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08/24/1999
Stress and Lithium-ion Batteries
I just finished reading an article in the July/August issue of
RxEMEDY magazine. The article mentioned a study of over a
hundred sufferers of rheumatoid arthritis or chronic asthma. In
the study one group was told to write for 20 minutes a day for
three days about very upsetting events in their lives while the rest
were told to just write about their plans for the day. The results
of later (up to 4 months) checkups indicated that there was a
tendency for those who had written about the stressful events to
show marked improvements in their asthma or arthritis. It was
suggested that writing about stressful events might alleviate
hormonal changes accompanying trauma and actually improve
long term health. Accordingly, I''ve decided to write about my
own stressful events of the past week or so. Forgive me for
burdening you but, hey, it''s worth it for me!
In June, my colleagues Al, Jack and I gave our 3-day short
course on modern battery technology in Amsterdam. Last week,
we gave the same course here in New Jersey. Each of us gives
four lectures, mine being on lithium batteries, battery
applications and other energy sources. I am the course director
and all went well except for a couple of minor items. First, none
of my transparencies (with all my crib notes and various goodies)
arrived from Amsterdam, although Al and Jack received their
materials in good shape. Second, I''m at the lectern to introduce
Jack for his first lecture on lead-acid batteries. Slight problem -
no Jack! Today, 6 days later, still no word from him so, Jack, if
you should read this, what happened? (We weren''t able to
contact you, not knowing your Jersey shore vacation phone
number or address.)
I found myself giving Jack''s talk on fuel cells, a subject on which
I hadn''t lectured before. In addition, I had only followed fuel
cells in a casual manner and have no expertise in the field. I felt
like Johnny Carson, doing his tap dance while dying during his
opening monologue on the Tonight Show. (I''m in bed at 10:30
these days so don''t know what Jay Leno does in this situation.)
Fortunately, I conned good old Al into giving Jack''s talks on
electric vehicles, lead-acid batteries and supercapacitors and he
did a superb job. You remember Al from my "memory" column
and also from my mention of the fact he was quoted a couple
weeks ago in Business Week (August 9, pg. 88) in an article,
"Battle of the Batteries". He said he got more calls on that quote
in Business Week than on any of his hundreds of technical
articles.
Well, there - I''ve written about it and feel better already!
However, as a result of being totally exhausted by this
experience and talking about a subject in which I have no
expertise, I''ve decided to fall back for this column on a subject I
know well, lithium-ion batteries. Chances are you''ve heard of or,
quite likely, have actually used lithium-ion batteries in your
laptop computer, cellular phone or one of those $2500 robot dogs
that Sony is marketing. It''s really neat how that dog can be laid
on its back and very gracefully get back on its feet!
To appreciate the lithium-ion battery we need a bit of
background. If you have read my earlier column on lithium
cardiac pacemaker batteries, you know that lithium is a very
reactive metal. The pacemaker battery is a primary, or
nonrechargeable battery. When I started working on
rechargeable lithium batteries at Bell Labs in 1972, we used pure
lithium metal for one of the electrodes. Lithium is the negative
electrode, so called because the negatively charged electrons are
generated at this electrode when lithium atoms give up electrons
and dissolve in the electrolyte as lithium ions. It''s these electrons
that constitute the current traveling through your flashlight or
laptop computer, ending up back at the positive electrode of the
battery. At this electrode, the lithium ions pick up those
electrons as they enter the positive electrode material.
Because lithium reacts with water, we can''t use the usual
electrolytes such as sulfuric acid or alkaline electrolytes found in
our car batteries or alkaline dry cells. Instead, we use organic
solvents with lithium salts dissolved in them. A typical solvent
is something called propylene carbonate and we''ll just call it PC.
Actually, lithium metal reacts with almost anything, including
PC, and you might rightly suggest that a lithium battery would be
impossible because the PC would just chew up the lithium.
