NMR Analysis of Battery Electrolytes: Quantifying Ion, Solvent, and Additive Composition

Abstract:

Controlling the composition of sodium- and lithium-ion batteries is important to ensure that solvents, additives and charge-carrying ions match the required formulation for effective battery performance. This Application Note demonstrates how quantitative nuclear magnetic resonance spectroscopy (qNMR) using the Oxford Instruments X-Pulse benchtop NMR spectrometer can measure different electrolyte components. Examples include relative quantification of solvent composition using ¹H and ¹³C qNMR, absolute quantification of lithium concentration using ⁷Li NMR with external calibration, and quantification of [PF₆]^- and [BF₄]^- anions using ¹⁹F qNMR with an internal standard. The results demonstrate how both relative and absolute qNMR methods can support battery electrolyte formulation development, quality assurance, and quality control.

Introduction

Lithium-ion (Li-ion) batteries have become ubiquitous in daily life, providing power for a diverse range of applications, ranging from mobile phones, computers, power tools, and medical devices to the green technologies including electric cars and solar energy storage. As our usage of lithium-ion batteries has grown, so has the need to optimise their performance and to ensure reliability over a long lifetime. At the same time new technologies including sodium-ion (Na-ion) batteries are in development.

Electrolytes play a crucial role in the performance and reliability of Li-ion batteries, providing the medium that allows anions and cations to carry charge between the electrodes, as shown in Figure 2.1. In the current generation of commercial batteries, the electrolyte is typically made up of organic small-molecule liquid solvents, commonly a mixture of alkyl carbonates, combined with a lithium salt. Additionally, various additives are used to ensure specific chemical and electrochemical properties.

Figure 2.1 Diagram of a lithium-ion battery, showing major components including the anode, cathode, separator, and electrolyte.

While lithium-ion batteries have played a major role in transforming our daily lives, benchtop NMR spectroscopy has similarly begun to revolutionise quality assurance and quality control (QA/QC) processes by making a powerful technique easier and more accessible for routine analysis in any laboratory. Benchtop NMR has many potential lithium-ion battery QA/QC applications, as the well-resolved spectra of the small organic molecules and ions are ideally suited for quick, convenient analysis.

The Oxford Instruments X-Pulse broadband benchtop NMR spectrometer (available with a 60 MHz or 90 MHz permanent magnet) provides significant advantages for analysis of battery electrolytes. In this Application Note we look at how benchtop NMR can be used for quantitative analysis of Li-ion battery electrolytes. Since the exact composition of the electrolyte will affect the performance of the battery, ensuring the electrolyte matches its specification is essential for quality control in the manufacture of Li-ion batteries.

Quantitative NMR as an Analytical Method

When performed correctly, NMR is an inherently quantitative method, in which case it’s usually referred to as qNMR. It therefore allows for not only the qualitative composition of a sample to be determined, but also to accurately quantify the species present.

When performing quantitative NMR (qNMR), there are three important considerations:

• ensuring the NMR spectrum acquired is actually quantitative
• if you’re performing relative or absolute quantification
• if you’re performing absolute quantification, if you’re using an internal or external standard

These are summarised in the following sections.

It’s important to recognise that the accuracy of qNMR, like that of any other quantitative measurements, can be affected by every stage of the measurement process, from sample preparation through to data processing. Important sources of error include incomplete dissolution or extraction, losses during transfer, contamination, uncertainty in weighing or volumetric preparation, and the purity and stability of calibration standards. Additionally errors may arise from the measurement itself, although these are usually minimal for NMR.

Throughout this application note, these sources of error haven’t always been accounted for. Therefore errors in the measurement in practice are usually going to be greater than those indicated, if present.

Requirements for Accurate Quantitative NMR

To ensure that an NMR spectrum is quantitative, every signal used for analysis must be acquired under conditions where its integral is proportional to the number of nuclei contributing to that resonance.

The most important requirement is sufficient relaxation between scans: The relaxation delay should normally be at least five times the longest spin-lattice relaxation time (T₁), so that all resonances have effectively returned to equilibrium before the next pulse. Pulse angles should be calibrated, receiver gain kept within the linear range, and enough scans acquired to provide adequate signal-to-noise without compromising the relaxation delay. The selected signals should be well resolved and free from overlap or treated consistently by peak fitting where overlap is unavoidable, and the final spectrum should be carefully phased, baseline corrected and integrated over reproducible limits.

When these acquisition and processing conditions are controlled, the measured integrals can be used with confidence for relative or absolute quantification. All the spectra in the Application Note have been acquired on the X-Pulse 90 and processed in accordance with these guidelines.

