Optimized performance of Na_0.67Mn_0.5Fe_0.5O₂@TiO₂ and presodiated hard carbon (Pre-SHC) full-cells using direct contact method

We report the synthesis and electrochemical performance of an optimized core-shell structure composed of P2-type Na_0.67Mn_0.5Fe_0.5O₂ coated with TiO₂. The structural properties are characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), which confirm the successful formation of the core-shell structure. Electrochemical performance is evaluated through tests on half-cells and full-cells. Na_0.67Mn_0.5Fe_0.5O₂@TiO₂ as cathode and sodium metal as anode are used in half-cells, while in full-cells, presodiated hard carbon (Pre-SHC) anodes are prepared via a direct-contact method. Cyclic voltammetry (CV) tests show similar redox behavior for uncoated and TiO₂-coated Na_0.67Mn_0.5Fe_0.5O₂. Galvanostatic cycling tests are performed using two different voltage windows of 1.5–3.5 V and 1.5–4.3 V and capacity retention values are compared. Performance analysis of the full-cells reveals the best conditions for the presodiation process for the hard carbon (HC) anode. The first charge and discharge capacity values are used to determine the optimized presodiation conditions. Long-term cycling tests for both uncoated and TiO₂-coated Na_0.67Mn_0.5Fe_0.5O₂ cathodes show significantly improved capacity retention and stability for the Na_0.67Mn._0.5Fe_0.5O₂ @TiO₂ cathode over 500 cycles at 0.5 and 1.0C rates. This study highlights the effectiveness of the TiO₂ coating in enhancing the electrochemical performance and stability of Na_0.67Mn_0.5Fe_0.5O₂ cathode material.