III. Quinone derivatives as bifunctional SODm
3.1.2 Results and discussion
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The electrochemical fingerprints of two different isomers of the benzoquinone derivative (DB-p-BQ and DB-o-BQ) were identified voltammetrically (Figure 3.2b to d). Two reversible reactions were found in both isomers: the first electron transfer for Q/Q- (Q = quinone) followed by the second electron transfer for Q-/Q2- along cathodic scan. The isomeric configuration change from para to ortho shifted the reduction potentials of both reactions positively from 2.63 VLi to 2.94 VLi for the first electron transfer reaction Q/Q- (Figure 3.2b). From the para’s reduction potential at 2.63 VLi more negative than Eo(O2/LiO2) at 2.74 VLi, therefore, it is expected that the Q/Q- electrochemistry of the para form was appropriate for the DRM function to facilitate electron transfer between electrode and molecular oxygen for accelerating superoxide formation (out of the yellow SODm region in Figure 3.2b). More interestingly, the positive potential shift of Q/Q- beyond Eo(O2/LiO2) by the isomeric transformation encouraged the Q/Q- electrochemistry to be responsible for mediating electrons between superoxides or disproportionating superoxides (the SODm function) rather than the DRM function (within the yellow region in Figure 3.2b).
In the presence of oxygen, as expected, the Q/Q- peak of the para at 2.63 VLi was amplified since Q was regenerated from Q- by reducing oxygen to superoxide, which confirmed the DRM function (Figure 3.2c).7-11 On the other hand, the ortho in the presence of oxygen disallowed the Q/Q- peak intensity at 2.94 VLi changed at all while the second peak for Q-/Q2- was amplified instead of the first peak (Figure 3.2d). The unchanged Q/Q- peak within the SODm region reflected the chemical (i.e., non- electrochemical) nature of the disproportionation reaction where electrons from electrodes were not involved. Q is regenerated by another superoxide molecule after Q is reduced to Q- by a superoxide molecule so that the concentration of the ortho determining current is invariant. The second reduction process for Q-/Q2- of the ortho in the presence of oxygen played the same DRM role as the first reduction process for Q/Q- of the para.25 Their peak potentials were almost identical around 2.5 VLi out of the SODm region, showing the current increase in the presence of oxygen with respect to argon.
The anodic peaks for Q-/Q2- and Q-/Q at 2.37 VLi and 2.68 VLi for the para and 2.72 VLi and 3.01 VLi
for the ortho in the absence of oxygen were not observed up to 3.0 VLi in the presence of oxygen (Figure 3.2c and d). The electrodes were passivated by non-conductive ORR products including Li2O2 generated during the previous cathodic scan. The passivated surface did not allow the contact between the redox- active species and electrodes. As the electrode was exposed to electrolyte by oxidative decomposition of the deposited ORR products during the further anodic scan above 3 VLi, however, the para as well as the ortho was oxidized to increase the anodic currents in the presence of oxygen when compared to the oxygen-absent situation.
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Figure 3.2 Electrochemical fingerprints in cyclic voltammograms (CVs) at 100 mV s-1. 0.1 M LiTFSI in TEGDME (tetraethylene glycol dimethyl ether) was used as the electrolyte unless indicated otherwise.
10 mM additives were used. (a) 0.1 M LiTFSI in DMSO in the presence of oxygen. The potential turning point of the cathodic scan was changed. (b) DB-o-BQ and DB-p-BQ without oxygen. (c) DB- p-BQ. (d) DB-o-BQ. (e) Potential dependency of functionality.
86 LOB cell performance
From the voltammetric fingerprints, therefore, we concluded that the ortho has the SODm/DRM bifunctionality based on Q/Q- and Q-/Q2- electrochemistry while the para has the DRM mono- functionality based on Q/Q- (Figure 3.2e). The additional SODm functionality of the ortho was expected to be possibly beneficial to the durability of LOBs by converting the reactive superoxide species to harmless products in a facile manner as well as the discharge capacity by facilitating the solution pathway over the surface one. However, the ortho was significantly inferior to the para in terms of discharge capacities when the quinone-based additives were compared in LOB cells (Figure 3.3a): e.g., discharge capacity at 0.2 mA cm-2 = 3.5 mAh cm-2 for the para > 1.8 mAh cm-2 for the ortho > 1.0 mAh cm-2 for the additive-free. No capacities realized in the absence of oxygen (Ar in Figure S3.2) confirmed that the oxygen-related reactions were responsible for the capacities. On the other hand, the charge-to- discharge efficiency of the ortho was higher than that of the para (Figure S3.3).
The slower DRM kinetics of the ortho over the para, indicated by the smaller discharge capacities at a fixed rate (Figure 3.3a), was considered to come from their anionic valency difference. Di-valency (Q2-) is required to reduce molecular oxygen to superoxide in the presence of the ortho while mono-valency (Q-) is enough for the DRM function of the para. Two electron (2e) transfer for Q/Q2- is bound to be kinetically slower than 1e transfer for Q/Q-. Before going to Q2-, also, the monovalent species (Q-) of the ortho return back to Q0 since it dumps its electron to superoxide for SODR. Moreover, more negative driving force is required for Q-/Q2- of the ortho (Eo = 2.58 VLi) than Q/Q- of the para (Eo = 2.63 VLi).
For those reasons, the Q2- of the ortho was produced less than the Q- of the para so that the ortho was not superior to the para in terms of the DRM function.
