Place the sweet potatoes, onion, bell pepper, and garlic in the slow cooker.
Add the black beans, diced tomatoes, and vegetable broth.
Sprinkle in the chili powder, cumin, cayenne pepper, and salt. Stir everything together until evenly combined.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the sweet potatoes are tender when pierced with a fork.
Ladle into bowls and top each serving with cilantro, a squeeze of lime, and avocado slices.