Results and discussion

Chapter 3. Design of organic single-ion conductor based on electrophoretic

3.1. Electrophoretic regulation of polycations for artificial solid-

3.1.3. Results and discussion

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Figure 3-1. Schematic illustration depicting the direct printing-driven fabrication procedure of the pSEI-Li, along with its chemical structure.

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Figure 3-2. Viscosity (as a function of shear rate) of the pSEI paste at room temperature.

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Figure 3-3. Structural characterizations of the pSEI. (a) A photograph of the pSEI-Li. Inset is a cross- sectional SEM image, showing the uniform deposition of pSEI (~ 1 μm) on the thin Li metal electrode (~ 25 μm). (b) FT-IR spectra of the pSEI and its components (ETPTA and DADMA-TFSI). (c) SEM and EDS elemental mapping images (O, N and S atoms) of the pSEI. (d) FT-IR spectra of acrylic C=C bonds of the pSEI, before and after UV irradiation.

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The ion conduction behavior of the pSEI was investigated as a function of its composition ratio (i.e., ETPTA/DADMA-TFSI). As a model study, a free-standing pSEI film was prepared. Its ionic conductivity and tLi+ were measured after swelling with a liquid electrolyte (1 M LiPF6 in EC/DEC = 1/1 (v/v)). Both ionic conductivity and tLi+ tended to increase with the DADMA-TFSI content (Figure 3-4a). Further increase of DADMA-TFSI content at more than 70 wt% failed to fabricate a self-standing pSEI film (inset of Figures 3-4a), demonstrating a viable role of ETPTA as a mechanical framework of the pSEI. At a composition ratio of ETPTA/DADMA-TFSI = 30/70 (w/w), the pSEI showed the highest ionic conductivity (1.210^-3 S cm^-1) and tLi+ (0.69). In addition, a high mechanical modulus (5.23 GPa, Figure 3-4b), which is a sufficiently high value to prevent the growth of Li dendrites,¹⁸ was observed at the pSEI. These results indicate that ETPTA acts as a mechanical framework and DADMA-TFSI makes preferential transport of Li ions possible. To better understand the high tLi+ value, FT-IR analysis of the pSEI was conducted. An appreciable shift of the characteristic peak assigned to the P-F vibration of the PF6- anions was observed at the pSEI (Figure 3-4c). This result indicates that the PF6- anions could be electrostatically trapped¹⁹ by the positively charged ammonium groups of the DADMA, indicating the electrophoresis-driven ion-rectifying effect.^20,21 Future studies will be devoted to further investigating the effects of the ionic medium, unit charges, and the dimension of ion channels on tLi+.

Such unusual ion transport phenomena in the pSEI were examined in more detail. As a model study, a pSEI (ETPTA/DADMA-TFSI = 30/70 (w/w)) was fabricated on a Cu foil. Then, the pSEI- deposited Cu (pSEI-Cu) was subjected to Li plating/stripping. The pSEI made reversible Li plating/stripping behavior possible (Figure 3-5a) without structural disruption (Figure 3-5b).

Meanwhile, fast/homogeneous Li-ion flow toward Li metals is needed to ensure uniform electrodeposition.^2,5,11 Unfortunately, conventional electrochemical characterization techniques are not adequate for identifying localized ion flow to Li metal electrodes because of their averaged electrochemical response.²² I suggested a new analysis technique based on local electrochemical impedance spectroscopy (LEIS²²) to elucidate the localized ionic topology and resistance. The pSEI- Cu showed uniform ionic topology with a low local resistance (~ 97 Ω cm²) after five Li plating/stripping cycles (Figure 3-5c), whereas the bare Cu showed an inhomogeneous ionic topology with random and high local resistance resulting from side reactions between the plated Li and liquid electrolytes (Figure 3-5d). This result demonstrates that the pSEI acts as an ion-rectifying protective layer and, thus, enables facile/uniform Li-ion flow toward Li metals, as illustrated in Scheme 1. In addition, a Li/Li symmetric cell test was conducted with pSEI-Li (Figure 3-5e). The stable Li plating/stripping behavior was observed for longer than 450 h, in comparison with a bare Li, which showed severe and irreversible voltage fluctuations.

