context: Fig. 3 shows the electrochemical performances of a half-cell in a three-electrode system and a NVP//NTP full cell. As shown in Fig. 3a and b, the NVP half-cell exhibits a pair of redox peaks with good symmetry at 0.68 V and 0.42 V vs. Ag/AgCl, which can be ascribed to the reversible conversion of the V3+/V4+ couple, corresponding to the reversible extraction/insertion of Na+ in the NVP electrode. The half-cell delivers a specific charge and discharge capacity of 110 mA h g−1 and 96 mA h g−1 in the first cycle, and exhibits good reversibility for the first time. The irreversible charge capacity mainly resulted from the dissolution of the V-ions until the appearance of O2 evolution which was recorded in CV curves. For NTP, a pair of Ti4+/Ti3+ redox peaks at −0.55 V and −0.82 V vs. Ag/AgCl can be found in the CV curves. As seen in Fig. 3c, the CV curves show that the hydrogen evolution reaction still does not occur when the potential is −1.1 V vs. Ag/AgCl, which suggests that the Na–H2O–urea–DMF electrolyte can inhibit the side reaction well. As shown in Fig. 3d, the NTP in the Na–H2O–urea–DMF electrolyte delivers a pristine discharge capacity of 118 mA h g−1, concurrently exhibiting a perfect platform and good reversibility, which is better than that of a NTP anode with cationic doping. As shown in Fig. S10 and S11,† the NTP anode exhibits an impressive cycling stability in a conventional non-aqueous electrolyte. However, the coulombic efficiency and cycling performance of the NVP cathode at 2C in a conventional non-aqueous electrolyte are poor due to the dissolution of V-ions and change of structure.
answer: NTP