Energon cube

This is the safest form of Energon, used for transportation and storage.

G1 Animated Series

At first, only Soundwave seemed to be able to create the empty cubes from the frame on his chest. Later, though, all manner of Transformers were seen to store energon as cubes, including the Autobots.

Another aspect of the cubes disappeared early in the series: Originally, energy was converted and filled up pink-coloured cubes. Three cubes would then be flattened together to make a 'rainbow'-coloured cubes. Soon enough, though, the series would always show cubes as solid pink.

Beast Wars

In Beast Wars, it seems that energon cubes also occurred naturally, as a 'stable' form of natural energon. However these were a fraction of the size of the G1 Energon Cubes and seemed to be solid Energon rather than a cube containing it.

From Teletraan I: The Transformers Wiki, a Wikia wiki.