Oxygen Vacancy Introduction to Increase the Capacity and Voltage Retention in Li‐Excess Cathode Materials

Abstract

Li‐rich rocksalt oxides are promising cathode materials for lithium‐ion batteries due to their large capacity and energy density, and their ability to use earth‐abundant elements. The excess Li in the rocksalt, needed to achieve good Li transport, reduces the theoretical transition metal redox capacity and introduces a labile oxygen state, both of which lead to increased oxygen oxidation and concomitant capacity loss with cycling. Herein, it is demonstrated that substituting the labile oxygen in Li‐rich cation‐disordered rocksalt materials with a vacancy is an effective strategy to inhibit oxygen oxidation. It is found that the oxygen vacancy in cation‐disordered lithium manganese oxide favors high Li coordination thereby reducing the concentration of unhybridized oxygen states, while increasing the theoretical Mn capacity. It is shown that in the vacancy‐containing compound, synthesized by ball milling, the Mn valence is lowered to less than +3, providing access to more than 300 mAh g−1 capacity from the Mn2+/Mn4+ redox reservoir. The increased transition metal redox and decreased O oxidation are found to improve the capacity and voltage retention, indicating that oxygen vacancy creation to remove the most vulnerable oxygen ions and reduce transition metal valence provides a new opportunity for the design of high‐performance Li‐rich rocksalt cathodes.

Oxygen vacancy chemistry in oxide cathodes

“…This is the deepest microscopic reason for the changed thermodynamic characteristics of solid oxide materials. The most direct effect of intrinsic OVs on the electronic structure is the breakage of charge balance in the system, which leads to reduced oxidation states for TMs such as Mn, Ni, and Co. 32,43,95 Cui et al 25 observed that intrinsic OVs produced during sintering resulted in a lower Mn ion valence (Mn 3.93+ vs. Mn 3.98+ ) in the bulk of OV-enriched Li-rich oxide Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2Àx synthesized through constant hightemperature sintering compared to non-isothermal sintering. After soaking the Li-rich material Li 1.2 Mn 0.6 Ni 0.2 O 2 with hydrazine hydrate solutions, Liu and co-workers detected a small amount of Mn 3+ produced on the surface of the treated oxygendeficient sample Li 1.2 Mn 0.6 Ni 0.2 O 2Àd based on Mn L-edge soft X-ray absorption spectra (sXAS) in the total electron yield (TEY) mode.…”

“…Band structure: reproduced with permission. 95 Copyright 2022, John Wiley & Sons, Inc. Phase structure: reproduced with permission.…”

“…This results in a smaller overlap between the Li-O-Li state and the Mn e g * state, increasing the charge compensation contribution of Mn ions while limiting the oxidation of lattice oxygen in charged Li 1.25 Mn 0.75 O 1.625 . 95 Since there are no systematic studies on the effect of intrinsic OVs on the electronic structure in TM oxide materials, there are unverified possible conjectures. For example, OVs with different concentrations may have different effects on the band structure and orbital energy level of TMs.…”

“…However, it severely suppresses the ultra-high-capacity anionic oxygen redox from the Li-O-Li configuration, resulting in the Li 1.25 Mn 0.75 O 1.625 cathode delivering a lower capacity (317 vs. 334 mA h g À1 ) compared to pristine Li 1.25 Mn 0.75 O 2 . 95 In contrast, for TM oxide cathodes that do not involve anion redox, intrinsic OVs have been recently found to enhance the reversible specific capacity of P2-type sodium layered oxide cathodes by triggering Mn redox contribution. 33 Note that intrinsic bulk OV-induced cation mixing (such as Li + /Ni 2+ exchange) tends to cause the limited availability of Li + sites in the Li layer.…”

Impact of rare earth (La, Pr, Eu) impurities on the perovskite SrTiO3 for efficient photocatalytic activity

“…Therefore, doping material is an efficient strategy for the enhancement of the absorbed light in the visible region which is favourable for effective solar cells, photo-electrochemical water splitting, and hydrogen storage [32,33]. However, other research works suggested the creation of oxygen vacancies to improve photocatalytic properties [34][35][36], furthermore, manipulation and control of the Oxygen's interstitial sites could help researchers produce more active O-rich oxide photocatalysts for water oxidation [37]. On the other hand, DFT simulation is designed to predict many aspects of energy harvesting and light up-conversion such as dye-sensitised solar-cells [38][39][40], thermoelectricity [41][42][43], and photocatalysis [36,44], also, it is regarded as a valuable tool with good reproducibility [45].…”