Sodium-ion batteries (SIBs) have attracted significant attention as a highly promising alternative to lithium-ion batteries for large-scale energy storage systems, owing to the crustal abundance and cost-effectiveness of sodium resources. However, the development of high-performance SIBs is hindered by the lack of negative electrode (anode) materials that deliver both structural stability and fast sodium-ion diffusion kinetics. Currently, conventional carbon-based materials suffer from safety risks associated with sodium dendrite growth at low potentials, while alloying-type materials undergo severe structural degradation due to massive volume changes. Sodium titanates have emerged as attractive candidates because of their safe operating potentials and near-zero-strain volume stability during cycling, but their practical development is constrained by poor intrinsic electronic conductivity, and a detailed understanding of their structural evolution during sodium-ion insertion and extraction remains limited.
Herein, a cobalt-doping strategy was implemented into a tunnel-structured sodium titanate to regulate its electronic structure and enhance its conductivity. A monoclinic Na4.5Co0.5Ti4.5O12 precursor was first synthesised via a solid-state reaction, which was then converted into a cobalt-doped NaCo0.5Ti7.5O8 phase through a proton-exchange process followed by high-temperature thermal calcination. This structural rearrangement produces a Freudenbergite-type structure constructed from edge and corner sharing (Ti,Co)O6 octahedra, which enclose one-dimensional (1D) open tunnels for rapid Na+ transport.
Although Rietveld refinement against powder X-ray diffraction (XRD) data confirmed the tunnel structure, it could not distinguish the precise occupancy and position of Co and Ti. To overcome this limitation, we used neutron powder diffraction (NPD) to leverage the strong contrast between their positive and negative neutron scattering lengths. X-ray absorption spectroscopy (XAS) and magnetometry (VSM) measurements then unambiguously confirmed the presence of high-spin Co2+ within the framework. When evaluated in sodium half-cells, the NaCo0.5Ti7.5O8 anode demonstrated good cycling stability. Finally, we used micro-electron diffraction (microED) to characterise the structures of the cycled electrode materials, revealing that the Freudenbergite-type framework and crystal symmetry remained highly stable even after long-term cycling.