Phytic acid assisted formation of P-doped hard carbon anode with enhanced capacity and rate capability for lithium ion capacitors

The unsatisfactory capacity and rate capability of hard carbon still restrict its practical application for lithium ion capacitors (LICs) and heteroatoms doping is an effective strategy to improve the electrochemical performance of carbon. Herein, the P doped hard carbon (PHC-4) with high P content (3.44 at.%) is synthesized via the esterification reaction between starch and phytic acid, following by the carbonization. Phytic acid acting as three function including the crosslinker, the activation agent and the hetetoatom dopant, can suppress orderly stacking of graphene layers, increase specific surface area and improve the P content, which not only provides more active sites availing fast surface redox reactions but also facilitates the insertion/extraction of Li ions. As a result, PHC-4 presents a high reversible capacity of 1040.2 mA h g−1 at 0.05 A g−1 and superior rate capability (231.7 mA h g−1 at 6.4 A g−1). The kinetic analysis demonstrates that lithium ion storage is dominated by the surface-induced capacitive process. Furthermore, PHC-4//AC LICs assembled using pre-lithiated PHC-4 anode and activated carbon (AC) cathode deliver an energy density of 104 Wh kg−1, a power density of 11.9 kW kg−1 and a cyclic stability of 84.7% retention after 5000 cycles at 1.6 A g−1.