Power packed

Power packed – NMR, EPR and MRI in lithium-ion battery R&D. By Clemens Anklin

In the short time since the development of the first lithium ion batteries (LIBs) in the 1970s, and the commercial availability of the first rechargeable product in 1991 (Sony), the world has become accustomed to the convenience of portable power – for the ever increasing range of devices that are in use in daily life. Today, LIBs are ubiquitous in our industrialized society. 

Now, the emphasis of LIB research has shifted in response to consumer demand and is focused not only on boosting performance and shortening charging times, but also on incorporating new (and sustainable) materials, improving safety, and reducing size and weight. The motivation behind many of these factors is the growing popularity of electric vehicles.
Central to achieving these goals, and developing next-generation technology that can overcome current energy limits of LIBs, is the need for a deeper understanding of the underlying chemistry of the materials at a researcher’s disposal, and important aspects of the key reactions happening in LIBs.  
Where techniques such as electron and optical microscopy offer high resolution imaging, they are often limited to surface imaging and are difficult to interpret quantitatively. Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies are both non-invasive methods with quantitative capabilities, and research is continuing to improve sensitivity and increase resolution. In addition, associated imaging techniques such as magnetic resonance imaging (MRI) are being utilized in a new multi-technique analytical paradigm.

How do rechargeable LIBs work?

Rechargeable batteries depend on electrochemical reactions, where chemical energy is converted to electrical energy, and vice versa, through the movement of ions and electrons in an electrolyte between two electrodes – the anode and the cathode.
During discharge, Li-ions carry the current within the battery from the anode to the cathode through the electrolyte and separator. When charging, an external electrical power source applies a higher voltage than the battery produces, forcing a charging current to flow within the battery from the cathode to the anode. The Li-ions then move from the cathode to the anode, where they become intercalated in the porous anode material, effectively ‘holding’ the charge for future release.

The role of NMR, EPR and MRI?

For many battery systems, NMR spectroscopy can be used to reveal structural details (including electronic structure) for phase identification of intermediates and to study dynamics in battery materials, including possible alternative electrode materials and electrolyte components (Li salts, solvents and additives, and solid forms), for example. 
Importantly, advances in understanding the processes governing the critical solid–electrolyte interphase (SEI) and dendrite formation that occur on first charge of a LIB have been assisted by developments in NMR analysis. The formation of a stable SEI determines many parameters that impact the performance and longevity of a battery. During charging, when Li ions move towards the anode, they may undergo plating, leading to the formation of dendrites, which can cause the battery to short-circuit and catch fire. Little is known currently about how to prevent dendrite formation. 
To investigate both of these significant factors, NMR allows separation and quantitative identification of many aspects of the layer. For example, 7Li and 19F magic angle spinning (MAS) NMR enables the identification and quantification of lithium fluoride (LiF) in the SEI at anodes and electrodes1. Dendritic growth can also be monitored and quantified. Changes in the intensity of the Li peak during cycling can be correlated with the growth of dendritic microstructures vs. smoothly deposited metal. One study found that in situ NMR could determine that up to 90% of Li deposited during slow charge of a Li/LiCoO2 battery was dendritic[2]. The technique can also be used to systematically test methods of dendrite suppression, such as electrolyte additives, advanced separators, cell pressure, temperature and electrochemical cycling conditions[3].  
EPR offers a useful compliment to analysis with NMR. It is well suited to studying the evolution of metallic Li species in operando. EPR spectroscopy can also semi-quantitatively detect deposited Li metal in a LIB with a metallic lithium anode and LiCoO2 cathode; and EPR imaging is being used to investigate the formation and disappearance of radical oxygen species in new batteries as a function of current rates, potentials, resting times, electrolytes, or temperatures.  
MRI is a powerful, non-invasive technique to provide time-resolved and quantitative information about the changes occurring within the electrolyte and electrodes of a LIB. In the same way as NMR, MRI is capable of detecting and localizing lithium microstructure build-up, but has the additional benefit of providing spatial information, allowing specific structural changes to be localized. In one study, researchers have been able to reconstruct 3D images of growing Li dendrites, elucidating their growth rate and fractal behavior (Figure 1)[4]. 
The benefits of MRI technology for researching new battery materials and cell designs are increasingly recognized and future applications could also include studying LIB capacity fade, examining cells after a large number of cycles, and high stress and accelerated aging testing.

Towards the batteries of the future

As discussed above, a range of strategies are in development and under review in order to push the performance limits of current LIBs. In parallel, research into more radical alternatives is accelerating too. All-solid-state batteries are a good example of these new approaches.
Solid-state batteries would represent a major shift in terms of battery technology. In all-solid-state batteries, the liquid electrolyte is replaced by a solid compound which nevertheless allows lithium ions to migrate within it. This concept is far from new, but over the past 10 years new families of solid electrolytes have been discovered with very high ionic conductivity, similar to liquid electrolyte. Such designs would offer a marked improvement in safety: solid electrolytes are non-flammable when heated, unlike their liquid counterparts. In addition, it permits the use of innovative, high-voltage high-capacity materials that may overcome performance issues thought to be due to high internal resistance for Li-ion over the solid-solid electrode-electrolyte interfaces. The result can be a battery that offers significantly increased energy density and improved cycling performance. 
Whatever the future of battery technology holds, it seems likely that research scientists – perhaps the subsequent production control and QC of finished products too – will rely on analysis using NMR, EPR and MRI as they work to ensure our future energy storage needs are met.
Clemens Anklin is Vice President NMR Applications & Training at Bruker BioSpin
1. B.M. Meyer, N. Leifer, S. Sakamoto, S.G. Greenbaum and C.P. Grey, “High field multinuclear NMR investigation of the SEI layer in lithium rechargeable batteries”, Electrochem. Solid-State Lett. 8(3), A145–A148 (2005). https://doi.org/10.1149/1.1854117 
2. R. Bhattacharyya, B. Key, H. Chen, A.S. Best, A.F. Hollenkamp and C.P. Grey, “In situ NMR observation of the formation of metallic lithium microstructures in lithium batteries”, Nat. Methods 9, 504–510 (2010). https://doi.org/10.1038/nmat2764. 
3. O. Pecher, J. Carretero-Gonzáelz, K.J. Griffith and C.P. Grey, “Materials’ methods: NMR in battery research”, Chem. Mater. 29, 213–242 (2016). https://doi.org/10.1021/acs.chemmater.6b03183. 
4. A.J. Ilott, M. Mohammadi, C.M. Schauerman, M.J. Ganter and A. Jerschow, “Rechargeable lithium-ion cell state of charge and defect detection by in-situ inside-out magnetic resonance imaging”, Nat. Commun. 9, 1776 (2018). https://doi.org/10.1038/s41467-018-04192-x.   

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