Results and discussion

Chapter 2. Design of organic single-ion conductor based on relaxation dynamics

2.1. Ion relaxation dynamics of G-quadruplex for single-ion conduction

2.1.3. Results and discussion

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Figure 2-3. Self-assembly of the LiGQ. The ordered π stacks of G-quartets offer 1D conduction pathways for Li⁺ transport. For a clear representation, hydrocarbons and bithiophenes are blurred.

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Figure 2-4. Crystal structure of LiGQ. (a) Synchrotron WAXD patterns of monomer and LiGQs fabricated by 2 different methods. LiGQ fabricated by nonsolvent diffusion shows a hexagonal columnar ordering with the highest (001) peak intensity. (b) TEM image of the LiGQ showing clear π- π stacking distance. MAS (c) ¹H and (d) ⁷Li NMR spectrum of LiGQ. Ideally dissociated Li cations are stabilized by ion-dipole interaction with G-quartets.

23 Figure 2-5. FT-IR spectra of monomer, Li salt, and LiGQ.

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Table 2-1. Elemental analysis (C, H, N, S) results for LiGQ (C253H404F3LiN20O19S9)).

C (wt%) H (wt%) N (wt%) S (wt%)

calcd. 69.34 9.29 6.39 6.58

found 69.57 9.39 6.76 6.51

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Based on the structural understanding of the LiGQ described above, I explored its feasibility as a single-ion conductor for potential use in Li batteries. The cyclic voltammogram showed stable and reversible Li plating and stripping through the LiGQ (Figure 2-6a). The details of the sample preparation and measurement conditions are described in “Cyclic voltammetry (CV)” in the Experiment. I investigated chronoamperometry profiles of cells containing the LiGQ at a direct current polarization of 100 mV (Figure 2-6b). In a blocking symmetric cell (Sus|LiGQ|Sus), a steady-state current was ascribed to electronic leakage (𝑖_𝑒). Meanwhile, a steady-state current of a nonblocking symmetric cell (Li|LiGQ|Li) is the sum of electronic leakage (𝑖_𝑒) and Li⁺ conduction (𝑖_𝐿𝑖). By comparing the profiles of the blocking and non-blocking cells, the ionic transference number of the LiGQ (= 𝑖_𝐿𝑖/(𝑖_𝑒+ 𝑖_𝐿𝑖))³¹ was estimated to be 0.95. This result demonstrates that the LiGQ is ionically conductive and electronically insulating, which fulfills a requirement for a reliable ion conductor.

From a potentiostatic polarization analysis, the Li⁺ transference number of the LiGQ was estimated to be 0.86, exhibiting the predominant contribution of Li⁺ to the ion conductivity. The Arrhenius plot shows a proportional increase in the logarithmic ionic conductivity with temperature, yielding an activation energy (Ea) of 0.13 eV along with a room temperature ionic conductivity of 8 × 10⁻⁵ S cm⁻¹ (Figure 2-6c). This ion conduction behavior of the LiGQ was compared with those of previously reported single-ion conductors, such as polyanions, inorganic conductors, covalent organic frameworks, and metal organic frameworks (Table 2-2). Our particular attention is given to the substantially low Ea of the LiGQ, which represents facile ion conduction with short tortuosity and is a solid evidence for the directional ion conduction^15,32 based on Li⁺ slippage. Meanwhile, I synthesized a Na⁺-centered G-quadruplex (NaGQ) using the same technique used for the preparation of the LiGQ to explore the feasibility of the G-quadruplex as a versatile ion-conducting medium. The synthesized NaGQ showed a Hoogsteen hydrogen bond and hexagonal ordering similar to those of the LiGQ (Figure 2-7a and 2-7b). The Arrhenius plot of the NaGQ showed an ion conductivity of 4.3 × 10⁻⁶ S cm⁻¹ at room temperature and an Ea of 0.15 eV (Figure 2-7c), indicating the potential use of the G-quadruplex in the Na⁺ conductors similar to the Li⁺ conductors.

