Elemental composition analysis of LiFePO₄ cathode materials using XRF

LiFePO4, normally referred to as LFP, is a major cathode material used by lithium-ion battery industry. LFP has its advantage in superior safety and lower material cost compared to other popular chemistries like NMC (LiNixMnyCo1-x-yO2). Though LFP has lower energy density than NMC, this gap is diminishing fast with new battery manufacturing and assembling techniques, like Cell to Pack (CTP) and blade battery designs.

There are several approaches for the manufacturing of LFP, such as high-temperature solid-state fusion of FePO4 precursor with Li2CO3, co-precipitation of LiFePO4 precursor followed with high-temperature sintering, carbothermal reduction of Li, P and Fe precursors, and hydrothermal synthesis. Each approach has its own pros and cons in the process and the final product quality control. Whatever is the manufacturing method, a strict control on the molar ratios of the raw materials and the elemental composition of the final product is critical to the performance of the assembled battery.

X-Ray Fluorescence (XRF) spectroscopy, with its advantage of high stability and simple sample preparation, is an ideal technique to analyze major elements (except Li) in LFP cathode manufacturing, from raw material to the final product. A study is shown in this report as an example.

Introduction

LiFePO4, normally referred to as LFP, is a major cathode material used by lithium-ion battery industry. LFP has its advantage in superior safety and lower material cost compared to other popular chemistries like NMC (LiNixMnyCo1-x-yO2). Though LFP has lower energy density than NMC, this gap is diminishing fast with new battery manufacturing and assembling techniques, like Cell to Pack (CTP) and blade battery designs.

There are several approaches for the manufacturing of LFP, such as high-temperature solid-state fusion of FePO4 precursor with Li2CO3, co-precipitation of LiFePO4 precursor followed with high-temperature sintering, carbothermal reduction of Li, P and Fe precursors, and hydrothermal synthesis. Each approach has its own pros and cons in the process and the final product quality control. Whatever is the manufacturing method, a strict control on the molar ratios of the raw materials and the elemental composition of the final product is critical to the performance of the assembled battery.

X-Ray Fluorescence (XRF) spectroscopy, with its advantage of high stability and simple sample preparation, is an ideal technique to analyze major elements in LFP cathode manufacturing, from raw material to the final product. A study is shown in this report as an example.

Instrumentation

Malvern Panalytical Zetium floor standing WDXRF spectrometer equipped with an Rh anode SST-mAX tube was used in these measurements. Designed to meet the most demanding process control and R&D applications, the Zetium XRF spectrometer leads the market in high-quality design and innovative features for sub-ppm to percentage level analysis of elements ranging from Be to Am. The Zetium is equipped with the latest version of our SuperQ software, including our Virtual Analyst that enables even non-expert users to easily set up applications. 

Sample preparation

Samples were prepared in two ways: Borate fusion and pressed pellets. Due to the limitation in matrix match, calibration samples were prepared only for the fusion application. The method precision of the pressed pellet application was evaluated with a single-point calibration.

Fusion preparation

To create a calibration for the fusion method, synthetic reference materials were prepared using Fe2O3 and Li3PO4 mixtures in varying stoichiometric ratios. 

To achieve an optimal precision for Fe in the sample, Co was used as an internal standard. A flux doped with 10% Co2O3 is used during the fusion. A dilution ratio of 1:10 was used for the calibration standards, while 0.8:10 was used for the samples in order to match the concentration. The samples were then fused with TheOX automatic fusion machine at 1120℃.

Pellet preparation

5g of LFP powder was dried and pressed into pellet with boric acid as backing.

Measurement procedure

Below measurement conditions were used for fuse bead samples. Total measurement time was 160 seconds per sample. Li2O is used as a balanced compound.

ItemTubeOptical pathTime (s)
CompoundLinekVmATube filterCollimatorX-talDetector
(Co)KA6040None150 µmLiF 220Scint.20
Fe2O3KA60

40

None150 µmLiF 220Scint.60
P2O5KA24100None550 µmGe 111-CFlow80

Table 1. Measurement conditions

The same measurement conditions were used for press pellet samples, except that Co was not present in these. 

Calibration and results

Figures 1 and 2 show the calibrations using fuse bead synthetic reference samples. A single-point calibration (not shown here) was made for the pressed pellet samples.

[Figure 1 AN220215-Elemental-composition-LiFePO4-XRF.png] Figure 1 AN220215-Elemental-composition-LiFePO4-XRF (2).jpg

Figure 1. Fusion calibration for Fe2O3

[Figure 2 AN220215-Elemental-composition-LiFePO4-XRF.png] Figure 2 AN220215-Elemental-composition-LiFePO4-XRF.jpg

Figure 2. Fusion calibration for P2O5

Since data consistency is quite important for process control, sample preparation and measurement repeatability were tested to validate the XRF application on LiFePO4 samples. The results are given in tables 2 and 3 below.

Pressed pelletFused bead
SampleP2O5Fe2O3(Li2O)SampleP2O5
Fe2O3
(Li2O)
147.64048.2004.160147.67748.1084.215
247.67948.1294.193247.67548.2194.105
347.68348.0954.223347.67748.1064.217
447.46548.0844.588447.61848.0474.335
547.78948.1644.047547.68348.1394.179
Average (%)47.62448.1344.242Average (%)47.66648.1244.210
Minimum result (%)47.46548.0844.047Minimum result (%)47.61848.0474.105
Maximum result (%)47.78948.2004.588Maximum result (%)47.68348.2194.335
RMS (%)0.1740.0480.204RMS (%)0.0270.0630.083
Relative RMS (%)0.370.104.82Relative RMS (%)0.060.131.97

Table 2. Sample preparation repeatability for pressed pellet and fusion sample preparations

Pressed pelletFused bead
MeasurementP2O5
Fe2O3
(Li2O)
MeasurementP2O5
Fe2O3
(Li2O)
147.78948.1644.047147.73648.1064.158
247.78048.1524.068247.73748.1634.100
347.75948.1774.064347.73148.2014.068
447.75748.1734.070447.74048.1554.105
547.77048.1524.078547.73648.2434.020
647.77948.1524.069647.72448.2034.073
747.78548.1224.093747.72148.2434.037
847.79648.1254.079847.70948.2834.008
947.76248.1584.080947.71648.2294.055
1047.67248.1814.1471047.68648.2474.067
Average (%)47.765
48.1564.080Average (%)47.723
48.2074.069
Minimum result (%)47.672
48.1224.047Minimum result (%)47.68648.1064.008
Maximum result (%)47.796
48.1814.147Maximum result (%)47.740
48.2834.158
RMS (%)0.035
0.0200.027RMS (%)0.0170.0530.044
Relative RMS (%)0.07
0.040.65Relative RMS (%)0.04
0.111.09

Table 3. Sample measurement repeatability for pressed pellet and fusion sample preparations

Summary and Conclusion

Above data demonstrate the capability of XRF in LFP material analysis. It shows high precision in sample analysis loop with fully automatic sample preparation and measurement. With further refinement, it is possible to reach precision similar to titration methods.




 

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