Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode

Abstract

Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na₂MO₃ (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na₂MO₃ are poorly established. Here using two polymorphs of Na₂RuO₃, we demonstrate the critical role of honeycomb-type cation ordering in Na₂MO₃. Ordered Na₂RuO₃ with honeycomb-ordered [Na_1/3Ru_2/3]O₂ slabs delivers a capacity of 180 mAh g⁻¹ (1.3-electron reaction), whereas disordered Na₂RuO₃ only delivers 135 mAh g⁻¹ (1.0-electron reaction). We clarify that the large extra capacity of ordered Na₂RuO₃ is enabled by a spontaneously ordered intermediate Na₁RuO₃ phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na₂MO₃ cathodes.

Introduction

Li-ion batteries power almost all of today’s portable electronics and their ability to store energy is increasingly important for large-scale applications such as electric vehicles and power grids. However, Li resources are limited and unevenly distributed geographically; hence, alternatives need to be found. Na-ion batteries have garnered great attention as candidates for large-scale applications owing to the abundance and low cost of sodium. Accordingly, Na-ion (de)intercalation compounds have been explored intensively^{1,2,3,4,5,6,7,8}.

Among the various positive electrode materials that have been investigated, layered transition metal oxides NaMO₂ (M=transition metal) are considered to be the most promising^{3,9,10,11,12,13}, in part owing to their large theoretical capacities and the simple analogies from the successful application of LiMO₂ in Li-ion battery systems. However, the stably cyclable capacity of NaMO₂ is too small for practical use because deintercalation of a large amount of Na ions from the Na layer between the MO₂ slabs usually causes irreversible structural changes^14,15.

Replacing M atoms in the MO₂ slab with Na (for example, Na[Na_1/3M_2/3]O₂ or Na₂MO₃) is an appealing strategy to suppress these structural changes over a wider compositional range. The [Na_1/3M_2/3]O₂ slabs supply additional Na ions, thereby increasing the reversible capacity while suppressing the over-deintercalation of Na ions from the Na layer. This ‘A_2−xMO₃’ strategy has already been adopted in many lithium systems such as Li₂MnO₃–LiMO₂ and Li₂RuO₃, in which enhanced capacities exceeding a M⁴⁺/M³⁺ one-electron reaction have been achieved through additional oxygen redox contribution^16,17,18.

In our previous work, we applied this strategy to sodium systems, using a model compound, Na₂RuO₃ (Na[Na_1/3Ru_2/3]O₂)¹⁹. However, despite the presence of [Na_1/3Ru_2/3]O₂ slabs, Na₂RuO₃ delivered a modest capacity of 135 mAh g⁻¹, limited to the Ru⁵⁺/Ru⁴⁺ one-electron reaction. In contrast, Rozier et al.²⁰ very recently reported that Na₂RuO₃ can deliver a capacity greatly exceeding that of the Ru⁵⁺/Ru⁴⁺ one-electron reaction. At present, the discrepancy between the reported electrochemical properties is an open question. More generally, the requisite conditions for successful application of the ‘A_2−xMO₃’ strategy for large-capacity sodium systems are poorly understood.

Here we demonstrate that a highly stabilized intermediate phase with honeycomb-type cation ordering in the [Na_1/3M_2/3]O₂ slab is critical for the effective use of the ‘A_2−xMO₃’ strategy, by comparing two polymorphs of Na₂RuO₃; namely, ‘ordered’ Na₂RuO₃ with honeycomb-ordered [Na_1/3M_2/3]O₂ slabs and ‘disordered’ Na₂RuO₃ with randomly distributed [Na_1/3M_2/3]O₂ slabs (Fig. 1).

Figure 1: Structure of ordered and disordered Na₂RuO₃.

Oxide ions (red) stack in the manner of ABCABC while both Na (yellow) and Ru (grey) occupy octahedral sites for both Na₂RuO₃. Ordered Na₂RuO₃ has the honeycomb-type cation ordering in the [Na_1/3Ru_2/3]O₂ slab. Disordered Na₂RuO₃ has the randomly distributed [Na_1/3Ru_2/3]O₂ slab.

