The battery is strongly glued into place.

Peel up the black pull tab at the top edge of the battery, and slowly pull it away from the battery at about a 45-degree angle to release the command strips (a.k.a. stretch-release adhesive) underneath.

If the command strips break or the black pull tab is missing, you can still remove the battery using heat and/or isopropyl alcohol to weaken the adhesive.

Apply some high concentration (>90%) isopropyl alcohol under each corner of the battery and allow it to penetrate for several minutes to help weaken the adhesive.

Alternatively, apply a freshly heated iOpener to the back of the phone directly behind the battery for two minutes to help soften the adhesive. Re-heat and reapply the iOpener as necessary.