Lithium-rich oxide materials are promising candidates for high-energy lithium ion batteries, but currently have critical challenges of poor cycle performance and voltage drop induced by undesirable phase transformation. To resolve these problems, it is necessary to identify the origins and mechanism of phase transformation in Li 2 MnO 3 , a key component of Li-rich oxides. In this work, the phase transformation of bulk Li 2 MnO 3 is investigated by thermodynamic and kinetic approaches based on first-principles calculations and validated by experiments. Using the calculated thermodynamic energies, the most stable structure is determined as a function of Li extraction for Li 2Àx MnO 3 : monoclinic (x ¼ 0.0-0.75), layered-like (x ¼ 1.0-1.25), and spinel-like (x ¼ 1.5-2.0) structures. The phase transformation becomes kinetically possible for Li 2Àx MnO 3 (x > 1.0). Atomic scale origins and the mechanism of phase transformation are elucidated by the thermodynamically stable and kinetically movable tetrahedral coordination of Mn 4+ in the transition state. These theoretical observations are validated by ex situ X-ray photoelectron spectroscopy combined with electrochemical experiments for Li 2Àx MnO 3 with various Li contents upon cycling. The mechanistic understanding from theoretical calculations and experimental observations is expected to provide a fundamental solution and guidelines for improving the electrochemical performance of Li-rich oxides and, by extension, the battery performance.

As a promising hybrid energy storage system, lithium ion capacitors (LICs) have been intensively investigated regarding their practical use in various applications, ranging from portable electronics to grid support. The asymmetric LIC offers high-energy and high-power densities compared with conventional energy storage systems such as electrochemical double-layer capacitors (EDLCs) and lithium ion batteries (LIBs). To enable suitable operation of the LIC, the negative electrode should be pre-lithiated prior to cell operation, which is regarded as a key technology for developing self-sustainable LICs. In this work, we have demonstrated the potential use of Li 6 CoO 4 as an alternative lithium source to metallic lithium. A large amount of Li + can be electrochemically extracted from the structure incorporated into the positive electrode via a highly irreversible process. Most of the extracted Li + is available for pre-lithiation of the negative electrode during the first charge. This intriguing electrochemical behaviour of Li 6 CoO 4 is suitable for providing sufficient Li + to the negative electrode. To obtain a fundamental understanding of this system, the electrochemical behaviour and structural stability of Li 6 CoO 4 is thoroughly investigated by means of electrochemical experiments and theoretical validation based on first principles calculations.