Unveiling the Influence of Carbon Impurity on Recovered NCM622 Cathode Material

Abstract

With the proliferation of market demand for lithium-ion batteries (LIBs) over the past decades, battery recycling has aroused extensive attention due to the environmental, supply, and economic issues caused by waste batteries. The hydrometallurgical recycling method has been widely adopted to recover cathode materials as a result of its wide applicability and high productivity. However, it is hard to completely eliminate impurities such as copper, aluminum, and carbon, which could bring significant impacts on recovered materials. Here, the influence of the carbon impurity on recovered LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode material is systematically investigated. It shows that the carbon impurity promotes nucleation during coprecipitation and forms holes in the cathode secondary particles after sintering which could enhance cyclability of the NCM622 cathode. The cathode with 0.2 atom % of carbon impurity displays the highest capacity of 159.9 mAh/g with a striking capacity retention rate of 97.9% after 100 cycles at 0.33C, but performs worse at high rates. Nonetheless, excess carbon (5 atom %) results in severe cation disorder and lattice distortion which significantly deteriorates the electrochemical properties of the NCM622 cathode. Therefore, it is important to strictly control carbon impurity during the recycling process for spent LIBs.

Positive Role of Fluorine Impurity in Recovered LiNi0.6Co0.2Mn0.2O2 Cathode Materials

“…To obtain cathode materials, precursors were mixed with Li 2 CO 3 in a mortar at a stoichiometric ratio of 1:1.05 (5% excess of lithium salt was used in order to compensate for the loss of lithium ions during sintering). Next, the mixture was subjected to a two-step sintering process: (I) heated up to 450 °C for 5 h, then cooled down to room temperature; (II) heated up to 850 °C for 18 h, followed by a same cooling method (the ramp rate was fixed to 2 °C per minute). ,, In the end, a total number of four cathode materials were obtained: LiNi 0.6 Co 0.2 Mn 0.2 O 2 (VNCM) and LiNi 0.6 Co 0.2 Mn 0.2 F x O 2– x ( x = 0.002, 0.01, and 0.05, named as 0.2FNCM, 1FNCM, and 5FNCM, respectively).…”

“…Next, the mixture was subjected to a two-step sintering process: (I) heated up to 450 °C for 5 h, then cooled down to room temperature; (II) heated up to 850 °C for 18 h, followed by a same cooling method (the ramp rate was fixed to 2 °C per minute). 15,18,19 In the end, a total number of four cathode materials were obtained: LiNi 0.6 Co 0.2 Mn 0.2 O 2 (VNCM) and LiNi 0.6 Co 0.2 Mn 0.2 F x O 2−x (x = 0.002, 0.01, and 0.05, named as 0.2FNCM, 1FNCM, and 5FNCM, respectively). 2.2.…”

“…According to earlier reports, a better ordered structure or slightly expanded lattice triggered by tiny ion substitution leads to the enhancement of rate capability and capacity retention, whereas excess impurity ions would interfere with the structural arrangement, resulting in a bad cathode morphology as well as the formation of an impurity phase, which causes significant deterioration in the cathode electrochemical performance. , Carbon, a common insoluble impurity coming from conductive additives, is proved to be an adverse factor in hydrometallurgical recycling. Although the carbon impurity does not participate in coprecipitation, cathode particles will show a much higher surface Co 2+ concentration in the presence of carbon after sintering, which contributes to the worse cation mixing and poor average capacity . Beyond those impurities mentioned above, another critical concern is the anion impurity, for example, fluorine released by the LiPF 6 lithium salt − but little effort has been made to investigate the impacts on synthesized products via hydrometallurgical recycling due to the existence of F – in the solution.…”

“…Although the carbon impurity does not participate in coprecipitation, cathode particles will show a much higher surface Co 2+ concentration in the presence of carbon after sintering, which contributes to the worse cation mixing and poor average capacity. 19 Beyond those impurities mentioned above, another critical concern is the anion impurity, for example, fluorine released by the LiPF 6 lithium salt 20−22 but little effort has been made to investigate the impacts on synthesized products via hydrometallurgical recycling due to the existence of F − in the solution. Hence, designing a series of trial reactions with different amounts of fluorine impurity to monitor the change in the recovered precursors and cathodes is essential to achieve a better understanding of impurity control in hydrometallurgical recycling.…”

The Effects of Phosphate Impurity on Recovered LiNi0.6Co0.2Mn0.2O2 Cathode Material via a Hydrometallurgy Method

“…Cation impurities such as Al 3+ , , Fe 3+ , and Cu 2+ , have drawn extensive attention in the past. Based on our previous work, insoluble carbon is determined to be a negative factor in hydrometallurgical recycling . Anion impurities such as fluorine (F – ) is found to be beneficial to the process, resulting in a degree of performance improvement for the recovered cathode compared to virgin .…”