From the picture it is a security Allan key. A hex key with dimension across the flatsand a hole drilled in the centre. Most likely 4mm or 5mm.
Since you don't need a quality forged tool check your LiDL or discounter tools house for something like
If you have a vernier caliper you can measure across the flats reliably; if not, let's see who come up with the info here.
Other TomTom's use Torx screws and when Gooleing for battery replacement on those models vendors often offer a package for tools an pry 'bar'.
Have you tried to search for battery replacement on your model?