High performance electrochemical energy storage is a key component of the renewable energy transition. Li-ion batteries represent an increasingly important contribution across a wide range of energy applications. However, Li-ion electrochemistry is almost entirely dominated by a small handful of active materials. On the positive electrode, these include LiCoO 2 , LiFePO 4 , LiMn 2 O 4 spinel, and their isostructural analogues. Nevertheless, this meager selection of crystal structures has generated an incredibly diverse array of active materials; by varying doping, coating, aggregation state, defects, and other crystallographic parameters. These microstructural motifs play a critical role in tuning Li intercalation phase transitions and ultimately the performance of devices, including energy efficiency, capacity, and stability. Despite their importance, remarkably little is known about the phase transitions at the nanoscale in most materials, and the mechanisms by which microstructure influences them.