The development of solid state electrolytes is very important for future high energy batteries. Among the different strategies, Li metal is a key electrode material for high energy batteries with a specific energy much higher than 300 Wh kg⁻¹. Despite of intensive efforts, significant challenges remain in direct utilization of Li metal anode in realistic high energy cells. This talk will summarize our current understanding of the scientific and technological challenges, discuss recent progress and propose potential directions of solid state electrolytes based on a high-energy cell design, fabrication and testing. The fundamental relationship between the Li anode and other cell components, including all active and inactive materials, is explored at the cell level in order to inspire more new ideas to effectively address the grand challenges in high energy and low cost batteries.