Separation of nickel from cobalt and manganese in lithium ion batteries using deep eutectic solvents

The authors would like to thank the Faraday Institution (grant codes FIRG005 and FIRG006) for funding (Project website https://relib.org.uk). This research also received funding from the European Commission's H2020 – Marie Sklodowska Curie Actions (MSCA) − Innovative Training Networks within the SOCRATES project under the grant agreement no. 721385 (Project website: https://etn-socrates.eu).A cornerstone of the decarbonisation agenda is the use of lithium ion batteries, particularly for electric vehicles. It is essential that effective recycling protocols are developed and this includes the ability to selectively digest and recover components of the cathode materials, most commonly including manganese, cobalt and nickel. This study shows a method by which nickel oxide can be efficiently separated from cobalt and manganese oxides using an oxalic acid-based deep eutectic solvent. The subsequent addition of water to the pregnant solution enables the co-precipitation of cobalt and manganese oxalates. This permits a route to the reformulation of the active materials from high cobalt and manganese content to high nickel content.Publisher PDFPeer reviewe

Metal oxides processing using deep eutectic solvents

Metal oxides are the form from which most metals are extracted. They are found in natural ores and many industrial residues and end-of-life products, which makes their efficient processing an important topic. The state-of-the art processes for the extraction of metal oxides include either the pyrometallurgy or hydrometallurgy, which both have a significant environmental footprint. The investigation of more efficient alternatives for their extraction is crucial, in order to develop sustainable flowsheets for recycling materials like cathodes from lithium ion batteries.The understanding of the dissolution mechanism of selected metal oxides was attempted using deep eutectic solvents. In general, Pourbaix diagrams have shown that metal oxides can be digested either through protonation, complexation or by redox processes. The two former methods were investigated to determine their effect on solubility, and it was shown that the surface complexation had a greater impact on their solubility compared to the proton activity of the solvent. Speciation is known to be the key to designing selective processes, so the ability to tune the deep eutectic solvents to selectively dissolve some metals over others is a great asset. The selective extraction of Co and Mn over Ni from cathode materials of lithium ion batteries and Y and Eu from spent fluorescent lamp phosphors was demonstrated.Apart from the chemical dissolution, the electrochemical oxidation of metal oxides was also investigated as this was previously shown to be efficient for the dissolution of metal sulfides, tellurides and selenides. It was found that a significant enhancement of the metal oxide dissolution rate could be obtained in solvents that are neither acidic nor consist of complexing agents. Indeed, the rate of dissolution, which was dependent upon the band gap of the metal oxides, was strongly enhanced, sometimes even more than 10000 times. </div

Electrochemical oxidation as alternative for dissolution of metal oxides in deep eutectic solvents

Metal oxide dissolution is central to most metal processing and is generally carried out using mineral acids. An alternative approach has been demonstrated for metal sulfides using electrochemical oxidation. Here the so-called paint casting method was employed to investigate the electrochemical dissolution and recovery of selected metal oxides. Cyclic voltammetry showed that all of the metal oxides were electrochemically active in deep eutectic solvent media, and the majority displayed a redox couple that could be potentially assigned to the redox behaviour of the oxide moiety. Bulk anodic dissolution was carried out on the metal oxides, and it was shown that dissolution was enhanced to up to >10000 times in pH neutral solutions. The rate of electrochemical dissolution was shown to be influenced by the band gap of the compounds. It is proposed that oxidation of the oxide moiety, potentially to the superoxide, enables the solubilisation of the metal ions. The metal speciation appears to remain the same as for chemical dissolution

The effect of pH and hydrogen bond donor on the dissolution of metal oxides in deep eutectic solvents

The dissolution behaviour of a series of d- and p-block metal oxides was investigated in deep eutectic solvents, DES, to determine the effect of pH and hydrogen bond donor (HBD) coordination strength on their solubility. The solubility of metal oxides was found to be increased by a higher proton activity in DESs composed of poorly complexing HBDs, due to the ability of H+ to act as an oxygen acceptor. However, the employment of HBDs that have stronger complexing abilities was proven to have a greater effect on the solubility. The strongly coordinating HBDs increased metal oxide solubility via surface complexation reactions followed by ligand exchange for chloride in the bulk solvent. Differing selectivity for leaching of metal oxides was demonstrated, with solubility shown to be broadly dependent on lattice energy and Gibbs energy of formation of the metal oxide, while dissolution kinetics for metal oxides are shown to vary significantly. The results provide pathways to separation of metal oxides by dissolution in DES.</p

Accessing polyanionic redox in high voltage Li-rich thiophosphates

In the search for novel positive electrode materials for lithium-ion cells, Li-rich sulfides are attracting increasing interest. Despite the success of polyoxyanion-based cathodes such as LiFePO4, their thiophosphate counterparts have remained largely unexplored. Here, the Li-rich thiophosphate Li2FeP2S6, which exhibits the highest known voltage (3 V) for a sulfide electrode, is investigated in a solid-state configuration. Through examination of isostructural transition-metal substitutions, we identify a novel Mn-substituted compound, Li2Fe0.8Mn0.2P2S6, with higher capacity than the parent Fe system while maintaining the high voltage. Hard X-ray Photoelectron Spectroscopy and ab initio molecular dynamics simulations indicate that Mn substitution activates P2S6 polyanionic redox involving interlayer S--S bond formation with no evidence of Fe or Mn cation migration, and increases capacity beyond the formal transition-metal redox limit. This demonstration of polyanionic redox in a thiophosphate material highlights the opportunity to explore alternative Li-rich thiophosphate structures as high-capacity lithium-ion cathodes