Results

Structural characterization of Na₂RuO₃

Ordered Na₂RuO₃ (hereafter denoted O-Na₂RuO₃) was synthesized by a thermal decomposition method, in which Na₂RuO₄ was annealed at 850 °C for 48 h under an Ar atmosphere²¹. Disordered Na₂RuO₃ (hereafter denoted D-Na₂RuO₃) was synthesized for comparison according to our previously reported procedure¹⁹. The ⁹⁹Ru Mössbauer spectra of both compounds (insets in Fig. 2a,b) show a singlet absorption peak with an isomer shift around −0.3 mm s⁻¹, which is a typical value for Ru⁴⁺ (for example, −0.25 mm s⁻¹ for Y₂Ru₂O₇ and −0.33 mm s⁻¹ for SrRuO₃)²², suggesting successful formation of stoichiometric Na₂RuO₃ compositions from both syntheses. The synchrotron X-ray diffraction (XRD) pattern of D-Na₂RuO₃ (Fig. 2a) is fully indexed to the O3 structure (R$\overline{3}$m space group, a=3.0969(3) Å and c=15.970(2) Å); all diffraction peaks are well-fitted by Rietveld refinement with a structural model in which Na and Ru are randomly distributed in the [Na_1/3Ru_2/3]O₂ slabs (Fig. 1 and Supplementary Table 1). Na in the Na layer occupies octahedral sites and the oxide ions are stacked in an ABCABC arrangement (O3 structure)²³. The selected area electron diffraction (SAED) pattern along the [001]_hex zone axis (Fig. 2c) shows diffraction spots fully indexed by the R$\overline{3}$m model with disordered [Na_1/3Ru_2/3]O₂ slabs.

Figure 2: Structural characterization of disordered and ordered Na₂RuO₃.

Observed and calculated (Rietveld method) synchrotron X-ray diffraction patterns for (a) disordered and (b) ordered Na₂RuO₃. Red crosses: experimental, black line: calculated, blue line: difference and green bars: Bragg positions. The black arrows in b indicate the superstructure peaks that were not considered for the refinement. Insets of a,b correspond to the ⁹⁹Ru Mössbauer spectra recorded at 4.2 K for both pristine materials. Vertical error bars represent 1σ s.d. of counting statistics. Selected area electron diffraction (SAED) patterns in the (c) [001]_hex zone axes of disordered Na₂RuO₃, and in the (d) [001]_hex and (e) [110]_hex zone axes of ordered Na₂RuO₃. The red circles and arrows, respectively, indicate the central and fundamental diffraction spots, which are common to disordered Na₂RuO₃. Un-marked diffraction spots in between correspond to superstructure peaks. Scale bars, 10 nm⁻¹.

The synchrotron XRD pattern of O-Na₂RuO₃ (Fig. 2b and Supplementary Table 1) is similar to that of D-Na₂RuO₃ and is almost fitted by Rietveld refinement in the R$\overline{3}$m space group (a=3.1195(5) Å and c=15.989(4) Å) assuming disordered [Na_1/3Ru_2/3]O₂ slabs. However, diffuse scatterings remain, as highlighted by the black arrows in Fig. 2b. Similar characteristics have been reported for many A₂MO₃ (A=Li, Na and M=transition metal(s)) materials, and usually arise from honeycomb ordering in the MO₂ slabs^16,17,20. Indeed, the SAED pattern along the [001]_hex zone axis (Fig. 2d) shows many extra diffraction spots, which are not predicted using the standard R$\overline{3}$m model with disordered [Na_1/3Ru_2/3]O₂ slabs. These spots can be indexed to a supercell arising from the honeycomb ordering of Na and Ru in the [Na_1/3Ru_2/3]O₂ slabs (Fig. 1). In contrast, the SAED pattern along the [110]_hex zone axis (Fig. 2e) shows extra streaks along [001]_hex. This feature is characteristic of stacking faults in honeycomb-ordered [A_1/3M_2/3]O₂ slabs along the c_hex direction, explaining the diffusive nature of the superstructure reflections in Fig. 2b (ref. 24).

