A range of electronic products, such as medical devices, smartphones, notebooks and electrical vehicles (EVs), are run by lithium-ion batteries (LIBs). As such, they have become an integral part of our lives. Due to the growing demand for LIBs, one of the key issues is handling the spent LIB waste. According to CSIRO, Australia and the European Union (EU) annually recycle only 2 percent and 5 percent of LIB waste respectively.
With the rise of EVs, the world is facing a huge battery waste issue. As LIBs have various valuable resources, such as cobalt (Co) and lithium (Li), recycling is the answer to the problem. Since there are limited supplies of Li, Co and other materials used in LIBs, and the fact that those can be hazardous materials, it is essential to develop an environmentally friendly process for treating the spent LIBs. This is a fundamental aspect of establishing an electronic-waste circular economy.
Scientists from Nanyang Technological University (NTU) in Singapore have developed a new method for recycling precious metals from spent LIBs by using fruit peel waste. This method is in line with the circular economy approach with zero waste, as it covers food and electronics waste simultaneously.
According to a technical paper published in Environmental Research Letters, it is estimated that 1.3 billion tons of food waste and 50 million tons of electronic waste are generated annually at the global level. In the EU, it is estimated that a person produces 123 kg of food waste annually, of which 60 percent is fruit and vegetable waste. Most of it is disposed of in a landfill or burnt.
Conventional Methods for Recycling Spent Batteries
The battery recycling process involves several steps. The first is shredding the used batteries and forming a crushed material called black mass from which the valuable metals are extracted. Currently, the available battery recycling methods are not fully environmentally friendly because they require high-energy requirements or strong acids or produce hazardous secondary pollutants.
The current methods for LIB recycling include pyro-, bio- and hydrometallurgy. The first one is the most widely used in the industry. This method includes thermal treating of the spent batteries to smelt valuable metals. The process involves extreme heat (over 500°C), which is energy-intensive and results in the emission of hazardous gases.
The bio-metallurgy method uses acid-generating microbes (bacteria/fungi) to extract heavy metals from LIB waste and causes minimal environmental and health impacts. Commonly used microbes include Acidithiobacillus Ferro-oxidants, Acidithiobacillus thio-oxidants and Aspergillus niger. However, because of the inefficient bioleaching process and sensitivity of microbes to the toxins from the metals, this method is not widely used.
Hydro-metallurgy uses water as a solvent providing a more direct metal recovery route. The processing temperatures used are much lower (10 to 200°C) than in the pyro-metallurgy process. When compared to the bio-metallurgy, the efficiency of this method is higher because the process is independent of the growth kinetics of the microbes. The high recovery rate and low energy consumption make this approach for treating LIB waste highly attractive.
Conventional hydrometallurgical processes use combined strong inorganic acids (H2SO4, HCl, and HNO3) and reductants (e.g., hydrogen peroxide, or H2O2) for extracting metals from spent cathode materials. The industrial-scale use of strong acids can present safety and health risks due to generating a significant amount of secondary pollutants, such as sulfur trioxide (SO3), chlorine gas (Cl2) and nitrogen oxides (NOx). Mild organic acids, when combined with H2O2 as the reducing agent, are environmentally safe and as effective as mineral acids. A technical paper reports that leaching efficiencies of Li and Co from discarded lithium cobaltite (LiCoO2), or LCO, in citric acid (H3Cit) have been increased from 54 percent to 99 percent for Li, and from 25 percent to 91 percent for Co when H2O2 (1.0 vol %) was added to the lixiviant. Additionally, when the optimized combination of tartaric acid (0.6 mol/L) and H2O2 (3 vol %) is used, it is possible to achieve 99.1 percent of Li selective leaching from LIB cathode materials. However, this solution is not sustainable because H2O2 is highly explosive, hazardous and unstable. This has resulted in the search for greener alternatives to H2O2 in recent years.
Scientists from NTU Singapore have demonstrated that it is possible to do so with biodegradable substances.
A New Approach for Recycling LIBs
NTU Assistant Professor Dalton Tay stated that using strong chemicals on an industrial scale results in the generation of a significant amount of secondary pollutants which, despite their advantages when compared to conventional methods, carry considerable health and safety risks. This was a trigger to start several studies that have tackled the issue of exploring the use of less hazardous weak organic acids (formic, salicylic, citric, gluconic, itaconic, succinic, and acetic acids) to replace the strong ones. NTU scientists discovered that an over-dried orange peel ground into powder and combined with citric acid (H3Cit) lixiviant can achieve the same results. It was discovered that this combination can be used as a green reductant for the acid leaching of valuable materials from LIB cathode materials. This is published in their technical paper for Environmental Science and Technology.
