Gassing Behavior of High‐Entropy Oxide Anode and Oxyfluoride Cathode Probed Using Differential Electrochemical Mass Spectrometry

Multicomponent materials may exhibit favorable Li-storage properties because of entropy stabilization. While the first examples of high-entropy oxides and oxyfluorides show good cycling performance, they suffer from various problems. Here, we report on side reactions leading to gas evolution in Li-ion cells using rock-salt (Co₀.₂Cu₀.₂Mg₀.₂Ni₀.₂Zn₀.₂)O (HEO) or Li(Co₀.₂Cu₀.₂Mg₀.₂Ni₀.₂Zn₀.₂)OF (Li(HEO)F). Differential electrochemical mass spectrometry indicates that a robust solidelectrolyte interphase layer is formed on the HEO anode, even when using an additive-free electrolyte. For the Li(HEO)F cathode, the cumulative amount of gases is found by pressure measurements to depend strongly on the upper cutoff potential used during cycling. Cells charged to 5.0 V versus Li⁺/ Li show the evolution of O₂, H₂, CO₂, CO and POF₃, with the latter species being indirectly due to lattice O₂ release as confirmed by electron energy loss spectroscopy. This result attests to the negative effect that lattice instability at high potentials has on the gassing.