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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 1.2. Anion redox layered transition-metal oxides Eunjeong Kim1,3, Begoña Silv ́an2,3, Nuria Tapia-Ruiz2,3 and A Robert Armstrong1,3 1 EastChem School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom 2 Chemistry Department, Lancaster University, Lancaster LA1 4YB, United Kingdom 3 The Faraday Institution, Quad One, Harwell Science and Innovation Campus, OX11 0RA, United Kingdom Status Oxygen redox-based reactions have gained interest among NIB researchers, due to the prospect of obtaining extra capacity at high voltages beyond those available from conventional transition-metal redox reactions. The study of anionic redox reactions in sodium layered cathode materials is especially relevant, due to their intrinsically lower energy density when compared to their Li counterparts. As with oxygen reactions observed in Li-rich cathode materials for LIBs, these are typically kinetically limited, and thus, research efforts are devoted to improving their reversibility [16]. To date, most studies in this area have focussed on P2-type materials (where Na occupies trigonal prismatic sites), accounting for over 60% of the reported compounds. Anionic redox reactions can be described in terms of band structure, where the overlap of bonding (M–O) and antibonding (M–O)∗ orbitals leads to a continuum band which enables the removal of electrons from oxygen ions (figure 3(b)). In contrast, separated M d and O p bands may be observed in pure cationic redox-driven compounds (figure 3(a)). From a proof-of-concept perspective, the formation of strong covalent bonds and consequently favourable M–O orbital overlap was first demonstrated with 4d (Ru) and 5d metals (Ir). However, for practical reasons, oxygen redox research has naturally shifted toward the use of low-cost and non-toxic 3d metals, such as manganese-rich compounds. When up to one third of the manganese is replaced by another more electronegative and active element (e.g., Ni2+, Fe3+, Co3+), the strong overlap with the O 2p states favours electron transfer, often via a reductive coupling mechanism, while the substituted inactive elements (e.g., Li+, Zn2+, Mg2+) allow O 2p states to be placed at the top of the valence band [17]. Furthermore, the more covalent Mn–O bonds tend to stabilise the crystal structure, thereby avoiding unwanted phase transitions while reducing the inherent structural instability caused by Jahn–Teller active Mn3+ ions and parasitic disproportionation reactions. A challenge encountered in these materials is their large hysteresis, typically explained by the irreversible migration of certain metals (including Mn) towards the Na layers or within the MO2 planes [18]. This has been partially alleviated by designing materials with undercoordinated oxygen ions (i.e., with either ‘direct’ or ‘indirect’ vacancies – mobile Li dopants that leave the transition-metal layers upon charging), where the presence of non-bonding oxygen states translates into a new non-bonding O band (ONB) above the (M–O) band, where extra electrons can be located (figure 3(c)). These vacancies need to be well dispersed and thus isolated from a nearby vacancy to avoid the formation and release of O2 gas, which is favoured by the under-bonded O ions [19]. Current and future challenges While oxygen redox represents an effective way of enhancing energy density in NIBs, limited reversibility on cycling needs to be overcome for most oxygen redox-active compounds. In general, the structural transformation from P-type to O-type structures is induced by gliding of the transition-metal layers upon excess deintercalation of Na ions [20]. The lattice stress caused by this phase transition as well as the partial irreversibility of the structural transition are often responsible for a reduction in oxygen redox activity. In addition, cation migration, either from transition-metal layers to Na layers, or within transition-metal layers, may lead to large voltage hysteresis. The local coordination around oxygen anions has a direct effect on the voltage at which the discharge plateau occurs [19]. Moreover, increasing the upper cutoff voltage beyond ∼4.1 V accelerates the removal of cations from the transition-metal layers, which may result in lattice oxygen loss due to uncoordinated oxygen atoms [21]. Finally, since oxygen redox occurs in the high-voltage region where classical organic electrolytes are unstable, detrimental reactions associated with the decomposition of electrolytes are also a significant problem. In order to gain deep insights into oxygen redox behaviour in cathode materials, spectroscopic techniques have been heavily used (figure 4). It is important to select appropriate techniques to distinguish bulk vs. surface phenomena, since cathode-electrolyte reactions tend to dominate the surface chemistry. Laboratory X-ray photoelectron spectroscopy (XPS) has been widely applied to probe chemical changes in oxygen anions, although the technique is limited to surface information. Furthermore, synchrotron X-ray techniques, such as hard and soft X-ray absorption spectroscopy (XAS), hard X-ray photoelectron spectroscopy (HAXPES), and resonant inelastic x-ray scattering (RIXS) have been exploited for bulk characterisation. In addition to the depth of analysis, the different techniques provide a range of information. In this regard, soft XAS and RIXS are promising tools for demonstrating oxygen redox, whilst hard XAS provides indirect evidence of the participation of oxygen anions in the charge-compensation mechanism. Additional laboratory-based characterisation techniques, such as differential electrochemical mass 9PDF Image | 2021 roadmap for sodium-ion batteries
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