Interlayer-Expanded Titanate Hierarchical Hollow Spheres Embedded in Carbon Nanofibers for Enhanced Na Storage

Layered titanates are of great potential for hybrid Na-ion capacitors (NICs). However, the poor conductivity and sluggish reaction kinetics are the critical issues for the practical applications of titanates. Herein, an approach to synthesize magnesium titanate hierarchical hollow spheres embedded in carbon nanofibers (denoted as MTO@C) by electrospinning coupled with interlayer engineering processes is reported. 3D conductive carbon framework helps to enhance the electronic conductivity for binder-free electrode, while the expanded interlayer spacing of titanate hierarchical hollow spheres via the incorporation of Mg²⁺ ions help to reduce the charge transfer resistance and expose more active sites for Na storage. The interconnected hollow spheres can effectively accommodate the volume expansion during the repeated cycles. The results have shown that the MTO@C electrode can deliver a high capacity of 136 mAh g^-1 at 1 A g^-1 with long lifespan. The assembled NIC device with MTO@C as anode and active carbon as cathode produces a high energy density of 110.3 Wh kg^-1 at 112 W kg-¹ and a high power density of 5380 W kg^-1 at 41.9 Wh kg^-1 , together with a high capacity retention of 80% after 5000 cycles.