Low cycle efficiency and dendrite growth are two critical barriers for rechargeable batteries using Li metal as negative electrodes. Current research efforts are mostly focused on how to suppress dendrite growth to improve the abuse tolerance, while the low cycle efficiency is temporally fixed by using large amount of extra lithium. However, the low cycle efficiency remains a significant technical challenge to achieve the high energy density (>350 Wh/kg) for Li-S or Li metal rechargeable batteries, and in new systems based on solid electrolytes, where the amount of excess Li must be minimized. we have developed a comprehensive set of in-situ diagnostic techniques and a model SEI/Li electrode system to understand the coupled mechanical/chemical degradation of SEI layer during Li cycling at different material level with controlled Li capacity. Based on those fundamental understanding, we developed some coating design strategies to achieve high cycle efficiency and extend the cycle life of high energy density batteries for EV application.