Saturday, December 6, 2014

41. LCO, LFP, NMC...CRYPTIC LIVES OF THE CATHODE                              

What flavors does the lithium ion battery come with? The answer is: quite a few. They represent the various combinations of materials that are used for the two electrodes, the anode and the cathode.

It turns out the available choices for the anode are quite limited. Different forms of carbon, in particular graphite, are the common choice in all commercial applications. Silicon and silicon-carbon composites as well as tin are candidates for future anode materials but they are not presently in wide commercial use.

But the cathode has enjoyed a longer list of candidate materials, typically known by their acronyms.  All of these materials are lithium metal composite alloys, with many them using heavy metals such as cobalt and nickel.Each material seems to fit a particular application. Lithium-cobalt-oxide (LCO) is most commonly used in applications where high energy density and high capacity are needed, in particular mobile devices. Lithium-iron-phosphate (LFP) is of particular interest to the automotive industry especially in China. Its safety, long cycle life and low cost make it attractive to electric vehicles. Nickel and manganese composites have been rather limited in their utilization primarily due to cost considerations. For example, the Tesla Model S uses 18650 cells made by Panasonic with nickel-cobalt-aluminum (NCA) alloy for the cathode material.

Another salient difference between these materials is their open circuit voltage; in other words, how their terminal voltage varies with the amount of charge stored in the cell (the state of charge or SOC). The graph below shows that dependence during charging. As more charge is added to the battery, its terminal voltage rises.

One will quickly notice that LFP has the lowest average voltage, near 3.2V, considerably lower than the voltage for NMC or LCO, both hovering near 3.7V. In fact, modern cells made with LCO have a maximum voltage of 4.35V, up from 4.2V, thus raising the average voltage to 3.8V. This higher voltage, and consequently higher energy, makes LCO an attractive material for use in mobile devices.

Additionally, the voltage behavior near empty (below 15% SOC) plays a big role in the utility of the material. LCO can sustain a useful voltage above 3.6V down to about 5% remaining charge, whereas NMC drops below 3.5V when the SOC is at 10%. This is yet another reason why the mobile industry continues to choose LCO.

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