  We report constraints on light dark matter through its interactions with
shell electrons in the PandaX-II liquid xenon detector with a total 46.9
tonne$\cdot$day exposure. To effectively search for these very low energy
electron recoils, ionization-only signals are selected from the data. 1821
candidates are identified within ionization signal range between 50 to 75
photoelectrons, corresponding to a mean electronic recoil energy from 0.08 to
0.15 keV. The 90% C.L. exclusion limit on the scattering cross section between
the dark matter and electron is calculated based on Poisson statistics. Under
the assumption of point interaction, we provide the world's most stringent
limit within the dark matter mass range from 15 to 30 $\rm MeV/c^2$, with the
corresponding cross section from $2\times10^{-38}$ to $3\times10^{-37}$ cm$^2$.
