A Thermodynamic Model for the Insertion Electrochemistry of Battery Cathodes

Abstract

The authors dedicate this paper to the memory of Alexander Milchev (1943Milchev ( -2022, a pioneer of nucleation/growth science and a researcher with highest human qualitiesThe 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.

Autocatalytic solid-state electrochemical reactions: a non-linear kinetic theory of batteries

“…This system is an example of a battery electrode that undergoes phase separation. 10 Fig. 3a (red curve) shows the CV of the Ni(OH) 2 /NiOOH electrode in the first and second cycles vs.…”

“…An example of a non-linear dynamic phenomenon is autocatalysis that has been used frequently to describe the dynamics of biochemical systems such as the evolution of life from simple molecules, 6,7 and recently, for studying the stability of reactive mixtures of relevance to batteries by Bazant et al 8,9 In a recent paper, we reported a thermodynamic treatment of phase separation in batteries. 10 Here, we study the consequences of this phase separation for solid-state electrochemical reactions using an autocatalytic model. This model reveals the autocatalytic nature of Ni(OH) 2 /NiOOH and Mn 2+ /MnO 2 reactions, as well as the origin of their asymmetric charge and discharge reactions, which may be a source of charge-discharge hysteresis, and hence, loss of energy efficiency in batteries.…”

“…This model reveals the autocatalytic nature of Ni(OH) 2 /NiOOH and Mn 2+ /MnO 2 reactions, as well as the origin of their asymmetric charge and discharge reactions, which may be a source of charge-discharge hysteresis, and hence, loss of energy efficiency in batteries. 10,11 The present autocatalytic kinetic model assumes the following: (1) no nucleation activation, i.e., a nucleus is always present, which reduces the model to an initial value problem, (2) no diffusion limitation, i.e., the electrode surface controls the reaction dynamics, (3) no ohmic resistance, and (4) no microscopic reversibility, i.e., the forward and backward reactions are always completely unidirectional, which is the core concept of autocatalysis. Finally, the chemical equations written here do not refer to a conventional chemical reaction, but to a ''reaction-interaction''; † hence, elimination law does not apply.…”

“…In a recent paper, we reported a thermodynamic treatment of phase separation in batteries. 10 Here, we study the consequences of this phase separation for solid-state electrochemical reactions using an autocatalytic model. This model reveals the autocatalytic nature of Ni(OH) 2 /NiOOH and Mn 2+ /MnO 2 reactions, as well as the origin of their asymmetric charge and discharge reactions, which may be a source of charge–discharge hysteresis, and hence, loss of energy efficiency in batteries.…”

“…This model reveals the autocatalytic nature of Ni(OH) 2 /NiOOH and Mn 2+ /MnO 2 reactions, as well as the origin of their asymmetric charge and discharge reactions, which may be a source of charge–discharge hysteresis, and hence, loss of energy efficiency in batteries. 10,11…”