By devising a method that provides a detailed view inside a lithium-ion battery while it is running, researchers have come up with a way to gather reconnaissance information about processes that could trigger disastrous battery failure. Under some charging conditions, lithium can accumulate on a battery’s anode, leading to uncontrolled growth of needlelike metal dendrites that can cause hazardous short circuiting. Lithium metal, an ideal anode material based on its exceptional charge-storage capacity, carries a high dendrite risk. So manufacturers use lower capacity carbon anodes, which are safer but not fully dendrite-proof. To study this poorly understood process, Andrew J. Ilott and Alexej Jerschow of New York University and coworkers developed an 1H magnetic resonance imaging (MRI) method and used it to watch dendrites grow inside a Li-ion battery as it was being charged (Proc. Nat. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1607903113). Unlike an earlier Li MRI method developed by the team, which images dendrites directly, the new method measures “shadows” formed by the magnetic properties of the growing dendrites in the surrounding electrolyte solution. Because the shadows are roughly 20 times as large as the dendrites, the new method provides much improved spatial resolution. It also boosts the time resolution, owing to the relatively shorter scan times for 1H NMR.