Role of Fluorine in Chemomechanics of Cation-Disordered Rocksalt Cathodes

Abstract

The role of nanoscale chemomechanical behavior in macroscopic performance of functional materials is well recognized. For lithium-ion battery cathodes, tremendous effort has been devoted to the development of new chemistry and structure, yet fundamental understanding in the correlation between redox processes and mechanical properties of the novel materials lags 1 behind. In the present study, we prepare large discrete single-grains of Li-excess cationdisordered rocksalts (DRX) and investigate their chemomechanical behavior at the particle-level, using nano-resolution X-ray and electron-based spectro-imaging and chemical mapping techniques. While irregular cracking upon lithium extraction leads to the eventual breakdown of the baseline DRX oxide (Li 1.2 Ti 0.4 Mn 0.4 O 2 ) particles at high delithiation state, the fluorinated-DRX (Li 1.3 Ti 0.3 Mn 0.4 O 1.7 F 0.3 ) clearly displays aligned cracking along the <001> direction. The resulting periodicity in the cracking pattern enables the particles to retain their integrity and consequently, improved electrochemical stability. Density functional theory (DFT) calculations showed that fluorination leads to increased concentration of Li + on the (001) planes and preferential Li movements along the <001>-family directions, revealing the underlying mechanism for directional cracking. Our study demonstrates the unique role of fluorine in modulating nanoscale chemomechanics, which in turn influences the evolution of charge and strain heterogeneity at the particle level. These insights provide important design guidelines in further improving DRX cathode materials.

Equilibrium Particle Shape and Surface Chemistry of Disordered Li-Excess, Mn-Rich Li-ion Cathodes through First-Principles Modeling

“…Experimental work in the literature report DRX materials exposing {100} and {110} type facets, in good agreement with our computational results. Kwon 49 To elucidate the effect of surface chemistry on surface energy we employ a bootstrapping statistical method, which uses results found on a collection of smaller random samples to infer properties of the entire population, using replacement during the sampling process. For clarity, when referring to the surface, only the outermost layer of atoms is considered.…”

“…show high-resolution scanning transmission electron microscope images observing a {110} type facet of the DRX material Li 1.2 Ni 0.333 Ti 0.333 Mo 0.133 O 2 . In the DRX material Li 1.3 Ti 0.3 Mn 0.4 O 1.7 F 0.3 Chen et al observed directional cracking along the ⟨100⟩ family of directions exposing {100} type planes …”

Towards commercialization of fluorinated cation-disordered rock-salt Li-ion cathodes

“…Interestingly, fluorination seems to affect the cracking behavior of the DRXs. Chen et al reported that the cracking of DRXs is non-directional, fluorination leads to the aligned cracking by increasing the concentration of Li on (001) plane (Chen et al, 2021b). However, it is difficult to rule out the possibility that the improved electrochemical performance is due to other effects of fluorination.…”

Activity of Metal-Fluorine States upon Delithiation of Disordered Rocksalt Oxyfluorides

“…2 Further permutations were enabled by the substitution of O by F, a strategy with an established record in the quest for new cathode materials. 3 In disordered rocksalts, this substitution is accommodated by a lower oxidation state of the metals (M) in the initial lithiated state than in the equivalent oxide, which, coupled to the Li excess, enables the reversible utilization of M(II)/M(IV) redox couples, as in Li 2 Mn 2/3 Nb 1/3 O 2 F. 3 Further, the presence of F has been associated with improvements in long-term cycling stability, 4 linked to a greater chemical, 5 mechanical, 6 and interfacial 7 stability of the charged (oxidized) state. However, its electronic underpinnings have not been evaluated in depth.…”