Elemental composition analysis of Nickel-Manganese-Cobalt cathodes and their precursor materials using Epsilon 4 ED-XRF spectrometer

Introduction

In recent years, lithium-ion batteries have revolutionized the energy storage landscape by powering portable electronics, transportation, and renewable energy storage systems. Among the various types of lithium-ion batteries, lithium nickel manganese cobalt oxide (Li-NMC) batteries have emerged as a prominent choice for many high-end applications. NMC cathodes offer a high energy density, making them ideal for applications requiring long battery life, such as electric vehicles (EVs) and portable electronics. The combination of nickel, manganese, and cobalt allows for fine-tuning of properties like capacity, stability, and thermal safety. Nickel enhances energy capacity, cobalt provides stability and structural integrity, while manganese contributes to safety and affordability. This balance makes NMC cathodes a preferred choice for manufacturers seeking customized batteries for diverse range of applications.

Elemental composition analysis is a critical aspect in the development and optimization of Nickel Manganese Cobalt (NMC) cathode materials used in lithium-ion batteries. The relative concentration of these elements directly influences the electrochemical performance, stability, and longevity of the batteries. Accurate determination of the elemental composition is essential for ensuring the quality and consistency of the cathode materials during production.

There are many analytical techniques that can analyze elemental composition, of which ICP and X-ray fluorescence (XRF) are most significant. Comparing XRF spectrometry to other elemental analysis techniques like inductively coupled plasma (ICP) spectroscopy, XRF spectrometers enable simpler and faster analysis while delivering high-quality results in terms of precision and accuracy. This makes XRF a practical solution for process and quality control in battery cathode and precursor production and in battery recycling.

This application note outlines the development and use of NCM reference standards to calibrate XRF instruments. The use of these calibration standards allows similar or better accuracy and precision on Epsilon 4 EDXRF instrument compared to that of ICP. This can be leveraged to simplify the elemental composition analysis in battery production and R&D environment.

NCM CRMs Package

XRF can analyze elemental composition in two ways. The first is the standardless screening of input materials, which provides detection and a semiquantitative estimate of the elemental composition. However, for the accuracy desired in cathode materials production process and quality control, the second method involving calibration using standard samples needs to be deployed. In the latter case, the accuracy of XRF results relies on the availability of high-quality calibration materials, and there is a clear lack of commercially available calibration standards for battery cathode materials. Malvern Panalytical has designed and produced a set of Nickel-Cobalt-Manganese (NCM) Certified Reference Materials (CRMs) for its XRF calibration, along with using our sample preparation systems and expertise, which can deliver highly accurate and reliable results on NCM cathode materials.

The NCM CRMs package comprises 12 synthetic mixes specifically designed for preparing XRF fused bead specimens. The package also includes a fusion recipe and an XRF application method template. The CRMs are made from pure chemicals using a gravimetric approach for metrological traceability and adherence to ISO 17034. The elemental composition of the NCM CRMs and corresponding minimum and maximum concentrations are given in Table 1.

The NCM CRMs package is also suitable for cathode materials such as Lithium Nickel Cobalt Aluminum Oxide, Lithium Cobalt Oxide, and Lithium Manganese Oxide and their precursors. For high accuracy and repeatability, it is recommended to use fusion sample preparation method. However, these CRMs can also be used for making secondary calibration standards in the form of pressed pellets.  

* Lithium or Lithium oxide cannot be measured directly by XRF instruments, but they are added to the composition to simulate battery cathode mixes.

** The lowest calibration point should not be considered as the minimum concentration that can be reported. Instead, the Limit of Quantification (LOQ) is used for this purpose. LOQ depends on sample preparation, XRF instrument, measurement conditions, and measurement time. LOQ values for each element of interest are calculated during the instrument calibration process. Typically, LOQ values for the elements listed in Table 1 are in the range of 50 – 300 ppm.

Spectrometer description Epsilon 4

The Epsilon 4 is a high-performance benchtop, energy-dispersive X-ray fluorescence (EDXRF) spectrometer, powered by the latest advances in excitation and detection technology. Designed to be reliable and simple to operate, it has outstanding analytical performance from carbon (C) to americium (Am) in solids, liquids, loose- and pressed-powders and filters.

A high-performance, metal-ceramic X-ray tube has been specially developed for the Epsilon 4 at PANalytical’s tube manufacturing facility. A choice of anode materials (Rh, Ag and Mo on request), flexible voltage settings from 4.0 to 50 kV and a maximum current setting of 3.0 mA can be used to define optimum application-specific excitation across the periodic table.

The Epsilon 4 is equipped with the latest in silicon-drift detector technology with up to 30mm² high-resolution SDD silicon drift detector. Pulse-reset electronics give a count rate capacity of over 1,500,000 cps and a count rate independent resolution, typically 135 eV.

An exclusive deconvolution algorithm, automatic line-overlap and matrix corrections, advanced environmental fundamental parameter control and condition optimizer provide reliable results for many different types of materials.

Table 2: Key specifications and options for the Epsilon 4 spectrometer used in this application study

This application may be run on a spectrometer with different tube power or detector without sacrificing performance.

Sample preparation

Specimens of calibration standards and validation CRM were prepared in the form of diameter 32 mm fused beads, particularly with Lithium Borate fusion technique. A dilution ratio of 1 mass part of a sample to 10 parts of a lithium borate flux (with an integrated non-wetting agent) was used for preparation. Fusion of beads was done at 1100°C with the Eagon 2 automatic fusion machine¹. The total fusion time of the “cold-to-cold” operation cycle takes about 30 minutes. Particular details of the fusion recipes, used chemicals and fusion instructions could be provided on request or at purchase of calibration standards.

Calibration standards

Since most of the XRF instruments rely on high-quality calibration materials and there is a clear lack of commercially available calibrants for battery cathodes, Malvern Panalytical has designed and produced a set of Nickel-Cobalt-Manganese (NCM) Certified Reference Materials (CRM) for calibration purposes.

The set of NCM CRMs consists of 12 synthetic mixes specifically designed to be used for XRF instrument calibration in combination with fused bead specimen preparation. The CRMs are made from pure chemicals using a gravimetric approach for metrological traceability and adherence to ISO 17034. The elemental composition of the NCM CRM set and corresponding minimum and maximum concentrations are given in table 1. 

Measurement program

The measurement time of one specimen during the application study was about 5 minutes for 8 measured compounds present in the calibrants. Depending on the method precision requirements, instrument configuration and a number of compounds of interest the measurement time could be reduced to 3-5 slightly decreased or increased. 

Lithium oxide could not be measured directly by the XRF technique but an average expected value for it might be sufficient to obtain accurate results for the other measurable elements. 

Calibration

The examples of obtained calibrations for Ni, Co, Mn and S are given in the graphs below. 

Measurement precision 

A measurement precision (measurement instrument repeatability error) was estimated by measurement of a single specimen (bead) 21 consecutive times. The results are shown in the Table below. 

Validation of application. Results Accuracy and Trueness

A final validation of the method's trueness was done by measuring fused beads of commercially available BAM-S014 Certified Reference Material of Li-NMC 111 Cathode Material. 7 beads of the CRM were prepared on 3 different days. See the individual results and the summary in the Table below. 

* Allowed differences calculated according to the ISO Guide 35 requirements. 

¹ There are also proven fusion recipes for LeNeo, TheOx and FORJ automatic fusion machines.