As a promising electrochemical energy storage system, rechargeable aluminum batteries face critical challenges in their quest for commercial viability. While the design of suitable cathodes has attracted much attention, their chemical composition and purity has been less of a concern. This is especially true for carbon cathodes, where the presence of metallic impurities is often overlooked. Herein, we demonstrate the influence that transition metals exert on the electrochemistry of carbon nanotube cathodes for non-aqueous aluminum batteries. In the presence of chloroaluminate electrolytes, these synthesis by-products originate inflated capacities, increased self-discharge and misleading electrochemical signatures, among others. Thus, our findings affirm the need for strict control of the composition and purity of materials (and components) used in non-aqueous aluminum batteries.