A Multi Scale Multi Domain Approach For The Modelling Of Large Format Lithium-Ion Battery Cells

Summary:

The physical processes in a lithium-ion battery (LIB) cell take place on various length scales. While MSMD models have been established to calculate the electrochemistry on the micrometer scale and map e.g. the temperature distribution in the entire battery cell on the mesoscale at the same time, two challenges remain for modeling large format cells: (i) a complex model geometry representing hundred(s) of electrochemically active layers (ii) a physicochemical submodel requiring a system of nonlinear partial differential equations, i.e., as the P2D model. These challenges are addressed in this work by presenting a MSMD model containing a novel homogenization approach for large-format cells, which enables the consideration of electrochemical-thermal details related to microstructural features of electrodes in a large format LIB at affordable computational costs.

The model is divided into three levels: (i) the cell level for heat transport in the cell and current distribution in the current collectors, (ii) the electrode level for charge transport in the electrolyte and in the active material and (iii) the particle level for solid-state diffusion in the particle and charge transfer at its surface. The cell level contains a novel homogenization approach, which transfers the idea of volume averaging and homogenization of the commonly known Newman Model to the layer structure in large format battery cells. Thereby, the layer structure of the cell is considered by effective, anisotropic transport parameters without resolving it geometrically. The levels (ii) and (iii) are implemented by an extended homogenized model, which extends the Newman Model by considering particle size distributions.

The resulting highly efficient model is parameterized for a battery cell consisting of graphite anode and NCA/LCO-blend cathode and is applied to four different cooling conditions: no cooling, base cooling, tab cooling and side cooling. The full coupling of electrochemistry and temperature allows conclusions to be drawn not only about the occurring temperature profile in the cell, but also about the resulting local SOC distribution and current densities during 2C discharge. Thus, it is shown that the local temperature distribution in large format batteries results in inhomogeneous current and SOC distribution yielding local peak discharge rates up to 2.8C depending on the cooling concept. These inhomogeneities evolve transient and originate from the temperature profile of the cell as well as from the interaction of voltage losses in the current collectors, emerging and decaying SOC inhomogeneities and the slope of the OCV curve. Overall, the MSMD model enables the investigation local multiphysical interactions in large format LIB.