Modeling and Experimental Validation on a Bipolar Membrane Electrodialysis Stack for Lithium Sulfate Separation
Authors: Zhu Jiyong , Asadi Anahita , Chuang Po-Ya Abel , Zhang Ruiming , Jung Joey Chung-Yen , Sui Pang-Chieh 
Published: August 2024

Recycling lithium-ion battery materials is a crucial step to achieving a circular economy. Among a few methods for recycling spent battery materials, a novel hydrometallurgical recycling process with leaching by acids and precipitation by bases was proposed [1]. Lithium sulfate (Li2SO4) leachate solution is generated at the end of the closed-loop recycling process. Such Li2SO4 solution can be separated by electrodialysis (ED) to form H2SO4 and LiOH solutions, which are reused for leaching and precipitation of the recycling process. The present study attempts to model this ED process and validate the model with experimental data.

An ED stack based on bipolar membranes (EDBM); see Fig. 1a, was built with repetitive unit cells, each consisting of a bipolar membrane, anion exchange membrane (AEM), and cation exchange membrane (CEM). Li2SO4 enters the dilute channel and splits into SO4
2- and Li+, which travel through the AEM and CEM, respectively. Water electrolysis occurs within the bipolar membrane to produce H+ and OH-, which react with SO4
2- and Li+ to form H2SO4 and LiOH, respectively; see Fig. 1b. A two-dimensional (2D) multiphysics model is developed to elucidate the coupled transport processes within the unit cell. Conservation equations for mass, momentum, and species are solved. The transport of ions is described by the Nernst-Planck equation. Electroneutrality equations are satisfied in these membranes and channels. Electro-osmotic water transport across the membranes is considered.

The effective cross-sectional area of the stack was 121.8 cm2 (70mm×174mm). The flow channel between consecutive membranes was 0.8 mm, with a mesh inserted to facilitate mixing. Experiments were carried out with the EDBM stack over different conditions such as current density, number of cell pairs, and flow rates. All experiments were operated at constant current mode [2]. The feed, acid, and base solutions were recirculated through their tanks at initial concentrations of 1.1, 0.1, and 0.1 mol/L. The ion concentrations of the solutions were measured periodically with inductively coupled plasma (ICP) mass spectrometry. The weight and the pH value of the solution tanks were also recorded.

The concentration of H2SO4 is found to increase with time linearly, whereas that of LiOH levels off due to a significant electro-osmosis that drags water through the CEM into the concentrate channel. This drag reduces the performance of EDBM due to the dilution of water into the concentrate compartment. However, electro-osmotic water transport also leads to a velocity increase in the concentrate channel, which is consistent with the simulation results in our previous ED model [3]. It is found that the voltage loss across the bipolar membrane ca. 1V accounts for a significant part of the unit cell of about 1.27V.

The migration of ions in electrodialysis mainly relies on diffusion and electric drive. The streamlines and the concentration distribution of Li+ under different voltages indicate that increasing voltage accelerates ion migration, see Fig. 1(c). Furthermore, the effect of the potential and concentration variations on EDBM performance is also investigated. The effects of applied current density and inlet velocity are studied on species concentrations and fluxes. Increasing the stack current density accelerates the separation process, see Fig. 1(d). It is found that increasing the solution flow rate can increase LiOH solution concentration, ion recovery rate, and current efficiency. However, the impact of the flow rate is low.

This work demonstrates that EDBM can be a simple and energy-saving alternative to the current chemical precipitation method to produce LiOH from Li2SO4. The present study provides new insights into optimal operation and design for Li2SO4 EDBM. The findings are helpful in determining how the stack can be scaled up for practical application.

[1] A review of recycling spent lithium-ion battery cathode materials using hydrometallurgical treatments, JCY Jung, PC Sui, J Zhang, J. Energy Storage 35, 102217, 2021.

[2] Electrodialysis of a Lithium Sulphate Solution: An Experimental Investigation. B Kang, D Kang, JCY Jung, A Asadi, PC Sui, J. Electrochem. Soc. 169 (6), 063515, 2022.

[3] A Comprehensive Computational Fluid Dynamics Modeling of Lithium Sulphate Electrodialysis, A Asadi, HB Harandi, JCY Jung, PC Sui, J. Electrochem. Soc. 170(9), 093502, 2023.

Figure 1