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Pollutant-free pyrolysis strategy for direct upgrading of cathode materials from spent lithium-ion batteries.Abstract
Highlights
- This strategy eliminates the formation of HF during the pyrolysis of spent LIBs.
- Fluorinated pollutants are directly converted into stable metal fluorides.
- Mechanical stirring promotes the separation of cathode material from Al foil.
- The recovered cathode material still has a complete spherical morphology.
The recycling of lithium-ion batteries (LIBs) has been dogged by air pollutants containing fluoride (e.g. HF, PF5, POF3). Pyrolysis is a technique that can eliminate polyvinylidene fluoride (PVDF) from the cathode electrode sheets of spent LIBs, effectively separating the cathode material from the aluminum (Al) foil. Nonetheless, the HF gas generated during pyrolysis not only corrodes equipment but also presents serious environmental risks. To address this, a novel, eco-friendly strategy is introduced for the direct upgrading of cathode active materials (CAM). The strategy’s cornerstone involves incorporating a minor amount of calcium into the original cathode material’s coating, and it leverages mechanical stirring during the waste battery material separation process to ensure the electrode is fully detached from the current collector at a reduced temperature. The pyrolysis mechanism elucidates that fluorine-containing organic pollutants are converted into metal fluorides and deposited on the surface of cathode particles during aerobic pyrolysis, thereby improving the interfacial stability of lithium nickel cobalt manganese oxide (NCM) materials, reducing transition metal dissolution. This strategy not only eliminates the release of fluorine-containing organic pollutants during pyrolysis but also achieves direct regeneration of CAM. This work underscores the importance of the cathode materials’ manufacturing process in facilitating the recycling of spent LIBs and provides an environmentally friendly and economically viable solution for the battery recycling industry.
Graphical abstract
Introduction
Lithium-ion batteries (LIBs) have been widely used in various electronic products such as mobile phones, digital cameras, and laptops due to their excellent electrochemical properties (e.g., high voltage, high energy density, long cycle life, and no memory effect) [1], [2], [3]. At the same time, a large number of electric vehicles use LIBs as power sources [4], [5], which has led to a rapid increase in global demand for LIBs. Generally speaking, the average service life of LIBs is 5 to 8 years [6], [7]. However, extreme usage conditions or excessive charging and discharging can greatly shorten the life of LIBs [8]. It is reported that between 2017 and 2030 alone, more than 11 million tons of spent LIBs will be generated globally [9], [6]. If these spent LIBs cannot be disposed of properly, the toxic and harmful electrolytes and heavy metal elements contained in them will be released into the environment, posing a serious threat to the ecosystem or human health [10]. Additionally, spent LIBs contain a variety of transition metal elements, for example, lithium nickel cobalt manganese oxides (LiNixCoyMn1?x-yO2, NCM) batteries contain a large number of Li, Ni, Co, Mn, and other metal resources with high added value [11]. Hence, recycling these critical metals from spent LIBs not only helps environmental protection but may also bring huge economic benefits.
In the industrial recycling of spent LIBs, the current focus is on recovering valuable metals from spent cathode [12], [13], [14]. LIBs are usually composed of a cathode, anode, electrolyte, separator, and casing. The percentage of each component in total mass is shown in Fig. S1. The cathode is usually composed of 90 wt% lithium-containing transition metal oxide (e.g. LiCoO2, LiFePO4, LiNixCoyMn1?x-yO2), 7 wt% conductive agent (e.g. acetylene black, carbon black) and 3 wt% organic binders (polyvinylidene fluoride, PVDF), and coated on the current collector (Al foil) with a thickness of about 0.1 mm [15]. PVDF has excellent thermal stability, chemical stability, and mechanical strength, and is therefore widely used as an organic binder to attach cathode materials to Al foil [3], [16] The high stability and strong adhesion of PVDF make it extremely difficult to separate cathode materials and Al foil, which poses a severe challenge to the recycling of spent LIBs [17], [18]. In recent years, pyrolysis-based pretreatment processes have been widely adopted and have become one of the important research topics in the field of spent LIBs recycling [3], [19]. Pyrolysis can inactivate the binder PVDF (lose its adhesiveness), thereby achieving the separation of cathode material and Al, which is beneficial to the subsequent hydrometallurgy or direct regeneration of electrode materials. For example, [20], [21] proposed that the use of pyrolysis on the cathode electrode sheets can completely achieve the separation of cathode powder and Al foil. Hanisch et al. [5] proposed that pyrolysis of organic compounds such as PVDF binder can reduce the cohesion of cathode particles and weaken the adhesion between cathode particles and Al foil. Recently, [22] quantitatively analyzed the gas production during the pyrolysis of spent LIBs and found that pyrolysis has the advantage of avoiding the disordered dispersion of organic matter into subsequent processing steps. Although pyrolysis has many advantages, the biggest drawback of this process is that PVDF and organic electrolytes will release toxic and harmful fluorine-containing gases (e.g., HF) during the pyrolysis process.