Electrochemical properties of Na₂RuO₃

Having demonstrated the differences between O- and D-Na₂RuO₃ in terms of the in-plane ordering and stacking sequences of the [Na_1/3Ru_2/3]O₂ slabs, the Na⁺ (de)intercalation properties of both materials were studied to clarify the influence of the honeycomb ordering. Figure 3a,b shows the charge–discharge curves measured between 1.5 and 4.0 V versus Na/Na⁺ at 30 mA g⁻¹ (a rate of ∼C/5). Here charging is an anodic process (Na deintercalation) and discharging is a cathodic process (Na intercalation). D-Na₂RuO₃ (Fig. 3a) delivers a reversible capacity of 135 mAh g⁻¹, corresponding to (de)intercalation of 1.0 Na⁺, which is consistent with our previous report¹⁹. In contrast, the reversible capacity of O-Na₂RuO₃ exceeds 180 mAh g⁻¹, indicating reversible (de)intercalation of 1.3 Na⁺, beyond a Ru⁵⁺/Ru⁴⁺ one-electron redox process. This behaviour is in agreement with that of Na₂RuO₃ reported by Rozier et al.²⁰. Importantly, the voltage profile of O-Na₂RuO₃ significantly differs from the S-shaped voltage profile of D-Na₂RuO₃. O-Na_xRuO₃ (0.7≤x≤2) exhibits a staircase-like charge profile with a first voltage plateau around 2.5 V for 1.0≤x≤2.0 and a second voltage plateau around 3.6 V for 0.7≤x≤1.0; the second voltage plateau is related to the extra capacity of O-Na₂RuO₃, which exceeds that of the Ru⁵⁺/Ru⁴⁺ one-electron reaction. Although the plateau at 3.6 V shows gradual narrowing with repeating the cycles presumably due to slight loss of the crystallinity (Supplementary Fig. 1a,b), O-Na₂RuO₃ shows excellent capacity retention of 160 mAh g⁻¹ after 50 cycles (Fig. 3c), which indicates the remarkable stability of the redox processes that contribute to the increased capacity. Indeed, the voltage plateau around 3.6 V is clearly observed for every charge process, suggesting occurrence of the accumulative oxygen redox reaction (Supplementary Fig. 1c,d)¹⁸. Therefore, the available capacity significantly exceeds one-electron redox reaction for O-Na₂RuO₃ even after 50 cycles. It is surprising that the in-plane honeycomb-type cation ordering in O-Na₂RuO₃ is solely responsible for the drastic changes in the electrochemical properties that results in 30% higher capacity. To the best of our knowledge, this is the first demonstration of the critical role of honeycomb-type cation ordering in [A_1/3M_2/3]O₂ slabs in determining the primary electrochemical properties.

Figure 3: Electrochemical properties of disordered and ordered Na₂RuO₃.

Galvanostatic cycling curves recorded at 30 mA g⁻¹ for (a) disordered and (b) ordered Na₂RuO₃ with the first cycle highlighted in blue. Insets show the coordination environment of Na at x=1.0 for each phase. (c) Capacity retentions for (blue squares) disordered and (red circles) ordered Na₂RuO₃.

Structural change during (de)sodiation

To clarify the reaction mechanisms in Na₂RuO₃, we studied the structural changes of D-Na₂RuO₃ and O-Na₂RuO₃ during cycling (Fig. 4a–d). As reported previously¹⁹ and further supported by ex situ XRD patterns (Supplementary Fig. 2), D-Na_xRuO₃ undergoes a structural change from O3 to P3 (with Na in prismatic sites owing to the ABBCCA oxide-ion stacking) on charging²³. This O3→P3 transition is commonly observed in O3-NaMO₂ materials through gliding of the [Na_1/3Ru_2/3]O₂ slabs from ABCABC (O3) to ABBCCA (P3) stacking^9,25. The synchrotron XRD pattern of the charged state (D-Na₁RuO₃; Supplementary Fig. 3 and Supplementary Table 2) is fully fitted by the Rietveld refinement assuming a P3 structure (R$\overline{3}$m space group, a=2.927(2) Å and c=16.774(12) Å), in which Na ions in the Na layer are located in prismatic sites (inset in Fig. 3a). The interlayer distance is significantly increased from 5.323(1) Å (x=2) to 5.591(4) Å (x=1) because of the high aspect ratio of the prismatic Na sites in the P3 structure (Fig. 4a).

Figure 4: Structural changes of disordered and ordered Na₂RuO₃ during charge.

