When fuel and air burn inside the cylinder of an internal-combustion engine, the energy being released comes from the electronic bonds that bind atoms together to form molecules. The bond energy of unburned hydrocarbon fuel and diatomic oxygen from the air is higher than that of the products of complete combustion, which are water and carbon dioxide. Upon combustion, this energy difference appears (mainly) as heat. This heat raises the pressure of the gases in the cylinder, driving the piston downward to turn the engine’s crankshaft.
The very same kind of electronic bond energy stores and delivers the
power we take from batteries. A battery at its simplest consists of a
positive and a negative electrode, exposed to an electrolyte. In the
case of today’s powerful Lithium-ion batteries, the electrolyte consists
of Lithium salts dissolved in an organic liquid. Just as table
salt—sodium chloride or NaCl—separates into oppositely charged sodium
and chlorine ions when dissolved in water, so the Lithium salts release
Li+ ions into solution.
During charging, negative electrons are supplied by the charger to
the battery’s negative electrode (anode). Because the electrolyte is an
insulator to electrons, the only way charging current can move through
it from one electrode to the other is by the movement of Li+ ions. They
move to the anode (whose commonest material is carbon) where they
wriggle between the layers of carbon, one Lithium ion nestling
comfortably into each available six-carbon ring. This process has the
wonderful name “intercalation.” Taking up an electron in the process of
nestling into the carbon, the Li ion becomes neutral.
During discharge, Li atoms each give up an electron as they emerge
from the carbon anode and migrate through the electrolyte to the
cathode. In the first successful Li-ion batteries, the cathode was
Cobalt Oxide. There, the ion enters the layered structure of the Cobalt
Oxide. The electrons released in the discharge process move through the
external circuit to power a cellphone, laptop computer, or other
The voltage difference between positive and negative electrodes,
which drives electrons through the external circuit and its load (the
motor of an electric TT bike?) is the difference between the “electron
affinities” of the two metals, their different electro-chemical
potentials. In the experiment so many of us performed in school, two
electrodes in the form of a strip of zinc and a strip of copper are
inserted into a lemon. The water and mild acetic acid content of the
lemon act as an electrolyte, allowing ions to move it to create an
easily measured voltage difference between the two electrodes. The lemon
has become a simple battery cell.
How We Feel
Why does battery power please some people and deeply offend others?
Those who are pleased are those who see that battery electric vehicles
could, if they became cheap enough to reach a mass market, clean up
urban air. Many are also attracted to the high efficiency of electric
motors, which, depending on price, varies between 90 and 97 percent.
Electrics seem modern and progressive.
And those who are offended? Even though combustion power and battery
power come from the same basic source—the electric charges that hold
molecules together—the sound and fury of combustion give it romantic
appeal. Understandably to these romantics, the hum of electric power is
anticlimax, turning vehicles into appliances. Electrics seem like the
leading edge of an era of standardized automatic vehicles, driving
themselves identically in ranks and rows.
Many alternative cathode and anode chemistries have been discovered
since that first commercial Li-ion battery hit the market in 1991. The
original Cobalt Oxide cathode’s vulnerability to overheating, producing
oxygen, and possibly catching on fire led to the 2006 “era of flaming
laptops”. Meanwhile, other cathode types such as Lithium Manganese Oxide
(LMO) and Lithium Iron Phosphate (LPO) were developed, offering greater
resistance to overheating but having less energy density (measured in
kilowatt-hours per kilogram, or kWh/kg). These types have become the
principal players in the electric vehicle field.
How They’re Made
What are these batteries, physically? First of all, they are completely
sealed and contain no water (lithium and water react violently). Each of
the two electrodes is usually implemented as a thin metal foil carrying
a layer of the electrode material in powder form, held together and
onto the foil by a polymer binder. The positive electrode begins with a
thin aluminum foil to function as a current collector, with electrode
material and binder on its surface. The negative electrode material is
commonly carbon, again held in place by polymer binder but on a thin
copper foil current collector. The active surfaces of the two face each
other, separated by a thin (0.001 inch) polymer membrane separator whose
cost can be half of total cell cost. Electrolyte wets both electrodes.
The two obvious packaging schemes are cylindrical and flat. In a
cylindrical cell, such as the “billions served” 18650, the electrode
material is made in the form of long strips, which are sandwiched over
the separator. This is then rolled up to fit in the cylindrical
container. The 18650 is so called because it is nominally 18mm in
diameter (a little under 3/4 inch) and 65mm long (a bit over 2 1/2
inches). A potential advantage of a flat format is that the cell
container can be a flexible flat bag or a pouch that packages densely.
Although much is made of the possible scarcity and high cost of
materials such as Cobalt or Lithium, material cost is said to be only a
small element in finished cell price.
Problems and Solutions
Many problems have beset Lithium-ion batteries, and many problems remain
to be solved. Back in the 1980s, researchers found that if the cell was
charged too rapidly, Lithium ions did not obediently wriggle between
the layers of the carbon anode but instead plated out on its surface.
Then the plated surface developed bumps, which developed into
whisker-like dendrites. Such dendrites could either grow right through
the separator membrane, shorting out the cell, or they could become
loose particles during discharge, causing loss of lithium that had to be
made good by providing more than just necessary for normal operation.
Lithium’s burrowing act also had consequences. Each time the cell was
charged, the carbon anode swelled up as Lithium ions took up their
positions between its many layers, then shrank again as Lithium departed
during discharge. This, over time, led to cracking and the shedding of
particles. In response, the industry has developed other anode
chemistries such as LTO, or Lithium Titanate Oxide, which eliminates
dendrite formation and speeds charging but reduces cell voltage and
If aggressive charging went on too long, it drove reactions between
the Lithium and electrolyte. Such reactions gradually consumed Lithium,
causing a drop in cell properties, and ultimately releasing oxygen.
