In intercalation and an Si-containing protective layer enhance the electrochemical performance of NaNi_0.5Mn_0.5O₂ for sodium-ion batteries

To enhance the electrochemical performance of NaNi_0.5Mn_0.5O₂ (NM) cathode material for sodium-ion batteries, a novel indium-intercalated NaNi_0.5-<i>x</i>Mn_0.5In_<i>x</i>O₂ cathode material enriched with oxygen vacancies was first synthesized <i>via</i> a high-shear co-precipitation method. Subsequently, a thin Si-containing protective layer was produced on the surface through the interfacial reaction between oxygen vacancies and tetraethyl orthosilicate. Results of performance evaluation and characterization tests, including <i>in situ</i> XRD, Ar⁺ sputtering XPS, and HAADF-STEM, indicated that the optimal sample Si_<i>y</i>@NaNi_0.497Mn_0.5In_0.003O_2-<i>y</i> exhibited superior initial discharge capacity of 125.0 mA h g^-1 (0.1 C) and excellent capacity retention of 98.4% after 100 cycles at 1 C. In particular, the Si<i>@</i>In-doped sample offered larger lattice spacing, higher Na⁺ diffusion rate and better conductivity than the In-intercalated sample and pristine NM. DFT calculations illustrated that In preferentially substituted the Mn site, while Si preferred to substitute the Ni site closer to the In-intercalating location. The Na⁺ diffusion energy barrier was greatly reduced with In intercalation and Si doping. Such a facile strategy to augment the lattice spacing of the O3-layer cathode while producing a thin protective layer using the oxygen vacancies on the surface has promising applications to explore new cathode materials with high electrochemical performance for sodium-ion batteries.