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

## TL;DR

This paper shows how to make peroxodisulphuric acid using an electrolyser and tests its use for recovering materials from old lithium-ion batteries.

## Contribution

The study introduces a flow electrolyser for synthesizing peroxodisulphuric acid and explores its application in battery recycling.

## Key findings

- Peroxodisulphuric acid was synthesized at 180 g L−1 with 1.5 Wh g−1 energy consumption.
- The acid extracted nearly all lithium but only partially dissolved transition metals.
- A three-step process is proposed to improve metal recovery using the acid.

## Abstract

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) in situ 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.

A coaxial flow electrolyser generates peroxodisulfuric acid at 180 g L−1 concentration with 1.5 Wh g−1 energy consumption. Applied to spent battery black mass, the acid extracts lithium quantitatively but shows limited transition metal recovery.