Low cycle efficiency and dendrite growth are two critical barriers for rechargeable batteries using Li metal as negative electrodes, mainly due to the coupled mechanical/chemical degradation of the SEI layer formed on Li metal surface. We have developed a comprehensive set of in situ diagnostic techniques combined with atomic/continuum modeling schemes to investigate and understand the coupled mechanical/chemical degradation of the SEI layer/Li system including fundamentally understanding the surface and interface phenomena. We have found that the mechanical incompatibility between SEI and soft Li leads to the complicated mechanical behaviors of the lithium metal electrode during the plating and stripping process. We systematically investigated the relationship between surface morphology and current density distribution which results in an inhomogeneous Li plating/stripping process. Based on the fundamental understanding from the integrated in-situ diagnostics, atomic simulation, and continuum frame work, we have developed a new coating design strategy to achieve high cycle efficiency/dendrite free and extend the cycle life of lithium rechargeable batteries.