Cylindrical vs. rectangular batteries' power (C-rates)

[BlahHead]

Hi CPF'ers,

I was wondering why cylindrical batteries (18650, etc.) have significantly less C-rates than square batteries used for R/C toys. Is it because the R/C batteries are specifically designed for stress?

Is it significantly hard to increase the C-rates?

Side question: I've looked at the new 18650 Panasonic cells with 3100 mAh and discovered that they are relatively safe. Some sellers claim that they have the technology of ... well, let me just quote.

Callies Kustoms says: "A solid solution technology that allows the high capacity of a standard Lithium-Ion battery and the safety of an IMR battery."

They even have a video of a battery short-circuit torture test. Is this technology really safer than our previous batteries?

Thanks guys, and happy illuminating! :D

 

[Th232]

Generally cylindrical cells are Li-ions, while what you're referring to as rectangular cells are Li-Po. Different chemistry -> different discharge rate.

 

[BlahHead]

Nice! That clears many things up.

Now, why aren't 18650's made out of lithium polymer? Wouldn't that provide higher discharge rates?

 

[45/70]

I believe the higher current rate capability of LiPo cells has more to do with the surface area of the electrodes, than the actual chemistry. In fact, LiPo and LiCo cells are basically the same chemistry, lithium cobalt. The only difference is in the "polymer like" properties of the electrolyte. Also note that LiPo cells are "Li-Ion cells", as are LiMn, LiFePO4 etc. They all are part of the "lithium-ion" family.

What makes the Panasonic cells (Callie's, Redilast, AW 2900-3100mAh) a bit safer than conventional LiCo cells, is an added nickel, or "safety layer" on the electrode. These are still pretty much "LiCo" cells, only modified somewhat with this added layer, which improves cell stability.

Dave

 

[Battery Guy]

I believe the higher current rate capability of LiPo cells has more to do with the surface area of the electrodes, than the actual chemistry. In fact, LiPo and LiCo cells are basically the same chemistry, lithium cobalt. The only difference is in the "polymer like" properties of the electrolyte. Also note that LiPo cells are "Li-Ion cells", as are LiMn, LiFePO4 etc. They all are part of the "lithium-ion" family.

Dave's response is right on the mark. So-called "lithium polymer" cells (more accurately referred to as "pouch" cells) are simply lithium-ion cells with a polymer additive to act as an adhesive between the electrode layers. This is needed in pouch cells because they lack a rigid can to hold the electrodes together.

From a rate capability perspective, it is easier to make a high rate pouch cell than a high rate cylindrical cell. This has to do with the current collection within the electrode. All high rate pouch cells use a stacked electrode plate design, which means that the maximum distance current travels in the cell is the distance between the current collection tab and the far corner of the cell. For cylindrical cells, current must travel the entire length of the electrode, which can be close to a meter in high power 18650 and 26650 cell. If you only had one current collect tab in this design, you would have too much voltage drop along the electrode. To get around this, high power cylindrical cells use multiple tabs, but this is complicated from a manufacturing perspective, and can result in reliability problems. By far the easiest and cheapest way to make a high power lithium-ion cell is to use the stacked plate, pouch configuration, and this is why "LiPo" tends to by synonymous with "high power".

Cheers,
BG

 

[xul]

For a given surface area of the battery container, a cylinder contains more volume than a rectangular solid, but this advantage can be cancelled out by technology. A spherical battery would be best in this regard but who would buy it?
Whatever sells decides the issue.