Pseudocapacitive Ti-Doped Niobium Pentoxide Nanoflake Structure Design for a Fast Kinetics Anode toward a High-Performance Mg-Ion-Based Dual-Ion Battery.

Magnesium-ion batteries (MIBs) have received increasing attention for next-generation energy storage recently because of the natural abundance, high capacity, and dendrite-free deposition of Mg. However, their applications are hindered by irreversible Mg anode plating in conventional electrolytes and the lack of cathode materials, demonstrating high working voltage, satisfactory Mg2+ diffusivity, and long cycling life. In this work, we first developed a novel magnesium-ion based dual-ion battery (Mg-DIB) by utilizing expanded graphite as the cathode and Ti-doped niobium pentoxide nanoflakes (Ti-Nb2O5 NFs) as the anode. The Ti-Nb2O5 NFs showed hierarchical structures of microspheres with diameters of 4-5 μm assembled by nanoflakes. For the first time, the Mg-ion storage mechanism in Ti-Nb2O5 NFs was investigated. Benefiting from the hierarchical structure design and pseudocapacitive intercalation behavior of Mg ions, the Ti-Nb2O5 NF anode exhibited fast Mg-ion diffusion. Consequently, the Mg-DIB exhibited a high discharge capacity of 93 mA h g-1 at 1 C (1 C corresponding to 100 mA g-1), along with good long-term cycling performance with a capacity retention of 79% at 3 C after 500 cycles. The Mg-DIB also demonstrated a capacity retention of 77% at 5C, indicating its good rate performance. Moreover, the Mg-DIB exhibited a high discharge medium voltage of ∼1.83 V, thus enabling a high energy density of 174 W h kg-1 at 183 W kg-1 and 122 W h kg-1 at a high power density of 845 W kg-1, among the best of the reported magnesium-ion full batteries. Our work provides a new strategy to improve the performance of MIBs and other rechargeable batteries.