<p>This study explores the demethylation of methylene blue in an acidic pH, utilizing a spent cathode from Li-ion batteries. The spent cathode has a composition of LiMn₂O₄ and a pzc close to pH = 2.1. In contact with methylene blue in an acidic medium (pH = 2), the spent cathode adsorbed and demethylated methylene blue to form thionine. This was confirmed by UV–Vis and ESI measurements of the resulting solutions. The intermediate <i>m</i>/<i>z</i> = 300 was crucial for proposing the demethylation mechanism that is similar to a demethylation mechanism catalyzed by hemoproteins. FTIR measurements confirmed the adsorption of methylene blue onto the spent cathode. The rise in pH observed during demethylation can explain the incomplete degradation of methylene blue stopping at the thionine stage. Consequently, the reaction involving the spent cathode of the Li-ion battery based on LiMn₂O₄ may be applicable to the demethylation of other molecules in future research.</p>