Decomposition of LiPF6 in High Energy Lithium-Ion Batteries Studied with Online Electrochemical Mass Spectrometry

The chemical and electrochemical instabilities of LiPF6 in carbonate electrolytes for Li-ion batteries were studied with online electrochemical mass spectrometry (OEMS). Decomposition of carbonate electrolytes based on LiPF6 eventually results in the formation of POF3, which is readily detected and followed in situ during operation of Li-rich HE-NCM-based Li-ion cells. Electrode potentials above similar to 4.2 V leads to carbonate solvent oxidation and presumably the formation of ROH species, which subsequently hydrolyze the LiPF6 salt and initiate a thermally activated autocatalytic electrolyte decomposition cycle involving POF3 as a reactive intermediate. Activation of the Li2MnO3 domains of the Li-rich cathode contributes along with electrolyte and separator impurities to further POF3 generation. Electrode potentials below similar to 2.5 V vs. Li+/Li impede POF3 formation and presumably also further electrolyte decomposition by scavenging reactive intermediate species. As a result, much less POF3 gas was detected upon the 2nd charge when using Li metal counter electrode, contrary to delithiated LiFePO4. In situ OEMS confirm that the parasitic reactions involving LiPF6 constitute an intricate reaction scheme, but more importantly, provide further evidence about what the components of this scheme are and how these may interact with each other. (C) The Author(s) 2016. Published by ECS. All rights reserved.