Nikkei Electronics Asia -- July 2007
Feature
SK Develops Li-Polymer Batteries for Hybrid Cars

"We will complete a prototype line capable of manufacturing 3,000 cells a month before the end of 2007," said a source at SK Corp of Korea, which is jumping into the rechargeable automotive battery market in a big way. After the prototype line is set up in 2007, the batteries will undergo evaluation by vehicle manufacturers in 2008 and 2009, and full-scale volume production is scheduled to begin in 2010. SK is a major chemicals manufacturer under SK Telecom Co Ltd of Korea, which is involved in mobile phones. With proven performance in the energy sector, including petrochemicals, SK is aiming at the storage device market and hoping to grab a chunk of future market growth.

An intense competition is under way in Korea concerning rechargeable batteries for hybrid and other vehicles, between SK, LG Chem Ltd, Samsung SDI Co, all of Korea, and other industry majors. SK says it has already sample-shipped to two automobile manufacturers in Korea. The developed battery unit has already been installed in a Prius hybrid vehicle from Toyota Motor Corp of Japan, and is undergoing evaluation (Fig 1).

High Performance Polymer
The weight energy density of the developed cell is 110Wh/kg, which is double or more than that of the NiMH rechargeable battery used in the Prius, and 10Wh/kg to 20Wh/kg or so higher than the Li-ion rechargeable batteries under development by Japanese battery manufacturers for hybrid vehicles. The output density is 4,000W/kg for a depth of discharge (DOD) of 50%, or about 1,000W higher than the cells being developed by Japanese manufacturers. A single cell is capable of discharge at 200A (about 27C discharge), for the high-current discharge demanded by hybrid vehicles.

What is most intriguing about SK's cells is that this high performance is provided by a polymer electrolytic: in other words, Li-polymer rechargeable batteries. Normally polymer designs offer improved safety, but lower energy density and output density than liquid electrolyte designs. Even so, SK has managed to develop a polymer cell with higher performance than one using liquid electrolyte.

According to the firm, development priority was assigned to safety, durability and other points emphasizing automotive applications, and the end result was a polymer electrolyte. Jeon Keun Oh, leader, HEV Battery Project, Corporate R&D Center at the firm, explained: "We improved safety not only by using polymer electrolyte, but by developing all major components specifically for automotive use, including anode, cathode and separator."

Instead of the more common cylindrical or rectangular cells, the firm uses a laminated design for better performance, including output density and heat radiation. The cathode is spinel LiMn2O4 (lithium manganese oxide) and the anode a carbon-based material. Compared to the LiCoO2 (lithium cobalt oxide) cathodes used in Li-ion rechargeable batteries for mobile phones, the new material offers improved safety, such as in the event of overcharging.

The polymer electrolyte is a mixture of diethylcarbonate (DEC) and LiPF6. While details on the polymer have not been disclosed, it is said to be unlike gels currently used for Li-polymer rechargeable batteries in that a chemical reaction occurs in the monomer state, and covalent bonding is used to produce a chemically stable gel.

The separator is an in-house product with high heat resistance. Conventional separators melt at about 150*C, losing functionality in "meltdown". The SK separator, however, does not suffer meltdown until 190*C, heightening safety in high temperature environments.

Evaluation Trials: Prius
SK is trialing a battery unit using the newly-developed cell in a Prius. The existing Prius battery unit controller, cooling system and other equipment are used "as-is", with only the standard NiMH rechargeable battery swapped out for the new one. Battery unit voltage was adjusted to the same 201.6V as the NiMH rechargeable battery unit so that existing Prius systems would function normally. As a result, the capacity of the battery unit is 1.6kWh, or slightly higher than the 1.3kWh of the NiMH rechargeable battery unit.

Battery unit weight is about 10kg less than the original battery unit, or about 25kg. The weight energy density is 60Wh/kg, and output density 2,200W/kg. These values are low compared to the per-cell values because existing Prius equipment is being used. If the system were designed for the Li-polymer rechargeable battery, the manufacturer believes that the battery unit would only be one-half to two-thirds the size, weight, etc of the NiMH rechargeable battery unit.

SK performed repeated abrupt start/stop trials with the Prius using the new battery unit (Fig 2), confirming that temperature in the battery unit did not exceed about 40*C. The original NiMH rechargeable battery unit, on the other hand, exceeded 50*C, eventually triggering the protect circuit to curtail battery discharge current. The temperature is less likely to rise in the SK battery unit because the laminate cell design enjoys superior heat radiation performance, and also because the internal resistance is 200mOhm, or half that of the original NiMH rechargeable battery unit.

According to SK's Oh, now that battery characteristics have been improved somewhat, future development will focus on how to make the battery unit control system less expensive, including the protect circuit handling problems like cell overcharge and overdischarge.

by Kouji Kariatsumari

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