Black mass, derived from the recycling of spent lithium-ion batteries (LIBs), is a complex mixture containing valuable metallic ions such as lithium (Li⁺), nickel (Ni²⁺), manganese (Mn²⁺), cobalt (Co²⁺), cupper (Cu²⁺) and aluminum (Al³⁺). These metals are critical for the production of new high-energy-density batteries, particularly in the context of the growing demand for electric vehicles (EVs) and renewable energy storage systems. Our preliminary study focuses on the recovery of high-purity lithium hydroxide monohydrate (LiOH.H₂O) from the black mass of spent NMC811 cathodes provided by HONDA R&D Co., Ltd., which consist of Ni, Mn, and Co in a ratio of 8:1:1. This material is of particular interest due to its high lithium content and its relevance to next-generation battery technologies. The process began with acid leaching using sulfuric acid (H₂SO₄), which effectively dismantled the cathode structure, dissolving Al³⁺, Cu²⁺, Ni²⁺, Mn²⁺, Co²⁺, and Li⁺ into a leaching solution. To remove Fe³⁺, Al³⁺, and Cu²⁺, potassium hydroxide (KOH) was introduced, raising the pH from 6 to 11. This step indicates precipitated Al³⁺, Ni²⁺, Mn²⁺, and Co²⁺ as their respective hydroxides (Al(OH)₃, Ni(OH)₂, Mn(OH)₂, and Co(OH)₂), leaving a solution enriched with Li+ but contaminated with high concentration of K+ ions. The presence of K+ posed a significant challenge due to its chemical similarity to Li+, necessitating a highly selective separation method. To address this, we employed solvent extraction using Solvay's CYANEX® 936P, a phosphorus-based extractant designed explicitly for selective Li recovery. The extractant was diluted 5 times with a diluent of Exxsol™ D80, and the extraction pH was carefully adjusted using 48% KOH. Optimal extraction conditions were achieved at an organic-to-aqueous (O/A) ratio of 1/1 at pH 12, resulting in an impressive Li extraction efficiency of 99%. The subsequent back-extraction of the organic phase using 0.5M hydrochloric acid (HCl) yielded a high-purity lithium solution (>99% purity). To convert the lithium chloride (LiCl) solution into lithium hydroxide (LiOH), we utilized strong anion exchange chromatography, which facilitated the exchange of chloride (Cl⁻) for hydroxyl (OH⁻) anions. Finally, the high-purity LiOH solution was subjected to an evaporation-crystallization process, resulting in LiOH.H2O crystals. This product meets the stringent purity requirements for use in advanced battery manufacturing. This study demonstrates a scalable and efficient method for recovering high-purity lithium from spent NMC811 batteries, contributing to the circular economy and reducing reliance on primary lithium resources. The proposed process not only addresses the environmental challenges associated with battery waste but also supports the sustainable production of high-performance energy storage materials.