  Pure spin-current is of central importance in spintronics. Here we propose a
two-dimensional (2D) spin-battery system that delivers pure spin-current
without an accompanying charge-current to the outside world at zero bias. The
principle of the spin-battery roots in the photogalvanic effect (PGE), and the
system has good operational stability against structural perturbation, photon
energy and other materials detail. The device principle is numerically
implemented in the 2D material phosphorene as an example, and first principles
calculations give excellent qualitative agreement with the PGE physics. The 2D
spin-battery is interesting as it is both a device that generates pure
spin-currents, also an energy source that harvests photons. Given the versatile
operational space, the spin-battery should be experimentally feasible.
