The existence of LiH and its formation mechanism in Li metal anodes are still poorly understood, partly due to the difficulty in direct observation of environmentally reactive and electron beam sensitive lithium metal and SEI . Aurbach and colleagues first proposed the possibility of hydrogen reacting at the Li metal anodes to generate LiH . In 2017, Cui and colleagues reported the first atomic resolution image Li metal lattice and SEI enabled by cryo-EM . In 2018, Kourkoutis and colleagues used cryogenic scanning transmission electron microscopy (cryo-STEM) electron energy loss spectroscopy (EELS) to directly identify the existence of LiH in Li metal anodes based on H and Li K-edges . In 2021, Wang and colleagues reported the existence of LiH in SEI with XRD characterization on cycled SEI in Li metal batteries . Despite reports on LiH formation in the Li metal anodes , little is known about where and when LiH is formed. There are hypotheses suggesting that hydrogen, as a byproduct of SEI formation and growth, may contribute to the growth of LiH in Li metal anodes. Therefore, bridging the gap in understanding interfacial reactions between Li metal anode, its SEI, and corresponding LiH formation bears great significance.