Relative vs Absolute Quantitative NMR

In quantitative NMR (qNMR), signal integrals are directly related to the number of resonant nuclei, allowing NMR spectra to be used not only for identification but also for measuring composition.

Relative quantification compares the integrals of signals within the same spectrum to determine the ratios of components in a mixture, making it well suited to applications such as solvent composition, impurity profiling and additive screening where absolute concentration is not required.

Absolute quantification extends this approach by comparing analyte signals against a material of known amount or concentration, either added to the sample as an internal standard or measured separately using an external calibration.

In both cases, reliable results depend on appropriate signal selection, sufficient relaxation delay, good signal-to-noise and careful integration so that the measured peak areas accurately reflect the true number of nuclei present.

Internal vs External Standards

For absolute quantification by NMR, the analyte signal must be compared with a standard of known amount or concentration. This can be achieved using either an internal or an external standard.

In the internal-standard approach, an accurately measured reference compound is added directly to the same NMR sample as the analyte, so the analyte and standard experience the same acquisition conditions and the concentration calculation is based on their integral ratio. This often gives high precision, provided that the standard is pure, soluble, chemically inert and gives a well-resolved signal that does not overlap with the analyte.

In the external standard approach, the reference material is prepared and measured separately, placing greater emphasis on accurate preparation, stable instrument response and consistent acquisition conditions.

The choice between the two approached therefore depends on the analytical priority: internal standards are generally preferred when precision and simplicity of calculation are most important, whereas external standards are useful when the sample must remain unchanged or when addition of a standard is impractical.

Examples of Quantitative NMR Analysis of Battery Electrolytes

Using a series of Li-ion battery electrolyte related samples, various methods for quantitative NMR can be demonstrated.

Measuring Battery Electrolyte Solvent Composition by qNMR

The solvents for Li-ion battery electrolytes generally comprise a mixture of alkyl carbonates in various proportions. Quantitative NMR is an ideal method for accurately measuring their amounts. An initial sample was prepared containing approximately a 1:1:1:1 ratio (by volume) of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC) and vinylidene carbonate (VC) (Sample I). A ¹H NMR spectrum with 13C decoupling was acquired with quantitative parameters (Figure 2.2) with an acquisition time of 4 minutes.

To determine the ratio of these signals, and hence the ratio of the different compounds, the area of each signal must be determined. The traditional approach is to integrate each signal within the spectrum. Another approach is the fit the signals to individual peaks and determine the signal area by summing the areas of the appropriate peaks. Both approaches have been used in this case.

Figure 2.2 ¹H{¹³C} qNMR, spectrum of Li-ion battery electrolyte, comprising a mixture of DMC/DEC/EC/VC (spectrum in dark blue, fitted peaks in light blue)

The ratio of alkyl carbonates can also be determined with quantitative ¹³C NMR. However, due to the lower receptivity of ¹³C compared to ¹H, coupled with longer T₁ relaxation times, acquiring the spectrum takes considerably longer, with a quantitative ¹³C{¹H} spectrum of the same sample (Figure 2.3) acquired in 21 hours 20 minutes.

Even in this case the signal-to-noise ratio (SNR) is low enough that integrating individual peaks would have an unacceptably high error. However, by fitting the peaks, specifically those of the carbonate carbons, the area of each signal and hence amount of each compound, can still be determined.

Figure 2.3 ¹³C{¹H} qNMR, spectrum of Li-ion battery electrolyte, comprising a mixture of DMC/DEC/EC/VC; insert showing carbonate region (spectrum in dark blue, fitted peaks in light blue)

By converting the volume ratio used to prepare Sample I into a normalised molar ratio, which is also the expected signal ratio of the ¹³C carbonate signals, we can predict signal ratios of the signals in the ¹H{¹³C} NMR spectrum (Table 2.1) and compare these with the compositions obtained from the NMR spectra. In this case, both the ¹H and ¹³C NMR spectra can be used to obtain the actual sample composition.

For the ¹H spectra, measuring the signal areas with peak fitting appears to give more consistent values than obtained by signal integration since it minimises the effect of the signal overlap.

Table 2.1 EC/DMC/DEC/VC molar & signal ratios comparing predicted values derived from the volume ratio and the measured values from the ¹H{¹³C} & ¹³C{¹H} qNMR spectra

A more realistic sample composition is a 1:1:1 mixture of EC/DMC/DEC, with a small amount of VC added. Four more samples were prepared Sample II, III, IV, & V, containing gradually increasing amounts of VC. This can clearly be shown in Figure 2.4, with the VC peak at δ_H 7.7 ppm.

