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The lithium iron phosphate (LiFePO4) battery is a type of rechargeable battery based on the original lithium ion chemistry, created by the use of LiFePO4 as a cathode material. It is not yet widely in use.
LiFePO4 cells have higher discharge current, do not explode under extreme conditions and weigh less, but have lower voltage and energy density than normal Li-ion cells. Because this type of battery is not widely in production, little performance information is available. Pihsiang Energy Technology Co. LiFePO4 batteries (distributed by Valence) have already begun shipping in some scooters and electric bicycles.
LiFePO4 was proposed by researchers at the University of Texas in 1997 because of its low cost, non-toxicity, the high abundance of iron, its excellent thermal stability, safety characteristics, good electrochemical performance, and high specific capacity (170 mA·h/g) but was unusable due to the low conductivity of LiFePO4 until 2002, when Yet-Ming Chiang and his coworkers at MIT tried doping the cathode with conductive materials — such as aluminum, niobium, and zirconium. The increased conductivity allowed development to move forward.
Recent research has included carbon doping — with both carbon black and graphite. In carbon doping, specific capacities of up to 130 mA·h/g were achieved.
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.
LiFePO4 is an intrinsically safer cathode material than LiCoO2. 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.
- ^ a b "Bigger, Cheaper, Safer Batteries: New material charges up lithium-ion battery work" (html).
- ^ a b . "Building safer Li ion batteries" (html).
- ^ . "Effect of conductive additives in LiFePO4 cathode for lithium-ion batteries" (pdf).
Categories: Rechargeable batteries | Lithium