Capacity fade of Sony 18650 cells cycled at elevated temperatures: Part II. Capacity fade analysis

A complete capacity fade analysis was carried out for Sony 18650 cells cycled at elevated temperatures. The major causes of capacity loss were identified and a complete capacity fade balance was carried out to account for the total capacity loss of Li-ion battery as a function of cycle number and temperature. The three most significant parameters that cause capacity loss were loss of secondary active material (LiCoO₂/carbon) and primary active material (Li⁺) and the rate capability losses. Intrinsic capacity measurements for both positive and negative electrode has been used to estimate the capacity loss due to secondary active material and a charge balance gives the capacity lost due to primary active material (Li⁺). Capacity fade has been quantified with secondary active material loss dominating the other losses.     

Photovoltaic solar cells performance at elevated temperatures

It is well known that efficiency of photovoltaic solar cells decreases with an increase of temperature, and cooling is necessary at high illumination conditions such as concentrated sunlight, or cosmic or tropical conditions. The purpose of present study was to investigate the opposite option: to make a cell work at relatively high temperature (around 100–200 °C) and use the excessive heat in a hybrid system of some kind to increase the total efficiency of solar energy utilization. We studied the temperature dependence of the solar cell parameters both theoretically and experimentally, for the basic cells with p–n junction and the Schottky barrier, taking account of the different carrier transport mechanisms and recombination parameters of the cell material. The possibility of usage of the concentrated sunlight was also taken into account. The experiments conducted in the temperature interval of 25–170 °C and the calculated data show a real possibility of construction of a two-stage solar-to-electric energy converter with high-temperature second stage, having the overall conversion efficiency of 30–40%.     

Storage and cycling performance of Cr-modified spinel at elevated temperatures

The influences of partial substitution of Mn in LiMn₂O₄ with Cr³⁺ and Li⁺ on their charge/discharge profiles were quite different: Cr³⁺ affected it only in the high-voltage region, while Li⁺ showed in the both high and low voltage regions. Either Cr³⁺ or Li⁺ doping significantly improved the storage and cycling performance of spinel LiMn₂O₄ at the elevated temperature, specially both doped spinel. Li_1.02Cr_0.1Mn₂O₄ shows very low rate of capacity rention, 0.1% per cycle, and maintained a steady discharge capacity of 114 mAh/g, 95% of the initial discharge capacity over 50 cycles at 50°C. The chemical analysis and X-ray diffraction measurement indicate that the capacity losses of LiMn₂O₄ is mainly due to the dissolution of Mn into electrolyte, further transformation to lithium-rich spinel Li_1+xMn₂O₄. The improvements in their electrochemical profiles for the Cr³⁺ and Li⁺ modified spinel is attributed to that the partial substitution of Mn stabilize its structure, thus minimizing the dissolution of Mn into electrolyte, as well as maintaining its original morphologies.     

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.     

Thermal abuse performance of high-power 18650 Li-ion cells

High-power 18650 Li-ion cells have been developed for hybrid electric vehicle applications as part of the DOE Advanced Technology Development (ATD) program. The thermal abuse response of two advanced chemistries (Gen1 and Gen2) were measured and compared with commercial Sony 18650 cells. Gen1 cells consisted of an MCMB graphite based anode and a LiNi_0.85Co_0.15O₂ cathode material while the Gen2 cells consisted of a MAG10 anode graphite and a LiNi_0.80Co_0.15 Al_0.05O₂ cathode. Accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC) were used to measure the thermal response and properties of the cells and cell materials up to 400 °C. The MCMB graphite was found to result in increased thermal stability of the cells due to more effective solid electrolyte interface (SEI) formation. The Al stabilized cathodes were seen to have higher peak reaction temperatures that also gave improved cell thermal response. The effects of accelerated aging on cell properties were also determined. Aging resulted in improved cell thermal stability with the anodes showing a rapid reduction in exothermic reactions while the cathodes only showed reduced reactions after more extended aging.     

