Testing New Battery Materials in Standard Cells
Building a better battery is a key goal for those who would like to see electric and hybrid electric vehicles (EVs and HEVs) become viable options in the car market. However, progress toward this goal has been slow. Many labs are seeking battery anode (negative electrode) and cathode (positive electrode) materials that will last longer, suffer less degradation, and operate safely over wider temperature ranges than is currently possible.
As part of this battery research effort, a unique cell development program has been under way at Lawrence Berkeley National Laboratory (Berkeley Lab) for the past three years, led by Kathryn Striebel, a scientist in the Lab's Environmental Energy Technologies Division (EETD). This project uses standardized cells to assess, in a working battery, the performance of promising new materials. The project aims to bridge the gap between materials research and transfer to a battery developer.
EETD has long studied advanced materials for batteries. The work is currently funded by the U.S. Department of Energy (DOE) Batteries for Advanced Transportation Technologies (BATT) program of the Office of FreedomCAR and Vehicle Technologies. BATT, which is administered by EETD for DOE, consists of six research tasks involving Berkeley Lab and a number of other institutions and national laboratories.
Standard Cells Test Realistic Conditions
"The idea of this research element," says Striebel, "is to take new materials from labs and build them into test cells for new batteries. We build new materials from different sources into these test cells and run a set of standard tests to see how they perform under realistic conditions. Then, we disassemble the test cells, and, after some additional electrochemical testing of our own, we send samples to the Berkeley Lab researchers focusing on diagnostic techniques, such as Raman and Fourier Transform Infrared Spectroscopy [FTIR] and many others." The testing helps determine why electrode materials fail or degrade. Experimental materials come from labs all over the world, including EETD's own electrochemistry labs.
To be successful, a battery for automotive applications must meet DOE criteria for features such as weight, cost, power density, and operating temperature range. These criteria include a 10-year life, $150/kWh cost, ability to operate between -40° and 50°C, and a lifetime loss of capacity of no more than 20 percent. Batteries for HEVs differ slightly from those for EVs in that they also need to be able to provide numerous pulses of power for acceleration and accept charge during regenerative braking.
Currently, lithium-ion-based cells are promising canditates for meeting these performance goals. One option is based on lithium iron phosphate (LiFePO4) and natural graphite (NG).
"The central goal for us," Striebel says, "is to determine which materials work the best, and, when they fail, to answer the question 'why'?"
LiFePO4 material has some advantages: it is stable and flame retardant, it has a long cycle life, and it shows promise for meeting the goal of no more than 20- percent capacity degradation over the battery's lifetime. However, the capacity of LiFePO4 batteries is currently insufficient for use in vehicles.
No material currently meets all of DOE's goals for automotive batteries. One important reason is that the performance of existing materials degrades significantly after many chargedischarge cycles. EETD's "strength is in our understanding of degradation mechanisms in battery materials," says Striebel. "If we can nail down the mechanisms of degradation, it will be a great help to everyone working in the field."
Test cells are small, thin pouches: 12 square centimeters and just larger than an inch (about 3.5 cm.) on each side (see Figure 1). They can store an average of 12 milliampere-hours of charge. The effort to make a cell starts with 5 to 20 grams of an experimental material—an amount that is considered large for a new material that may exist in only tiny quantities in a single lab. The material is mixed with carbon, a binder polymer, and a solvent to form a slurry. This slurry is cast in thin layers onto a foil current collector and dried extensively (see Figure 2). One anode and one cathode are placed in a flexible pouch with a porous separator and transferred to a helium-filled glove box for finishing. At this point, electrolyte is added, and the pouch is sealed to protect the cell from water vapor during testing. The pouch is then compressed and mounted on a test device (see Figure 3), usually along with many other cells that are undergoing testing.
The device tester can charge and discharge up to 64 test cells simultaneously, according to any specification. For example, it can run through continuous charge-discharge cycling at constant current, letting the cells rest between half-cycles, which is the procedure for determining baseline cell performance, or it can charge and discharge with short, high-current pulses, simulating the conditions that an HEV battery might encounter.
The tester measures current, voltage, and other parameters, and, for each test cell, provides impedance characteristics, capacity, and power as a function of cycle number or time. After a cell reaches a pre-determined end-of-life limit (low capacity or power), additional diagnostic cycles are carried out before the cell is removed to the glove box for disassembly.
Once the cell is disassembled, Striebel and her colleagues might subject the experimental material to a range of additional tests to investigate its degradation mechanisms. These tests might use Raman, FTIR, and other spectroscopic methods; X-ray diffraction; or transmission electron microscopy.
"The testing is an ongoing program," says Striebel. "We continue to test new materials as they are developed. The results allow us to compare the performance of different materials with one another. We have also been working with John Newman's group [of EETD and the University of California, Berkeley], which develops computer models of the performance of batteries. This really helps us isolate why these materials perform the way they do."
Test results are presented at U.S. and international meetings and published in peer-reviewed journals, so the data are available to the scientific community as well as battery developers. "Recently, we used the computer modeling directly to help in the comparison of six different sources of LiFePO4 from around the world. This approach generated a lot of interest at the most recent meeting of The Electrochemical Society, in Orlando Florida," says Striebel.
For more information, contact:
- Kathryn Striebel
- (510) 486-4385; fax (510) 486-7303
This research is supported by the U.S. Department of Energy's Office of FreedomCAR and Vehicle Technologies.
Funding for this research was provided by U.S. DOE, and the California Energy Commission.