Reassessing H2O2 in Malonic Acid Leaching of Spent-LIB Black Mass: Fenton-Like Ni–Co Oxalate Reprecipitation

Abstract

Hydrometallurgical processing of spent lithium-ion battery cathode mass with organic acids has traditionally relied on the addition of hydrogen peroxide as a reducing agent to convert Co3+ and Mn4+ into soluble form. Here we show that, for real industrial high-nickel black mass (NCA with a minor NMC admixture), this recommendation not only fails but proves counterproductive. Without any external reducing agent, 2.0 M malonic acid (70 °C, 2 h, S:L = 1:30) achieves recoveries of 90.3 wt % Ni, 97.7 wt % Mn, 99.9 wt % Li, and 69.3 wt % Co through the intrinsic reductive activity of its active methylene group. The addition of H2O2 progressively lowers transition-metal recovery (Ni from 90.3 to 50.9 wt %, Co from 69.3 to 45.8 wt % at 8 vol %), whereas Li recovery remains essentially unchanged (98–100 wt %). Using a combination of independent methods (XRD, SEM-EDS, FT-IR), the decrease is shown to arise not from cathode passivation but from Fenton-like oxidative degradation of malonic acid to oxalic acid, initiated by the impurity metals Cu and Fe, with subsequent reprecipitation of the dissolved Ni2+ and Co2+ as the mixed oxalate dihydrate (Ni,Co)C2O4·2H2O. The radical nature of the process is confirmed by complete suppression of the effect upon addition of methanol (an HO• scavenger) and by its absence when H2O2 is replaced with mild reductants (ascorbic acid, glucose), which afford nearly quantitative Ni and Co recovery. These findings revise the role of H2O2 in systems with organic chelating acids and, at the same time, offer a single-step route to an oxalate precursor for cathode materials.