Probing sodium storage mechanism in hollow carbon nanospheres anode using liquid phase transmission electron microscopy

Abstract text (incl. figure legends and references)

Carbon-based electrode materials are considered promising anode materials for sodium-ion batteries. Understanding the Na transport and storage mechanism is crucial for developing more reliable anode materials. However, it has remained elusive. [1] The combination of an enclosed chamber with other stimuli in TEM brings advances in in-situ/operando detecting of dynamic phenomena in the liquid phase with high temporal and spatial resolution without risking sample modification in vacuum. Here, we synthesized hollow carbon spheres (HCSs) with tailored composition and desirable morphology for probing the Na-storage behavior in carbon-based electrode materials by means of in-situ TEM. [2]

We tracked the absorption of Na ions at the slope region of discharging nitrogen-doped porous HCSs (NPHCSs), which is followed by Na deposition at the potential plateau under the overpotential condition, by step-wise SEM-EDX experiments. NPHCSs surface remains partially covered after Na stripping (Fig. 1). We monitored the dynamic procedure of SEI formation and Na deposition within and around the NPHCSs by running micro batteries liquid phase TEM experiments (Fig. 2a-c). We observed the carbon shell expansion as a result of Na filling, SEI layer assisted NPHCSs binding, and partially reversible Na deposition (Fig. 2d-f). Na filling into carbon shells was further confirmed by energy-filtered TEM. Moreover, the electronic and ionic distribution after sodiation suggests the existence of Na0 and Na+1 within NPHCSs (Fig. 2g-h).

At the beginning of sodiation, Na absorbs on the surface of the nanospheres, accompanied by SEI formation essentially filling gaps between nanospheres within the NPHCS-based electrode. Consequently, this facilitates the Na ions' transport into the hollow spheres or through the tunnels between them until the current collector is reached. Although the curvature-induced electron density gradient in the hollow spheres can partially drive Na deposition inside the spheres at a low cycling rate, most Na deposition takes place underneath the electrode layer. Both Na storage paths contribute to improving the safety of sodium-ion batteries by preventing dendrite growth on the anode.

We gratefully acknowledge financial support of the Max Planck Society and European Research Council (ERC).

Reference:

[1] C. Vaalma, et al. Nat. Rev. Mater. 2018 34 2018, 3, 1.

[2] Z. L. Wang, et al. J. Phys. Chem. Solids 2000, 61, 1025.

Fig. 1: SEM images of NHPCSs at different stages of sodiation/disodiation and the schematic illustration of sodiation procedure with Na content detected by EDX.

Fig. 2: (a-c) In-situ STEM image of partially sodiated NPHCSs during cycling. Na is highlighted in yellow. Hollow arrows in (a) indicate the empty spheres while solid arrows in (b) indicate the Na-filled spheres. (d) STEM-HAADF image of pristine NPHCSs. STEM-BF(e) and HAADF(f) image of cycled NPHCSs. (g) Low-loss EELS comparison of pristine and cycled particles. (h) EFTEM comparison of pristine (black) and cycled particles (red). Electron density and sodium mapping are corresponding to the selected energy range between 4-7eV and 30-33 eV respectively, as highlighted in yellow in (g).