II. TEMPO derivatives as bifunctional SODm
2.1.2 Results and discussion
SODm/CRM bifunctionality of TEMPO-COOH
TEMPO-COOH is a functionalized version of TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl). TEMPO was previously reported as a CRM25 while TEMPO-COOH was found in a SODm list for aqueous systems27. The expected bifunctionality of TEMPO-COOH was based on the following reactions (TC stands for TEMPO-COOH):
SODm (discharge): O2•- + O2•- + TC → O2 + O22- + TC (1) CRM (charge): TC → TC+ + e- (2a) 2TC+ + Li2O2 → 2TC + 2Li+ + O2 (2b).
The reversible electrochemistry of TC+/0 required for its CRM functionality (equation 2) was clearly shown in cyclic voltammograms (Figure 1.2a). TEMPO-COOH (or TC+/TC) had its reduction potential Eo(TC+/TC) at 3.79 V vs. Li+/Li with the peak-to-peak potential gap ΔEp = 106 mV in 1 M LiTFSI in TEGDME, inheriting the quasi-reversible electrochemistry of TEMPO+/TEMPO (or T+/T; Eo(T+/T) = 3.72 V vs. Li+/Li; ΔEp = 123 mV). The shape and location of the cathodic and anodic peaks were identical between Ar and O2 environments. The value of Eo(TC+/TC) satisfied the potential requirements of CRMs. It was estimated to be more positive than the thermodynamic reduction potential of O2/Li2O2 (2.96 V vs. Li+/Li) so that TC+ can oxidize Li2O2 to O2 (equation 2b). Also, the Eo(TC+/TC) was more negative than the potential required for Li2O2 decomposition in the absence of TEMPO- COOH (4.25 V vs Li+/Li in this work) so that we could obtain a charge overpotential gain by using TEMPO-COOH.
The oxidative decomposition of Li2O2 by TC+ (equation 2b) was visually confirmed when the air cathode of the LOB cell having lithium peroxide deposit were immersed into the solution of the electrochemically pre-oxidized TC (therefore, TC+) (Figure 1.3a). By the help of electrochemically generated TC+, the solid deposit (Li2O2 in the upper left of Figure 1.3b) was chemically decomposed completely so that such a clean surface of the air cathode was exposed (the upper right of Figure 1.3b;
similar to the surface of the air cathode after charging in the lower left). This peroxide decomposition was clearly the CRM-functional heritage from pre-oxidized TEMPO (or T+ in Figure 1.3c). In the presence of the reduced form of TC (not TC+ but TC0), however, the Li2O2 discharge product on the air cathode was not decomposed (the lower right of Figure 1.3b) like in T0. Any chemical change of TC0 was not detected spectroscopically in the presence of Li2O2 (ultraviolet-visible spectroscopy in Figure S1.1).
14
The SODm functionality of TEMPO-COOH, which TEMPO did not have, was ensured by confirming the chemical disproportionation reaction of O2- to O22- and O2 in the presence of TEMPO-COOH without applied potentials. Oxygen was evolved from the dimethyl sulfoxide (DMSO) solution of potassium superoxide (KO2) in the presence of TEMPO-COOH (TC0 in Figure 1.3d). When the TEMPO-COOH was replaced by TEMPO (T0), however, there was no signal of oxygen detected by the mass spectrometer. The SODm functionality of TEMPO-COOH was recognized electrochemically by comparing cyclic voltammograms between different gas environments and between with and without TEMPO-COOH (Figure 1.2). In the absence of additives (None in Figure 1.2a and 1.2b), both a broad cathodic peak for oxygen reduction (C) and an anodic doublet peak for lithium peroxide oxidation (A) were observed. At the cathodic peak between 2.5 V and 2.0 V, diatomic oxygen is reduced to superoxide and then peroxide (O2 → LiO2 → Li2O2).29 Even if direct conversion of oxygen to peroxide (O2 to O22-
at 2.96 V) is thermodynamically more favorable than superoxide formation (O2 to O2- at 2.75 V), the direct 2e transfer is entropically unfavorable.30-32 The anodic peak A could be deconvoluted into two peaks. A previous report33 suggested that unstable and imperfect under-stoichiometric lithium peroxide (Li2-xO2) is oxidized earlier at more negative potentials (A1) and then lithium peroxide (Li2O2) is decomposed at more positive potentials (A2). There was no significant difference relevant to oxygen electrochemistry observed after TEMPO was introduced into the same aerated electrolyte (TEMPO in Figure 1.2a and 1.2c). The cathodic peak (C) and the first anodic peak (A1) of the unchanged intensity and shape were recorded while the second anodic peak was overlapped with the anodic peak of TEMPO (T → T+). It indicates that TEMPO do not affect oxygen electrochemistry and therefore there are no chance of functioning as SODm.
