  Normal operations of electrochemical devices such as solid oxide fuel cells
(SOFC), solid oxide electrolyzer cells (SOEC) and lithium ion batteries (LIB)
sometimes fail because of unexpected formation of internal phases. These phases
include oxygen bubbles at grain boundaries inside the zirconia electrolyte of
SOEC, isolated Li metal islands inside the (garnet type) Li7La3Zr2O12
electrolyte of all-solid-state LIB, and similar Na metal islands inside the
Na-beta-alumina and NASICON electrolytes of Na-S batteries. Remarkably,
although the devices can operate in both polarities, the propensity for failure
depends on the polarity. Here we explain these and other phenomena in nominally
ionic solid electrolytes and mixed-conducting electrodes in simple
thermodynamic and kinetic terms: the unexpected internal phases are caused by a
large potential jump that is needed to push a constant ion or electron flow
through its internal transport bottleneck. Definite rules for internal phase
formation including its polarity dependence are formulated to help predict and
mitigate it, which leads to microstructural instability, efficiency
deterioration and breakdown.
