J.R. Selman

Thermal modeling and design considerations of lithium-ion batteries

Abstract A simplified one-dimensional thermal mathematical model with lumped parameters was used to simulate temperature profiles inside lithium-ion cells. The model makes use of heat-generation parameters established experimentally for the Sony (US18650) cell. The simulation results showed good agreement with temperature measurements at C/2, C/3, and C/6 discharge rates, while some deviation was noticed for the C/1 discharge rate. The model was used to simulate temperature profiles under different operating conditions and cooling rates for scaled-up cylindrical lithium-ion cells of 10 and 100 A h capacity. Results showed a strong effect of the cooling rate on cell temperature for all discharge rates. A significant temperature gradient inside the cell was found only at higher cooling rates, where the Biot number is expected to be >0.1. At lower cooling rates, the cell behaves as a lumped system with uniform temperature. To establish the limits of temperature allowable in scale-up by the simplified model, commercial lithium-ion cells at different open circuit potentials were tested inside an accelerated rate calorimeter (ARC) to determine the onset-of-thermal-runaway (OTR) temperatures. Sony (US18650) cells at 4.06, 3.0, and 2.8 V open circuit voltage (OCV) were tested and their measured OTR temperatures were found to be 104, 109, and 144°C, respectively. A sharp drop in the OCV, indicating internal short circuit, was noticed at temperatures close to the melting point of the separator material for all open circuit voltages.

Electrochemical‐Calorimetric Studies of Lithium‐Ion Cells

An accelerated rate calorimeter in combination with a battery cycler and a precision multimeter was used to measure the heat dissipation from, and heat accumulated in, commercially available lithium-ion cells during cycling over a range of operating parameters within the limits recommended by the manufacturer. An integral energy balance was used to determine the total heat generated in the test cell during cycling. From the measurements during temperature transients the heat capacity of the test cell was found to be relatively independent of temperature, ranging from 0.82 to 1.07 J g -1 K -1 . This value agrees relatively well with separate measurements using an adiabatic calorimeter which yield slightly higher values. DC current interruption technique was used to determine the time-dependent area-specific impedance, , of the cell which was well correlated with steeply increased heat dissipation rate at the end of discharge. The reversible (entropic) heat effect derived from an energy balance was found to be exothermic during discharge and endothermic during charge. Using two different methods, values were obtained for the entropy of reaction (AS) during discharge of the cell. The resulting values obtained with method II, depending on the discharge rate, varied from -41.19 to -80.98 J K -1 per g mole of Li and showed a weak dependence on temperature, in the 35 to 55°C range. The rate dependence of the AS values needs further examination in a future study. By extrapolating to zero rate, the reversible entropy of the faradaic reaction for this cell was found to be -37 ± 3 J K -1 per g mole of Li within the temperature range. The entropic heat effect and the heat effect associated with nonfaradaic reactions are appreciable and should be included in thermal modeling.

Thermal modeling of secondary lithium batteries for electric vehicle/hybrid electric vehicle applications

Abstract A major obstacle to the development of commercially successful electric vehicles (EV) or hybrid electric vehicles (HEV) is the lack of a suitably sized battery. Lithium ion batteries are viewed as the solution if only they could be “scaled-up safely”, i.e. if thermal management problems could be overcome so the batteries could be designed and manufactured in much larger sizes than the commercially available near-2-Ah cells. Here, we review a novel thermal management system using phase-change material (PCM). A prototype of this PCM-based system is presently being manufactured. A PCM-based system has never been tested before with lithium-ion (Li-ion) batteries and battery packs, although its mode of operation is exceptionally well suited for the cell chemistry of the most common commercially available Li-ion batteries. The thermal management system described here is intended specifically for EV/HEV applications. It has a high potential for providing effective thermal management without introducing moving components. Thereby, the performance of EV/HEV batteries may be improved without complicating the system design and incurring major additional cost, as is the case with “active” cooling systems requiring air or liquid circulation.

Solar desalination with a humidification-dehumidification technique — a comprehensive technical review

Major desalination processes consume a large amount of energy derived from oil and natural gas as heat and electricity. Solar desalination, although researched for over two decades, has only recently emerged as a promising renewable energy-powered technology for producing fresh water. Solar desalination based on the humidification-dehumidification cycle presents the best method of solar desalination due to overall high-energy efficiency. This paper provides a comprehensive technical review of solar desalination with a multi-effect cycle providing a better understanding of the process. Discussion on methods to improve system performance and efficiency paves the way towards possible commercialisation of such units in the future.

