Electrode structure analysis and surface characterization for lithium-ion cells simulated low-Earth-orbit satellite operation: II: Electrode surface characterization

As a sequence work to investigate the performance-degradation mechanism of an aged commercial laminated lithium-ion cell experiencing 4350-cycle charge–discharge in a simulated low-Earth-orbit (LEO) satellite operation, we performed the surface characterization of LiCoO₂ cathode and graphite anode by Fourier transform infrared-Attenuated total reflection (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) analysis in this work. Overall, the graphite anode had a larger change in surface chemistry than that of the LiCoO₂ cathode. Except the common surface components, we detected Co metal at the aged graphite surface in the first time. This Co metal deposition was believed to originate from Co²⁺ dissolution from LiCoO₂ cathode during prolonged cycling, and detrimental to structure stability of LiCoO₂ cathode which was a main cause of cell capacity loss. The amount of surface-film component was also estimated by FTIR analysis. Though the total amount of surface film increased, the organic (inorganic) surface film decreased (increased) with prolonged cycling.     

Cycle-life testing of large-capacity lithium-ion cells in simulated satellite operation

We are conducting cycle-life testing of 10–100 Ah-class lithium-ion cells in a simulated satellite operation at the Japan Aerospace Exploration Agency (JAXA). This paper reviews the latest test results of these lithium-ion cells. Thus far, we have verified impressive life performance exceeding 30,000 cycles in a simulated low-earth-orbit (LEO) mode and 1800 cycles in a simulated geostationary-earth-orbit (GEO) mode for some of these cells. We optimized the thickness of the electrode layer to suppress cell-internal impedance and found that a lithium-ion cell with a thin electrode layer exhibited promising cycling performance in a simulated LEO operation. Since the electrode material is an important factor affecting the charge–discharge behavior of a lithium-ion cell, we also compared the cycling performance of lithium-ion cells with different cathode materials.     

Development and on-orbit operation of lithium-ion pouch battery for small scientific satellite “REIMEI”

A lithium-ion battery was developed using off-the-shelf pouch cells and launched with a small scientific satellite “REIMEI.” The cells were potted with polyurethane or epoxy resin to protect the battery from vacuum in space. Preliminary experimental test results of pouch cells potted in a soft aluminum cap suggested that the cells tended to swell in vacuum, although they had been reinforced with the resins. Bread board models (BBMs), in which pouch cells were potted with resins in a hard aluminum case, were fabricated for cycle life performance tests in the laboratory. The test results indicated that the performance of epoxy-potted BBM was superior to that of the polyurethane-potted BBM. The measured cell resistance implied that the electrolyte solution leaked through the polyurethane resin, resulting in premature deterioration. The epoxy resin was used for the flight battery. The end-of-discharge-voltage (EoDV) trend of the flight battery on orbit was compared with the laboratory test results corrected based on a post-launch cycle test using a fresh cell. The corrected EoDV trend in the laboratory was in good agreement with the on-orbit trend for the early cycle period, indicating that the on-orbit battery was not inadvertently affected by conditions in space.     

Comparison of commercial supercapacitors and high-power lithium-ion batteries for power-assist applications in hybrid electric vehicles: I. Initial characterization

Commercial supercapacitors, also known as ultracapacitors or electrochemical capacitors, from Saft, Maxwell, Panasonic, CCR, Ness, EPCOS, and Power Systems were tested under constant current and constant power discharges to assess their applicability for power-assist applications in hybrid electric vehicles (HEVs). Commercial lithium-ion batteries from Saft and Shin-Kobe were also tested under similar conditions. Internal resistances were measured by electrochemical impedance spectroscopy (EIS), as well as by the “iR drop” method. Self discharge measurements were also recorded. Compared with earlier generations of supercapacitors, the cells showed improved current and power capability. However, their energy densities are still too low to meet goals set by Partnership for a New Generation of Vehicles (PNGV) for HEV propulsion. Cells that use acetonitrile as the electrolyte solvent yield better performance, although safety issues need to be addressed. New high-power lithium-ion batteries show high energy densities, with high power capabilities.     

