A symmetric sodium-ion battery based on P2-Na0.67[ZnMn1-]O2 as both positive and negative electrode materials

Sodium-ion batteries have been explored extensively due to its abundant reserve and low cost. However, reports on full symmetric battery with the same electrode materials are relatively less than asymmetrical battery. In this work, symmetric sodium-ion battery based on layered P2-Na0.67[ZnxMn1-x]O2 (x = 0.1, 0.2, 0.28, 0.34) as both positive and negative electrode materials are studied comprehensively. This active material shows two electrochemical working windows with reversible lattice oxygen redox and Mn4+/Mn3+ redox couple. The positive side Na0.67[Zn0.28Mn0.72]O2 provides a highly reversible capacity of 170 mAh g−1 at 0.05C (10 mA g−1) between 2.0–4.2 V and outstanding capacity retention of 77.2% after 100 cycles. Na0.67[Zn0.28Mn0.72]O2 electrode also exhibits excellent rate performance up to a high rate of 5C which is the best among the pure anionic-redox based cathodes. In addition, an average voltage of about 2.0 V versus Na+/Na is obtained as negative electrode material, with a reversible capacity of 99 mAh g−1. This full symmetric battery also exhibits excellent rate performance and cycle stability which are related to Zn doping. As a cost-effective element, zinc has been used as inactive atom to induce the anionic redox and enhance the crystal structure. This is the first time to explore symmetric sodium-ion battery with anionic redox.