Simulation and Validation Testing of the Effect of Novel Electrolyte Design on Thermal Propagation of Lithium-Ion Batteries

Abstract

Lithium-ion batteries are an increasingly prevalent source of power in grid storage applications and electric powertrains, given its high energy density and extended cycle life. However, lithium-ion batteries carry significant safety risks and are a growing hazard due to possible fires that can be caused by overheating, physical cues (i.e., crushing or penetration of cell), overcharging and short-circuits. These risks are greatly increased when focus shifts from a single cell to either modules or packs, where an individual cell entering thermal runaway propagates to adjacent cells. In spite of multiple protections battery packs have in recent times been known to fail.It is well understood that thermal runaway occurs when the self-heating rate in the cell exceeds the heat dissipation out of the cell. This self-heating is the product of undesired side-reactions in the cell which are onset by abuse conditions such as excess heat, internal short circuits, etc. The key to inhibiting thermal runaway is to modify the cell such that the self-heating rate under abuse conditions is significantly reduced. Reducing the self-heating rate is important when considering battery packs that consist of many cells as is the case in electric vehicles, so that if one cell fails in a pack the thermal runaway does not propagate to adjacent cells. Our approach therefore is to modify the electrolyte system to reduce the overall cell self-heating rate. This is a tunable, drop-in solution to addressing the need for improving battery safety.The incorporation of NOHMs novel safe electrolyte formulation is shown to impact the abuse tolerance of cells on both the material level, i.e. the electrolyte-electrode interaction and on the cell level. The presented electrolyte systems demonstrate reduced overall energy and peak heat flow in the exotherm profiles of NMC cathode materials as well as lithiated graphite via differential scanning calorimetry (DSC). With this approach, we demonstrate significantly lower self-heating rates in an 18650 lithium-ion cell via an accelerating rate calorimeter (ARC).Using the self-heating rate data produced via ARC testing we developed a thermal runaway model to simulate thermal propagation in battery packs consisting of seven 18650 lithium-ion cells arranged in a honeycomb configuration. The self-heating rate is translated into thermal power generation for individual cells surrounding a ‘trigger cell’, a cell that has entered thermal runaway. The model suggests that changes in the electrolyte system as measured by ARC mitigates cell-to-cell thermal propagation in event of an individual cell thermal runaway (Figure 1). This simulation is then validated with a real pack by inducing thermal runaway in the center cell with a nail penetration test. NOHMs novel safe electrolyte development carries significance in not only safer individual cells but also in safer overall battery packs. Figure 1