The energy transition to renewable energy resources strongly depends on energy storage systems like lithium-ion batteries. Because of their high energy density, low self-discharge rates, and good cycle life, these batteries have increased in use for a range of system scales from consumer electronic devices to utility scale power storage systems. While these battery systems are generally safe, there are possibilities that a single cell might have a manufacturing defect, or the battery, might be subjected to electrical, mechanical, or thermal abuse. In these compromised conditions, a lithium-ion cell might undergo a process known as thermal runaway wherein the internal exothermic reactions exceed the cooling rate of the cell. Temperatures as high as 900 C are measured for cells undergoing thermal runaway. In many systems, cells are arranged in planar arrays to improve packing efficiency and volumetric energy density. In the absence of thermal separators/barriers, the thermal contact area between cells makes it very likely that once a single cell has failed it will initiate failure in adjacent cells in a process known as thermal runaway propagation. Experimental results are presented for the rate of thermal propagation through five 10 Ah nickel-manganesecobalt-( oxide) NMC-532 cells for four different cell-separation conditions: a baseline of direct cell-to-cell contact, a 1 mm aluminum plate separator, a single layer of a 1.6 mm ceramic woven fiber, and a triple layer of the same woven fiber. Thermocouples are placed either between the cells (baseline case) or around the separator (thermal barrier) material. The temperature changes between cells provides an estimate for the rate of propagation. As each cells fails, it releases a significant volume of gas. The influence of the thermal barriers on the rate of propagation is presented. Results show that the rate of propagation is decreased by a factor of three between the baseline case and the aluminum plate separator. A computational model was developed to better understand which properties of the separators (thermal barriers) are most important in affecting the thermal propagation rate. A heat transfer energy budget is provided to clarify the relative roles of energy storage, conduction, convection, and radiative cooling on the rate of propagation.