Lithium-ion batteries, which power most of today’s portable electronics and many vehicles, rely on electrodes that combine lithium with the oxides of cobalt, nickel, manganese, and other metals. Pure lithium electrodes provide higher charge capacity than mixed metal forms. But lithium metal’s reactivity leads to performance-reducing reactions with common battery electrolytes and poses safety hazards, especially during charging. Aiming to exploit the metal’s advantages while bypassing its problems, Stanford University scientists searched for a thin film to protect the electrode surface. The film had to be electrochemically stable; free from pinholes that allow dendrites to grow and short-circuit the battery, potentially igniting it; flexible enough to stretch and shrink during charging cycles without cracking; and able to allow unimpeded flow of lithium ions to and from the metal electrode. The team, which includes Yayuan Liu, Steven Chu, and Yi Cui, met those requirements by using a low-cost deposition method to grow a two-layer, nanometer-thin diamond film (Joule 2018, DOI: 10.1016/j.joule.2018.05.007). The double-layer design, which effectively blocks pinholes, led to energy-efficient cells that remained stable through more than 400 charge cycles.