High-gradient and high-efficiency acceleration in plasma-based accelerators has been demonstrated, showing its potential as the building block for a future collider operating at the energy frontier of particle physics. However, generating and accelerating the required spin-polarized beams in such a collider using plasma-based accelerators have been a long-standing challenge. Here we show that the passage of a highly relativistic, high-current electron beam through a single-species (ytterbium) vapor excites a nonlinear plasma wake by primarily ionizing the two outer $6s$ electrons. Further photoionization of the resultant ${\mathrm{Yb}}^{2+}$ ions by a circularly polarized laser injects the $4{f}^{14}$ electrons into this wake, generating a highly spin-polarized beam. Combining time-dependent Schr\"odinger equation simulations with particle-in-cell simulations, we show that a subfemtosecond, high-current (4 kA) electron beam with up to 56% net spin polarization can be generated and accelerated to 15 GeV in just 41 cm. This relatively simple scheme solves the perplexing problem of producing spin-polarized relativistic electrons in plasma-based accelerators.