Thin solid electrolyte interface on chemically bonded Sb₂Te₃/CNT composite anodes for high performance sodium ion full cells

Nanostructured metal chalcogenides (MCs) and their composites are studied for high performance sodium-ion batteries (SIBs). Herein, we report the assembly of an emerging MC, Sb₂Te₃, with functionalized carbon nanotubes (CNTs) to form composite anodes. The role of oxygenated functional groups on CNTs in fostering the chemical interactions with Sb₂Te₃ for enhanced structural integrity of electrodes is elucidated by density functional theory combined with ab-initio molecular dynamics simulations and X-ray photoelectron spectroscopy analysis. Remarkably, cryogenic transmission electron microscopy (TEM) analysis reveals a uniform and thin solid electrolyte interface (SEI) layer of ~19.1 nm on the Sb₂Te₃/CNT composite while the neat Sb₂Te₃ presents an irregular and ~67.3 nm thick SEI. The ex-situ X-ray diffraction (XRD) and ex-situ/in-situ TEM analyses offer mechanistic explanations of phase transition and volume changes during sodiation. The Sb₂Te₃/CNT composite electrode with an optimal content of 10 wt% CNTs delivers excellent reversible gravimetric and volumetric capacities of 422 mA h g⁻¹ and 1232 mA h cm⁻³, respectively, at 100 mA g⁻¹ with ~97.5% capacity retention after 300 cycles. The excellent high-rate capability of 318 mA h g⁻¹ at 6400 mA g⁻¹ corroborates the structural robustness of the composite electrode. Sodium-ion full cells (SIFCs) are assembled by pairing the above anode with a Na₃V₂(PO₄)₂F₃ cathode, which exhibit remarkable energy density of ~229 Wh kg⁻¹ at 0.5 C and excellent cyclic stability of over 71% and 66% capacity retention at 5 C and 10 C, respectively, after 200 cycles. Even at 40 C, an ultrahigh power density of 5384 W kg⁻¹ is delivered. Furthermore, the pouch-type SIFCs prove excellent flexibility with ~85% capacity retention after 1000 bending cycles and satisfactory operation under temperatures ranging from 40 to −20 °C. The design strategy developed here can also be employed to other electrode materials to achieve better SEI stability and excellent Na storage performance.