These unusual Li⁺ transport phenomena of the LiGQ were investigated in more detail. As a model study, I prepared a ⁶Li|LiGQ|⁶Li symmetric cell and monitored its galvanostatic Li plating and stripping behavior. The symmetric cell showed stable cycle performance over 360 h without unstable and irreversible voltage fluctuations (Figure 2-8a), demonstrating the reliable Li⁺ conduction through the LiGQ. The LiGQ was further characterized by MAS ⁷Li-/¹H-NMR and synchrotron WAXD. The gradual decrease in the singlet ⁷Li peak during the cycle test (Figure 2-8b) confirms the Li⁺ conduction through the LiGQ.³³ In addition, the Hoogsteen hydrogen bond and hexagonal columnar ordering of the LiGQ remained almost unchanged (Figure 2-8c and 2-8d), exhibiting its structural durability during the

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cycle test. Meanwhile, the Li metal electrode after the cycle test showed a clean and smooth surface without dendritic Li growth (Figure 2-8e). This result reveals that the single-ion conducting LiGQ enables the suppression of anion migration (causing unwanted interfacial side reactions with Li metals) and the uniform Li⁺ flux during the cycle test. This results in the homogeneous nucleation and growth of Li without random and irregular deposition which is commonly observed in typical liquid electrolytes that allow conduction of cations and anions.^34,35

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Figure 2-6. Bulk ion transport behaviors. (a) Cyclic voltammogram, showing reversible Li redox. (b) Chronoamperometry profiles of ion-blocking (black, Sus|LiGQ|Sus) and non-blocking (red, Li|LiGQ|Li) cell configurations. (c) Arrhenius plot for the ion conductivity, exhibiting Ea of 0.13 eV.

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Table 2-2. Comparison in ion conduction properties between the LiGQ (this study) and previously reported single Li⁺ conductors.

Ion conductors Ea (eV) tLi+

Conductivity

@ RT (S/cm) Reference

LiGQ 0.13 0.86 8.1×10^-5 This work

Polysulfonate

(Lithiated Nafion) 0.30 0.93 ~10^-6 J. Power Sources

2018, 382, 179-189.

Polysulfonylimide - 0.91 ~10^-8 Angew. Chem. Int. Ed.

2016, 55, 2521-2525.

Garnet

(Li7La3Zr2O12) 0.34 - 5.1×10^-4 Angew. Chem. Int. Ed.

2007, 46, 7778-7781.

Antiperovskite

(Li3OCl) 0.35 - 2.0×10^-4 Adv. Mater.

2016, 3, 1500359.

LGPS-type sulfide

(Li9.6P3S12) 0.24 - 1.2×10^-3 Nat. Energy

2016, 1, 16030.

COF

(Sulfonated COF) 0.18 0.9 2.7×10^-5 J. Am. Chem. Soc.

2019, 141, 5880-5885.

COF (ICOF-2, filled with PC)

0.24 0.8 3.1×10^-5 Angew. Chem. Int. Ed.

2016, 55, 1737.

MOF

(HKUST-1,

filled with LiClO4/PC)

0.18 0.65 3.8×10^-4 Adv. Mater.

2018, 30, 1707476.

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Figure 2-7. Structural and electrochemical characterizations NaGQ: (a) MAS ¹H NMR, (b) Synchrotron WAXD patterns, (c) Arrhenius plot. The NaGQ was synthesized using the same method used for the LiGQ. The NaGQ showed hexagonally ordered G-quadruplex assembly with ion conductivity of 4.3×10^-6 S/cm and Ea of 0.15 eV, demonstrating the validity of G-quadruplex as an ion conductor.

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Figure 2-8. Electrochemical characteristics of LiGQ: (a) Galvanostatic Li stripping/plating profile of

6Li symmetric cell. (b) MAS ⁷Li NMR spectra during the cycle test. The gradual decrease of singlet ⁷Li peak demonstrates that the voltage profile of the cycle test was yielded by Li⁺ transport through the only LiGQ. (c) Synchrotron WAXD and (d) MAS ¹H-NMR of LiGQ after the cycle test. The unchanged Hoogsteen hydrogen bond and hexagonal ordering of LiGQ exhibited structural durability of the LiGQ during the electrochemical test. (e) SEM image of Li anode after the cycle test, showing smooth surface without dendritic Li growth.

