In this paper, a sustainable process to recover valuable metals from spent lithium ion batteries (LIBs) in sulfuric acid using ammonium chloride as reductant was proposed and studied. Being easily reused, ammonium chloride is found to be efficient and posing minor environmental impacts during the overall process. By investigating the effects of a wide range of parameters, e.g., H2SO4 concentration, NH4Cl concentration, temperature, leaching time, and solid-to-liquid mass ratio, the leaching behaviour of Li, Ni, Co, and Mn was systematically investigated. And the leaching mechanism and kinetics were determined by mineralogically characterization of residues at various reaction times and by fitting using different kinetic models. With this research, it is possible to provide a win-win solution to improve the recycling effectiveness of spent LIBs by using waste salt that is easily reused as the reductant.