Electrochemical Charge/Discharge Properties of Li Pre-doped Si Nanoparticles for Use in Hybrid Capacitor Systems

Abstract

To explore the utility of Si negative electrodes (NEs), we fabricated a new hybrid capacitor system using a Li pre-doped Si (Li alloyed Si) NE, denoted Si-CAP [Li alloyed Si | activated carbon (AC)]. The Si-CAP cell exhibited a relatively high charge/discharge capacity and higher cell voltage than that of a reference electric double-layer capacitor (EDLC) [AC | AC]. The effects of additives (vinylene carbonate or fluoroethylene carbonate (FEC)) in Li pre-doping on cell performance were also investigated, with the goal of forming suitable solid electrolyte interphase films on the Si NE surface. The discharge capacity per weight of Si, i.e., Si utilization ratio, of the cells was increased by the additives, especially FEC. Vacuum pressure impregnation (VPI) treatment prior to Li pre-doping was also performed to further increase the Si utilization ratio. The combination of both FEC addition and VPI resulted in a discharge capacity of ca. 700 mAh g(Si)−1, corresponding to a high energy density of 114 Wh kg(NE + positive electrode)−1. The Li pre-doping method also improved the rate capability; >60 Wh kg−1 was attained even at 1.0 mA cm−2 (ca. 2500 mA g(Si)−1), which was three times larger than that of the reference EDLC.