Carbon-Coated Na2.2V1.2Ti0.8(PO4)3 Cathode with Excellent Cycling Performance for Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries (SIBs) show promising applications in fields of sustainable energies such as wind and solar, owing to wide availability of sodium resources and incombustibility of aqueous electrolytes. However, they are still challenged by poor cycling performance of Na-storage electrode materials. Herein, carbon-coated Na2.2V1.2Ti0.8(PO4)(3) is reported as a novel cathode for high-performance aqueous SIBs, which is obtained by studying the effect of Ti substitution on the charge/discharge properties of a Na3-xV2-xTix(PO4)(3) (0 <= x <= 1) system in 6 M NaClO4 aqueous electrolyte. Experimental results indicate that the material is composed of approximately 200 nm Na2.2V1.2Ti0.8(PO4)(3) crystals uniformly coated with a thin layer of carbon (ca. 4 nm). During Na extraction/insertion, it undergoes a one-step, two-phase reaction mechanism through the reversible electrochemistry of the V4+/V3+ redox couple, showing a reversible capacity of around 62 mAhg(-1) at the current rate of 1 C and a well-defined potential plateau of 0.50 V (vs. Ag/AgCl). More importantly, it achieves outstanding high-rate capability with a discharge capacity of approximately 40 mAhg(-1) at the current rate of 10 C, and excellent cycling performance with capacity retention of 93.4% after 500 cycles. The good electrochemical performance is attributed to the unique Ti-substituted composition, thin-layered carbon coating and high-concentration electrolytes.