Understanding cation-disordered rocksalt oxyfluoride cathodes

Partial fluorine (F) substitution into the oxygen (O) sublattice has been shown to improve cycling stability of cation-disordered Li-excess rocksalt oxide (DRX) cathodes. Detailed understanding on failure mechanisms and key optimization knowledge of fluorinated-DRX (F-DRX), however, are lacking. In the present study, we incorporate different amounts of F into a baseline DRX system, Li1.2Ti0.4Mn0.4O2 (LTMO3.0, 3.0 denotes nominal Li/Mn ratio), and synthesize two oxyfluoride compounds, Li-rich Li1.3Ti0.3Mn0.4O1.7F0.3 (LTMOF3.25) and Mn-rich Li1.2Ti0.2Mn0.6O1.8F0.2 (LTMOF2.0) with an increased and reduced Li/Mn ratio of 3.25 and 2.0, respectively. Through careful monitoring of chemical and structural evolution, we show that cycling-induced changes are manifested not only by Mn reduction and degradation of its local coordination environment, but also by F enrichment and formation of LiF-type of domains on the surface. A concerted-densification based failure mechanism, involving atomic-level changes in both transition-metal cationic sublattice and oxygen/fluorine anionic sublattice, is proposed for the degradation in F-DRX cathode materials. The study reveals that increasing F content accompanied by reduced Li/Mn ratio mitigates the degradation process, offering key design strategies in achieving balanced cathode capacity and stability.

Automated summary

This study investigates the role of fluorine in improving cycling stability of lithium-ion battery cathodes. Increasing fluorine content and reducing the Li/Mn ratio can mitigate degradation processes, offering key design strategies for achieving balanced cathode capacity and stability.