(180j) Ultrasonically Enhanced Hydrometallurgical Process for Li-Ions Battery Cathode Recycling

Recycling metals from wasted lithium-ion (Li-ion) batteries has been gaining momentum in recent years due to the substantial increase in commercialised batteries, mainly fueled by the expansion of the electric vehicles market. Li-ion batteries are used in various electronic devices and electric vehicles, representing the best in-practice energy storage method. The cathode of such batteries consists of about one-third of the total battery mass and holds more than half its value. The cathode comprises valuable metals such as cobalt, manganese and lithium. Recycling these metals reduces the need for new mining, minimises environmental pollution from battery disposal, preserves natural resources, and contributes to a circular economy. With the increasing demand for LI-ion batteries and the growing concern for environmental impact, efficient recycling practices will create a more sustainable and greener future.

Currently three processes have been proposed for recycling Li-ion batteries: direct recycling, pyrometallurgical process, and hydrometallurgical process. Among these three, the hydrometallurgical process is receiving significant attention from academia and industry. This attention is due to its potential for achieving the highest recovery rates and purity of metals. The hydrometallurgical process involves leaching metals from shredded battery cathode materials using acid solutions, followed by various separation and purification techniques to isolate metals like lithium, cobalt, nickel, and manganese. Leaching presents several challenges, such as the need for strong acid solutions, resulting in safety concerns due to potential toxic vapour emissions. Secondly, the time scale required for cathode dissolution is lengthy, often taking hours, impacting industrial plants' economic viability and capacity.

The application of ultrasounds for leaching intensification might impreve the process by enabling the utilisation of mild organic acid and reducing the residence time required for the cathode dissolution. Ultrasonic process intensification relies on the physical phenomena called cavitation. Ultrasonically induced cavitation entails the formation and subsequent fate of vapor-filled cavities in a liquid medium irradiated by a source of high-frequency acoustic waves. When the waves pass through a liquid, they create alternating high and low-pressure zones. The fluid changes phase in low pressure regions, forming small vapor-filled bubbles or cavities. As the pressure waves oscillate, these bubbles grow until they reach a critical size and collapse violently. This collapse is associated with severe local conditions where the energy stored in the bubble is released through localised shockwaves, heat, and liquid jets, leading to various effects. In particular, UIC can have several beneficial effects on the mixing and reactivity of heterogeneous systems:

• Enhanced Mixing: Cavitation leads to the formation and collapse of nano-scale bubbles, creating intense local turbulence and agitation within the liquid. This effect significantly enhances the mixing of different components in a heterogeneous system, promoting uniform distribution and contact between reactants.
• Increased Surface Area: The collapse of cavitation bubbles near solid surfaces or interfaces generates high-speed microjets and shockwaves. These improve transport properties around the surface of the particles. The latter break up due to increased acid erosion, effectively increasing the overall surface area available for reactions in the system.
• Improved Mass Transfer: The turbulent flow and microstreaming induced by cavitation bubbles facilitate mass transfer processes such as diffusion and convection. This accelerates the transport of reactants to active sites and enhances the overall reactivity of the system.
• Disruption of Boundary Layers: Cavitation disrupts boundary layers that may hinder mass transfer or reaction rates near solid surfaces. This effect promotes the penetration of reactants into confined spaces or porous media, improving the system's overall efficiency.
• Energy Efficiency: Ultrasonic cavitation can achieve efficient mixing and reactivity with relatively low energy input compared to mechanical agitation or high-pressure methods. This energy efficiency is advantageous for industrial processes aiming to reduce energy consumption and operating costs.

This work experimentally investigated the effect of ultrasonically induced cavitation on the leaching process of cathodic material. Lithium cobalt oxide, purchased from Sigma Aldrich, was used as a surrogate to mimic the cathode material during the experimental campaign. The experiments were performed using about 200 ml of leachate solution placed in 400 ml quartz Becher. Acetic acid was ultilized as acidic medium for the investigation.

A parametric study investigated the ultrasonically enhanced leaching at different operative conditions to understand the effect of parameters such as acid concentration, addition of a reducing agent, and intial solid to liquid ratio to optimize the cathode dissolution.

The ultrasonic intensification resulted in a substantial reduction in the residence time needed for the leaching process. Specifically, ultrasonically enhanced leaching requires less than ten minutes, whereas conventional leaching typically takes a few hours (usually 120 to 150 minutes). The leaching efficiency, defined as the percentage of the starting solid mass dissolved in the acid solution, is used to compare with other literature studies.

Figure 1 compares the leaching efficiency of ultrasonically enhanced leaching with a similar study available in the literature (Xiao, Xiong, et al. "Ultrasound-assisted extraction of metals from Lithium-ion batteries using natural organic acids." Green Chemistry 23.21 (2021)). The superior results derived by the different methodology of ultrasound application to the solution. Specifically, our study directly applied ultrasound through an ultrasonic horn, whereas other works in the literature used an ultrasonic bath, thus limiting effective cavitation activities.

Different state-of-the art analytical techniques were adopted to investigate the leachate and the solid particles morphology after exposure to ultrasonically induced cavitation. In detail:

• Inductively coupled plasma - optical emission spectrometry (ICP-EOS) was adopted to quantify metals concentration in the leachate.
• Scanning electron microscope permited the visualization of solid particles before and after exposure to the ultrasonically induced cavitation.
• BET surface area and porosimetry analysis.

The analysis aided in comprehending the interactions between particles and collapsing cavitation bubbles. Specifically, the intense fluid dynamic conditions generated during bubble collapse enhance the system's mixing, preventing the formation of stagnant regions along particles irregular surface and resulting in a more homogeneous consumption of the particles. Moreover, the deeper erosion enabled by the enhanced transport properties results in particle breaking up, exposing more surface to the action of the acid.

The ultrasonically enhanced leaching was scaled from lab scale to a prototype that processed 250 L/day of liquid solution with an in-house ultrasonic reactor. The scale-up was aimed at developing a first-of-its-kind continuous leaching process. The ability to process cathode materials continuously was enabled by the shorter residence time required for a substantial dissolution of the cathodic material.

In conclusion, the hydrometallurgical process has emerged as a promising method for recycling lithium-ion batteries due to its potential for achieving high recovery rates and purity of metals. However, this process faces challenges such as using strong acid solutions and lengthy residence times, highlighting the need for innovative solutions. Applying ultrasonic-induced cavitation in leaching processes shows promise in overcoming these challenges.