Pollutant-free pyrolysis strategy for direct upgrading of cathode materials from spent lithium-ion batteries.

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 (LiNi_xCo_yMn_1?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, LiNi_xCo_yMn_1?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.