Unveiling Li ⁺ Partitioning Thermodynamics in Electrolysis-Driven Li Metal Recovery

Securing a reliable lithium (Li) metal supply is vital for future energy needs. Electrolysis-based approaches show promise for Li recovery from brine, but the incorporation of hybrid electrolytes complicates Li+ transport across immiscible phases. Here, we develop a two-chamber electrolysis cell with a Li+-selective membrane separating synthetic brine anolytes from dimethyl carbonate (DMC) catholytes with varying hexafluorophosphate (LiPF6) concentrations. Real-time inductively coupled plasma mass spectrometry (ICP-MS) analyses combined with finite element analysis (FEA) elucidate the thermodynamics of Li+ transport at the brine/DMC interface. We find that [PF6]− counterions in DMC are essential for Li recovery by maintaining charge neutrality and enabling Li+ mass transfer. However, excess Li+ in the DMC suppresses Li+ partitioning from brine (low partitioning coefficient, K = ∼0.15) and drives back-diffusion, undermining recovery efficiency. These findings highlight that effective electrolysis-based Li recovery requires both appropriate counterions and maintaining the concentration ratio (CDMC:Cbrine) below K.