Lithium Metal Batteries and the Critical Function of Solid Electrolyte Interphases: An Interview with Lauren Marbella of Columbia University

Lithium metal batteries (LMBs) are an advanced type of rechargeable battery distinguished using lithium metal as the anode. Compared to conventional lithium-ion (Li-ion) batteries, which use graphite or other intercalated materials for the anode, LMBs offer significantly higher energy density and specific capacity (1). This makes them particularly promising for applications in electric vehicles, portable electronics, and grid storage.

The performance of LMBs is heavily dependent on the composition and properties of the solid electrolyte interphase (SEI) (2). The SEI plays a crucial role in battery function by being electronically insulating and ionically conductive, facilitating uniform Li-ion flux to the electrode to prevent filament growth, and accommodating the large volume changes during lithium electrodeposition (2). However, difficulties in studying this fragile composite layer have impeded the practical development of LMBs (2).

Lauren Marbella, an associate professor of chemical engineering at Columbia University, is conducting research on LMBs, focusing on nuclear magnetic resonance imaging (MRI) and spectroscopy to study changes in material properties in real time, revealing chemical mechanisms behind degradation in lithium and other battery systems (3). Her group has developed detailed images of ion dynamics, uncovered new electrolyte decomposition mechanisms, and developed advanced techniques to study commercial batteries during use (3). Marbella has received several prestigious awards, including the Cottrell Scholar Award (2022), the NSF CAREER Award (2021), and the Scialog Collaborative Innovation Award (2019).

She earned her PhD in chemistry from the University of Pittsburgh in 2016 under Jill Millstone (3). In 2017, she became a Marie Curie Postdoctoral Fellow at the University of Cambridge with Clare Grey and was named the Charles and Katharine Darwin Research Fellow. Marbella joined the chemical engineering faculty at Columbia University in 2018 (3).

Spectroscopy recently sat down with Marbella to discuss her current research, as well as the future of LMB technology.

Lauren Marbella of Columbia University | Photo Credit: © Columbia University

Can you explain the critical functions of the solid electrolyte interphase (SEI) in the performance of lithium (Li) metal batteries?

The SEI is a very thin (~10 nm) layer that passivates the surface of the lithium metal electrode in a lithium metal battery. It forms because no liquid electrolytes in lithium batteries are thermodynamically stable when placed in contact with lithium metal; rather, they can only reach a metastable state through the formation of the SEI. That metastable state is achieved because once the liquid electrolyte is put in contact with the SEI, it starts to decompose, and those decomposition products precipitate out of the solution and deposit on the lithium metal electrode (they are essentially tarnishing the electrode surface. This is ideally a self-limiting process, per question #2). Because the SEI is made from electrolyte decomposition products, the electrolyte composition can be deterministic in terms of how the SEI and the battery behaves.

Why is it important for the SEI to be electronically insulating and ionically conductive?

To maintain that metastable state, the SEI cannot let through any electrons, as those electrons would cause the electrolyte to decompose (the lithium metal surface is electron-rich, and it is those electrons that have caused the electrolyte to decompose in the first place!). At the same time, for the battery to charge and discharge, lithium ions need to be able to pass through the SEI to make their way from the electrode to the electrolyte and back again.

What are the primary challenges in probing the SEI layer in Li metal batteries, and how do these challenges affect the development of practical applications?

The SEI is very thin (only a few nanometers thick), so advanced analytical techniques are needed to detect it and even then, there is not a lot of material there, especially compared to its surroundings (like the electrode and the electrolyte) that are present at several orders of magnitude higher that can overwhelm its signal. It is air sensitive, so if you try to analyze it outside of the battery, you might damage it. In addition, the SEI is made up of many different types of electrolyte decomposition products all mixed together, and very few techniques can measure samples that are heterogeneous, disordered mixtures that make up a very small fraction of the overall system.