Figure 2.4 ¹³C{¹H} qNMR, spectrum of Li-ion battery electrolyte, comprising a mixture of DMC/DEC/EC, with varying amounts of VC

By analysing Sample II, the EC/DMC:DEC molar ratio can be accurately measured and confirmed to match the predicted value of the basis of the 1:1:1 volume ratio used to prepare the sample (Table 2.2).

Table 2.2 EC/DMC/DEC molar & signal ratios comparing predicted values derived from the volume ratio and the measured values from the ¹H{¹³C} qNMR spectra

Using the process already shown, the ¹H{¹³C} spectra of samples III, IV & V can be analysed and the actual percentage vinylidene carbonate present in the sample can be determined (Table 2.3). While the actual amount of VC in Sample III was consistent with the target amount, the amount of VC in both samples IV & V was significantly higher that the target value, suggesting an error is sample preparation.

This degree of variation in sample composition is sufficient to affect the performance of a Li-ion battery, demonstrating both the importance of quality control of the alkyl carbonate solvent and the value of benchtop NMR for this analysis.

Table 2.3 VC content in an EC:DMC:DEC solution, as determined by ¹H{¹³C} qNMR

Quantifying Lithium Content using External Standards

A ⁷Li NMR spectrum of Li⁺ comprises a single peak at around δ_Li 0 ppm. To accurately quantify the total amount of lithium present in the sample, it’s necessary to use an external calibration.

A series of samples containing accurately measured amounts of Li⁺ were prepared at a range of concentrations ranging from 200 mmol/ℓ to 1.00 mol/ℓ, with identical ⁷Li NMR spectra acquired for each. Previously the T₁ relaxation time for Li⁺ has been determined as 22.1 s, therefore spectra were acquired with a relaxation time of 2 minutes using 16 scans.

Examples of the resulting spectra are shown in Figure 2.5, clearly showing the linear relationship between lithium concentration and the intensity of the NMR signal.

Figure 2.5 ⁷Li qNMR spectra for varying Li⁺ concentrations

These measurements were repeated three times, with a standard deviation between the repetitions of less than 0.2 %. By plotting the lithium concentration against the signal areas, an equation can be determined for the resulting linear correlation (Figure 2.6).

Figure 2.6 Calibration of Li⁺ concentration with Integrated Signal Areas by ⁷Li qNMR

Using this calibration ‘curve’ and equation, we can then analyse samples with an unknown Li⁺ concentration, and determine the concentration from the signal.

Table 2.4 Lithium concentration in unknown samples, determined by ⁷Li qNMR

Quantifying Electrolyte Anions using an Internal Standard

The third method for performing qNMR, is through absolute quantification using an internal standard. In this case, an accurately measured amount of 99.9% pure α,α,α-trifluorotoluene was added to a battery electrolyte containing a mixture of [PF₆]⁻ and [BF₄]⁻ salts. Initially, ¹⁹F inversion recovery experiments were performed to determine the T₁ relaxation times of the three species (Table 2.5).

Table 2.5 T₁ relaxation times, measured by a ¹⁹F inversion recovery experiment

To perform accurate quantitative measurements, the relaxation delay should be at least 5 times the longest T₁. Therefore, setting a relaxation delay of 20 seconds, along with acquiring 16 scans, allows for the acquisition of a ¹⁹F{¹³C} qNMR spectrum in under 5 minutes (Figure 2.7).

Figure 2.7 ¹⁹F{¹³C} qNMR spectrum of a mixture of [PF₆]⁻ & [BF₄]⁻ plus α,α,α-trifluorotoluene as an internal standard

Table 2.6 Determination of anion concentrations using ¹⁹F{¹³C} qNMR and a C₆H₅CF₃ internal standard of known concentration

The accuracy of this calculation could be further improved by considering the accuracy of the concentration calculated for C₆H₅CF₃ with the accuracy in the measured volumes / masses propagated through to the concentration, taking in account the purity of C₆H₅CF₃ and the affect that has on its concentration, and considering the reproducibility of the NMR measurement, for which the standard deviation is expected to be of less than 0.2 %. Together, these considerations would allow for the concentration and associated standard error to be determined.

Summary

The X-Pulse 90 and X-Pulse 60 broadband benchtop NMR spectrometers are ideal tools for the quantitative analysis of lithium-ion battery electrolytes. When performed correctly, NMR in an inherently quantitative technique, which can be used for either relative or absolute quantification, using either internal or external standards. This allows for the most appropriate quantification method to be chosen depending on the chemistry of the sample. All these methods are applicable to the different components of a Li-ion or Na-ion battery electrolyte.