Spent lithium ion batteries (LIBs) are piling up from the electric-vehicle revolution and the increased demand in portable electronics. Currently, there is no environmentally friendly LIB recycle process really commercialized. This paper describes a series of experiments to advance the knowledge about recovery of metals from the spent battery cathode materials and to develop a novel environmentally friendlier closed-loop hydrometallurgical process. The leaching conditions are optimized by the bench scale experiments and various options for recovering critical LIB cathode metals are investigated. The semi-continuous locked-cycle campaigns document the dynamics of the recycled streams and yield much useful data. The behaviour of the leached cobalt is strongly affected by sulphate supersaturation and the location of sodium sulphate crystallization, but successful operation can be maintained when the sodium sulphate level is carefully controlled. These solution properties are the key factors when recycling spent cathode metals using systems based on sulphuric acid and sodium salts. Experimental results also show that the systems may reach supersaturation when circulating loads are incorporated, demonstrating the significance of sulphate levels in such systems. The closed-loop flowsheets are developed to recover metals, reduce discharges, and minimize environmental impact in the recycling of the spent cathode materials.