Fluorination‐Enhanced Surface Stability of Cation‐Disordered Rocksalt Cathodes for Li‐Ion Batteries

Abstract

Cation-disordered rocksalt (DRX) materials have emerged as a class of novel high-capacity cathodes for Li-ion batteries. However, the commercialization of DRX cathodes will require reducing their capacity decay, which has been associated with oxygen loss during cycling. Recent studies show that fluorination of DRX cathodes can effectively reduce oxygen loss and improve cycling stability; however, the underlying atomic-scale mechanisms remain elusive. Herein, using a combination of electrochemical measurements, scanning transmission electron microscopy, and electron energy loss spectroscopy, the correlation between the electrochemical properties and structural evolution in Mn-redox-based DRX cathodes, Li 1.2 Ti 0.4-x Mn 0.4+x O 2.0-x F x (x = 0 and 0.2) is examined. It is found that fluorination strongly suppresses structural amorphization and void formation initiated from the particle surface, therefore greatly enhancing the cyclability of the cathode. A novel rocksalt-to-spinel-like structural transformation in the DRX bulk is further revealed, which surprisingly contributes to a gradual capacity increase during cycling. The results provide important insight for the design of novel DRX cathodes with high capacity and long cycle life.

Fluorination‐Enhanced Surface Stability of Disordered Rocksalt Cathodes

“…In comparison, for the cycled LMNOF 0 , a high density of blackdot-like nanoregions is observed in the STEM HAADF image (Figure 3b). The corresponding EELS elemental maps show that these black-dot-like nanoregions should be void structures with local material loss, similar to those previously observed in Li-rich layered [21] and disordered [18] rocksalt structured cathodes. In those cathodes, it is believed that the O redox activity during the electrochemical cycling facilitates the O migration from the bulk lattice toward surface, while O vacancies are injected to the surface and penetrated into the bulk.…”

“…In the pristine LMNOF 0 particle (Figure 2c), while the Nb distribution is mostly uniform from the bulk to the surface, there is a thin (3 nm) O-deficient and Mn-rich surface layer. Similarly, O-deficient surface layers in pristine DRX cathodes have also been observed in our previous study of Li-Mn-Ti-O DRX cathodes, [18] indicating that the surface O loss might be a common phenomenon during the preparation of DRX cathode materials, where high calcination temperature and inert atmosphere are involved.…”

“…In those cathodes, it is believed that the O redox activity during the electrochemical cycling facilitates the O migration from the bulk lattice toward surface, while O vacancies are injected to the surface and penetrated into the bulk. [ 18,21 ] It should be also noted that voids are often seen to be formed in layer‐structured NMC cathodes during the cycling, [ 22 ] the origin of which has been recently identified as a consequence of cycling‐induced accumulation of intrinsically existing vacancies dispersed in the bulk of the cathode. [ 23 ] In the present case of the LMNOF 0 cathode, we believe that the mechanism of surface O vacancy injection should operate to boost the void formation due to the high‐level of O redox participation from the initial cycling (Figure 1a,b).…”

“…[12a] Correspondingly, the increased TM redox capacity accommodated upon fluorination reduces the capacity reliability on O redox and thus mitigates irreversible O loss. [2b,5,9,12a,13] Due to the presence of local TM‐poor, Li‐rich environments in the DRX lattice that are highly favorable for F – , [ 13,14 ] substantial levels of fluorine (F) substitution have been achieved in various DRX materials, such as Li 2 VO 2 F, [ 15 ] Li 1.2 Mn 0.7 Nb 0.1 O 1.8 F 0.2 , [ 5 ] Li 1.2 Mn 0.75 Nb 0.05 O 1.7 F 0.3 , [ 16 ] Li 1.2 Mn 0.625 Nb 0.175 O 1.325 F 0.675 , [ 17 ] Li 2 Mn 2/3 Nb 1/3 O 2 F, [2b] Li 1.2 Ti 0.2 Mn 0.6 O 1.8 F 0.2 , [ 18 ] and Li 2 MoO 2 F. [ 19 ] In general, fluorination has been demonstrated as a very effective route for improving the cycling stability in all these fluorinated DRX (F‐DRX) cathodes.…”

“…[12b] Furthermore, using scanning transmission electron microscopy (STEM), we previously discovered that in a Li 1.2 Ti 0.4 Mn 0.4 O 2 DRX cathode, extensive cycling results in a high level of structural degradation including amorphization and void formation at the particle surface, which impedes Li transport and causes fast capacity loss; while in a highly fluorinated Li 1.2 Ti 0.2 Mn 0.6 O 1.8 F 0.2 DRX cathode, the cycling‐induced surface degradation is much less prominent, and the cycling stability is greatly improved. [ 18 ] These previous studies indicate that the chemical and structural stability near the surface of DRX cathode materials plays a critical role in determining the cycling performance. However, a detailed understanding of the cycling‐induced chemical reconstruction and TM oxidation states evolution associated with the structural evolution near the surface of the DRX cathodes remains elusive.…”