(a) Interlayer distance of the different phases involved on desodiation as a function of x in disordered and ordered Na_xRuO₃. (b) Crystal structure of ilmenite-type Na₁RuO₃. All Na ions displace cooperatively towards Na vacancies in the honeycomb planes. (c) Observed and calculated (Rietveld method) synchrotron X-ray diffraction patterns for ordered Na₁RuO₃. Red crosses: experimental, black line: calculated, blue line: difference and green bars: Bragg positions. The inset shows the selected area electron diffraction (SAED) pattern for ilmenite-type Na₁RuO₃. Scale bar, 10 nm⁻¹ (d) Ex situ XRD patterns showing the evolution of the superstructure peaks for ordered Na_xRuO₃ on charge.

It should be emphasized that O-Na₂RuO₃ exhibits a completely different structural evolution from D-Na₂RuO₃ on charging. The in situ and ex situ XRD patterns of O-Na₂RuO₃ (Supplementary Figs 4 and 5) at the first plateau show that an intermediate phase ‘X’ with an unknown structure (presumably a P-type phase based on the interlayer distance of around 5.5 Å) appears during the initial stages of the desodiation, and that the O3 phase is eventually transformed to a new phase at x=1 with an extremely short interlayer distance of around 5.2 Å (Fig. 4a). These complex equilibria among the O3, X and O1 phases are responsible for the long voltage plateau observed in the 1≤x≤2 composition range of O-Na_xRuO₃. On charging, the diffuse scatterings of the superstructure gradually disappear, while new, well-resolved peaks appear (Fig. 4d). The well-resolved character of the new superstructure reflections suggest that the disordered stacking of the honeycomb lattice is spontaneously adjusted towards the ordered state during electrochemical desodiation. The Rietveld refinement and SAED pattern of O-Na₁RuO₃ (Fig. 4b,c and Supplementary Tables 2 and 3) confirm that the [Na_1/3Ru_2/3]O₂ layers maintain the original honeycomb-type in-plane ordering. As shown in Fig. 4b, the ABCABC stacking sequence of oxygen is transformed to an ABAB sequence, in which Na in the Na layer occupies octahedral sites (O1 structure; inset in Fig. 3b). This ordered structure is isomorphic with the ilmenite FeTiO₃-type structure²⁶.

In the refined structure of O-Na₁RuO₃ (R$\overline{3}$ space group, a=5.2492(1) Å and c=15.6201(6) Å), the Na site in the [Na_1/3Ru_2/3]O₂ slabs is vacant, which means Na was extracted from the [Na_1/3Ru_2/3]O₂ slabs prior to the Na layers. In this very unique, stable intermediate ilmenite-type Na₁RuO₃, all NaO₆ octahedra in the Na layer share faces with a RuO₆ octahedron and a ‘□O₆’ octahedron of the adjacent [□_1/3Ru_2/3]O₂ slabs (□: Na vacancy). As shown in Fig. 4a, the interlayer distance of O-Na₁RuO₃ (5.2067(2) Å) is much shorter than that of D-Na₁RuO₃ (5.591(4) Å) as a result of substantial displacement of Na in the Na layer towards □O₆ octahedra by Coulombic attraction (Fig. 4b). Importantly, the RuO₆ octahedron is strongly distorted: the shortest neighbouring O–O distance is 2.580(4) Å, whereas the longest is 3.080(6) Å (Fig. 5 and Supplementary Table 3).

Figure 5: Reaction mechanisms of disordered and ordered Na₂RuO₃.

Schematic representation of the structural changes during charge–discharge for disordered Na₂RuO₃ and ordered Na₂RuO₃. Ordered Na₂RuO₃ can distort cooperatively to raise the energy level of the antibonding σ* orbital of the O–O bond, leading to the oxygen redox reaction. Disordered Na₁RuO₃ cannot accommodate the RuO₆ distortion due to strain frustration, which prevents the oxygen redox reaction.

The ilmenite structure of O-Na₁RuO₃ is stabilized by the following mechanisms: (1) the ordered Na arrangement minimizes Na⁺–Na⁺ repulsion; (2) the Na vacancy □ in the [□_1/3Ru_2/3]O₂ slab strongly attracts Na ions in the face-sharing octahedra in adjacent Na layers; (3) the ordered and displaced arrangement of Na ions minimizes Ru⁵⁺–Na⁺ repulsive interactions; and (4) the cooperative distortion of the RuO₆ octahedra can minimize the total strain energy in the ordered honeycomb lattice. All these mechanisms lead to the formation of the ilmenite phase with well-ordered stacking of the honeycomb lattices.

