Microstructural Analysis of the Effects of Thermal Runaway on Li-Ion and Na-Ion Battery Electrodes

Abstract

Thermal runaway is a phenomenon that occurs due to self-sustaining reactions within batteries at elevated temperatures resulting in catastrophic failure. Here, the thermal runaway process is studied for a Li-ion and Na-ion pouch cells of similar energy density (10.5 Wh, 12 Wh, respectively) using accelerating rate calorimetry (ARC). Both cells were constructed with a z-fold configuration, with a standard shutdown separator in the Li-ion and a low-cost polypropylene (PP) separator in the Na-ion. Even with the shutdown separator, it is shown that the self-heating rate and rate of thermal runaway in Na-ion cells is significantly slower than that observed in Li-ion systems. The thermal runaway event initiates at a higher temperature in Na-ion cells. The effect of thermal runaway on the architecture of the cells is examined using X-ray microcomputed tomography, and scanning electron microscopy (SEM) is used to examine the failed electrodes of both cells. Finally, from examination of the respective electrodes, likely due to the carbonate solvent containing electrolyte, it is suggested that thermal runaway in Naion batteries (NIBs) occurs via a similar mechanism to that reported for Li-ion cells.

Spatially resolved ultrasound diagnostics of Li-ion battery electrodes

“…Direct measurements of internal changes in the structure of a battery can be performed using X-ray techniques; [10][11][12] however, this is an expensive and often time-consuming process and is not practical in many real-world applications. In contrast, the use of ultrasound techniques to understand changes in the battery architecture is relatively low in cost and can be performed in operando.…”

Multiscale tomographic analysis of the thermal failure of Na-Ion batteries

“…Previous work has shown that the thermal runaway process in Na-ion systems is less rapid than in Li-ion cells [44]. The substantial reduction of the cathode as shown via EDX in this work may suggest that this reduced rate is inherent to the materials rather than as a result of a variation in the pathways of thermal runaway or the microstructural behaviour of the positive electrode.…”

“…Thermal runaway has previously been shown to cause significant morphological changes to electrode structures in Li-ion batteries [43]; however, previous work [44] has shown that thermal runaway is a less exothermic event in comparable Na-ion cells. The temperature profile obtained as a result of thermal runaway can be seen in Figure 1, with the maximum temperature observed being limited to 280 °C as a result of hardware limitations, as demonstrated in previous work [52]. When the cell was removed, it was observed that the side of the pouch had ruptured during thermal runaway and that the cathode material had partially delaminated from the aluminium current collector.…”

“…This is a standard method that involves heating the sample to the start temperature, following which a period is observed for the environmental temperature to stabilise followed by the measurement of any self-heating. After thermal runaway, the calorimeter was left to cool to ambient temperature before the electrodes of the cell were harvested [44].…”

“…Studies of the thermal runaway process in such Na-ion cells has recently been reported by the authors [44], showing that the maximum temperature reached by Na-ion cells during thermal runaway is slightly lower than for comparable Li-ion batteries. The thermal stability of materials has also been studied by Xia and Dahn [45] who found that NaCrO 2 is a less reactive positive electrode material than LiFePO 4 in non-aqueous electrolyte.…”