Electrochemical characterization of polymer electrolyte membrane fuel cells and polarization curve analysis

This paper presents the diagnostic results of single polymer electrolyte membrane fuel cell assemblies characterized by polarization curves. Single PEM fuel cell assemblies were investigated through accelerated voltage cycling test at different values of relative humidity. The fuel cells are tested at different humidity level. The cells are discussed in this paper with analysis results at different relative humidity at atmospheric pressure. This represents a nearly fully humidified, a moderately humidified, and a low humidified condition, respectively. This technique is useful for diagnosing the main sources of loss in MEA development work, especially for high temperature/low relative humidity operation where several sources of loss are present simultaneously. All the fuel cells showed better performance in terms of limiting current density value through polarization curves when oxygen was fed to the cathode side of each cell instead of air. The results indicate that the performance of the fuel cell could be depressed significantly by decreasing RH from 100 to 33%. Decrease in RH can result in slower electrode kinetics, including electrode reaction and mass diffusion rates, and higher membrane resistance.     

Coupling SOFCs to biomass gasification - The influence of phenol on cell degradation in simulated bio-syngas. Part I: Electrochemical analysis

Feeding solid oxide fuel cells (SOFCs) with gas from biomass gasification is promising with regard to highly efficient power generation. But it is also intricate since biogenic contaminants are harmful to state-of-the-art anode materials. In this work the influence of phenol as a biogenic model contaminant on the performance of single solid oxide fuel cells was studied under realistic conditions. For this purpose Ni/YSZ anode supported cells were operated with simulated bio-syngas, applying an electrical load of 0.34 A/cm². Over a duration of several hundreds of hours phenol was periodically added to the fuel gas. The tests showed that for the lowest concentration of phenol no accelerated degradation could be observed regarding cell potential and electrical impedance measurements, but disintegration of the Ni/YSZ support took place. Metal dusting of the anode support was found to be the most important mechanism of degradation.     

锂离子电池电极结构参数对单体能量与功率的影响

以模型化仿真技术为基础的电池正向设计方法可替代大量制样、实验选优的试错方法,从而显著缩短产品研发周期、降低物料与能源的成本消耗、提高产品创新能力.本文基于热-电化学耦合的三维跨尺度模型,在单体尺寸与容量不变的约束条件下探究了电极结构参数如电极涂层厚度、孔隙率等对单体主要性能参数如功率、能量、单位质量与体积比功率、单位质...     

Composite nonwoven separator for lithium-ion battery: Development and characterization

A new type composite nonwoven separator has been developed by combining a polyacrylonitrile (PAN) nano-fiber nonwoven and ceramic containing polyolefin nonwoven. The physical, electrochemical and thermal properties of the separator were investigated. The separator has mean pore size of about 0.8 μm as well as narrow pore-size distribution. Besides, the separator possesses higher porosity and air permeability than a conventional microporous membrane separator. The separator showed tensile strength of 46 N 5 cm⁻¹ at 10% elongation. Any internal short circuit was not observed for cells with the separator during charge-discharge test, and the cells showed stable cycling performance. Moreover, the cells showed better rate capabilities than cells with the conventional one. On a hot oven test at 150 °C, the composite nonwoven separator showed better thermal stability than the conventional one. In addition, internal short circuit by thermal shrinkage of separator was not observed for a cell with the separator at 150 °C for 1 h.     

Electrochemical analysis for cycle performance and capacity fading of a lithium-ion battery cycled at elevated temperature

Laboratory-size LiNi_0.8Co_0.15Al_0.05O₂/graphite lithium-ion pouch cells were cycled over 100% DOD at room temperature and 60 °C in order to investigate high-temperature degradation mechanisms of this important technology. Capacity fade for the cell was correlated with that for the individual components, using electrochemical analysis of the electrodes and other diagnostic techniques. The high-temperature cell lost 65% of its initial capacity after 140 cycles at 60 °C compared to only a 4% loss for the cell cycled at room temperature. Cell ohmic impedance increased significantly with a elevated temperature cycling, resulting in some of loss of capacity at the C/2 rate. However, as determined with slow rate testing of the individual electrodes, the anode retained most of its original capacity, while the cathode lost 65%, even when cycled with a fresh source of lithium. Diagnostic evaluation of cell components including X-ray diffraction (XRD), Raman, CSAFM and suggest capacity loss occurs primarily due to a rise in the impedance of the cathode, especially at the end-of-charge. The impedance rise may be caused in part by a loss of the conductive carbon at the surface of the cathode and/or by an organic film on the surface of the cathode that becomes non-ionically conductive at low lithium content.     

