Crystallization of cobalt sulfate within a typical hydrometallurgical process for the recycling of Ni-Mn-Co oxide or Ni-Co-Al oxide Li-ion batteries has been investigated. The cobalt sulfate salt was obtained by eutectic freeze crystallization (EFC) from a synthetic acidic cobalt strip liquor after solvent extraction. The ternary phase diagram of CoSO4–H2SO4–H2O was first established by the mixed-solvent electrolyte (MSE) model to predict and reveal the changes in the system during the freezing process and to assess the conditions required for EFC. Batch EFC experiments were then conducted for the cobalt strip liquor, which contained a low concentration of impurities. It is shown that with suitable control of supersaturation, seeding, and stirring, pure ice and salt crystals can be recovered as separate phases at the eutectic temperatures, with the crystalline salts in the form of a heptahydrate. The crystallization process can be described using the ternary phase diagram, but with certain deviations. The deviations might be related to insufficient data at the low temperatures in the MSE model and acid entrapment in crystals during the crystallization process. Finally, the performance of the EFC process has been compared to that of an evaporative crystallization (EC) using the same strip liquor. It was found that the CoSO4·7H2O product obtained by EFC was of slightly higher quality considering purity and crystal shape compared to that from EC.