Selective recovery of valuable cobalt-nickel alloys and inorganic compounds from spent lithium ion battery cathodes for open and closed loop recycling

Abstract

Lithium-ion batteries (Li-ionBs) have been extensively deployed as the primary electrochemical power source for many applications, and their demand has increased significantly over the past decade, with the market projected to reach 116 billion United States dollars (USD) by 2030. Consequently, a considerable amount of end-of-life Li-ionBs are disposed of as waste annually. The current processes utilized for recycling spent Li-ionBs involve high-cost, energy-intensive, and eco-hazardous processes and materials. This has resulted in only a mere 5% of the spent Li-ionBs currently being recycled. Therefore, the development of an effective, low-cost, lowenergy- intensive, and eco-friendly recycling process route for recovering valuable metals (i.e., lithium [Li], cobalt [Co], nickel [Ni], and manganese [Mn]) is imperative and imminent. The cathode, in this study composed of Lithium Nickel Manganese Cobalt Oxide (LiNixMnyCozO2, or NMC) material, is a key determinant of a NMC Li-ionB’s cost and performance. While NMC is one of the predominant cathode chemistries used in Li-ionBs due to its favourable balance of performance, stability, and cost, other types of cathode materials are also widely employed in various applications. NMC cathode combines the valuable metals Ni, Co, and Mn in varying ratios (x, y, z in LiNixMnyCozO2) for optimal performance, stability, and cost. For example, NMC 532, used in this study, has a Ni:Mn:Co ratio of 5:3:2, corresponding to the formulation LiNi0.5Mn0.3Co0.2O2. Conventional pure metal recovery, from Li-ionB waste, processes include solvent extraction, ion exchange, and selective precipitation, followed by galvanostatic electrowinning to obtain solid metal deposits. In this work, valuable Ni-Co alloys from spent Li-ionBs were selectively recovered from multi-ion (Ni2+, Co2+, Mn2+, and Li+) NMC 532 leachate solutions through potentiostatic electrowinning. Since the postelectrowinning spent liquor still contains traces of Ni and Co and significant amounts of Li and Mn, an additional sodium (Na)-based chemical precipitation unit operation was added to recover the nickel hydroxide (Ni(OH)2) , manganese hydroxide (Mn(OH)2) and cobalt hydroxide (Co(OH)2) composite mixture (0.6[Ni(OH)2].0.3[Mn(OH)2].0.1[Co(OH)2]), Mn(OH)2, and lithium carbonate (Li2CO3) materials. The rationale of this research was based on the elimination of cost and energy-intensive hydrometallurgical intermediate processes like solvent extraction, ion exchange, and selective precipitation (to extract Ni and Co selectively), utilization of potentiostatic techniques (instead of conventional galvanostatic techniques) to selectively extract specific metals and enhance purity, integration of rotating cathodes to increase the deposition rate, and utilization of Platinum (Pt)-coated Titanium (Ti) dimensionally stable anodes (DSA) to reduce deposit contamination and consequently levelize the cost of operation. It is believed that by applying a constant potential suitable for Co and Ni reduction reactions (potentiostatic electrowinning), valuable Ni and Co can be selectively deposited and separated from less valuable Li and Mn. Recovered Ni-Co alloys can be used as feedstock for Lithium Nickel Manganese Cobalt Oxide (NMC) cathode production processes. Ni-Co alloys can also be used in the production of magnetic films, electrocatalysis materials, and other various technological applications. In the first experimental phase, this research demonstrated the applicability of inorganic acidreductant leachant-based leaching of NMC 532 to effectively leach and recover all valuable metals in the cathode material. This approach provides quantitative recovery data for each element of the entire particle population at different operational parameters: reductant and inorganic acid concentration, solid-to-liquid (S/L) ratio, reaction time, and temperature. The quantification of elemental recovery data was done through Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Energy Dispersive Spectroscopy (EDS). Phase composition was assessed through X-ray Diffraction (XRD) analysis. Morphology and particle size were analysed using High-Resolution Scanning Electron Microscopy (HR-SEM). By utilising optimised leaching parameters, aluminium (Al) foil and carbon (C) flakes were freed from the cathode matrix and peak leaching recovery efficiencies of 98.9% for Li, 97.1% for Co, 96.9% for Ni, and 95.7% for Mn were attained. In the second experimental phase, the effects of key electrowinning parameters were quantified and studied, and alternative electrodes were tested to suppress the extent of scaling, electrode resistivity, operational and capital costs, and life cycle duration limitations. This optimisation study was conducted using synthetic Ni, Co, Mn, and Li sulphate solutions mimicking the NMC 532 ratio of elements. The optimised electrowinning parameters such as applied potential, temperature, pH, Co-Ni, Na₂SO₄ and buffer dosage, and electrode distance and active area were then utilised to recover Ni0.65Co0.35 alloy from real NMC leachate at a minimum rate of 0.06 g/cm².hr with 88% current efficiency. 90% of the Co and 77% of the Ni in the leachate were recovered in a 3-hour electrowinning run. In the last phase of the experiments, the metals remaining in the post-electrowinning electrolyte from the electrowinning were recovered through multi-stage precipitation to recover Mn(OH)₂ and Li₂CO₃, and a hydroxide composite formulation of Ni, Mn, and Co (0.6[Ni(OH)2].0.3[Mn(OH)2].0.1[Co(OH)2]) at over 99% precipitation efficiency. Approximately 19% of Ni, 7.5% of Co, and 95% of Li were recovered during multi-stage precipitation. The varied and optimized key parameters for pH-based precipitation were temperature, reactants ratio, and pH. The semi-closed loop recycling cost (R/kg) of the cathode was calculated to be R 153/kg, which is at least 50% lower than R 360/kg, R 308/kg, and R 258/kg recycling costs for direct recycling, pyrometallurgical, and hydrometallurgical processes, respectively. A significant fraction of the process resultant solution from the whole process can be recycled for use in the leachate pH adjustment stage, while the minority portion will need further treatment (to remove sulphate (SO2-4) and sodium (Na+) before being discarded per National Environmental Protection Agency (NEPA) regulations since it contains negligible and environmentally tolerant metal concentrations of Mn, Li, Ni and Co. The obtained results demonstrate the feasibility of a semi-closed loop spent Li-ionB cathode recycling process comprising battery pre-treatment, single-stage leaching, single-compartment electrowinning cell, and Na-based precipitation. The main objective of this work, high purity Ni0.65Co0.35 alloy recovery, was successfully achieved. The recovered Ni0.65Co0.35 alloy, Li2CO3, Mn(OH)₂, and 0.6[Ni(OH)2].0.3[Mn(OH)2].0.1[Co(OH)2] are versatile compounds with applications ranging from the production of Li-ionBs to medicine and material production, among other applications.