To meet the objectives of the Intergovernmental Panel on Climate Change nations are adopting policies to encourage consumers to purchase electric vehicles. Electrification of the automobile industry reduces greenhouse gases but active metals for the cathode-LiCoO₂ and LiNiO₂-are toxic and represent an environmental challenge at the end of their lifetime. LiFePO₄ (LFP) is an attractive alternative that is non-toxic, thermally stable, and durable but with a moderate theoretical capacity and a low electrical conductivity. Commercial technologies to synthesize LFP are energy-intensive, produce waste that incurs cost, and involve multiple process steps. Here we synthesize LFP precursor with lignin and cellulose in a sonicated grinding chamber of a wet media mill. This approach represents a paradigm shift that introduces mechanochemistry as a motive force to react iron oxalate and lithium hydrogen phosphate at ambient temperature. Ultrasound-assisted wet media milling increases carbon dispersion and reduces the particle size simultaneously. The ultrasound is generated by a 20 kHz,500 W automatic tuning ultrasound probe. The maximum discharge rate of the LFP synthesized this way was achieved with cellulose as a carbon source, after 9 h milling, at 70% ultrasound amplitude. After 2.5 h of milling, the particle size remained constant but the crystal size continued to drop and reached 29 nm. Glucose created plate-like particles, and cellulose and lignin produced spindle-shaped particles. Long mill times and high ultrasound amplitude generate smoother particle surfaces and the powder densifies after a spray drying step.