When TEMPO-COOH was introduced into the aerated electrolyte, on the contrary, the cathodic peak below 3 V significantly increased from the background current obtained in argon-purged electrolyte (Figure 1.2d). It was ensured that the background-eliminated ORR current increase of TEMPO-COOH was larger than that of TEMPO (Figure 1.2d versus 1.2c) even if the irreversible process relevant to the reduction tail below 2.5 V under argon was not identified.29 The increase in cathodic peak current supports that the solution mechanism guaranteeing more peroxide formation was more favored rather than the surface mechanism when TEMPO-COOH was introduced. Larger cathodic peak area results from more Li2O2 formation on air cathode during discharge.
More interestingly, the anodic peak A1 responsible for lithium-deficient peroxide oxidation disappeared when TEMPO-COOH was introduced to the aerated electrolyte (Figure 1.2a and Figure 1.2d). We think that this is another evidence of the solution mechanism. The surface mechanism is based on direct electron transfer from electrode to oxygen species on electrode surface. Too fast electron transfer would not allow time for forming thermodynamically stable phase of lithium peroxide. Lithium-deficient
15
peroxide (Li2-xO2), oxidized at A1 peak, is thought to be the result of kinetic formation rather than thermodynamic formation. On the other hand, there is every possibility that thermodynamically and therefore stoichiometrically more favorable phase of lithium peroxide is formed via solution mechanism rather than surface mechanism. The main reason is that the solution mechanism is based on kinetically- slow non-electrochemical processes including mass transfer and disproportionation reaction. Therefore, no A1 peak for Li2-xO2 oxidation is possibly obtained in the presence of TEMPO-COOH.
16
Figure 1.2 Electrochemical evidence of the SODm/CRM bifunctionality in cyclic voltammograms at 100 mV s-1. The second-cycle voltammograms were presented. (a) 10 mM TEMPO-COOH and TEMPO in 1 M LiTFSI in TEGDME under O2 atmosphere. (b) 1 M LiTFSI in TEGDME. (c) 10 mM TEMPO in 1 M LiTFSI in TEGDME. (d) 10 mM TEMPO-COOH in 1 M LiTFSI in TEGDME: working electrode = Au; counter = lithium metal; reference = lithium metal; O2 or Ar purged.
17
Figure 1.3 Chemical evidence of the SODm/CRM bifunctionality. (a) Schematic illustration of the Li2O2 oxidation experiment. 50 mM TEMPO-COOH or TEMPO (TC0 or T0) was electrochemically oxidized on Pt mesh as a working electrode with lithium metal as a counter electrode in 1 M LiTFSI / TEGDME. The air cathode (H2315 cabon paper) obtained from the LOB cell discharged by 0.1 mA cm-2 up to 0.25 mAh cm-2 capacity was introduced into the solution of TC+ or T+. (band c) The SEM images of the discharged air cathodes (upper left); (lower left) after charging; (upper right) after storing in TC+ or T+; (lower right) after storing in TC0 or T0. (d) The O2 gas evolution rate. Inset: Schematic illustration of the experiment. A DMSO solution of TC or T was injected into a glass vial containing a KO2/DMSO solution. The Argon flow carried the evolved gas into the mass spectrometer for gas analysis.
