Energy density maximization of Li-ion capacitor using highly porous activated carbon cathode and micrometer-sized Si anode

• Very-high-energy-density Li-ion capacitor (LIC) developed • Highly porous activated carbon cathode and micrometer-sized Si anode used • 400 Wh kg −1 maximum energy density at 32 W kg −1 power density achieved • > 10 kW kg −1 high power density available even at 100 Wh kg −1 energy density • Adequate current response of micrometer-sized Si for LIC anode recorded A very-high-energy-density Li-ion capacitor (LIC) was developed using commercially available activated carbon (AC) having a high surface area of 3041 m 2 g −1 and conventional 2 μm-sized Si as the cathode and anode material, respectively, at an AC:Si mass ratio of 6.37. The LIC delivered a maximum energy density of 400 Wh kg −1 at a power density of 32 W kg −1 and exhibited excellent rate stability (200 Wh kg −1 at 6 kW kg −1 ) over the cell voltage range of 1.0−4.3 V; these values were based on the total mass of the active materials in the cathode (AC) and anode (Si). However, its low cycling stability induced a 79.9% decrease in its energy density (278 to 56 Wh kg −1 ) following 1000 cycles at ∼650 W kg −1 . The potential variations of the cathode and anode were investigated during charge/discharge cycling. Postmortem electrode analyses indicated that the anode permitted the delamination of Si particles and the formation of a thick passive solid electrolyte interphase layer (60 nm) mainly constituted of LiF. Reducing the cell voltage range to 2.0−4.0 V and the mass ratio to 3.30 reduced the energy density to 183 Wh kg −1 but improved the sustainability (176 to 156 Wh kg −1 , 88.6% retention) and maintained the power density (> 10 kW kg −1 at 100 Wh kg −1 ) following 2000 cycles at ∼1 kW kg −1 . Graphical abstract