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Considering that the ion–dipole interaction is weaker than the ion–ion interaction,^36,37 I expect that the intermolecular interaction of Li⁺ (ion) with G-quartet (dipole) in the LiGQ could be weaker than those of traditional single-ion conductors bearing negatively charged moieties (Nafion with sulfonates, garnet with oxygen sublattices, and others), eventually facilitating the transport kinetics of Li⁺. According to the bond strength-coordination number fluctuation (BSCNF) model,^38,39 the mean residence time of an ion placed in a potential well is proportional to the product of ion binding energy and ion coordination number. I assume that Li⁺ in the LiGQ, upon exposure to an electric potential, may have a lower mean residence time than those of other single-ion conductors owing to its weak binding strength. To experimentally verify this BSCNF model-based ion conduction mechanism of the LiGQ, I conducted a local electrochemical impedance spectroscopy (LEIS) analysis. The local interfacial impedance (𝑍₀) is defined as follows:⁴⁰

𝑍0(𝜔) =𝑉(𝜔) − Φ₀(𝜔)

𝑖_𝑙𝑜𝑐(𝜔) ≈𝑉(𝜔) − Φ_ℎ(𝜔)

𝑖_𝑙𝑜𝑐(𝜔) =𝑉(𝜔) − Φ_ℎ(𝜔) Δ𝑉_{𝑝𝑟𝑜𝑏𝑒}(𝜔)

𝑑 𝜎

where 𝑉(𝜔) − Φ₀(𝜔) is the potential difference between electrode and electrolyte measured at the limit of the diffusion layer, 𝑉(𝜔) − Φ_ℎ(𝜔) is potential difference between electrode and the bi- electrode, located at a distance of h from the electrode, and 𝑖_𝑙𝑜𝑐(𝜔) is the local AC-current density defined by potential difference between bi-electrode (Δ𝑉_{𝑝𝑟𝑜𝑏𝑒}(𝜔)), electrolyte conductivity (𝜎), and bi- electrode distance (𝑑). The LEIS analysis involves the potential difference between the electrode surface and the electrolyte measured at the limit of the diffusion layer, theoretically (Figure 2-9a). Thus, the local interfacial impedance is strongly affected by the bulk electrode resistance, electrode/electrolyte interfacial resistance, and IR drop in the diffusion layer. To clear investigate the local interfacial ionic responses of two different solid electrolytes having different carrier density and mobility, 1) bulk ionic conductance is controlled to exclude a perturbations in fabrication and 2) salt-free solvent was selected as an electrolyte for LEIS measurement and assume the electrolyte conductivity is unity to minimize an ionic perturbation in the diffusion layer. As a control sample, a lithiated Nafion (Li-Nafion), which shows single Li⁺ transport behavior based on the ion-ion interaction, was prepared using chemical lithiation.⁴¹ The conductance of the LiGQ, which was calculated from its ion conductivity and thickness, was 3.2×10⁸ Ω. This information reveals that the thickness of 7.8 μm is needed for the Li-Nafion to synchronize its conductance with that of the LiGQ. To obtain such a thin Li-Nafion specimen, the Li- Nafion cut by microtome, precisely. The LEIS Bode plot (Figure 2-9b) of the LiGQ showed a significantly higher peak frequency (3 × 10⁴ Hz) than that of the lithiated Nafion (4 × 10² Hz) chosen as a control sample. Considering that the characteristic frequency (𝑓) of the ion migration is given by the reciprocal of ion relaxation time (𝜏, 𝜏⁻¹= 2𝜋𝑓),^42,43 the LEIS result shows that the Li⁺ conduction through the LiGQ is driven by the faster Li⁺ relaxation, which is consistent with the lower binding

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energy of Li⁺ in the LiGQ discussed in the theoretical investigation. This result confirms the viable role of the ion–dipole interaction in the directional Li⁺ slippage-driven fast ion transport of the LiGQ.

Figure 2-9. Facile local ion relaxation of LiGQ. (a) Schematic illustration showing the conductance- controlled LEIS experiment. (b) LEIS bode plots of LiGQ and Li-Nafion. The profiles were repeatedly measured ten times to ensure reliability and depicted in averaged values with error bars. The vertical dotted line is characteristic frequency for ion relaxation time.

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