18 Solution mechanism promoted by TEMPO-COOH
The scan-rate (ν) dependency of the cathodic peak current (ip) responsible for the oxygen reduction reaction to form Li2O2 supported that the TEMPO-COOH promoted the solution mechanism. The slope of ip versus ν in log-log scale (Δlog ip / Δlog ν) should be 1/2 for the totally diffusion-limited processes as indicated by the Randles-Sevcik equation (ip ~ ν1/2).34 On the other hand, the ip of the surface-confined processes is simply proportional to the scan rate: Δlog ip / Δlog ν = 1.0. The electrochemical oxygen reduction process in LOBs was expected to go via the mixed mechanism of the surface mechanism and the solution mechanism (Figure 1.4a). The surface mechanism is the diffusion-limited oxygen-to- superoxide process followed by the surface-confined superoxide-to-peroxide process. Oxygen diffusion to the electrode surface is required for the oxygen-to-superoxide process while the surface-immobilized lithium superoxide is reduced to lithium peroxide on the electrode surface. Therefore, the log-log slope of the surface mechanism depends on which step is the rate-determining step (0.5 or 1.0) or its value was expected to be between those extreme values when the steps are kinetically competitive. In the solution mechanism, on the other hand, only the diffusion-limited oxygen-to-superoxide process was considered because the subsequent superoxide disproportionation step was the non-electrochemical process. Therefore, the electrochemical oxygen reduction process in LOBs was expected to be between 0.5 and 1.0 in terms of Δlog ip / Δlog ν as the result of the mixed mechanism (Figure 1.4a). The value of Δlog ip / Δlog ν decreased from 0.79 to 0.68 when TEMPO-COOH was employed (Figure 1.4b and c). That is to say, the diffusion-limited solution mechanism became more favored in the presence of TEMPO-COOH when compared with the systems including no additives and TEMPO.
19
Figure 1.4 Scan-rate dependency of the oxygen reduction reaction current. (a) Oxygen reduction reaction in LOBs via the surface and solution mechanisms. (b) The voltammograms during cathodic potential sweep without (none) and with TEMPO (T) or TEMPO-COOH (TC). Electrolyte = oxygen- saturated 1 M LiTFSI in TEGDME; Additive = 10 mM T or TC; Working electrode = Au; Counter and reference electrode = lithium metal. (c) Scan-rate dependency of the peak current of the oxygen reduction reaction.
20 LOB cell performances
As expected, the SODm/CRM-bifunctional additive dramatically improved the cycle life of LOB cells from 45 cycles to 190 cycles when the cells were discharged and then charged by 0.1 mA cm-2 up to 0.2 mAh cm-2 (Figure 1.5a and Figure S1.2). Superoxide was converted to less toxic oxygen species in a faster manner during discharge by the SODm function while the rechargeability was improved by the CRM function. The improvement was much more significant than those reached by TEMPO as a CRM and 2,5-Di-tert-butyl-1,4-benzoquinone (DBBQ) as a DRM, emphasizing the synergetic effect of the bifunctionality of TEMPO-COOH (Figure 1.5a and Figure S1.2). The SODm function of TEMPO- COOH during discharge made a difference between TEMPO-COOH and TEMPO. DBBQ did not improve cycle life at all even if it improved discharge capacity.
