Simulation of Cell Aging and Degradation of Li-Ion Batteries

Simulation of Cell Aging and Degradation

Simulations of our base models in BEST provide solution fields for the local lithium concentration and potentials inside Li-ion batteries. These fields either directly provide information about local risks of degradation or can be further coupled to electrochemical and mechanical models to capture complex interactions between cell aging and performance.

During operation and even storage, Li-ion batteries are exposed to a variety of degradation processes which cause cell aging. As a result, the cell often suffers from capacity and power fade, as well as increased heating during operation and higher risks of thermal runaway. Within different research projects we have used our simulation software BESTto consider three of the major degradation processes inside Li-ion batteries:

Simulation of the Solid Electrolyte Interphase (SEI) Layer
Detection of Lithium Plating Through Simulation
Simulation of Mechanical Degradation

The following sections go into more detail regarding these topics. Click on the headings to go directly to the relevant paragraph.

Simulation of the Solid Electrolyte Interphase (SEI) Layer

The SEI is a thin, porous layer which forms on top of common anode active materials such as graphite and silicon as a product of electrolyte reduction reactions. While the passivating nature of the SEI allows for a stable cell operation, its continuous formation and growth reactions over the lifetime of the battery are major contributors to capacity fade.

Within BEST we offer an SEI model which is able to capture the long-term growth behaviour of the SEI during storage (calendar aging) and operation (cyclic aging). The model is supported in both BESTmicro and BESTmeso and accounts for the SEI-induced capacity and power fade. Depending on the model, the inhomogeneous SEI thickness profile is resolved along the surface of the anode microstructure (BESTmicro) or across the homogenized electrode scale (BESTmeso).

The Solid Electrolyte Interphase (SEI) — © Fraunhofer ITWM
Qualitative representation of cell ageing simulations by SEI growth in BESTmicro and BESTmeso. Starting from an initial, thin SEI layer, both models depict the long-term SEI growth during storage and dynamic operation. The detailed simulation in BESTmicro resolves the inhomogeneous SEI layer thickness along the complex surface of the anode microstructure, while the more efficient BESTmeso simulation describes the effective layer thickness on the homogenized electrode.

Detection of Lithium Plating Through Simulation

If a Li-ion battery approaches its maximum state of charge during charging or is being fast-charged, there is a risk of a sharp drop in the anode potential. If the potential falls below a certain limit, the formation of metallic lithium is more energy-efficient than the intercalation, i.e. the chemical storage of lithium in the active material, which is essential for the opertaion of Li-ion batteries.

As a result, lithium is deposited on the anode surface, which is at least temporarily removed from the cell's cyclable lithium inventory. This process is referred to as »lithium plating« and usually exhibits spike-like growth in the form of dendrites. Apart from the loss of capacity, excessive lithium plating carries the risk of the separator being punctured by the growth of dendrites, which can result in a short circuit followed by a thermal runaway.

The risk of lithium plating occurring is determined via the anode potential relative to a lithium reference electrode. If the resulting voltage falls below zero, there is a risk of lithium plating. As these potentials are an essential part of the simulation results of our software »BEST«, this so-called plating condition can be easily monitored and evaluated. This enables the assessment of the plating risk when testing different cell designs and operating protocols.

Simulation of Mechanical Degradation

During charging and discharging of the battery, lithium is intercalated and deintercalated into the active materials. This refers to the process of reversible chemical storage of Lithium-ions inside the molecular structure of the active particles which form the porous electrodes. Consequently, a breathing of the active particles can be observed. This induces expansion and contraction of the particles and results in inner mechanical stresses on particle level, but also on cell level. While the breathing effect is rather small for traditional materials, such as graphite, it becomes prominent when materials like silicon are used as active material.

Simulation of a stress-induced damage field in a graphite-silicon microstructure. © Fraunhofer ITWM

In order to capture the interaction of electrochemical and mechanical behaviour of Li-ion batteries we combine our two simulation tools »BEST« and »FeelMath« to determine the cell expansion based on the state of charge of the cell. Furthermore, we developed a damage model which computes a damage field based on the internal stresses, which can be used to modify the electrochemical transport properties accordingly.