  The recent discovery of crystalline pentazolates marks a major advance in
polynitrogen science and raises prospects of making the long-touted potent
propellant N$_5$$^{+}$N$_5$$^{-}$ salt. However, despite the synthesis of
cyclo-N$_5$$^{-}$ anion in pentazolates, counter cation cyclo-N$_5$$^{+}$
remains elusive due to the strong oxidizing power of pentazole ion; moreover,
pure N$_5$$^{+}$N$_5$$^{-}$ salt is known to be unstable. Here, we devise a new
strategy for making rare cyclo-N$_5$$^{+}$ cation and assembling the
long-sought N$_5$$^{+}$N$_5$$^{-}$ salt in tailored ionic compounds, wherein
the negative/positive host ions act as oxidizing/reducing agents to form
cyclo-N$_5$$^{+}$/N$_5$$^{-}$ species. This strategy is implemented via an
advanced computational crystal structure search, which identifies XN$_5$N$_5$F
(X = Li, Na, K) compounds that stabilize at high pressures and remain viable at
ambient pressure-temperature conditions based on \textit{ab initio} molecular
dynamics simulations. This finding opens an avenue for creating and stabilizing
N$_5$$^{+}$N$_5$$^{-}$ salt assembly in ionic compounds, where cyclo-N$_5$
species are oxidized/reduced via co-matching with host ions of high/low
electronegativity. The present results demonstrate novel polynitrogen
chemistry, and these findings offer new insights and prospects in the design
and synthesis of diverse chemical species that exhibit unusual charge states,
bonding structures, and superior functionality.
