The progress of Li-ion batteries is severely hindered by the complexity of the chemical and physical processes and most importantly, the difficulty of observing these processes in situ during operation. Direct observation of Li ions in a battery in a non-destructive way is not possible by any conventional material analysis tech­nique. The consequence is that a large number of practical and fundamental questions remain including: how do Li-ion battery degradation mechanisms depend on battery conditions? How do the structural changes and electrochemical processes depend on the (dis)charge rate, and what actually determines the (dis)charge rate? An understanding of the interplay between structure, electrochemistry, and reaction mechanisms is required for battery design.

To answer these questions we require time-resolved and non-destructive struc­tural information including Li-ion positions and the Li distribution under operando conditions. These possibilities are offered by in situ neutron techniques that have been realized by the recent developments in neutron sources, detectors, and analysis methods. In situ neutron diffraction enables researchers to follow the structural changes and Li-positions upon all possible electrochemical manipulations in both the positive and negative electrodes. In situ neutron depth profiling determines Li — ion concentrations with high resolution in flat electrodes giving direct insight into the cylindrical batteries. Finally in situ neutron imaging allows a full three­dimensional picture of Li distribution in the battery to be determined. Recent advances in these three techniques will be discussed, including one of the major challenges, the cell design.