This work investigates a circular hydrometallurgical process for recycling lithium-ion battery materials using a multicomponent synthetic solution designed to mimic typical recycling leachates. The process integrates transition metal recovery, impurity removal, solvent recovery, and lithium carbonate production. Guided by thermodynamic simulations, the conditions for selective precipitation of mixed Ni–Mn–Co oxalates were optimized by adjusting oxalic acid concentration, temperature, and presence of chelating agent (NH4OH), reaching removals up to 98% for Co2+ and Ni2+, and 43% for Mn2+. Sulfates, considered an impurity of the process, were removed by adding CaO, raising the pH to a suitable level for Li2CO3 precipitation. Evaporation allowed the recovery of circa 70% of the system's water. Lithium was subsequently recovered as Li₂CO₃ by controlled Na₂CO₃ addition, with optimization of temperature and Na/Li ratio: 5% Na2CO3 excess at 70 °C, reaching 94.1% purity. The recovered oxalates were converted to oxides and used to resynthesize LNMC cathode material, confirmed through SEM and XRD analyses. The resynthesized NMC electrodes showed reversible intercalation and deintercalation of Li in the Li-ion cell. Overall, the results demonstrate a robust and promising route for producing battery materials from end-of-life cathodes, strengthening process circularity and reducing dependence on primary critical metals.