Potential jumps at transport bottlenecks cause instability of nominally ionic solid electrolytes in electrochemical cells

TL;DR

This paper explains how potential jumps at transport bottlenecks in ionic solid electrolytes cause internal phase formation, leading to device failure, and provides rules to predict and mitigate these phenomena.

Contribution

It introduces a thermodynamic and kinetic framework to understand and predict internal phase formation in ionic electrolytes based on potential jumps.

Findings

Potential jumps cause internal phase formation at transport bottlenecks.

Polarity influences the likelihood of internal phase formation.

Rules are formulated to predict and mitigate phase formation.

Abstract

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.