If you're still having trouble removing the battery, apply a few drops of high concentration (90% or higher) isopropyl alcohol under the edge of the battery in the area of the broken adhesive strip(s).

Tilt the phone up so the isopropyl alcohol can flow toward the adhesive strip.

Hold for 1–2 minutes to allow the isopropyl alcohol to weaken the adhesive.

Use an opening pick or the flat end of a spudger to slowly pry up the battery.