Masakazu Haruta

In situ Scanning Electron Microscopy of Silicon Anode Reactions in Lithium-Ion Batteries during Charge/Discharge Processes

A comprehensive understanding of the charge/discharge behaviour of high-capacity anode active materials, e.g., Si and Li, is essential for the design and development of next-generation high-performance Li-based batteries. Here, we demonstrate the in situ scanning electron microscopy (in situ SEM) of Si anodes in a configuration analogous to actual lithium-ion batteries (LIBs) with an ionic liquid (IL) that is expected to be a functional LIB electrolyte in the future. We discovered that variations in the morphology of Si active materials during charge/discharge processes is strongly dependent on their size and shape. Even the diffusion of atomic Li into Si materials can be visualized using a back-scattering electron imaging technique. The electrode reactions were successfully recorded as video clips. This in situ SEM technique can simultaneously provide useful data on, for example, morphological variations and elemental distributions, as well as electrochemical data.

Fluoroalkyl ether-diluted dimethyl carbonate-based electrolyte solutions for high-voltage operation of LiNi0.5Co0.2Mn0.3O2 electrodes in lithium ion batteries

The energy density of lithium-ion batteries can be increased by improving the battery voltage and/or specific capacity. However, conventional electrolyte solutions decompose oxidatively at high voltages. The stability against oxidation of electrolyte solutions could be enhanced by increasing the concentration of lithium salts. The nearly saturated 8.67 mol kg−1 LiBF4/dimethyl carbonate (DMC) electrolyte solution enabled high-voltage operation of LiNi0.5Co0.2Mn0.3O2 positive-electrodes at 4.6 V, and extended the discharge capacity to ca. 200 mA h g−1. BF4− anions, as well as DMC solvents, were stabilized at the high concentration. To reduce the viscosity and Li concentration without losing the high stability against oxidation toward practical use, the highly concentrated electrolyte solution was diluted with fluorinated co-solvents having a low donor ability. Raman spectroscopy revealed that the solvation structure of LiBF4/DMC was maintained after dilution with a specific fluoroalkyl ether. The resultant low-viscosity and -concentration electrolyte solution was highly stable against oxidation at the LiNi0.5Co0.2Mn0.3O2 electrodes as long as the DMC/LiBF4 molar ratio was kept low. The charge/discharge performance was much better than that for a conventional 1 M LiPF6/ethylene carbonate-based electrolyte solution and the energy density far exceeded that of 5 V class LiNi0.5Mn1.5O4 electrodes.