LFP or NMC battery?
Batteries are booming business. In the energy world, the Lithium-Iron Battery (LFP) still has a lot of advantages. Because of its low energy density, this battery is pre-eminently suited for Energy buffers (stationary storage).
However, we see that NMC batteries are slowly taken the position of the LFP because of their higher energy density and because prices are leveling.
Lithium Iron Phosphate (LFP)
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. (See BU-808: How to Prolong Lithium-based Batteries). 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 Nickel Manganese Cobalt Oxide (NMC)
One of the most successful Li-ion systems is a cathode combination of nickel-manganese-cobalt (NMC). Also known as lithium-manganese-cobalt-oxide batteries, or NMC, lithium nickel manganese cobalt oxide batteries are made of several materials common in other lithium iron batteries. These involve a cathode combination of nickel, manganese and cobalt
NMC in the same cell optimized for specific power has a capacity of only about 2,000mWh but delivers a continuous discharge current of 20A.
A silicon-based anode will go to 4,000mAh and higher but at reduced loading capability and shorter cycle life. Silicon added to graphite has the drawback that the anode grows and shrinks with charge and discharge, making the cell mechanically unstable.
Like other varieties of lithium-ion batteries, NMC batteries can have either a high specific energy or high specific power. They cannot, however, have both properties. This battery is most common in power tools and in powertrains for vehicles.
The cathode combination ratio is usually one-third nickel, one-third manganese and one-third cobalt, meaning that the raw material cost is lower than for other options, as cobalt on its own can be quite expensive. According to Battery university, this battery is also commonly preferred for electric vehicles due to its very low self-heating rate.
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