High Operating Potential Induces Conversion in Li-Rich Chalcogenides

Lithium-ion batteries are central to the renewable energy transition, yet the capacity of commercial cathode materials limits further improvement in performance. Li-rich cathodes offer a promising route to higher capacities by leveraging both cationic and anionic redox. However, the full theoretical capacity is yet to be realized in the Li-rich materials. Here, we evaluate conditions to attempt full delithiation, which requires higher operating potentials. We focus on the Li-rich sulfide and selenide Li₂FeCh₂ (Ch = S, Se), since high potential required to access anion redox in oxides leads to severe degradation of electrolyte which obscures intrinsic cathode processes. Both materials exhibit poor Coulombic efficiency and rapid capacity loss during galvanostatic measurements that reach >3 V vs Li/Li⁺. The materials undergo conversion reactions at these potentials, which is corroborated by ex situ laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and X-ray diffraction (XRD) measurements. For Li₂FeS₂, conversion to polysulfides causes material loss. However, if the upper voltage cutoff is below 3 V vs Li/Li⁺, there is no evidence of dissolved S even though persulfides are formed in the solid Li_2-xFeS₂. In contrast, conversion of Li₂FeSe₂ to γ-Se is limited to the solid-state, though formation of protons via electrolyte decomposition causes active-material dissolution. The conversion reactions occur >1 V positive of the reversible anion redox reactions in both materials which makes it easy to avoid such parasitic side reactions by potential limited cycling.