and cracks between the Li metal and the SE (such as Li 7 La 3 Zr 2 O 12 , LLZO, and Li phosphorus oxynitride, LiPON), and eventually penetrate the SE. [5][6][7] Another critical problem is the interfacial instability arising from the contact loss at the Li/SE interface during stripping, which lowers the battery's cyclability and ultimately causes cell failure. [8][9][10][11][12] Thus, the dynamic behavior of the mechanical contact at the Li/SE interface needs to be understood to design a better battery cell.A challenge to maintain mechanical contact at the Li/SE interface is void formation. [13][14][15][16] Void formation leads to interfacial porosity, surface roughness, and consequently contact loss. [17][18][19] Recently, experimental characterization has shown that the stack pressure is an important factor in preventing void formation during stripping in SSBs. [18,20,21] It has been proposed that the pressuredriven creep deformation of Li metal replenishes the void at the Li/SE interface. [18,22] However, void formation at the solid-solid interface involves stress, contact, reaction, and Li/Li + transport, which are challenging to observe and measure experimentally. Therefore, the understanding of contact issues during stripping is still in its infancy. Specifically, the fundamental questions as to how the external pressure and current as well as intrinsic material properties impact the internal void formation at the Li/SE interface are unanswered.Taking a deeper insight into the mechanism of interfacial void formation, when applying current density and stack pressure, the stripping current removes electrons from Li metal and releases Li + into the SE to migrate away from the interface (i.e., the flux of Li + migration away from the interface, J migration ). This generates a large number of vacancies in Li metal near the interface. The flux of the vacancies contributed by the Li metal creep, J creep , and diffusion, J diffusion , can transport the vacancies away from the interface and towards the bulk Li metal, as illustrated in Figure 1a. Recent kinetic Monte Carlo (KMC) simulations [23] show that for an ideal flat Li/SE interface, J diffusion is high enough to transport the vacancies away from the interface and maintain a smooth Li/Li 2 O surface even without the stack pressure (i.e., J diffusion > J migration where J # represents the magnitude of the flux), as illustrated in Figure 1b. However, such an ideal flat interface is unlikely due to the limitation of the experimental conditions and techniques, and pre-existing interfacial defects such as Interfacial instability from void formation at the solid-solid interface is one of the crucial challenges in solid-state batteries. However, the fundamental mechanism as to how stress is generated in lithium and thus impacts void formation has not been established. A general creep/contact electro-chemomechanical model is herein developed to reveal the mechanisms of void formation at the Li/solid electrolyte (SE) interface during stripping. Li stress calculation is ...