Thermal aging of electrolytes used in lithium-ion batteries – An investigation of the impact of protic impurities and different housing materials

• Thermal aging of 1 M LiPF6 in EC:DEC (1:2, v/v) at 60 °C and ambient temperature in two different packaging materials.
• Hardly any decomposition products are detected if aging occurred in pouch bags sealed under argon atmosphere.
• The influence of water and water-containing LFP on electrolyte aging is shown.
• Quantification of decomposition product OPF2(OH) by 19F NMR spectroscopy.

Thermal degradation products in lithium-ion batteries result mainly from hydrolysis sensitivity of lithium hexafluorophosphate (LiPF6). As organic carbonate solvents contain traces of protic impurities, the thermal decomposition of electrolytes is enhanced. Therefore, resulting degradation products are studied with nuclear magnetic resonance spectroscopy (NMR) and gas chromatography mass spectrometry (GC–MS). The electrolyte contains 1 M LiPF6 in a binary mixture of ethylene carbonate (EC) and diethylene carbonate (DEC) in a ratio of 1:2 (v/v) and is aged at ambient and elevated temperature. The impact of protic impurities, either added as deionized water or incorporated in positive electrode material, upon aging is investigated. Further, the influence of different housing materials on the electrolyte degradation is shown. Difluorophosphoric acid is identified as main decomposition product by NMR-spectroscopy. Traces of other decomposition products are determined by headspace GC–MS. Acid–base and coulometric titration are used to determine the total amount of acid and water content upon aging, respectively. The aim of this investigation is to achieve profound understanding about the thermal decomposition of one most common used electrolyte in a battery-like housing material.