Whitepaper

Optimising battery electrolytes
with benchtop NMR
In this white paper, we highlight why broadband
benchtop Nuclear Magnetic Resonance (NMR)
Spectroscopy is now a key technique to exploit
across the complete product lifecycle.

Introduction
The development of higher performance, moresustainable energy storage devices has been widely
recognised as a key challenge for the scientificcommunity and industrial manufacturers. The
market for Lithium-ion batteries which currently
provide the highest energy density levels is projected
to grow to $116 billion by 2030. Improvements inenergy and power densities, charge and discharge
times, cost, lifetime and safety are all critical to thesuccess of next generation batteries.
Figure 1 shows a schematic of the components
within a typical lithium ion-battery. The current
carrying electrolyte has a critical role in defining allthese performance, safety and cost parameters ofcommercial battery systems. The mixture of different
solvents, additives, and lithium salts contained
within complex electrolyte systems needs to befinely tuned to deliver optimal performance andsustainability. Hence, accurate, quantitative, andreliable electrolyte characterisation and analysis
methods are needed - right through from incoming
raw materials checking to development of new
formulations and quality control within high volume
manufacturing.

Figure 1: Schematic of a lithium-ion battery

Benchtop NMR Spectroscopy
In NMR (Nuclear Magnetic Resonance) spectroscopy
particular isotopes of elements in the sample, mostly
in the liquid state, are probed by a radio frequency
pulse while exposed to an external magnetic field.
This provides information about the chemical
structure, concentration and local chemical
environment of the probed nuclei. To obtain adetailed structural understanding of your analyte(electrolyte in the case of batteries), the probingof many different nuclei across a broad frequency
range is needed. Uniquely, broadband benchtop
NMR spectroscopy now enables the qualitative andquantitative analysis of all constituents in a complex
electrolyte mixture. Previously, instruments wererestricted to just a few nuclei and had blind spots formany compounds.

Portable benchtop NMR instruments operating atroom temperatures using permanent magnets now
remove the accessibility barriers (cost, maintenance,
space requirements and skilled operators) oflarge cryogenic NMR systems. This ease of use
makes them ideal for rapid development, reaction
monitoring and quality control, where quick
and accurate results are important for instantfeedback and optimisation. Oxford Instruments
X-Pulse benchtop NMR is the only true broadband
spectrometer that gives the freedom to investigate
any electrolyte design and composition of choice.
While the H/F channel can collect spectra from1
H and 19F, its broadband channel excites anddetects a wide range of nuclei including 13C, 7Li,
31P, 11B, 29Si, 27Al and 23Na. all those with a Lamour
frequency between 25 and 11 MHz in a 1.4 Tesla
magnetic field. The capability to perform twodimensional spectral methods with pulsed field
gradients and shaped pulses enables a multitude ofadvanced experiments to quantify specific chemicaland physical properties that define electrolyte
performance.

Battery performance, where does NMR fit?
Battery technology is often considered a balancing
act between 5 key parameters: charge density,
the amount of energy stored in the cell; power
density, how quickly that energy can be extracted
from the cell; lifetime, sometimes known as charge
cycle life time and a measure of how quickly thecharge density is reduced; safety, how safe thebattery is through its lifetime and cost, how muchenergy per unit currency. Trying to maximise all ofthese parameters for a single battery chemistry isat the forefront of todays materials research. An
example of how some typical battery chemistries(defined by the cathode material) balance theseFigure 2: Chart showing the balance of critical parametersfor 3 common battery chemistriesparameters is shown in figure 2. Benchtop NMR
can assist the development of new electrolyte
chemistries by giving rapid understanding of thephysical and chemical properties that underpin
those critical performance parameters, these will be
discussed in detail through this white paper. NMR
is inherently quantitative allowing measurementof ion concentration (Li+, Na+, etc) which underpins
energy density, solvent composition or degreeof degradation which ensures safety, and thesecan be performed in seconds minimising cost.
Using a quick set of measurements, the diffusion
properties of electrolyte components, and therefore
the ionic conductivity, can be calculated allowing
understanding of potential power density as thechemistry is being developed.

