Porous Si@C ball-in-ball hollow spheres for lithium-ion capacitors with improved energy and power densities

The rate capability of battery-type anode materials has become a great limitation to the energy and power densities of lithium-ion capacitors. Besides the electronic conductivity, it is crucial to enhance the electrochemical rate by facilitating the diffusion and transportation of Li+. Herein, a new kind of porous Si@C ball-in-ball hollow structure was designed to build a stable electrode–electrolyte interface, overcome the volume change of Si and enhance the mass diffusion and conductivity simultaneously. The obtained porous Si@C ball-in-ball hollow spheres as Si anodes show great cycling stability for 1000 cycles and good rate capability with 943 and 586 mA h g−1 at 16.0 and 32.0 A g−1, respectively. As a result, lithium-ion capacitors based on porous Si@C ball-in-ball hollow spheres as anode materials give high energy densities of 239 and 154 W h kg−1 at the power densities of 1376 and ∼69 600 W kg−1, respectively. Furthermore, the lithium-ion capacitors also show a stable cycling performance for 15 000 cycles at 6.4 A g−1.