Without third-party tools a backup only encryption is difficult. However, you can use Transparent data encryption (TDE), what I am unsure of is what versions TDE is available in, probably not anything less than Standard. But I didn't do the research.

Also, I would ask a larger question. Why? For what reason are you looking at encrypted backups? Are you trying to meet a business requirement or a regulatory requirement? What is the underlying problem you are trying to solve?

CEWII