Rapid assessment of electrolytecomposition
In case of a non-aqueous electrolyte an organic
solvent or a mixture of solvents, the majority
compound will be visible in simple 1H NMR spectra.
1
H nuclei of different species and positions within
the molecules result in distinct signals withcharacteristic chemical shifts. Some signals might besplit into doublets, triplets or other multiplets, which
gives valuable information about the nature of other
NMR active nuclei in the vicinity, such as the isotope(element), distance and quantity.
An easy example for a spectrum like that is given in
figure 3, where a simple 1H spectrum was taken ofa mixture of typical solvents contained in lithiumion batteries: ethylene carbonate (EC), dimethylcarbonate (DMC) and diethyl carbonate (DEC).

Figure 3: 1H spectrum of a simple solvent mixture typicalfor electrolytes

The resulting peaks are distinct from each other,with EC and DMC showing one distinct singlet eachtypical their chemical composition and structure. As
we have two 1H positions in DEC, we can see twosignals in the spectrum, with the signal for the CH3
group being split into a triplet, while the CH2 signal
is split into a quadruplet. This is explained throughthe n+1 rule for spin 1H and similar nuclei, where apeak will split into n+1 signals if n equivalent NMR
active nuclei are close enough to cause coupling.
This is the case here, as the three protons in CH3
cause the splitting of the CH2 signal into n+1 = 4,
while the CH2 group causes the CH3 position to besplit in three. This splitting can be interpreted togather information about the structure of unknown
compounds.

multinuclear NMR spectra taken from a commercial
battery electrolyte are shown in figure 4. Again, allsignals are distinct and well defined in all spectra.
Species with the nuclei 13C, 19F, 31P, 11 B and 7Li
could be detected. The n+1 rule holds true again for
19
F and 31P even in the case of coupling between
different nuclei. The splitting of the 31P signal into
seven suggests six coupling nuclei, while the 19F
signal at −69 ppm is split into two, which indicates
one coupling species. As the distance between
the peaks in the doublet and septet are the same(710.1Hz), these nuclei can be assigned to couple toeach other. This is characteristic for [PF6]− a common
anion used in electrolytes. This is an example of
how the structure of even unknown species can bereconstructed through NMR spectroscopy.

It is further possible to quantify the components
against each other or a standard, as the area
beneath the peaks is proportional to the amount of
nuclei present. Using this integration result, a ratio of
1:1:1 by mass could be determined.

This depth to the NMR technique and the ability tolocate it in the lab next to where you are developing
new electrolyte formulations allows can significantly
reduce develop time and cost allowing users toconfirm their new formulations .

The same principles can be applied to more
complex mixtures and to nuclei beyond 1H

Figure 4: Multinuclear NMR spectra of the same electrolyte taken on a single X-Pulse spectrometer with broadband
channel.

Quality control and reaction monitoring
to maximise safety and lifespanDue to its high sensitivity and accessibility, theX-Pulse is uniquely suited for quick reaction
monitoring and quality control. Its broadband
channel enables the detection of compounds
invisible in routine 1H and 13C spectra. In a casestudy, two electrolyte solutions of supposedly
the same constituents differed significantly inperformance when used to build a test cell. There
were, however, no visible differences, so the question
arose as to whether it is possible to test electrolyte toensure performance before spending time buildinga cell. The 1H spectrum of the samples is identical,
giving no clues to what might be wrong with one of
the mixtures but suggesting that the organic solvent
mixture is not the root cause (figure 5).

Figure 5: 1H benchtop NMR spectra of two electrolytes
presenting different performance characteristics.

