An Advanced Symmetric Sodium Ion Cell Based on O3-Type NaNi0.33Li0.11Ti0.56O2

P2-type layer Na 0.66 Ni 0.33 Ti 0.67 O 2 is a promising active host for symmetric sodium ion cell as both anode and cathode material due to the high-voltage redox couple Ni 2+ /Ni 4+ (about 3.6V) and the low-voltage redox couple Ti 4+ /Ti 3+ (about 0.6V) [1] . In this work, a new compound of O3-type NaNi 0.33 Li 0.11 Ti 0.56 O 2 was proposed and synthesized by introducing Li atom into Ti sites in P2-type Na 0.66 Ni 0.33 Ti 0.67 O 2 . A symmetric sodium-ion cell based on the O3-type NaNi 0.33 Li 0.11 Ti 0.56 O 2 was further assembled. The cell exhibits a high voltage of 3.0 V and a reversible charge/discharge capacity of 80 mAh g -1 . The electrochemical performance of the as proposed cell is superior to that of the symmetric cell based on P2-type Na 0.66 Ni 0.33 Ti 0.67 O 2 . As shown in Fig. 1, the crystal structure and phase purity of the compounds are confirmed from powder XRD patterns. The particle size of the NaNi 0.33 Li 0.11 Ti 0.56 O 2 material varies in the range from 2 to 5 μm. Fig. 2a shows the schematic of the symmetric sodium-ion cell. Fig. 2b and 2c represent the CV and typical charge-discharge profiles of the NaNi 0.33 Li 0.11 Ti 0.56 O 2 material as anode, cathode and full cell, respectively. In contrary to P2-type Na 0.66 Ni 0.33 Ti 0.67 O 2 , Li-substituted NaNi 0.33 Li 0.11 Ti 0.56 O 2 shows an improved cycling stability, especially as cathode (Fig. 3b). It is obvious that the NaNi 0.33 Li 0.11 Ti 0.56 O 2 -based symmetric full cell has a good cycle life and excellent charge/discharge capacity (Fig. 3c). [1] R. Shanmugam, W. Lai, ECS Electrochemistry Letters, 3(4), A23-A25 (2014). Figure 1