Since the liquid part of the electrolyte is an inflammable organic, the
combination of fuel, oxygen, and heat is a recipe for fire. To prevent
this, the high-power-density Lithium Cobalt Oxide cells are provided
with electronic battery-management systems to supervise and control
charge/discharge rates and monitor temperature. Such systems add
Also, fire retardants may be added to electrolytes. In the celebrated
case of Boeing’s 787 “Dreamliner,” the engineers’ inability to
understand and overcome battery overheating led to placing each $16,000
cell assembly inside a fire-resistant steel box, vented outside the
aircraft. Sadly, the weight saved by adoption of energy-dense Li-ion
cells was neutralized by the weight added as containment.
Intensive development work on every aspect of the Lithium-ion cell is
ongoing around the world. Many kinds of high surface area electrode
materials—extremely fine powders, spinel-structured minerals, and
nano-wires—seek to provide so much area onto which Li-ions can attach
that they need not burrow into layered solids such as carbon or silicon,
causing swelling, cracking, and flaking. You will find announcements of
such work almost every day on sites such as greencarcongress or gizmag. As one battery expert put it in 2009, Li-ion batteries are “boxes of technical trade-offs and compromises.”
For some, this intensive research fuels a certainty that any day now,
a complete solution will be found—compatible anode and cathode
chemistries offering near-zero heating, record energy density, long
cycle life, high cell voltage, fast charge, and low cost. For others,
the modest gains achieved by all this work and investment suggest the
work must continue for a long time yet.
Stanley Wittingham, an original pioneer of Lithium-ion, has said
electric vehicles will be used only for trips of less than 100 miles. He
expects energy density to eventually double, but not much more. J.B.
Straubel, an engineer at electric automaker Tesla, says battery
technology improves by “of the order” of double in 10 years, which
implies a rate of improvement of about seven percent per year.
The greater the energy density of a cell system, and the more tightly it
is packaged, the greater is its need of active cooling. Because the
charge/discharge cycle cannot be 100 percent efficient, heat is
generated. Standard sources list charge/discharge efficiency as 66
percent for Ni-metal hydride batteries such used on Toyota’s hybrid
Prius, and 80 to 90 percent for Lithium-ion.
Cooling can be as simple as spacing cells apart enough to allow air
circulation or placing strips of aluminum sheet between flat “pouch”
cells, leaving part of the sheet projecting into circulating air that
carries away heat. In the most intensive systems, liquid coolant is
circulated to an external radiator by pump. Reminds me of what former
Rolls-Royce CEO Lord Hives said when told of the simplicity of the gas
turbine, “We’ll soon design the simplicity out of it!”
Change of Auto Industry Emphasis
When I reviewed my back issues of Automotive Engineering, the
magazine of the Society of Automotive Engineers (SAE), I saw that
articles on battery technology, electric motors, and motor controllers
peaked in 2008/09 and declined thereafter. In conversations with auto
engineers, I have learned that attitudes have changed. It is now clear
that there is little market demand for electric vehicles at present
price levels; the “electric-car buzz” has arisen mainly from government.
Because the industry now faces the mandated 54-mile-per-gallon fleet
average fuel economy, it has for the present chosen a dual-path
strategy. One path is to continue to improve the internal combustion
engine and the other is to develop hybrids, which are of two basic
kinds, parallel and serial:
1. In a parallel hybrid, either the IC engine or the electric motor
can propel the vehicle. The electric motor is used at low speeds and
loads at which the IC engine is least efficient, and the IC engine
propels it the rest of the time.
2. In a serial hybrid, the wheels are driven by an electric motor
drawing power from a battery, but the battery is charged by what
marketeers are now calling a “range extender.” That is an IC engine, so
in fact this system’s prime mover is an IC engine, driving through a
“transmission” consisting of battery and electric motor.
Either type of hybrid may become a “plug-in hybrid” by carrying a
charging system that can pull power from a 120V household outlet (power
limited to 1500 W, meaning long charging time), a 220V stove/drier
outlet, or a dedicated charging point.
Hybrids cost about 30 percent more than equivalent
all-combustion-powered vehicles yet can reach much more of the market
than can expensive present-day pure electrics. Hybrids have what
electrics presently most lack: range and quick refueling from the
hundreds of thousands of existing gasoline stations.
Lithium-ion batteries have been expensive, around $500 to $650 per
kilowatt-hour (kWh) of capacity. That would price an electric
motorcycle’s 14-kWh battery at $8,000. Tesla’s recently announced new
battery plant aims to bring the price down to $300 per kWh or $4,200 for
a notional electric motorcycle’s battery. And $150 to $200 per kWh is
regarded as a possible turning point in the market competitiveness of
pure electric cars.
Can we believe recent announcements that Li-ion prices are about to
drop by 50 percent? Or do we extrapolate Bloomberg’s price-versus-year
graph, which shows Li-ion battery price dropping at just five percent a
year, a rate that requires 14 years to achieve that 50 percent price
Other Battery Futures
One way to compare battery chemistries is by their theoretical
properties, unmodified by the compromises of usable, affordable
commercial products. Comparing in this way, a Lithium-air battery is
tantalizing, as its numbers suggest an energy storage device that could
be close in energy density to that of hydrocarbon fuels.
Lithium is among the very lightest of the elements and the air
doesn’t have to be carried; it comes from the atmosphere. In 2009, there
was intense interest in Lithium-air, peaking in 2012, but more
recently, major labs such as those of IBM and Argonne have all but given
up Lithium-air as unworkable. Li-air work continues at St. Andrews
University in Scotland.
Now, some believe more actual performance can be realized from a
Sodium-air battery, despite its having only half as much theoretical
There is much work to be done.