Reactivity with Water and Bulk Ruthenium Redox of Lithium Ruthenate in Basic Solutions

Abstract

The reactivity of water with Li-rich layered Li 2 RuO 3 and partial exchange of Li 2 O with H 2 O within the structure has been studied under aqueous (electro)chemical conditions. Upon slow delithiation in water over long time periods, micron-size Li 2 RuO 3 particles structurally transform from an O3 structure to an O1 structure with a corresponding loss of 1.25 Li ions per formula unit. The O1 stacking of the honeycomb Ru layers is imaged using high-resolution HAADF-STEM, and the resulting structure is solved from X-ray powder diffraction and electron diffraction. In situ X-ray absorption spectroscopy suggests that reversible oxidation/reduction of bulk Ru sites is realized on potential cycling between 0.4 V RHE and 1.25 V RHE in basic solutions. In addition to surface redox pseudocapacitance, the partially delithiated phase of Li 2 RuO 3 shows high capacity which can be attributed to bulk Ru redox in the structure. This work demonstrates that the interaction of aqueous electrolytes with Li-rich layered oxides, can result in the formation of new phases with (electro)chemical properties that are distinct from the parent material. This understanding is important for the design of aqueous batteries, electrochemical capacitors and chemically stable cathode materials for Li-ion batteries.

What X‐Ray Absorption Spectroscopy Can Tell Us About the Active State of Earth‐Abundant Electrocatalysts for the Oxygen Evolution Reaction**

“…Other transition metal oxides, in particular those containing Ni, are known to incorporate Fe from alkaline electrolytes, which strongly modifies their electronic and thus catalytic properties [102–105] . Also, in the absence of Fe, exposure to hydroxide solutions may change the surface and even the bulk phase of transition metal oxides after sufficiently long exposure [106] . Finally, elements may dissolve from the electrocatalyst into the electrolyte changing the composition and thereby other properties, such as redox potentials or the catalytic reaction's overpotential [9, 107, 108] .…”

Oxygen Redox Versus Oxygen Evolution in Aqueous Electrolytes: Critical Influence of Transition Metals

“…Importantly, the suppression of the anodic side reactions aids the activation of oxygen redox reactions, resulting in a capacity as large as ≈300 mAh g ‐1 (Figure S4b , Supporting Information). However, during repeated charge/discharge cycling, the reversible capacity of Li 2 RuO 3 decreases steadily, presumably because of damaging parasitic reactions such as the Li + ‐H + exchange, [ 26 ] or the oxygen evolution reaction (OER). [ 27 ]…”

“…Importantly, the suppression of the anodic side reactions aids the activation of oxygen redox reactions, resulting in a capacity as large as ≈300 mAh g -1 (Figure S4b, Supporting Information). However, during repeated charge/discharge cycling, the reversible capacity of Li 2 RuO 3 decreases steadily, presumably because of damaging parasitic reactions such as the Li + -H + exchange, [26] or the oxygen evolution reaction (OER). [27] The onset potential is further up-shifted to 1.7 and 1.8 V versus Ag/AgCl using Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 and Li 1.2 Ni 0.2 Mn 0.6 O 2 , respectively, in a hydrate-melt electrolyte (Figure 1a,b).…”

What X-ray absorption spectroscopy can tell us about the active state of earth-abundant electrocatalysts for the oxygen evolution reaction

“…[84][85][86][87] Also, in the absence of Fe, exposure to hydroxide solutions may change the surface and even the bulk phase of transition metal oxides after sufficiently long exposure. 88 Finally, elements may dissolve from the electrocatalyst into the electrolyte changing the composition and thereby other properties, such as redox potentials or the catalytic reaction's overpotential. [89][90][91] Dosaev et al 92 recently studied Mn-bases spinels as synthesized, in the ink suspension and after soaking in hydroxide electrolyte, which oxidized Mn 3 O 4 but not MgMn 2 O 4 .…”