Dr. Bortrum
10/30/2008
Lithium in Your Auto?
If the following column seems even more disjointed than usual, yesterday I spent an hour or so trying to figure out why my computer suddenly began to have about a 30 second delay in obeying my pushing of the delete, backspace and space keys. As well as not letting me save the file to disc! Not a good situation when I had not saved most of the column! Today, as you will see at the end of this column, an even weirder computer uprising has given me trouble just before posting the column.
In the past few months I’ve seen various news reports, TV programs and TV commercials about hybrid or electric vehicles and the use of lithium-ion batteries in these vehicles. The TV ad that caught my attention is one in which an ExxonMobil scientist says that his most important project in his career has been his work on separators for lithium-ion batteries. A recent "60 Minutes" program featured work at Tesla Motors, a very small company, and at General Motors, a bigger but troubled one, on quite different approaches to future vehicles, both anticipating lithium-ion batteries as power sources.
Honestly, I was shocked and alarmed to think of lithium-ion batteries in cars, be they hybrid or purely electric. We’ve all heard of incidents ts involving fires and/or explosions of lithium-ion batteries and the recall of millions of lithium-ion batteries because of manufacturing problems resulting in small shards of metal that could short circuit the batteries internally. You can understand my reluctance to want to drive a lithium-ion powered vehicle.
I once considered myself an expert on lithium batteries but, in the past few years, I’ve admittedly not kept up with developments in the field. So, I’ve reconnoitered the Web searching for anything to reassure me on the safety issues. First, let’s talk about the separator, the subject of the Exxon Mobil commercial. In lithium-ion batteries the separator has been typically a very thin sheet or envelope of a polymer material with many tiny pores to allow the lithium ions to travel through the separator from one electrode to the other. (On discharge of the battery lithium in the anode gives up an electron and goes into the electrolyte as a positively charged lithium ion. The electron goes into the wire connected to the anode and travels through the load, say the vehicle’s motor, to the cathode. The lithium ion travels through the electrolyte to the cathode, where it picks up the electron that has come through the load from the anode. This is an oversimplification in that the same lithium ion won’t pick up the same electron it gave up at the anode, but the overall reaction is the same.)
The separator is necessary to prevent the two electrodes from touching, which would short out the battery and lead to explosion or fire. The typical separator is much the same as the plastic wrap you use in your kitchen - it’s fairly tough but a shard of metal can easily poke through the plastic and cause a short. One common separator is/was Celgard, the trade name for a particular type of polypropylene sheet only about a thousandth of an inch thick. When I worked with this material at Bell Labs, I found that if I pulled a sheet of it in one direction it was pretty tough; however, if I pulled it in the other direction it "cleaved", just as a crystal cleaves, pulling apart cleanly in straight lines. This could be a problem in a battery if on cycling the electrodes change dimensions and put stress on the separator, causing a small tear or hole and the possibility of a connection or actual contact between the electrodes that results in a short circuit.
At Bell Labs we often had shorts in our test cells and some of our best cycling results were on cells with "soft" shorts. Not recommended in the real world of lithium-ion batteries! A "hard" short is fire or explosion. So, what has Exxon Mobil done that is so special that it merit’s the ICIS 2008 Innovation Award? OK, even after visiting the ICIS Web site, I don’t know what ICIS stands for, if anything! The ExxonMobil site was somewhat more revealing but I’m still puzzled.
The separator is apparently either a two-layer separator or perhaps a multilayer separator. The ExxonMobil Web site has a news release dated May 8, 2007 describing a co-extrusion, bi-orientation process for making this separator. At Bell Labs, we had the idea of putting two layers of Celgard together with the cleavage directions at right angles to each other so that if one cleaved the other would not. The ExxonMobil release talks about higher melting properties and a tougher separator but does not specify the materials. In the end, I’m frustrated and hope to track down a patent or some other information. The battery business is not known for its "transparency", a popular word today in regard to other matters!
The Tesla Motors Roadster was the subject of part of the "60 minutes" interview by Leslie Stahl. This small vehicle, manufactured in Silicon Valley by the inventor of PayPal is a very small auto that runs with a battery composed of over 6,000 laptop-size lithium-ion batteries! Needless to say, those thousands of lithium-ion batteries caught my attention! The Tesla Motors Web site claims the assembly is safe and points out that they have deliberately set on fire individual batteries nested in the middle of the 6,000 plus group of cells. By monitoring each of the over 6,000 cells individually, Tesla claims to be able to isolate any troublesome cell and shut down the whole battery if necessary. This to me is somewhat mind boggling but then each lithium-ion cells typically has its own silicon chip to monitor the charging regime so as to keep any cell from being charged at a higher than recommended voltage. However, taking care of an internally shorted cell about to catch fire seems to me to be a somewhat daunting task. Hopefully, Tesla and others will solve the problem.
I would also hope that the lithium-ion battery in other electric or hybrid cars will not be made by sticking together thousands of laptop batteries! There’s gotta be a better way than that!
I had planned to go into other lithium battery developments that I’ve found in my searches but a strange computer problem has brought me up to the deadline for posting this column. Regular readers will know I’ve had my share of problems with modern technology in the past months. However, today as I was finishing this column, I came across something new to me. Not only was every movement of my cursor accompanied by highlighting of the sections covered by the cursor movement, which frightened me enough, but suddenly the "." and "," keys only printed the normally shifted ">" and "<" characters! I could not print a period or a comma no matter whether or not the Shift key was pressed! Obviously, it’s hard to write a paragraph with no punctuation marks!
I finally resolved the problem by restarting the computer but I am left totally mystified by this occurrence. If I don’t post a column next week you’ll know the stresses of caregiving and computer malfunctions have finally overcome Old Bortrum.
Allen F. Bortrum