On discharge, in situ XRD patterns (Supplementary Fig. 4) prove that the phase transformation is almost reversible except for a small asymmetry, that is, the appearance of the X phase only at charge. Presumably, this parasitic asymmetric behaviour is observed under non-equilibrium conditions such as the microscale heterogeneity.

Electronic structure of Na_2−xRuO₃

After the O3→O1 transition, O-Na₂RuO₃ delivers additional capacity, exceeding that of the Ru⁵⁺/Ru⁴⁺ one-electron reaction at the higher voltage plateau of around 3.7 V. To clarify the overall redox mechanism in O-Na₂RuO₃, we conducted Ru L₃-edge and oxygen K-edge X-ray absorption spectroscopy (XAS) in the partial fluorescence yield mode at various charge depths (Fig. 6, Supplementary Figs 6 and 7). The Ru L₃-edge XAS directly probes the 4d orbitals through the Laporte-allowed 2p→4d transition²⁷. Furthermore, because the O 2p orbital strongly hybridizes with the Ru 4d orbital, oxygen K-edge XAS can be used to monitor the hole created on the O 2p and Ru 4d orbitals on charging²⁸. Note that the probing depth of the partial fluorescence yield mode is about 100 nm; hence, the spectra are bulk sensitive²⁹.

Figure 6: Electronic structure changes of ordered Na₂RuO₃ upon charge.

(a) States of charge of the samples for X-ray absorption spectroscopy. (b) Ruthenium L₃-edge and (c) oxygen K-edge X-ray absorption spectra for various O-Na_xRuO₃ compositions: (i) x=2.0 (black), (ii) x=1.5 (blue), (iii) x=1.0 (green) and (iv) x=0.62 (red). The asterisked peak corresponds to O 2p-Na 3p. The calculated oxygen K-edge spectra for ordered Na₂RuO₃ (black) and ilmenite-type NaRuO₃ (green) are also shown.

The Ru L₃-edge spectrum for pristine O-Na₂RuO₃ (t_2g⁴e_g⁰, neglecting the trigonal distortion of Ru for clarity), Fig. 6b, shows two absorption peaks corresponding to the excitation from the 2p orbital to the unoccupied t_2g and e_g orbitals. The peak position for the 2p→e_g absorption (2841.5 eV) is similar to that reported for Ru⁴⁺ (2841.6 eV) in SrRuO₃ and RuO₂ (ref. 27), confirming Ru⁴⁺ in pristine O-Na₂RuO₃. On charging to ilmenite-type Na₁RuO₃, two absorption peaks shift to higher energy. Since the peak position of the 2p→e_g absorption for ilmenite-type Na₁RuO₃ (2,843 eV) agrees with that of Ru⁵⁺ in Sr₄Ru₂O₉ (ref. 27), oxidation from Ru⁴⁺ to Ru⁵⁺ occurs at the first potential plateau. However, further desodiation at the second plateau does not shift the 2p→e_g absorption peak, suggesting no substantial change in the valence state of Ru.

The O K-edge spectrum of pristine O-Na₂RuO₃ (t_2g⁴e_g⁰) shows a large peak at 534 eV, which is ascribed to the unoccupied hybridized orbital of O 2p–Na 3p, and its peak intensity simply reflects the amount of Na in the lattice³⁰. The calculated density of states for O-Na₂RuO₃ (Fig. 7a) also indicates that the O 2p–Na 3p hybridized orbital exists in this energy range. For the lower energy region that represents the redox reaction, the calculated oxygen K-edge spectrum agrees well with the spectra from 528 to 533 eV. Thus, the peaks around 529 eV and 532 eV can be ascribed to the unoccupied hybridized orbitals of O 2p–Ru t_2g and O 2p–Ru e_g, respectively^31,32. At the voltage plateau of 2.5 V, desodiation diminishes the O 2p–Na 3p signal, whereas the peak intensity of O 2p–Ru t_2g (around 529 eV) increases on charging, indicating hole generation on the O 2p–Ru t_2g orbital. The increase in the peak intensity around 529 eV is also in good agreement with the calculated oxygen K-edge spectrum for ilmenite-type Na₁RuO₃.

Figure 7: Calculated electronic structure of ordered Na₂RuO₃ upon charge.

Calculated density of states (DOS) for ordered (a) O3-Na₂RuO₃ and (b) ilmenite-type Na₁RuO₃.

