With the impending surge in retired lithium-ion batteries, developing efficient strategies for recovering valuable elements has attracted significant attention. This study presents an innovative closed-loop recycling method that integrates NH₄Cl reductive roasting with a selective ammonia leaching system to achieve the complete recovery and regeneration of lithium and manganese from spent LiMn₂O₄ cathodes. The reaction mechanism was elucidated using XRD, SEM-EDS, and XPS demonstrating that Mn⁴⁺/Mn³⁺ in the LiMn₂O₄ spinel structure is simultaneously reduced and chlorinated by NH₄⁺ from molten ammonium salts. This synergistic process efficiently converts the cathode material into water-soluble LiCl and (NH₄)₂MnCl₄. Notably, residual nitrogen is stored and recycled as NH₄⁺, with no impurity cations introduced during roasting. Under optimized roasting conditions (350 °C, 15 min, w(s-LMO)/w(NH₄Cl) = 1:2.5), the chlorination extent of manganese reached 88%, with the residual fraction stabilized as Mn₃O₄, while the lithium conversion efficiency approached 96%. Subsequent leaching in an NH₃·H₂O–H₂O system enabled the nearly complete separation of Li and Mn, yielding battery-grade Li₂CO₃. The incorporation of 2 % H₂O₂ as an oxidizing agent facilitated the selective precipitation of over 99 % of the manganese in the form of spherical nano-crystalline Mn₃O₄, while the lithium leaching efficiency remained virtually quantitative. The overall recovery rate for lithium reached 96%, while that for manganese approached 100%. Thermodynamic analysis and comprehensive characterization reveal the underlying mechanisms governing this selective manganese precipitation. Finally, the regenerated LiMn₂O₄ cathode material synthesized via the closed-loop process exhibited excellent structural integrity and electrochemical performance, confirming the viability and sustainability of the proposed methodology.