Monitoring the Composition of NCM cathodes in Li-ion Batteries

Summary

Optimising and controlling the cathode material is one of the main differences between battery solutions for different applications in the current generation of Li-ion battery technology. Traditional methods for determining the chemical composition of the commonly used cathode materials are complicated and require expertise. This application note demonstrates a simple and automated method which does not rely on operator expertise — using a Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) to determine the composition of powder particles used for Li-ion battery cathodes.

Introduction

Cathode materials are the most expensive component of Li-ion batteries and have the greatest impact on battery performance. The development of cathode materials is one of the most competitive areas in the Li-ion battery materials industry, attracting high levels of investment.

Common cathode materials include lithium cobalt oxide (LiCoO₂), lithium manganate (LiMn₂O₄), lithium iron phosphate (LiFePO₄) and ternary cathode materials. Ternary cathode materials are composite cathode materials, the two main classes of which are: lithium nickel cobalt aluminate (NCA) (LiNi_xCo_yAl_1-x-yO₂) and lithium nickel cobalt manganate (NCM) (LiNi_xCo_yMn_1-x-yO₂). These ternary cathodes have been found to give batteries excellent overall performance and great development potential.

The structure and electrochemical performance of ternary cathode materials varies with the proportion of transition metals within them. For example, the atomic ratios of Ni, Co and Mn in common NCM cathodes are 1:1:1, 5:2:3, 6:2:2, and 8:1:1. The proportion of transition metals needs to be controlled in the design and production process of lithium-ion batteries, to ensure that the desired performance is achieved at the right cost. The increased demand for NCM cathode material means that these metals are now produced more widely by many suppliers leading to variations in composition and quality.

The current, mainstream manufacturing process for ternary cathode materials is to synthesize a ternary precursor via a co-precipitation method and use high-temperature solid-phase sintering to produce the final product. The uniformity of the distribution of transition metals in each particle is affected by various factors at all stages of this process. The content of transition metals in the finished ternary materials has often been measured by a chemical analysis method where the electrode is first dissolved, and the ratio of elements determined afterwards. This measurement process is complicated, destructive and requires high levels of experience.

This application note demonstrates an alternative method to determine the ratio of transition metals in each ternary cathode particle. The analysis of the NCM particles is performed using an Oxford Instruments Ultim Max 170 Energy Dispersive Spectrometer (EDS) equipped with dedicated automated particle analysis software (AZtecFeature).

Sample Preparation

Good sample preparation is required for accurate quantitative analysis. In this case the NCM particles were first mixed with a carbon slurry and were then dried and polished with an ion beam. The polished NCM particles show flat cross-sections of their broadly spherical morphology, as shown in figure 1.

Fig 1. The morphology of NCM particles and carbon slurry after mixing and ion beam polishing is clearly visible in the secondary electron image (SE).

Results

Particles are identified using thresholds based on their brightness in the backscattered electron (BSE) image and an EDS compositional measurement is performed automatically for each particle. To improve quantitative accuracy, smaller particles were automatically excluded, with only particles with an equivalent circular diameter of 2.4 μm or greater included in the analysis. Morphological and compositional information were presented and could be interpreted in real time during the automated run.

AZtecFeature can automatically control the SEM stage and thereby cover large sample areas in order to obtain good statistical coverage. On this sample a total area of 63,072 μm² (292×216 μm) was covered and 756 particles were detected and analysed in less than 25 minutes. Figure 3 shows the particle distribution together with the results table which shows the statistical information for all particles including compositions and morphology as well as statistical averages for the whole dataset.

Fig2. AZtecFeature analysis of NCM particles in a single field of view

Fig 3. Detected particles shown in random colour together with results table providing compositional data of the NCM particles

In order to assist in understanding the relative proportions of the transition metals in each particle, the oxide content of the transition metals can be plotted on a ternary phase diagram, as shown in figure 4. In this plot it can be clearly seen that the composition distribution of the particles is clustered around the 523 ratio with a small proportion of particles being outliers.

Another way of locating outliers quickly would be to use chemical thresholds to define the expected material. In that way all particles which do not match the expected chemistry would automatically be identified and classified. AZtecFeature can easily relocate to any of these particles and perform more detailed secondary analysis manually or automatically if required.

Fig 4. Figure 4 - Ternary phase diagram of transition metal composition in NCM particles

Conclusion

An Oxford Instruments EDS system with the automatic particle analysis software AZtecFeature can automatically perform accurate quantification analysis of cathode material particles. The analysis process is simple and fast and does not require an expert user. Based on accurate and statistically valid results, the ratio of transition metals can easily be determined, and the process provides a powerful monitoring tool for controlling the composition of ternary cathode materials during production.