Lithium batteries have profoundly impacted our daily life, with extensive applications in portable electronics, electrical automotive and grid−scale energy storage applications. In order to achieve much higher energy density than the state−of−the−art, chemistries beyond Li−ion are currently being investigated and need to be made viable for commercial applications. Using metallic Li is among the most prominent choices for next−generation Li batteries, such as Li−S and Li−air systems. After falling into oblivion for several decades because of safety concerns, metallic Li is now ready for a revival. In this talk, I will present my fundamental studies on the failure mechanisms of Li metal, as well as the rational material designs to tackle the problems. In the first chapter, the backgrounds regarding lithium battery research will be introduced. More specifically, the basic principles and the current stages of lithium battery, the future direction of its development, the chemistries at the battery electrode interfaces, and the failure mechanisms will be thoroughly discussed. These aspects lay the foundation for the research presented in this dissertation. In the second chapter, I will present the new findings and fundamental understandings on Li metal failure mechanisms. In the studies, infinite relative volume change of the conventional lithium metal electrode was first identified to be a key contributor to its failure. Then, the failure was further studied from the corrosion points of view, where a new corrosion pathway was identified as the dominant origin of the corrosion in the long term. In addition, typical Kirkendall voids were observed in the corroded Li. The findings were further rationalized by detailed analysis of solid electrolyte interphase formed on the surfaces. In the third part, I presented our efforts in stabilizing Li metal anode by Adv. Mater. development. Based on the new understandings presented in Chapter 2, the stable "host" design for Li metal was proposed and demonstrated. Interfacial modification technology was also developed to further stabilize the electrode/electrolyte interfaces. The two methodologies were proven to be very powerful in stabilizing Li metal. In the next part, battery safety issue was tackled by materials design in advanced battery separators and solid−state electrolytes, both of which were prominent for future batteries employing high energy battery chemistries. On one hand, strong separators with Li dendrite sensing function was developed. On the other hand, solid−state electrolytes with highly improved ionic conductivity and modulus were demonstrated. In the final part of the dissertation, I will present my perspectives and outlooks for the future research in this field. To commercialize the high−energy and safe batteries based on Li metal chemistry requires continuous efforts in various aspects, including electrode design, electrolyte engineering, development of advanced characterization/diagnosis technologies, full−battery engineering, and possible sensor design for safe battery operation, etc. Ultimately, the combinations of various approaches might be required to make Li metal anode a viable technology.