Add sweet potatoes, beans, tomatoes, onion, poblano, garlic, broth, chili powder, tomato paste, cumin, paprika, oregano, salt and cayenne to a 6-quart slow cooker; stir to combine. Cover and cook until the sweet potatoes are tender, about 4 hours on High or 7 hours on Low.
Divide the soup among 8 shallow bowls. Top with avocado slices and serve with lime wedges.
