Solid-state synthesis improves stability and cycling performance of layered sodium oxide cathodes: A solid-state NMR study

Improving the electrochemical performance and stability of layered sodium oxide cathode materials is of essential importance for developing sodium -ion batteries (SIBs). Recent studies showed that additional capacity can be obtained by triggering the O2-/(O2)n- redox reactions at high voltages (>4.0 V) for a novel Li -doped P2Na2/3Ni1/3Mn2/3O2. Here, we present that surprisingly, such an involvement of lattice oxygen redox reaction can be suppressed by using solid-state synthesis, which leads to improved cycling stability upon simplification of the preparation procedure. Specifically, P2-Na0.79Li0.11Ni0.21Mn0.67O2 was prepared via a solid-state synthetic approach and showed no sign of lattice O2-/(O2)n- redox reaction at high voltages (>4.0 V) compared to the solgel synthesized analogy. The former yields higher crystallinity and much less defects or impurities, as demonstrated by 23Na and 7Li solid-state NMR spectroscopy. Such a material remains more stable at > 4.0 V, evidenced by the inhibition of LiTM to LiAM migration (transition metal layer to alkaline metal layer), which is usually a sign of phase change or meta -phase changes upon charging.