When Ag is replaced by alkali metal Li in AgGaS 2 , the thermal conductivity is apparently increased from 1.2 W/(m•k) (AgGaS 2 ) to 5.1 W/(m•k) (LiGaS 2 ), despite the fact that the reduced crystal symmetry could decrease the phonon lifetime as well as thermal conductivity. The underlying sciences and the intrinsic source of the confusing large thermal conductivity of LiGaS 2 are studied in detail by anharmonic lattice dynamic theory based on the first-principle calculation. It is confirmed that large phonon relaxation time of acoustic and lowenergy optical phonons in LiGaS 2 account for the increased lattice thermal conductivity. Physical mechanism of phonon lifetime are discussed on both the number and strength of individual three-phonon scattering channels. Large value w-JDOS of LiGaS 2 is observed due to more optical phonon modes in LiGaS 2 than that in AgGaS 2 , which could reduce phonon relaxation time. However, low magnitude of mode Gruneisen parameters of LiGaS 2 are demonstrated in the lowfrequency domain especially for three acoustic phonon modes (−5.7, −3.8, −3.1 for AgGaS 2 and −1.9, −1.2, −0.4 for LiGaS 2 ), which is considered as the driving force of the increased phonon relaxation time and lattice thermal conductivity of LiGaS 2 . To essentially understand the difference in phonon anharmonic behavior, electronic study of both materials are provided by ELF and chemical bonding analysis. The results show that the introduction of lithium atom changes the generation of low frequency phonons from Ag−S bond to Ga−S bond vibration. Due to the stronger covalent nature of Ga−S bond, weak phonon anharmonicities are aroused in the low frequency domain, which can ultimately lead to high phonon relaxation time and large thermal conductivity of LiGaS 2 .

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