As expected from the SODm functionality of the ortho, on the other hand, the ortho was better than the para in terms of durability even if the improvement by the quinone additives were not very significant (Figure 3.3b and S3.4). It should be notified that the charging process to decompose the discharge product grows to be more important than the discharging process to produce solid deposits on electrodes as the discharge capacity increases. The superiority of the ortho to the para would be recognized only when the discharging process determines cell durability without problems relevant to charging process.
Therefore, we employed (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) as a CRM16 in the presence of the quinone additives in LOB cells. Cyclic voltammograms (Figure 3.3c) showed insignificant changes in the Q/Q-/Q2- electrochemistry of the para and the ortho in the presence of TEMPO while TEMPO showed its reversible redox activity at Eo = ~3.8 VLi in the CRM region (> 3.17 VLi in Figure 3.1). The TEMPO improved durable rechargeability of the LOB cells having the quinone additives, more importantly, amplifying the difference between the ortho and the para. The cyclability of the ortho-present cell increased from 14 cycles to 85 cycles by employing the CRM when it was discharged
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and charged repeatedly at 0.2 mA cm-2 up to 0.2 mAh cm-2 (Figure 3.3b and S3.4). On the other hand, the para-installing cell was rechargeable only for 30 cycles. The benefit of the SODm functionality of the ortho to convert the reactive oxygen species (superoxide) to less harmful oxygen species (oxygen and peroxide) was clearly confirmed in the presence of the CRM.
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Figure 3.3 LOB cells. 1 M LiTFSI in TEGDME was used as the electrolyte with the P50 carbon electrode as the air cathode. (a) Potential profiles. Cells were fully discharged at 0.2 and 0.5 mA cm-2 with 30 mM additives. (b) Capacity retention along cycle. Cells were discharged and charged repeatedly at 0.2 mA cm-2 for 0.2 mAh cm-2 with 15 mM TEMPO + 15 mM DB-o-BQ or DB-p-BQ. (c) CVs of the quinone-based additives in the presence of TEMPO as a CRM at 100 mV s-1. 10 mM TEMPO + 10 mM DB-o-BQ or DB-p-BQ were used in 0.1 M LiTFSI in TEGDME. The electrolytes were purged by argon with gold disk as the working electrode, platinum foil as the counter and lithium metal as the reference.
89 Discharge products on air cathode
The SODm functionality of the ortho was confirmed by X-ray photoelectron spectra (XPS in Figure 3.4a). The ortho was expected to disproportionate superoxide molecules to less reactive molecules incapable of triggering carbon corrosion. There were less amount of side products found on the air cathode sampled from the cell having the ortho when compared with other cells having no additives and the para. The carbonate biproducts were found on the air cathodes of the cells in the absence of additives and in the presence of the para, which were confirmed by the peaks at 532.1 eV in O1s, 290 eV in C1s and 55.2 eV in Li1s spectra. Lithium peroxide was clearly dominant when the ortho was employed.
The DRM functionality improves the superoxide production rate (r+) while the SODm functionality facilitates the superoxide decomposition rate (r-). The net rate of superoxide between production and consumption (r+ - r-) determined the discharge product morphology (scanning-electron-microscopic (SEM) images in Figure 3.4b). In the absence of DRM and SODm (r+ > r-), the surface mechanism was dominant in the earlier stage of Li2O2 deposition to form a dense peroxide film (Figure S3.5).21 As superoxide is accumulated in electrolyte, then, toroidal Li2O2 deposits in 100 nm, characteristic of the solution mechanism, were grown from the layer of particulate deposits in 10 to 20 nm on the carbon electrode (No additives in Figure 3.4b). The crystal nature of the toroids was confirmed by X-ray diffraction (XRD in Figure S3.6a) while Raman signal of Li2O2 was not found due to its dimension less than 514 nm, the laser wavelength used in Raman spectroscopy (Figure S3.6b). The positive net rate of superoxide brought the surface-to-solution mechanism transition leading to the morphological evolution from the dense film to the particulate aggregates to the big toroids.
When the para was employed as the DRM to increase the superoxide production rate (r+ >> r-), the solution mechanism fully governed the Li2O2 formation process during discharge to deposit bigger toroids in 200 nm (DB-p-BQ in Figure 3.4b). Superoxide ions (O2-) over a critical concentration, produced in a faster manner by the DRM, were allowed to stay in electrolyte long enough to form toroidal crystals on the surface of electrode.11 The crystal nature of the toroids was confirmed by X-ray diffraction (XRD in Figure S3.6a). Also, Li2O2 Raman signals were clearly observed to confirm its large dimension in several hundreds of nm (Figure S3.6b).
In the presence of the ortho, on the other hand, the Li2O2 deposit formed the layer having tiny particles in several nanometer feature size agglomerated (DB-o-BQ in Figure 3.4b and Figure S3.7). Even if XPS spectra confirmed that the particulate deposits were Li2O2 (Figure 3.4a), the amorphous nature of the discharge product and its tiny size disallowed the XRD and Raman spectra to show Li2O2-characteristic peaks (Figure S3.6). The ortho boosts up the SODR rate by its SODm functionality while its superoxide
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production rate by the DRM functionality was slower than the para’s (r+ < r-). Too fast peroxide formation was expected to disallow the thermodynamically preferred Li2O2 toroids formed, encouraging tiny particles to be agglomerated. Such a morphology was found in another SODm, carboxylated TEMPO.21 The film-like particulate agglomerate provides a good contact with electrodes so that lithium peroxide was more easily decomposed during charging to improve the LOB rechargeability after deep discharge (Figure S3.3).
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Figure 3.4 Discharge product on air cathodes after being discharged. (a) XPS spectra after the 30th discharging. (b) SEM images after the first discharging (H23 gas diffusion layer).
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