On further charging at the second plateau (around 3.6 V), the peak intensity of the O 2p–Ru t_2g is almost constant, whereas the peak around 532 eV broadens as a result of the new absorption band emerging around 533 eV. This spectral change cannot be explained by Ru⁵⁺→Ru⁶⁺ oxidation (hole generation on O 2p–Ru t_2g). One possible explanation is the formation of a new chemical bond involving oxygen. For example, formation of an O–O bond (that is, peroxo/superoxo-like groups O₂ⁿ⁻ (1≤n≤3)) has been proposed as the oxygen redox mechanism for many Li₂(M,M′)O₃ cathode materials (M,M′=Fe, Ru, Sb, Sn or Ir)^18,33,34. Here the hole generated by further charging is stabilized on the antibonding σ* orbital of the O–O bond which is covalently bonded to the transition metal. It should be noted that the ⁹⁹Ru Mössbauer spectrum of O–Na_0.62RuO₃ (Supplementary Fig. 8) shows absorption with an isomer shift of δ=+0.21 mm s⁻¹, which is a typical value for Ru⁵⁺ (+0.11 mm s⁻¹ for Na₃RuO₄ and +0.19 mm s⁻¹ for Ca₂EuRuO₆)²², supporting that no further Ru⁵⁺→Ru⁶⁺ oxidation occurs.

On discharge, while the 2p→t_2g and 2p→e_g signals in the Ru L₃-edge spectra shift to the initial lower energy region, the O–O bond and O 2p–Ru t_2g signals in the O K-edge spectra decrease reversibly (Supplementary Figs 6 and 7). Furthermore, although the diffraction peaks of the ex situ XRD pattern after cycling exhibit weak broadening due to the slight loss of the crystallinity, O-Na₂RuO₃ still shows characteristic diffuse scatterings similar to the pristine compound (Supplementary Fig. 1b), which suggests that the honeycomb-type in-plane ordering is maintained during the charge–discharge cycles for all Na compositions, where 0.7≤x≤2. Importantly, this is the first simultaneous achievement of extra oxygen redox capacity and structural integrity leading to totally reversible oxygen participation. All other A_2−xMO₃-type electrodes suffer from severe structural rearrangement on the first charge and from lowering of the operating voltage during subsequent cycles^{16,17,34,35,36}. This structural rearrangement occurs because M migrates from octahedral sites in the M layer to face-sharing tetrahedral sites in the adjacent A layer³⁵. Presumably, this inter-site migration of Ru is suppressed in Na₂RuO₃ because of incompatibility of large Ru with the tetrahedral site. Furthermore, the interlayer O–O bond formation, which is predicted theoretically for delithiated Li₂MnO₃ to destabilize the oxygen redox reaction³⁷, should be inhibited by the large interlayer distance in Na_xRuO₃ (for example, 5.2−5.5 Å for Na_xRuO₃ and 4.7−4.8 Å for Li_xRu_0.75Mn_0.25O₃)³⁶. As shown in Supplementary Fig. 9, the initial Coulombic efficiency of O-Na₂RuO₃ is higher (95%) than that of other A_2−xMO₃ (for example, 70% for Li₂MnO₃ and 90% for Li₂RuO₃)^17,36,38,39.

Discussion

The critical role of honeycomb ordering in Na₂RuO₃ based on the aforementioned original observations is summarized in Fig. 5. The ordered [□_1/3Ru_2/3]O₂ slabs in ilmenite O-Na₁RuO₃ accommodate the distortion of the RuO₆ octahedra cooperatively, whereby the shortest O–O distance is 2.580(4) Å. Because the short O–O distance raises the energy level of the antibonding σ* orbital of the O–O bond to the Fermi level, the oxygen redox reaction is triggered. Indeed, the calculated density of states for ilmenite O-Na₁RuO₃ (Fig. 7b) shows that the occupied antibonding orbitals of the O–O bond exist near the Fermi level. The naturally derived scenario is that, once the hole is generated on the antibonding σ* orbital of the O–O bond, the O–O distance becomes shorter⁴⁰. This short distance leads to the higher energy level of the antibonding σ* orbital of the oxygen–oxygen bond and an acceleration of the oxygen redox reaction. This hypothesis was recently reinforced by McCalla et al.³⁴: scanning transmission electron microscopy and neutron diffraction on fully delithiated Li₂IrO₃ (Li_0.5IrO₃) determined the shorter O–O distance (2.45 Å) after the oxygen oxidation. However, as demonstrated in this work, the short O–O distance is required before the oxygen oxidation to achieve the extra capacity. For example, the disordered [Na_1/3Ru_2/3]O₂ slabs in D-Na₁RuO₃ cannot accommodate the cooperative distortion of the RuO₆ octahedra owing to strain frustration. Thus, the O–O distances in D-Na₁RuO₃ are long enough (2.751(6) and 2.927(1) Å) so that the energy level of the antibonding σ* orbital of the O–O bond is well below the Fermi level, which inhibits the extra oxygen redox process within the electrolyte stability window. Therefore, honeycomb-type cation ordering in the [Na_1/3M_2/3]O₂ slab is a structural requisite condition to trigger the stable oxygen redox capacity in Na₂MO₃.

