The 2H-to-1T' phase transition in transition metal dichalcogenides (TMDs) has been exploited to phase engineer TMDs for applications in which the metallicity of the 1T' phase is beneficial. However, phase-engineered 1T'-TMDs are metastable; thus, Stabilization of the 1T' phase remains an important challenge to overcome before its properties can be exploited. Herein, we performed a systematic study of the 2H-to-IT' phase evolution by lithiation in ultrahigh vacuum. We discovered that:by hydrogenating the intercalated Li to form lithium hydride (LiH), unprecedented longterm (>3 months) air stability of the 1T1 phase can be achieved. Most importantly, this passivation method has wide applicability for other alkali: metalS and TMDs. Density functional theory calculations' reveal that LiH is a good electron donor and stabilizes the IT' phase against 2H conversion, aided by the formation of a greatly enhanced interlayer dipole dipole interaction. Nonlinear optical studies reveal that air-stable 1T'-TMDs exhibit much stronger optical Kerr nonlinearity and higher optical transparency than the 2H phase, which is prornising for nonlinear photonic applications.