LFP (lithium iron phosphate)

LFP cells use an iron-phosphate cathode and are known for long cycle life, good thermal stability and lower sensitivity to high SOC than nickel-based cells. They store less energy per kilogram and have a flat voltage curve that makes SOC estimation harder.

Strengths

In comparative testing of commercial cells, LFP showed the longest cycle life of the chemistries tested. Cycle-ageing studies on LFP found that capacity loss depends on throughput, temperature, C-rate and depth of discharge, but that the cells tolerate cycling well overall. The iron-phosphate cathode is also structurally stable and less prone to oxygen release than nickel-rich cathodes.

For calendar ageing, LFP cells generally show a weaker SOC dependence than NMC or NCA. Even so, calendar studies on LFP found that higher SOC and higher temperature accelerate ageing, so time at high SOC is not irrelevant.

Limitations and trade-offs

LFP has a lower energy density, so packs are heavier or offer less range for the same size. Cold-weather performance can be weaker, and research on LFP cells shows SEI growth dominating when warm while lithium plating becomes important when cold and at higher charging currents.

Like all lithium-ion cells, LFP cells vary from one to another. A large study of fast-charged LFP cells found wide differences in cycle life between nominally identical cells.

The flat voltage curve and full charges

LFP's voltage changes very little across most of its SOC range. This makes it harder for the BMS to estimate SOC from voltage alone, and the estimate can drift over time. Reaching a full charge gives the BMS a clear reference point to recalibrate.

For this reason LFP vehicles are often advised to charge to 100 % periodically; follow the guidance in your vehicle's documentation. Charging to 100 % and driving soon afterwards adds relatively little time at high SOC, which fits well with LFP's lower SOC sensitivity.

How Voltgevity models LFP

Voltgevity uses LFP-specific calendar and cycle parameters informed by LFP ageing studies, with a weaker SOC dependence than for nickel-based chemistries. Because public data covers specific cells rather than every vehicle pack, results are shown with uncertainty ranges.

Key takeaways

LFP generally offers the longest cycle life of common EV chemistries and lower SOC sensitivity than nickel-based cells.
Higher SOC and temperature still accelerate LFP calendar ageing, just less steeply.
The flat voltage curve makes periodic full charges useful for SOC calibration; check your vehicle's guidance.
Lower energy density and sensitivity to cold charging are LFP's main trade-offs.