  Carrier doping of quantum spin liquids is a long-proposed route to the
emergence of high-temperature superconductivity. Electrochemical intercalation
in kagome hydroxyl-halide materials shows that samples remain insulating across
a wide range of electron counts. Here we demonstrate through first-principles
density functional calculations corrected for self-interaction the mechanism by
which electrons remain localized in various Zn-Cu hydroxyl-halides,
independently of the chemical identity of the dopant - the formation of
polaronic states with attendant lattice displacements and a dramatic narrowing
of bandwidth upon electron addition. The same theoretical method applied to
electron doping in cuprate Nd2CuO4 correctly produces a metallic state when the
initially formed polaron dissolves into an extended state. Our general findings
explain the insulating behavior in a wide range of doped quantum magnets and
demonstrate that new quantum spin liquid host materials are needed to realize
metallicity borne of a spin liquid.
