Post-Leaching Water, Ultrasonic and Mild-Acid Washing for Purifying Graphite Recovered from Spent NMC111 Lithium-Ion Batteries

Recovered graphite from spent lithium-ion batteries is an important secondary resource that can reduce reliance on primary graphite and lower the environmental footprint of battery production. In this work, graphite obtained as a carbon-rich residue after industrial hydrometallurgical leaching of NMC111 black mass (2 M H2SO4 + 3% H2O2) is subjected to three post-leaching washing treatments to assess how far simple, low-intensity steps can further clean the leach residue while preserving the carbon structure. The washing routes are water washing (GW), water washing with ultrasonication (GU) and mild sulfuric-acid washing with 0.1 M H2SO4 (GA). ICP-OES and SEM–EDX show that, relative to the leached black mass, all washing treatments reduce residual transition-metal contents by two to three orders of magnitude, and that the mild acid wash provides the lowest bulk metal levels, with several elements at or below detection limits. X-ray diffraction and Raman spectroscopy indicate graphite-dominated patterns and improved structural order, with the ID/IG ratio decreasing from 0.62 (GW) to 0.11 (GA) and the corresponding in-plane crystallite size increasing from 30.6 nm to 168 nm. Overall, the mild acid washing step is the most effective low-impact post-leaching purification route, yielding a thoroughly cleaned low-metal graphite fraction that preserves the graphite framework and constitutes a suitable intermediate for further upgrading or reuse in secondary applications.