Heat olive oil in a soup pot over medium heat. Add onion, celery, bell pepper, and garlic. Cook and stir for 3 to 5 minutes. Add softened sweet potatoes, remaining broth, chickpeas, tomatoes, lemon juice, ginger, cumin, curry powder, coriander, chili powder, salt, and pepper. Bring to a boil, reduce heat, and cover pot. Simmer soup until vegetables are tender, about 20 minutes.