Characterization of commercial Li-ion batteries using electrochemical-calorimetric measurements

Abstract Commercial Li-ion cells of Type 18650 dimensions and prismatic designs from different manufacturers have been tested to evaluate their performance and to study their thermal behavior using electrochemical–calorimetric methods. All cells tested in this work showed good performance and cyclability under normal operating conditions. The measured heat effect for the cells were exothermic during discharge and partially endothermic during charge. Cell impedance was measured for selected cells and showed some dependence on the state of charge or depth of discharge, with significant increase at the end of discharge due to concentration polarization. The entropy coefficient (d E eq /d T ) for the A&T (18650) and Panasonic (CGR 18650) cells were measured using potentiometric methods at different depths of discharge (DOD). The measured values for both cells showed some dependence on the DOD with some perturbation near 4.0 V, where the measured d E eq /d T for Panasonic cell had an unexpected positive value. This was found to be consistent with the measured endothermic heat effect during discharge for the Panasonic cell near E eq =4.0 V. This may be related to a phase change in the LiCoO 2 cathode material, as reported in the literature, and to structural transformation in the graphite used as anode material, in the Panasonic cell.

Entropy Changes Due to Structural Transformation in the Graphite Anode and Phase Change of the LiCoO2 Cathode

Electrochemical-calorimetric measurements, in combination with the dc current interruption technique, were used to determine the change of entropy during discharge and charge of the Panasonic Li-ion cell CGR 18650H. The entropy coefficient dE eq /dT was found to vary from -0.6 mV K -1 in the discharged state to -0.1 mV K -1 in the fully charged state. An unexpected variation of large amplitude (from -0.1 to 0.25 and -0.75 back to -0.1 mV K -1 ) was observed in a narrow range of depth of discharge (DOD) near 0.23, between 3.95 and 4.10 V, curing charge as well as discharge. Thus, changes in the reversible heat, TAS, appear to be mainly responsible for the transient endothermic heat effect observed during discharge of these Li-ion cells, near DOD 0.23, between 4.035 and 3.975 V. This interpretation of the calorimetric measurements is confirmed by the fact that dE eq /dT measured at 4.0 V was positive (0.14 mV K -1 ). The unexpected strong variation of dE eq /dT near DOD 0.23 is due to the nearly simultaneous occurrence of a phase change in the cathode and a structural transformation (quasi-phase change) in the graphite anode.

Solar desalination with a humidification-dehumidification cycle: mathematical modeling of the unit

Abstract Solar desalination is gradually emerging as a successful renewable energy source of producing fresh water. Solar Multi-Effect Humidification (MEH) units based on the humidification-dehumidification principle are considered as the most viable among solar desalination units. A simulation study of these units leads to a better understanding of the performance of such type of desalination units. This study therefore focuses on studying and analysing the effects and performance of various components involved in the process along with the study of the effect of water feed flow rate on the desalination production. To our knowledge, there is no such comprehensive model available in the literature. This study could lead a step further in the commercialisation of solar desalination units based on the humidification-dehumidification principle.

Application of ac impedance in fuel cell research and development

In applying ac impedance to fuel cells and their porous (gas diffusion) electrodes the emphasis lies on different fuel cell components, and their properties, according to the fuel cell type. The focus has been directed at the electrode/electrolyte interface in MCFC and PAFC, whereas in SOFC and PEMFC the ionic/electronic conductivity of the electrolyte or the characteristics of its composite with the electrocatalyst is of primary interest. The limitations of ac impedance in fuel cell application are in part due to difficulties of interpretation and in part due to experimental difficulties because of the generally fast electrode reaction kinetics. Further research directions are indicated.

Passive thermal management using phase change material (PCM) for EV and HEV Li- ion batteries

Researchers at the Illinois Institute of Technology (IIT, Chicago, IL) have successfully demonstrated a passive thermal management system using phase change materials (PCM) in Li-ion batteries for electric vehicle and scooter applications. Thermal characterization of Li-ion battery modules using PCM is presented and discussed. In addition, a battery pack design for plug-in hybrid vehicles (PHEV) is proposed and discussed.

Electrical conductivity and chemical diffusivity of NiAl2O4 spinel under internal reforming fuel cell conditions

NiAl2O4 spinel was formed by solid state reaction. Its electrical conductivity was measured in the temperature range of 680–940 °C and under various oxygen-rich environments, as well as under reducing conditions. From the temperature dependence of the conductivity, the activation energies for conduction increase for decreasing oxygen partial pressures. From the partial oxygen pressure dependence, the defect structure of the material was analysed. The conductivity change with respect to PO2 can be attributed to singly and doubly ionized nickel vacancies. The chemical diffusivity of the oxide was determined by conductivity relaxation upon abrupt change in PO2 in the surrounding atmosphere. The oxygen chemical diffusion coefficient is of the order of magnitude of 10−4 cm2 s−1.