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Page 2 of 10 Liu et al. Energy Mater 2023;3:300011 https://dx.doi.org/10.20517/energymater.2022.68

INTRODUCTION
Li-ion batteries are approaching their theoretical limit in energy density, although they have truly changed
[1]
the world by serving as either portable or large-scale energy storage system . Nonaqueous lithium-oxygen
(Li-O ) battery, an emerging next-generation energy storage system, shows an extremely high theoretical
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[2]
energy density that is almost several times that of state-of-the-art Li-ion batteries . The electrochemistry of
a nonaqueous Li-O battery is simply based on the reaction between oxygen and Li on a porous cathode .
[3-6]
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Nevertheless, such a simple reaction conceals quite complicated aspects which affect the final performance
of a Li-O battery. With a certain electrolyte, the cathode and its structure and type of catalyst largely govern
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the formation route and the structure and morphology of the Li O , which in turn influences the capacity,
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overpotential and lifespan of Li-O batteries.
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So far, two main types of Li O formation route have been studied and the intrinsic relationships of their
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structure and the electrochemical behaviors have been widely accepted by researchers [3,4,7-11] . The Li-O
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battery with toroidal-structured Li O usually delivers a high discharge capacity because of its large size,
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which of course leads to a large charge overpotential to decompose. Film-or shape-structured Li O are
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rather easier to decompose during charging process, but the cathode surface will be fully passivated once
such a thin film is formed and the discharge process will be terminated, leading to a low capacity. As we all
expect, large discharge capacity and low charge overpotential is the ultimate goal of developing Li-O
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battery, which implies that the ideal Li O should have premium size, crystalline and in particular the
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cathode surface should still feature a porous structure after being generated [9,12-14] . Therefore, a variety of
strategies to regulate the Li O formation have been proposed, including rational cathode design, surface
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engineering, multicomponent catalyst employment, and so on [15-19] . With these measures in hand, in recent
years, Li O with different favorable morphologies have been discovered, which helped decrease the
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overpotential and enlarge the capacity to some extent. However, the cycling performance is still far from
satisfactory and needs more attention [15-17] . As we all know, carbon materials show their overwhelming
advantage in serving cathode in Li-O batteries due to their low price and excellent electron
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conductivity [20,21] , but carbon corrosion upon battery cycling is an unavoidable factor that leads to battery
failure [22-24] . Therefore, designing a cathode that could regulate favorable Li O formation/ decomposition
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and meanwhile could retain the merit of carbon materials while avoiding the side reactions will be of high
significance to building long-life Li-O batteries [25,26] .
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Herein in this work, a rational design of TiO /CNTs cathode is presented. A thin and loose TiO layer
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partially coats the CNTs surface, followed by decorating ultrafine Ru nanoparticles on the TiO /CNTs.
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Several advantages can be revealed in this cathode design: (i) The CNTs skeleton offers multiple three-
+
dimensional channels for the rapid transportation of oxygen, Li and electrons; (ii) The large surface area
provides huge space for Li O accommodation; (iii) The thin-loose TiO layer effectively inhibits the carbon
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corrosion but still could let mass transfer; and (iv) The ultrafine Ru nanoparticles serves as catalytic active
sites. Taking these four aspects above in consideration, a long-life Li-O battery is constructed.
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EXPERIMENTAL
Preparation of the catalyst
A certain amount of the commercial multiwall carbon nanotubes (MWCNTs) was firstly oxidized with a
solution (200 mL) that contained 75 mL nitric acid (65%-68%) and purified water under vigorous stirring
overnight to remove impurities. Then it was washed with deionized water and ethanol several times,
followed by drying at 80 °C in air.