Evaluation of commercial lithium-ion cells based on composite positive electrode for plug-in hybrid electric vehicle applications. Part I: Initial characterizations

Evaluating commercial Li-ion batteries presents some unique benefits. One of them is to use cells made from established fabrication process and form factor, such as those offered by the 18650 cylindrical configuration, to provide a common platform to investigate and understand performance deficiency and aging mechanism of target chemistry. Such an approach shall afford us to derive relevant information without influence from processing or form factor variability that may skew our understanding on cell-level issues. A series of 1.9 Ah 18650 lithium ion cells developed by a commercial source using a composite positive electrode comprising {LiMn_1/3Ni_1/3Co_1/3O₂ + LiMn₂O₄} is being used as a platform for the investigation of certain key issues, particularly path-dependent aging and degradation in future plug-in hybrid electric vehicle (PHEV) applications, under the US Department of Energy's Applied Battery Research (ABR) program. Here we report in Part I the initial characterizations of the cell performance and Part II some aspects of cell degradation in 2C cycle aging. The initial characterizations, including cell-to-cell variability, are essential for life cycle performance characterization in the second part of the report when cell-aging phenomena are discussed. Due to the composite nature of the positive electrode, the features (or signature) derived from the incremental capacity (IC) of the cell appear rather complex. In this work, the method to index the observed IC peaks is discussed. Being able to index the IC signature in details is critical for analyzing and identifying degradation mechanism later in the cycle aging study.     

Electrochemical studies on LiCoO2 surface coated with Y3Al5O12 for lithium-ion cells

Synthesized yttrium aluminum garnet (YAG) sol was coated on the surface of the LiCoO₂ cathode particles by an in situ sol–gel process, followed by calcination at 923 K for 10 h in air. Based on XRD, TEM, and ESCA data, a compact YAG kernel with an average thickness of ∼20 nm was formed on the surface of the core LiCoO₂ particles, which ranged from ∼90 to 120 nm in size. The charge–discharge cycling studies for the coated materials suggest that 0.3 wt.% YAG-coated LiCoO₂ heated at 923 K for 10 h in air, delivered a discharge capacity of 167 mAh g⁻¹ and a cycle stability of about 164 cycles with a fading rate of 0.2 mAh cycle⁻¹ at a 0.2C-rate between 2.75 and 4.40 V vs. Li/Li⁺. The differential capacity plots revealed that impedance growth was slower for YAG surface treated LiCoO₂, when cells were charged at 4.40 V. DSC results exemplified that the exothermic peak at ∼468 K corresponded to the release of much less oxygen and greater thermal-stability.     

Model validation and performance analysis of regenerative solid oxide cells: Electrolytic operation

The use of regenerative, high temperature solid oxide cells (SOCs) as energy storage devices has the potential for round-trip efficiencies that are competitive with other storage technologies. The focus of the current study is to investigate regenerative SOC operation (i.e., working in both fuel cell and electrolysis modes) through a combination of modeling and numerical simulation. As an intermediate step, this paper focuses on the electrolysis mode and presents a dynamic cell model that couples the reversible electrochemistry, reactant chemistry, and the thermo-fluidic phenomena inside a cell channel. The model is calibrated and validated using available experimental and numerical data for button cells, single cells, and multi-cell stacks supplied with either steam or syngas. Parametric studies are also performed to show how the investigated parameters affect model validity. The results show that the present model can accurately simulate the electrolytic cell behavior, especially in the low current range, which is a favored operating point in practical systems. It is observed that improvements in stack-level model precision require further investigation to better represent the contact resistance of the stack components and to improve the estimation of the activation polarization throughout the operating envelope. It is also concluded that the CO₂ electrochemical reaction can be neglected when the concentration of the steam supplied to the cell is high enough to support the water–gas shift reaction.     

