Researchers led by a team at Temple University have developed a soft solid electrolyte—(Adpn)₂LiPF₆ (Adpn, adiponitrile)—that exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. A paper on their work is published in Nature Materials.

Adiponitrile is an organic compound that is an important precursor to the polymer nylon 66.

The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. The material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries.

Crystal structure of soft solid co-crystalline (Adpn)₂LiPF₆ electrolyte a, Representation of the basic structural unit of (Adpn)₂LiPF₆ showing tetra-coordinated Li⁺ ions with four Adpn molecules, each shared with a second symmetry-equivalent Li atom; PF6⁻ anions occupy the available interstitial pocket in the crystal structure. Grey, C; yellow, Li; blue, N; red, P; orange, F.; colored box shows the dimensions of unit cell; red, a-crystallographic direction; blue, c-crystallographic direction. b, Packing diagram of (Adpn)₂LiPF₆ showing the channels of Li⁺ ions in the low-affinity matrix in the crystal structure. Prakash et al.

A substantially high ion conductivity (~10⁻⁴ S cm^–1) and lithium-ion transference number (0.54) are obtained due to weak interactions between ‘hard’ (charge dense) Li⁺ ions and the ‘soft’ (electronically polarizable) –C≡N group of Adpn.

Molecular simulations predict that Li⁺ ions migrate at the co-crystal grain boundaries with a (preferentially) lower activation energy E_a and within the interstitial regions between the co-crystals with higher E_a values, where the bulk conductivity is a smaller but extant contribution.

These co-crystals establish a special concept of crystal design to increase the thermal stability of LiPF₆ by separating ions in the Adpn solvent matrix, and also exhibit a unique mechanism of ion conduction via low-resistance grain boundaries, which contrasts with ceramics or gel electrolytes.

Resources

• Prakash, P., Fall, B., Aguirre, J. et al. (2023) “A soft co-crystalline solid electrolyte for lithium-ion batteries.” Nat. Mater. 22, 627–635 doi: 10.1038/s41563-023-01508-1