The battery is strongly glued into place.

Apply 1-2 drops of high-concentration (>90%) isopropyl alcohol under each corner of the battery and allow it to penetrate for several minutes to help weaken the adhesive.

Using too much alcohol can damage the display.

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