Although magnesiothermic reduction has attracted immense attention as a facile route for the fabrication of mass-scale Si nanostructures for high-capacity lithium-ion battery applications, its low conversion yield (<50%) and the discovery of a sustainable and low-cost precursor remain challenging. Here, an unprecedentedly high final conversion yield (>98%) of magnesiothermic reduction based on control of reaction pressure is reported. The successful use of sand as a nearly infinite and extremely low-cost source for the high-yield fabrication of nanostructured Si electrodes for Li-ion batteries is demonstrated. On the basis of a step-by-step analysis of the material's structural, morphological, and compositional changes, a two-step conversion reaction mechanism is proposed that can clearly explain the phase behavior and the high conversion yield. The excellent charge–discharge performance (specific capacities over 1500 mAh g-1 for 100 cycles) of the hierarchical Si nanostructure suggests that this facile, fast, and high-efficiency synthesis strategy from ultralow-cost sand particles provides outstanding cost-effectiveness and possible scalability for the commercialization of Si electrodes for energy-storage applications.