@sapstar, no battery based zigbee device would act as a repeater (parent). All battery devices would only act as endpoints.
@brumster, I’ve previously performed some analysis of the mesh using…
… and have noticed that the Xiaomi sensor do not seem to want to repeat via the smartthings outlet nor any other powered devices other than via the Xiaomi Outlets or the Orvibo RGB relays. I do not use my Xiaomi outlets anymore due to their incompatibility with ST sensors so all of my Xiaomi devices either connect directly to the hub or via the orvibo relay.
I’ve noticed the range of the orvibo to be impressive also, so you might have some luck with these to extend your mesh.