When the cells were discharged to a higher capacity from 0.2 to 0.5 mAh cm-2, the absence of TEMPO- COOH brought serious capacity decay along repeated cycles (Figure S1.3). However, only the use of TEMPO-COOH did not solve the capacity decay problem due to too much discharge product deposition on electrode surface. A portion of discharge product (Li2O2) was not completely decomposed during charging so that Li2O2 was accumulated on electrode. Morphological engineering of air cathodes to provide higher surface is additionally required. Also, the concentration of TEMPO-COOH as well as the amount of electrolyte should be optimized for a specific form factor of LOB cells. We calculated the turnover frequency (TOF) of TEMPO-COOH for estimating its effective availability for the given form factor of the LOB cell used in this work. The TOF was calculated by comparing between the total amount of the homogeneous catalyst (100 μl x 50 mM) and the total amount of reaction within the fully discharging time. In the presence of 100 μl of 50 mM TEMPO-COOH (the standard condition of this work), its TOF was 1.6 x 10-4 mol e (mol TC)-1 s-1. The amount (100 μl) was believed to be enough for our LOB cell form factor because the same charging and discharging profiles were obtained with 50 μl of the same concentration electrolyte (Figure S1.4). With 20 μl of the electrolyte, however, the charging profile deviated from those of the enough-electrolyte cases and moreover its cyclability turned to be very poor. Therefore, the intrinsic TOF of TEMPO-COOH was thought to be between 3.2 x 10-4 s-1 and 8.0 x 10-4 s-1.
The charge overpotential was significantly reduced from 4.25 V (None) to 3.5 V in average by using TEMPO-COOH, which clearly supported the CRM function (Figure 1.5b). The range of the charge voltage from 3 V to 3.8 V in the cell voltage profile at 0.1 mA cm-2 corresponded to the potential range of the cathodic peak of TC+ to TC in the cyclic voltammogram (Figure 1.1). During discharge, TEMPO- COOH increased the discharge capacity significantly (more than four times at 0.1 mA cm-2) due to its SODm function to encourage the solution mechanism (Figure 1.5c). The capacity improvement was not
21
observed in the absence of O2 (Figure S1.5). Also, it improved the kinetics of the discharging process.
The overpotential at a fixed current density decreased in the presence of TEMPO-COOH (Figure 1.5c).
As the second evidence of the SODm role and the main reason of the improved cyclability, TEMPO- COOH suppressed side reactions during discharge. Byproducts such as Li2CO3 are generated during discharge by superoxide-triggered side reactions: e.g., 2 Li+ + C (air cathode) + 3/2 O2- → Li2CO3 and/or electrolyte + O2- → Li2CO3 + LiRCO3.35 The carbonates are decomposed during the following charging period so that CO2 gas is evolved during the charge.11, 35-37 In situ differential electrochemical mass spectra (DEMS)38, 39 of the gas released from discharged LOB cells (at 0.13 mA cm-2 for 5 h to 0.65 mAh cm-2 capacity prior to the charge) during charging at 0.13 mA cm-2 evidently indicate the decrease in the amount of CO2 release in the presence of TEMPO-COOH (Figure 1.5d) and therefore less decomposition of cell components including electrolyte and electrode. A clear difference between TEMPO-COOH and TEMPO was observed at the latter part of charging (Figure 1.5b). The TEMPO- present cell showed an abrupt increase in overpotential after 0.15 mAh cm-2. The process requiring such higher overpotentials was most probably carbonate decomposition. However, the TEMPO-COOH did not exhibit the overpotential increase because its SODm function suppressed the carbonate formation in the previous discharging step. The decrease in the amount of carbonate reduces overpotential during charge (overpotential gain) and therefore suppresses the oxidative decomposition of cell components (durability gain).
22
Figure 1.5 LOB cell performances in the absence and presence of 50 mM TEMPO or TEMPO-COOH in electrolyte. LOB cells were discharged and charged repeatedly at 0.1 mA cm-2 up to 0.2 mAh cm-2 capacity. (a) Cell voltages at 0.1 mAh cm-2 along cycle. (b) Cell voltage profiles at the first cycle. (c) Full discharge voltage profiles at three different current densities. Current densities were indicated in mA cm-2. (d) CO2 evolution from LOB cells during the galvanostatic charge at 0.13 mA cm-2. LOB cells were galvanostatically discharged at 0.13 mA cm-2 for 5 h to 0.65 mAh cm-2 capacity prior to the DEMS measurement.
