Ensuring Lithium Battery Performance: A Guide to Electrolyte Analysis

The elemental examination of lithium battery electrolytes is crucial in guaranteeing the performance and quality of contemporary energy storage systems. Lithium batteries are essential in various technical applications, from mobile devices to electric vehicles.

Electrolytes are essential for lithium battery performance. Their efficacy and longevity depend on the purity and specific composition of the electrolytes used. In lithium batteries, the main function of the electrolyte is to enable ion flow between electrodes.

The in-depth and consistent quality control of electrolyte solutions is vital in ensuring ideal performance. The Chinese industrial standard HG/T 4067-2015 denotes the comprehensive requirements and processes for the chemical analysis of lithium battery electrolytes.1

The standard guarantees that all applicable elements in the electrolytes are correctly identified and quantified, maximizing lithium battery performance and safety. It also offers a homogeneous method that enables compliance with global quality standards and comparability of results.

Lithium hexafluorophosphate (LiPF6) is used in lithium batteries because of its superb conductive properties. The standard referred to above defines a process for assessing the LiPF6 electrolyte and specifies that a mix of methyl ethyl carbonate, ethanol, and water in the ratio 1:4:5 should be used to prepare calibration and sample solutions.

This article elucidates the practical implementation of the Chinese standard, focusing on the analytical procedures for determining various elements in lithium battery electrolytes. Under the standard, three electrolyte samples were assessed for 14 elements, each using the high-resolution ICP-OES PlasmaQuant 9100 Elite (PQ9100).

As LiPF6 results in the formation of hydrofluoric acid, the measurement system was fortified with a hydrofluoric acid-resistant sample introduction kit (HF kit).

The carbon-rich matrix creates spectral overlaps on some analytical lines, an effect that was corrected using the Correction of Spectral Interferences (CSI) software tool. This led to an improved baseline without spectral disturbances and an enhancement of the results’ reliability.

Methods

The reagents and samples used were LiPF6 electrolytes, a multi-element standard solution for ICP (100 mg/L Al, As, Ca, Cd, Cr, Cu, Fe, Mg, Na, Ni, Pb, Zn), single-element standard solutions for Hg and K (1000 mg/L each), ethanol, and ethyl methyl carbonate.

The test samples were diluted by weight by a factor of 10. Under the standard, the diluent was prepared in a ratio of 1:4:5 with methyl ethyl carbonate, ethanol, and deionized water.

Due to the small amount of sample material provided, analysis was performed in manual operation mode (without the auto-sampler) on the PQ9100 fortified with an HF kit. Table 1 summarizes the individual settings and components; Table 2 provides comprehensive information about the method parameters and settings.

External calibration standards were made using single- and multi-element standard solutions with the diluent. Table 3 lists the concentrations of the calibration standards; examples of the resulting calibration functions are shown in Figure 1.

Table 1. Instrument configuration and settings. Source: Analytik Jena US

Parameter Specification
Plasma power 1450 W
Plasma gas flow 15 L/min
Auxiliary gas flow 0.5 L/min
Nebulizer gas flow 0.35 L/min
Nebulizer parallel path, PFA, 1 mL/min
Spray chamber cyclonic, 50 mL, PTFE
Outer tube / inner tube ceramic/ceramic (alumina)
Injector alumina, 2 mm id
Pump tubing PU (sample: black/black, waste: red/red)
Pump rate 0.2 mL/min
Fast pump 0.2 mL/min
Delay time/rinse time 100 s/100 s
Torch position 0 mm

 

Table 2. Method parameters. Source: Analytik Jena US

Element Line [nm] Plasma view Integration Read time [s] Evaluation
Pixel Baseline fit Poly. deg. Correction
Al 308.215 axial Spectrum 3 3 ABC auto -
As 193.698 axial Spectrum 3 3 ABC auto CSI
Ca 317.933 axial Spectrum 3 3 ABC auto -
Cd 228.802 axial Spectrum 3 3 ABC auto CSI
Cr 205.552 axial Spectrum 3 3 ABC auto CSI
Cu 324.754 axial Spectrum 3 3 ABC auto -
Fe 259.940 axial Spectrum 3 3 ABC auto -
Hg 184.886 axial Spectrum 3 3 ABC auto CSI
K 769.897 axial Spectrum 3 3 ABC auto -
Mg 285.312 axial Spectrum 3 3 ABC auto -
Na 589.592 axial Spectrum 3 3 ABC auto -
Ni 231.604 axial Spectrum 3 3 ABC auto -
Pb 220.353 axial Spectrum 3 3 ABC auto CSI
Zn 213.856 axial Spectrum 3 3 ABC auto -

