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Researchers discover why adding carbon to new type of solid electrolyte improves diffusion

In new research by an international collaboration jointly led by Lawrence Livermore National Laboratory (LLNL) scientist Brandon Wood and Mirjana Dimitrievska of the National Institute of Standards and Technology (NIST), the team discovered why substituting one boron atom for one carbon atom in a key battery electrolyte material made lithium ions move even faster, which is attractive for a more robust solid-state battery.

This is an example of what scientists refer to as “frustration”: the dynamics of the system ensure that lithium is never satisfied with its current position, so it’s always moving around. The research appears in the journal Advanced Energy Materials.

Solid-state lithium-ion batteries can provide improved safety, voltage and energy density compared with today’s batteries, which use liquid components. However, solid-state batteries are still in an early stage of development, with very few commercialized to date.

One of the key obstacles to the viable commercialization of solid-state batteries is the small number of candidate solid electrolyte materials that can shuttle lithium ions efficiently between the electrodes. This is easily done through a liquid, but known solid materials that can do this are few. Some of the available materials have stability issues. Others are difficult to process. Most of the remaining candidates are simply too slow at moving lithium ions around, which means they must be made very thin to be effective.

The new work focuses on one material within a new class of materials, closo-borates, that was recently discovered to have fast lithium ion mobility. According to Wood, closo-borates are electrochemically stable and can be easily processed, offering some significant advantages over the competition. Although there are still some remaining barriers to commercialization—higher thermal stability, mechanical strength and cyclability are the current focus—this new class is an attractive potential replacement for current solid electrolytes.

The icosahedral CB11H12 (carba-closo-dodecaborate) anion is one of many poly- hedral boron-derived anions that are characterized by high chemical stability and rather weak chemical coordination, making them intriguing molecules for use in organometallic catalysis and as electrolyte anions in metal-ion battery technologies. For the latter, the incor- poration of such weakly coordinating anions into a solid-state electrolyte might also be expected to facilitate higher ionic conductivities due to lesser interactions with the surrounding, translationally diffusing cations within the interstitial sublattice network.

… The uniquely high cation translational mobility within these disordered compounds has been attributed to several possible factors. Here we use a combination of ab initio molecular dynamics (AIMD) and quasielastic neutron scattering (QENS) to more comprehensively address the influence of carbon incorporation and anion reorientations on cation conductivity in LiCB11H12 and NaCB11H12 salts.

—Dimitrievska et al.

Another key advantage to closo-borates is their inherent tunability. They can be readily alloyed, as well as structurally and chemically modified. In many cases, these changes can dramatically alter their behavior.

—LLNL postdoc Patrick Shea, who developed some of the analysis tools used in the study

The electrolyte material is a salt comprising positively charged lithium cations and negatively charged closo-borate anions. The research showed that the closo-borate anions reorient themselves rapidly, spinning around in the solid matrix as they alternate between specific preferred directions.

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Artist rendering of the solid electrolyte material, showing lithium atoms (purple) moving within a matrix of anions composed of boron (green), carbon (gray) and hydrogen (white) atoms. Image by Joel Varley/LLNL. Click to enlarge.

The addition of carbon to the closo-borate anion creates what’s known as a dipole, which repels lithium in the local vicinity of the carbon atom. As the anion spins, the carbon atom faces different directions, each time forcing lithium to move away to a nearby site in the solid matrix. Because the salt is full of spinning anions, this results in very rapid motion of lithium.

We propose that the broken symmetry of the anions introduced by C substitution has three primary effects: first, modification of the local static cation–anion interaction via introduction of an anion dipole; second, modification of the orientational preferences of the anions; and third, modification of the rotational dynamics of the anions on timescales accessible to diffusion. In each case, the C atom introduces a source of frustration that complicates the ability of the phase to order into a stable ordered structure, lowering the superionic transition temperature and enhancing cation diffusion. Our results point to the role of broken anion symmetry and atomic substitution as a more general strategy for enhancing thermal disorder and cation ionic conductivity in this emerging class of materials.

—Dimitrievska et al.

Now that we understand the beneficial consequences, we can start to think about how to introduce similar effects by chemical modification of the anion itself. We also can start to think about how structure and chemistry are interrelated, which may give clues into how structural modifications of the material could generate further improvements.

—Brandon Wood

It’s an early step toward developing a new class of robust solid electrolytes with ultra-high lithium ion mobility, offering an attractive alternative for current solid-state battery designs. The general design principle also may be useful for optimizing other solid electrolyte materials where molecular rotations play a role.

—Joel Varley, an LLNL materials scientist and co-author

Resources

  • M. Dimitrievska, P. Shea, K. E. Kweon, M. Bercx, J. B. Varley, W. S. Tang, A. V. Skripov, V. Stavila, T. J. Udovic, B. C. Wood, (2018) “Dynamics as Synergistic Drivers for Ultrafast Diffusion in Superionic LiCB11H12 and NaCB11H12Adv. Energy Mater. doi: 10.1002/aenm.201703422

Comments

SJC

Thank goodness for electro chemists.

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