Unveiling the Thermal Stability of Sodium Ion Pouch Cells Using Accelerating Rate Calorimetry

Abstract

The thermal stability of ∼420 mAh Na0.97Ca0.03[Mn0.39Fe0.31Ni0.22Zn0.08]O2 (NCMFNZO)/hard carbon (HC) pouch cells was investigated using accelerating rate calorimetry (ARC) at elevated temperatures. 1 m NaPF6 in propylene carbonate (PC):ethyl methyl carbonate (EMC) (1:1 by volume) was used as a control electrolyte. Adding 2 wt% fluoroethylene carbonate to the electrolyte improves the cell’s thermal stability by decreasing the self-heating rate (SHR) across the whole testing temperature range. The selected states-of-charge (SoC), including 70%, 84%, and 100%, exhibit minimal impact on the exothermic behavior, except for a slight decrease in SHR after ∼275 °C at 70% SoC. When compared to traditional lithium-ion batteries operating at 100% SoC, NCMFNZO/HC pouch cells demonstrate inferior thermal stability compared to LiFePO4 (LFP)/graphite pouch cells, displaying a higher SHR from 220 to 300 °C. LiNi0.8Mn0.1Co0.1O2/graphite + SiOx pouch cells exhibit the worst safety performance, with an early onset temperature of ∼100 °C and the highest SHR across the entire temperature range. These results offer a direct comparison of the impact of SoC and electrolyte compositions on the thermal stability of SIBs at elevated temperatures, highlighting that there is still room for improvement in SIBs safety performance compared to LFP/graphite chemistry.