Oxidized carbon nanotubes, oxCNTs, were formed when multi-walled carbon nanotubes were treated by chemical oxidation with acidic potassium permanganate. The synthesis and characterization of oxidized multi-walled carbon nanotubes of five diameter ranges for use as anode materials in lithium-ion battery applications is detailed herein. Thermogravimetric analysis and typical spectroscopic methods confirmed extensive oxidation, while X-ray photoelectron spectroscopy comparisons provided quantification of specific components. Images from high-resolution transmission electron microscopy indicated a change in structure associated with the oxidation as X-ray diffraction peaks indicated broadening interplanar spacing. This study confirmed that the oxidative process successfully led to oxCNTs, with a reversible capacity demonstratively higher than their parent multi-walled carbon nanotubes. Higher capacities were demonstrated in smaller-diameter oxCNTs, and associated with structural and compositional differences between the samples.