Lithium Polymer Traction Batteries, Good or Bad?

February 23, 2011 - via EVWorld

A large and increasing number of traction battery manufacturers are now offering what they call lithium polymer (LiPo) versions. Lithium polymer batteries are being eagerly adopted in a rapidly widening variety of applications. That includes in both pure electric and hybrid vehicles whether on land, in water or in the air. Battery manufacturers in Canada, the USA, Russia, China, Korea, Taiwan and elsewhere are involved.

Many suppliers
Many of these lithium polymer battery suppliers are vertically integrated, making cells and modules and sometimes complete battery packs as well. They include include ABAT, ChinaBAK, Dow Kokam, Electro-Energy, Electrovaya, Kokam, LGChem, LiCo Technology, Lishen, Lithium Energy, Pihsiang Energy Technology, Rusnano/Thunder Sky, SEEO, Thunder Power, Thunder Sky and Xiamen Powerlong Industry.

Varied applications
Such a battery cell construction is valued in very different vehicles. For instance ABAT LiPo batteries are in pure electric motorcycles. Dow Kokam ones are in a Cessna light aircraft modified to be hybrid electric, Electrovaya has them in a military off-road vehicle, Kokam ones are in an EADS manned stunt aircraft that is pure electric. LGChem has LiPo traction batteries designed into several mainstream hybrid cars but Thunder Sky has them in buses and Autonomous Underwater Vehicles AUVs.

What most people call lithium polymer
Lithium polymer batteries use a thick polymer separator such as polyethylene oxide or polyacrylonitrile composite. Polymer gels are added to make performance at ambient and low temperature adequate, so these are not of fully solid construction. However, leakage is less likely and absence of liquids should improve upper temperature range and life according to proponents. Most lithium polymer cells are contained in pouches - heat sealed plastic laminated bags akin to the aluminised ones used in the food industry. This makes them exceptionally compact, unlikely to explode and easily manufactured in huge numbers. These cells are then packed into modules. Plastic or lightweight aluminium casing for modules can replace the strong, heavy steel cases of traditional lithium-ion traction battery modules. The resulting assembly is still very tolerant of pressure and impact.

Pouch problems?
In 2010, BMW carried out extensive tests of certain pouch based cells for lithium-ion traction batteries and found damning evidence of leakage of electrolyte out and moisture in. This can affect life and safety. Time will tell if there really is a problem with latest delivered product, and certainly improved pouches, are of interest. Anyway, most of the lithium polymer traction battery suppliers now offer prismatic flat alternatives suitable for other forms of packaging at the cell level. Prismatic construction is a very compact assembly also employed with some new wet lithium-ion traction battery cells. Challenges from movement of both layered and prismatic cells during use seem to have been overcome.

Uses today
Lithium polymer traction batteries are favoured in Autonomous Underwater Vehicles AUVs because they tolerate pressure and in Unmanned Aerial Vehicles UAVs because they are lightweight - no need for a heavy metal case - and they can be thin and even shaped. In a collision of manned light electric aircraft they should not spew acid or explode. Many land vehicles incorporate them because they are light and tolerant of impact but there are other important benefits as well.

They often combine affordability and adequate performance and life because many chemistries and compromises are possible as we await superior solid state and other traction alternatives still stuck in the laboratory. For example, Hyundai uses LGChem lithium polymer batteries in its new hybrid family cars. They have spent thousands of hours testing the Hyundai Hybrid Blue Drive lithium polymer battery system. This testing has proven that Hyundai's lithium polymer technology has greater thermal and mechanical stability than existing systems, meaning better safety and performance.

Lithium polymer vs nickel metal hydride
Compared with the nickel-metal hydride batteries in today's leading hybrid cars, lithium polymer batteries deliver the same power with 20-30% less weight, 40% less volume and 10% greater efficiency.

Lithium polymer batteries offer 1.7 times more energy density than nickel-metal hydride batteries, allowing Hyundai engineers to devote less space and weight to the battery pack. Lithium polymer batteries hold their charge 1.25 times longer. Lithium polymer batteries also are more resistant to changes in temperature, which improves cycle life. Additionally, lithium polymer's self-discharge rate is less than a third of a nickel-metal hydride battery and it is not dependent on the lanthanum supplies for nickel metal hydride batteries that are cornered by China.

Lithium polymer vs. lithium-ion
Lithium-polymer has significant advantages over traditional wet lithium-ion traction batteries, including higher energy density and lower manufacturing costs. Lithium polymer is more resistant to physical damage and it can handle more charge-discharge cycles before storage capacity begins to degrade. Lithium polymer technology also offers significant advantages in thermal robustness and safety.

Traditional lithium-ion batteries are easy to handle, withstand mild internal pressures, and have been around in various forms since 1991. That means a manufacturing infrastructure is in place, and economies of scale are reasonably high. However, they do have several disadvantages. For example, their typically cylindrical shape reduces packaging efficiency and they are complicated to manufacture since they have so many small parts necessary to make them robust to thermal fluctuations, this adds significant cost and weight to the overall battery system.

Cell-to-cell consistency is extremely critical in a vehicle battery package, since the pack is only as robust as its weakest cell. Traditional lithium-ion wet batteries have considerable cell-to-cell variation, while well designed lithium polymer batteries deliver outstanding cell-to-cell consistency, Hyundai being one user that has independently verified this.

Another key engineering challenge for Hyundai Hybrid Blue Drive was assuring maintenance-free battery operation over the vehicle's life. At least 10 years, and 150,000 miles in all weather conditions is targetted. Heat is the enemy of battery cycle life but Hyundai's thermal imaging testing showed how much cooler a lithium polymer battery is compared to today's nickel-metal hydride battery or a conventional wet lithium-ion battery. Consumers will notice these advantages in improved useful life and lower maintenance costs, says the company.

Where next?
It would be wise to conclude that the adoption of lithium polymer traction batteries will continue to increase for several years. Any appraisal of the pros and cons must recognise that the term lithium polymer simply refers to a form of electrolyte containment when the performance and cost of the cell is particularly dependent on the cathode chemistry, with electrolyte, anode and cell geometry among the factors that are also very important. For example, the more temperature tolerant, lower cost lithium iron phosphate cathodes and their variants are gaining in popularity as are lithium titanate anodes for faster charge-discharge. They can be used in lithium polymer or wet chemistry.

Companies embracing lithium polymer construction for their manufacture of traction batteries are also variously employing lithium manganese, lithium nickel cobalt aluminium and other cathodes to achieve different price-performance compromises. Solid state, reel to reel assembled batteries will gain favour particularly for third generation batteries such as lithium sulphur and lithium air, gaining advances in such parameters as energy density but volume sales are several years away.

Customisation
It is imprudent to obsess about cars, because markets for electric vehicles and their components are often more profitable and innovate earlier. When developing and selling traction batteries, it is necessary to look at the very different needs of all electric vehicles - on-road, off-road, on water, under water and in the air, in each case including both manned (safety critical) and unmanned (less safety critical) pure electric and hybrid vehicles.

Understand the timelines - for example, the next generation plug in hybrids will optimise energy density in batteries as longer ranges are demanded so it will no longer be all about power density with hybrids. Uniquely, the IDTechEx global series of events Electric Vehicles: Land, Sea, Air Conference & Exhibition provides the full picture, the next one being in Stuttgart, Germany on June 28-29, visit www.IDTechEx.com/evEurope.

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Author:Peter Harrop