The role of O2 in O-redox cathodes for Li-ion batteries

Abstract

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidising O 2triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau replacing it with a lower discharge voltage, as well as a loss of O2 accompanied by densification at the surface. Here we consider various models for O-redox proposed in the literature before describing a single unified model involving O 2oxidation to form O2, which is trapped in the bulk with the balance evolving from the surface. The model extends the O2 formation and evolution at the surface, which is well-known and well-characterised, into the electrode particle bulk as caged O2 that can be reversibly reduced and oxidised. This converged understanding allows us to propose practical strategies for avoiding O-redox-induced instability offering potential routes towards more reversible high energy density Li-ion cathodes.Since the discovery of 'anomalous' extra capacity to store charge in 3d transition metal oxide Li-rich cathode materials in the early 2000s, 1-4 there has been intense research interest seeking to understand the origin of the effect. [5][6][7][8][9] Over the years, these compounds have grown in number extending to include materials based on 4d and 5d transition metal oxides. [10][11][12][13] In the case of a conventional Li transition metal oxide intercalation cathode, Li + ions are extracted on charging, with charge-compensation by oxidation of the transition metal ion, the process is reversed on discharge, e.g. Li1-xMn2O4 (0 < x < 1). In contrast, the Li-rich cathodes, such as Li1.2Ni0.2Mn0.6O2, Li1.3Nb0.3Mn0.4O3, Li2RuO3 and Li2Ir0.5Sn0.5O3, extend the capacity to store charge by oxidation of the O 2ions. 10,[13][14][15][16] The ability of anions to undergo redox reactions is not without precedent, for example the S 2-/S2 2reaction in sulphides is well known but the phenomenon was not recognised in oxides until more recently. 17 The oxidation of O 2in cathode materials is typically accompanied by a high voltage plateau (usually ~4.5 V vs Li + /Li for 3d cathodes) on charge followed by an S-shaped discharge profile, Fig. 1. Early models posited that the charging plateau was associated completely with the irreversible loss of oxygen from the lattice (O-loss), which alongside extraction of Li + gives rise to the net loss of Li2O. 2,4 Online mass spectrometry showed that O2 gas was released from the surface of the material. 18 Later work showed that there was an insufficient degree of reduction observed of the transition metal ions on re-lithiation to explain the large discharge capacity 19 and quantitative studies also revealed an insufficient amount of evolved O2 to account for the charging capacity associated with the plateau. 14,20 Consequently, the idea was developed that reversible oxidation and reduction of O 2ions in the bulk compensate for the extraction and reinsertion of Li + beyond the limit of TM redox. 5,...

Lattice Oxygen Instability in Oxide‐Based Intercalation Cathodes: A Case Study of Layered LiNi1/3Co1/3Mn1/3O2

“…Additionally, previous works suggested surface densification due to oxygen loss during charge and discharge. [ 45–47 ] Oxygen loss during electrochemical cycling is non‐equilibrium phenomena and preferentially proceeds at the cathode surface. Due to concentrated oxygen vacancy and related structural changes near the surface, cathode surface can be densified selectively after the charge/discharge cycle.…”

Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

“…As discussed previously, [22] a possible strategy to suppress the voltage hysteresis in O-redox cathodes is to prevent O 2 formation by inhibiting TM migration; i.e., kinetically trapping the charged structures in a metastable state with bulk localised, 'split-polaron' or delocalised Oholes. [16,58,66] We suggest that this strategy is unlikely to be effective in Li 2 MnO 2 F because of relatively easy Mn migration.…”

“…[41] An alternative approach to the design of Li-rich disordered rocksalt cathodes will be to allow O 2 formation and then aim to achieve O-O dimerisation with reversible TM migration while preventing irreversible TM migration. [22] By allowing O 2 formation, the cathode is rendered much more stable on charge, since unstable lattice Ospecies are not trapped in the structure. To suppress irreversible TM migration, the displacement of Mn into interstitial sites is important because the Mn can return to their original sites on discharge.…”

“…[13][14][15][16][17][18][19][20][21] The atomic-scale mechanisms of O-redox that contribute to the hysteresis, particularly the nature of the oxidised O species formed on charge and the role of atomic-scale rearrangements, are not well understood and remain topics of considerable debate. [22][23][24][25][26] For Li-rich cathodes, such as the disordered rocksalt materials, to be harnessed for technological use, the mechanisms of O-redox that contribute to the hysteresis must be identified so that strategies to mitigate the loss of energy density can be found. [27][28][29][30][31][32][33][34][35] The disordered rocksalt cathode Li 2 MnO 2 F exhibits a large capacity comparable to that of Li-rich ordered layered oxides.…”

Tuning Bulk O2 and Nonbonding Oxygen State for Reversible Anionic Redox Chemistry in P2‐Layered Cathodes

“…Bulk O 2 is stably trapped in the vacancy cluster in the deep bulk of the material, while surface O 2 is easily lost in charge and discharge processes. It is well known that surface modification can inhibit the production of molecular O 2 on the material surface and further reduce gaseous O 2 release and oxygen loss, which improves the stability of the oxygen redox process [12a] . However, the molecular O 2 mechanism has not been clearly discussed in various sodium‐ion layered cathode materials, and oxygen release inhibition by the mechanism is not fully understood.…”