Electro-Thermal Planar Dynamics and Control of Prismatic Lithium-ion Battery Cells

Project duration: 2012-2015

Owing to their large gravimetric and volumetric energy densities, large-format prismatic lithium-ion (Li-ion) cells are becoming ubiquitous in civilian vehicle applications and are also under consideration for heavy military vehicle hybridization and robotic platforms. Limited availability of cooling or warm-up auxiliaries poses a challenge for battery management schemes. In prismatic cells, electro-thermal coupling of the in-plane distributions of temperature and current density have also been shown to cause self-heating, which can lead to the onset of thermal instability. Temperature within a cell can increase due to poor heat transfer at cell edges, or because heat transfer is slowed by local changes in properties such as electric conductivity, reaction rate constants, or thermal conductivity that occur as the distributions of temperature or electrolyte content vary. Under certain conditions (say, if electric conductivity rises with temperature as thermal conductivity stays constant) constructive feedback can develop, causing a potentially catastrophic “thermal runaway” situation where temperature rises without bound. Also, new cathodes tend to have poor capacity or efficiency at low temperatures. Electrolytic solutions can be frozen and more viscous at low temperatures, degrading conductivity.

The key motivations for this ongoing project were 1) to rationalize how coupling between charge and heat transfer impacts thermal stability in prismatic Li-ion battery cells, and further, 2) to understand what material properties or control techniques resist thermal runaway or leverage positive electro-thermal feedback to facilitate cold start.

In the first phase of the proposed work we have defined a multi-phase finite-element battery-cell model with sufficient complexity to allow accurate predictions of the transient thermal response of large-format prismatic cells. We have also targeted model simplicity, identifying a few key dimensionless parameters (there were 4 in the 2013 implementation, expanded to 8) that control the electro-thermal response. This balance of simplicity and complexity leads to a model that predicts the 3D dynamic distributions of heat, current, and voltage accurately, but whose computations are fast enough to allow goal-seeking approaches to parameterization. Using various experimental measurements of the dynamic thermal response of 15Ah LFP battery cells, we have determined accurate values of interior material properties that contribute to the observed distributed thermal response. At present, the model predicts maximum, minimum, and average temperature on the battery surface within the error of the thermocouples at charge/discharge currents from 15A (the minimum rate at which significant temperature elevation is observed) to 75A.

The 2013 effort produced techniques for rapid, accurate model parameterization and complimentary experimental approaches that make such parameterization possible. This approach was met with approval by referees at the annual review. In 2014 the experimental and theoretical efforts were extended to establish the temperature dependences of material properties. This data will underpin the parameter functionalities assumed when assessing stability under different operating boundary conditions, which remains the ultimate project goal.

Publications:

• S.U. Kim, P. Albertus, D. Cook, C.W. Monroe, and J. Christensen, “Thermoelectrochemical simulations of performance and abuse in 50-Ah automotive"cells”, Journal of Power Sources 268 (2014) 625-633.
• H.N. Chu and C.W. Monroe, “Electromechanical behavior of large-format prismatic lithium-ion batteries”, ECS Annual Meeting, Fall 2014.
• L. Secondo, J. Siegel, A. Stefanopoulou, and C. W. Monroe, “Simplifying Electrothermal Dynamics of 15-Ah Prismatic Li-Ion Batteries”, ECS annual meeting, Fall 2014.
• S.U. Kim, L. Secondo, and C.W. Monroe, “Coupling of Dynamic Electrical and Thermal Processes in Prismatic batteries”, ECS annual meeting, Fall 2013.
• S.U. Kim, L. Secondo, and C.W. Monroe, “Key Parameters for Electrothermal Dynamics and Control of 15-Ah Prismatic Li-Ion Batteries”, AIChE annual meeting, Fall 2013.