Counteracting thermal degradation of LiPF6-based electrolyte with additives or lithium salts: A gas analysis revealing the impact of NMC

One of the weaknesses of the LiPF6-based electrolyte is its poor thermal stability. It can result in an accelerated battery capacity fading and CO2 gas generation due to the formation of PF5 which are highly reactive towards the electrolyte solvents. As the formation of PF5 is inevitable, efforts are dedicated to inhibiting its deleterious impact by adding a Lewis base to form a complex. However, no study investigates the PF5-complexation efficiency through gas analysis. Here, gas analysis shows that N-containing additives are effective in reducing gas generation upon electrolyte storage at 55 °C. Out of our expectations, the trend is reversed when it comes to thermal storage of NMC-graphite batteries, involving competitive chemisorption processes on the NMC acid and basic sites. It turned out that, NMC surface can be more effective than additives in mitigating the thermal degradation of the electrolyte. Furthermore, the gas level thermally generated does not decrease while replacing the culprit LiPF6 salt by mixtures of LiFSI + LiPF6 or LiFSI + LiODFB. Especially in presence of LiODFB, water triggers hydrolysis reactions that also lead to gas evolution.