@nicole.faerber said in August 2019 that Purism will sell replacement batteries for the Librem 5. I’m not sure if that will be available when Evergreen starts shipping.
A bigger question is whether there is any device that can charge up the extra battery when it isn’t inserted in the Librem 5. Until Purism enables good power management (suspend to RAM and wakeup on call), some people will want to carry two batteries to get through a full day. Most people, however, will just carry around a USB-C power bank, which is heavier, but a cheaper solution.