Place your chicken breasts in the bottom of a greased slow cooker.

In a small bowl, combine the lemonade concentrate, ketchup, brown sugar, and rice wine vinegar. Whisk until smooth.

Pour the sauce mixture over the chicken breasts, using the back of a spoon to spread it evenly over the surface of the chicken.

Place the lid on the slow cooker and cook on high for 2-3 hours, or on low for 4-6 hours. Check the internal temperature of the chicken to ensure it is cooked through. Chicken needs to reach and internal temperature of 165F to be safe to eat.

Remove the chicken from the slow cooker and keep warm by covering with aluminum foil.

Add the cornstarch to a small bowl. Pour in the cold water and whisk vigorously with a fork.

Add the cornstarch and water mixture to the juices remaining in the slow cooker. Whisk to combine.

To thicken the sauce, you can either remove the liquid from the slow cooker and pour into a saucepan OR you can thicken the sauce in the slow cooker by cooking on high for 30 minutes or so. The quickest method (and most effective) is to use the stovetop to thicken the sauce. It takes just a few minutes over medium heat to thicken, whisking often.

Serve the chicken over a bed of rice or vegetables, and pour on the sauce. Serve immediately.