A chemically bonded NaTi2(PO4)3/rGO microsphere composite as a high-rate insertion anode for sodium-ion capacitors

Abstract

We report on the synthesis of a high rate NaTi₂(PO₄)₃/graphene composite for use as an anode material for high power Na-ion hybrid capacitors with the following characteristics; (1) reduction of the particle size of NaTi₂(PO₄)₃ to the nanometer scale in order to reduce the Na⁺ ion diffusion length, (2) chemical bonding between NaTi₂(PO₄)₃ nanoparticles and graphene in order to improve electrical conductivity, and (3) interconnected nanoporous structures in order to allow easy access of Na⁺ ions to NaTi₂(PO₄)₃. For this, the NaTi₂(PO₄)₃/rGO microsphere composite was prepared via a facile spray drying method using a solution mixture of graphene oxide, NaH₂PO₄·2H₂O, Ti(OC₂H₅)₄ and NH₄H₂(PO₄)₃, in which all the components of the titanium were present as ionic species in order to facilitate the chemical bonding between NaTi₂(PO₄)₃ and rGO in the composite. The NaTi₂(PO₄)₃/rGO microsphere composite had a Ti–O–C bond between NaTi₂(PO₄)₃ nanoparticles (<80 nm) and rGO and interconnected nanoporous structures. The NaTi₂(PO₄)₃/rGO microsphere composite exhibited a near theoretical specific capacity of 133 mA h g⁻¹ at a 0.1 C-rate and excellent rate capability (70% capacity retention at a 50 C-rate) with very stable cycling performance (only 2% capacity loss after 200 cycles at a high rate of 10C). Furthermore, the energy density and power density of the NHC assembled with a NaTi₂(PO₄)₃/rGO anode and an AC-based cathode are far better than those of other NHCs assembled using other metal oxide-based anodes and AC cathodes.