Capacity fading with oxygen loss for manganese spinels upon cycling at elevated temperatures

The nominal LiMn₂O₄ and Li-doped spinels with different oxygen stoichiometry were prepared and investigated for capacity fading upon cycling at elevated temperatures. The discharge plateau at 3.2 V originating from oxygen defects in manganese spinels is observed to grow very quickly to nearly a maximum scale in initial 15 cycles at 60 °C. Meanwhile, the majority of capacity fading is lost. Therefore, the quick capacity fading in the initial stage is associated with the increase of oxygen deficiencies or oxygen loss upon cycling. It is proposed that the oxygen loss is originated from the decomposition of instable spinel phases that containing little Li cations on the 8a sites ([□₁]_8a[Mn_2−x]_16d[O_4−δ□_δ]_32e, etc.), which are formed upon charging to the upper voltage limit. This phenomenon is much severe for nominal LiMn₂O₄ spinels with oxygen deficiencies. After partial substitution of Mn with Li, part of the Li cations on the 8a sites will be retained upon charging to the upper voltage limit. Thereafter, the cycling performance can be improved for the stabilized spinel phases formed upon charging.     

Degradation analysis of a Ni-based layered positive-electrode active material cycled at elevated temperatures studied by scanning transmission electron microscopy and electron energy-loss spectroscopy

We investigate the local structural changes in a positive electrode of a lithium ion secondary battery (LiNi_0.8Co_0.15Al_0.05O₂ (NCA) as the active material) associated with charge-discharge cycling at elevated temperatures by scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). STEM-EELS spectral imaging reveals the evolution of a NiO-like phase localized near the surface and grain boundary regions after many cycles. The amounts of capacity fading and resistance increase are discussed based on the results of the semiquantitative estimation of NiO-like and other product phases. We also identify the chemical state of lithium in the NiO-like phase substituting for Ni.     

Coal conversion submodels for design applications at elevated pressures. Part I. devolatilization and char oxidation

Numerous process concepts are under development worldwide that convert coal at elevated pressure. These developments rely heavily on CFD and other advanced calculation schemes that require submodels for several stages of coal chemistry, including devolatilization, volatiles combustion and reforming, char oxidation and char gasification. This paper surveys the databases of laboratory testing on devolatilization and char oxidation at elevated pressure, first, to identify the tendencies that are essential to rational design of coal utilization technology and, second, to validate two well-known reaction mechanisms for quantitative design calculations.Devolatilization at elevated pressure generates less volatile matter, especially tar. Low-rank coals are no less sensitive to pressure variations than bituminous coals; in fact, coal quality is just as important at elevated pressure as it is at atmospheric pressure. Faster heating rates do not enhance volatiles yields at the highest operating pressures. The FLASHCHAIN^® predictions for the devolatilization database depict the distinctive devolatilization behavior of individual samples, even among samples with the same nominal rank. The only sample-specific input requirements are the proximate and ultimate analyses of the coal. There were no systematic discrepancies in the predicted total and tar yields across the entire pressure range. Char oxidation rates increase for progressively higher O₂ partial pressures and gas temperatures, but are insensitive to total pressure at constant O₂ mole fraction. Char burning rates become faster with coals of progressively lower rank, although the reactivity is somewhat less sensitive to coal quality at elevated pressure than at atmospheric pressure. An expanded version of the carbon burnout kinetics model was able to represent all datasets except one within useful quantitative tolerances, provided that the initial intrinsic pre-exponential factor was adjusted for each coal sample.     

Electromechanical properties of langasite resonators at elevated temperatures

Langasite is a single crystalline material with potential applications as piezoelectric transducer at elevated temperatures. Anticipated applications include resonant gas sensors with the ability to detect the gas composition in, e.g. SOFCs or their gas reformers. The material exceeds significantly the operation temperature limit of quartz and exhibits bulk oscillations at temperatures of at least 1400 °C. Commonly used Y-cut shear-mode resonators show a strong dependence of the resonance frequency on the operation temperature. As long as variable operation temperatures have to be encountered, a frequency compensation using the third overtone of the resonator or other elaborated compensation methods have to be applied. The data acquisition can be simplified for fixed operation temperatures occurring often during routine measurements. Under such circumstances, the application of temperature compensated crystal cuts is favourable. This paper focuses on the determination and discussion of all components of elastic and piezoelectric tensors of langasite at elevated temperatures. The resonance spectra of several langasite samples have been measured and fitted with the impedance calculated from a one-dimensional physical models of piezoelectric bodies vibrating in several modes. In order to extract the electromechanical parameters, different resonator geometries and orientations are used. These properties are determined at temperatures up to 900 °C. Based on this data, a crystal cut with minimised temperature dependence is predicted.     

