# Electochemically Engineered Mesoporous Sn-Oxide Rods for Anode Materials in Lithium-Ion Batteries
## 1 Introduction  
The demand for eco-friendly electric vehicles (EVs) has increased significantly in recent years due to stricter environmental regulations on carbon emissions [1, 2]. Lithium-ion batteries (LIBs) are considered the most suitable option for EVs because of their high energy density, power density, and long cycle life [3, 4, 5]. However, the energy density of commercial LIBs (150-200 Wh/kg) remains insufficient to completely replace fossil fuel-based systems [5, 6]. To address this limitation, significant research efforts have focused on developing next-generation electrode materials, including high-voltage cathodes and high-capacity anodes, to enhance energy density [7, 8, 9, 10].  
Among the promising anode materials, Li-alloying materials, such as silicon (Si), tin (Sn), and phosphorus (P), offer theoretical specific capacities significantly higher than graphite (372 mAh/g) [11, 12, 13, 14]. Sn-based anodes, particularly, are attractive due to their high theoretical capacity (994 mAh/g), low cost, and non-toxic nature [15, 16, 17, 18]. However, Sn undergoes substantial volume change (\(\sim\)260%) during lithiation/delithiation, which has long hindered its application in LIBs [15, 16, 17, 18, 19]. Repeated expansion and contractionof the Sn anode cause a loss of contact between the active material and the conductive additive. In severe cases, this results in the pulverization of the active material and the delamination from the current collector, which in turn causes rapid capacity fading over repeated charge-discharge cycles [19; 20; 21].  
Various strategies have been explored to mitigate volume change issues in Sn-based anodes. Sn-alloys [15; 16; 17; 18; 22; 23; 24] and Sn-oxides [25; 26; 27; 28; 29; 30; 31; 32] incorporating inert elements have been investigated as alternatives to pure Sn. In Sn-alloys, inert metals act as a buffer matrix, absorbing stress and preserving electrode integrity [15; 16; 17; 18; 22; 23; 24]. In Sn-oxides (SnO\({}_{\rm x}\)), Li oxide (Li\({}_{2}\)O) formed during the initial charging also serves as a buffer matrix, helping maintain the electrode structure [25; 26; 27; 28; 29; 30; 31; 32]. Another approach to preventing mechanical degradation is microstructural control, which involves reducing the particle size to the nanoscale or designing porous, hollow, sheet, and rod structures [16; 17; 18; 29; 30; 31; 32; 33; 34; 35; 36]. These engineered structures relieve internal stress, minimize volume changes, and improve electrochemical performance by shortening lithium-ion diffusion paths. However, most studies have relied on hydrothermal synthesis for fabricating nanoparticles, which requires high temperatures, elevated pressures, and long processing times [29; 30; 31; 32; 33]. In contrast, electrochemical approaches such as electrodeposition and anodization, which have not been widely studied, enable precise microstructural control under ambient conditions in a short time. These methods offer greater cost-effectiveness and convenience while allowing for adjustable material properties by varying electrochemical parameters [34; 35; 36; 37; 38; 39].  
In this study, we report the fabrication of core-shell structures, where an Sn rod is covered with a mesoporous Sn-oxide shell, using electrodeposition followed by anodization. One-dimensional (1-D) Sn micro-rods were fabricated through single-step electrodeposition under different voltages, current densities, and deposition durations. Subsequently, mesoporous Sn-oxide layers with varied pore sizes and wall thicknesses were formed on the Sn rod surface through anodization. These fabricated materials were evaluated as anode active materials for LIBs, and their electrochemical performances were analyzed in relation to Sn-oxide microstructure. Finally, we discuss the advantages and limitations of applying the core-shell structures as Sn-based anode materials for LIBs.