Depth-dependent oxygen redox activity in lithium-rich layered oxide cathodes

Abstract

Lithium-rich materials, such as Li 1.2 Ni 0.2 Mn 0.6 O 2 , exhibit capacities not limited by transition metal redox, through the reversible oxidation of oxide anions. Here we offer detailed insight into the degree of oxygen redox as a function of depth within the material as it is charged and cycled. Energy-tuned photoelectron spectroscopy is used as a powerful, yet highly sensitive technique to probe electronic states of oxygen and transition metals from the top few nanometers at the near-surface through to the bulk of the particles. Two discrete oxygen species are identified, O nÀ and O 2À , where n < 2, confirming our previous model that oxidation generates localised hole states on O upon charging. This is in contrast to the oxygen redox inactive high voltage spinel LiNi 0.5 Mn 1.5 O 4 , for which no O nÀ species is detected. The depth profile results demonstrate a concentration gradient exists for O nÀ from the surface through to the bulk, indicating a preferential surface oxidation of the layered oxide particles. This is highly consistent with the already well-established core-shell model for such materials. Ab initio calculations reaffirm the electronic structure differences observed experimentally between the surface and bulk, while modelling of delithiated structures shows good agreement between experimental and calculated binding energies for O nÀ .

Inhibiting Oxygen Release from Li‐rich, Mn‐rich Layered Oxides at the Surface with a Solution Processable Oxygen Scavenger Polymer

“…[ 35 ] The increased peak intensity of the surface species confirms that the carbonate electrolyte is decomposed by reaction with evolved ROS during the initial charge, [ 4b,8 ] resulting in accumulation of oxygenated products on the electrode surface which form the CEI. The new component at 530.5 eV on charge to 4.8 V is assigned to the oxidation of surface oxide ions O 2− to O n − ( n < 2), [ 36 ] and/or localized oxygen hole states, based on previous studies. [ 5 ] On discharge, this feature essentially disappears as expected, whereas the oxy‐organic species continue to grow.…”

Recent Advances in Electrode Materials with Anion Redox Chemistry for Sodium-Ion Batteries

“…This phenomenon has been observed in overstoichiometric lithium compounds, where lithium partially replaces transition metals (TMs) with typical feature of Li[Li x TM 1-x ]O 2 (TM: Ni, Co, Fe, Cu, etc.) [12][13][14] or Li 1 +x TM 1-x O 2 (TM: Ru and Ir) [15,16]. These materials provide a higher capacity than the theoretical value obtained from a redox pair TM.…”

“…The anionic redox process enables delivery of additional capacity, such that sodium ions can be additionally de/intercalated from/into the structure; namely, the combination of cationic and anionic redox reactions provides more capacity. This type of chemistry has been demonstrated in Li-rich manganese oxide systems (Li 2 MnO 3 [7][8][9], Li 1.2 TM 0.8 O 2 [10][11][12], and their derivatives [13][14][15][16]) that have provided capacities to their theoretical limit. These anionic redox can contribute to additional capacity, thereby increasing the specific energy density of the battery.…”

“…Further, the formation of (S 2 ) ndimers was occurred, and it was triggered by noncoordinated S 3p states (Equation ( 2)). The weakening of Cr-S electrostatic repulsion caused Cr migration from TM layer to Na layer, resulting in 12 Energy Material Advances P3-O1' phase transformation. The direct observation of Cr migration was obtained using STEM.…”

Stabilization of Li-Rich Disordered Rocksalt Oxyfluoride Cathodes by Particle Surface Modification

“… 1 , 2 Many of such materials have been found to exhibit anionic redox reactions, in addition to the transition metal redox chemistry, to compensate for the excess lithium extracted. 3 However, Li-rich layered materials, such as Li[Li 0.2 Ni 0.13 Co 0.13 Mn 0.54 ]O 2 , are often subject to oxygen loss, phase transformations, densification, and metal dissolution during cycling, leading to poor practical performance. 4 , 5 …”