<p>The challenge of dendrite growth limiting metal battery lifespan, despite extensive chemical composition optimizations for artificial solid electrolyte interphases (ASEIs), necessitates the development of ASEI design routes. Regulating the ASEI crystallographic microstructure offers a promising yet underexplored solution with efficacy uncertainty. Critical issue lies in what is the optimal crystallographic microstructure state. Using the ZnS ASEI in aqueous Zn batteries as a case study, here we report the effects of grain orientation and grain boundary density on the performance of Zn negative electrode. The results reveal the existence of an optimal microstructure state—predominant in-plane (111) orientation coupled with a critical grain boundary density of ~55 μm/μm²—which delivers an 18-fold lifespan extension, and over 3400 cycles with a Coulombic efficiency of 99.92% at 5 mA cm^-2, surpassing the efficacy of most chemical composition manipulations. Mechanistically, the (111) orientation integrates higher electrochemical kinetics and mechanical strength. As grain boundary density increases to tens of μm/μm², the enhancement in electrochemical kinetics coincides with compromised mechanical strength, with their trade-off delineating the critical density that maximizes electrode cycling stability. Our findings exemplify an efficient ASEI design route—crystallographic microstructure engineering.</p>