Gaining a thorough understanding of the reactions on the electrode surfaces of lithium batteries is critical for designing new electrode materials suitable for high-power, long-life operation. A technique for directly observing surface structural changes has been developed that employs an epitaxial LiMn(2)O(4) thin-film model electrode and surface X-ray diffraction (SXRD). Epitaxial LiMn(2)O(4) thin films with restricted lattice planes (111) and (110) are grown on SrTiO(3) substrates by pulsed laser deposition. In situ SXRD studies have revealed dynamic structural changes that reduce the atomic symmetry at the electrode surface during the initial electrochemical reaction. The surface structural changes commence with the formation of an electric double layer, which is followed by surface reconstruction when a voltage is applied in the first charge process. Transmission electron microscopy images after 10 cycles confirm the formation of a solid electrolyte interface (SEI) layer on both the (111) and (110) surfaces and Mn dissolution from the (110) surface. The (111) surface is more stable than the (110) surface. The electrode stability of LiMn(2)O(4) depends on the reaction rate of SEI formation and the stability of the reconstructed surface structure.

Structural changes at electrode/electrolyte interface of a lithium cell were studied by X-ray reflectometry and two-dimensional model electrodes with a restricted lattice plane of LiMn 2 O 4 . The electrodes were constructed with an epitaxial film synthesized by the pulsed laser deposition method. The orientation of the film depends on the substrate plane; the ͑111͒, ͑110͒, and ͑100͒ planes of LiMn 2 O 4 grew on the ͑111͒, ͑110͒, and ͑100͒ planes of the SrTiO 3 substrates, respectively. The ex situ reflectometry indicated that a thin impurity layer covered the lattice plane of the as-grown film. The impurity layer was dissolved and a solid-electrolyteinterface-like phase appeared after the electrode was soaked into the electrolyte. A defect layer was formed in the ͑111͒ plane, whereas no density changes were detected for the other lattice planes. The in situ observation clarified that the surface reactivity depended on the lattice planes of the spinel; the defect layer at the ͑111͒ plane was stable during the electrochemical reaction, whereas a slight decrease in the film thickness was observed for the ͑110͒ plane. Our surface characterization of the intercalation electrode indicated that the surface structure changes during the pristine stage of the change-discharge processes and these changes are dependent on the lattice orientation of LiMn 2 O 4 .Because the lithium-ion configuration composed of carbon anodes and intercalation cathodes has been widely accepted for lithium secondary batteries, significant efforts have been devoted to attain high energy and power densities to produce an excellent energy storage system. 1 In particular, recent progress in pure electric vehicles ͑EVs͒ and hybrid electric vehicles ͑HEVs͒ require high power density operation for the current battery systems. The power characteristics of the battery system are closely related to the nature of electrode reactions, which is composed of several reaction steps proceeded in series: lithium diffusion in the electrolyte, adsorption of solvated lithium on the cathode surface, desolvation, surface diffusion, charge-transfer reaction, intercalation from the surface to the bulk, and the bulk diffusion of lithium in the electrode material. Recent electrochemical studies have claimed that the desolvation process was the rate-determining step of the whole electrode reaction. 2,3 It is well known that electrode surfaces are almost covered with a passive surface layer, which is generally called the solid electrolyte interface ͑SEI͒. The idea of the SEI layer was originally introduced on the alkali and alkaline earth metal in organic electrolytes, 4 and then it is believed that the layer plays a key role in the electrochemical performance, particularly the calendar life of lithium batteries. Many experimental techniques such as X-ray photoelectron spectroscopy ͑XPS͒, 5-8 IR spectroscopy, 9,10 nuclear magnetic resonance ͑NMR͒, 11 and ellipsometry 12 have been employed to study the nature and formation mechanism of the SEI layer.Among the materials prop...

Potential-dependent surface structures of Au (111) and Au(100) single-crystal electrodes in a 50 mM H 2 SO 4 solution were investigated at an atomic level using in situ surface X-ray scattering (SXS) techniques. It was confirmed that both the Au(111) and Au(100) surfaces were reconstructed with an attached submonolayer of an oxygen species, most probably water, at 0 V (vs Ag/AgCl). Results at +0.95 V supported a previously suggested model for both the Au(111) and the Au(100) electrodes that, based on infrared and scanning tunneling microscopy measurements, the surfaces were a (1 × 1) structure with the coadsorbed sulfate anion and hydronium cation (H 3 O + ). At +1.05 V, where a small amount of an anodic current flowed, adsorption of a monolayer of oxygen species was observed on both surfaces. When the single-crystal gold electrodes were electrochemically oxidized at +1.40 V, the expansion of the gold surface by about one monolayer of Au atoms was observed, suggesting the penetration of oxygen into the surface gold layers (i.e., the formation of two layers of surface oxide). When the surface oxide was reduced at +0.65 V, the surface structure returned back to the structure observed at +0.95 V before the oxide formation (i.e., a (1 × 1) structure with coadsorbed sulfate anion and H 3 O + ). When the potential was reduced to 0 V, the surfaces were reconstructed again but with slightly more random structures than those before the potential cycle.

ABSTRACT:In situ electrochemical X-ray absorption fine structure (XAFS) measurements were performed at the Pt L 3 and Ce L 3 edges of the Pt−CeO x /C catalyst, which was prepared by a combined process of precipitation and coimpregnation methods, as well as at the Pt L 3 edge of the conventional Pt/C catalyst in oxygen-saturated H 2 SO 4 solution to clarify the role of CeO x in the reduction of the overpotential for the oxygen reduction reaction (ORR) at the Pt−CeO x nanocomposite compared with the conventional Pt/C catalyst. XAFS measurements clearly show that the enhancement of ORR activity is attributed to the inhibition of Pt oxide formation by the CeO x layer, of which Ce 3+ was oxidized to Ce 4+ instead of Pt at the Pt oxide formation potential.

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The structure of Bi adlayers on the Au(111) electrode surface during the course of O 2 reduction has been investigated by in situ surface X-ray scattering. Under oxygen reduction conditions, both the low-coverage (2 × 2)-Bi and the close-packed (p × 3)-2Bi adlayer structures are stable. We developed an electrochemical drop cell that provides a more uniform potential distribution during high current conditions compared with commonly used thin-layer cells. Our results clearly indicate distinctly different catalytic properties for the two ordered bismuth adlayer phases. In the potential region corresponding to the (2 × 2)-Bi phase, O 2 reduction is promoted to a four-electron reaction, albeit with relatively slow kinetics. The close-packed (p × 3)-2Bi phase, formed at more negative potentials, appears to have a limited number of sites available for fourelectron reduction, but the reaction kinetics on this adlayer is enhanced by an increased overpotential.

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