Batteries usually have two terminals — an anode and a cathode. Engineers are trying to remove the anode from solid-state batteries (with solid electrolytes) to make them lighter and more capacious. During charging, metal should deposit itself as a layer on the contact, and then vanish completely during discharge. But after a few cycles, capacity drops.
The VE-LEEM method allowed a peek into this hidden space. It turned out that metal growth is universal: like a snowfall, where only the density of falling flakes matters, not the surface below. Just like in industrial thin-film deposition (standard model of film growth). Lithium and sodium behave almost identically.
But the disappearance is asymmetric. It starts along the boundaries of microscopic grains — as if snow melts, leaving behind an icy crust. This residue, just a few atoms thick, is robust and prevents a new layer from taking hold, accumulating entropy (disorder) and damaging the battery. VE-LEEM helps engineers find materials where this crust is minimal.
A film just a few atoms thick left after lithium dissolves prevents new metal from attaching — like an invisible layer of soot that kills the battery.