The key technology that may pave the way for widespread,
safe use of electric vehicles is a little-known polymer film system that
separates the anode and the cathode in lithium-ion batteries.
The separator itself does not produce any electrochemical
reactions, but it has a major impact on energy density, power density, cycle
life and safety of the battery.
Exxon, which commercialized the first rechargeable
lithium-ion battery, is one of the technology leaders in development of
specialty polyolefin films that perform well as separators. Pat Brant, the
chief polymer scientist at ExxonMobil Chemical,
headed a global research team that developed a separator system that can
withstand more demanding hybrid-vehicle battery conditions - the type that will
be encountered when GM launches the Chevy
Volt in a few months.
"It's a true breakthrough that can help deploy more hybrid vehicles
faster," says Brant. Hybrid vehicles use less fuel, than traditional vehicles.
Replacement of 10 percent of the gas-powered cars in the U.S. with
hybrid-electric vehicles would result in carbon-dioxide reductions equivalent
to taking 5 million cars off the road.
A new announcement signals that the technology will be
moving to a much larger scale. ExxonMobil Chemical's affiliate TonenGeneral and
Toray Industries have agreed to establish a global joint venture for the
battery separator film business. It will develop, manufacture and sell lithium
ion battery (LIB) separator film and introduce next-generation films The joint
venture will combine Toray's plastic film processing and polymer science
capabilities with Tonen's lithium ion battery separator film business and
"We believe the joint venture will accelerate the
development of separator film technology to support the rapidly evolving
lithium ion battery market faster than either company could do alone,"
says Jim P. Harris, senior vice president ExxonMobil Chemical Co.
Detailed agreements are being prepared in anticipation of
the joint venture formation in January, 2010. TonenGeneral and Toray Industries
will each hold a 50 percent interest in the joint venture, with headquarters
located in Tokyo.
"We are confident that Toray's innovative ideas and
technologies will enhance the capability of the joint venture to capture growth
and meet the needs of the growing LIB market," says Sadayuki Sakakibara, president,
CEO and COO, Toray Industries Inc. Toray is a major producer of plastic film,
particularly polyester, and is well known as the largest global producer of
Other major technology innovations are coming at another
Japanese-based company, Ube Industries, which has also developed a
polyolefin-based separator film it calls U-Pore. Its polyolefin material is
said to possess excellent solvent and chemical resistance.
The battery separator is a porous sheet placed between the
positive and negative electrodes in a liquid electrolyte, a gel electrolyte or
a molten salt battery. It prevents physical contact of the positive and
negative electrodes while serving as an electrolyte reservoir to enable free
ionic transport. The more porous they are, the more energy can move to the
electric motor. They are also key to safety. Ruptured membranes can cause
thermal runaway activity. Lithium-ion batteries developed for cars require a
higher order of technology because of the large sizes of the cells as well as the
need to allow very high power flow.
Role of Trilayer
Trilayer constructions of separator films have become a
particular commercial success, due, in part, to the separator's puncture
strength and the separator's ability to reduce the risk of internal short
circuit within the battery. The puncture strength is important, in part,
because it enables the separator to withstand the rigors of battery
The exact technology used by companies is a tightly kept
secret, both in terms of polymer chemistry and laminate construction. Treatment
of the films is particularly important.
Patent applications; however, provide some technical clues.
trilayer separator is a porous film made of a
polypropylene-polyethylene-polypropylene construction. The Ube process includes
the extrusion of a polypropylene non-porous precursor, extrusion of a
polyethylene non-porous precursor, formation of the
polypropylene-polyethylene-polypropylene non-porous trilayer precursor and
bonding of the trilayer precursor at a temperature ranging from 120-140C
between nip rollers and then taking up the precursor for subsequent processing.
The bonded precursor is then annealed at a temperature range
from 110-140C. The final critical step is a stretching of the bonded, annealed
precursor to form the porous, trilayer separator. The stretching step consisted
of three discrete steps: 1) cold-stretching at a temperature ranging from -20
to 50C and a draw ratio of 5-200 percent; 2) hot-stretching at a temperature
ranging from 70-130C and a draw ratio of 100-400 percent; and 3) heat-treating
at a temperature ranging from 75-175C.
The separator's thickness is about 1 mil (about 25 microns),
and it has a permeability, as measured by Gurley,
of less than 50 sec and a puncture strength of at least 300 gm. The separator's
pores have an average area in the range of 0.003 to 0.010 square microns with
an aspect ratio ranging from 3 to 5.
One of the goals of recent R&D has been to increase the
manufacturing throughput while minimizing the capital investment.
Better Thermal Stability
ExxonMobil introduced two new developmental grades of
co-extruded battery separator films in 2009, with the goal of improving safety
for hybrid and electric vehicles, power tools and electronic devices including
laptop computers. They feature improved thermal stability and lower shutdown
temperature for a higher safety margin. They are based on a proprietary wet
bi-orientation manufacturing process.
Developmental grade one extends thermal stability with an
improvement in transverse direction shrinkage to 8 percent, at 130C for 30 min,
up from 35.5 percent in previous-generation separator films. Developmental
grade two has a significantly improved shutdown temperature of 128C, compared
to 134C for a standard V series grade, and maintains a longer insulation period
with higher meltdown functionality, according to ExxonMobil.
"These developmental grades demonstrate our technology
leadership advancement and commitment to the LIB industry in developing safer
and better-performing batteries," says Exxon Mobil's Harris. "The
enhanced performance characteristics of these new separator films will enable
battery companies to develop smaller and more powerful batteries, including
those required for next-generation, lower-emission vehicles."
ExxonMobil Chemical currently manufactures battery separator
films in Nasu, Japan. A new plant in Gumi, Korea,
will be completed in 2010.
One OEM partnering with ExxonMobil is Electrovaya, which has
developed what it calls Lithium Ion SuperPolymer battery technology. The first
car to be developed with the technology is the Indica EV from India's Tata
Motors. Electrovaya announced it is powering the plug-in hybrid electric Hummer
H3 showcased at the 2009 SAE World Congress.
"As we look ahead to fiscal 2010, we are optimistic about
our opportunities for continued growth as the macro-environment for our
business improves and interest in our unique zero-emission, toxin-free
manufacturing process is higher than it has ever been," says CEO Sankar Das
Gupta. The company became profitable in 2009.