May 26, 2020

Improving Batteries by Modeling the Materials Space

  Batteries are complex systems of materials. For example, in a lithium…
Avatar Stephen TODD

Batteries are complex systems of materials. For example, in a lithium-ion battery the electrodes contain an active material such as graphite or a mixed metal oxide combined with a polymer binder, whilst the electrolyte is a complex formulation of organic and organometallic materials (see Figure 1). As the battery charges and discharges, many chemical reactions occur, causing the underlying chemistry of the battery to change. This leads to formation of the solid-electrolyte interphase (SEI) which protects the anode but also contributes to capacity fade and can lead to battery failure. This changing chemistry makes batteries difficult to model but also makes the challenge more interesting!

Figure 1: Composite model showing the fully charged anode, solid-electrolyte interphase layer, electrolyte and cathode.

The effect of the changing chemistry also means your models need to work at different length and time scales. For example, if you are trying to model how the constituents of the electrolyte might degrade, you need a solver that can model the reactions in the system. However, if you want to calculate the conductivity of the electrolyte, you have to calculate how the lithium ions diffuse. BIOVIA’s chemical and materials modeling tools have solvers that can perform simulations across multiple time and length scales. These can be used to gain insight into the behavior of the battery materials and calculate properties that are relevant to battery material engineers.

Specialist additives are included to modify the properties of the electrolyte and control the formation of the SEI. These additives should not change the flash point of the formulation as this would have a negative effect on the flammability of the electrolyte. BIOVIA’s COSMOtherm product can predict many relevant thermodynamics properties of potential additives such as flash point and viscosity and you can use it to screen libraries of candidate molecules.

Also, the additives must not inhibit the flow of lithium ions across the electrolyte as this would reduce the conductivity of the battery. Using classical simulations solvers from BIOVIA Materials Studio, you can apply molecular dynamics to model the diffusion of lithium ions in different formulations. You can predict properties such as conductivity and transmission function for a formulation. You can also use these properties to parameterize Newman models for battery cell simulations available from the Dymola tool of Dassault Systèmes’ CATIA brand. These simulations provide a direct connection from the electrolyte formulation to the change in temperature, cell voltage and cell power loss.

Besides allowing screening of materials, the electrolyte simulations also provide insight into why the conductivity changes in different formulations. Examining the precise movement of the lithium ions reveals the effect of the local environment on diffusivity, facilitating new design rules for the development of the next generation of additives.

Figure 2: Simulations showing the effect of increasing the Lithium ion concentration on the crystal lattice in an anode.

A major challenge in the development and integration of electrode materials is the Lithium-induced expansion and compression during charging and discharging. If the volume change is too large, the electrode will likely suffer from fracturing, electrical disconnection and eventual failure. Battery materials engineers can accurately simulate the effect of intercalation of lithium ions on the electrode volume using solvers based on electronic structure models in BIOVIA Materials Studio (see Figure 2). They can then use this information to provide input parameters for Abaqus calculations and understand the macro-structure implications of an expanding electrode on battery cell performance. Besides calculating the volume change and lattice expansion as a function of lithium intercalation, the simulations also provides information about the change in bonding between the layers. You can use this to estimate the level of intercalation within a layer that can lead to exfoliation and failure of the electrode.

The discrete modeling tools provided in BIOVIA Materials Studio enable battery materials engineers to screen new candidate materials, explore the wide materials space and understand material behaviour. Discrete modeling tools accelerate the development of the next generation of battery materials supporting future electric vehicles.

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For more details on multiscale simulations of electrolytes click on the button above.

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