Si has the highest theoretical gravimetric and volumetric capacity of any alloying element but suffers from poor electrical conductivity and large volumetric expansion that causes rapid cell failure. Conductive carbons can improve cycle life, however, all of the synthetic routes to Si/C composites reported to date have significant drawbacks ranging from lacking scalability, use of materials with lower earth abundance or high carbon impact, high cost, high-temperatures, low yield, requiring HF treatment, and requiring carbon coating. Here we report a scalable one-step synthetic method for nanocrystalline Si/C composites from earth abundant materials by low-temperature reaction in inexpensive solvents in the presence of a biomass derived, CO2 negative, conductive carbon (multiwall graphene nanoshells, MGNS). The resulting Si/MGNS composite is air-stable with very low surface oxide content without the need for HF treatment due to surface stabilization by Al, and can be purified by a simple HCl wash with synthetic yields greater than 90%. Li-ion battery anodes prepared with the Si/MGNS (40% Si) displayed excellent performance, with a reversible galvanostatic cycling capacity of 3454 mAh/g Si at 179 mA/g. Stable cycling at rates as high as 1.79 A/g was demonstrated over 75 cycles, with Coulombic efficiencies exceeding 99%. (C) 2017 The Electrochemical Society. All rights reserved.