  We report the superconductivity in the layered Zintl phase LiSn$_2$As$_2$,
which is isostructural to NaSn$_2$As$_2$ and has a transition temperature
($T_{\mathrm{c}}$) of 1.6 K. Despite similar $T_{\mathrm{c}}$ and Debye
temperatures, substituting of Na with Li considerably increases the upper
critical field. Based on a systematically comparation of Sn$_4$As$_3$, NaSnAs,
NaSn$_2$As$_2$,Na$_{1-x}$Sn$_2$P$_2$, SrSn$_2$As$_2$, and LiSn$_2$As$_2$, we
propose that carrier doping, intimately related to the formation of lone-pair
electrons, controls superconductivity in layered SnAs-based compounds rather
than chemical pressure. The current findings provide a thorough and
comprehensive understanding of Sn-based Zintl phase.
