Last Month's Most Accessed Feature: Impure state: Separating iron oxidation states in lithium-ion batteries

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  • Published: Mar 1, 2018
  • Categories: Electrophoresis
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Magnetic impurities

Impure state: Separating iron oxidation states in lithium-ion batteries

Lithium-ion batteries increasingly power our modern world, from phones to laptops to cars, but they are not without their problems, such as their propensity for losing capacity over time with repeated charging and discharging. Scientists are looking at various way to slow or even halt this reduction in capacity, but one of the simplest is to prevent the formation of impurities in lithium-based cathodes.

In particular, magnetic impurities in cathodes made from lithium iron phosphate, a commonly used material in lithium-ion batteries, are known to degrade both the capacity and charging rates. There are two main causes of these magnetic impurities, both of which relate to iron (Fe) at the incorrect oxidation state.


Fe(II) and Fe(III)

The main precursor in the production of lithium iron phosphate is iron phosphate, and this should comprise only Fe(III). Any traces Fe(II) in the iron phosphate causes the formation of magnetic impurities. In contrast, the final lithium iron phosphate should only comprise Fe(II), as any traces of Fe(III) are only found in impurities. So to ensure the production of high-quality lithium-ion batteries, manufacturers need to ensure the iron phosphate and lithium iron phosphate don’t contain any iron at the wrong oxidation state.

Several analytical techniques can help with this, such as titration and atomic absorption spectrometry, but they tend to be quite complex and time-consuming. Now, though, chemists from Peking University in China, led by Zhiwei Zhu, have shown that capillary electrophoresis (CE) can offer a quick and simple alternative for determining whether either of these compounds contains any iron at the wrong oxidation state.

This first required the chemists to overcome several stumbling blocks. One is that Fe(II) and Fe(III) are difficult to separate by CE, as they obviously only differ slightly in charge. Another is that one of them will only be present at trace concentrations. So Zhu and his team decided to utilize compounds that prefer to form complexes with one of the oxidation states over the other, in order to enhance the separation. As a further complication, even though always separating Fe(II) from Fe(III), they found they had to use a different set of complexing agents for each compound.


Complexing agents

For iron phosphate, where they’re trying to separate trace amounts of Fe(II) from much larger amounts of Fe(III), they were able to use a single complexing agent, phenanthroline (phen), which binds to Fe(II) much more readily than Fe(III). But they couldn’t just use phen with lithium iron phosphate, because now they’re trying to separate trace amounts of Fe(III) from much larger amounts of Fe(II). Furthermore, although a complexing agent such as ethylenediaminetetraacetic acid (EDTA) will bind bind to Fe(III) more readily than Fe(II), the difference isn’t enough for a clear separation when Fe(II) is present at a much greater concentration than Fe(III). So for lithium iron phosphate, they had to use both phen and EDTA, with a higher concentration of phen than EDTA to soak up all the Fe(II) and prevent it forming complexes with the EDTA.

To ensure this CE method would be sensitive enough to separate trace amounts of each oxidation state, Zhu and his team also employed field-enhanced sample injection (FESI) to concentrate the iron in the two compounds. This involved injecting a plug of water into the background electrolyte (BE) before the sample, with the difference in conductivity between the water and the BE causing the analytes to congregate at the junction between them.

They tested this CE method with the relevant complexing agents on samples of iron phosphate and lithium iron phosphate spiked with the incorrect oxidation state. This revealed that the method was able to detect Fe(II) in iron phosphate at concentrations down to 2.5nM and Fe(III) in lithium iron phosphate at concentrations down to 0.1μM. By ensuring that iron at the wrong oxidation state isn’t present above these concentrations, this novel CE method could help to produce lithium-ion batteries able to power the world for a bit longer between charges.

Related Links

Talanta, 2018, 179, 822–827: "Quality monitoring methods of initial and terminal manufacture of LiFePO4 based lithium ion batteries by capillary electrophoresis"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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