Peroxodisulphuric acid synthesis in a flow electrolyser and its potential utilisation for black mass leaching

This study demonstrates the electrochemical synthesis of peroxodisulfuric acid (H₂S₂O₈) in a coaxial flow-type electrolyser. It evaluates its potential as a leaching agent for the black mass from spent lithium-ion batteries. The optimised synthesis (conditions: flow rate, current density) achieved high concentrations of (H₂S₂O₈) (≈180 g dm^-3) at a specific energy consumption of nearly 1.5 Wh g^-1. The leaching performance of H₂S₂O₈ was compared with that of conventional systems, including aqua regia and 2 M H₂SO₄ + H₂O₂. While aqua regia completely dissolved the NMC phase, and the H₂SO₄/H₂O₂ mixture ensured nearly full transition metal leaching, H₂S₂O₈ leaching resulted in only partial dissolution of Ni (≈61%), Co (≈61%), and Mn (≈5%). However, lithium was fully extracted (≈99.6%) due to dual dissolution from residual electrolyte salts and chemical deintercalation from the cathode lattice. Mechanistic analysis using XRD, AAS, and Pourbaix diagrams revealed that the poor transition metal recovery originates from the extreme oxidising environment of H₂S₂O₈, which stabilises insoluble high-valent oxides and prevents reductive dissolution pathways. The results highlight that direct application of H₂S₂O₈ is less practical than H₂SO₄/H₂O₂ for transition metal extraction but could be exploited for selective Li recovery or integrated into a three-step process: (i) <i>in situ</i> H₂SO₄/H₂O₂ generation, (ii) controlled hydrolysis to H₂SO₄ + H₂O₂, and (iii) reductive leaching. This approach offers industrial advantages, including on-site oxidant production and the elimination of H₂O₂ transport hazards.