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Lithium Phosphate Batteries

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JinThursday, April 24, 2008



Li-ion batteries are now widely used as a staple for cell phones, laptops and some other gadgets. Their convenient, lightweight and rechargeable nature makes it the ideal battery chemistry for such devices. Unfortunately, all these positive capabilities come with some major drawbacks. The positive electrode, or cathode, in these batteries is typically made of lithium cobalt oxide, which is expensive and requires electronic circuitry to keep devices from overheating when charged. This also limits the size of the batteries. For these reasons, researchers started to experiment with new materials.

In 1997, researchers at the University of Texas in Austin proposed a new cathode made from lithium iron phosphate (LiFePO4). This material was cheaper and safer than lithium cobalt oxide; however, it had a horribly low electronic conductivity.

In response, researchers have found a way to bind lithium iron phosphate with small amounts of metal irons. This fused material has a conductivity of 10 million times that of unaltered lithium iron phosphate, putting them on par with conventional cathode materials.

This breakthrough cathode is the key ingredient in lithium phosphate batteries. Although these types of batteries are not yet widely in use, their high specifications show that they will be very popular once they gain more exposure.

Valence Technology has already taken advantage of this chemistry and created a successor to sealed lead acid batteries. Their U24 U-Charge® battery claims to have the following specifications:

U24 U-Charge
System Performance

 

Standard Lead acid

Technology Improvement

Operating Voltage

12.8V

12V

6% higher

Capacity (Ah@ C/2)

100

45.6

2.2X

Nominal Energy (Wh)

1280

528

2.4X

Dimensions

Group 24

Group 24

Equivalent

Nominal Weight (kg)

15.8

24.5

35% lighter






On another note, Lithium Technology Corp. announced in May 2007 the immediate availability of cells large enough for use in hybrid cars, claiming they are "the largest cells of their kind in the world," and that a Toyota Prius powered by their batteries obtained 125+ MPG.

This type of battery is also being used in the One Laptop per Child project.
As you can see, this chemistry is already on its way to becoming a major part of new technology these days.

Advantages and disadvantages
Being a lithium-ion-derived chemistry, the LiFePO4 chemistry shares many of the advantages and disadvantages of lithium ion chemistry. Key differences are safety and current rating. Cost is claimed to be a major difference, but that cannot be verified until the cells are more widely accepted.

Lithium iron phosphate batteries do have some drawbacks. The capacity/size ratio of the LiFePO4 battery is somewhat lower than LiCoO2 battery. Battery manufacturers across the world are currently working to find a way to get the maximum storage performance out of smaller size/weight.

Safety
LiFePO4 is an intrinsically safer cathode material than LiCoO2 and Manganese spinel. The Fe-P-O bond is stronger than the Co-O bond so that when abused, (short-circuited, overheated, etc.) the oxygen atoms are much harder to remove. This stabilization of the redox energies also helps fast ion migration. Only under extreme heating (generally over 800 °C) does breakdown occur, which prevents the thermal runaway that LiCoO2 is prone to.

As lithium migrates out of the cathode in a LiCoO2 cell, the CoO2 undergoes non-linear expansion, which affects the structural integrity of the cell. The fully lithiated and unlithiated states of LiFePO4 are structurally similar, which means that LiFePO4 cells are more structurally stable than LiCoO2 cells.

No lithium remains in the cathode of a fully charged LiFePO4 cell — in a LiCoO2 cell, approximately 50% remains in the cathode. LiFePO4 is highly resilient during oxygen loss, which typically results in an exothermic reaction in other lithium cells.

The development of a safer cathode material may solve problems that researchers have encountered while working to manufacture large lithium-ion batteries; however, this chemistry still needs more technical fine-tuning, before it can be used to power commercial hybrid vehicles.
 
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