What is a Lithium-Ion, Polymer Battery?

I’ve posted quite a few times now, about Lithium-Ion Batteries, without ever answering the question of how Lithium-Ion Polymer Batteries differ. And I think that I should write a posting about that subject, which this time around, will contain no links to other articles.

My previous postings assumed that standard, lithium-ion batteries are being examined, which were not of the polymer variety, but those postings did mention plenty of possible electrode materials. Well, batteries are not solely defined by their electrode materials, but are sometimes defined as much, by the choice of electrolyte which Engineers put their trust in.

In a standard, lithium-ion battery, most of the time, the electrolyte needs to be kept under pressure, in order to be liquid. In fact, this means that the standard battery variety also has a pressurized container around it, from which its electrodes are insulated electrically, but that adds bulk. The electrolytes in question are not Brønsted acids, as was once the case with lead – lead oxide batteries, but are very flammable.

In a polymer-variety battery, the electrolyte is the polymer, but the same assortment of electrode materials is still available. The favorite composition for the positive electrode seems to be lithium-iron-phosphate. Because the electrolyte is the polymer, it counts as a solid, which does not need to be kept under pressure, and through which lithium ions effuse, even though this solid is also flexible. As soon as this option presents itself, it creates advantages on two fronts:

  1. Energy-to-mass ratio,
  2. Safety.

(Updated 10/25/2018, 13h25 : )

(As of 10/21/2018 : )


 

1)  One benchmark by which progress in advanced batteries is measured, is whether they can achieve an equal or greater energy-to-mass ratio, to that of gasoline burning in air. And the answer to that question is that the electrode materials often can, but that when the mass of the pressurized container is taken into account, this ratio is no longer achieved. Since the polymer-variety batteries do not need such a container, they can be made lightweight enough, to achieve this ratio. And this is especially true, if the material of the positive electrode, is lithium-cobalt-oxide, which is also known to hold the highest charge densities. But, this would no longer be the safest electrode-material to use.

2)  When a standard-variety battery does in fact fail, not only can the electrolyte catch fire, but inevitably, the pressurized container also breaches, which means that burning liquid shoots out. A polymer-variety battery can fail in the same way. But at least, when it does, it will not act as a flamethrower. A polymer-variety battery could become red-hot and then charred, can emit smoke, but still won’t be as dangerous as the standard-variety battery was.


 

Now I suppose that the question could be asked separately, of whether the standard lithium-ion batteries are at increased risk of erupting on airplanes, because the cabin-pressure decreases. And my answer would be, ‘Not, if the designers of the pressurized container knew what they were doing.’ There is a slight increase in relative pressure, just because the exterior pressure decreases, but this should not be enough to breach the container. Breaches tend to happen for thermal reasons – i.e., because batteries become overheated.

But the people who manage airline flights will flag any type of item that could catch fire, simply because a fire in the passenger cabin of an airplane in-flight, is especially dangerous. Appliances that caught fire on airplanes, typically also caught fire on the ground…


 

One mistake which some people may make, is to assume that all Lithium-Ion Polymer Batteries, are flexible batteries. This is an easy mistake to make, because most polymers are flexible. But of course, as long as the only part of the battery which is a polymer is the electrolyte, what follows is that the composition of either electrode could still be rigid.

The concept of flexible batteries also exists, but is separate from the concept of Li-Ion batteries of either type.

(Update 10/22/2018, 21h55 : )

What has become deprecated, is the use of a metallic lithium negative electrode. It’s been replaced in most cases by graphite, which is able to store lithium ions. This change has cost each battery about 0.2V of resulting voltage. Or, it has eased the voltage with which each battery must be charged, reducing it, by 0.2V .

But, I’ve observed that in the case of gray-market batteries, that are being sold as drop-in replacements for internal tablet and laptop batteries, often, the manufacturer will actually use metallic lithium as the negative electrode, presumably because doing so is cheaper to manufacture than batteries using lithium-graphite. This often means that the gray-market replacement batteries end up with an over-voltage of 0.2 V /Cell, i.e., end up with cell-voltages of 3.8V rather than 3.6V …

But graphite is not really flexible either.

Further, I’ve read that a liquid, pressurized electrolyte which would often get used, is a mixture of phosphorus pentafluoride and lithium hexafluorophosphate. This would be a liquid under pressure. The composition of the polymeric electrolyte is often such, that a passive polymer such as poly-(vinyldiene fluoride), or polyacrylonitrile, or poly-(ethylene oxide)…, is actually imbued with some quantity of the hexafluorophosphate -based salt, which forms a solid-solid solution. I.e., it does not appear to be due to special chemical properties of the actual polymer, that it can function. It just needs to be a polymer in which such a salt can dissolve, and which will neither react with this salt, nor with either electrode.


 

I suppose that yet another question which could arise, would challenge how inert PF5 really is. If it was to act on metallic lithium as a Lewis acid, this would suggest that a fluoride ion could separate, leaving a positive site on the phosphorus atom, which in turn could either bond to or react with lithium. I’m assuming that this doesn’t happen, as it should not.

I can imagine that the remaining 4 fluoride ions could arrange themselves in a tetrahedral configuration around the P+5 ion, and still be large enough to prevent access to it, by other atoms, molecules and electron-pairs.

What will happen though, if PF5 ever comes into contact with water, is that hydrogen fluoride will form, which is a Brønsted acid, along with phosphoric acid forming.

Further, an antimony homologue to PF5 exists, which would be written SbF5, and which should actually be liquid under ambient conditions. But maybe the cost of antimony and its toxicity, along with a smaller supply of it, convince manufacturers to stick with using PF5. Additionally, the fact that Sb+5 ions are larger than P+5 ions, suggests that maybe, SbF5 would react with lithium, as a Lewis acid, which again, nothing in the electrolyte should do.


 

(Update 10/25/2018, 13h25 : )

I am analyzing the phosphorus compounds according to the concept of coordinate covalence. In practice, these compounds exist part-way between being ionic and being covalent.

A substance distantly related to PF5 exists, that is known as phosphorus pentachloride ( PCl5 ) , and it undergoes a decomposition reaction, in which the phosphorus transitions from being oxidized as P+5 to only being oxidized as P+3 . In return, this compound will release two of its initial Cl-1 ions, in the form of a Cl2 molecule. And one reason this happens may be, the fact that the P+5 is just too small an ion, to hold on to all 5 chloride ions, that are much larger.

Obviously, once a cation with a charge of (+3) has formed, surrounded by 3 anions in the planar geometry, the central cation will be exposed, and the compound will not be compatible for use in this type of battery, with metallic lithium.

On that basis, a reaction pathway could also exist, by which the phosphorus in PF5 can go into its (+3) oxidation state, thereby absorbing two electrons from metallic lithium, and losing two fluoride ions. Yet, the potential does not exist for F2 molecules to form, and the LiF salt which would form, would be identical to the same salt, that has been absorbed into LiPF6. Therefore, the presence of an excess of both PF5 and LiPF6 could manage this decomposition reaction.

Further, PF3 is not a planar molecule, due to the unshared electron-pair on the P+3 . But, when PF3 comes into contact with the positive electrode, the phosphorus could just become oxidized to P+5 again. What this would suggest, is that the level of PF3 in the electrolyte, may contribute to the self-discharge rate of the battery.

Dirk

 

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