23 Discharge products on air cathodes
The formation and decomposition of Li2O2 as discharge product on air cathode were investigated by scanning electron microscopy (SEM in Figure 1.6a to d and Figure S1.6). X-ray photoelectron spectra (XPS) confirmed that the dominant discharge product on air cathode was Li2O2 both in the presence and absence of TEMPO-COOH (Figure 1.6e).40-42 Carbonate contents were not observed significantly for both cases when the air cathodes were discharged once. TEMPO-COOH changed the morphology of the discharge product from a dense film indicating the surface mechanism (Figure 1.6d and Figure S1.6) to a porous aggregate of particles indicating the solution mechanism (or at least the solution mechanism more preferred to the surface mechanism) (Figure 1.6a to c). The aggregates of Li2O2
particles were cleared after recharging (Figure S1.6). This rechargeability to regenerate a clean surface of cathode is the merit of the CRM role of TEMPO-COOH. It is difficult to obtain such a high rechargeability in the absence of a CRM. For comparison, discharge product on air cathode of the cell containing DBBQ as DRM was investigated. Toroidal Li2O2 was developed during discharge by the help of DBBQ (Figure S1.7). In the following charging process, however, the electrode surface was not totally cleaned so that rechargeability of the cells were not guaranteed. A whole mass of Li2O2 was not oxidized due to the non-conductive nature of Li2O2.
The non-toroidal morphology of Li2O2 developed in the presence of TEMPO-COOH (Figure 1.6b and c) was interesting. Unclear X-ray diffraction peaks responsible for Li2O2 indicated the amorphous nature of the porous aggregate of Li2O2 particles in the presence of TEMPO-COOH (Figure S1.8). As shown in the DBBQ case (Figure S1.7), the toroidal shape of Li2O2 has been believed to indicate dominancy of solution mechanism over surface mechanism.21, 43 It was demonstrated in our previous work that toroids were highly developed in the presence of MA-C60 as another SODm. Before confirming the non-toroidal morphology and amorphous characteristics of discharge products, therefore, we had expected the toroidal structure in the presence of TEMPO-COOH because the voltammo- fingerprint, the increase in capacity and cyclability and the decrease in carbonate formation (Figure 1.5) supported the SODm role of TEMPO-COOH. One of the possible explanations to understand this non- toroidal morphology and amorphous characteristics is too fast rate of disproportionation reaction accelerated by TEMPO-COOH. An analogy was found in the cases that fast galvanostatic discharging leading to fast Li2O2 formation increased the amorphous nature of Li2O2.44-46
The fast rate of disproportionation reaction accelerated by TEMPO-COOH was expected from its SOD activity higher than that of MA-C60 in aqueous media: SOD activity (1 / MSOD /s) = 7.54 for TEMPO- COOH > 6.30 for MA-C60.27 Superoxide disproportionation rates in an aprotic solvent were measured by using the rotating ring-disk electrode (Figure S1.9). 0.1 M TBATFSI in DMSO was used as a standard aprotic electrolyte for the analysis (TBATFSI = tetrabutylammonium bis-
24
trifluoromethanesulfonimidate; DMSO = dimethylsulfoxide). The non-lithium salt (TBATFSI) was used to avoid lithium peroxide deposit formation on the disk electrode while the high-DN solvent (DMSO) was used to solvate superoxide efficiently. The superoxide produced from the disk electrode during a cathodic potential scan was oxidized at the ring electrode held at 3.3 VLi+/Li. The currents of disk and ring electrodes (id and ir) were used as the measures of the amount of superoxide generated from the disk electrode and the remaining amount of superoxide detected on the ring electrode after disproportionation reaction, respectively. Similar disk currents were obtained independent of additives.
However, the ring current of TEMPO-COOH was significantly lower than those of TEMPO and None.
