The recovery of spent lithium-ion batteries is essential for the sustainable development of renewable resources and environmental protection. Low lithium recovery efficiency and high energy consumption are the main problems in the current recycling technologies for black mass of spent NCM batteries. In this work, flash joule heating (FJH) method was innovatively applied to preferentially recover lithium from the actual raw material in industry. In particular, we firstly revealed the Li-phase conversion behavior in a nonequilibrium and high temperature state. Under the optimal FJH condition (1400 ℃, 90 s), the main Li phases in the product are converted into Li2O and Li5AlO4, while the transition metals are reduced to elemental forms and low valence oxides. The Li-phase conversion behavior and FJH characteristics matched excellently, so 91.8% of Li could be selectively extracted by water. Ni, Co, and Mn are recovered by sulfuric acid leaching with the efficiencies over 98%. This innovative method combines the phase transition behavior of lithium with the advantages of rapid joule heating, offering a new pathway for recycling spent LIBs in both theory and technology, and is extremely appealing owing to its energy conservation and high recovery efficiency.