Lithium-rich, composite-layered, transition metal oxides have been intensively investigated recently. However, poor rate capability has severely hindered the commercial development of these materials. We produce nanoscale hydroxide precursors for these cathodes using ultrasonic-assisted co-precipitation. The ultrasonic irradiation of a liquid leads to the generation of a cavitation phenomenon comprised of unique reaction fields, which assists in the synthesis of nanoparticle materials. This is confirmed by scanning and transmission electron microscopy. X-ray diffraction and Rietveld refinement results indicate that the ultrasonic-assisted Li1.3Ni0.21Mn0.64O2+d material has a larger lithium slab space. Electrochemical studies indicate that the ultrasonic-assisted material exhibits a higher initial discharge capacity (251.2 mAh g−1 at 0.1 C) and better cycling performance (200.4 mAh g−1 after 50 cycles) compared with the material synthesized using traditional co-precipitation. Surprisingly, the ultrasonic-assisted material has a stable capacity of 140 mAh g−1 at 5 C and 115 mAh g−1 at 10 C after 50 cycles. First-principles calculations are used to confirm that the specific lattice structure leads to a faster lithium-ion mobility speed.