Silicon Nanowires Diameter Modification for the Lithium-Ion Batteries

Abstract

Silicon has been considered a prospective anode material for lithium-ion batteries (LIBs) due to its abundant reserves and high theoretical specific capacity (about 10 times higher than the graphite anode). However, the significant volumetric expansion of bulk silicon anode during cycling, which causes cracking and pulverization, results in irreversible capacity decay and a shorter lifetime compared to other anode materials.[1, 2, 3] The general strategy for performance and stability enhancement is the use of nanostructured silicon materials. Various nanostructured silicon electrodes had been developed, such as nanoparticles, nanorods, nanotubes, and hollow nanostructures.[4, 5] In this work, we demonstrate silicon nanowires (Si NWs) directly grown on current collecting substrates by a vapor-liquid-solid (VLS) approach that is capable of achieving a reversible capacity of over 2200 mAh g-1 with good cycling stability. The improved electrochemical performance, compared to previous studies, may be due to faster electron transport, higher surface reactivity and mechanical stability with the growth substrate. Additionally, in this study, we investigate a feasible approach for controlling Si NWs diameters to optimize electrochemical performance. Figure 1. Si nanowires growth scheme. (I) Bare stainless-steel mesh, which was (II) coated by different size tin (Sn) catalyst before the reaction, (III) the dense Si nanowires were anchored on the substrate.The vapor-liquid-solid (VLS) approach that we used is more economic and safer in producing pure Si NWs compared to the other methods, such as metal-assist etching, chemical vapor deposition, and oxide-assisted/magnesium thermal evaporation.[4] The difficulties for Si NWs synthesis are that it has a high melting point and good mechanical property as a semiconductor, which makes it harder to break it down from bulk Si material to nanowires.[4] On the other hand, various silicide might be formed instead of pure Si in the synthesis reaction, as the decomposition of Si precursor and nanowires nucleation happen at the same time, causing the undesirable specific capacity in the further application.[6] As shown in Figure 1, Si NWs were grown on the Sn-coated stainless-steel mesh after injecting the Si precursor. During the reaction, the specific size Sn catalyst is melted and formed as droplets on the substrate, the silane (vapor) is decomposed from phenyl silane and absorbed into the Sn droplet (liquid), Si NWs (solid) are then gradually extracted from the droplet when it got saturated.The electrochemical performance of the self-supported Si NWs/Stainless-steel mesh (SSM) electrodes show that Si NWs with the bigger diameter (SSM01 ~ 120 nm, SSM02 ~ 80 nm) delivered higher initial specific capacity (3761 mAh g-1 and 2871 mAh g-1 separately) but poor stability and decayed ~ 50% at the 100th cycle. The largely irreversible capacity loss may be due to bigger nanowires losing electronic contact after the initial volume change. The smaller wires (SSM03 ~ 41 nm, SSM05 ~ 45 nm) maintained ~ 80% capacity retention but achieved lower initial specific capacity (2201 mAh g-1 and 1872 mAh g-1 separately). This result illustrates that the stability can be improved by nano-sizing the electrode material, as the smaller diameter nanowires may result in faster ion transfer, while the unique electrode architecture leads to electrodes with higher surface reactivity, conductivity and improved robustness and electrode stability. This approach not only eliminated the use of excess explosive gas and the possibility of silicide impurity production but also avoids the energy-intensive top-down process.