Material Degradation
Cracking and Li inventory loss
Charge photometry can capture how the morphology of individual particles changes during cycling. By identifying fragmentation and spatially resolving local variation in Li-ion concentrations within individual particles, it is possible to track degradation mechanisms.
Extending battery lifetime is a key economic and environmental goal. However, battery capacity and rate performance degrades over long-term battery operation. The ability to understand and monitor the extent of material degradation within an electrode is crucial to solving these issues.
Charge photometry enables the causes of degradation and capacity loss to be investigated by identifying the development of Li-ion inhomogeneities and particle fragmentation, in real time during battery cycling.
Mechanical degradation of the high-rate anode material, NWO (Nb14W3O44), was investigated over successive fast (dis)charge cycles. Changes to particle morphology during cycling, such as particle fragmentation, can be identified and quantified by charge photometry. Figure 1 is an optical video exemplifying this, in which a rod-shaped active particle of NWO is seen to crack during delithiation.
Figure 2 (left) shows how the optical detection of cracks enabled quantification of the number of fragmented and intact particles within a fixed population over repeated cycling. The number of observed cracks initially increased with successive 5C cycles, before starting to plateau after the 5th cycle. Increasing the delithiation rate to 20C then caused an additional sharp increase in the number of cracks, indicating that faster delithiation induces more particle fragmentation in NWO. Morphological parameters can be derived from the optical images, including particle length and width. Figure 2 (right) reveals that particles of longer length were more prone to cracking.
Figure 1. Optical video showing a NWO particle cracking during delithiation. Three NWO particles are shown. During rapid delithiation at 5C, the formation of a crack is observed in one particle. The orange arrow indicates the location of the crack.
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Figure 2. Monitoring NWO particle fragmentation over 15 (de)lithiation cycles. (left) The number of observed cracks increased over successive cycles, showing a sharp increase when the delithiation rate was increased. (right) Active particles of longer length were more likely to crack.
Figure 3 depicts some example images of fragmented NWO particles in a delithiated electrode, in which some fragments display significantly different optical intensities from the rest. Since optical intensity changes as a function of Li-concentration in the material, this shows that the fragments have differing Li-content. The comparatively bright fragments contain more Li-ions than the majority of the electrode: they have become electrically disconnected and now contain trapped Li. This would contribute to a loss of Li-inventory and a loss of cell capacity.
Figure 3. Identification of trapped Li in NWO particle fragments. In these optical images of NWO particles, a brighter intensity indicates a higher local Li-concentration. The brighter fragments contain trapped Li, indicating these fragments had become disconnected during cycling. Scale bar – 5 μm.
For more detail on how charge photometry can be used to investigate degradation and Li-inventory loss