Figure 4. a) Rietveld refined XRD patterns for disordered (top) and ordered (bottom) spinel. b) Raman and c) FTIR spectra of the disordered (Fd3m,black) and ordered spinel (P4 3 32, red) material. Reproducedf rom Refs. [35, 37] with

An increase in the amount of nickel in LiMO2 (M = Ni, Co, Mn) layered system is actively pursued in lithium‐ion batteries to achieve higher capacity. Nevertheless, fundamental effects of Ni element in the three‐component layered system are not systematically studied. Therefore, to unravel the role of Ni as a major contributor to the structural and electrochemical properties of Ni‐rich materials, Co‐fixed LiNi0.5+xCo0.2Mn0.3–xO2 (x = 0, 0.1, and 0.2) layered materials are investigated. The results, on the basis of synchrotron‐based characterization techniques, present a decreasing trend of Ni2+ content in Li layer with increasing total Ni contents. Moreover, it is discovered that the chex.‐lattice parameter of layered system is not in close connection with the interslab thickness related to actual Li ion pathway. The interslab thickness increases with increasing Ni concentration even though the chex.‐lattice parameter decreases. Furthermore, the lithium ion pathway is preserved in spite of the fact that the c‐axis is collapsed at highly deintercalated states. Also, a higher Ni content material shows better structural properties such as larger interslab thickness, lower cation disorder, and smoother phase transition, resulting in better electrochemical properties including higher Li diffusivity and lower overpotential when comparing materials with lower Ni content.

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Tin oxide-based materials, operating via irreversible conversion and reversible alloying reaction, are promising lithium storage materials due to their higher capacity. Recent studies reported that nanostructured SnO 2 anode provides higher capacity beyond theoretical capacity based on the alloying reaction mechanism; however, their exact mechanism remains still unclear. Here, we report the detailed lithium storage mechanism of an ordered mesoporous SnO 2 electrode material. Synchrotron X-ray diffraction and absorption spectroscopy reveal that some portion of Li 2 O decomposes upon delithiation and the resulting oxygen reacts with Sn to form the SnO x phase along with dealloying of Li x Sn, which are the main reasons for unexpected high capacity of an ordered mesoporous SnO 2 material. This finding will not only be helpful in a more complete understanding of the reaction mechanism of Sn-based oxide anode materials but also will offer valuable guidance for developing new anode materials with abnormal high capacity for next generation rechargeable batteries. ■ INTRODUCTIONLithium-ion batteries have been recognized as one of the most promising power source for various applications including portable electronics, electric vehicles, and power storage systems of renewable energy. 1 Major challenges of lithium ion batteries for these applications include high energy density, excellent capacity retention, safety, and low cost. 1−3 In order to achieve higher energy density of lithium ion battery than that of currently commercialized lithium ion battery, 4−7 metal oxides are being investigated as alternative anode materials due to their high energy density achieved by conversion and alloying reactions. 8,9 Especially, tin oxide-based materials, including SnO and SnO 2 , are being considered as one of the best anode materials due to their higher specific lithium storage capacities. 10 Previous studies show that SnO 2 goes through an irreversible conversion reaction during the initial cycle, which leads to formation of Sn metal and Li 2 O matrix, followed by a reversible alloying/dealloying reaction of Sn with lithium. 11−17

The reaction mechanism of a high capacity lithium-and manganese-rich metal oxide, 0.4Li 2 MnO 3-0.6LiMn 0.5 Ni 0.5 O 2 , has been investigated at the atomic level. High-resolution synchrotron X-ray powder diffraction (HRPD) and X-ray absorption spectroscopy (XAS) were used, respectively, to evaluate the electrochemical charge and discharge reactions in terms of local and bulk structural changes, and variations in the oxidation states of the transition metal ions. Ni Kedge XAS data indicate the participation of nickel in reversible redox reactions, whereas Mn K-edge absorption spectra show that the manganese ions do not participate in the electrochemical reactions. Rietveld refinements of the oxygen occupancy during charge and discharge provide evidence of reversible oxygen release and re-accommodation by the host structure; this unique oxygen participation is likely the main reason for the anomalously high capacity of these electrodes. The HRPD data also show that during the early cycles, characteristic peaks of the Li 2 MnO 3 component disappear when charged to 4.7 V, but reappear on discharge to 2.5 V, consistent with a reversible lithium and oxygen extraction process. The results provide new insights into the charge compensation mechanisms that occur when high capacity, lithium-and manganese-rich electrode materials are electrochemically cycleda topic that is currently being hotly debated in the literature.