Electrochemical behavior of polyamides with cyclic disulfide structure and their application to positive active material for lithium secondary battery

Polyamides (DTA-I, DTA-II, and DTA-III) containing cyclic disulfide structure were prepared by condensation between 1,2-dithiane-3,6-dicarboxylic acid (DTA) and alkyl diamine, NH₂–(CH₂)_n–NH₂ (DTA-I; n=4, DTA-II; n=6, DTA-III; n=8) and their application to positive active material for lithium secondary batteries was investigated. Cyclic voltammetry (CV) measurements under slow sweep rate (0.5 mV s⁻¹) with a carbon paste electrode containing the polyamide (DTA-I, DTA-II, or DTA-III) were performed. The results indicated that the polyamides were electroactive in the organic electrolyte solution (propylene carbonate (PC)-1,2-dimethoxyethane (DME), 1:1 by volume containing lithium salt, such as LiClO₄). The responses based on the redox of the disulfide bonds in the polyamide were observed.Test cells, Li/PC-DME (1:1. by volume) with 1 mol dm⁻³ LiClO₄/the polyamide cathode, were constructed and their performance was tested under constant current charge/discharge condition. The average capacity of the test cells with the DTA-III cathode was 64.3 Ah kg⁻¹ of cathode (135 Wh kg⁻¹ of cathode, capacity (Ah kg⁻¹) of the cathode×average cell voltage (2.10 V)). Performance of the cell with linear polyamide containing disulfide bond (–CO–(CH₂)₂–S–S–(CH₂)₂–CONH–(CH₂)₈–NH–, GTA-III) was also investigated and the average capacity was 56.8 Ah kg⁻¹ of cathode (100 Wh kg⁻¹ of cathode, capacity (Ah kg⁻¹) of the cathode×average cell voltage (1.76 V)). Cycle efficiency of the test cell with the DTA-III cathode was higher than that with the GTA-III cathode.     

Experimental analysis of a domestic electric hot water storage tank. Part I: Static mode of operation

In this paper, the experimental analysis of the static mode of operation of a full-scale Domestic Electric Hot Water Storage Tank (DEHWST) with a capacity of 150 l is reported. The main purpose of the analysis is to determine the thermal behaviour of the DEHWST for the static heating and cooling periods in order to characterize its performance. The analysis is based on experimental data taken from a tank provided by the manufacturer and equipped with an appropriate data acquisition system. The storage tank, the data acquisition system and the experimental methodology are described. Performance parameters to evaluate the energy and exergy efficiencies and thermal stratification are defined and obtained from the experimental data. The results of the analysis for different heating powers and tank pressures are shown and discussed.     

Model validation and performance analysis of regenerative solid oxide cells for energy storage applications: Reversible operation

The need for electrical energy storage (EES) is being driven by the deployment of increasing amounts of intermittent renewable energy resources. In addition to their fuel flexibility, high efficiency, scalability, and long-term cost outlook, reversible (regenerative) solid oxide cell (rSOC) systems have the potential for round-trip efficiencies competitive with the other available energy storage technologies. Accordingly, the focus of the current study is to investigate modeling methods for high temperature rSOCs in order to facilitate future endeavors related to establishing optimal operating conditions and system designs. Previously developed solid oxide fuel cell (SOFC) and solid-oxide electrolytic cell (SOEC) models are integrated to form a general rSOC model. The model is first calibrated and validated based on the available experimental data in the extant literature. The validation results show that the fitting parameters extracted from the calibration study can precisely simulate the cell behavior under various operating conditions. The effects of key operating parameters, such as temperature, gas composition and fuel utilization, on the voltage–current density performance characteristic and thermoneutral voltage are then investigated. It is also observed that the total electrochemical losses of the cell can be significantly different in each operating mode (fuel cell and electrolyzing) under certain operating conditions. Advantages of pressurized operation on thermal management are also discussed.     

