- Joined
- Jun 12, 2010
- Messages
- 892
- Points
- 0
So, many of you remember me from the Lithium Ion explosion thread. Well, I'm back with more battery news. And surprisingly, this time it's good news too.
Many of you here would be familiar with your lithium-ion cells, mainly in the form of 18650s. These blighters run at 3.7V, powering your FlexDrives and MicroBoosts day in, day out. But every now and again, they go bang. And when they do go bang, they usually end up taking quite a fair bit of equipment and surroundings out with them, and sometimes even causing injury.
Now, what if I told you there were lithium cells that couldn't explode, even if abused? Or lithium cells that have a 25% charge time for a given capacity than a lithium-ion cell? Or lithium cells that are immune to thermal runaway, even when shorted? Or that all this is done by an unprotected cell?
You'd probably think I was lying, or I had too much to smoke again. Either that, or I'm being paid by some cheap company to advertise their goods. But I'm not. Behold, the lithium iron phosphate LiIon cell.
Exactly the same form factor as standard 18650s, they can also be found in other common form factors such as CR123A and 10440/14500.
And at first sight, you wouldn't be able to tell it apart from a standard Li-Ion 18650.
The difference between standard Li-Ion cells and LiFePO4 cells is the composition of the cathode. While normal lithium ion cells use a lithium cobalt oxide as a cathode, lithium iron phosphate cells use lithium iron phosphate as a cathode, hence their name.
It's change in cathode that is responsible for the thermal and chemical stability that renders the cell virtually inert, even when abused and misused.
The reason behind the explosions commonly seen in Li-Ion cells is caused by the release of oxygen from the decomposition of lithium cobalt oxide. This outgassing can occur under high temperatures, overcharging, or reverse polarity charging. Eventually, an explosion (resulting from an overpressure condition) and fire will result.
Lithium cobalt oxide breaks down at temperatures above 200C. Although it seems high, in a thermal runaway condition, it is possible that this will go unnoticed until it is too late. Also, the decompositon is an exothermic reaction (giving out heat), which will eventually make the condition worse. Lithium iron phosphate, however, requires temperatures exceeding 800C to break down. Even when decomposition does occur, lithium iron phosphate isn't affected as badly as lithium cobalt oxide is, and the fire (if any) will be significantly eaiser to contain. The reason for this? The iron and phosphorus atoms hold onto the oxygen atoms much tighter than the cobalt can, hence reducing reactivity.
Another major advantage is shelf life- lithium iron phosphate has a significantly longer shelf life than Li-ion cells, nor are they affected by storage on a full charge. The power density difference between lithium iron phosphate and lithium ion is quickly offset after a year or two, through usage or storage.
Unfortunately, these benefits don't come free. With Lithium iron phosphate cells, you're trading off two things- power density, and voltage.
You'd have noticed that on the cell, it's marked at 3.2V. It's not a typo, LiFePO4 cells only run at 3V nominal voltage. They are charged at 3.6V, and 2.8V is considered empty. Although not all portables will be able to take a sudden drop in voltage, anything running from a FlexDrive or MicroBoost will be unaffected. This means the cell is capable of delivering less power than standard Li-Ion cells.
The second disadvantage is the low energy density. You'd have noticed that most LiFePO4 cells have a little more than half the capacity of an equivalent-sized Li-ion cell. This translates into shorter runtimes between charges, and less current deliverable by a cell. At this stage, lithium-ion is still the superior option for builds running >1W diodes.
There's the tradeoff- power density versus safety and fast charge times. It's up to you- if you want power, or peace of mind.
Many of you here would be familiar with your lithium-ion cells, mainly in the form of 18650s. These blighters run at 3.7V, powering your FlexDrives and MicroBoosts day in, day out. But every now and again, they go bang. And when they do go bang, they usually end up taking quite a fair bit of equipment and surroundings out with them, and sometimes even causing injury.
Now, what if I told you there were lithium cells that couldn't explode, even if abused? Or lithium cells that have a 25% charge time for a given capacity than a lithium-ion cell? Or lithium cells that are immune to thermal runaway, even when shorted? Or that all this is done by an unprotected cell?
You'd probably think I was lying, or I had too much to smoke again. Either that, or I'm being paid by some cheap company to advertise their goods. But I'm not. Behold, the lithium iron phosphate LiIon cell.
Exactly the same form factor as standard 18650s, they can also be found in other common form factors such as CR123A and 10440/14500.
And at first sight, you wouldn't be able to tell it apart from a standard Li-Ion 18650.
The difference between standard Li-Ion cells and LiFePO4 cells is the composition of the cathode. While normal lithium ion cells use a lithium cobalt oxide as a cathode, lithium iron phosphate cells use lithium iron phosphate as a cathode, hence their name.
It's change in cathode that is responsible for the thermal and chemical stability that renders the cell virtually inert, even when abused and misused.
The reason behind the explosions commonly seen in Li-Ion cells is caused by the release of oxygen from the decomposition of lithium cobalt oxide. This outgassing can occur under high temperatures, overcharging, or reverse polarity charging. Eventually, an explosion (resulting from an overpressure condition) and fire will result.
Lithium cobalt oxide breaks down at temperatures above 200C. Although it seems high, in a thermal runaway condition, it is possible that this will go unnoticed until it is too late. Also, the decompositon is an exothermic reaction (giving out heat), which will eventually make the condition worse. Lithium iron phosphate, however, requires temperatures exceeding 800C to break down. Even when decomposition does occur, lithium iron phosphate isn't affected as badly as lithium cobalt oxide is, and the fire (if any) will be significantly eaiser to contain. The reason for this? The iron and phosphorus atoms hold onto the oxygen atoms much tighter than the cobalt can, hence reducing reactivity.
Another major advantage is shelf life- lithium iron phosphate has a significantly longer shelf life than Li-ion cells, nor are they affected by storage on a full charge. The power density difference between lithium iron phosphate and lithium ion is quickly offset after a year or two, through usage or storage.
Unfortunately, these benefits don't come free. With Lithium iron phosphate cells, you're trading off two things- power density, and voltage.
You'd have noticed that on the cell, it's marked at 3.2V. It's not a typo, LiFePO4 cells only run at 3V nominal voltage. They are charged at 3.6V, and 2.8V is considered empty. Although not all portables will be able to take a sudden drop in voltage, anything running from a FlexDrive or MicroBoost will be unaffected. This means the cell is capable of delivering less power than standard Li-Ion cells.
The second disadvantage is the low energy density. You'd have noticed that most LiFePO4 cells have a little more than half the capacity of an equivalent-sized Li-ion cell. This translates into shorter runtimes between charges, and less current deliverable by a cell. At this stage, lithium-ion is still the superior option for builds running >1W diodes.
There's the tradeoff- power density versus safety and fast charge times. It's up to you- if you want power, or peace of mind.
Last edited:
