Theory and analysis of module-scale thermal runaway propagation

Lithium-ion batteries (LIBs) are ubiquitous in consumer and industrial applications due to their high energy density and cycle life. Under nominal use conditions, they provide a safe means of energy storage. If abused, they can undergo thermal runaway (TR), i.e., rapid localized heating that propagates as a thermo-chemically reacting front, generating combustible gas, increasing internal pressure, leading to venting. Battery Energy Storage Systems (BESS) are collections of LIBs which stabilize power grids with a high penetration of variable and intermittent power sources. If one LIB in the BESS undergoes TR, the immediately neighboring LIBs will be abused thermally and may undergo TR, resulting in a cascade of cell-to-cell failures known as thermal runaway propagation (TRP). Matters are further complicated by the potential for external flame heating or explosion if the vented gases mix with ambient air and ignite.

We examine TRP across a LIB stack parametrically in a non-dimensional setting. Pouch-format LIBs are considered since these are used in some BESS applications and permit a quasi-one-dimensional unsteady analysis. From the governing equations, key non-dimensional groups controlling the solution behavior are identified. The mean consumption rate is identified as a scalar value mapping the non-dimensional groups to a hazard metric for a LIB stack. Solution of the system is carried out numerically and the parametric dependencies of the hazard metric on the non-dimensional groups are demonstrated. We identify a regime in the parameter space where TRP is inhibited, constituting a passively safe design space for LIB stacks. Another regime is identified in which flame heating results in bi-directional TRP, doubling the mean consumption rate and exacerbating the hazard. This work provides designers, engineers and scientists with a formalized, non-dimensional framework and compact parameter set to gain an intuitive understanding of, qualitatively compare, and potentially ameliorate the TR hazards posed by different LIB stacks.

Novelty and significance

Thermal runaway is the principal failure mode for large-scale battery energy storage systems used increasingly worldwide for power grid stabilization. A formalized framework with a compact non-dimensional parameter set is needed to effectively assess and ameliorate hazards posed by different lithium-ion battery modules. The main novelty of this work is that it formulates, solves and analyzes for the first time the problem of thermal runaway initiation and propagation in a module of multiple lithium-ion battery cells in an unsteady, non-dimensional setting suitable for detailed physical analysis. This framework allows the derivation of a hazard metric, from which non-dimensional parameter regimes of passive safety due to non-propagation, and exacerbated hazard from bi-directional propagation, are identified and their physical mechanisms elucidated.