Place the cubed sweet potatoes, diced onion, minced garlic, and diced red bell pepper into the slow cooker.
Add the drained black beans, diced tomatoes with their juices, and vegetable broth.
Sprinkle in the chili powder, cumin, cayenne pepper, salt, and black pepper. Stir everything together until the spices are evenly distributed.
Cover and cook on low for 6 to 7 hours, until the sweet potatoes are soft and the flavors have melded.
Taste and adjust salt or cayenne if needed. Ladle into bowls and top with fresh cilantro and a squeeze of lime.