Guest post by Marca Doeff
While Venkat is off
It all
started around the mid 70’s or so, when Stan
Whittingham at Exxon announced his newly discovered phenomenon of lithium
ion intercalation into TiS2, in a letter to the journal Science. It turns out that TiS2,
which has the layered structure shown below, could be reduced electrochemically
while simultaneously inserting lithium ions between the layers. Better yet,
this process was entirely reversible! Everyone in those days was interested in making
a secondary battery with a lithium metal anode work. That announcement set off
a race in research labs everywhere to look for other intercalation compounds.
The layered structure
of TiS2.
In 1980, John
Goodenough, who was then at Oxford, wrote a paper in the Materials Research
Bulletin describing another layered compound, LiCoO2 or LCO. This
one was made with lithium already in the structure, so it had to be charged up
before you could use it in a battery. The battery companies at the time didn’t
like this, so they said “Goodenough, it’s not good enough!” (haha, be kind to
me, it’s lonely in the lab.) LCO also was pretty oxidizing once you started
taking the lithium ions out, and the electrolytes of the day just couldn’t
handle it. The last laugh was on the battery companies that dissed John
Goodenough, though, because just a few years later, someone figured out how to
make a graphite anode work, and the lithium-ion battery was born, and then
commercialized by Sony in 1991. Having lithium in the cathode structure for
that configuration turned out to be just what was needed, so that you could
assemble the cell in the discharged state and then charge it up. Moreover, by
that time, there were better electrolytes that didn’t fall apart so readily.
The higher potential at which LCO operated compared to TiS2 was an
asset, not a liability, since it meant higher energy densities in cells.
Batteries with TiS2 and the problematic lithium anode were out, and
Li-ion batteries with LiCoO2 cathodes and graphite anodes were in!
End of
story? No, that was really just the beginning. Cobalt is awfully expensive and
somewhat scarce. Everyone wanted a cathode that was cheaper with more abundant
elements in it, like iron or manganese or nickel. People like Jeff Dahn in Canada, Claude
Delmas in France, and Tom Ohzuku in Japan started looking at other layered
compounds with various combinations of nickel and cobalt and manganese in them
and sometimes a soupรงon
of aluminum or magnesium. They and many other scientists around the world
fiddled around with the formulas to get the best energy density, safety, and
performance. These layered compounds are often called by their initials, like
NCA (nickel cobalt aluminum) and NMC (nickel manganese cobalt) and are among
the most technologically important cathodes we have today.
But let’s backtrack a bit. A few
years after LiCoO2 was discovered, Mike Thackeray, who was living in
South Africa at the time, visited John Goodenough’s lab and started fooling
around with manganese oxides. Only problem was that the compounds he was
looking at weren’t layered but had spinel structures instead. Strictly
speaking, they weren’t intercalation compounds, because “intercalation” really
refers to the insertion of ions between layers, like leaves of a calendar
(“inter”=between and “calation” is related to the word “calendar” from the
Latin word “calends” for the first day of the month). Nevertheless, lithium
ions could be removed from lithium manganese oxide (LiMn2O4,
LMO) and inserted back in again through three-dimensional diffusional pathways
in the structure. Nowadays, the use of the term “intercalation” has expanded to
mean insertion of ions not only into layered structures (the original meaning)
but other types of structures as well. I guess if you are a grammar
prescriptivist, this shift of the language is an offense against all that is
good and holy, but if you are a grammar descriptivist like I am, it’s simply a
useful word to use to describe the general phenomenon of ion insertion into all
kinds of structures.
Another surprise was the olivine-structured
LiFePO4 or LFP (Goodenough, again). It wasn’t even electronically
conducting! Most intercalation compounds are mixed conductors; that is, both
ions and electrons can move through the structure, which is necessary for them
to function. A few of them start out nearly electronically insulating, but
become more conductive as they undergo redox (notably, the spinel anode
material Li4Ti5O12). All you have to do is get
the reaction started (say, on particle surfaces) and then it can propagate. In
contrast, what you get when you oxidize LiFePO4 is another nearly
insulating compound, FePO4. To this day, it is somewhat of a mystery
of how and why it works, and scientists spend lots of time dreaming up exotic
experiments to explain its behavior and arguing over what is really happening.
Hey, we have to remain employed somehow!
On the left, the spinel structure
of LMO and on the right, that of the olivine LFP. The yellow spheres represent
Li ions in the structures.
It turns out that lots of
structures with transition metals in them can insert lithium ions, even some
that are completely disordered. Of course, the majority of these materials fail
on some metric or other; some of them don’t have high enough capacity or energy
density, some have rare or toxic metals in them, or they do something weird
like dissolve or change their voltage characteristics when they are cycled. Of
the hundreds of compounds that have been studied over the past forty years,
only a few have passed muster. A modified version of LCO is used in consumer
electronic batteries, and, depending on manufacturer, hybrid electric, plug-in
hybrid, and electric vehicle batteries contain LFP, NCA, NMC, LMO, or mixtures
of the last two.
Will something better come along?
It’s hard to beat what we have now, but researchers are still trying. To quote
the immortal Yogi Berra, it’s tough to make predictions, especially about the
future!
Marca Doeff
Marca Doeff
