Hollow silica spheres encapsulated with nitrogen-doped carbon frame as an anode for high performance Li-ion capacitor: Correlating experimental and theoretical studies

The sluggish reaction kinetics of the silica (SiO ₂) electrode due to low electrical conductivity and high-capacity degradation due to volume expansion result in poor electrochemical performance. To overcome these drawbacks, we have fabricated hollow SiO ₂ microspheres (HSiO ₂) encapsulated in an N-doped carbon frame (C[sbnd]N) using a simple template-based method followed by the carbonization treatment. The significance of such a unique structural architecture is that the HSiO ₂ microspheres provide additional space for volume expansion which restores the structural integrity of the hollow SiO ₂ microspheres through buffering effect. At the same time, the C[sbnd]N enhances electrical conductivity, which accelerates Li ⁺ diffusion and promotes high charge accommodation. As a result, the collective contribution of HSiO ₂ and C[sbnd]N results in superior electrochemical performance in terms of improved capacity, accelerated reaction kinetics (rate capability), and higher cyclic stability. The density functional theory (DFT) validates the enhanced interactive characteristics of the HSiO ₂/C[sbnd]N composite electrode with Li ions, which include improved charge transfer kinetics, high binding energy, and low migration barrier. The HSiO ₂/C[sbnd]N composite electrode employed in the lithium-ion capacitor (LIC) demonstrates exceptional performance, with a remarkable energy density of 161.76 Wh/kg, a high power density of 23.52 kW/kg, and an outstanding stability of 87 % for 20,000 cycles, outperforming most of the previously reported anodes based on sulfides and oxides.