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Materials Characterization for Lithium Ion Battery Technology

Table of Contents

Introduction
Significance of Materials Characterization
Pristine Materials
Morphology and Uniformity
Surface Chemistry and Composition
Cycled Cell Characterization
About Evans Analytical Group
Introduction

Dedicated characterization tools can be used to gain insight into material behavior during battery research and development, production and operation. Matching the measurement technique with both the data required and the dimensions of the feature or layer of interest is a key step in achieving desired goals. Evans Analytical Group provides analytical methods and expertise to better understand the materials utilized in the anodes, cathodes, and electrolytes of existing lithium ion batteries and next generation of batteries for automotive applications.
Significance of Materials Characterization

Since batteries have found use in new markets, gaining insight into material problems is imperative. The commercialization of new battery technologies demands a suite of mechanical, electrical and environmental stress testing. Moreover, gaining insight into the materials employed and their behavior throughout the life cycle of the battery system can provide useful data about the processes taking place within the battery systems and their potential failure mechanisms.

Modern batteries consist of a variety of organic, inorganic and metallic components, with sizes in the range of between Å and cm. It is essential to use the appropriate tools to analyze these components. Evans Analytical Group has an extensive range of analytical instruments to tackle materials problems related to all stages of battery development.
Pristine Materials

It is important to control the consistency of pristine electrode materials to ensure that lithium ion batteries have desirable energy, power and lifespan. Instrumental gas analysis (IGA) is used to study elements such as S, O, N, C, and H, while inductively coupled plasma-optical emission spectrometry (ICP-OES) enables measurement of the composition of major elements. Glow discharge mass spectrometry (GDMS) is suitable to monitor impurity levels for a complete mass range elemental analysis with lower detection limits or ppm. X-ray diffraction (XRD) is an ideal technique to measure the phase identification and crystallinity of pristine cathode materials. The analysis of a pristine NCA cathode is shown in Table 1.

Table 1. Analysis of a pristine NCA cathode
Elements Measured Elemental Composition wt%
Li 7.6
O 30.9
C 0.31
N 0.22
S <0.001
Al 1.2
Mn <0.005
Co 8.9
Ni 46.6

Tab and electrode alignment is typically monitored using a real-time X-ray (RTX). An RTX image of 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 understand the parameter that have an impact on the cycle life of a battery. Microscopy approaches such as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are ideal techniques to analyze particle size, morphology and particle agglomeration and shortcomings during materials acceptance, process development and actual production. An SEM image of a pristine LiFePO4 cathode is shown in Figure 2, demonstrating a good particle size distribution and 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 size and 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 the thin carbon coating which improve the electrical conductivity, with thickness measurements. The crystallinity of the particle is also illustrated in Figure 4, enabling to verify thickness consistency by surveying several particles.

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 spectroscopy (EELS) enables obtaining compositional information from thin layers or very small areas, as shown in Figure 5.

Figure 5. An EELS linescan across the outer edge of the particle confirms the coating is composed of carbon only.
Surface Chemistry and Composition

The electrolyte utilized in lithium ion batteries forms a film called as the solid electrolyte interphase (SEI) on the surfaces of electrodes during battery cycling. Methods that provide elemental details and inorganic and molecular data are suitable to investigate the film growth process and decomposition chemistry.

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