Interconnected mesoporous Na₂FeSiO₄ nanospheres supported on carbon nanotubes as a highly stable and efficient cathode material for sodium-ion battery

Nanostructured sodium metal orthosilicates hold a lot of promise as next-generation cathodes but their practical application is hindered by the complex crystal structure and electrochemical behaviour. Herein, cubic polymorphs of Na₂FeSiO₄ with F-43 m symmetry decorated on carbon nanotubes have been synthesised successfully using a novel two-step process. The composite material with 0.08 wt% of carbon nanotubes demonstrate a capacity corresponding to a 1.25 sodium-ion exchange process resulting in 172.9 mAh g⁻¹ at 0.1C cycled in the voltage range between 1.5 and 4.5 V. This work also highlights the mechanism of insertion/extraction process using complementary techniques of X-ray photoelectron spectroscopy, and X-ray diffraction revealing the involvement of Fe 3 d band and conversion of Fe⁺² to Fe⁺³ during first sodium-ion extraction while second sodium-ion extraction is possible due to further oxidation of Fe⁺³. The discharge capacity is remarkable even at the high current rate of 20C, where we obtained the final capacity of 109.3 mAh g⁻¹. The excellent electrochemical performance reported here is due to the high structural stability of Na₂FeSiO₄ composed of rigid corner-sharing tetrahedra, improved electronic conductivity by carbon nanotubes, reduced particle size, increased defect structure, high sodium-ion diffusion coefficient and the decreased charge transfer resistances.