Influence of temperature on corrosion behavior,wettability, and surface conductivity of 304 stainless steel in simulated cathode environment of proton exchange membrane fuel cells

The effects of temperature on corrosion behavior, wettability, and surface conductivity of 304 stainless steel (SS304) in simulated cathode environment of proton exchange membrane fuel cells (PEMFC) are investigated systematically using electrochemical tests and surface analyses. The results indicate that although the corrosion resistance of SS304 is decreased with the rising of solution temperature, the current density of SS304 at the working potential in the simulated PEMFC cathode environment can still meet the 2025 U.S. Department of Energy (DOE) technical target (i_corr < 1 μA cm^?2). Meanwhile, the surface wettability and ICR of SS304 samples after potentiostatic polarization show a continuous increase with the rise of the simulated solution temperature. The surface conductivity of SS304 both before and after polarization cannot reach the 2025 DOE technical target (<0.01 Ω cm²) and needs to be improved by surface modification.     

Three-dimensional micro/macroscale simulation of planar,anode-supported,intermediate-temperature solid oxide fuel cells: I. Model development for hydrogen fueled operation

Intermediate-temperature solid oxide fuel cells (IT-SOFCs) are promising SOFC technologies that can solve many problems of high-temperature SOFCs (HT-SOFCs), such as the stringent restriction on material selection, accelerated degradation of electrode activity, limitation in thermal cycling, and requirement for long start-up times. In this study, a comprehensive three-dimensional micro/macroscale model is developed for simulating planar, anode-supported IT-SOFCs fueled with hydrogen. Many constitutive sub-models for electrode microstructure, detailed charge-transfer processes, and heat/mass transport in three-dimensional interconnect plate/gas channel geometries are combined to investigate the performance and operating characteristics of IT-SOFCs with rather standard materials (such as nickel, YSZ, LSM, and stainless steel). The current−voltage performance curves are presented along with the contribution of activation, concentration, ohmic, and contact overpotentials to total potential loss. In addition, the spatial distributions of temperature, current density, and species concentrations are also investigated for co- and counter-flow configurations. The results clearly demonstrate the capabilities of the present three-dimensional micro/macroscale model as an accurate and efficient design tool for optimizing the operating conditions, electrode microstructures, and cell geometries of planar, anode-supported IT-SOFCs.     

A diesel fuel processor for stable operation of solid oxide fuel cells system: II. Integrated diesel fuel processor for the operation of solid oxide fuel cells

Post-reforming experimental results for the complete removal of light hydrocarbons from diesel reformate are introduced in part I. In part II of the paper, an integrated diesel fuel processor is investigated for the stable operation of SOFCs. Several post-reforming processors have been operated to suppress both sulfur poisoning and carbon deposition on the anode catalyst. The integrated diesel fuel processor is composed of an autothermal reformer, a desulfurizer, and a post-reformer. The autothermal reforming section in the integrated diesel fuel processor effectively decomposes aromatics, and converts fuel into H₂-rich syngas. The subsequent desulfurizer removes sulfur-containing compounds present in the diesel reformate. Finally, the post-reformer completely removes the light hydrocarbons, which are carbon precursors, in the diesel reformate. We successfully operate the diesel reformer, desulfurizer, and post-reformer as microreactors for about 2500 h in an integrated mode. The degradation rate of the overall reforming performance is negligible for the 2000 h, and light hydrocarbons and sulfur-containing compounds are completely removed from the diesel reformate.     

Three-dimensional earth-contact heat flows: A comparison of simulated and measured data for a buried structure

If renewable energy sources are to be successfully utilised within the built environment then accurate simulation tools have an important role to play. This paper addresses a very specific part of the overall simulation process: that of earth-contact heat transfer which can be the cause of significant modelling errors. Using a three-dimensional numerical model, simulation results are compared with experimental data for a test room. It has been shown that for this case, neglecting climatic conditions (e.g. snow and rain) causes discrepancies between measured and calculated temperatures. A sensitivity study has also shown that thermophysical properties (like the soil thermal conductivity) have to be accurately known.