Abstract:

With the development of clean and sustainable energy sources, the demand for large-scale electrochemical energy storage systems has rapidly increased over the last few years. Rechargeable Na-ion batteries (NIBs), one of the most promising energy storage technologies, have received a great deal of attention. Titanium-based P2-type layered oxides are attractive candidates for NIB anode materials, owing to their suitable redox potential, low cost, air stability and high safety. The exposed large interlayers of P2 configuration provide facile channels for Na⁺ insertion/extraction when employed as electrode materials for room temperature, non-aqueous NIBs. In this paper, a novel P2-type Na_0.65Li_0.13Mg_0.13Ti_0.74O₂ is synthesized by a solid-state reaction method. An orthorhombic phase of Na_0.9Mg_0.45Ti_1.55O₂ is observed with the increase in calcination time. During the long calcination process, it is speculated that some lattice Na⁺ and Li⁺ of the previously formed P2 phase compound would be volatilized or extracted by O₂, forming a low Na-content orthorhombic phase based on the layered host structure. In particular, when the precursor was calcined at 1273 K for 24 h, a perfect biphasic hybrid composite was synthesized. The Na storage performance of the pure P2 compound and hybrid composite were evaluated respectively in sodium half cells with voltage range of 0.2–2.5 V. The P2-type electrode can deliver a reversible capacity of 85.1 mAh·g^-1 (theoretical capacity of approximately 108.5 mAh·g^-1), whereas, the sample with the orthorhombic phase shows an enhanced initial reversible capacity of 96.3 mAh·g^-1. Both of the curves are smooth with no observed plateau, indicating the good structural stability of the electrode during cycling. Thus, the hybrid composite exhibits better cycling performance (capacity retention of 89.7% vs. 84.4% for pure P2, after 400 cycles at current density of 1C) and better rate capability (56.6 mAh·g^-1 at 5C vs 47.1 mAh·g^-1 at 2C). These results can be attributed to the introduced second phase, which improves the electron and bulk ion conductivity and helps stabilize the structure. Therefore, this novel two-phase intergrowth composite could serve as a promising anode candidate for the large-scale energy storage application of NIBs. Moreover, this structural design strategy could be used for other layered oxides to improve their energy density and cycling stability.

Key words: Na-ion battery, P2-type layered structure, Anode material, Second phase, High reversible capacity