These challenges present issues for a few reasons. We can take the example of Li-ion batteries. Li-ion batteries have been a successful technology for over 30 years through the serendipitous creation of a stable SEI. To this day, the SEI in Li-ion batteries is the only part of the device where there is ambiguity in its function and degradation. We know what we want the SEI to do (electronically insulate, ionically conduct), but given what we know about SEI composition, we have no idea how it is doing that and in fact, it is quite counterintuitive how the SEI in a Li-ion battery works. Because we know so little about the only commercial success, it is incredibly difficult to realize future applications that are more challenging, such as lithium metal.

How does nuclear magnetic resonance (NMR) spectroscopy provide unique advantages in studying the SEI of Li metal batteries?

NMR spectroscopy is unique in that it is one of the only techniques that can probe the chemical composition of the SEI as well as lithium-ion dynamics within the SEI at the same time. It does this without disturbing the sample within the original battery environment, and it shows how the disordered regions of the sample change as a function of electrolyte formulation.

In what ways does NMR spectroscopy help connect the local structural properties of the SEI to its electrochemical behavior?

With certain in situ experimental setups, you can concurrently collect NMR spectra while you are running electrochemical measurements to provide a direct correlation between what you are seeing in the spectroscopic measurement and the electrochemistry. Even if experiments are not performed in situ, but rather ex situ, one can provide a robust correlation between the electrochemical behavior that is observed in a battery (for example, Coulombic efficiency) and how this is related to changes in specific structures in the SEI. For example, we show that the structure of lithium fluoride in the SEI changes dramatically as a function of Coulombic efficiency. As lithium metal Coulombic efficiency goes up via electrolyte engineering, disordered lithium fluoride phases emerge in the SEI.

How does understanding ion dynamics through NMR contribute to improving the design of Li metal surfaces?

NMR is one of the few methods that can provide a readout of what chemical compounds lead to a given set of lithium transport properties and how those are connected to the cyclability of Li metal.

How can insights gained from NMR studies of the SEI influence the development of next-generation, high-energy-density batteries?

Results from NMR can direct how battery electrolytes are designed as well as show how SEIs are formed during battery operation. For example, we may not have to change the electrolyte at all, but rather, we can use different electrochemical protocols to alter the structure of the SEI and change lithium metal battery performance that way.

What specific molecular-level insights from NMR are most crucial for enabling reactive anodes in Li metal batteries?

The most important insights from NMR are the rate of lithium transport in the SEI, the phases that enable desirable transport properties, and how the structure of SEI compounds change as a function of performance.

What are some of the practical applications where advancements in Li metal battery technology, facilitated by NMR spectroscopy, can make the most significant impact?

Some of the practical applications in Li metal battery technology that will make the most significant impact are within the electric transportation sector. Lithium metal batteries may enable electrification of heavy-duty vehicles (for example, semi-trucks) or even small aircrafts.

How do you envision the future of Li metal batteries evolving with the integration of advanced characterization techniques like NMR?

I hope that by using NMR, we can accelerate electrolyte and SEI design, potentially through screening new electrolytes and/or analyzing the SEI to gain a better understanding of the features that are necessary to enable Li metal.

Are other analytical techniques useful or practical for this type of analysis?

Absolutely! Complementing these NMR measurements with other methods of analysis like X-ray techniques, electron microscopy, mass spectrometry, electroanalytical tools, vibrational spectroscopy, and computational modeling is incredibly valuable and necessary.

References

(1) Xu, W.; Wang, J.; Ding, F.; et al. Lithium Metal Anodes for Rechargeable Batteries. Energy Environ. Sci. 2014, 7, 513–537. DOI: 10.1039/C3EE40795K

(2) Svirinovsky-Arbeli, A.; Juelsholt, M.; May, R.; Kwon, Y.; Marbella, L. E. Using NMR Spectroscopy to Link Structure to Function the Li Solid Electrolyte Interphase. Joule 2024, ASAP. DOI: 10.1016/j.joule.2024.04.016

(3) Columbia University Engineering, Lauren Marbella. Columbia.edu. Available at: https://www.cheme.columbia.edu/faculty/lauren-marbella (accessed 2024-06-18).