Oxygen-participated electrochemistry of new lithium-rich layered oxides Li3MRuO5 (M = Mn, Fe)

Abstract

We describe the synthesis, crystal structure and lithium deinsertion–insertion electrochemistry of two new lithium-rich layered oxides, Li₃MRuO₅ (M = Mn, Fe), related to rock salt based Li₂MnO₃ and LiCoO₂. The Li₃MnRuO₅ oxide adopts a structure related to Li₂MnO₃ (C2/m) where Li and (Li_0.2Mn_0.4Ru_0.4) layers alternate along the c-axis, while the Li₃FeRuO₅ oxide adopts a near-perfect LiCoO₂ (Rm) structure where Li and (Li_0.2Fe_0.4Ru_0.4) layers are stacked alternately. Magnetic measurements indicate for Li₃MnRuO₅ the presence of Mn³⁺ and low spin configuration for Ru⁴⁺ where the itinerant electrons occupy a π*-band. The onset of a net maximum in the χ vs. T plot at 9.5 K and the negative value of the Weiss constant (θ) of −31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g⁻¹ (2.3 Li per f.u.) and 144 mA h g⁻¹ (1.38 Li per f.u.) is observed for Li₃MnRuO₅ and Li₃FeRuO₅, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge–charge cycling points to partial reversibility (ca. 160 mA h g⁻¹ and 45 mA h g⁻¹ for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li₃MRuO₅ reveals that in the Li₃MnRuO₅ oxide, Mn³⁺ and Ru⁴⁺ are partially oxidized to Mn⁴⁺ and Ru⁵⁺ in the sloping region at low voltage, while in the long plateau, O²⁻ is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O²⁻ ligand in the reversible redox processes. In the Li₃FeRuO₅ oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O²⁻ (plateau) while Fe seems to retain its 3+ state.