ABC: Automatic Baseline Correction, CSI: Correction of Spectral Interferences

Table 3. Concentrations of the calibration standards. Source: Analytik Jena US

Element Concentration [mg/L]
Cal. 0 Std. 1 Std. 2 Std. 3 Std. 4
Al, As, Cd, Cr, Cu, Hg, K, Mg, Ni, Pb, Zn 0 0.02 0.06 0.12 0.2
Ca, Fe, Na 0 0.2 0.6 1.2 2.0

 

Examples for calibration functions

Figure 1. Examples for calibration functions. Image Credit: Analytik Jena US

Results and Discussion

Table 4 shows the results for the three electrolyte samples. After the sample analysis, an independent QC standard of 0.12 mg/L was also prepared and quantified. The recovery is listed in the results table.

Table 5 shows the method-specific detection limits (MDL) of the analysis. These values were determined using the reagent blank method (three times the standard deviation of 11 repeated measurements of the reagent blank). The calculation of the results and the MDL includes the sample preparation dilution factor of 10.

Table 4. Measuring results and QC standard recovery. Source: Analytik Jena US

Element Measured values [mg/kg] QC std. recovery
[%]
Electrolyte 1 Electrolyte 2 Electrolyte 3
Al <MDL < MDL 0.02 103
As <MDL <MDL <MDL 109
Ca 0.61 0.711 0.60 107
Cd < MDL < MDL < MDL 101
Cr < MDL < LOQ < LOQ 101
Cu < MDL < MDL < LOQ 99.0
Fe 0.21 0.477 0.45 103
Hg < LOQ < MDL < LOQ 94.0
K 0.85 1.12 0.80 95.0
Mg 0.04 < LOQ < LOQ 104
Na 1.36 1.65 0.92 109
Ni < LOQ < LOQ < LOQ 103
Pb < LOQ < MDL < LOQ 96.0
Zn < MDL < MDL < MDL 104

MDL/LOQ: Method specific Detection Limit/Limit Of Quantification (3 or 9 times the standard deviation of 11 reagent blank measurements)

Table 5. Method-specific detection limits (MDL). Source: Analytik Jena US

Element/Line
[nm]
MDL
[mg/kg]
Element/Line
[nm]
MDL
[mg/kg]
Al308.215 0.15 Hg184.886 0.11
As193.698 0.13 K769.897 0.02
Ca317.933 0.06 Mg285.312 0.01
Cd228.802 0.01 Na589.592 0.01
Cr205.552 0.07 Ni231.604 0.03
Cu324.754 0.02 Pb220.353 0.11
Fe259.940 0.01 Zn213.856 0.01

MDL: Method-specific Detection Limit

Summary

The high matrix tolerance, combined with the high resolution and measurement sensitivity of the PQ9100, allows robust and interference-free analysis of lithium battery electrolytes. Software tools, including automatic baseline correction, facilitate spectra evaluation and offer consistent results.

Some lines of analysis show a spectral overlay by matrix-related emission bands. The CSI software tool, which is based on the least squares model, was applied to eradicate this structured background. A spectrum of a pure sample matrix solution (diluent) was logged at the related wavelengths and saved into a database. This correction spectrum was subtracted from the recorded sample spectra.

The created correction model can be employed in the method for automatic application during routine measurements. Figure 3 shows the high spectral resolution of the PQ9100 (2 pm at 200 nm) and the effect of using the CSI tool on the example of mercury (184 nm).

PlasmaQuant 9100 Elite

Figure 2. PlasmaQuant 9100 Elite. Image Credit: Analytik Jena US

Effect of the CSI software tool on the example of Hg184

Figure 3. Effect of the CSI software tool on the example of Hg184. Image Credit: Analytik Jena US

Table 6. Overview of recommended devices, accessories, and consumables. Source: Analytik Jena US

Article Article number Description
PlasmaQuant
9100 Elite
818-09101-2 High-resolution ICP-OES
Teledyne Cetac
ASX 560
810-88015-0 Teledyne-Cetac ASX-560 autosampler for ICP-OES and ICP-MS
HF-Kit 810-88007-0 HF-resistant sample introduction kit
Consumable set
HF Kit
810-88042-0 Consumables Set HF Kit for PlasmaQuant 9x00 series
PU pump tubing (sample) 418-13-410-528 PU pump tubing (black/black) for sample
PU pump tubing (waste) 418-13-410-529 PU tubing (red/red) for waste

 

References

  1. Chinesestandard.net. (2015). HG/T 4067-2015 English PDF. [online] Available at: https://www.chinesestandard.net/PDF/English.aspx/HGT4067-2015 [Accessed 8 Nov. 2024].

Image

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.

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