  Extended defects, including exposed surfaces and grain boundaries, are
critical to the properties of polycrystalline solid electrolytes in
all-solid-state batteries (ASSBs). These defects can significantly alter the
mechanical and electronic properties of solid electrolytes, with direct
manifestations on the performance of ASSBs. Here, by building a library of 590
surfaces and grain boundaries of 11 relevant solid electrolytes $-$including
halides, oxides, and sulfides$-$ their electronic, mechanical, and
thermodynamic characteristics are linked to the functional properties of
polycrystalline solid electrolytes. It is found that the energy required to
mechanically ``separate'' grain boundaries can be significantly lower than in
the bulk region of materials, which can trigger preferential cracking of solid
electrolyte particles in the grain boundary regions. The brittleness of ceramic
solid electrolytes, inferred from the predicted low fracture toughnesses at the
grain boundaries, contributes to their cracking under local pressure imparted
by Lithium or Sodium penetration in the grain boundaries. Extended defects of
solid electrolytes introduce new electronic ``interfacial'' states within
bandgaps of solid electrolytes. These interfacial states alter and possibly
increase locally the availability of free electrons and holes in solid
electrolytes. Factoring effects arising from extended defects appear crucial to
explain electrochemical and $-$mechanical observations in ASSBs.
