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 thatseparates the anode and the cathode in lithium-ion batteries.

The separator itself does not produce any electrochemicalreactions, but it has a major impact on energy density, power density, cyclelife and safety of the battery.

Exxon, which commercialized the first rechargeablelithium-ion battery, is one of the technology leaders in development ofspecialty polyolefin films that perform well as separators. Pat Brant, thechief polymer scientist at ExxonMobil Chemical,headed a global research team that developed a separator system that canwithstand more demanding hybrid-vehicle battery conditions - the type that willbe encountered when GM launches the ChevyVolt in a few months.

"It's a true breakthrough that can help deploy more hybrid vehiclesfaster," says Brant. Hybrid vehicles use less fuel, than traditional vehicles.Replacement of 10 percent of the gas-powered cars in the U.S. withhybrid-electric vehicles would result in carbon-dioxide reductions equivalentto taking 5 million cars off the road.

Scale Expands

A new announcement signals that the technology will bemoving to a much larger scale. ExxonMobil Chemical's affiliate TonenGeneral andToray Industries have agreed to establish a global joint venture for thebattery separator film business. It will develop, manufacture and sell lithiumion battery (LIB) separator film and introduce next-generation films The jointventure will combine Toray's plastic film processing and polymer sciencecapabilities with Tonen's lithium ion battery separator film business andtechnology.

"We believe the joint venture will accelerate thedevelopment of separator film technology to support the rapidly evolvinglithium 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 ofthe joint venture formation in January, 2010. TonenGeneral and Toray Industrieswill each hold a 50 percent interest in the joint venture, with headquarterslocated in Tokyo.

"We are confident that Toray's innovative ideas andtechnologies will enhance the capability of the joint venture to capture growthand 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 ofcarbon fiber.

Other major technology innovations are coming at anotherJapanese-based company, Ube Industries, which has also developed apolyolefin-based separator film it calls U-Pore. Its polyolefin material issaid to possess excellent solvent and chemical resistance.

The battery separator is a porous sheet placed between thepositive and negative electrodes in a liquid electrolyte, a gel electrolyte ora molten salt battery. It prevents physical contact of the positive andnegative electrodes while serving as an electrolyte reservoir to enable freeionic transport. The more porous they are, the more energy can move to theelectric motor. They are also key to safety. Ruptured membranes can causethermal runaway activity. Lithium-ion batteries developed for cars require ahigher order of technology because of the large sizes of the cells as well as theneed to allow very high power flow.

Role of Trilayer

Trilayer constructions of separator films have become aparticular commercial success, due, in part, to the separator's puncturestrength and the separator's ability to reduce the risk of internal shortcircuit within the battery. The puncture strength is important, in part,because it enables the separator to withstand the rigors of batterymanufacturing.

The exact technology used by companies is a tightly keptsecret, both in terms of polymer chemistry and laminate construction. Treatmentof the films is particularly important.

Patent applications; however, provide some technical clues.The Ubetrilayer separator is a porous film made of apolypropylene-polyethylene-polypropylene construction. The Ube process includesthe extrusion of a polypropylene non-porous precursor, extrusion of apolyethylene non-porous precursor, formation of thepolypropylene-polyethylene-polypropylene non-porous trilayer precursor andbonding of the trilayer precursor at a temperature ranging from 120-140Cbetween nip rollers and then taking up the precursor for subsequent processing.

The bonded precursor is then annealed at a temperature rangefrom 110-140C. The final critical step is a stretching of the bonded, annealedprecursor to form the porous, trilayer separator. The stretching step consistedof three discrete steps: 1) cold-stretching at a temperature ranging from -20to 50C and a draw ratio of 5-200 percent; 2) hot-stretching at a temperatureranging from 70-130C and a draw ratio of 100-400 percent; and 3) heat-treatingat 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'spores have an average area in the range of 0.003 to 0.010 square microns withan aspect ratio ranging from 3 to 5.

One of the goals of recent R&D has been to increase themanufacturing throughput while minimizing the capital investment.

Better Thermal Stability

ExxonMobil introduced two new developmental grades ofco-extruded battery separator films in 2009, with the goal of improving safetyfor hybrid and electric vehicles, power tools and electronic devices includinglaptop computers. They feature improved thermal stability and lower shutdowntemperature for a higher safety margin. They are based on a proprietary wetbi-orientation manufacturing process.

Developmental grade one extends thermal stability with animprovement in transverse direction shrinkage to 8 percent, at 130C for 30 min,up from 35.5 percent in previous-generation separator films. Developmentalgrade two has a significantly improved shutdown temperature of 128C, comparedto 134C for a standard V series grade, and maintains a longer insulation periodwith higher meltdown functionality, according to ExxonMobil.

"These developmental grades demonstrate our technologyleadership advancement and commitment to the LIB industry in developing saferand better-performing batteries," says Exxon Mobil's Harris. "Theenhanced performance characteristics of these new separator films will enablebattery companies to develop smaller and more powerful batteries, includingthose required for next-generation, lower-emission vehicles."

ExxonMobil Chemical currently manufactures battery separatorfilms in Nasu, Japan. A new plant in Gumi, Korea,will be completed in 2010.

One OEM partnering with ExxonMobil is Electrovaya, which hasdeveloped what it calls Lithium Ion SuperPolymer battery technology. The firstcar to be developed with the technology is the Indica EV from India's TataMotors. Electrovaya announced it is powering the plug-in hybrid electric HummerH3 showcased at the 2009 SAE World Congress.

"As we look ahead to fiscal 2010, we are optimistic aboutour opportunities for continued growth as the macro-environment for ourbusiness improves and interest in our unique zero-emission, toxin-freemanufacturing process is higher than it has ever been," says CEO Sankar DasGupta. The company became profitable in 2009.