However, in a 19F NMR spectrum we can observean additional doublet in one of the spectra
corresponding to a decomposition product of the[PF6]− (figure 6). From the chemical shift we canattribute this to Difluorophosphoric acid, OPF2(OH),a common breakdown product of [PF6]− in thepresence of water. Another species seen in thespectrum is LiF which is a biproduct of the hydrolysis
reaction of [PF6]− to OPF(OH)2 and confirms theelectrolyte failure has been caused by a hydrolysis
reaction in the presence of unwanted water.
The X-Pulse is also well equipped to quantify
impurities below a quantity of 1% in your electrolyte.
It is possible to easily perform acquisition of spectrawith carbon decoupling. This removes the carbon
satellites which might overlap or be confused withlow concentration impurities or additives, as isshown in figure 7.

Figure 6: 19F benchtop NMR spectra of two electrolytes
presenting different performance characteristics

Figure 7: 1H benchtop NMR spectra of the same sample
with (top) and without (bottom) 13C decoupling.

Beyond just identifying and quantifying degradationand impurities the form factor of benchtop NMR
means that you can continue to take measurements
as your formulations age, are exposed to the typicallab environment or are cycled in test cells. This
enables understanding of the processes and kinetics
that drive electrolyte degradation, whether throughhydrolysis as previously discussed, interaction withelectrodes, exposure to high voltages or changes intemperature. Understanding these processes allows
the formulation of new electrolyte and additive
combinations to minimise parasitic reactions
and significantly increase cell life time. Figure 8
demonstrates this, by integrating the peak areasof 19F spectra collect periodically after exposing a
combination of Li[PF6]and DMC to a small amount of
H2O the hydrolyses reaction can be followed and thekinetics understood.

Using Diffusion to understand Powerdensity and performance
Equipped with pulsed field gradients, the X-Pulse
can perform experiments that go beyond
identification and quantification of materials. ThePulsed field Gradient Spin Echo (PGSE) experimentenables the determination of diffusion coefficientsfor each species in solution. Due to the broadband
capabilities, even the inorganic compounds canbe investigated. Figure 9 shows such experimentsfor Li[PF6] in DMC, in which a series of spectra withincreasing field gradient strength are collected,
which results in a decline of the signal strength
related to the diffusion coefficient of the molecules.
This value can also be used to obtain some crucial
key properties for electrolytes like ionic conductivityor the cation transference number. By thisexperimental feedback loop, the physical properties
of electrolytes can be optimized by specificallytargeting the most impactful components one byone. Through the variable temperature options on
the X-Pulse, all those parameters can be determinedover most of the operation temperature range ofthe cell.
This allows experiments to confirm predictions
based on models before building test cells and cansignificantly shorted formulation optimisation.

Figure 8: 19F peak integrals for different compounds
collected every 30 minutes after exposing Li[PF6] in DMC
to H2O.

Figure 9: PGSE series for the determination of the diffusion coefficient of Li[PF6] in DMC.

Summary
Through its unique broadband NMR capabilities,
the X-Pulse can provide accurate analysis
for all compounds commonly used inelectrolytes in any workspace. This analysis
can also be performed across the rangeof typical battery operating temperatures.
Hence, the performance of your electrolyte
can be optimised at realistic conditions.
The measurements that can be performedby X-Pulse support all five of those critical
performance parameters discussed earlier:•Energy Density- Ion concentration (Li+, Na+)•Power Density- Ionic Conductivity and Cation 		transference over a range of 			temperatures•Safety- Raw material quality- Electrolyte quality and consistency- Additive development•

Lifespan- Understanding of electrolyte degradation- Additive development•Cost- Rapid low-cost analysis of new 		
formulations- Reduced cost of poor quality

This makes benchtop NMR a perfect fit to support
this industry which will be crucial to ensuring thefuture sustainability of our planet.

Visit nmr.oxinst.com/batteries© Oxford Instruments Industrial Products Ltd trading as Oxford Instruments Magnetic Resonance,
2021. All rights reserved. Do not reproduce without permission. Part no: MR/236/1121

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