In a 6- to 8-quart slow cooker, combine the sweet potatoes, black beans, tomatoes, orange juice, oil, chipotles and adobo sauce, brown sugar, lime juice, garlic cloves, and dried spices. Stir in 2 teaspoons salt, a generous amount of pepper and 1 cup water. Cook on low until the sweet potatoes are tender, about 8 hours. The chili holds well on the warm setting.
Just before serving, stir in the corn and let it warm through, about 5 minutes. Taste and add more salt and pepper if necessary. Top with avocado and red onion.