An Observation about Modern Lithium-Ion Batteries

I have visited this subject before, but feel that I should post about it again.

The way Lithium-Ion batteries first became popular, they either had metallic lithium, or graphite as their negative electrode, and lithium-cobalt-oxide as their positive electrode. This stored much energy, but also presented an initial cause for alarm, especially since some of the then-new batteries were prone to catch fire, when over-charged. In response, there existed a trend followed by some companies and manufacturers, to switch to lithium-manganese-oxide, either in the layered or the spinel form, as the next-best positive electrode. ( :2 )

It would seem that the lithium-cobalt-oxide batteries produce 4.2V when fully charged, while the lithium-manganese-oxide batteries only produce 3.7V when fully charged ( :1 ) , and the latter battery-type was deemed ‘safer’.

Additionally, there exists a battery-type which has lithium-iron-phosphate, which is even safer than the 3.7V batteries, and which only produces 3.6V when fully charged. This third family of batteries is used in Segways and some electric cars, where it would be exceptionally unfortunate if the batteries could explode, simply due to a traffic accident – a hypothetical collision.

All the voltages which I’m citing here are relative to a lithium-graphite negative electrode.

What seems to have happened – and I don’t have proof – would be called a ‘trend reversal’. Some manufacturers have switched back to using the lithium-cobalt-oxide batteries, simply because those store more energy.

foxy_153982435955

Why do consumers need to know this? So that they don’t place 3.7V batteries – which are labeled identically to the other type – into 4.2V chargers, and leave them there. That’s all.

I suppose that a valid question which some readers might have would be, ‘What has become of the safety / over-charging issue?’ And my answer would be that most of today’s charging circuits have become ‘smarter’, and less prone actually to over-charging the batteries. The best example of this is the smart-phone. However, if some people buy separate batteries for ‘Vapers’, then those devices have a reputation of ‘no charging intelligence’, i.e., of sometimes over-charging the battery.

The typical behavior of a dumb charger is, to ‘Apply a constant voltage of 4.2V, and when the current which the battery draws falls below a certain amount of current, give an indication that the battery is fully charged. But keep applying 4.2V, even after the LED has changed color.’ The lithium-manganese-oxide batteries will also tolerate such charging voltages for brief periods of time. And the lithium-cobalt-oxide batteries will realize their maximum held charge that way.

The thing not to do, is to keep whichever batteries in their dumb charger for long periods of time, after the LED indicates they are charged.

I also want to add, that this posting is meant to voice an issue, with the low-budget lithium-ion batteries, in the modern era. I understand that high-budget, big-ticket items exist, such as…

(Updated 10/21/2018, 22h55 … )

(As of 10/17/2018 : )

According to the latest WiKiPedia article, “lithium-nickel-manganese-cobalt-oxide” has been proposed since 2008. In reality, I just see this as a compromise between the manganese-oxide and the cobalt-oxide compositions. ( :4 )

How would this help a small-time consumer understand the labeling of individually bought, online-bought 18650 batteries, not to put in his 4.2V charger?

(Update 10/18/2018. 13h50 : )

I suppose that this observation raises the potential question, of what happens if, a smart-phone contains ‘safer’ lithium-manganese-oxide electrodes, but if its charging circuit raises the battery-voltage all the way to 4.2V. And after some consideration I’ve concluded that very little happens, except for the possibility, that as soon as charging has stopped, the battery voltage will seem to drop from that 4.2V target very quickly, even though very little charge or current has been drawn from the battery. This will also result in the charge-indicator dipping below 90% much faster, than it will later dip from 70%-60%…

The real problem I see happening, would be if the smart-phone contained lithium-cobalt-oxide batteries, let’s say ‘just because the manufacturer wanted to increase the battery capacity’, and if the charging circuit did the same thing. In that case, to leave the lithium-cobalt-oxide batteries continuously connected to 4.2V may eventually lead to the famous problem, ‘of their catching fire’.

(Update 10/18/2018, 21h45 : )

1: )

I suppose that for people who are not familiar with concepts in Electronics, I should add that voltage and current are two separate parameters, and charge is more-closely related to current, than it is to voltage.

When people loosely say, that a battery operates ‘at a specific voltage’, such as ‘at 3.7V’, what this means can confuse some people. What this really means is, that at voltages close to 3.7V, the charge of the battery changes the fastest, and that due to voltage-changes near such a voltage, the amount of current that flows will be greatest. The result should be a sideways s-shaped curve…

hypbattliion_1

Thus, even if a battery that functions most-efficiently at 3.7V is over-charged modestly, all it really means, is that due to further voltage-increases, its charge-level changes progressively less. It’s only due to more-considerable over-charging, that a stable battery will fail.

However, it would be hypothetically possible to have a battery-type the charge-level of which just keeps increasing with voltage. While some people would see this as an unstable device, other people might just see it as an opportunity, to store more and more energy.

