High-performance symmetric sodium-ion batteries using a new, bipolar O3-type material, Na_0.8Ni_0.4Ti_0.6O₂

Based on low-cost and rich resources, sodium-ion batteries have been regarded as a promising candidate for next-generation energy storage batteries in the large-scale energy applications of renewable energy and smart grids. However, there are some critical drawbacks limiting its application, such as safety and stability problems. In this work, a stable symmetric sodium-ion battery based on the bipolar, active O3-type material, Na_0.8Ni_0.4Ti_0.6O₂, is developed. This bipolar material shows a typical O3-type layered structure, containing two electrochemically active transition metals with redox couples of Ni⁴⁺/Ni²⁺ and Ti⁴⁺/Ti³⁺, respectively. This Na_0.8Ni_0.4Ti_0.6O₂-based symmetric cell exhibits a high average voltage of 2.8 V, a reversible discharge capacity of 85 mA h g^-1, 75% capacity retention after 150 cycles and good rate capability. This full symmetric cell will greatly contribute to the development of room-temperature sodium-ion batteries with a view towards safety, low cost and long life, and it will stimulate further research on symmetric cells using the same active materials as both cathode and anode.