Abstract
The surface structure of a lithium-rich layered material and its relation to intercalation properties were investigated by synchrotron X-ray surface structural analyses using Li2RuO3 epitaxial-film model electrodes with different lattice planes of (010) and (001). Electrochemical charge–discharge measurements confirmed reversible lithium intercalation activity through both planes, corresponding to three-dimensional lithium diffusion within the Li2RuO3. The (001) plane exhibited higher discharge capacities compared to the (010) plane under high rate operation (over 5 C). Direct observations of surface structural changes by in situ surface X-ray diffraction (XRD) and surface X-ray absorption near edge structure (XANES) established that an irreversible phase change occurs at the (010) surface during the first (de)intercalation process, whereas reversible structural changes take place at the (001) surface. These experimental findings suggest that the surface reconstructed phase limits lithium intercalation between the electrode and the electrolyte, leading to the poor rate capability of the (010) film. Surface structural changes at the initial cycling therefore have a pronounced effect on the power characteristics and stability of lithium-rich layered materials during battery operation.
Highlights
The surface structure of a lithium-rich layered material and its relation to intercalation properties were investigated by synchrotron X-ray surface structural analyses using Li2RuO3 epitaxial-film model electrodes with different lattice planes of (010) and (001)
Direct observations of surface structural changes by in situ surface X-ray diffraction (XRD) and surface X-ray absorption near edge structure (XANES) established that an irreversible phase change occurs at the (010) surface during the firstintercalation process, whereas reversible structural changes take place at the (001) surface. These experimental findings suggest that the surface reconstructed phase limits lithium intercalation between the electrode and the electrolyte, leading to the poor rate capability of the (010) film
The in-plane 060 surface diffraction peak shows a decrease in intensity compared to the RuO2 002 peak, indicating that a structural change occurs at the surface when the electrode is in contact with the electrolyte solution
Summary
Lithium-rich layered oxides, xLi2MO3-(1Àx)LiMO2 (M 1⁄4 3d and/ or 4d transition metal) have attracted signi cant attention as promising intercalation cathodes for use in lithium batteries, since they allow high discharge capacities over 200 mA h gÀ1 and high operating voltages (over 3.5 V vs. Li/Li+ on average).[1,2,3,4,5,6] The discharge voltage of such cathodes, is gradually reduced during electrochemical cycling, resulting in the fading of energy density.[1,2,7,8] Another de cit is their poor rate capability compared to that obtainable with LiCoO2.9–11 These drawbacks make it difficult to consider lithium-rich layered oxides as viable candidates for the replacement of conventional layered rock salt cathodes (Li(Co, Ni, Mn)O2). The surface structure of a lithium-rich layered material and its relation to intercalation properties were investigated by synchrotron X-ray surface structural analyses using Li2RuO3 epitaxial-film model electrodes with different lattice planes of (010) and (001).
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