Choose bananas that are still slightly green or just barely yellow - they should feel firm when squeezed. Overripe bananas will be too soft and will not crisp properly. Peel the bananas and slice them into thin rounds about 1/8 inch thick, keeping the slices as uniform as possible.
Place the banana slices in a bowl. Add the lemon juice and melted coconut oil and toss gently until every slice is lightly coated. The lemon juice helps prevent browning and adds a subtle tang.
Preheat the air fryer to 350 degrees F for 2 minutes.
Arrange the banana slices in a single layer in the air fryer basket, making sure they do not overlap. You will need to work in several batches.
Air fry at 350 degrees F for 8 to 10 minutes, flipping the chips halfway through. Keep a close eye on them during the last 2 minutes as they can go from golden to burned quickly.
Remove the chips and immediately sprinkle them with the sea salt and cinnamon, if using. The chips will continue to crisp as they cool.Let them cool completely on a wire rack for maximum crunch. Store in an airtight container for up to 3 days.