Synthesis and Electrochemical Activity of Some Na(Li)‐Rich Ruthenium Oxides with the Feasibility to Stabilize Ru⁶⁺

Abstract

The capacity of Li-ion cathode materials has recently been greatly improved by the feasibility to trigger both cationic and anionic redox reactions within the same material. This concept has rapidly been implemented to Na-ion batteries to boost their energy density. The electrochemical properties of Na 3 RuO 4 with Li 3 RuO 4 are reported and compared herein. Strikingly, it is found that 3 Na can be extracted from Na 3 RuO 4 with the charge compensation mechanism enlisting first the oxidation of Ru 5+ to Ru 6+ , leading to Na 2 RuO 4 , and then the oxidation of oxygen during the rest of the charge. This drastically contrasts with the behavior of the Li counterpart since Ru never reaches that high oxidation state during lithium removal. By comparing the phase diagrams of A x RuO 4 (A being Li, Na, or K) together with density functional theory calculations, this finding is rationalized and it is demonstrated that this difference is mainly rooted in the size of the alkali cation. The bigger the alkali, the lower the coordination of Ru will be, stabilized by the same higher oxidation states. This work highlights the difference between Li and Na materials toward anionic redox and suggests the unusual coexistence of Ru 6+ and (O 2 ) n− , hence setting new challenges to theoreticians and opening opportunities for materials design.

Quantification of Anionic Redox Chemistry in a Prototype Na-Rich Layered Oxide

“…The XRD patterns are very similar to those reported in the literature, suggesting the successful synthesis of high-purity Na3RuO4. [22][23] As summarized previously, Na3RuO4 adopts a Na-rich layered structure where the stacking of alternating Na + and (Na1/2Ru1/2)O2layers along the c-axis, and the isolated tetramers of edge-sharing RuO6 octahedra was separated by sodium atoms in the (Na1/2Ru1/2)O2layer. [22][23] The SEM images of Na3RuO4 (Figure 1b and c) show nonuniform nanometer-sized particles induced by the solid-state reaction process.…”

“…[22][23] As summarized previously, Na3RuO4 adopts a Na-rich layered structure where the stacking of alternating Na + and (Na1/2Ru1/2)O2layers along the c-axis, and the isolated tetramers of edge-sharing RuO6 octahedra was separated by sodium atoms in the (Na1/2Ru1/2)O2layer. [22][23] The SEM images of Na3RuO4 (Figure 1b and c) show nonuniform nanometer-sized particles induced by the solid-state reaction process. In addition, the corresponding energy dispersive spectra (EDS) results clearly demonstrate that Ru, Na and O atoms are uniformly distributed throughout the sample (Figure 1d−g).…”

“…[20][21] Most recently, Na3RuO4 was introduced as a pure prototype Na-rich layered oxide for SIBs, which delivers an outstanding electrochemical performance. [22][23] However, the charge compensation mechanism of Na3RuO4 undergoes different hypotheses: (i) Qiao et al…”

“…proposed that during the charge and discharge processes Ru cations are inert and merely oxygen anions contribute to the reversible capacity, 22 and (ii) Otoyama et al believed that both Ru and oxygen are involved into the charge compensation process with Ru 6+ and peroxo-like species as the oxidation products during the first few charge cycles. 23 It is clear that the understanding of the cationic and anionic redox reactions of Na3RuO4 is still limited, mainly because (i) there is still no consensus on whether Ru cations are electrochemically active or not; (ii) the charge compensation mechanism was investigated merely for the first cycle without quantification; and (iii) more importantly, despite prevailing O-K spectroscopies, particularly X-ray absorption spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), have been popularly applied to investigate the anionic redox reaction mechanism, direct and reliable evidence for validating the participation of intrinsic oxygen redox process is still rarely reported due to the shallow probe depth of XPS and intricate TM-O hybrid effect in XAS. [24][25] In this work, we have investigated the cationic and anionic redox mechanism of Na3RuO4 over extended electrochemical cycles by using bulk-sensitive XAS and full-range mapping of resonant inelastic X-ray scattering (mRIXS) techniques.…”

Stabilizing Reversible Oxygen Redox Chemistry in Layered Oxides for Sodium‐Ion Batteries

“…Compared with pristine state, the peaks denoting average RuRu and RuO distances tend to decrease upon charge, which is consistent with the shrinkage of the unit cell caused by Na + deintercalation. [ 24 ] The obvious variation of Ru–O peak at two potential plateaus region is due to the change of O–O distances derived from anionic redox reactions, which also causes slight distortion in the MO 6 octahedral environment at high voltage range meanwhile. [ 25 ] Generally speaking, the slope regions within the initial charge and final discharge stages (blue region, Figure 3c) can be assigned to the Ru‐involved cationic redox reaction processes while the two charge and discharge plateaus (green region, Figure 3c) can be attributed to the Ru‐free redox process, which can be ascribed to the oxygen‐related anionic redox reaction processes.…”