Battery Power - September/October 2013 - (Page 16)

Feature Highly Accurate Li-Ion Battery Simulation Mikael Cugnet French Atomic and Alernative Energy Commission (CEA) Whether in cell phones, hybrid/electric vehicles or airplanes, batteries have become virtually indispensable to modern life. Traditional means of evaluating battery performance with advanced chemistries such as lithium-ion don’t provide sufficient information to allow researchers to better optimize them. Thus, they are turning to simulation software to get a deeper understanding of what is going on inside the cells, information they can use in the design of batteries that are more reliable and safe. EIS and ECM Methods Used Primarily Until Now In vehicles, battery management systems (BMSs) are designed to protect the battery, predict vehicle range and update the range prediction depending on driving conditions. These BMSs use circuit models often derived from electrochemical impedance spectroscopy (EIS), a widely used technique to characterize batteries. With readings from an EIS system, it is possible to construct an electronic component model (ECM) that consists of resistances and capacitors connected both in series and in parallel (see Figure 1, bottom). With the results from an ECM study (see Figure 1, top) you can, for instance, determine a battery’s internal resistance, which in turn dictates how much energy it can deliver: is it good enough to propel a vehicle, enough to light up an emergency exit sign, or enough to power a cell phone? Some people attempt to get additional information from used batteries, but it is understandably difficult to do an accurate study on a burned-out battery. With an ECM, you get component values that mix the contributions of various phenomena occurring in the cell. However, there is a gap between the meaning of the electrical components in the equivalent Figure 1. Using Electrochemical Impedcircuit models and ance Spectroscopy (EIS), battery impedthe physical equaance is measured at a range of frequentions characterizing cies in the milliHertz to kiloHertz range. the batteries. In our From this impedance plot (top), it is possible to construct an equivalent circuit case, an ECM does model (bottom). not provide any infor- 16 Battery Power • September/October 2013 mation about important cell properties like the electrode active material resistance, the reaction rate, the specific capacitance, and the diffusion coefficient. We can get this information from a multiphysics model. Realistic Multiphysics Simulation Rather than work with an equivalent circuit model, we at INES decided to create a physics-based model of a LiFePO4/Li half cell. Its output is likewise a plot of impedance vs. frequency so we could compare its results to those of EIS measurements for verification. This model, though, gives us a great deal more information that we can use in the design of batteries that are more reliable and safe. Because I am very familiar with the equations governing battery behavior, I built my own model from scratch in COMSOL Multiphysics to give me full control of all parameters and even deeper understanding from the simulation. The physical battery model is a half cell in the shape of a button battery (see Figure 2). I need to study the half cell rather than a conventional battery in order to separate the electrodes and get a more precise evaluation of their physical properties; if I were to work with a complete cell, I would get a mix of all the phenomena occurring in each electrode without knowing Figure 2. Half cell used as the basis which electrode I should for modeling and for verification. attribute the resulting parameter values. The corresponding simulation actually consists of two coupled 1D models (see Figure 3). The first model represents the macroscopic level. It is made up of two domains: the working electrode, plus the separator between the iron-phosphate electrode and the lithium foil, which also serves as the counter-electrode (see Figure 3, left). The second model represents the microscopic level, which has only one domain. It models a spherical particle of iron phosphate, which is the main component of the working electrode’s active material (see Figure 3, right). Everything Done with PDEs Entered Through the GUI Both models were created exclusively with partial differential equations (PDEs). The macroscopic model uses equations for the conservation of current for the electronically conducting solid phase, the conservation of current for the ionically conducting liquid phase, and the material balance on the salt LiPF6 www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - September/October 2013

ABB to Build World’s Largest Network of EV Fast-Charging Stations in the Netherlands
Duracell Powermat and Starbucks Expand Wireless Charging in Silicon Valley Area
Thermal Management of an Electrical Vehicle Battery Packing Using 1D and 3D CFD
New Battery Technologies - Added Value Electronics Can Turn Normal Batteries into Smart Battery Packs
A Novel Single-Compartment Concentration Cell Driven by Natural Evaporation for Green Energy Harvesting
Highly Accurate Li-Ion Battery Simulation
Batteries
ICs & Semiconductors
Charging, Testing & Monitoring
Industry News
Marketplace
Application Profile

Battery Power - September/October 2013