Improvement of leaching efficiency of cathode material of spent $LiNi_xCo_yMn_zO_2$ lithium-ion battery by the in-situ thermal reduction

Green cars and electronic products consume lots of lithium-ion batteries (LIBs), and massive spent LIBs are yielded due to performance degradation. This paper provides an economical and environmentally friendly approach to recover valuable metals from cathode materials of the spent LIBs. It combines the in-situ thermal reduction (self-reduction by polyvinylidene fluoride (PVDF) and residual electrolyte in cathode material) and sulfuric acid leaching. Elements of high valent are reduced by the binder (PVDF) and the residual electrolyte on the surface of $NCM(LiNi_xCo_yMn_{1-x-y}O_2)$ material at high temperatures. Moreover, the changes in substance type, element valency, and contents of cathode materials reduced with various terminal temperatures and retention time are analyzed by Xray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results show that the optimal terminal temperature for in-situ thermal reduction is 600 °C, and the optimum retention time is 120 min. Under the best in-situ thermal reduction conditions, the results from XRD confirm that part of $Ni^{2+}$ is converted to simple substance $Ni$, $Co^{3+}$ is reduced to $Co$, and $Mn^{4+}$ is reduced to $Mn^{2+}$ and elemental $Mn$, which are confirmed by XRD. Analyzed results by XPS indicate that the content of $Ni^{2+}$ decreases to 67.05%, and $Co^{3+}$ is completely reduced to $Co$. $Mn^{4+}$ is reduced to 91.41% of $Mn^{2+}$ and 8.59% of simple substance $Mn$. In-situ thermal reduction benefits the leaching processes of cathode materials. The leaching efficiencies of $Ni$, $Co$, and $Mn$ increase from 53.39%, 51.95%, and 0.71% to 99.04%, 96.98%, and 97.52%, respectively