Finally, we mention the role of Ru in the oxygen redox reaction. Very recently, Saubanère et al.³⁷ theoretically demonstrated that the covalent bond between the peroxo-like (O₂)ⁿ⁻ 2p and Ru 4d orbitals in Li₂RuO₃ (reductive coupling mechanism) is essential to facilitate the oxygen redox reaction and to suppress the oxygen gas release, whereas the M(3d)-O(2p) overlap is too small to enable this mechanism. For O-Na₂RuO₃, in addition to the stable structural nature inherent in the Na system by suppressing both the inter-site Ru migration and the interlayer O–O bond formation as discussed above, the similar reductive coupling mechanism through the large orbital overlap between the Ru 4d and O 2p orbitals should suppress the oxygen gas release, which explains the high cycle stability of O-Na₂RuO₃ compared with that of any other Li excess materials (Supplementary Fig. 9).

In conclusion, the present work demonstrates the critical role of honeycomb-type cation ordering in Na₂MO₃ to achieve enhanced reversible capacity based on the oxygen redox process. In comparison to its disordered polymorph, O-Na₂RuO₃ with in-plane honeycomb ordering exhibits a significantly different voltage profile, leading to a 30% extra capacity, spurred by the electrochemically driven, further ordered intermediate, NaRuO₃, which accommodates cooperative distortion of the RuO₆ octahedra. The short O–O distance in distorted RuO₆ induces frontier orbital reorganization, triggering the oxygen redox reaction.

To the best of our knowledge, this is the first report that clarifies the underlying structural requirement to trigger the oxygen redox reaction in A_2−xMO₃. By further exploiting the compositional versatility of A₂MO₃ while meeting this requirement, there is great potential to develop oxygen redox electrodes for superior batteries. Considering the impact of the stable intermediate on the overall electrode properties, the peculiar phenomena and concepts demonstrated here may give general implications for a deeper understanding and improved utilization of intercalation materials.

Methods

Synthesis of Na₂RuO₃

Ordered Na₂RuO₃ was synthesized by the thermal decomposition method according to the literature where Na₂RuO₄ was annealed at 850 °C for 48 h under Ar atmosphere²¹. Disordered Na₂RuO₃ was synthesized according to the procedure we previously reported¹⁹. The NaHCO₃ and RuO₂ precursors were calcined at 850 °C for 12 h under Ar atmosphere. Both compounds were handled in a globe box filled with Ar (dew point below −110 °C).

Materials characterization

Powder X-ray diffraction patterns were recorded on a Rigaku TTR-III diffractometer equipped with Cu Kα radiation in 0.02° steps over the 2θ range of 10–80°. XRD patterns were measured at SPring-8 BL02B2 and Photon Factory BL-8B. The samples were handled without air exposure throughout the experiments. Rietveld refinements were carried out using Jana2006 (ref. 41) or TOPAS-Academic software. The profile of Na₁RuO₃ was better described using independent crystallite size for 00l reflections, Popa’s sphere harmonics for h0l and hkl reflections and different crystallite sizes for (h0k)=3n as implemented in TOPAS in order to take into account the stacking faults affecting part of the diffraction peaks. The crystal structures were drawn using VESTA⁴². SAED patterns were recorded using an electron microscope (HF-3000S; Hitachi Ltd.) operated at 300 kV and an imaging plate system (FDL-5000; Fujifilm). The camera length was calibrated with a Si crystal. ⁹⁹Ru Mössbauer spectroscopy was carried out with the ⁹⁹Rh source (T_1/2=15 days), which was prepared by the ⁹⁹Ru(p,n)⁹⁹Rh reaction at the RIKEN AVF cyclotron. The ⁹⁹Ru(p,n)⁹⁹Rh reaction was conducted with a ⁹⁹Ru (95% enriched) target under 12 MeV proton irradiation. Both the source and the samples were cooled in liquid He during the measurements. The samples were transferred into the liquid He cryostat without air exposure. The Doppler velocity was calibrated by measuring a ⁵⁷Fe Mossbauer spectrum of α-Fe, and the zero velocity was determined by the isomer shift of Ru metal at 4.2 K.

