What exactly does "IMR" mean?

Battery Guy - Greetings Everyone,

I have seen a lot of threads recently on "IMR" lithium-ion cells. DFiorentino and myself have posted some test results recently (seen here and here, respectively). I also have seen a lot of comments recently that indicate a general misconception of what IMR cells actually are, especially with respect to rate capability and safety. The purpose of this thread is to clarify the issues surrounding the "IMR" designation for lithium-ion cells. I will try to keep the techno-jargin to a minimum, but please feel free to ask clarification questions if I lose you.

Unlike most battery chemistries whose name defines the chemistry of the anode and cathode, the term "lithium-ion" refers to an ever growing class of cell chemistries that utilize different materials to host lithium-ions in the charged and discharged state of the cell. Most lithium-ion cells use graphite or hard carbon as the negative electrode host material (typically referred to as the anode), but the selection of the positive electrode material (typically referred to as the cathode) is much more varied. The following three-letter designations for lithium-ion cell denote what cathode material is used in the cell:

ICR = LiCoO2 (also called LCO)
IMR = LiMn2O4 (also called LMO)
IFR = LiFePO4 (also called LFP)

If you take away one thing from this thread, remember this: The term "IMR" simply indicates that LiMn2O4 is a major component of the cathode. It does not necessarily indicate that a cell has exceptional rate capability or improved safety characteristics, properties often incorrectly associated with cells with the IMR designation.

You can stop reading now, but if you want that statement explained, please read on.

The selection of a particular cathode material generally has implications for the performance (capacity, energy density, rate capability, etc...), reliability (cycle life, calendar life) and safety characteristics. The impact of LiMn2O4 on these properties is discussed below.

Performance

The crystal structure of LiMn2O4 has three-dimensional tunnels that enable rapid diffusion of Li+ through LiMn2O4 particles, and therefore lithium-ion cells made with this material can have better theoretical drain rate capability. Conventional lithium-ion cells use LiCoO2 or variations of that material. Cells with LiCoO2 cathodes are typically designated with the term "ICR". The LiCoO2 type materials have a layered crystal structure that only enables 2D diffusion within the layers, so the overall Li+ diffusion rate is slower. Interestingly, LiFePO4, which is commonly used in high rate cells (think A123), has 1D diffusion of Li+ and an overall low diffusion rate. The only way to make LiFePO4 work is to make the particles very small (i.e. "nano") to reduce the distance that Li+ needs to diffuse. "Why not just make LiCoO2 or LiMn2O4 nano size to get even better rate capability?" I hear you ask. The only reason nano LiFePO4 works is that this material has a lower oxidizing potential (which is why the cell voltage is lower) and therefore it does not chemically react with the electrolyte. LiCoO2 and LiMn2O4 actually oxidize the electrolyte slowly with use, so using a nano-particulate cathode with these materials would greatly accelerate aging due to the higher surface area of the cathode, and the cells would have terrible cycle life.

Now back to LiMn2O4. This material has a lower crystalline density and therefore a lower inherent capacity (mAh/ml) than LiCoO2. If you do a direct replacement of LiCoO2 with LiMn2O4, the capacity of the cell will be reduced by ~15%. But here is the interesting thing: simply doing a direct replacement with LiMn2O4 will not significantly increase the rate capability of a cell. In order to get a real increase in rate capability, you need to design a cell with higher electrode surface area and thinner electrodes because Li+ diffusion in the cathode particles is not the only bottleneck to getting current out of the cell. Using thinner electrodes further reduces the capacity of the cell because you will have a higher fraction of "inert" materials like the separator and current collector. This is why IMR cells typically have ~40% lower capacity than their ICR cousins.

Safety and Reliability

An additional property of LiMn2O4 is that it cannot be overcharged. When a "true" (and I will explain why I put that in quotes later) IMR cell is charged beyond ~4.25 V, no additional capacity will go into the cell. The voltage will simply spike. This is exactly the same case for LiFePO4 cells. Cells that use LiCoO2 and its layered-metal oxide cousins can be overcharged, leading to significant safety issues when these cells are charged above their specified voltages. Cells with LiMn2O4 and LiFePO4 cathodes can be damaged by overcharging (oxidation of the electrolyte leads to increased internal resistance, loss of capacity and shorter cycle life), but overcharging to higher voltage does not make them less safe. This inability to overcharge, combined with the lower total energy density, is why lithium-ion cells with LiMn2O4 and LiFePO4 are generally considered to be "safer".

The last property of LiMn2O4 that I will mention is that in its pure form it tends to have very poor cycle life. There has been a tremendous amount of research that has gone into improving the stability of LiMn2O4 to improve the cycle life of IMR cells, and this has produced cells with reasonably good long term performance. However, the easiest way to get great cycle life out of LiMn2O4 is to blend it with a separate cathode material that contains nickel. This includes the layered cathode materials Li(Nix,Cox,Mnx)O2 (aka NCM or 333), Li(NixCoyAlz)O2 (aka NCA) and Li(NixCoy)O2 (aka NCO). I won't bore you with the details, but it turns out that the oxides containing nickel acts to change the local chemistry around the LiMn2O4 particles and helps to improve stability.

The end result is that most cells called "IMR" actually have a significant fraction of nickel-containing layered metal oxide as a co-cathode. This means that these cells can in fact be overcharged, thus minimizing the perceived safety advantage of the cell.

Summary

Not all IMR cells are created equal. One cannot and should not make generalized statements about their performance, reliability or safety characteristics.

I hope this helps dispel some of the mystery surrounding these cells.

Cheers,
BG

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