium-doped SrZrO3 (SZY) thin films, a type of secondary phase that most frequently occurs at interfaces of solid oxide fuel cells utilizing yttria-stabilized zirconia (YSZ) electrolytes. Electrochemical impedance results of symmetrical cells prepared with LSCF cathodes were evaluated using DRT (distribution of relaxation times) analysis, and 18O isotope exchange depth profiling with SIMS (secondary ion mass spectrometry) analysis was performed on post-tested samples to examine the ionic transport across heterointerfaces. Results showed high polarization resistance contributed by interfacial resistance associated with interfaces with SZY, i.e., LSCF/SZY, SZY/YSZ or SZY/GDC (gadolinia-doped ceria). The careful selection of heterostructure configurations lead to the conclusion that the interfacial contact of SZY with LSCF, YSZ and GDC results into high interfacial resistances, possibly due to an ion blocking effect.

All-solid-state lithium-ion batteries (ASSBs) are considered as next generation energy storage systems. A model might be very useful, which describes all contributions to the internal cell resistance, enables an optimization of the cell design, and calculates the performance of an open choice of cell architectures. A newly developed one-dimensional model for ASSBs is presented, based on a design concept which employs the use of composite electrodes. The internal cell resistance is calculated by linking two-phase transmission line models representing the composite electrodes with an ohmic resistance representing the solid electrolyte (separator). Thereby, electrical parameters, i.e. ionic and electronic conductivity, electrochemical parameters, i.e. charge-transfer resistance at interfaces and lithium solid-state diffusion, and microstructure parameters, i.e. electrode thickness, particle size, interface area, phase composition and tortuosity, are considered as the most important material and design parameters. Subsequently, discharge curves are simulated, and energy- and power-density characteristics of all-solid-state cell architectures are calculated. These model calculations are discussed and compared with experimental data from literature for a high power LiCoO2-Li10GeP2S12/Li10GeP2S12/Li4Ti5O12-Li10GeP2S12 cell.

The outstanding oxygen permeability of the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and its applicability as a cathode in solid oxide fuel cells are remarkable, yet have been hindered by the formation of secondary phases at T < 840 °C and the subsequent degradation. The other main drawback to BSCF is related to the formation of carbonates in the presence of CO2. These degrade its excellent oxygen surface-exchange kinetics. In this work, 10% Y-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF10Y) is electrochemically, microstructurally, and chemically characterized in O2- and CO2-containing atmospheres as porous cathodes in symmetrical cells with Gd-doped ceria as electrolyte. Experiments in oxygen/nitrogen gas mixtures (pO2 = 0.02–0.40 atm) at T = 600–900 °C showed high performance with a cathode specific resistance of 49.9 mΩ cm2 at 600 °C in air (pO2 = 0.21 atm), which is very comparable to 47.8 mΩ cm2 for undoped BSCF, but which deteriorates constantly under the same conditions. Moreover, adding significant amoun...

Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) exhibits a very high oxygen permeability in its cubic perovskite phase, making it a promising candidate for high-temperature energy-related applications such as oxygen-transport membranes. It suffers, however, from a pronounced phase instability at application-relevant temperatures below 840 degrees C which is presumed to result from a valence change of B-site cobalt. In an attempt to stabilize the cubic BSCF phase, monovalent Y3+ was doped in small concentrations (1-10 mol-% yttrium) onto its B-site. The influence of this doping on the physico-chemical properties (electrical conductivity, reductive stability, lattice constant), on the sintering behavior, and on the oxygen permeation of BSCF has been systematically investigated. Despite a slightly adverse effect to permeability (decrease in oxygen permeation by about 20-30%), a doping concentration of 10 mol-% Y is found to completely suppress secondary-phase formation and, hence, stabilize the cubic BSCF system at 800 degrees C. These findings are extremely promising with regard to a long-term operation of BSCF in atmospheres free of acidic impurity gases.

Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840 degrees C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0...10 mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700 degrees C and 800 degrees C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.

This work presents a non-isothermal 2D FEM gas channel model, capable of performance predictions for hydrocarbon-fueled SOFC stack layers. Therefore, a previously developed isothermal model, incorporating relevant loss mechanisms for SOFC operated on hydrocarbons, was extended by implantation of the energy balance equations. Heat transport is described in a physically meaningful way by heat conduction, convection and radiation. This enables the model to predict the spatial temperature distribution within Ni/YSZ-based SOFC while taken into account the different loss mechanisms. Furthermore, the deactivation of active catalyst surface area via sulfur poisoning is considered by implementing surface area-dependent reforming kinetics. This global kinetic approach was determined by measuring the conversion of fuels containing different amounts of H2S in a specialized test rig with gas extraction and temperature tracking probes along the gas channel. The presented results show how poisoning of the Ni surface will affect the reforming-activity.

(La, Sr)(Co, Fe)O3-delta (LSCF) and the composite (La, Sr)MnO3 (LSM) - Y2O3 doped ZrO2 (YSZ) are the most commonly used cathode materials for solid oxide fuel cells (SOFCs). If electrochemical and microstructural parameters of these electrodes are known from appropriate techniques, the performance can be modeled and compared for different operating conditions. Advanced imaging techniques such as focused ion beam/scanning electron microscopy (FIB/SEM) tomography have shown their potential for microstructure quantification. This contribution will show how FIB/SEM tomography can be used to identify microstructural parameters such as porosity, particle size, surface area and tortuosity. A comparison between LSCF and LSM-YSZ cathodes is given, including a demonstration of how a change in tomography parameters allows distinguishing the different phases especially in composite cathodes to overcome challenges in reconstruction and parameter acquisition.

This work aims at the developing of a battery model, which simulates the operating voltage U-op of a high power lithium-ion battery at varying operating conditions. U-op results from the open circuit voltage U-ocv and the sum of overvoltages eta(I) that exist when the battery is under load: U-op = U-ocv (T, SOC) + eta(0) (T, SOC,I) + eta(CT,C) (T, SOC, I) + eta(CT/SEI,A) (T, SOC, I) + eta(Diff,A/C) (T, SOC, I) The open circuit voltage U-ocv is determined with a quasi-stationary method. The ohmic loss eta(0) and the interface losses eta(CT.C) and eta(CT/SEI,A) are measured and separated by the application of electrochemical impedance spectroscopy measurements in the high and middle frequency range and a corresponding DRT-analysis. Using an equivalent model (ECM) enables the quantification of those loss processes and provides their resistance and time constant which are required to calculate the overvoltage. The lithium solid state diffusion losses eta(Diff,A/C) are studied by a current interruption method in the time domain which is the method of choice in the low frequency range and which equally provides the resistance and the time constant of the process. Finally, a continuous discharge curve is simulated. The ECM also serves as a basis for physically motivated fractional identification methods which estimate the impedance parameters out of time domain data. These methods, in turn, can be used for online parametrization of the presented battery model.

Advanced experimental setups and measurement techniques are crucial for investigating and systematically improving interface characteristics. In this study we introduce an interface-cell, which allows the systematic analysis of liquid/solid electrolyte interfaces via four-point measurements. The functionality of this setup is demonstrated by analysing the impact of parameter variations on the aqueous interface of lithium lanthanum titanate (Li3xLa2/3-x-TiO3, LLTO) solid electrolytes as used in Lithium-air batteries.By variation of real operating conditions their impact on cycling performance is highlighted. Examples include temperature (0 to +25 degrees C) and state of charge (SoC), which induces an alteration of pH (here pH approximate to 6-14) and Li+-concentration (here 0.057 mol L-1 to 10.62 mol L-1). Interestingly, a change of the polarization resistance greater than two orders of magnitude (250 Omega cm(2)-25,000 Omega cm(2)) could be identified.The proven dependency of the interface to both the pH and Li+-concentration is explained by an H+/Li+-exchange reaction at the LLTO/H2O-interface. Additionally, we were able to determine the solely grain boundary resistance (similar to 250 Omega cm(2) at 25 degrees C) of the LLTO samples, without the impact of blocking electrodes. A temperature variation revealed the activation energies of the processes to be 0.4 eV (grain boundary) and 0.46 eV (interface).These results were then critically evaluated in the context of aqueous Lithium-air batteries. (C) 2016 Elsevier B.V. All rights reserved.

