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In 1996, the University of Texas (and other contributors) discovered phosphate as cathode material for rechargeable lithium batteries. Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are high current rating and long cycle life, besides good thermal stability, enhanced safety and tolerance if abused.

 Li-phosphate is more tolerant to full charge conditions and is less stressed than other lithium-ion systems if kept at high voltage for a prolonged time. As a trade-off, the lower voltage of 3.2V/cell reduces the specific energy to less than that of Li-manganese. With most batteries, cold temperature reduces performance and elevated storage temperature shortens the service life, and Li-phosphate is no exception. Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. Figure 9 summarizes the attributes of Li-phosphate.

 Li-phosphate is often used to replace the lead acid starter battery. Four cells in series produce 12.80V, a similar voltage to six 2V lead acid cells in series. Vehicles charge lead acid to 14.40V (2.40V/cell) and maintain a topping charge. With four Li-phosphate cells in series, each cell tops at 3.60V, which is the correct full-charge voltage. At this point, the charge should be disconnected but the topping charge continues while driving. Li-phosphate is tolerant to some overcharge; however, keeping the voltage at 14.40V for a prolonged time, as most vehicles do on a long drive, could stress Li-phosphate. Cold temperature operation starting could also be an issue with Li-phosphate as a starter battery.

lithium iron phosphate battery

Snapshot of a typical Li-phosphate battery.

Li-phosphate has excellent safety and long life span but moderate specific energy and elevated self-discharge.

Courtesy of Cadex


Lithium Iron Phosphate: LiFePO4 cathode, graphite anode    

Short form: LFP or Li-phosphate

Since 1996

Voltages

3.20, 3.30V nominal; typical operating range 2.5C3.65V/cell

Specific energy (capacity)

90C120Wh/kg

Charge (C-rate)

1C typical, charges to 3.65V; 3h charge time typical

Discharge (C-rate)

1C, 25C on some cells; 40A pulse (2s); 2.50V cut-off (lower that 2V causes damage)

Cycle life

1000C2000 (related to depth of discharge, temperature)

Thermal runaway

270C (518F) Very safe battery even if fully charged

Applications

Portable and stationary needing high load currents and endurance

Comments

Very flat voltage discharge curve but low capacity. One of safest

Li-ions. Used for special markets. Elevated self-discharge


                            Characteristics of lithium iron phosphate.
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