New membranes based on ionic liquids for PEM fuel cells at elevated temperatures

Proton exchange membrane (PEM) fuel cells operating at elevated temperature, above 120 °C, will yield significant benefits but face big challenges for the development of suitable PEMs. The objectives of this research are to demonstrate the feasibility of the concept and realize [acid/ionic liquid/polymer] composite gel-type membranes as such PEMs. Novel membranes consisting of anhydrous proton solvent H₃PO₄, the protic ionic liquid PMIH₂PO₄, and polybenzimidazole (PBI) as a matrix have been prepared and characterized for PEM fuel cells intended for operation at elevated temperature (120–150 °C). Physical and electrochemical analyses have demonstrated promising characteristics of these H₃PO₄/PMIH₂PO₄/PBI membranes at elevated temperature. The proton transport mechanism in these new membranes has been investigated by Fourier transform infrared and nuclear magnetic resonance spectroscopic methods.     

Structures at elevated temperatures: How long will they last?

In principle, questions about the behaviour of high-temperature structures can be answered by analysis in conjunction with material creep properties. However, because of the wide range of geometries in use, simplified methods have been developed which avoid lengthy computations yet isolate the important factors controlling component behaviour. This paper describes one approach which is to define a reference stress such that the component life is equal to the life of a simple specimen tested at the reference stress. A substantial body of work has shown that the reference stress can often be established quite simply even for complex components containing cracks. The approach then provides a simple framework for assessing structures which operate at high temperatures.     

Titanium dioxide for solar-hydrogen III: Kinetic effects at elevated temperatures

The present work considers the equilibration kinetics for the _O2/_TiO2${\mathrm{O}}_2/{\mathrm{TiO}}_2$ system and the related chemical diffusion coefficient. It is shown that the equilibration involves two kinetic regimes: Kinetic Regime I (rapid kinetics) and Kinetics Regime II (slow kinetics). The Kinetic Regime I is related to the diffusion of fast defects, including oxygen vacancies and Ti interstitials. The gas/solid equilibrium during routine testing of defect-related properties corresponds to the Kinetic Regime I. The Kinetic Regime II is related to gas/solid reactions that are rate-controlled by all kinds of point defects, including Ti vacancies. Therefore, the kinetic data within the Kinetic Regime II may be used for the imposition of a homogeneous distribution of all defects, while the Kinetic Regime I allows equilibration of only oxygen vacancies and titanium interstitials. The significance of the gas/solid kinetics, that is considered in terms of the chemical diffusion coefficient, in the processing of well-defined _TiO2${\mathrm{TiO}}_2$ specimens with controlled nonstoichiometry and the related defect disorder, is outlined.     

Subsonic compressible flow in two-sided lid-driven cavity. Part I: Equal walls temperatures

This paper presents a numerical study of the laminar, viscous, subsonic compressible flow in a two-dimensional, two-sided, lid-driven cavity using a multi-domain spectral element method. The flow is driven by steadily moving two opposite walls vertically in opposite directions. All the bounding walls have equal temperatures. The results of the simulations are used to investigate the effects of the cavity aspect ratio, the Reynolds number and the Mach number on the flow. At lower Reynolds numbers, the flow pattern consists of two separate co-rotating vortices contiguous to the moving walls. For higher Reynolds numbers, initially a two-vortex flow is formed, which eventually turns into a single elliptical vortex occupying most of the cavity. For a higher aspect ratio, the flow patterns are dissimilar in that the streamlines become more and more elliptic. For aspect ratios as high as 2.5, at high Reynolds numbers, a three-vortex stage is formed. It is found that the compressibility effects are not very significant for Mach numbers less than 0.4. Dissipation of kinetic energy into internal energy changes the temperature field especially near the boundaries. Boundary layer studies suggest that the velocity and temperature boundary layer thicknesses are lower for higher Reynolds numbers. For engineering purposes, these thicknesses can be approximated by the existing flat-plate solutions.     

