What is inside the SPIKE Essential battery?

This battery was glued when assembled, and it can only be opened destructively. The best way seems to be cutting with a knife around the seam to break all of the tabs that are glued in place.

Here's how it looks with the top removed:

The guts include two LiPo cells in series to give us the 7.3V nominal voltage.

The two cells are taped together with a charging circuit.

We can see on the cells that they each have a 2.44Wh capacity at a nominal voltage of 3.65V:

Here's a photo of the board with the connector:

And the back of the board showing the protection circuitry and cell connection points:

There is some additional information about this battery in TLG's official test report. That document includes a few photos of the components as well:

The materials safety datasheet (MSDS) may also be of interest. Both the testing document and the MSDS come from TLG's battery documentation.