Development of Porous Silicon Nanowires with Conductive Paths and Large Specific Surface Area for Application to Lithium-Ion Batteries

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The potential of silicon-based anodes, with their high specific capacity and abundant reserves, has sparked significant interest as a potential alternative to commercialized graphite anodes, paving the way for a promising future for energy materials. However, there are major challenges in the use of silicon. There are two main challenges with applying Si anodes: huge volume variations during lithiation and delithiation processes, and unstable surface electrolyte interphase (SEI) films, which cause pulverization and low cycling efficiency. The silicon nanostructure is a solution for preventing damage caused by large volume variations. However, a simple nanoparticle structure cannot obtain cycle characteristics that are conducive to practical use. Due to the volume change, the adhesion among the silicon nanoparticles, the binder, and the conductive material, and the cycle characteristics deteriorate. In contrast, porous silicon nanowires, with their unique structure and properties, offer a potential solution to these challenges, demonstrating their superiority over other materials.In this study, we introduced a groundbreaking innovation: porous silicon nanowires. These nanowires, created using the pioneering metal-assisted chemical etching method, possess a significant surface area and conductive path. This original development holds vast potential for energy materials and lithium-ion technology, stirring curiosity and anticipation in the scientific community.Silver nanoparticles were fabricated on silicon wafers by electroless plating. The silicon wafer was then immersed in an HF + H2O2 solution. The porous silicon nanowires detached automatically from the silicon wafer. Composite films of porous silicon nanowires and carbon nanotubes were fabricated without a binder or conductive material. We characterized the film removed from water and dried using an FE-SEM (JSM-7001F) and an optical microscope (VHX-7000). By contrast, the porous silicon nanowire's mean pore diameter and surface area were used to determine its porosity. The Brunauer–Emmett–Teller (BET) method (BELSORP-MINIX) was used to determine the surface area. Constant current charge/discharge cycling was carried out in a glove box.Our FE-SEM analysis revealed that the porous silicon nanowires were 70 nm in diameter and 10 μm in length, with pores in the range from 1 nm to 10 nm. These experiments achieved 480 m2/g of a groundbreaking specific surface area of silicon. By combining these porous silicon nanowires with carbon nanotubes, we formed a negative electrode without the need for a binder, as they were naturally intertwined. Our study demonstrated that the reversible capacity showed significantly higher stability during the first 100 cycles and remained at 1,966 mAh g-1 after 100 cycles. In combination with the higher silicon doping concentration, the larger number of pores may provide more stability for lithiation/delithiation. Because the charge/discharge characteristics did not alter the film structure, we concluded that porous silicon nanowires hold promise as negative electrodes in practical applications. Acknowledgment This work was supported by JST GTEX Program (Grant Number JPMJGX23S8 and JPMJGX23S5, Japan)