Engineering of a TiO2 anode toward a record high Initial coulombic efficiency enabling high-performance low-temperature Na-ion hybrid capacitors†
Abstract
Initial coulombic efficiency (ICE) is an important evaluation index to weigh the applicability of electrode materials, but it is not acknowledged by many researchers. Herein, S-doped TiO2 nanosheets with porous and layered structures are utilized as anodes to fabricate hybrid sodium-ion capacitors in an ether electrolyte with high performance, especially through dramatically improved ICE. For Na-ion half cell tests in a DME electrolyte, S-doped TiO2 nanosheets display record high ICE of 88.6%, excellent rate capability and good cycling performance. We also find that TiO2 material with a large surface area is helpful for reducing the first irreversible capacity in an ether electrolyte, which is different from that observed using a traditional ester-based electrolyte. This could be due to excellent electronic conductivity with charge resistance of ∼1 Ω through the construction of an ultrathin solid electrolyte interphase (SEI) layer. Coupling with an Na3V2(PO4)3 cathode, we verify a successful Na-ion hybrid capacitor, delivering high energy and power density values of 158 W h kg−1 and 1075 W kg−1, respectively, at room temperature. Moreover, it also exhibits satisfactory performance of 82 W h kg−1 at −20 °C and outstanding cycling performance with over 95% retention after 800 cycles even at 1 A g−1 charge and discharge rate.