Removal of Polyvalent Cations from Aqueous Solutions Prepared By Roasting and Dissolving Spent Lithium-Ion Batteries By Ion-Exchange Membrane Electrodialysis
Authors: Segami Yukari , Shin-mura Kiyoto , Kitada Ayumi , Sasaki Kazuya 
Published: December 2024

Background and purpose

Efforts to curb CO2 emissions, which cause global warming, are spreading around the world, and countries and regions are attempting to ban new sales of gasoline and diesel vehicles that emit CO2. Therefore, it is expected that the spread of electric vehicles will increase in the future, and that the number of used lithium-ion batteries will increase accordingly. In order to reduce the environmental impact of these battery products, Europe adopted the European Battery Regulation in July 2023, making it mandatory to recycle used batteries. The regulations require recycling of 50% of lithium materials from spent batteries by the end of 2027 and 80% by the end of 2031. In addition, by August 2031, 6% of the materials must be made from recycled lithium. Recycling technology has not yet been established for large lithium-ion batteries, such as those used in cars, because their exteriors are more durable and the risk of ignition is higher than for smaller ones used in mobile phones. Therefore, early establishment of recycling technology is necessary.

Lithium is typically recycled by roasting the used lithium-ion batteries, dissolved, and separated through hydrometallurgy. In addition to lithium, polyvalent cations of Cu, Mg, Ca, Mn, Al and so on, which are impurities, are present in high concentrations in the roasted and dissolved solutions of lithium-ion batteries. In hydrometallurgical refining, there is a concern that a high concentration of impurity polyvalent cations may cause a decrease in the recovery rate of lithium. Therefore, it is necessary to reduce the concentration of polyvalent cations in the battery roasting and dissolved solution as much as possible. The purpose of this study was to clarify the possibility of reducing polyvalent cations contained in the roasted and dissolved solution of lithium-ion batteries by electrodialysis using a monovalent permselective cation exchange membrane.

Previous research has shown that the monovalent permselective cation exchange membrane used in this study allows small amounts of multivalent ions to permeate. If the solution on the negative electrode side is basic, there is a concern that multivalent ions will precipitate as hydroxides on the surface of the cation exchange membrane and inhibit the permeation of ions. Therefore, in this study, we considered using an acidic solution as the negative electrode solution of the cation exchange membrane. In this study, we use a Ni electrode, which is inexpensive and easy to use industrially. However, the Ni electrode dissolves in acidic solutions. To prevent its dissolution, the electrode was placed in a basic solution separated from the acidic solution by a bipolar membrane.

Experimental method

Chronoamperometry (CA) was performed using an electrochemical cell with four solution baths separated by two bipolar membranes and a cation exchange membrane. A schematic diagram is shown in Fig.1. The electrode solutions were lithium hydroxide, and the initial solution in the acid solution bath was 1wt% hydrochloric acid. The stock solution tank contained an aqueous solution of roasted and dissolved lithium-ion batteries. The nickel electrode used was a 3 cm × 4 cm nickel mesh (100 mesh, φ 0.10 mm) with a nickel wire (φ 0.30 mm) attached. A voltage of 5 V was applied to CA for 24 hours. Inductively coupled plasma optical emission spectrometer was performed on the solutions in the stock solution and acid solution after the test and the amount of lithium transferred, and the removal rate of polyvalent cations were calculated.

Results

The ratio of multivalent ions to lithium in the acid solution after the test was decreased compared to the stock solution before the test, indicating that the multivalent ions were successfully reduced. There was a concern that precipitates would form on the surface of the cation exchange membrane, but no precipitate was formed. However, when a voltage was applied for a long time, the cations precipitated on the surface of the bipolar membrane faces the acidic solution, even though the pH of the acid solution after the test was between 1 and 2 where in principle they would not precipitate. In the presentation, we will report on the amount of lithium permeation, the removal rate of polyvalent cations, and the estimated mechanism of precipitation formation.

Fig.1 Schematic diagram of ion-exchange membrane electrodialysis.

BP: Bipolar membrane, C: Monovalent permselective cation exchange membrane

References

(1) the 91st ECSJ Annual Meeting S11_1_03.

(2) the European parliament and the council of the European union. Regulation (EU) 2023/1542 of the European parliament and of the council. 33-34,191 (2023).

Figure 1