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Figure 3-4. Structural characterizations of the pSEI. (a) Ionic conductivity and Li⁺ transference number (tLi+)) of the pSEI film swelled with a liquid electrolyte (1 M LiPF6 in EC/DEC = 1/1 (v/v)) as a function of its composition ratio. Inset is a photograph of the uncured SEI containing 70 wt% DADMA-TFSI.

(b) A plot of mechanical modulus of the pSEI (ETPTA/DADMA-TFSI = 30/70 (w/w)) as a function of penetration depth during the nanoindentation test. (c) FT-IR spectra of the pSEI and ETPTA films, in which both films were swelled with a liquid electrolyte (1 M LiPF6 in EC/DEC = 1/1 (v/v)).

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Figure 3-5. Li plating/stripping behavior through the pSEI. (a) CV profiles of the asymmetric cell (pSEI-Cu/Li) under a sweep rate of 10 mV s^-1 in a voltage range of -0.5 – 3.0 V (vs. Li/Li⁺). (b) Cross- sectional SEM images of the pSEI-Cu: (left) after Li plating and (right) after Li stripping. LEIS area scan of (c) pSEI-Cu and (d) bare Cu foil after 5 Li plating/stripping cycles (at current density of 1.0/1.0 mAh cm^-2). (e) Galvanostatic Li plating/stripping profile of the pSEI-Li/pSEI-Li symmetric cell (vs.

Bare Li) at a current density of 0.5 mA cm^-2 and capacity of 1.0 mAh cm^-2.

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The effect of the pSEI on the anti-oxidation stability of Li metals was systematically investigated. In general, Li metals are easily corroded by moisture and organic solvents, which becomes more vigorous in thin Li metals with a high surface area to volume ratio, which are preferred for high- energy-density Li metal batteries. Upon contact with a water droplet, the bare Li was severely oxidized and suffered from unwanted gas evolution, thereby failing to provide a definite value for the water contact angle (Figure 3-6a). In contrast, the pSEI exhibited a water contact angle of 65° (Figure 3-6b), demonstrating its water-repelling capability. Meanwhile, the water contact angles of the pSEI films were measured with different composition ratios of ETPTA/DADMA-TFSI (Figure 3-6b). The water contact angle tended to decrease with increasing DADMA-TFSI content, indicating that the hydrophobicity of the pSEI predominantly originated from the ETPTA.

The surface tarnish test of the pSEI-Li was conducted as a function of exposure time under humid environments. At 20% relative humidity (RH) and room temperature, the surface tarnish of pSEI- Li was remarkably delayed compared with that of the bare Li (Figure 3-7a). The pSEI-Li maintained its superior tarnish stability, even under a harsh environment (e.g., 50% RH) (Figure 3-7b). To better understand this beneficial effect of the pSEI, the Li metal surface was analyzed before and after exposure to humid air (50% RH) for 120 s. The Raman spectra (Figure 3-7c) showed that two strong peaks at 657 and 1060 cm^-1, which were assigned to LiOH and Li2CO3,²³ respectively, were detected for the bare Li after the tarnish test. By comparison, the pSEI-Li showed a negligible increase in the intensity of these characteristic peaks, confirming its strong moisture tolerance. As further evidence for this improvement, the change in the bulk structure of pSEI-Li and bare Li was examined. The thickness and overall structure of pSEI-Li remained almost unchanged (Figure 3-7d). In sharp contrast, the bare Li showed the formation of randomly distributed micro-sized pores/cracks (contaminated by oxidized byproducts²⁴) in the through-thickness direction (Figure 3-7e), resulting in a significant increase in the thickness by 160% (~ 40 μm).

The electrochemical activity of the pSEI-Li after exposure to humid air (50% RH) was investigated. Galvanostatic Li extraction of the pSEI-Li toward a Cu foil was analyzed as a function of exposure time. Figure 3-8a shows that the capacity retention of the bare Li after exposure for 120 s was only 1.3% with a large overpotential of 430 mV, revealing the serious loss of electrochemical activity.