Thermal stabilities of nanoporous metallic electrodes at elevated temperatures

Currently Pt-based metals are the best catalytic electrodes for fuel cells at operating temperatures below 500 °C. Pure platinum electrodes suffer degradation of microstructure over time at elevated temperatures due to Ostwald ripening. In this paper, better thermal stability of Pt–Ni nanoporous thin films relative to pure Pt is reported. Based on ab initio calculations, it was found that both the surface energy of a Pt_0.7Ni_0.3 cluster and the energy change of the Pt–Ni alloy cluster upon ripening on yttria stabilized zirconia (YSZ) solid electrolyte were lower than pure Pt. This suggested a better thermal stability of Pt_0.7Ni_0.3 than Pt. In addition, annealing impacts on microstructures and properties of nanoporous Pt and Pt–Ni alloy thin films were examined experimentally. SEM images show dramatic porosity reduction for pure Pt after annealing at temperatures of 400–600 °C but insignificant microstructure change for Pt–Ni nanoporous thin films. As a result, in solid oxide fuel cells using nanoporous Pt–Ni cathodic catalysts instead of pure Pt, better stability, lower electrode impedances, and higher power densities were achieved at elevated operating temperatures (350–500 °C).     

Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance

Mixtures of hydrogen and natural gas are considered viable alternative fuels to gasoline due to lower overall pollutant emissions but suffer from problems associated with on-board storage resulting in limited vehicle range. To date, vehicle engines have run on Hythane using a fixed hydrogen–natural gas fraction of 20% and fixed air-to-fuel ratio but it may be possible to leverage greater advantage from hydrogen addition due to its unique lean-burn properties. This study presents the results of a one-cylinder CFR engine test with mixtures of hydrogen in methane of 0, 20, 40 and 60% by volume. Each fuel was tested at speeds of 700 and 900 rpm, full and part loads, and equivalence ratios from stoichiometric to the partial burn limit. These results are used in a driving cycle simulation which is presented in a companion paper.     

Effect of crosslinking on the durability and electrochemical performance of sulfonated aromatic polymer membranes at elevated temperatures

End-group crosslinked sulfonated poly(arylene sulfide nitrile) (XESPSN) membranes are prepared to investigate the effect of crosslinking on the properties of sulfonated aromatic polymer membranes at elevated temperatures (>100 °C). The morphological transformation during annealing and crosslinking is confirmed by atomic force microscopy. The XESPSN membranes show outstanding thermal and mechanical properties compared to pristine and non-crosslinked ESPSN and Nafion^® up to 200 °C. In addition, the XESPSN membranes exhibit higher proton conductivities (0.011–0.023 S cm⁻¹) than the as-prepared pristine ESPSN (0.004 S cm⁻¹), particularly at elevated temperature (120 °C) and low relative humidity (35%) conditions due to its well-ordered hydrophilic morphology after crosslinking. Therefore, the XESPSN membranes demonstrate significantly improved maximum power densities (415–485 mW cm⁻²) compared to the ESPSN (281 mW cm⁻²) and Nafion^® (314 mW cm⁻²) membranes in single cell performance tests conducted at 120 °C and 35% relative humidity. Furthermore, the XESPSN membrane exhibits a much longer duration than the ESPSN membrane during fuel cell operation under a constant current load as a result of its improved mechanical and thermal stabilities.     

