Put the beans in a large bowl and cover with 3 inches of water. Soak the black beans overnight ( ~ 8 hours). In the morning drain and rinse the beans and add them to the slow cooker.
Add all remaining ingredients except the lime juice, spinach and guacamole and cover the slow cooker and set it to high for 6-7 hours. Stir a few times throughout cooking if possible.
When the beans are cooked and the sweet potato is tender the soup is done. Stir in the lime juice and baby spinach (if using) and continue to cook on high until the spinach is wilted.
Divide among bowls and garnish with guacamole or sliced avocado!
If you want a less spicy soup, use ½ teaspoon red pepper flakes. You can also use 2 cans of black beans instead of dried beans. If using canned beans, you can set the slow cooker to low and cook for 8 hours.