Lithium-Rich O2-Type Li_0.66[Li_0.22Ru_0.78]O₂ Positive Electrode Material

Abstract

Increasing the energy density of lithium-ion batteries is an important step towards flexible electricity supply, which can be achieved by developing large-capacity positive electrodes. Lithium-rich oxides have been a longstanding research target because of their large capacity involving extra oxygen-redox reactions. In this work, we report the synthesis, electrochemical properties, electronic structure, and structural evolution of O2-type lithium-rich layered oxide Li1.22‒x Ru0.78O2. A robust Ru‒O layered framework without Ru migration allows for unveiling the solid-state electrochemistry of O2-type lithium-rich layered oxides with possibility of a large yet stable extra capacity for oxygen-redox reaction. Using a combination of X-ray photoelectron spectroscopy, X-ray absorption/emission spectroscopy, and in situ/ex situ X-ray diffraction, we clarified that O2-Li1.22‒x Ru0.78O2 delivers a large capacity of 200 mAh g‒1 in association with Ru5+/Ru4+ and Ru4+/Ru3+ two-electron redox reactions under a solid-solution process, but with no contribution from the extra oxygen-redox reaction.

Effects of cation superstructure ordering on oxygen redox stability in O2-type lithium-rich layered oxides

“…36,37 Both electrodes could deliver a high discharge capacity of 239 and 244 mA h g −1 , respectively, with a remarkably small first-cycle irreversible capacities, which is hard to achieve with conventional O3-type lithium-rich layered oxide electrodes, indicating the superior electrochemical stability of O2-type oxygen redox electrodes. 14,23,24,26–28 While LLNMO and LLCMO electrodes exhibited similar initial electrochemical properties to each other, a noticeable difference was observed in the voltage retention behavior over extended cycling experiments. Fig.…”

“…It shows that the average discharge voltages were relatively well maintained for both LLNMO and LLCMO electrodes, which is in contrast to the O3-type counterparts that exhibited rapid voltage decay within a few cycles, consistent with previous literature reports. 14,23,24,26–28 Nevertheless, a slightly greater degree of the voltage decay was evidenced for the LLCMO electrode (93.8% retention) compared with the LLNMO (98.4%), even though they commonly shared the O2-type structure that is effective in suppressing the voltage decay. 14,23–29 Considering that the voltage decay is primarily coupled with the irreversible transition metal migration, 13–17 it may be inferred that the LLCMO electrode has undergone the structural transition involving the irreversible transition metal migration during cycles, which will be verified in detail later.…”

“…And, these efforts have led to the identification of several O2-type lithium-rich layered oxides that could surpass their conventional O3-type counterparts. 14,23–29 Nevertheless, studies also found that the degrees of voltage decay are slightly dependent on the species of substituents and the composition in the transition metal layer, even though they still could display a lower degree of voltage decay over O3-type electrodes. Moreover, some of the O2-type electrodes exhibited partially disordered structure with extended cycles, implying the presence of additional factors that trigger the irreversible transition metal migrations.…”

Stacking Faults in an O2-type Cobalt-Free Lithium-Rich Layered Oxide: Mechanisms of the Ion Exchange Reaction and Lithium Electrochemical (De)Intercalation

“…13 where the Ru migration is successfully blocked owing to the nature of the O2 stacking. 17 O2-type Li-rich layered oxides are metastable and cannot be obtained by common high-temperature solid-state lithiation methods (>600 °C). Their synthesis requires much lower temperatures of 120−300 °C that can only be achieved by soft chemistry routes.…”

“…The migration of TM cations would inevitably occur, but the strong electrostatic repulsions would restrict the movement of TM ions in the lithium interslab space and enhance their reversible return within their own original layers . Paulsen et al reported that the layered-to-spinel structural transition did no longer occur in O2-type Li 2/3 [Li 1/6 Mn 5/6 ]­O 2 ; more recently, Umeno et al studied the O2-type Li-rich layered ruthenium oxide Li 1.22– x Ru 0.78 O 2 where the Ru migration is successfully blocked owing to the nature of the O2 stacking …”