In comparison, the pSEI-Li maintained the extraction capacity after exposure for 120 s, along with a negligible increase in the overpotential (Figure 3-8b). This result was further verified by observing the cross-sectional structure of bare Li and pSEI-Li after galvanostatic Li extraction. For the bare Li, a substantial amount of oxidized Li still remained on the Cu foil (Figure 3-8c). By sharp contrast, only the pSEI on the Cu foil was observed at the pSEI-Li (inset of Figure 3-8c), demonstrating that the extraction of Li was almost complete.

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Figure 3-6. Water contact angle of (a) bare Li and (b) the self-standing pSEI films as a function of DADMA-TFSI content.

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Figure 3-7. Anti-oxidative characteristics of the pSEI-Li under humid conditions. Surface tarnish tests of the pSEI-Li and bare Li as a function of exposure time at (a) 20% and (b) 50% R.H. in room temperature. (c) Raman spectra of the pSEI-Li and bare Li before/after exposure to humid air (50%

R.H.) for 120 s. Cross-sectional SEM and EDS elemental mapping images of d) pSEI-Li and e) bare Li before/after exposure to humid air (50% R.H.) for 120 s.

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Figure 3-8. Electrochemical activity of the pSEI-Li after exposure to humid air (50% R.H.). Voltage profiles upon galvanostatic Li extraction to a Cu foil as a function of exposure time: (a) bare Li and (b) pSEI-Li. (c) Cross-sectional SEM image of the moisture (50% R.H. for 120 s)-exposed pSEI-Li after the galvanostatic Li extraction. Inset is a result of the exposed bare Li.

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Application of the moisture-exposed pSEI-Li as an anode for Li metal full cells was explored, with a focus on its anti-oxidation stability. As a proof-of-concept, Li metal anodes were assembled with LCO cathodes (areal capacity = 1.2 mAh cm^-2; the details are described in the Experimental section) and conventional carbonate liquid electrolyte (1 M LiPF6 in EC/DEC = 1/1 (v/v) without any additives).

After exposure to humid air (50% RH) for 120 s, the pSEI-Li still exhibited stable charge/discharge behavior comparable to that of the unexposed bare Li (Figure 3-9a), in contrast to the bare Li, which showed a serious capacity loss and large overpotential resulting from the formation of oxide byproducts, as shown in Figure 3-8. The discharge capacity of the exposed pSEI-Li was ~ 99.7% relative to that of unexposed bare Li. This electrochemical viability of the pSEI-Li, in combination with its structural stability (in particular, dimensional tolerance), is expected to benefit the volumetric energy densities of Li metal full cells. Under the same thickness (25 μm) of initial Li metals, the volumetric energy densities of the full cells containing the exposed pSEI-Li (and the exposed bare Li) were 99.7% and 57.0%, respectively, relative to that of the unexposed bare Li (Figure 3-9b). The lower value of exposed bare Li was attributed to the increase in thickness and electrochemical inertness of the oxidized Li. This result verifies that strict attention to the storage conditions of Li metals is needed, and the pSEI can play a viable role in the development of practical Li metal full cells. The advantageous role of the pSEI was further verified by GITT analysis. Figure 3-9c shows that the rise in the cell polarization upon the repeated current stimuli was remarkably suppressed for the pSEI-Li. The exposed pSEI-Li showed higher (cell volume-based) capacity retention with cycling than the exposed bare Li (Figures 3-9d).

Further improvement in the cyclability of the full cells will be conducted in our future studies through in-depth consideration of other cell components (e.g., liquid electrolytes and cathodes) at the same time.

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Figure 3-9. Application of the moisture-exposed pSEI-Li to Li metal full cells. (a) Charge/discharge voltage profiles at 1st cycle (under charge/discharge current density of 0.2 C/0.5 C), in which the Li metal anodes were assembled with LCO cathodes and conventional carbonate liquid electrolyte (1 M LiPF6 in EC/DEC = 1/1 (v/v) without any additives). (b) Comparison in the relative volumetric energy density of the full cells. The volumetric energy density was estimated based on the total volume of the Li metal anode, LCO cathode and separator membrane. (c) GITT profiles upon repeated current stimuli (at a current density of 78.5 mA g^-1 and interruption time between each pulse of 1 h). (d) Cycling performance under charge/discharge current density of 0.2 C/0.5 C, in which the capacity was expressed based on cell volume.

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