Role of Redox‐Inactive Transition‐Metals in the Behavior of Cation‐Disordered Rocksalt Cathodes

Abstract

Owing to the capacity boost from oxygen redox activities, Li‐rich cation‐disordered rocksalts (LRCDRS) represent a new class of promising high‐energy Li‐ion battery cathode materials. Redox‐inactive transition‐metal (TM) cations, typically d0 TM, are essential in the formation of rocksalt phases, however, their role in electrochemical performance and cathode stability is largely unknown. In the present study, the effect of two d0 TM (Nb5+ and Ti4+) is systematically compared on the redox chemistry of Mn‐based model LRCDRS cathodes, namely Li1.3Nb0.3Mn0.4O2 (LNMO), Li1.25Nb0.15Ti0.2Mn0.4O2 (LNTMO), and Li1.2Ti0.4Mn0.4O2 (LTMO). Although electrochemically inactive, d0 TM serves as a modulator for oxygen redox, with Nb5+ significantly enhancing initial charge storage contribution from oxygen redox. Further studies using differential electrochemical mass spectroscopy and resonant inelastic X‐ray scattering reveal that Ti4+ is better in stabilizing the oxidized oxygen anions (On−, 0 < n < 2), leading to a more reversible O redox process with less oxygen gas release. As a result, much improved chemical, structural and cycling stabilities are achieved on LTMO. Detailed evaluation on the effect of d0 TM on degradation mechanism further suggests that proper design of redox‐inactive TM cations provides an important avenue to balanced capacity and stability in this newer class of cathode materials.

A Fluorination Method for Improving Cation‐Disordered Rocksalt Cathode Performance

“…[ 5,26 ] Using a combination of electrochemical characterization techniques, synchrotron X‐ray diffraction, soft and hard X‐ray absorption, resonant inelastic X‐ray scattering and differential electrochemical mass spectroscopy, previous studies have primarily attributed region A and B to the cationic Mn redox and anionic O redox, respectively, with the latter involving the oxidation of O 2− to O n − (0 < n < 2) and O 2 gas evolution. [ 15,26,30,41,42 ] The two regions are also indexed as A’ and B’ in the d Q/ d V plot (Figure 3b). Careful comparison shows that O redox activities in the B’ region tends to decrease with increasing F content in DRX, with the changes between F5 and F10 particularly significant.…”

Li-rich cathodes for rechargeable Li-based batteries: reaction mechanisms and advanced characterization techniques

“…However, except for the densified layers (Figure 13b), the structure evolutions of DLROs are difficult to be detected due to their complex structural characteristics, which brings difficulties to mechanistic studies of voltage fade in DLROs. Kan et al 210 211 They found no obvious increase of Mn 2+ in discharged electrodes cycled five times compared to pristine when cycling within 1.5-4.2 V. However, when the upper cut-off voltage increased to 4.8 V, the amount of Mn 2+ increased with cycling, which suggest that the Mn reduction during cycling is induced owing to the anionic reactions above 4.2 V. Although the reduction of TM ions with cycling in anionic redox active DLROs has been profoundly observed 211,355 and should obviously be related to the voltage fade phenomena in DLROs. However, the intrinsic causes of the TM reduction and their detailed connection with electrochemical performances are rarely studied.…”

Increasing Capacity in Disordered Rocksalt Cathodes by Mg Doping

“…Figure 3a shows the frequency with which several types of cation environments around F and Nb 5+ are both electrochemical inactive, they can affect atomic ordering and thus the percolating Li-content of DRX cathodes. 37,38 To rule out that the capacity improvement…”