output. [2] Li metal has been regarded as an ideal anode material because of its ultrahigh theoretical specific capacity (3860 mAh g −1 ) and very low electrochemical redox potential (−3.040 V vs standard hydrogen electrode). [3] However, the practical utilization of Li metal anode is greatly hampered by the formation of Li dendrites during repeated charge/discharge cycles and the low Coulombic efficiency. The most serious concern is the direct contact of Li metal with the cathode material after Li penetrates the battery separator layer, which causes short circuiting and can lead to serious safety hazards. [4] Therefore, significant efforts have been devoted to detect, understand, and prevent the Li dendrite formation processes within liquid electrolytes. [5] The use of solid-state electrolytes (SSEs) in Li metal batteries (LMBs) is considered attractive as most SSEs are not volatile or flammable. [6] SSEs are also thought to be less susceptible to the growth of Li dendrites. However, recent studies have shown that Li dendrites can form in the voids and grain boundaries of Li 7 La 3 Zr 2 O 12 (LLZO) based SSEs. [7] Once the dendrites penetrate through the SSEs, the interactions between Li metal and cathode materials set off a chain of highly energetic reactions. Mechanistic pathways of such far-fromequilibrium processes are not understood and approaches to their control and/or mitigation are missing.Overlithiation can also occur when the LIBs are overdischarged or during synthesizing Li-rich cathodes. For example, even though some Li-rich cathode materials (e.g., Li[LiNi 1/3 Mn 1/3 Co 1/3 ]O 2 (Li-rich NMC)) have shown greatly increased reversible capacity, [8] Li x CoO 2 (LCO), the most widely used cathode material, does not have a stable Li-rich (x > 1) form. [9] Overlithiation in LCO causes degradation in structural integrity and device performance. It has been reported that LCO will eventually be converted to Co metal during the overdischarge process, and various reaction intermediates, including Li 1+x CoO 2−y , Co 3 O 4 , and CoO have also been proposed through ex situ studies. [10] Recent in operando X-ray absorption spectroscopy (XAS) and spectroscopic transmission X-ray microscopy (TXM) studies reveal a core-shell conversion pathway from LCO to Li 2 O and Co metal during overlithiation of the cathode. [11] However, the atomic-scale structural and chemical evolution during the overlithiation reaction is still unclear.Nonuniform and highly localized Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries. However, the mechanisms of cathode failures upon contact with Li metal are far from clear. In this study, using in situ transmission electron microscopy, the interaction of Li metal with well-defined, epitaxial thin films of LiCoO 2 , the most widely used cathode material, is directly visualized at an atomic scale. It is shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to e...