In situ NMR observation of the lithium extraction/insertion from LiCoO2 cathode

Rechargeable lithium-ion batteries (LIBs) are currently accepted to be one of the most suitable energy storage resources in portable electronic devices because of their high gravimetric and volumetric energy density. To understand the behavior of Li+ ions on electrochemical lithium extraction/insertion process, we performed in situ7Li nuclear magnetic resonance (NMR) measurements for LiCoO2 cathode in a plastic cell battery, and the spectral evolutions of the 7Li NMR signal of LixCoO2 (0 ≤ x ≤ 1) were well investigated. Very narrow solid solution region of LixCoO2 (∼0.99 ≤ x < 1) was explicitly defined from the large intensity reduction of LiCoO2 signal at ∼0 ppm, which is related to the localized nature of the electronic spin of paramagnetic Co4+ ion formed at the very early delithiation stage. With further decreasing the signal intensity of LiCoO2, a Knight-shifted signal corresponding to an electrically conductive LixCoO2 phase emerged at x = 0.97, which then monotonously decreased in intensity for x < 0.75 in accordance with the electrochemical lithium de-intercalation from LixCoO2. These observations acquired in situ fully confirm the earlier studies obtained in ex situ measurements, although the present study offers more quantitative information. Moreover, it was shown that the peak position of the NMR shift for LixCoO2 moved as a function of lithium content, which behavior is analogous to the change in its c lattice parameter. Also, the growth and consumption of dendritic/mossy metallic lithium on the counter electrode was clearly observed during the charge/discharge cycles.

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