I recently found this video by El-Cell illustrating their in-operando optical imaging cell. I think this type of imaging is not only a good way to study battery materials with significant optical variation, but also a great way to visualize electrochemistry for specialists and non-specialists alike.
The bottom electrode is graphite, the anode (negative terminal) in nearly 100% of commercial lithium-ion batteries sold today (and the focus of my research). Discharged (i.e. empty) graphite is gray in color. As lithium ions insert inside the graphite particles, their color changes from gray to red to yellow.
The top electrode is lithium metal, which is commonly regarded as a “next-generation” anode material. The regions on the surface that grow and shrink are commonly called “dendrites”. After many battery cycles, a dendrite may reach the other battery electrode, which causes an electrical short. Controlling the formation of these dendrites is the key to safe, high-energy batteries and is the focus of significant research efforts today.