P2-type layer Na_0.66Ni_0.33Ti_0.67O₂ is a promising active host for symmetric sodium ion cell as both anode and cathode material due to the high-voltage redox couple Ni²⁺/Ni⁴⁺ (about 3.6V) and the low-voltage redox couple Ti⁴⁺/Ti³⁺ (about 0.6V) ^[1]. In this work, a new compound of O3-type NaNi_0.33Li_0.11Ti_0.56O₂ was proposed and synthesized by introducing Li atom into Ti sites in P2-type Na_0.66Ni_0.33Ti_0.67O₂. A symmetric sodium-ion cell based on the O3-type NaNi_0.33Li_0.11Ti_0.56O₂ 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_0.66Ni_0.33Ti_0.67O₂. 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_0.33Li_0.11Ti_0.56O₂ 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_0.33Li_0.11Ti_0.56O₂ material as anode, cathode and full cell, respectively. In contrary to P2-type Na_0.66Ni_0.33Ti_0.67O₂, Li-substituted NaNi_0.33Li_0.11Ti_0.56O₂ shows an improved cycling stability, especially as cathode (Fig. 3b). It is obvious that the NaNi_0.33Li_0.11Ti_0.56O₂-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).