A method is evaluated that determines the chemical oxygen surface exchange kδ and chemical bulk diffusion coefficient Dδ of mixed ionic–electronic conducting La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) by using electrochemical impedance spectroscopy. Each measured spectrum contains the Gerischer impedance, which represents the polarization characteristics of a porous LSCF cathode structure. Firstly, it was separated from the impedance data by a well-established equivalent circuit model. Second, the specific values for kδ and Dδ were calculated from the Gerischer impedance using the ALS (Adler, Lane, Steele) model. Third, the corresponding microstructure parameters, porosity, surface area and tortuosity, were quantified by focused ion beam (FIB) tomography. This allows a consideration of the actual sample characteristics. This approach was applied, for the first time, to follow the time- and temperature-dependent course of kδ and Dδ values for porous LSCF cathode structures, from the very beginning of cell operation up to several hundred hours. The microscopic structure characteristics remain constant, as indicated by scanning electron microscope analysis and proven by FIB tomography before and after measurements. At T = 600 °C the cathode polarization resistance changed substantially with time, which is associated with a strong decrease of both kδ and Dδ. At T = 750 °C the cathode polarization resistance changed nonlinearly resulting in a rather constant value for kδ but a distinct decrease for Dδ. For T = 900 °C the cathode polarization resistance increased only marginal, and so kδ and Dδ remained constant.

Graphite anode behaviour is of great interest for the optimization of Lithium-ion batteries. The improvement of battery performance depends on an understanding of lithium intercalation/deintercalation in graphite anodes, especially in regards to energy and power density.In this study we present a new approach for investigating graphite anode behaviour under equilbirum and nonequlibirum conditions. It is based on reflectance change measurements from a graphite anode surface, relative to state of charge (SOC). We have introduced an innovative optical test cell and used a photodiode.A reflectance change hysteresis occurs between lithium intercalation and de-intercalation, under equilbrium conditions. This was ascribed to specific lithium-carbon bonds at the graphite particle edge region. Charging and discharging have a unique reflectance change characteristic. This was assigned to SOC nonequilibrium in the anode and serves as clear evidence that limited lithium mobility in the porous microstructure is a signficant loss factor. We then performed a more detailed analysis of the anode loss processes by correlating the anode reflectance change with dynamic electrical excitation. This method, Optical Impedance Spectroscopy (OIS), provided detailed information about the frequency range of the loss processes. It also confirmed our assigning the low frequency range (1 mHz-3 Hz) to solid state diffusion and lithium transport in the electrolyte filled pores. The results are in excellent agreement with previous EIS studies. We conclude that OIS is applicable for validating physicaly-based graphite anode models and determining model parameters, in combination with EIS. (C) 2015 Elsevier Ltd. All rights reserved.

Electrochemical impedance spectroscopy (EIS) is an indispensable technique for the investigation of polarization processes in Lithium-ion Batteries. These cause performance limitation or degradation. A physically meaningful impedance model is key when drawing conclusions on further cell improvement.This study introduces an in-depth impedance analysis of a commercial high-power graphite anode. The impedance spectra measured between 0 degrees C and 30 degrees C and 0%-100% SOC were analyzed by the distribution of relaxation times (DRT-method), enabling a separation of loss processes by their individual time constants. Using this method, we separated charge transfer resistance and solid electrolyte interface resistance at medium frequencies (10 Hz-200 Hz) and the contact resistance anode/current collector in the at high frequency range (5 kHz-100 kHz). Two fundamentally different model structures were set up, either (i) two modifications of a serial model connecting RQ-elements and a Warburg element for solid state diffusion, or (ii) three modifications of a transmission line model with one-path or two-path design. The suitability of all serial and TLM model structures was tested, and the fitting procedure was supported using microstructure parameters gained from x-ray tomography. The favored one-path transmission line model reveals that the lithium-ion transport in the electrolyte contributes more to polarization than expected. Impediment of lithium-ion transport is caused by the pore structure and the tortuosity of the high-power graphite anode, and has to be considered for meaningful interpretation of impedance spectra. (C) 2015 Elsevier B.V. All rights reserved.

