Ion slippage through Li+-centered G-quadruplex

Abstract

Single-ion conductors have garnered attention in energy storage systems as a promising alternative to currently widespread electrolytes that allow migration of cations and anions. However, ion transport phenomena of most single-ion conductors are affected by strong ion (e.g., Li⁺)-ion (immobilized anionic domains) interactions and tortuous paths, which pose an obstacle to achieving performance breakthroughs. Here, we present a Li⁺-centered G-quadruplex (LiGQ) as a class of single-ion conductor based on directional Li⁺ slippage at the microscopic level. A guanine derivative with liquid crystalline moieties is self-assembled to form a hexagonal ordered columnar structure in the LiGQ, thereby yielding one-dimensional central channels that provide weak ion-dipole interaction and straightforward ionic pathways. The LiGQ exhibits weak Li⁺ binding energy and low activation energy for ion conduction, verifying its viability as a new electrolyte design.

Fig. 1.. LiGQ.

(A) Schematic comparison of ion transport phenomena in various ion conductors: traditional conductors versus an ideal conductor. (B) Theoretically simulated chemical structure of the single-strand LiGQ and its self-assembly procedure. The dotted line represents the ion-dipole interaction between Li⁺ and G-quartet. The close stacking of the G-quartets in the vertical direction leads to the formation of 1D central channels allowing straightforward Li⁺ conduction pathways in the LiGQ. To represent the LiGQ clearly, its hydrocarbons and bithiophenes are blurred.

Fig. 2.. Structural characterizations of the LiGQ.

(A) Synchrotron WAXD patterns of the LiGQs obtained by two different fabrication methods (nonsolvent diffusion and solution casting) and their monomer. The self-assembled LiGQ fabricated by the nonsolvent diffusion method shows a hexagonal columnar ordering with the distinct (001) peak. (B) TEM image of the LiGQ. The crystal lattice shows a π-π stacking distance of ~3.4 Å. Magic angle spinning (MAS) (C) ¹H and (D) ⁷Li nuclear magnetic resonance (NMR) spectra of the LiGQ. A characteristic ¹H NMR peak at 12 parts per million (ppm) exhibits the Hoogsteen hydrogen bond of G-quartet assembly. The deshielded singlet ⁷Li NMR peak at −0.044 ppm reveals the isolation of Li⁺ from its counter anion. (E) Contour plot of Li⁺ number density of the LiGQ under an electric field. Red bar indicates the 2D number density of Li⁺ projected to the yz plane.

Fig. 3.. Electrochemical characteristics of the LiGQ.

(A) Cyclic voltammogram of the SUS|LiGQ|Li asymmetric cell at a sweep rate of 1 mV s⁻¹ in a voltage range of −0.5 to 3.0 V (versus Li/Li⁺), showing the stable and reversible Li plating and stripping through the LiGQ. (B) Arrhenius plot for the ion conductivity, yielding an E_a of 0.13 eV. (C) Comparison in the conductivities (ion versus electron) of the LiGQ, yielding an ion transference number of 0.99997. (D) Galvanostatic Li stripping and plating profile of the ⁶Li|LiGQ|⁶Li symmetric cell under a current density of 5 μA cm⁻² for 5 min per cycle at room temperature. (E) Ex situ MAS ⁷Li NMR spectra during the cycle test. The gradual decrease in the singlet peak intensity with the cycling time verified the Li⁺ conduction through the LiGQ. The spectra of Li salt (short-dashed line) and solvated Li⁺ (FEC was chosen as a solvent, long-dashed line) were provided as a reference.

Fig. 4.. Theoretical and experimental elucidations of the Li⁺ slippage phenomena in the LiGQ.

(A) Schematic representation of the chemical structures and Li⁺ binding states of the LiGQ and conventional single-ion conductors. (B) PDOS of the ion-conducting moieties and Li⁺. Colored and uncolored areas represent PDOS of ion conducting moieties and Li⁺, respectively. (C) Li⁺ binding energy (expressed as E_MM, E_ele, and E_vdW) calculated using AMBER force field. The binding energy is normalized by the number of oxygen atoms coordinating with Li⁺. (D) Schematic illustration showing the LEIS experiment. LEIS Bode plots of (E) LiGQ and (F) Li-Nafion. The profiles were measured 10 times to ensure reliability and are depicted in averaged values with error bars. The vertical dotted line represents the characteristic frequency for ion relaxation time.