Mesoporous Mn-doped and carbon-coated NaTi₂(PO₄)₃ nanocrystals as an anode material for improved performance of sodium-ion hybrid capacitors

Sodium electrochemical energy storage systems including sodium-ion battery and hybrid capacitor are attracting increasing interest in large-scale electrical energy storage due to low cost and abundant sodium resources. In particular, sodium-ion hybrid capacitor with the merits of high energy and power densities as a typical next-generation energy storage device provides a promising alternative to the currently commercial lithium-ion battery. It is highly challenging to design and prepared suitable anode materials with extraordinary performance, thereby spurring its large-scale application. Herein, mesoporous Mn-doped and carbon-coated NaTi₂(PO₄)₃ nanocrystals are synthesized via a three-step process of solvothermal reaction, serum albumin decoration and thermal annealing. Synergistic effect rendered by NaTi₂(PO₄)₃ particles with mesoporous structure, Mn²⁺ doping and ultra-thin carbon coating endows the prepared composite with facile ion/electron transportation as well as excellent reaction kinetics as an anode material for sodium-ion hybrid capacitors. In the half-cell tests, the prepared composite exhibits high reversible capacity of 116 mA h g⁻¹ at 1 C, high-rate capability of 95 mA h g⁻¹ at 50 C, and long-term cycling stability with high capacity of 92 mAh g⁻¹ (88% capacity retention) after 1000 cycles at 20 C. Using the prepared composite as anode and activated carbon as cathode, the sodium-ion hybrid capacitor is assembled and it exhibits high energy/power density of 85.5 WhKg⁻¹/5531 WKg⁻¹. Our results indicate that Mn-doped and carbon-coated NASICON-type anodes have great potential in the sodium-based electrochemical energy storage systems.