###### Abstract
In general, the common co-milling reagents employed for organic decomposition can be divided into two main groups, reductive agents such as zero-valent metals (e.g., Fe, Al) [22; 23] and oxidative substances like persulfate and manganese dioxide [24; 25]. During the MC processes, zero-valent irons (ZVIs) are utilized as electron donors to accomplish the reductive dehalogenation of the target chemicals [26]. For oxidative degradation of halogenated pollution, the MC process is mainly activated by losing electrons to oxidants. Manganese dioxide is usually used as the electron acceptor of the redox couples Mn(IV)\(\rightarrow\)Mn(II) or Mn(IV)\(\rightarrow\)Mn(III) [27]. The above-mentioned MC methods are all conducted using a single milling reagent. Recently, the combination of reductive metals and oxides has been tested to improve the destruction efficiency of halogenated contaminants in MC treatment [28; 29; 30]. [PERSON] et al. researched the synergistic action of Bi\({}_{2}\)O\({}_{3}\) and Fe on the MC process for BDE 209 degradation [31]. The results showed that the destruction efficiency using Fe and Bi\({}_{2}\)O\({}_{3}\) is significantly higher than those using only Fe or Bi\({}_{2}\)O\({}_{3}\) under comparable operational conditions. Lithium cobalt oxides (LiCoO\({}_{2}\)), the cathode material of lithium-ion batteries (LIBs), have a certain oxidation capacity, as cobalt can be reduced from Co(III) to Co(II). [PERSON] et al. proposed an MC method for extracting the valuable metals Li and Co from LiCoO\({}_{2}\) with polyvinyl chloride (PVC) as a co-grinding reagent [32]. It is interesting to note that inorganic chlorides were detected in the MC products as about 90% of the chlorine in PVC had been transformed. This might suggest that LiCoO\({}_{2}\) could be applied to degrade halogenated organics in MC treatment. ZVI, as a cheaper but strong electron donor, has an excellent reductive dehalogenation efficiency in both liquid solvents and MC processes. In our previous study [33], reductive iron (Fe) powder could significantly promote the extraction efficiency of Li and Co from waste lithium cobalt oxide batteries via the MC process, and the valence state of Co was converted from Co(III) to Co(II) after an MC reductive process with Fe. Therefore, we anticipated that the combination of LiCoO\({}_{2}\) and Fe could improve the destruction of halogenated organics via the MC process. Hence, the objective of the present study was to investigate the degradation behavior of BDE 209 with the assistance of LiCoO\({}_{2}\) and Fe by implementing the MC method. The MC parameters including the molar ratio of LiCoO\({}_{2}\)/Fe, rotational speed, and ball-to-powder ratio were examined. The physical chemistry changes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) meter, X-ray photoelectron spectroscopy (XPS), and Fourier transform-infrared spectrometry (FT-IR). Ultimately, the intermediate products of BDE 209 were detected by GC-MS to illustrate the details of the mechanism of the BDE 209 degradation. As such, it will offer new insights with regard to both the resource utilization of the cathode material LiCoO\({}_{2}\) from the increasing number of spent LIBs and the elimination of POPs.