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The following reaction takes place in a lithium iron phosphate (LFP) battery:

$$\ce{LiFePO4 -> FePO4 + Li+ + e-}.$$

According to Wikipedia, $\ce{LiFePO4}$ is a grey, reddish grey, brown, or black solid, and $\ce{FePO4}$ is a yellow-brown solid. If you had a window into the LFP battery, could you see this color change occurring during use?

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    $\begingroup$ I am not sure, if there are (colored) salts of transition metals, that have the same color at different metal oxidation states. $\endgroup$
    – Poutnik
    Jul 22, 2021 at 6:16
  • $\begingroup$ Is LiFePO4 darker than FePO4 as described by Wikipedia solo because of the addition of carbon for conductivity in batteries? $\endgroup$
    – bryan dunn
    Jul 31, 2021 at 23:46

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Since there are carbon coatings on the $\ce{Li_xFePO4}$ particles and additional graphite/conductive carbon in the electrode, the electrode always appears to be black or dark grey, and color changes within the phosphate are hidden. Chen et al. write [1]:

Cathode materials, such as lithium iron phosphate with lower conductivity, are difficult to be observed because of the high light absorption of the carbon additives that are used to improve the conductivity of electrodes.

$\ce{LiFePO4}$ crystals have a blackish green color [2], cf. the color of the mineral triphylite which is described as bluish- to greenish-gray. I am not sure about the color of the $\ce{FePO4}$ in a battery since the usual $\ce{FePO4}$ described as yellow-brown solid might be influenced by its water content.

References

  1. Chen, B.; Zhang, H.; Xuan, J.; Offer, G. J.; Wang, H. Seeing Is Believing: In Situ/Operando Optical Microscopy for Probing Electrochemical Energy Systems. Advanced Materials Technologies 2020, 5 (10), 2000555. DOI: 10.1002/admt.202000555.
  2. Chen, D. P.; Maljuk, A.; Lin, C. T. Floating Zone Growth of Lithium Iron(II) Phosphate Single Crystals. Journal of Crystal Growth 2005, 284 (1), 86–90. DOI: 10.1016/j.jcrysgro.2005.06.024.
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