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

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 Li_1.3 Nb_0.3 Mn_0.4 O₂ (LNMO), Li_1.25 Nb_0.15 Ti_0.2 Mn_0.4 O₂ (LNTMO), and Li_1.2 Ti_0.4 Mn_0.4 O₂ (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 (O^{n -} , 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.