Researchers led by a team from MIT, with colleagues from Oak Ridge National Laboratory (ORNL), BMW Group, and Tokyo Institute of Technology have developed a fundamentally new approach to alter ion mobility and stability against oxidation of lithium ion conductors—a key component of rechargeable batteries—using lattice dynamics. The new approach, described in a paper in the RSC journal Energy and Environmental Science, could accelerate the development of high-energy solid-state lithium batteries, and possibly other energy storage and delivery devices such as fuel cells.

Replacing organic liquid electrolytes with solid lithium ion conductors in Li-ion batteries can boost the energy density and also increase battery safety. Current research and development of solid-state lithium ion batteries has been catalyzed by recent breakthroughs in solid lithium ion conductors that have ion conductivities rivaling that of conventional organic liquid electrolytes. However, known fast solid lithium ion conductors are not stable against lithium ion battery electrodes. Of significance, no fast lithium ion conductor known to date is stable against positive electrode materials in lithium ion batteries. Therefore, it is of great importance to design new lithium ion conductors having not only high Li conductivity but also being stable during battery operation.

Increasing ion mobility and stability of lithium solid conductors is not straightforward and progress in the past decades has been achieved primarily by trial and error. Structural and chemical tuning via isovalent or aliovalent substitution of cation and/or anion in given structural families has led to steady increase in the lithium ion conductivity, and recent discovery of superionic lithium ion conductors. In this article, we report correlations between lattice dynamics and ion mobility or stability against electrochemical oxidation, and highlight opportunities to search for fast, stable lithium ion conductors based on low lithium band center but high anion band center. With rapid advances in the computational capability, we envision these descriptors to be used in high-throughput studies to screen not only lithium ion conductors but also other technologically relevant ion conductors such as oxygen or sodium ion conductors.

—Muy et al.

The new approach relies on understanding the way vibrations move through the crystal lattice of solid lithium ion conductors and correlating that with the way they inhibit ion migration. This provides a way to discover new materials with enhanced ion mobility, allowing rapid charging and discharging. At the same time, the method can be used to reduce the material’s reactivity with the battery’s electrodes, which can shorten its useful life. These two characteristics—better ion mobility and low reactivity—have tended to be mutually exclusive.

Diagram illustrates the crystal lattice of a proposed battery electrolyte material Li3PO4. The researchers found that measuring how vibrations of sound move through the lattice could reveal how well ions could travel through the solid material, and therefore how they would work in a real battery. In this diagram, the oxygen atoms are shown in red, the purple pyramid-like shapes are phosphate (PO4) molecules. The orange and green spheres are ions of lithium. Image: Sokseiha Muy Click to enlarge.

The new concept was developed by a team led by MIT W.M. Keck Professor of Energy Yang Shao-Horn, graduate student Sokseiha Muy, recent graduate John Bachman PhD ’17, and Research Scientist Livia Giordano, along with their colleagues at the other institutions. The new design principle has been about five years in the making, Shao-Horn says.

The initial thinking started with the approach she and her group have used to understand and control catalysts for water splitting, and applying it to ion conduction—the process that lies at the heart of not only rechargeable batteries, but also other key technologies such as fuel cells and desalination systems.

While electrons, with their negative charge, flow from one pole of the battery to the other, positive ions flow the other way, through an electrolyte, or ion conductor, sandwiched between those poles, to complete the flow.

Typically, that electrolyte is a liquid. A; a lithium salt dissolved in an organic liquid is a common electrolyte in today’s lithium-ion batteries. However, that substance is flammable and has sometimes caused these batteries to catch fire. The search has been on for a solid material to replace it, which would eliminate that issue.

A variety of promising solid ion conductors exist, but none is stable when in contact with both the positive and negative electrodes in lithium-ion batteries, Shao-Horn says. Therefore, seeking new solid ion conductors that have both high ion conductivity and stability is critical.

Sorting through the many different structural families and compositions to find the most promising ones is a classic needle-in-a-haystack problem. That’s where the new design principle comes in.

The idea is to find materials that have ion conductivity comparable to that of liquids, but with the long-term stability of solids. The team asked, “What is the fundamental principle? What are the design principles on a general structural level that govern the desired properties?” Shao-Horn says. A combination of theoretical analysis and experimental measurements has now yielded some answers, the researchers say.

We realized that there are a lot of materials that could be discovered, but no understanding or common principle that allows us to rationalize the discovery process. We came up with an idea that could encapsulate our understanding and predict which materials would be among the best.

—Sokseiha Muy, lead author

The key was to look at the lattice properties of these solid materials’ crystalline structures. This governs how vibrations such as waves of heat and sound—phonons—pass through materials. This new way of looking at the structures turned out to allow accurate predictions of the materials’ actual properties.

Once the vibrational frequency of a given material is known, it can be used to predict new chemistry or to explain experimental results, Shao-Horn says.

The researchers observed a good correlation between the lattice properties determined using the model and the lithium ion conductor material’s conductivity. They found, in particular, that the vibrational frequency of lithium itself can be fine-tuned by tweaking its lattice structure, using chemical substitution or dopants to subtly change the structural arrangement of atoms.

The new concept can now provide a powerful tool for developing new, better-performing materials that could lead to dramatic improvements in the amount of power that could be stored in a battery of a given size or weight, as well as improved safety, the researchers say. Already, they used the method to find some promising candidates. The techniques could also be adapted to analyze materials for other electrochemical processes such as solid-oxide fuel cells, membrane based desalination systems, or oxygen-generating reactions.

The work was supported by BMW, the National Science Foundation, and the US Department of Energy.

Resources

• Sokseiha Muy, John C. Bachman, Livia Giordano, Hao-Hsun Chang, Douglas L. Abernathy, Dipanshu Bansal, Olivier Delaire, Satoshi Hori, Ryoji Kanno, Filippo Maglia, Saskia Lupart, Peter Lamp and Yang Shao-Horn (2018) “Tuning mobility and stability of lithium ion conductors based on lattice dynamics” Energy and Environmental Science doi: 10.1039/C7EE03364H