Statistical Physics-Based Model of Solid Electrolyte Interphase Growth in Lithium Ion Batteries

Abstract

The article presents a statistical physics-based model for the growth of the solid electrolyte interphase (SEI) in the negative electrode of lithium ion batteries. During battery operation, the SEI thickness grows by the reaction between lithium ions, electrons and solvent species on the surface of active particles at the negative electrode. The growth of the SEI layer causes a loss of lithium ions that induces capacity fade. In addition, it increases the ion transport resistance and decreases the total porosity. Our model employs a population balance formalism based on the Fokker-Planck Equation to describe the propagation of the particle density distribution function in the electrode. Structure-transforming processes at the level of individual particles are accounted for by using a set of kinetic and transport equations. These processes alter the particle density distribution function, and cause changes in battery performance. A parametric study of the model singles out the first moment of the initial SEI thickness distribution as the determining factor in predicting the capacity fade. The model-based treatment of experimental data allows analyzing processes that control SEI growth and extracting their controlling parameters. Lithium ion batteries (LIBs) are highly touted energy storage devices for portable electronics and electric vehicles.1 The LIB system consists of a negative electrode, a positive electrode, a separator, an electrolyte, and two current collectors. The most commonly used electrolytes are comprised of lithium salts, such as LiPF 6 in a solution of ethylene carbonate (EC) and dimethyl carbonate (DMC).1 The electrodes consist of randomly distributed and interconnected particles of active material, which store and release the lithium ions.Aging and degradation of LIBs have become major concerns for the operation of electric vehicles (EVs), which must fulfill exacting requirements in terms of durability, cyclability and overall lifetime. Battery aging is linked to the (electro-)chemical and mechanical degradation of the electrodes and the electrolyte.2 Three main degradation mechanisms prevail at the particle level during the cycling of LIBs:(1) growth of the solid electrolyte interphase (SEI) at the negative electrode, 2,3 (2) formation of cracks in the SEI layer at the negative electrode, 4 and (3) dissolution and isolation of nanocrystalline particles at the positive electrode.5 These processes lead to capacity fade and power loss.At the beginning of the battery life, particularly during the first cycle, the electrolyte undergoes reduction at the electrode/electrolyte interface, because the negative electrode operates at potentials that are outside of the electrochemical stability window of electrolyte components. This reduction is accompanied by the irreversible consumption of lithium ions.6 This process forms a passivating surface layer known as the solid electrolyte interphase. The SEI layer grows further during charging, thereby reducing the amount of active lithium ions. Moreover, this l...

Model Based Multiscale Analysis of Film Formation in Lithium‐Ion Batteries

“…In this work, the probability of an electron leaking through the surface film is assumed to exponentially decrease with film thickness. This is usually associated to electron tunneling, and has been applied frequently, e. g ,. This assumption provides good agreement to experiments shown in this work.…”

“…Coupling of continuum models with less detailed molecular simulations provides a feasible and promising way to cover and bridge long time and length scales. This can be achieved by adequate multiscale techniques [25] as has been demonstrated for several related electrochemical problems. [26,27] In our previous work we established the required methodology by detailed investigation of numerical implementations [28] and by demonstration for heterogeneous EC reduction within a continuum single particle model.…”

“…Moreover, MD and kMC models do not consider spatial distribution and time dependency of species concentration and potential. In contrast, continuum models have been used to simulate spatial distribution of the SEI thickness due to cycling aging, and for evaluation of statistical distributions using population balance equations . In these aging studies no significant distribution of thickness has been observed.…”

“…Since film properties have been assumed to be homogeneous and no morphological or structural aspects have been considered, thickness tends to equilibrate and spatial differences are only visible under extreme conditions, e. g., high currents or thick electrodes. Transport processes and growth limiting steps have been studied frequently with model based approaches [23,24] and strongly depend on the actual film morphology and molecular structure as will be discussed in more detail below. To conclude, single scale models presently do not enable simulation of the molecular buildup for long time scales in order to evaluate heterogeneity and spatial distribution of structural properties.…”

High‐Energy Aqueous Lithium Batteries

“…[157][158][159][160][161] The decomposition of the organic electrolyte at the electrode surface forms a protective SEI, which allows the cell stability over a broad range of potential. [157][158][159][160][161] The decomposition of the organic electrolyte at the electrode surface forms a protective SEI, which allows the cell stability over a broad range of potential.…”