materials characterization for lithium ion battery technology. materials

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6/2/13 Materials Characterization for Lithium Ion Battery Technology

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Tab and elect rode alignment is typically monitored using a real-time X-ray (RTX). An RTX imageof a lithium ion 18650 cell is shown in Figure 1.

Figure 1. RTX image of a lithium ion 18650 cell

Morphology and Uniformity

Gaining insight into the materials employed in cell production is important to better understandthe parameter that have an impact on the cycle life of a battery. Microscopy approaches suchas transmission elect ron microscopy (TEM) and scanning electron microscopy (SEM) are idealtechniques to analyze particle size, morphology and particle agglomeration and shortcomingsduring materials acceptance, process development and actual production. An SEM image of apristine LiFePO4 cathode is shown in Figure 2, demonstrating a good particle size distributionand the lack of excessive agglomeration of particles.

Figure 2. SEM image of an LiFePO4 cathode

An SEM image of a pristine graphitic carbon anode that has significant difference in particle sizeand a non-uniform distribution of carbon black particles is shown in Figure 3.

Figure 3. SEM image of a graphitic anode

A TEM image of a pristine LiFeP04 cathode particle is depicted in Figure 4, clearly showing thethin carbon coating which improve the electrical conductivity, with thickness measurements.The crystallinity of the particle is also illustrated in Figure 4, enabling to verify thicknessconsistency by surveying several particles.

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Figure 4. TEM image of an LiFePO4 cathode

The combination of superior resolution of TEM and energy dispersive X-ray spectroscopy (EDS)or electron energy loss spect roscopy (EELS) enables obtaining compositional information fromthin layers or very small areas, as shown in Figure 5.

Figure 5.An EELS linescan ac ross the outer edge of the partic le confirms t he coat ing is

composed of c arbon only.

Surface Chemistry and CompositionThe electrolyte utilized in lithium ion batteries forms a film called as the solid electrolyteinterphase (SEI) on the surfaces of electrodes during battery cycling. Methods that provideelemental details and inorganic and molecular data are suitable to investigate the film growthprocess and decomposition chemistry.

Auger Electron Spectroscopy (AES) is an ideal method for analyzing the surface of individualcathode or anode particles. The lateral distribution of elements of interest can be shown byelemental mapping with AES, as shown in Figure 6. With incremental sputtering utilizing anargon ion beam, it is possible to obtain elemental depth profiles through layers, such as the SEI.

Figure 6. SEM and AES images of battery electrode surfaces

Cycled Cell Characterization

The formation of SEI layer on electrode particles during cell cycling can be investigated usingthe superior lateral resolution of TEM. It is possible to obtain data on possible SEI growth

mechanisms by detec ting the composition of SEI layers using TEM-EELS or TEM-EDS. Gaininginsight into the chemical state of elements on the surfaces of electrodes is important tounderstand the electrochemical processes taking place at different components of the battery,such as current collectors, electrolyte components, and electrodes. A TEM image of an SEIlayer created on a cycled LiFeP04 cathode particle is shown in Figure 7.


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Figure 7. TEM image of an SEI layer on a cathode particle

Phosphorus chemistry on a graphitic anode prior to and after cell cycling can be investigatedusing X-ray photoelectron spectroscopy (XPS), as shown in Figure 8, where the cycled celldemonstrates an increased phosphate bonding in relation to LiPF6.

Figure 8.XPS analysis of graphitic anode

An analysis of a particle on the surface of a graphitic anode film after removing from a cycledcell is shown in Figure 9. P is present from the electrolyte consisting of LiPF6.

Figure 9. Graphite anode part icle analysis

A Time of Flight Secondary ion Mass Spectrometry (TOF-SIMS) spectrum that was obtainedfrom a LiCoO2 cathode after its removal from a cycled cell is shown in Figure 10, enabling toidentify a number of molecular species of interest. Applicat ions and appropriate techniques forlithium ion batteries are listed in Figure 11.


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Figure 10. TOF-SIMS spectrum from a LiCoO2 cathode

Figure 11. Summary of analysis methods for Li ion cells

About Evans Analytical Group

Evans Analytical Group (EAG) has been a global leader in surface analysis and materials

characterization for over 30 years, delivering innovative analytical services and solutions withuncompromising sc ientific integrity. Since our incept ion in 1978 as Charles Evans & Assoc iates,our guiding principal has been innovative problem solving underscored by a commitment toexcellence.

This information has been sourced, reviewed and adapted from materials provided by EvansAnalytical Group.

For more information on this source, please visit Evans Analytical Group.

Date Added: May 21, 2013 | Updated: May 21, 2013

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