One overarching theme is that Li intercalation into a crystal structure generates massive expansive strain, on the order of several percent. The active material's ability to repeatedly tolerate these huge internal stresses is directly related to the reversibility of (de)lithiation, and therefore the cycling endurance of a cell. Materials with abrupt phase transitions further concentrate this strain from lattice mismatch at the sharp interface between lithiation and delithiated phases. From a mechanical perspective, these metal oxides are characteristically hard and brittle, so it has remained unclear exactly how such enormous strains are readily accommodated without cracking and pulverization of the crystals.