Pseudocapacitance-Dominated Li-Ion Capacitors Showing Remarkable Energy Efficiency by Introducing Amorphous LiFePO4 in the Cathode

Li-ion capacitors (LICs) can be endowed with a satisfying energy density by introducing battery materials in the system, but the polarization of bulk-phase Li insertion, especially from phase transition, sacrifices the crucial energy efficiency at high-power delivery. We herein report the fabrication of a pseudocapacitance-dominated LIC showing advanced energy efficiency and rate capability. The key solution relies on the selective use of nanosized and amorphous LiFePO4 as a cathode, which exhibits pseudocapacitive Li insertion by a solid solution reaction mechanism, thus bypassing the undesired severe phase transition and sluggish ion diffusion in crystalline LiFePO4. After having matched with a highly amorphous carbon anode, the constructed pseudocapacitance-dominated LIC exhibits an excellent energy efficiency of 89% at a maximum energy density of 131 Wh kg–1. Even ultrafast charging at 25 000 W kg–1 (within 11 s), the energy density can still maintain up to 92 Wh kg–1 without the concession of energy efficiency, which is 62% higher than the conventional configuration based on an adsorption-type activated carbon cathode. This demonstrates the broad potential of the designed pseudocapacitance based on an amorphous structure for integrating energy density and energy efficiency in a power-type device like LIC.