Organic Electrode Materials

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Organic electrode materials are built from abundant elements such as carbon, hydrogen, nitrogen, oxygen and sulfur, and offer a sustainable alternative to the transition-metal oxides used in today’s batteries. Their molecules can also be tailored almost at will. In practice, however, most organic electrodes work at low voltages, conduct electrons poorly, and crystallize in structures that are hard to resolve, because the light atoms that matter most, lithium and hydrogen, scatter X-rays only weakly. We combine first-principles calculations with experiments by our collaborators to understand each of these limitations at the molecular level.

To raise the voltage, we have identified two molecular features that control it: the change in aromaticity when a molecule is reduced, and the number of oxygen atoms coordinating each lithium ion. Together they explain why one cathode, dithiin-fused naphthazarin, reaches a first reduction potential of about 3.7 V against Li⁺/Li, some 0.6 V above a close analogue, and point to design rules for new high-voltage cathodes.^[1] To find where the ions sit, we have developed algorithms that locate ion insertion sites in organic crystals from the charge density and electrostatic potential computed from first principles, inserting all ions at once. They reproduce the known sites in 16 organic materials and reveal low-energy sites previously overlooked in the electrode material Li₄NDC.^[2] On conductivity, we helped show that the long-range order of hydrogen bonds in a layered organic cathode material, set by how it is synthesized, is associated with delocalized radicals and a conductivity two orders of magnitude higher than that of a less ordered analogue.^[3]

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