Boosting tunnel-type manganese oxide cathodes by lithium nitrate for practical aqueous Na-ion batteries

Aqueous Na-ion batteries are proposed as cheap, safe, environmentally friendly systems for large-scale energy storage owing to the high abundance of sodium in earth's crust and the benign nature of most of its salts. Practical utilization, however, is limited by poor electrochemical performance due to the slow diffusion kinetics of large sodium ions. Here, lithium nitrate was added to the electrolyte solutions to boost the performance of sodium manganese oxide cathodes. Ultrafast rate capability, high ion diffusivity, and superior cycling stability are attributed to enhanced conductivity of the ions in the solution, cointercalation of Li and Na ions, and lower cathode surface resistance. Three-dimensional bond valence maps illuminate the intercalation mechanism of sodium ions in the host structure. Lithium ions establish additional diffusion paths that activate sodium sites. Multistack cells were constructed and showed good electrochemical performance and high mechanical flexibility, which can be exploited to elaborate very effective practical batteries.