The growing demand for sustainable energy has accelerated the recycling of critical materials from spent batteries. In this context, this study investigates eutectic freeze crystallization (EFC) in a synthetic aqueous NiSO₄–CoSO₄–H₂O system, with the long-term goal of applying the method to real industrial lithium-ion battery (LiB) leachate streams. Binary phase diagrams for NiSO₄-H₂O and CoSO₄-H₂O were identified experimentally at eutectic points. In mixed systems, eutectic points were determined from temperature–concentration variations by adding 1–12 wt% CoSO₄ to a ≈ 20 wt% NiSO₄ solution. Similarly, 1–12 wt% NiSO₄ was incrementally added to a ≈ 20 wt% CoSO₄ solution to examine the influence of nickel. The eutectic points of both systems converged as the concentration of the added metal increased.

Experimental analysis (XRD, SEM-EDS, ICP-OES) revealed the formation of separate NiSO₄·7H₂O and CoSO₄·7H₂O crystal phases, and notably, CoNi(SO₄)₂·12H₂O was detected for the first time at eutectic points. Comparative simulations deviated from experimental findings, highlighting the need for improved low-temperature models. After washing, the generated ice contained <0.2 wt% Ni and Co, enabling its reuse as process water. These findings demonstrate EFC's potential for high-purity metal salt recovery and provide the necessary thermodynamic framework for its application to real-world industrial battery recycling.