 

(Update 10/19/2018, 6h10 : )

I suppose there’s another observation to add. If the composition of the positive electrode, when discharged, Is LiCoO2, then it follows that this electrode only has a limited number of Lithium Ions to shed. This implies that:

  • The total amount of charge that such a battery can store, regardless of the hypothetical curve above, has a definite limit,
  • According to conventional wisdom, the material is only allowed to shed 1/2 its lithium ions, before its composition changes irreversibly,
  • CoO2 is not only oxidized, but is itself a chemically active oxidizer, which will donate oxygen to the electrolyte, and thereby start the reaction, where the battery catches fire,
  • If a derived electrode material is to be found that can actually shed a higher fraction of its available lithium ions safely, then this would need to be a material which without, also fails to become an active oxidizer.

 

(Update 10/20/2018, 22h40 : )

2: )

One fact which had originally caught my attention, was that The lithium-manganese-oxide batteries come in two varieties: Spinel and Layered, in the order of discovery. Regardless of what the voltage-curves are, according to the article just linked to, a major difference between these two crystal structures is that, the spinel variety has as formula LiMn2O4, while the layered variety has as formula Li2MnO3. What this also suggests, is that some advanced variety can potentially shed twice as many lithium ions, more or less, than the spinel variety. ( :3 )

This could be of interest to the manufacturers of high-budget devices such as smart-phones, which could benefit by storing twice the charge of earlier smart-phones.

But if a seller of separate batteries had as their aim, to mislabel the batteries they sell, such as at the low-budget end of the market, then this can give them plausible deniability. The reason for this would be, that achieving twice the charging capacity of a simpler type of battery, could now have two explanations, at least as long as this detail is not mentioned in the labeling of the battery:

  1. The battery sold could be an advanced, lithium-manganese-oxide battery, or,
  2. The battery sold could be a lithium-cobalt-oxide battery.

Both would seem to hold approximately twice the charge, that the spinel, lithium-manganese-oxide battery was able to hold. The consumer would not know, what he’s getting.

(Update 10/20/2018, 22h40 : )

3: )

I suppose that a legitimate question to ask next might be, ‘How can one estimate how much energy a battery with a given Chemistry stores?’

And then one observation to note would be, that most of the battery’s voltage stems from the (negative) electrode potential of the lithium, where all the electrode voltages are given relative to hydrogen, for which reason very little of the battery-voltage is in fact decided by the positive electrode. There could be some differentiation, but it would be minute.

When a reference article states a voltage of 4.5V, relative to metallic lithium, this is just equivalent to 4.3V, relative to lithium-graphite.

And so the next factor to estimate, would be how much volume one mole of the material, of the positive electrode, occupies. And for that purpose, I’d treat oxygen atoms as though they were equivalent to the transition-metal atoms – even though Chemically, they’re very different. In a regular crystal, either type of atom will occupy approximately the same amount of space. And so I’d add the number of transition-metal atoms, to the number of oxygen atoms, and the result I’d get would be:

  • LiMn2O4 -> 6 units of volume.
  • Li2MnO3 -> 4 units of volume.
  • LiCoO2 -> 3 units of volume.

Even though this estimation is only very coarse, it puts the energy densities in the correct order.

I would add, If Pure Li2MnO3 was to undergo a reaction in which it loses its second lithium ion, it would do so at a higher electrode voltage, from the voltage, at which it lost its first. An ability to do so would also imply the existence of manganese-trioxide, which does not exist as a known, stable substance.

But then, in the case of the advanced, lithium-manganese-oxide composition:

  • LiMn2O4 · Li2MnO3 -> 10 units of volume, yielding up to 3 available lithium ions (?) , as opposed to 6 units of volume, yielding up to 1 available lithium ion.

 

(Update 10/21/2018, 14h30 : )

4: )

If the electrode material is to be taken seriously, which I named lithium-nickel-manganese-cobalt-oxide, there are two observations to take into account:

  1. Every time the resulting crystal possesses a manganese atom, it basically replaces a cobalt atom. Every time it possesses a nickel atom, that atom replaces either a manganese or a cobalt atom.
  2. The actual nickel atoms do not contribute directly, to the ability of the crystal to offer lithium ions to the battery. They only improve the stability of the crystal, and maybe enable for the first time, cross-linking between manganese and cobalt atoms.

I seem to recall that a lithium-nickel-cobalt-oxide battery existed, earlier than the actual lithium-cobalt-oxide battery. The introduction of the latter type actually did away with the presence of nickel atoms, and increased the total amount of lithium ions the electrode could offer. Doing so may also have reduced the stability of the batteries in question.

 

Dirk

 

Print Friendly, PDF & Email

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

This site uses Akismet to reduce spam. Learn how your comment data is processed.