Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium-rich cathodes

Abstract

Reversible anionic redox has rejuvenated the search for high-capacity lithium-ion battery cathodes. Real-world success necessitates the holistic mastering of this electrochemistry’s kinetics, thermodynamics, and stability. Here we prove oxygen redox reactivity in the archetypical lithium- and manganese-rich layered cathodes through bulk-sensitive synchrotron-based spectroscopies, and elucidate their complete anionic/cationic charge-compensation mechanism. Furthermore, via various electroanalytical methods, we answer how the anionic/cationic interplay governs application-wise important issues—namely sluggish kinetics, large hysteresis, and voltage fade—that afflict these promising cathodes despite widespread industrial and academic efforts. We find that cationic redox is kinetically fast and without hysteresis unlike sluggish anions, which furthermore show different oxidation vs. reduction potentials. Additionally, more time spent with fully oxidized oxygen promotes voltage fade. These fundamental insights about anionic redox are indispensable for improving lithium-rich cathodes. Moreover, our methodology provides guidelines for assessing the merits of existing and future anionic redox-based high-energy cathodes, which are being discovered rapidly.

Surface Degradation and Chemical Electrolyte Oxidation Induced by the Oxygen Released from Layered Oxide Cathodes in Li−Ion Batteries

“…Conversely, although the manganese is not expected to take part in the oxidation process, it shows an apparent reduction feature at 640.3 eV. As explained by Assat et al, this change can be either due to the redox activity of the neighboring Ni and Co and/or to a local crystal structural change. It is important to emphasize here that the surface (electro‐) chemical behavior of the NCM111 is similar to that of the Li‐rich NCM, showing the disappearance of the reduced TMs (generated at OCP), due to either their electrochemical oxidation (i. e. delithiation) or dissolution in the electrolyte.…”

“…According to XAS in bulk sensitive mode, a feature at 530.8 eV related to the oxygen redox couple O 2− /O n− should increase, as a result of the oxygen oxidation along the second plateau ,,. The apparent absence of changes of the “O‐redox” component suggests that the oxidized oxygen is unstable at the surface and either reacts immediately with the electrolyte or is released, as O 2 gas, leaving reduced TMs on the cathode surface.…”

“…have first suggested that oxidation of oxygen generally results in the pairing of oxygen ions, leading to an effective 2O 2− /O 2 n− redox couple, which is stabilized against oxygen gas evolution due to the presence of 4d and 5d TMs . Recently, they also proved that a nO 2− /O n− couple is reversibly oxidized (on charge) and reduced (on discharge) in the bulk of a 3d TMs‐based material (i. e. Li‐rich NCM), by monitoring the O1s core level evolution using X‐ray photoelectron spectroscopy (XPS) ,. On a similar oxide, Bruce and coworkers, proposed instead a localized hole O 2− /O − mechanism based on O K‐edge absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS).…”

Chemical, Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni‐Rich NCM and Li‐Rich HE‐NCM Cathode Materials in Li‐Ion Batteries

“…However, even if all of the Li 2 MnO 3 is activated, the measured capacity (activated MnO 2 plus NCM) would not be sufficient to account for the "anomalously" large reversible discharge capacities achieved using this material class. A large variety of structural and/or electronic changes are associated with the irreversible changes during the initial activation process, e.g., reversible oxidation of oxygen species [159] and probably condensation to peroxide-like units, [159] transition metal migration into tetrahedral sites [81,160] and formation of a spinel-like surface layer, [161] formation of lithium/transition metal dumbbells, [70,162] Li + /H + exchange, [163] formation of dislocations [164] etc. [67][68][69]118,156] During the initial charge cycle of the pristine material (up to about 4.8 V), there is, at first, an s-shape-like voltage profile that is attributed to the oxidation of Ni and Co from +II and +III, respectively, to +IV.…”

Associating and Tuning Sodium and Oxygen Mixed‐Ion Conduction in Niobium‐Based Perovskites

“…These properties are strongly dependent on the chemical composition, the solid network architecture and, at the atomic scale, the chemical bonding which has direct implications on the electronic band diagram as well as chemical or electrochemical potential for electrons at the Fermi level . Such considerations are especially of great interest to assess anionic redox processes occurring at the cathode in Li‐ion batteries and their interplay with cationic electrochemistry . In all solid‐state batteries, research focus is primarily dedicated to electrolytes.…”