However, it turns out the lithium reacts with PC and many other
electrolyte solvents to form a film. This thin film protects the
lithium from the electrolyte and the reaction stops. This
"passivating" film "enables" the very existence of virtually all
lithium batteries. In the lithium pacemaker battery, the film is
the lithium iodide, which also serves as a solid electrolyte and
separates the negative and positive electrodes.
The "passivating" film is not an unusual item. Aluminum is also
a reactive metal but forms a very tough oxide film that protects it
from reacting further with air. I saw a vivid demonstration of its
toughness when I was at NACA in Cleveland working on the
atomic airplane (see earlier column). We had just constructed a
furnace in which we could levitate a metal object. I watched,
fascinated, as a 1-inch cube of aluminum was floated in midair
(midhelium actually) in the furnace and then heated until the
aluminum melted. For about a minute, the thin film was so
strong that it held its cubic shape while the molten aluminum was
sloshing around inside. It was amazing!
Back to our lithium metal negative electrode and an electrolyte
of PC and a lithium salt. We need a positive electrode. Most
positive electrode materials are oxides such as the manganese
oxide in your alkaline dry cells that you''re always replacing.
Back in 1972, when I joined the Battery Development
Department of Bell Labs, I was working on other positive
materials, particularly, a compound of selenium and niobium, a
metal somewhat like the tungsten used in light bulb filaments.
The particular compound I was asked to prepare formed little
platelets. However, I noticed that if I interrupted the process
midstream, I got fibers of what turned out to be another
compound containing 3 atoms of selenium instead of 2 for each
niobium. While I went on vacation, my colleagues John and
Frank found that this "niobium triselenide" was a very promising
positive electrode material when combined with a lithium metal
negative electrode. We got a patent and about a decade later our
management decreed that we would develop this system in a AA
cell. If you''re a typical battery user you buy more AA cells than
any other size battery.
Well, we formed a group to work on this "Faraday" cell and did
indeed come up with arguably the best rechargeable lithium AA
cell in the world. Another company, the Moli Energy Corp. in
Canada, was then marketing a rechargeable lithium battery that
was quite impressive and indeed made its way into cellular
phones in Japan. We were pulling for Moli to be successful in
order to pave the way for our own superior product. However,
AT&T upper management killed our plans to manufacture the
Faraday cell and a joint venture with a large battery company fell
through at the last moment. In retrospect, this was, as Martha
Stewart would say, a good thing. At least one cellular phone
user in Japan was using a phone employing one of Moli''s
batteries when it either caught fire or exploded. The batteries
were immediately removed from the Japanese market and Moli
Energy went bankrupt, to be acquired by a consortium of three
Japanese companies. We would almost certainly have had the
same experience with our battery had it been introduced into the
market. AT&T would not have gone bankrupt but it would not
have been a pleasant experience!
What was the problem? It''s that passivating film, the enabler of
the lithium battery but also its curse! In a rechargeable battery,
when you stick it in a cellphone and it discharges, we''ve seen
that some of the lithium metal leaves the negative lithium metal
electrode and goes through the electrolyte to be inserted into the
positive electrode material. All well and good. However, after
the lithium leaves the negative electrode, our passivating film is
formed on the surface of the remaining lithium. The trouble
begins when we charge this battery by reversing the current and
send the lithium from the positive electrode to plate back on the
pure lithium negative electrode. Ideally, the lithium would form
a nice, smooth shiny deposit back where it came from, but there''s
a problem. The lithium ions in the electrolyte now see a film, not
pure lithium, and don''t know where to sit down. As a result they
form a bunch of fine particles of lithium and each particle reacts
with electrolyte to form its own film. As a result some of these
particles are "dead" insofar as participating in the next discharge
and, as cycling proceeds, the sheet of lithium is converted into a
"mush" of these fine particles. Although the particles are dead
electrochemically, they are far from dead chemically. There''s
pure lithium inside each particle and the surface area is now
huge! If there''s any kind of short circuit that raises the
temperature of the battery, these fine particles are very reactive
and can react rapidly with the electrolyte with resulting fire or
explosion.
