Publications

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Accurate models of thermal runaway in lithium-ion batteries require quantitative knowledge of heat release during thermochemical processes. A capability to predict at least some aspects of heat release for a wide variety of candidate materials a priori is desirable. This work establishes a framework for predicting staged heat release from basic thermodynamic properties for layered metal-oxide cathodes. Available enthalpies relevant to thermal decomposition of layered metal-oxide cathodes are reviewed and assembled in this work to predict potential heat release in the presence of alkyl-carbonate electrolytes with varying state of charge. Cathode delithiation leads to a less stable metal oxide subject to phase transformations including oxygen release when heated. We recommend reaction enthalpies and show the thermal consequences of metal-oxide phase changes and solvent oxidation within the battery are of comparable magnitudes. Heats of reaction are related in this work to typical observations reported in the literature for species characterization and calorimetry. The methods and assembled databases of formation and reaction enthalpies in this work lay groundwork a new generation of thermal runaway models based on fundamental material thermodynamics, capable of predicting accurate maximum cell temperatures and hence cascading cell-to-cell propagation rates.

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Lithium-ion battery safety is prerequisite for applications from consumer electronics to grid energy storage. Cell and component-level calorimetry studies are central to safety evaluations. Qualitative empirical comparisons have been indispensable in understanding decomposition behavior. More systematic calorimetry studies along with more comprehensive measurements and reporting can lead to more quantitative mechanistic understanding. This mechanistic understanding can facilitate improved designs and predictions for scenarios that are difficult to access experimentally, such as system-level failures. Recommendations are made to improve usability of calorimetry results in mechanistic understanding. From our perspective, this path leads to a more mature science of battery safety.

Heat release that leads to thermal runaway of lithium-ion batteries begins with decomposition reactions associated with lithiated graphite. We broadly review the observed phenomena related to lithiated graphite electrodes and develop a comprehensive model that predicts with a single parameter set and with reasonable accuracy measurements over the available temperature range with a range of graphite particle sizes. The model developed in this work uses a standardized total heat release and takes advantage of a revised dependence of reaction rates and the tunneling barrier on specific surface area. The reaction extent is limited by inadequate electrolyte or lithium. Calorimetry measurements show that heat release from the reaction between lithiated graphite and electrolyte accelerates above ~200°C, and the model addresses this without introducing additional chemical reactions. This method assumes that the electron-tunneling barrier through the solid electrolyte interphase (SEI) grows initially and then becomes constant at some critical magnitude, which allows the reaction to accelerate as the temperature rises by means of its activation energy. Phenomena that could result in the upper limit on the tunneling barrier are discussed. The model predictions with two candidate activation energies are evaluated through comparisons to calorimetry data, and recommendations are made for optimal parameters.

Total hemispherical emissivities are a commonly used property in radiative heat transfer analysis. Measurements made in the course of testing become far more useful to thermal analysts if they are compiled with a sufficient level of detail, and summarized in a manner that allows the most appropriate value or trend to be located quickly. This report collects emissivity measurements from recent years, made in the course of testing metallic surfaces at Sandia's Radiant Heat Test Facility, and compares them to a selection of previous summary documents. These measurements are organized by material type, surface finish, and degree of oxidation. The comparisons also consider the temperature dependence of total hemispherical emissivity. Materials considered include Inconel 600, SS304, 17-4PH SS, silicon carbide, and aluminum alloys. A limited selection of high-temperature paints and other surface coatings are also considered. Recommendations are made for frequency of measurements and level of detail in reporting emissivities in future test series. A more limited scope is recommended for the use of high-temperature paints at Sandia's Radiant Heat Test Facility; pre-oxidation of Inconel and stainless steel surfaces is preferred in many circumstances. ACKNOWLEDGEMENTS Thanks are due to Fredd Rodriguez and Jim Nakos for promoting this work for the benefit of their own programs and many others. The greatest share of the data in this report was provided by Jill Suo-Anttila, Jim Nakos, Walt Gill, and Burl Donaldson. The thoughtful suggestions of Brantley Mills on making the summaries more useful to thermal analysts are greatly appreciated. Thoughtful reviews and assistance with formatting from Ethan Zepper and Monica Bigney were also very valuable.

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