2007 Prius car key hardly open up

congrats and welcome!

i think you're correct, it's probably glued together from age. keep prying, but be careful. if you break it, and it's your only one, it could get expensive.
maybe the prior owner glued it together

 

Perhaps run a q-Tip with some alcohol along the seams of the lid? Sweat can be quite the adhesive. Im guessing you are holding in the tab you use to remove the key?

 

Make sure the manual key is removed.

Hold your right hand in front of you with palm up.
Place the FOB near the middle of your 4 fingers with the buttons down and the raised lip (on the back of the FOB) away from you.
Place your right thumb on the fob Toyota symbol (the center of the back)
Use your left thumbnail to move and hold the slide release button completely to the right
Using your right thumb, push firmly toward the raised lip (away from you).
The back of the FOB should slide off.

 

if not, try a blowtorch

 

The first time I tried to open a Gen 2 fob, the difference from Gen 3 threw me.

The Gen 3 fob is simply built with two plastic case halves that snap apart (like all kinds of small electronic things).

But the Gen 2 fob has a case that really is glued together, all except for the removable back panel, which only slides off and only when you are releasing the small latch, just as TMR-JWAP describes.

 

For those following along at home, you don't want to open it that way.

Slide the little latch that lets you take the mechanical key out.

Keep holding the same little latch slid over, and the back cover of the fob will slide off.

Avoid prying apart the glued-together fob.

 

It is important to understand what Chap and TMR-JWAP are describing as to how that sliding cover works. Because if that slide cover hasn't been exercised in a good number of years, it can get stuck quite firmly and is really hard to get moving.

The good news is, once you do get it moved and give it a good clean (and little dry lube on the tracks would go amiss before reassembling), you shouldn't have any more problems opening to revel the battery in the future.