This safety problem would exist for any rechargeable lithium
battery having a pure lithium metal negative electrode.
Surprisingly, two independent workers, Basu in our own group at
Bell Labs and Yazami in France, in the late 70s and early 80s
had found a solution to the problem. Each had demonstrated that
carbon, in the form of graphite, could serve as a host for lithium
and that the resulting lithium-carbon compound could behave as
a negative electrode that would cycle in a rechargeable lithium
battery. Neither Yazami''s nor Basu''s efforts were appreciated by
their managements but Basu did receive two patents. Yazami''s
work was not even judged to be worthy of filing a patent. In
roughly the same time period, John Goodenough, now in Texas
but then in England, and his colleagues at Oxford came up with a
positive electrode material that when combined with a lithium
negative gave a voltage of 4 volts. The compound, a lithium
cobalt oxide, was patented.
In 1990, Sony announced the "lithium-ion" battery at what was
then a relatively obscure battery meeting in Florida. That was
the year of our first short course on batteries. Two of the
participants in the course had been to that Florida meeting and
we learned of this new battery in our course. What was Sony''s
lithium-ion battery? You''ve probably guessed, a carbon
(possibly not graphite) negative electrode, a lithium cobalt oxide
positive electrode and an electrolyte of PC mixed with another
organic solvent and a lithium salt. The lithium-ion battery not
only has a high voltage of about 3.6 volts but also has an energy
density of about three times that of nickel-cadmium batteries.
The use of the carbon host for the lithium is a key safety feature
in that the compound won''t melt, as might lithium when a hard
short circuit occurs. Lithium is a low melting metal, melting at
180 degrees Centigrade, roughly the melting point of some
solders.
Lithium-ion batteries are not totally benign from the standpoint
of safety, however. Unlike the nickel-cadmium battery, which
can be overcharged without damage or safety problems, the
lithium-ion battery cannot be charged or discharged
indiscriminately. For lithium-ion or lithium metal batteries, you
must not exceed the charging voltage limit, typically 4.1 - 4.2
volts, recommended by the manufacturer. Going above that
voltage limit can lead to decomposition of the electrolyte and
explosion and/or fire! In fact, electronic charge protection is
built into the Sony and other lithium-ion batteries in the form of
silicon chips that monitor each cell in a battery to ensure that the
batteries are never overcharged. As a public service, I will again
emphasize that one should never, never attempt to override any
built-in electronic charging devices and should adhere strictly to
any instructions by the manufacturer. Never try to charge a
lithium ion cell in an ordinary charger. (There may be an
exception for some new Moli Energy cells, which apparently
have built in circuitry within each cell.)
Since Sony''s announcement, the lithium-ion battery''s impact on
the battery market has been nothing less than phenomenal. The
sales value of lithium-ion batteries now exceeds that for nickel-
cadmium batteries, primarily due to the use in laptop computers
and cellular phones and other portable applications. The nickel-
metal hydride battery, lithium-ion''s other main competitor in this
area, has also fallen behind lithium-ion in sales value.
It has only been within the last few years that Basu''s patents have
been recognized, although Yazami has been considered for some
time the "father" of the lithium-ion battery. A couple years ago,
Bell Labs honored Basu along with others whose inventions
proved to be of importance over the years. A strange thing is
that, despite mention of one of Basu''s patents in one of Sony''s
earliest papers in a relatively obscure battery publication, his
work remained unknown, even by workers in the field. A few
years ago at an international meeting on lithium batteries in
Mnster, Germany I polled everyone I could collar and asked if
they knew of Basu''s work. None did, even Yazami. Could this
be another case of the Japanese paying attention to what''s going
on and capitalizing on it? They deserve kudos for putting
everything together and for coming up with just the right
electrolyte to make it work. Strangely, in the years since the
Sony announcement, it has been revealed that the success of the
lithium-ion battery is due in large measure to another thin
passivating film! This time on the graphite. Once more, this
film is an enabler but also somewhat of a curse. But more about
that in a later column.
Allen F. Bortrum
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