Contributed Commentary by Clemens Anklin, Vice President NMR Applications & Training, Bruker BioSpin
October 17, 2019 | Global surge in portable electronic devices and consumer demand for better performance is placing pressure on companies to innovate faster. This speed of innovation depends vastly on battery performance, but in order to develop a superior battery, the underlying chemistry of their materials must be understood. Advanced in situ measurement techniques are required to achieve this, and developments in magnetic resonance spectroscopy, including nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) spectroscopy, and imaging techniques such as magnetic resonance imaging (MRI), are paving the way for this progress.
Lithium Ion Batteries
Lithium’s high energy density and electrochemical potential make it one of the world’s most popular options for (Li) ion batteries (LIBs), and since their initial development in the 1970s, LIBs have enabled significant technological innovation, with the first rechargeable model launched in 1991 by Sony Corporation.
Rechargeable batteries depend on electrochemical reactions, where chemical energy is converted to electrical energy, and vice versa, via the movement of ions and electrons in an electrolyte between two electrodes, the anode and the cathode.
During the first charging cycle of a LIB, when Li-ions travel through the electrolyte towards the anode, some react with the degradation products of the electrolyte and form insoluble deposits on the anode. These deposits build up to form the solid-electrolyte interphase (SEI), which prevents the anode material from decomposing and is crucial to the long-term operation of the battery. The formation of a stable, ion conducting, electron-resistive SEI has shown to determine many performance parameters and is therefore of high interest for LIB research.
Investigating LIBs with NMR
For many battery systems, NMR can be used to reveal structural details (including electronic structure) for phase identification of intermediates, and to study dynamics in battery materials. NMR is well-suited to investigate the dynamics of alkali ions, which is a crucial element of battery material science. The power of solid-state NMR is harnessed to characterize local structures in LIB materials, elucidating the signal shifts of the material’s various chemical species, even in highly disordered systems. Lithium has two NMR active isotopes (6Li and 7Li), which enable direct investigation into the dynamics of lithium and the quantitative analysis of Li-ion motion.
Improving understanding of the SEI has been assisted by developments in NMR technology, which enables the separation and quantitative identification of many aspects of the layer. For example, 7Li and 19F magic angle spinning (MAS) NMR has allowed the identification and quantification of lithium fluoride (LiF) in the SEI at anodes and electrodes in rechargeable LIBs. Dendritic growth can also be monitored and quantified using NMR methods. 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. NMR can be used to systematically test methods of dendrite suppression, such as electrolyte additives, advanced separators, cell pressure, temperature and electrochemical cycling conditions. This, together with the quantitative measurement of SEI and novel battery materials in operando, positions NMR as a key driver of innovative LIB design.
EPR: A Complementary Technique?
Measuring the build-up of dendrites during cell operation presents a challenge, but is necessary for the continued investigation of alternative LIB designs and materials. In addition to NMR, EPR spectroscopy is well–suited to studying the evolution of metallic Li species in operando. EPR spectrometry has also shown to semi-quantitatively detect deposited Li metal in an LIB with a metallic lithium anode and LiCoO2 cathode.
EPR imaging is now 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.
Gaining Spatial Information With MRI
In addition to spectroscopy, 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. Similarly to 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. The benefits of MRI technology for researching new battery materials and cell designs are increasingly recognized. Applications could also include studying LIB capacity fade, examining cells after a large number of cycles, and high stress and accelerated aging testing.
One example of cutting-edge LIB research is the transition from liquid to solid electrolytes. The flammability of liquid electrolytes represents a safety risk, considering the possibility of short-circuiting in LIBs. The alternative use of solid-state electrolytes has been investigated for a number of years which, as well as improving safety, could also offer the potential for Li metal anodes that are resistant to dendrite formation, therefore increasing energy density. Although all-solid-state batteries are not a new concept, progress has so far been hindered by their poor rate capability and cycle performance, possibly due to high internal resistance for Li-ion transfer over the solid-solid electrode-electrolyte interfaces. Investigating interface reactions and charge transport is therefore crucial to harness the potential of these batteries, and NMR is ideally suited to this purpose.
NMR may also help characterize potential solid electrolyte materials, such as ceramics, which depend on ionic transport. NMR paired with conductivity measurements can analyze ion dynamics and help elucidate the relationship between local structure and dynamic parameters.
Batteries Of The Future
It is clear that developments in analytical technologies over the past 40 years have significantly impacted the battery industry. Where techniques such as electron and optical microscopy offer high resolution imaging, they are often limited to surface imaging and are therefore difficult to interpret quantitatively. NMR and EPR spectroscopy are both non-invasive methods with quantitative capabilities, and research is ongoing to improve their sensitivity and increase resolution.
A deeper understanding of possible alternative electrode materials, electrolyte components (Li salts, solvents and additives), and the processes governing SEI and dendrite formation, is paving the way for safer LIBs with higher energy densities. The rapid development of new materials, such as higher capacity cathodes with higher operating voltage, can pose challenges for electrolyte and interphase chemistry. Such innovations are being met with sophisticated analytical technology, such as EPR and NMR spectroscopy and MRI, to ensure that LIB research continues to give rise to the future’s energy storage solutions.
Dr. Clemens Anklin is the Vice President of NMR Applications & Training for Bruker. Dr. Anklin received his Ph.D. from the Swiss Federal Institute of Technology (ETH Zürich) and has been with Bruker since 1984. He can be reached at Clemens.Anklin@bruker.com.