Different Effects of Impurities on Recovered Cathode Materials from Hydrometallurgical Recycling
Authors: Wang Yan , Zheng Yadong 
Published: July 2022

In the recycling field of lithium-ion batteries (LIB), the hydrometallurgical method has attracted lots of attention in recent years due to its high productivity, low cost and wide applicability. Tremendous effort has been made to improve design and control of manufacturing process to enable a stable and high-quality output. It is clear that the impurities introduced during the recycling process could pose a major impact on the final recovered materials in a variety of aspects. In particular, there are three types of impurities remained in the leaching solution. First, cations such as Al3+, Cu2+ and Fe3+ which come from dissolved metal debris, battery cases or current collectors for instance. Second, anions like F-, S2- and PO4
3- which are introduced from electrode materials or electrolyte salts. Lastly, neutral insoluble substances, for example, carbon particles that are added during the manufacturing process. In our work, systematic investigation has been carried out to explore the influence of different impurities on the recovered NCM622 cathodes via hydrometallurgical approach. It is found that different types of impurities have a unique impact on the cathode properties, and, even extraordinary when they are at certain concentration level.

In general, cations have obvious negative impacts on cathode morphology and powder density because of the reaction with OH- in alkaline environment which interferes with the nucleation and growth of precursors. Ultimately, it is very likely to obtain cathodes with irregular particle shapes, reduced sizes and poor tap densities under a high concentration of cation impurities. In these aspects, on the contrary, anions and insoluble substances cause little side effects on the recovered materials. It is noted that the presence of some anions during the co-precipitation, fluoride ions for example, could promote the formation of holes inside particles which can be beneficial to the electrochemical performance of the cathodes.

No crystallinity damage or heterogeneous element distribution have been confirmed within as-prepared cathode materials, indicating that a limited range of impurity (maximum 5 at% in our work) could not affect the overall periodicity and consistency in the materials. However, extra phases could be detected in some cathodes when impurity level reaches 5 at%. The excess amount of cation impurities prevents uniform precipitation, resulting in the creation of extra phase Li2MnO3 in the cathodes. Besides, a special extra phase of Li3PO4 exists under the presence of PO4
3- ions due to the reaction with lithium salt during sintering. Other impurities such as carbon and fluorine do not show similar mechanism. Extra phases in the cathodes will trigger a considerable deterioration in the electrochemical performance which needs to be avoided. Thus, special attention is required when impurity concentration exceeds 1 at% level, especially for cations.

According to the analysis, some impurity elements could merge into the cathode lattice. Cations like Al3+ and Cu2+ form metal hydroxides during the precursor synthesis, then they are incorporated into the cathodes by occupying the sites of transition metals. A small amount of replacement (< 1 at%), on Ni2+ in particular, could inhibit the cation mixing in the cathodes which results in an improved material quality. In addition, fluoride ion inclusion occurs by the substitution for oxygen in cathodes. This phenomenon not only lowers the average oxidation state of Ni ions, but also raises the lithium diffusivity in the cathodes which significantly increases the cathode capacity and rate performance. In contrast, impurities like carbon nano-particles and PO4
3- do not have clear interactions with the cathode lattice.

At 1 at% impurity level, NCM622-Cu shows the highest discharge capacity of 170 mAh/g after 100 cycles at 0.33C (1C = 175 mAh/g). Under the positive influence of fluorine, the cathode NCM622-F also presents a high discharge capacity of 167 mAh/g after 100 cycles at 0.33C which is about 8% better than that of virgin. In terms of Al3+ impurity, 1 at% of NCM622-Al still maintains a similar performance that is comparable to the virgin counterpart. However, 1 at% of carbon impurity has a clear adverse effect on the cathode performance and such a level of ion impurities leads to an even worse deterioration in the properties of recovered NCM622-Fe.

To sum up, the characteristics of recovered cathodes via hydrometallurgical process could achieve different levels of variation, either positive or negative at one particular aspect, due to the effects from impurity within certain concentration range. Thus, it is important to figure out the connections and trends behind these factors as well as the possible outcomes that may obtain to enable proper adjustments of the recycling scheme.

Figure 1