The smaller ring current was attributed to the promoted disproportionation of superoxides to peroxide and oxygen by TEMPO-COOH. The superoxide disproportionation rates calculated from the accumulated charges on the disk and ring electrodes were 0.26 s-1 for TEMPO-COOH > 0.16 s-1 for TEMPO ~ 0.17 s-1 for None (refer to the equation in the caption of Figure S1.9).
25
Figure 1.6 Discharge products on air cathodes. LOB cells were fully discharged at 0.1 mA cm-2. (a to d) SEM images of deposits on air cathodes (H2315 carbon paper) (a to c) in the presence of TEMPO- COOH and (d) without TEMPO-COOH. (e) XPS spectra of discharged air cathodes (P50 carbon paper).
The spectra of the bare P50 carbon paper were included for comparison on the top.
26 Calculation
The density function theory (DFT) calculation was used to support the feasibility of TEMPO-COOH as a CRM. Energy levels of molecular orbitals (MO) including highest occupied molecular orbitals (HOMOs) and singly occupied molecular orbitals (SOMOs) were converted to reduction potentials versus Li/Li+ (see “calculation details” in the SI).24 The energy levels of TEMPO-COOH (TC) were estimated to be similar to those of TEMPO (T) as the previously reported CRM: TC vs. T = 3.94 V vs.
3.92 V for the SOMO; 7.19 V vs. 8.01 V for HOMO of their oxidized form (Figure S1.10). The energy levels of TC and TC+ were compared with the reduction potentials of O2/Li2O2 and electrolyte (TEGDME) decomposition (Figure 1.7a).24 The calculated value of SOMO of TC was 3.94 V, which approximated the experimental value obtained from its cyclic voltammogram (3.8 V in Figure 1.2a).
Therefore, it was confirmed that the reduction potential of TC was placed within the appropriate potential range of CRM between 3 V and 4 V. The energy level of the SOMO of TC was lower than that of O2/Li2O2 (2.96 V) so that electrons are spontaneously transferred from Li2O2 to the empty SOMO of TC+ as the oxidized form of TC. Therefore, the electrochemically oxidized TC (i.e., TC+) is able to decompose Li2O2 to Li+ and O2, returning to its reduced form (i.e., TC) during charging (equations 2a and 2b for CRM function). Also, the oxidation of TEGDME as a solvent molecule by TC+ was not allowed because the HOMO of TC+ (or the second energy level of TC) was fully filled even if the energy level of the HOMO of TC+ is lower than that of TEGDME.24
Also, the DFT calculation confirmed the SODm function of TEMPO-COOH. There were significant differences between TEMPO and TEMPO-COOH observed in the configuration of the superoxide complex formation and the adsorption energy (ΔEads) of the superoxide species (O2•- or LiO2) (Figure S1.11 and Figure S1.12). The –COOH was thermodynamically more favoured rather than –NO when the superoxide species were adsorbed on TEMPO-COOH. On the other hand, the superoxide species were adsorbed on –NO of TEMPO. The –COOH of TEMPO-COOH was significantly more preferred than the –NO of TEMPO (Figs. S1.13 and S1.14): ΔEads = -0.81 eV on TEMPO-COOH vs. -0.29 eV on TEMPO for O2•- adsorption; ΔEads = -1.08 eV on TEMPO-COOH vs. -0.66 eV on TEMPO for LiO2
adsorption. The carboxylate group of TEMPO-COOH played a role of hydrogen-bonding to the superoxide species.
This thermodynamically favored superoxide-TEMPO-COOH complex formation encouraged the solution mechanism. In the free energy diagrams for Li2O2 formation along reaction coordinate (Figure 1.7b and Figure S1.13 to Figure S1.15), the TEMPO-COOH showed a free energy gain over TEMPO and no catalyst. The thermodynamic barrier of the disproportionation step from LiO2 dimer to Li2O2
and solvated oxygen gas (Δ[ΔG] = ΔG(Li2O2+O2) ‒ ΔG({LiO2}2)) was calculated to be 0.74 eV without catalyst. The endothermicity of this barrier was slightly changed from 0.74 eV to 0.58 eV in the presence