The Distribution Function of Differential Capacity (DDC) is introduced as a supportive method for analyzing the capacitive tail in impedance spectra, which is related to the solid state diffusion of species into/out of battery electrodes. The DDC provides insight into the individual time-dependent utilization of differential capacity, as charging/discharging occurs in different electrode compositions, structures, particle sizes or blends thereof with different time constants. The aggregate differential capacity of an electrochemical system is decomposed by (i) measuring the electrode's impedance spectrum, (ii) computing the complex capacity spectrum, and, (iii) computing its distribution function DDC. This paper presents the mathematical derivation of the DDC and shows, using a simplified electrode model, how individual capacity contributions and associated time constants become accessible.

A commonly used class of impedance spectra validity tests is based on fitting serially connected parallel coupled pairs of resistor and capacitor (RC elements) to a measured impedance spectrum. If the model approximates the spectrum well, the measurement is considered valid. It is considered invalid if approximation is poor. Despite being widely used, theoretical justification is still missing. It is not clear which electrochemical processes could be approximated by such a model and hence, for which processes a poor approximation of the spectrum truly indicates a false measurement, instead of merely a lack of generality of the model. The scope of this paper is to derive from a system theory point of view, which class of systems can be approximated by serially connected RC elements and from that to conclude for which electrochemical systems the mentioned class of validity tests is applicable. Moreover, the results will yield theoretical justification for generalizing the concept of distribution function of relaxation times (DRT) by using positive and negative RC elements so that its benefits can be utilized not only for strictly capacitive, but for any possible non-oscillating electrochemical system.

Lithium plating is a common problem for charging in high-rate and low-temperature Li-ion battery applications. The current “standard anode” material graphite is especially susceptible to the formation of lithium metal on its surface instead of intercalation. In order to improve the understanding of this phenomenon, this investigation was conducted using pulse-relaxation experiments, scanning electron microscopy (SEM) and optical in-situ microscopy. For that purpose, current pulses up to 10 C were applied on graphite half-cells to induce plating on the anode's surface. The resulting characteristics (e.g., cell voltage and changes in surface morphology) were analyzed during pulses and subsequent relaxation. Several characteristic attributes could be detected whenever lithium plating occurred: i) a prominent kink of the voltage transient during charging, ii) a distinctive plateau in the subsequent relaxation of the cell voltage, iii) a gray deposit covering the anode surface which under high magnification shows iv) a net-like structure covering the carbon particles. These attributes may provide useful detection tools for Li plating. The observed characteristics for Li plating were explained regarding the involved microscopic processes. This model was used to understand dissolution of plated lithium on the particle surface after the charging pulse and could thus be confirmed by SEM investigations.

A rechargeable Li-air cell is promising due to its high theoretical energy density of about 11140 Wh/kg. A practical energy density of approximately 1000 Wh/kg [1] would be still about five times higher compared to a state-of-the-art Li-ion cell. However, there are manifold problems concerning cell chemistry, reaction kinetics and cycleability of Li-air cells. In this study, a mixed aprotic/aqueous Li-air cell with a Li3xLa2/3-xTiO3 (LLTO) solid electrolyte was characterized in a new interface-cell setup. This setup is capable to analyse the polarization contribution caused by the liquid/solid interface between the solid electrolyte and different liquid electrolytes. The cell design is inspired by Abe et al. [2], consisting of two compartments, which are in contact with the solid electrolyte. Four-point electrochemical impedance measurements are performed using two electrodes on both sides. The excitation signal is applied on the outer working electrodes (WE, CE) and the voltage is measured between the inner reference electrodes (RE1, RE2). The impedance spectra are further processed by calculating their distribution of relaxation times, to clearly separate the different processes [3]. The special cell design allows a selective investigation of a single liquid/solid interface process by assembling different symmetric cells, using the same liquid electrolyte in each compartment. By changing the ratio of conducting salt to solvent (Fig. 1), we clearly defined the interface process in a frequency range from 102 Hz to 103 Hz and determined an activation energy of 0,44 eV. Furthermore, the interface process is a major contribution to the overall polarization, which even exceeds the contribution of bulk electrolyte and grain boundary for low lithium concentration and low pH-value. References: [1] Jake Christensen et al., Journal of The Electrochemical Society, 159 (2) R1-R30 (2012) [2] Takeshi Abe et al., Journal of The Electrochemical Society, 152 (11) A2151-A2154 (2005) [3] Helge Schichlein et al., Journal of Applied Electrochemistry, 32 (8) 875-882 (2002) Figure 1