Abstract: The present invention describes systems and methods of a novel hydrometallurgical process to perform reductive-acid leaching and separation of constituent compounds from solid material generated from the electrodes of lithium-ion batteries, or other source material containing target high-value materials. The process method involves the initial reductive-acid leaching with sulfur dioxide and sulfuric acid of the source material which may be performed in a single or a multi-step embodiment. In a single-step embodiment, the reductive-acid leaching results in two outlet streams, a leachate solution and a bulk solid, such as graphite. In a two-step embodiment, a dilute reductive-acid leaching results in a lithium brine that may be bled as a product stream. The resulting liquor, or leachate, can be subjected to precipitation and oxidation steps to remove other compounds except, for example lithium, cobalt, and nickel.