Unveiling and identifying the overlooked fluoride hazard derived from spent lithium-ion battery recovery.

Introduction

New energy such as solar, wind, water, and heat supported by lithium-ion battery (LIB) is challenging traditional fossil fuels. How to achieve the sustainable development of LIB industry has become a hot spot in global competition [1], [2]. Herein, recycling waste LIB has attracted wide attention because it can construct the industry closed-loop [3], [4]. After decades of endeavors, the internal cycle of critical resources in the LIB industry has been feasible. Until December 2023, the handling capacity for waste LIBs by the authorized enterprises in only China has amounted to 3.793 million tons per year, far exceeding the global waste LIB production [5]. However, this rapid expansion has processed without adequate consideration of associated environmental impacts, constraining overall sustainability. Hence, exploring the underlying environmental pollution of LIB recycling is crucial for its sustainable development.

LIB recycling focused on the recovery of critical metals in cathodes, such as lithium (Li), cobalt (Co), nickel (Ni), and manganese (Mn) [6], [7], [8]. In addition to these metals, the cathode materials also contained organic binders [9]. However, due to the low content (1–3 wt%) [10], the binder had attracted little attention and its whereabouts are unclear during the LIB recycling. As a result, the binder has “lost” in the LIB recycling. Although the mass proportion of binder in cathode materials was low, its total amount could not be ignored. Global shipment of LIB binders reached 82,000 tons in 2022, with an annual increase of 60.8 % [11]. By 2030, the shipment is estimated to exceed 500,000 tons. Such a large number of organic binders being lost in the process of LIB recycling will pose a huge environmental security hazard. Therefore, it is urgent to reveal the whereabouts of binders during LIB recycling.

The cathode material contained active particles, conductive black, and polyvinylidene fluoride (PVDF) binder. Under the bonding force of PVDF, the active particles and conductive black were chemically bonded to the aluminum foil [12], [13]. Therefore, the cathode material was characterized as “one body and two sides”, posing both resource properties (active particles) and environmental hazards (PVDF). In other words, recycling of active particles corresponded to the removal of PVDF. Despite this, most researches focused on the recycling of active materials while little interest was put on the PVDF removal [13], [14], [15]. Thus, we tried to summarize the removal of PVDF from the related studies on active particles separation. It was found that methods including dissolution [16], [17] and decomposition could be used to lose PVDF’s adhesion, achieving the recovery of active particles. Therein, methods of molten salt [18], solvent chemistry [19], [20], and high voltage pulse [21] have been used to dissolve PVDF. Essentially speaking, however, the PVDF was just transferred by these methods and its final disposal was not introduced. Thus, these methods were far away from application [22]. Relatively, the PVDF pyrolysis using thermochemical methods was a promising industrial treatment way [23], [24]. But similarly, these methods still focused on the recycling of active particles, with scarce introduction to the characteristics and mechanism of PVDF pyrolysis. Consequently, a key unresolved question emerged: significant quantities of PVDF binder were not recovered during LIB recycling. PVDF pyrolysis in lithium-ion battery recycling releases toxic fluorides. Despite effective defluorination (>98 %) via strategies like in-situ sequestration [25] and catalytic co-pyrolysis [26], molecular mechanisms remain unresolved. Critical gaps include unidentified atomic-scale bond cleavage pathways, unmonitored dynamics of transient fluorinated species, and unquantified effects on kinetics. This mechanistic void impedes targeted emission control in industrial recycling. Therefore, it is very necessary to fully reveal and identify the pollution characteristics of PVDF in LIB recycling process, informing industry responses to under-characterized environmental hazards.

In this paper, we first analyzed the reasons for overlooking PVDF hazard in recycling waste LIB recycling, to clarify that the research difficulties lied in revealing the real thermochemical properties of low-content PVDF in waste cathode. To address this, we established a response mechanism of cathode particle separation to PVDF pyrolysis, accurately revealing the thermochemical properties of PVDF. Then, through simulation experiments, the pyrolysis products were in situ tracked and analyzed to further identify the pyrolysis characteristic of PVDF. To further unravel the pyrolysis mechanism, the kinetic mechanism of PVDF was clarified by kinetic calculation. Combined with DFT calculation, three pathways of PVDF pyrolysis were proposed. Finally, the potential PVDF emissions of the LIB recycling were estimated. Therein, the distribution of pyrolysis products was identified based on pyrolysis experiments, highlighting the underlying fluoride pollution in LIB recycling industry. In this paper, we brough back the “lost” PDVF in LIB recycling, and identified its pollution characteristics. The findings can provide a theoretical basis for the recovery or disposal of PVDF in the future.