In this talk, I will review the symmetry constraints of electrically generated spin current and demonstrate that an effective way to resolve this issue is by exploiting crystalline asymmetry in certain materials, such as WTe2. Unfortunately, the spin-to-charge conversion efficiency in WTe2 is much lower than in conventional transition metals like Pt. Here, we propose a van der Waals heterostructure, WTe2/PtTe2, as an efficient spin source that not only avoids the symmetry constraint but also exhibits a high effective spin Hall conductivity. We introduce a novel spin-to-spin conversion mechanism for the high spin Hall conductivity in the WTe2/PtTe2 multilayer and show that spin-to-spin conversion opens new possibilities in spintronics, which are difficult to be achieved with conventional charge-to-spin conversion mechanism.