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.
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.
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 LixMO2, 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
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.
The assistance of Samuel Roberts-Baca in implementing these calculations as a web-based tool is gratefully acknowledged.
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