Solving the puzzle of Li4Ti5O12 surface reactivity in aprotic electrolytes in Li-ion batteries by nanoscale XPEEM spectromicroscopy

The safe operation and long life-span of Li-ion batteries rely on a stable electrode-electrolyte interface. However, determining the thermodynamic stability window of such an interface is challenging due to the different (electro) chemical reactivities of the electrode components. Here we demonstrate a holistic experimental and theoretical approach to elucidate the nature and origin of the multiple reactions at such complex interfaces, which remain a major obstacle for the development of next generation Li-ion batteries. We applied X-ray photoemission electron microscopy (XPEEM) on Li4Ti5O12 electrodes to solve, with nanoscale resolution, its controversial surface reactivity in carbonate-based electrolytes. Local X-ray absorption spectroscopy (XAS) is performed upon cycling on individual carbon and Li4Ti5O12 particles, while maintaining their working environment, as in the commercial-like electrode composition. Despite the theoretical prediction of a stable electrochemical interface, we find that electrolyte reduction occurs solely on Li4Ti5O12 particles during lithiation at 1.55 V vs. Li+/Li. With the support of density functional theory (DFT) calculations, we show that this behavior is caused by the solvents adsorbed on the Li4Ti5O12 outer planes driven by the Li-ion insertion. The DFT results indicate that Li-ion insertion leads to a shift of the LUMO of the adsorbed solvents to energies below the Fermi level position of lithiated Li7Ti5O12 and thus to chemical instability. Simultaneously, at the same potential, we detect a competing reaction that leads to the partial dissolution of the electrolyte by-product layer. Such a finding has to be considered for other insertion materials and needs to be addressed in surface engineering to mitigate side reactions and design safe and long-lasting batteries.