Dissolution Mechanism of LiNi1/3Mn1/3Co1/3O2 Positive Electrode Material from Used Lithium-Ion Batteries
Authors: Billy Emmanuel , Laucournet Richard , Joulie Marion , Meyer Daniel , Boulineau Adrien , De Vito Eric 
Published: February 2020

Much attention has been focused on hydrometallurgical routes to recover valuable metals from spent Li-ion batteries (LIBs). A lot of works has demonstrated that this method is an effective approach toward the recovery of a large panel of metals constituting LIBs and particularly the positive electrode material. Despite the large number of studies on hydrometallurgical processes for LIBs recycling, the phenomenon taking place during positive electrode material dissolution remains unknown. Different behaviours are observed during active material of positive electrode dissolution in literature. The interest of using a reducing agent to favour material dissolution is underlined. However, a partial dissolution of such material can be achieved without reducing agent in the leachate. The aim of this study is to explain such dissolution in absence of reductive agent. LIBs represent an active research field in which alternative positive electrode materials are developed to replace LiCoO2 due to cobalt cost and safety issues. The solution found is to substitute partially or totally the cobalt by other transition metals such as nickel or manganese: LiFePO4 (LFP), LiNi1/3Mn1/3Co1/3O2 (NMC), LiNi0.8Co0.15A0.05O2 (NCA), and LiMn2O4 (LMO). In this study, we chose the representative LiNi1/3Mn1/3Co1/3O2 (NMC) material which contains the most common elements found in LIBs. This active material has good overall performance and excels on specific energy, which makes it a very good candidate for the electric vehicles, and it has the lowest self-heating rate.
This work is dedicated to a kinetic study of the dissolution reaction in acidic media. The relation between structural changes and dissolution mechanism is studied during the dissolution evolution. The surface composition analyses of residual particles (during the dissolution) are performed by X-ray photoelectron spectrometry (XPS), high-resolution transmission electron microscopy (HRTEM), electron dispersive X-ray (EX) spectrometry mapping and X-ray diffraction. Regarding the dissolution mechanism some electrochemical experiments allows to precise redox reactions taking place at the interface. The limitations of NMC dissolution in acidic media are also identified.
The results show two different steps of dissolution with different sources of limitation. During the first step, there is no change on the surface of particles (cf. attachment HRTEM images and EDX mapping: B) between 5 and 15 minutes of dissolution (step 1). After 18 hours of dissolution represents the beginning of the second dissolution step. The results reveal the beginning of surface enrichment in manganese. X-ray diffraction analysis demonstrates that the rich manganese phase is composed by MnO2. This crystalline phase is located on the surface of particles and creates during the dissolution. At the end of the second dissolution step (43 days), the figure (cf. attachment HRTEM images and EDX mapping: D) reveals a growth of a rich manganese phase on the NMC surface particles under MnO2 needles form.
The structural study performed by microscopy, spectroscopy and X-rays diffraction reveals the surface of NMC particles is free from a neoformed phase at the end of the first step. Thus, this step is not limited by the formation of a surface passivation film at the interface between the NMC material and the dissolution solution. The electrochemical results and the thermodynamic approach demonstrate a dissolution controlled by the material delithiation. Regarding the second step, all the previous results indicate the formation of MnO2 on the surface of NMC particles. To highlight the chemical reactions involved during the second dissolution phase, the particular behaviour of manganese is investigated. The results indicate that the addition of manganese to the dissolution solution allows to reactivate the dissolution of nickel and cobalt contained in the NMC material. It highlights that the oxidation of manganese under MnO2 form is linked with nickel and cobalt dissolution from NMC material. The dissolution can be reactivated after MnO2 layer formation on the surface of NMC particles. So, the stopping of the NMC dissolution is not related to a surface passivation by MnO2 layer. The stopping of the dissolution coincide with the Mn2+ depletion. Finally, the results allow defining new ways of treatment in reducing the energy requirements and the addition of reducing species.

Figure 1