Lithium iron phosphate (LiFePO 4)
In 1996, the University of Texas discovered that phosphate could be used as cathode material for rechargeable lithium batteries. Lithium phosphate has good electrochemical properties and low resistance. This is achieved by nanometer phosphate cathode materials. The main advantages are high rated current and long cycle life, good thermal stability, enhanced safety and tolerance to abuse.
If kept at high voltage for a long time, lithium phosphate is more tolerant to all charging conditions and has less stress than other lithium ion systems. The disadvantage is that the lower nominal voltage of 3.2V battery makes the specific energy lower than that of cobalt-doped lithium-ion battery. For most batteries, low temperature decreases performance, and higher storage temperature shortens service life, including lithium phosphate. Lithium phosphate has higher self-discharge than other lithium-ion batteries, which may lead to aging and balance problems. Although it can be compensated by using high-quality batteries or advanced battery management systems, both methods increase the cost of battery packs. Battery life is very sensitive to impurities in the manufacturing process and can not withstand water doping. Because of the existence of water impurities, some batteries have a minimum life of only 50 cycles. Figure 9 summarizes the properties of lithium phosphate.
Lithium phosphate is commonly used to replace lead-acid starter batteries. Four series batteries produce 12.80V, similar to six 2V lead-acid batteries. The vehicle charges the lead acid to 14.40V (2.40V/battery) and maintains the floating charge state. Floating charge is designed to maintain full charge level and prevent sulfation of lead-acid batteries.
By connecting four lithium phosphate batteries in series, the voltage of each battery is 3.60V, which is the correct full charge voltage. At this point, the charge should be disconnected, but continue to charge while driving. Lithium phosphate tolerates some overcharging; however, the mechanical stress of lithium phosphate batteries may increase because most vehicles maintain a voltage of 14.40V for a long time during long journeys. Time will tell us how long lithium phosphate can withstand overcharging as a substitute for lead-acid batteries. Low temperature also reduces the performance of lithium ions, which may affect the startup ability in extreme cases.
Figure 9: Spider diagram of typical lithium phosphate batteries.
Lithium phosphate has good safety and long life, moderate specific energy and enhanced self-discharge ability.
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Lithium iron phosphate: LiFePO4 cathode, graphite anode
Abbreviation: LFP or lithium phosphate, started in 1996
Voltage 3.20, nominal value 3.30V; Typical operating range 2.5-3.65V
Specific energy (capacity) 90-120 Wh/kg
Charging (C rate) 1C typical, charging to 3.65V; typical 3-hour charging time
Discharge (C rate) 1C, 25C is feasible on some cores; 40A pulse (2s); 2.50V cut-off (damage caused by less than 2V)
Cycle Life 1000-2000 (Dependent on Discharge Depth and Temperature)
Thermal runaway 270 C (518 F) battery is very safe even if it is full of electricity.
Application scenarios for portable and fixed applications requiring high load current and durability
Note: Very flat voltage discharge curve but low capacity. Safest
One of the lithium ions. For special markets. High self-discharge.
Table 10: Characteristics of lithium iron phosphate
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