About Thermodynamic Web Calculator

Introduction

An estimate of the quantity of heat that can be released is required to assess the potential severity of thermal runaway in lithium-ion batteries. Here we use thermodynamic properties of materials involved to provide a heat of reaction for bounding the potential heat release and for use in thermal runaway models to predict consequences of abuse scenarios.

Background

From the early 1990s up to the time this calculator was developed in the early 2020s, most commercial Li-ion batteries have used a graphite negative electrode (anode) with a layered metal oxide positive electrode (cathode) and an organic electrolyte composed of a mixed carbonate solvent and a salt (typically LiPF₆). The largest contributions to thermal runaway heat release within cells originate from reactions of these electrode materials with the organic solvents. The amount of chemical heat that can be released via these thermal runaway reactions increases with state of charge (SOC) because the heat release is proportional to the quantities of reactive electrode materials, namely lithiated graphite in the anode (LiC₆) and delithiated layered metal oxide in the cathode (MO₂).

Detailed thermal runaway models indicate that rapid heat release from the cathode in the presence of organic electrolytes is the principal driver of thermal runaway, but heat release from the lithiated anode reacting with electrolyte must also be accounted for to accurately predict the maximum temperature of the cell, which is required for reasonable predictions of cascading cell-to-cell failure in large battery systems. When lithiated graphite reacts with liquid ethylene carbonate (EC) from the electrolyte to form a solid coating and gases, the predicted exothermic heat of reaction from thermodynamics is -281.4 kJ/mol of lithium in the graphite.¹ However, the heat of reaction associated with layered metal oxide cathodes is a more complicated multi-step process associated with a wider variety of materials.

Objective

This web-based calculator estimates heats of reaction associated with thermal runway of layered metal oxide cathode materials based on the underlying thermodynamics for specific metal compositions, degrees of delithiation, and coexisting organic material (e.g. electrolyte solvent). This calculator implements methods described in a recent review² for cathodes of the form Li_xMO₂, where x is the degree of lithiation, (1-x) is the degree of delithiation that determines the amount of reactive material, and M is a metal or mixture of metals in the layered metal oxide. Another recent article has demonstrated how these thermodynamic calculations are in general agreement with a wide range of literature calorimetry data.³

Capabilities

This calculator predicts the total and stepwise heats of reaction for cathode materials where the user specifies the organic solvent of interest, the degree of lithiation x, and the metal composition M as some mixture of Ni, Co, Mn, and/or Al. Ethyl methyl carbonate (EMC) is the default solvent representing typical electrolyte solvent mixtures.² The stepwise version of these calculations assumes an initial solid phase transition with no change in mass followed by additional phase changes that release oxygen, which can react with the electrolyte solvents or other organics to produce extra heat.^2,3 For convenience, the results of this thermodynamic calculator are presented in a summarized form followed by an optional detailed form that includes several types of units (basis of mass and moles), with or without electrolyte oxidation. This calculator does not consider effects of heat release external to the cell during thermal runaway, which is caused by flaming reactions of venting electrolyte and gaseous decomposition products with atmospheric oxygen. An optional input of cathode mass enables an estimate of maximum full-cell heat release from decomposition of both the cathode and the anode with electrolyte.

Acknowledgements

The assistance of Samuel Roberts-Baca in implementing these calculations as a web-based tool is gratefully acknowledged.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This website describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the website do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

References:

Release History

Title: “Thermodynamic Reaction Heat Calculator for Layered Metal Oxide Cathodes in Organic Electrolytes” (September 2020). Excel spreadsheet posted to calculate thermal runaway heat release for cathode materials. Suitable for calculations similar to those used in Ref. 3 for comparisons to calorimetry measurements.
Title: “Lithium-ion Battery Thermodynamic Web Calculator” (February 2021). Initial release of web calculator. Outputs are similar to the spreadsheet. Includes an option to report heat release for a full cell (cathode + anode) when the cathode mass is supplied.

Possible Future Enhancements:

Include additional decomposition reactions for the electrodes
Allow the user to specify Amp-hours as an alternative to degree of lithiation
Include curve-fits that will convert voltage to degree of lithiation in both electrodes
Estimate the maximum temperature rise expected for a full cell in thermal runaway when the user supplies the necessary compositions
Include capability to produce plots on the web site
Include additional metallic elements for layered metal oxides
Include thermodynamic heat release for decomposition of cathodes other than layered metal oxides (e.g. lithium manganese oxide, lithium iron phosphate, etc.)
Include thermodynamic heat release for decomposition of anodes other than lithiated graphite
Include estimated kinetic parameters for decomposition of electrode materials