磷酸铁锂动力电池常温循环衰减机理分析

常温循环寿命是锂离子电池应用的重要指标,磷酸铁锂电池具有阴极结构稳定和电解液成分简单的特点,是研究锂离子电池工作机理的重要手段.研究磷酸铁锂电池的常温衰减机理对于完善锂离子电池衰减机理的认知和电化学性能提升有重要意义.本文以不同健康状态(SOH)的商业化磷酸铁锂电池为样本,研究其常温循环容量衰减的原因.使用电化学微分容...     

Effect of various coal contaminants on the performance of solid oxide fuel cells: Part I. Accelerated testing

The contaminants that are potentially present in the coal-derived gas stream and their thermochemical nature are discussed. Accelerated testing was carried out on Ni-YSZ/YSZ/LSM solid oxide fuel cells (YSZ: yttria stabilized zirconia and LSM: lanthanum strontium manganese oxide) for eight main kind of contaminants: CH₃Cl, HCl, As, P, Zn, Hg, Cd and Sb at the temperature range of 750-850 °C. The As and P species, at 10 and 35 ppm, respectively, resulted in severe power density degradation at temperatures 800 °C and below. SEM and EDX analysis indicated that As attacked the Ni region of the anode surface and the Ni current collector, caused the break of the current collector and the eventual cell failure at 800 °C. The phosphorous containing species were found in the bulk of the anode, they were segregated and formed “grain boundary” like phases separating large Ni patches. These species are presumably nickel phosphide/phosphate and zirconia phosphate, which could break the Ni network for electron transport and inhibit the YSZ network for oxygen ion transport. The presence of 40 ppm CH₃Cl and 5 ppm Cd only affected the cell power density at above 800 °C and Cd caused significant performance loss. Whereas the presence of 9 ppm Zn, 7 ppm Hg and 8 ppm Sb only degraded the cell power density by less than 1% during the 100 h test in the temperature range of 750-850 °C.     

Agglomeration characteristics of alumina sand-straw ash mixtures at elevated temperatures

The agglomeration characteristics of alumina sand-straw ash mixtures were investigated at various levels of ash content (0.0, 3.3, 27.8, and 43.5%) and temperature (620, 740, 850, and 1000°C) using scanning electron microscopy and energy dispersive X-ray analysis techniques. Agglomeration of alumina sand did not take place below the temperature of 850°C at all levels of ash content, but at the temperature of 850°C a weak bonding of particles was observed. However, at the temperature of 1000°C, the alumina particles agglomerated in the presence of straw ash at all levels of ash content as a layer of ash melt bonded the particles together. The temperature at which agglomeration occurred was within the range of the initial deformation (921°C) and softening (1054°C) temperatures of straw ash and the liquidus temperature of potash feldspar (990 ± 20°C). The softening of straw ash and the formation of low melting temperature eutectic (potash feldspar) are two possible mechanisms for the agglomeration of alumina sand.     

A shock tube study of pyrolysis of tetrahydrothiophene at elevated temperatures

A single pulse shock tube has been used to study the pyrolysis of a hydrogenated sulphur compound, tetrahydrothiophene over the temperature range 1686–1885 K and pressures between 2.4 and 3.5 bars. Product yield and composition was determined using capillary column gas‐chromatography with flame ionization detection and flame photometric sulphur selective detection. The principal hydrocarbon products at all temperatures were C₂H₄ and C₂H₂. Other hydrocarbon reaction products were CH₄, C₂H₆, C₃H₄, C₃H₆, C₄H₃, C₄H₆, C₄H₁₀, C₄H₄, C₆H₆, C₄H₂ and some traces of C₅ and C₆H₅ species. The sulphur compounds identified were hydrogen sulphide, carbon disulphide, thiophene and traces of ethyl mercapton. The pyrolysis experiments indicated that at lower temperatures the hydrogenated thiophene molecule reacts in two unimolecular channels to form C₂H₄+(CH₂)2–S in the major faster channel which may be the route for other products. However, a second lower route may be the formation of C₃H₆+CH₂S. The rate constant obtained for tetrahydrothiophene pyrolysis calculated for this study was k_dis(C4H8S)=1.26×10¹³ exp (316.9 kJ mol^?1) s^?1. Copyright © 2004 John Wiley & Sons, Ltd.