Decomposition of LiPF₆in High Energy Lithium-Ion Batteries Studied with Online Electrochemical Mass Spectrometry

The chemical and electrochemical instabilities of LiPF 6 in carbonate electrolytes for Li-ion batteries were studied with online electrochemical mass spectrometry (OEMS). Decomposition of carbonate electrolytes based on LiPF 6 eventually results in the formation of POF 3 , which is readily detected and followed in situ during operation of Li-rich HE-NCM-based Li-ion cells. Electrode potentials above ∼4.2 V leads to carbonate solvent oxidation and presumably the formation of ROH species, which subsequently hydrolyze the LiPF 6 salt and initiate a thermally activated autocatalytic electrolyte decomposition cycle involving POF 3 as a reactive intermediate. Activation of the Li 2 MnO 3 domains of the Li-rich cathode contributes along with electrolyte and separator impurities to further POF 3 generation. Electrode potentials below ∼2.5 V vs. Li + /Li impede POF 3 formation and presumably also further electrolyte decomposition by scavenging reactive intermediate species. As a result, much less POF 3 gas was detected upon the 2 nd charge when using Li metal counter electrode, contrary to delithiated LiFePO 4 . In situ OEMS confirm that the parasitic reactions involving LiPF 6 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.