  Motivated by the existence of superconductivity in pyrite-structure CuS$_2$,
we explore the possibility of ionic-liquid-gating-induced superconductivity in
the proximal antiferromagnetic Mott insulator NiS$_2$. A clear gating-induced
transition from a two-dimensional insulating state to a three-dimensional
metallic state is observed at positive gate bias on single crystal surfaces. No
evidence for superconductivity is observed down to the lowest measured
temperature of 0.45 K, however. Based on transport, energy-dispersive X-ray
spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and
other techniques, we deduce an electrochemical gating mechanism involving a
substantial decrease in the S:Ni ratio (over hundreds of nm), which is both
non-volatile and irreversible. This is in striking contrast to the reversible,
volatile, surface-limited, electrostatic gate effect in pyrite FeS$_2$. We
attribute this stark difference in electrochemical vs. electrostatic gating
response in NiS$_2$ and FeS$_2$ to the much larger S diffusion coefficient in
NiS$_2$, analogous to the different behaviors observed among electrolyte-gated
oxides with differing O-vacancy diffusivities. The gating irreversibility, on
the other hand, is associated with the lack of atmospheric S; this is in
contrast to the better understood oxide case, where electrolysis of atmospheric
H$_2$O provides an O reservoir. This study of NiS$_2$ thus provides new insight
into electrolyte gating mechanisms in functional materials, in a previously
unexplored limit.