Recently, studies by [3] and [23] have shown that alkali metal oxides or hydroxides (e.g., CaO and Ca(OH)2) exhibit excellent fluoride removal effects. Based on this, in this work, we specifically selected spent LIBs containing calcium (Ca) as the research object. We proposed a new strategy to effectively separate cathode materials from Al foil in spent LIBs by combining aerobic pyrolysis with mechanical stirring. Additionally, we used advanced characterization techniques such as thermogravimetric mass spectrometry (TG-MS) and thermogravimetric infrared (TG-IR) coupling to elucidate the in situ solidification mechanism of fluorine in spent cathode electrode sheets. During the pyrolysis process, no toxic or harmful gases, such as HF, were detected. However, it cannot be excluded that other types of gases may be released. The pyrolysis of cathode electrode sheets usually requires an argon environment at 550 to 600 °C. However, this strategy reduces the pyrolysis temperature required to separate the cathode material from the Al foil to 420 °C. During the pyrolysis process, fluorine-containing pollutants are directly converted into metal fluorides such as LiF, CaF2, and MgF2, etc, which are coated on the surface of the spent CAM. These fluoride coatings significantly improve the electrochemical performance of the direct regenerated CAM. This work not only converts fluorine-containing pollutants in spent LIBs into fluoride coatings for CAM in situ, thereby improving the electrochemical performance of directly regenerated electrode materials but also points the way toward the design of new battery electrode materials in the future.
Section snippets
Materials and reagents
The spent LIBs (LiNixCoyMn1?x-y, NCM) containing the Ca element used in this study were obtained from electric vehicle battery modules provided by Guangdong Bangpu Cycle Technology Co., Ltd. Firstly, to avoid the risk of short-circuiting or spontaneous combustion, spent LIBs were placed in 10 wt% NaCl solution and fully discharged, then cleaned and air-dried. Secondly, the spent LIBs were manually disassembled into a cathode electrode sheet, anode electrode sheet, separator, and case. The anode …
Micromorphology and properties of spent electrode sheet
The surface micromorphology of the spent cathode electrode sheet is analyzed using SEM and XPS, and the results are shown in Fig. 2. Fig. 2a shows that the organic binder tightly bonds the CAM together. EDS results exhibit that the electrode sheet contains Ni, Co, Mn, and O elements, which mainly come from spent CAM (Fig. 2b?e). However, C and F elements mainly come from conductive agents and organic binders, respectively (Fig. 2f,g). The C-C/C-H functional group was found in the XPS spectrum …
Conclusion
The pyrolysis of spent LIBs will lead to the decomposition of PVDF and organic electrolytes, releasing toxic and harmful gases such as HF. These gases not only corrode equipment but also pollute the environment. In this work, we propose a strategy to eliminate the HF gas generated during the pyrolysis of spent LIBs. Specifically, by adding a small amount of alkali metal oxides (e.g., CaO) as dopants during the electrode material manufacturing stage. These oxides can act as fluorine removal …
Environmental implications
The pyrolysis of spent lithium-ion batteries (LIBs) can emit hazardous gases like HF, posing environmental and equipment risks. This work introduces a novel approach using alkali metal oxides (e.g., CaO) added during battery manufacture to capture fluorine, preventing HF release during recycling. By transforming harmful gases into non-toxic metal fluorides, which also enhance the material’s electrochemical properties, this method effectively mitigates pollution. Additionally, it lowers the …
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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ABSTRACT ONLINE AT
https://www.sciencedirect.com/science/article/abs/pii/S0304389424031327?via%3Dihub
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