# Electochemically Engineered Mesoporous Sn-Oxide Rods for Anode Materials in Lithium-Ion Batteries
## 3 Results and Discussion
### Electrochemical Properties of Core (Sn Rod)-Shell (Mesoporous Sn-Oxide Layer) Anodes
The charge/discharge capacities and coulombic efficiencies of the first three cycles are summarized in Table 2. The initial discharge capacities of the core-shell anodes anodized at 6, 8, and 10 V were 796, 803, and 808 mAh/g, respectively, significantly higher than that of the Sn rod anode (645 mAh/g). However, the first-cycle coulombic efficiencies of the core-shell anodes were relatively low for all three samples (ranging from 52% to 57%), lower than that of the Sn rod anode (71%), and showed a decreasing trend with increasing anodization voltage. This reduction in the coulombic efficiency is likely due to an increase in irreversible capacity caused by Li2O formation during the initial charge. However, the core-shell anodes exhibited a smaller discharge capacity fade after the first cycle than the Sn rod anode, leading to a more pronounced capacity difference between the core-shell and the Sn rod anode over repeated cycles. Notably, the coulombic efficiencies of the core-shell anodes improved to approximately 87-89% after the first cycle, significantly exceeding that of the Sn rod anode (-83%). Figure 10a shows the discharge capacity retentions and coulombic efficiencies of the samples cycled at a current density of 100 mA/g after the initial three cycles. The Sn rod anode deteriorated rapidly from the beginning of cycling, with capacity retention decreasing to approximately 30% after the 10 th cycle. In contrast, the core-shell anodes showed a more gradual decline, with higher capacity retention observed at increased anodization voltage. It is speculated that, even with the use of a 1-D Sn rod structure, the pure Sn metal anode struggled to effectively accommodate volume changes. This limitation is attributed to the inability of surrounding conductive additives, binders, and electrode pores to absorb the volume expansion, leading to the Sn rods delamination. However, it is believed that by forming a porous oxide layer on the surface of the Sn rod, the active material itself was able to effectively accommodate the volume change, reducing active material delamination and improving cycle life. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Cycle Number**} & \multicolumn{3}{c}{**Specific Capacity (mA/g)**} & **Coulombic** \\ \cline{2-4}  & **Charge** & **Discharge** & **Efficiency (\%)** \\ \hline \multicolumn{4}{c}{**(a) Core-shell anode anodized at 6 V**} \\
**1** & 1395 & 796 & 57 \\
**2** & 836 & 725 & 87 \\
**3** & 734 & 662 & 90 \\ \hline \multicolumn{4}{c}{**(b) Core-shell anode anodized at 8 V**} \\
**1** & 1479 & 803 & 54 \\
**2** & 834 & 744 & 89 \\
**3** & 763 & 695 & 91 \\ \hline \multicolumn{4}{c}{**(c) Core-shell anode anodized at 10 V**} \\
**1** & 1566 & 808 & 52 \\
**2** & 865 & 765 & 88 \\
**3** & 800 & 726 & 91 \\ \hline \multicolumn{4}{c}{**(d) Sn rod anode**} \\
**1** & 907 & 645 & 71 \\
**2** & 632 & 526 & 83 \\
**3** & 514 & 441 & 86 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specific charge/discharge capacity and coulombic efficiency for the initial three cycles at a rate of 50 mA/g: core-shell anodes anodized at (**a**) 6, (**b**) 8, (**c**) 10 V; (**d**) Sn rod anode. Notably, higher capacity retention was observed in Sn-oxide layers formed at higher anodization voltages, which were characterized by larger pore diameters and thinner pore walls (Figure 5h). This strongly suggests that porous structures with open pores provide greater stress-relief effects compared with smaller pore diameters and thicker pore wall structures. Quantitatively, a porous Sn-oxide layer with surface pore diameters exceeding 50 nm--corresponding to films anodized at voltages above 8 V--is required to significantly enhance cycle life performance. Despite the notable improvement in capacity retention compared to the Sn rod anode, the still-low capacity retention rate (~73% at the 20 th cycle) indicates that further optimization is needed for practical applications. Figure 10b demonstrates the rate capabilities of the samples.