Optimization of Soft Carbon Negative Electrode in Sodium-Ion Batteries Using Surface-Modified Mesophase-Pitch Carbon Fibers (Supporting Information)
Current efforts to improve sodium-ion batteries are heavily focused on developing high performance carbon materials for the negative electrode. With significant research, hard carbons have come to show massive storage capacities and fast discharge rates. On the other hand, soft carbons have received very little attention, though they likewise encompass a wide variety of materials with structures highly dependent on the starting material and preparation temperature. In our contribution, we systematically evaluate the electrochemical performance of soft carbon electrodes made from mesophase-pitch carbon fibers (MCF). By using felt electrodes, we evaluate the cyclic voltammetry of MCFs prepared at 600–1300 °C and show the best performance with MCF prepared between 700–950 °C. In addition, using a surface modification step with silver showed significantly improved voltammetry for all the materials. Electrochemical impedance measurements further indicated that the surface modification step could decrease both of charge transfer resistances and film resistances attributed to the solid electrolyte interphase. Upon comparing lithium- and sodium-cell, it was revealed that sodium-cell demonstrated more significant increase in current density and decrease in resistance through surface treatment. We further verified our results with measurements on single-fiber electrodes; an increase in currents and a decrease in impedance were also observed by the surface modification, as with the felt electrodes. Overall, we speculate our surface modification removes inhibitors, such as functional groups or impurities, on the MCF surface to prevent sluggish ion transfer or trapping during sodium insertion/extraction.