Pour the lemonade concentrate, salt, and pepper into a resealable plastic bag. Add the chicken breasts, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for at least 1 hour or overnight.
Preheat an oven to 350 degrees F (175 degrees C). Spray a baking sheet generously with cooking spray.
Place the cornflakes in another resealable plastic bag and crush them into fine crumbs. Add the thyme, and mix thoroughly. Remove the chicken pieces from the marinade, and shake off excess. Discard the remaining marinade. Place the chicken pieces into the bag containing the cornflake crumbs and shake to coat well.
Place the chicken breasts on the prepared baking sheet so they don't touch, and spray each one lightly with cooking spray. Bake in the preheated oven for 30 to 45 minutes, until chicken juices run clear, and the top is browned and crunchy. Serve hot or cold.