(Invited) Engineering Approaches to Lithium-Ion Cell Design
Authors: Spotnitz Robert 
Published: February 2020

By far, most effort in lithium-ion technology is devoted to the development of new and improved materials such as intercalation compounds, electrolytes, and binders.  However, as applications of lithium-ion technology have expanded, so has interest in designing batteries using existing chemistries.  Notably, consumer electronics companies are maximizing energy density by using irregularly shaped cells, and in electric vehicle makers are producing battery packs that are thermally uniform in operation.  The challenge of engineering models is to add value in the design process in a practical way.  There has been some notable success in battery design based on the unit cell concept.

A “unit cell” is defined as a discrete volume element that contains the material between the positive and negative current collectors.  So, in a lithium-ion cell, the unit cell is comprised of the negative electrode coating, separator, and positive electrode coating.  The unit cell encompasses all the electrochemistry and mass transfer processes that occur during the life of a lithium-ion cell.  The behavior of a unit cell can be simulated by a number of different models depending on the objective of the study.  The unit cell model serves as a building block for a full battery model which is constructed by connecting unit cells to current collectors. 

Simple unit cell models based on electric circuit analogs have proved the most successful because of the ease in which their parameters can be readily measured and they are computational efficient.  Tables of parameters can be determined as functions of charge and discharge current, temperature, state of charge, and age, and then interpolation used to obtain values at intermediate points.  Electric circuit models have been successfully used for scale-up from small to large cells and simulation of battery packs.  Despite these successes, there is great interest in physics-based models, most notably the DUALFOIL model.

The DUAL model (the lithium-ion portion of the DUALFOIL model) uses conservation of mass and energy, non-ideal electrolyte transport theory, Butler-Volmer kinetics, Fick’s law, Kirchoff’s law, and Ohm’s law to simulate mass and charge transport in a lithium-ion cell.  In theory, the model can predict the effect of unit cell design variables such as porosity and electrode thickness, on performance; a number of academic studies have used the DUAL model for optimization.  The model has been successful in representing the behavior of lithium-ion cells probably because the parameters are adjusted to fit data sets, though a recent study has reported some success in actually predicting cell performance using independently measured parameters.  The difficulty in parameterizing the DUAL model makes it difficult for battery makers to use, and even more difficult for OEMs to use.  Still, there is great interest in the DUALFOIL model and numerous enhancements have been made.  The ever increasing computational power of computers and the widespread availability of the DUAL model make it highly likely that applications of this model will continue to expand.

Today’s engineering models are fairly successful for simulation of charge and discharge behavior of lithium-ion cells.  The growing acceptance of these models should create a positive feedback loop in which validated parameter sets drive use and use drives development of validated parameter sets, and so enable better designs of lithium-ion cells.