In lab experiments, the scientists managed to successfully extract 80−99 percent of Co, Li, Ni, and Mn (manganese) from spent LIBs. Thus, the efficiency of the reductive power of orange peel in extracting Co, Li, Mn and Ni from LIB black mass with added lixiviant is similar to using the H2O2 as the reducing agent.
Those optimistic results are obtained thanks to the cellulose from the orange peel. During the extraction process under the heat, the cellulose is converted into sugars that increase the recovery of metals from battery waste. Additionally, antioxidants found in orange peel (flavonoids and phenolic acids) also influence this enhancement. To validate this reductive potential of the cellulose and antioxidant-rich orange peel, the NTU researchers have used the 3,5-dinitrosalicylic acid and diammonium salt 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulfonic) acid assays. Leaching parameters, orange peel concentration, processing temperature, reaction duration and slurry density have been optimized to achieve high leaching efficiencies of Ni, Mn, Co and Li from the LIB black mass between 80−99 percent.
Preprocessing Spent LIBs
The NTU researchers have used standardized steps in the LIB recycling process. When the LCO cylindrical cell battery has reached the end of its usable life, its voltage is in the range from 3.1 to 3.4 V. To avoid battery flame or explosion, it is necessary to fully discharge the battery before shredding. This is done by submerging the battery in NaCl solution (20 wt %) overnight.
The next step is shredding the discharged battery under inert gas conditions at room temperature. After the shredded materials are air-dried, they are filtered from the plastic parts, creating the recycling material called black mass.
The orange peel was chopped and dried for three days at 60°C to remove moisture. The dried orange peel was pulverized and sieved. When the orange peel was ready, the measurements of reducing sugar and antioxidant capacity were performed.
The H2Cit lixiviant has been used to examine the reductive leaching performance of orange peel in a black mass.
According to the battery manufacturer recommended protocol, cell viability assay is used to analyze the toxic potential of the solid byproducts (leaching residues).
In situ precipitation of metal hydroxides is performed by preparing the leaching solution, which involves mixing black mass and orange peel with H3Cit and allowing these components to react for a certain period at the corresponding temperature. The leaching solution was centrifuged and filtered to remove the solid residues. Co is recovered from the black mass by using NaOH. NaOH pellets were added to adjust the pH of the black mass leachate to 12, enabling rapid precipitation of a hydroxide mixture that includes manganese hydroxide and nickel hydroxide along with a portion of cobalt hydroxide. After the solution is oven-dried, another round of precipitation of the resultant cobalt-containing supernatant was performed by adding ethanol at a certain temperature. Again, the precipitate was retrieved by centrifugation, washed with water and oven-dried. Energy-dispersive X-ray spectroscopy (EDX) analysis has been performed to confirm the success of the process.
The next step of the recycling process is the LCO battery regeneration. The recovered cobalt hydroxide from the previous step was mixed with Li2CO3. Additional Li was added to compensate for loss during the thermal treatment. The mixture was treated at high temperatures for a certain period to form LCO. The crystal structure of the obtained product was examined via X-ray diffraction (XRD). The preparation of electrode slurry was performed by mixing the recovered LCO, super P carbon and Polyvinylidene fluoride (PVDF) binder in N-Methyl-2-pyrrolidone (NMP) solvent. The prepared slurry was coated on an aluminum current collector and oven-dried in the vacuum. Li metal is used as the anode and LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) as the electrolyte. A final step was checking the cyclic performance of the recycled Li-ion coin cell batteries by using the galvanostatic charge−discharge testing at 0.1°C at a constant rate at room temperature.
Conclusion
In their lab experiments, the scientists from NTU Singapore have successfully assembled new LIBs with a similar charge capacity to commercial ones. They have used an innovative method in which fruit peel waste has been used to extract and reuse precious metals from spent LIBs. Approximately 90 percent of Co, Li, Ni and Mn were extracted from spent LIBs. The researchers succeeded in recovering Co(OH)2 from the green lixiviant and fabricating new LCO coin cell batteries. This approach which involves fruit peel waste to recover valuable metals from spent LIBs is effective and eco-friendly, thus, practically feasible for recycling spent LIBs on the industrial scale. This is a potentially effective and sustainable strategy to minimize the environmental impact of different waste.
Researchers are going to continue their work to improve this approach to the LIB recycling process. They believe that the charge-discharge cycling performance of newly fabricated batteries can be optimized.
The team is now looking to further improve the performance of their batteries generated from treated battery waste. They are also optimizing the conditions to scale up production and exploring the possibility of removing the use of acids in the process. Other types of cellulose-rich fruit and vegetable waste can potentially be used in this process. Other types of LIBs (lithium iron phosphate and lithium nickel manganese cobalt oxide) can also be recycled by using this new green circular economy approach.