Electrochemical measurement

For the electrochemical measurements, 80 wt% of Na₂RuO₃ was mixed with 10 wt% of acetylene black and 10 wt% of polyvinylidene in N-methyl pyrrolidone to make a slurry, which was then coated onto Al foil. The dried films were used as a cathode in the 2,032-type coin cells with Na metal as an anode. NaPF₆ (1 M) in ethylene carbonate/diethyl carbonate (1:1 v/v) was used as an electrolyte. The charge–discharge experiments were conducted at 30 mA g⁻¹ between 1.5−4.0 V versus Na/Na⁺, within the electrolyte stability window.

Samples for ex situ characterization

The electrodes for ex situ XRD, XAS and ⁹⁹Ru Mossbauer spectroscopy were prepared as described above. O-Na₁RuO₃ samples for synchrotron XRD and TEM studies were prepared following the same method except that the positive electrode consisted in a pellet (Ø 10 mm, 50 mg) of O-Na₂RuO₃ that was sintered at 850 °C for 12 h. The batteries were then cycled to a designated voltage that was maintained for a few hours (Na₁RuO₃) or until a given current has been applied. After cycling, the batteries were introduced into an Ar-filled glovebox and the collected electrodes were washed three to five times with anhydrous dimethyl carbonate.

In situ XRD

In situ XRD (operando mode) was carried out using an in situ cell purchased from Rigaku on a D8 ADVANCE powder diffractometer equipped with Co Kα radiation in 0.02° steps over the 2θ range of 17−25° (1.37 s per step, one pattern recorded every ≈12 min). The electrochemical part of the cell is similar to the one described above. The only exception lies in the preparation of the electrode whose slurry was coated on plastic films to be peeled off after drying. The current rate was C/20 (full (dis)charge theoretically achieved in 20 h), allowing us to record an XRD pattern every Δx=0.02. Only selected patterns are shown in Supplementary Fig. 4 for clarity concerns. Electrochemistry was controlled and recorded using a Biologic VMP3.

X-ray absorption

O K-edge XAS was carried out at BL07LSU of SPring-8. The samples were transferred to a vacuum chamber without air exposure. Bulk-sensitive partial-fluorescence yield (PFY) mode was used for the O K-edge XAS. Energy resolution was approximately 100 meV. All the spectra were recorded using a silicon drift detector at room temperature. The spectra were normalized by the peak intensity at 532 eV, because the peak at 532 eV corresponding to O 2p-Ru e_g may not be affected by the redox of Ru⁵⁺/Ru⁴⁺. Ru L₃-edge XAS was carried out at BL27SU of SPring-8. The samples were transferred to a vacuum chamber without exposure to air. The spectra were recorded in the bulk-sensitive PFY mode using a silicon drift detector at room temperature.

Ab initio calculation

Spin-polarized DFT+U calculations were performed using the VASP code⁴³ with U_eff for Ru ranging from 0.5 to 4.5 eV. Van der Waals density functional optPBE⁴⁴ was employed. The plane-wave energy cut-off was set to 520 eV and the Brillouin zone integration was done in a k-point grid chosen to converge the energy to 10⁻⁴ eV. Atomic coordinates and lattice parameters were relaxed using conjugate gradient method until the maximum force on atoms is <2 × 10⁻³ eV Å⁻¹. Initial geometry of Na₂RuO₃ in the C2/m symmetry was taken from the literature⁴⁵ while the initial geometry for NaRuO₃ in the R$\overline{3}$ symmetry is from the present work. Antiferromagnetic ordering of spin on Ru was assumed, the pattern was set to be Néel type. The oxygen K-edge X-ray absorption pattern was calculated by the WIEN2k code⁴⁶ employing the full potential linear augmented plane-wave method, dipole approximation to the golden rule for the transition probability was assumed.