Enhanced Performance of Recovered Cathode Materials with Fluorine Impurity in Hydrometallurgy Recycling

Abstract

With the growing lithium-ion battery (LIB) industry since its initial inception in the late 20th century, battery recycling is seen as an end-of-pipe process to resolve environmental and economic issues generated by the accumulation of waste batteries. In recent years, an increasing attention has been paid to the hydrometallurgy recycling due to its high recovery efficiency, low energy cost, as well as large-scale production capability. In order to achieve a high-quality recovered material, many resources have been poured into this field trying to develop better design and control of synthesis process. The impurities introduced in the recycling process is an important factor which could have unintended impacts on the final cathode product in a variety of aspects. The presence of fluoride ions in hydrometallurgy recycling, as one possible anion impurity which is released from the LiPF6 in electrolyte, aroused our research interest. In this work, the influence of the fluorine impurity on the recovered LiNi0.6Co0.2Mn0.2O2 (“NCM622”) cathode material via hydrometallurgy method is systematically investigated. The results show that the addition of fluorine impurity in the hydrometallurgy process improves the electrochemical properties of the recovered cathode materials.It is found that the fluorine impurity has lots of positive impacts on the synthesized cathode materials. First of all, the equilibrium of co-precipitation reaction is affected by the fluoride ions, resulting in the formation of holes in hydroxide precipitates. After sintering, cathode particles preserve a hollow structure which leads to an improved rate capability and cycle stability. In addition, surface oxygen sites of cathode materials are found to be substituted by impurity fluoride ions. As a result, an increased surface Ni2+ concentration contributes to a better cathode surface stability as well as a higher reversible capacity. The inclusion of fluoride (<1 at%) in cathode lattice is also beneficial to the bulk lithium-ion diffusion. Not only that, but the fluorine impurity has no clear detrimental effects on the particle morphology, crystal structure, transition metal composition, and element distribution of the cathode materials. Therefore, the electrochemical performance of synthesized cathodes has achieved different levels of improvement due to those positive influences from the fluorine impurity. Specifically, cathode with 0.2 at% impurity exhibits a discharge capacity of ~168 mAh/g with a remarkable retention of 98% at 1/3C after 100 cycles, about 8% higher than the capacity of virgin benchmark. Also, rate test outcomes of fluorine-affected samples are superior to that of virgin standard. In summary, the electrochemical properties of recovered cathode via hydrometallurgy process can be boosted by fluorine impurity. Unlike adverse impurities, fluoride’s positive role in hydro-recycling indicates that it is no need to strictly control them in pre-treatment steps. Figure 1