Silicon nanowires of high purity and regular morphology are of prime importance to ensure high specific capacities of lithium-ion batteries and reproducible electrode assembly process. Using nickel formate as a metal catalyst precursor, straight silicon nanowires (65–150 nm in diameter) were directly prepared by electrolysis from the Ni/SiO2 porous pellets with 0.8 wt% nickel content in molten CaCl2 at 900 °C. Benefiting from their straight appearance and high purity, the silicon nanowires therefore offered an initial coulombic efficiency of 90.53% and specific capacity of 3377 mAh/g. In addition, the silicon nanowire/carbon composite exhibited excellent cycle performance, retaining 90.38% of the initial capacity after 100 cycles. Whilst further study on the charge storage performance is still ongoing, these preliminary results demonstrate that nickel formate is an efficient and effective metal catalyst precursor for catalytic preparation of high purity straight silicon nanowires via the molten salt electrolysis, which is suitable for large-scale production.