A Thermodynamic Model for the Insertion Electrochemistry of Battery Cathodes

Phase separation in Ni batteries: This scheme shows that for a point between reduced and oxidized species, there are two possible states with two different energies depending on their history, i. e., hysteresis. We developed a thermodynamic model for battery cathodes that reveals this hysteresis as a consequence of phase separation.

Abstract

The transition to Ni-based battery cathodes enhances the energy density and reduces the cost of batteries. However, this comes at the expense of losing energy efficiency which could be a consequence of charge–discharge hysteresis. Here, a thermodynamic model is developed to understand the extent and origin of charge–discharge hysteresis in battery cathodes based on their cyclic voltammograms (CVs). This was possible by defining a Gibbs energy function that weights random ion insertion/expulsion, i. e., a solid solution pathway, against selective ion insertion/expulsion, i. e., a phase separation route. The model was verified experimentally by the CVs of CoOOH and Ni(OH)₂ as solid-solution and phase-separating cathodes, respectively. Finally, a microscopic view reveals that phase separation and hysteresis are a consequence of large ionic radii difference of the reduced and oxidized central metal atoms.

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