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The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li-O(2p)-Li interactions). Na[MgMn]O exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na[MgMn]O does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg remains in Na[MgMn]O, which stabilizes oxygen.

A new high performance cathode material for Li-ion batteries with a disordered rocksalt structure powered by manganese and oxygen redox.

Magnesium substituted P2-structure Na 0.67 Ni 0.3 Mn 0.7 O 2 materials have been prepared by a facile solid-state method and investigated as cathodes in sodium-ion batteries. The Mg-doped materials described here were characterised by X-ray diffraction (XRD), 23 Na solid-state nuclear magnetic resonance (SS-NMR) and scanning electron microscopy (SEM). The electrochemical performance of the samples was tested in half cells vs. Na metal at room temperature. The Mg-doped materials operate at a high average voltage of ca. 3.3 V vs. Na/Na + delivering specific capacities of ~ 120 mAh g -1 which remain stable up to 50 cycles. Mg doping suppresses the well-known P2-O2 phase transition observed in the undoped composition by stabilising the reversible OP4 phase during charging (during Na removal). GITT measurements showed that the Na-ion mobility is improved by two orders of magnitude with respect to the parent P2-Na 0.67 Ni 0.3 Mn 0.7 O 2 material. The fast Na-ion mobility may be the cause of the enhanced rate performance. INTRODUCTIONOver the last two decades, most of the portable electronic market has been dominated by lithium-ion batteries (LIBs). These batteries are now finding new market opportunities in the electric vehicle industry along with stationary energy storage [1]. Even though the high energy density of lithium-ion batteries makes them attractive for many applications, there is a demand for inexpensive technology for which the sources of the ores are more uniformly distributed across the globe. Sodium-ion batteries (SIBs) are a promising alternative, addressing the aforementioned issues related to the cost and sources. Following their inception in the 1980s these batteries are under reinvestigation as an alternative to LIBs for certain applications [2][3][4][5][6]. Various types of cathode and anode materials have been proposed and studied for SIBs, often mimicking their LIB counterparts. Among cathode materials, layered oxide compounds of the type Na-TM-O 2 (TM = 3d Transition Metal) have shown promise in terms of energy density and rate capability. The most studied layered compounds can be classified as P2-type and O3-type structures as described by Delmas' notation [7]. P and O refer to the Na coordination, i.e. trigonal prismatic or octahedral, respectively, while the number represents the repeated transition metal oxide stacking within the unit cell (ABBA, ABCABC). Of these two layered structures, P2-type compounds provide great promise as they undergo fewer structural transitions when (de)intercalating Na-ions [2].P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 was first reported in 2001 by Lu and Dahn [8,9]. This material has a relatively high theoretical capacity (173 mAh g -1 ) but more importantly, it shows an average operating voltage greater than 3.5 V (vs. Na/Na + ), attributed to the Ni 2+ /Ni 4+ redox couple [10,11]. The main drawback, however, is its poor cycle life which has been attributed to the detrimental P2-O2 transformation that occurs at high voltages. This transition is caused by gliding of the trans...

It is possible to increase the charge capacity of transition-metal (TM) oxide cathodes in alkali-ion batteries by invoking redox reactions on the oxygen. However, oxygen loss often occurs. To explore what affects oxygen loss in oxygen redox materials, we have compared two analogous Na-ion cathodes, P2-Na0.67Mg0.28Mn0.72O2 and P2-Na0.78Li0.25Mn0.75O2. On charging to 4.5 V, >0.4e – are removed from the oxide ions of these materials, but neither compound exhibits oxygen loss. Li is retained in P2-Na0.78Li0.25Mn0.75O2 but displaced from the TM to the alkali metal layers, showing that vacancies in the TM layers, which also occur in other oxygen redox compounds that exhibit oxygen loss such as Li­[Li0.2Ni0.2Mn0.6]­O2, are not a trigger for oxygen loss. On charging at 5 V, P2-Na0.78Li0.25Mn0.75O2 exhibits oxygen loss, whereas P2-Na0.67Mg0.28Mn0.72O2 does not. Under these conditions, both Na+ and Li+ are removed from P2-Na0.78Li0.25Mn0.75O2, resulting in underbonded oxygen (fewer than 3 cations coordinating oxygen) and surface-localized O loss. In contrast, for P2-Na0.67Mg0.28Mn0.72O2, oxygen remains coordinated by at least 2 Mn4+ and 1 Mg2+ ions, stabilizing the oxygen and avoiding oxygen loss.