Polyolefin Film is Critical to Electric Vehicle Success

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.

Scale Expands

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 technology.

"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 carbon fiber.

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 manufacturing.

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. The Ube 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.