Poor stability restricts the use of polyaniline as a transition-metal-free, high-capacity electrode material for electrochemical energy storage. To improve the durability, methods such as adding a carbonaceous support and thermal treatments have been suggested. Here, we combine both of these approaches and study their effects on the degradation of a composite of polyaniline and acid-treated multi-walled carbon nanotubes (PANI/a-MWNT) when used as a positive electrode in a lithium-ion cell. The composite is prepared through facile ultrasonic-assisted mixing of aqueous colloids, processed into binder-free electrodes and heat treated under vacuum (120–180 °C). PANI without the carbon support presents a poor cycling stability due to a decreasing conductivity and changes in the particle morphology. The a-MWNTs are shown to inhibit these changes and to enhance the electrochemical accessibility of PANI in the composite. The heat treatment, in turn, improves the stability of the composite during open circuit conditions, which is ascribed to cross-linking of the polymer. However, with prolonged cycling, the stabilizing effect is lost. Analysis of the electrodes after the cycling reveals that both the pristine and the heat-treated composite undergo dedoping and side reactions with the electrolyte. The extent of the changes is larger for the pristine composite.