This study evaluated three inorganic acids(HCl, HNO₃, and H₂SO₄) and three oxidizing additives(H₂O₂, KMnO₄, and K₂S₂O₈) to determine the optimal leaching conditions for an additive-assisted hydrometallurgical process aimed at the efficient and sustainable recovery of lithium from insoluble lithium phosphate(Li₃PO₄). Among the tested conditions, the combination of 0.5 M HNO₃ and 0.01 M H₂O₂ showed the most stable and effective performance, achieving nearly complete lithium leaching. More than 98.5% of Li recovered within 10 min, and extending the reaction time beyond 30 min did not significantly improve the leaching efficiency. Excessively strong oxidizing additives such as KMnO₄ and K₂S₂O₈ caused side reactions or reduced process controllability. These results provide fundamental insights into lithium recovery from insoluble lithium compounds and suggest practical directions for simultaneously improving process efficiency and environmental performance.