Modeling Benefits Design of Lithium-Ion Batteries

Thermal Management and Safety

Most of the losses in a battery, for example ohmic losses and activation overpotentials, generate heat. In addition, in cold weather and during startup, the battery system may require heating in order to work. The cooling and heating of the battery system require thermal management.

Using a physics-based model, the different sources of heat are directly available from the model. The advantage of using a thermal model is that the temperature inside the cell can be estimated from the measurement at the surface. This allows for studying unwanted effects such as internal short circuits, where hot spots may be the cause of thermal runaway.

Temperature variations are predominant within large cells, since uneven current distributions cause uneven heat productions. Heating and cooling design for normal operation and for regular startup is focused on minimizing weight and power consumption.

The design of the thermal management system in a battery system is substantially complicated by the fact that it has to be able to cope with malfunctioning cells. Malfunction is usually caused by short circuiting of the electrodes due to metal deposits at the cathode that grow across the electrolyte and make electronic contact with the anode.

Mechanical damage is another cause of short circuits in batteries. If a foreign metallic object penetrates the battery pack or if the battery pack is damaged by being squashed, it can provide an internal conduction path that creates a short circuit. One standard safety test for Li-ion batteries is the “nail test” in which a nail is driven into the battery to create a short circuit. The nail conducts the current as an external circuit with a very small load, while the area around the nail behaves as during a discharge.

Characterization and State-of-Health

Li-ion batteries lose capacity and the internal resistance increases over time. After a while, the battery is unable to deliver the energy or power that is demanded. The reactions that are responsible for this aging can be included in a performance model.

There are many factors that influence the performance, and it is often difficult to separate the effects of different design and operation parameters on performance. A key to separating the influence of the different involved phenomena is the fact that they often have different time constants. For example, electrochemical reactions are usually fast compared to molecular diffusion.

A method that is becoming more common for analyzing the state-of-health of batteries is electrochemical impedance spectroscopy (EIS). This method is based on measuring impedance at different frequencies, thus separating processes with different time constants.

Physics-based performance models of EIS may be combined with experimental measurements to study effects of aging and decay of the battery material at the cell level.

Beyond the Newman Model

The latest development for understanding the electrodes in batteries is to use heterogeneous models that treat the geometry of the material in detail, in contrast to the homogeneous model. This is achieved by constructing the geometry from micrographs.

The example above shows a hypothetical heterogeneous structure with the graphite particles described as ellipsoids and the pore electrolyte filling the void between the skeleton formed by the ellipsoids. A structural analysis coupled to the detailed electrochemistry, with volume expansion caused by lithium intercalation, reveals that the necks of the skeleton structure are subjected to the highest stresses and strains. Cracks may therefore form there over repeated cycles and increase the ohmic losses, which contribute to the deterioration of the battery performance.

Multiphysics Models and Partial Differential Equations

The most accurate way of describing the Li-ion battery is through physics-based models formulated with partial differential equations. Further development of these batteries requires new models and new formulations, such as the heterogeneous model exemplified above. Models must be able to describe the fundamental processes that determine battery performance in order to give the deeper understanding required for developing new materials and new designs. There is no way around this: models and simulations are the shortcuts.

Source: ecnmag