No, a power bank will not replace the battery.  And Dell does not sell batteries online - you either need to call Dell and ask for "spare parts" or repair service, or search for a battery elsewhere by system model number.
My laptop's battery health is poor and seems to be rapidly deteriorating. I've looked for a replacement battery on dell.com, but can only find a Power Bank which is compatible with my model. Does my original battery need to be in good health to work with a Power Bank, or does the Power Bank replace the original battery? Thanks.
Dell Vostro 15 laptop poor battery health