# Electochemically Engineered Mesoporous Sn-Oxide Rods for Anode Materials in Lithium-Ion Batteries
## 3 Results and Discussion
### Sn Rod Formation Using Electrodeposition
In contrast, under potentiostatic conditions, the diameter of secondary rods remained constant, even with increasing voltage, enabling the continuous growth of separated Sn rods without agglomeration. The observed correlation between electrodeposition mode and rod agglomeration is quite notable, but the underlying mechanisms remain unclear. Nevertheless, it is strongly suggested that this is due to differences in the actual current applied to the electrode surface, for the following reasons: During galvanostatic deposition, the surface area of Sn rods on the working electrode increases continuously with electrodeposition time. Since the apparent current remains constant, the actual current density applied to the reactive surface decreases as the electrodeposition progresses. As the current density decreases, the nucleation of new Sn rods is suppressed due to the lower nucleation driving force, favoring the growth of pre-existing rods and leading to agglomeration. In contrast, under potentiostatic conditions, a constant overpotential is maintained, ensuring a steady electrochemical reaction driving force regardless of the surface area change. This suggests that the driving forces for nucleation and rod growth remain nearly unchanged, resulting in a more consistent actual current density at the working electrode surface. The increase in the apparent current densities over potentiostatic deposition time (Figure 4) supports this explanation. layer, allowing dissolution into the electrolyte to become dominant, thereby preventing the formation of porous layers. Quantitative analysis of Sn-oxide surface characterization in samples anodized above 6 V is shown in Figure 5h. The pore diameter increased from 27.1 nm at 6 V to 62.4 nm at 10 V, while the pore wall thickness decreased from 22.7 nm to 8.2 nm. The formation of more open porous structures (wide pores) at higher anodizing voltages is most likely attributed to the enhanced oxygen evolution reaction at higher voltages [36; 38]. The pore size and wall thickness of the Sn-oxide layer exhibited an exponential relationship with the anodization voltage, as indicated by the dotted lines in Figure 5h. Based on the fitted exponential curve, changes in the pore size and wall thickness became less significant above 10 V. If the anodization voltage is further increased, the pore size and wall thickness are predicted to converge at ca. 75.1 nm and 7.4 nm, respectively, which represent just 120% and 90% of the corresponding values observed at 10 V.  
Figure 6a-c show the composition analysis of the anodized samples. The Sn/O atomic ratio at the surface was approximately 5.49 and 3.61 for non-porous samples anodized at 4 and 5 V, respectively, but decreased to 1.07 for porous samples anodized at 6 V. This indicates that for porous Sn-oxide formed under oxidizing conditions above 6 V, the stoichiometry is close to that of ideal stannous oxide (SnO). The crystallographic analysis indicated no significant alterations in the XRD patterns of the anodized Sn rod compared to the pristine Sn rod (Figure 6d), confirming the low crystallinity of the porous Sn-oxide layer [39]. Observation of the cross-section of specimens with oxide formation under anodizing conditions above 6 V showed a core (Sn rod)-shell (mesoporous Sn-oxide) morphology, as expected (Figure 7). Quantitative analysis of the porous Sn-oxide layer revealed that its thickness increased with anodization voltage, reaching approximately 162, 263, and 351 nm at 6, 8, and 10 V, respectively. These three specimens were evaluated as anode active materials for LIBs in the following section. Figure 5: SEM images of Sn-oxide formed on the Sn rod surface by anodization for 60 s under various voltages of (**a**) 4, (**b**) 5, (**c**) 6, (**d**) 7, (**e**) 8, (**f**) 9, and (**g**) 10 V. The inset figures in (**c–g**) are magnified images. (**h**) Variations in pore diameter and pore wall thickness of Sn-oxide at different anodizing voltages, along with black and red dotted lines indicating their respective trends.