Determining the Facile Routes for Oxygen Evolution Reaction by

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Determining the Facile Routes for Oxygen Evolution Reaction by In situ Probing of Li–O Cells with Conformal LiO Films 2

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Misun Hong, Chunzhen Yang, Raymond A Wong, Aiko Nakao, Hee Cheul Choi, and Hye Ryung Byon J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02003 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Journal of the American Chemical Society

Determining the Facile Routes for Oxygen Evolution Reaction by In situ Probing of Li-O2 Cells with Conformal Li2O2 Films Misun Hong,†,‡,§,∥ Chunzhen Yang,‡ Raymond A. Wong,‡ Aiko Nakao,⊥ Hee Cheul Choi,*,§,∥ Hye Ryung Byon*,†,‡,¶ †

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291, Daehak-ro, Yuseonggu, Daejeon 34141, Republic of Korea ‡

Byon Initiative Research Unit (IRU), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

§

Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science, 77 Cheongam-ro, Nam-gu, Pohang 37673, Republic of Korea ∥Department

of Chemistry, Pohang University of Science and Technology, 77, Cheongam-ro, Nam-gu, Pohang 790784, Republic of Korea ⊥Bioengineering

Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

¶

Advanced Battery Center, KAIST Institute NanoCentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Supporting Information Placeholder ABSTRACT: An ongoing challenge with lithium–oxygen (Li– O2) batteries is in deciphering the oxygen evolution reaction (OER) process related to the slow decomposition of the insulating lithium peroxide (Li2O2). Herein, we shed light on the behavior of Li2O2 oxidation by exploiting various in situ imaging, gas analysis and electrochemical methods. At the low + potentials of 3.2–3.7 V (vs Li/Li ), OER is exclusive to the thinner parts of the Li2O2 deposits, which leads to O2 gas evolution, followed by the concomitant release of superoxide species. At higher potentials, OER initiates at the sidewalls of the thicker Li2O2. The succeeding lateral decomposition of + Li2O2 indicates the preferential Li and charge transport occurring at the sidewalls of Li2O2. To ameliorate the OER rate, we also investigate an alternative approach of rerouting charge carriers by using soluble redox mediators. Our in situ probes provides insights into the favorable charge-transport routes that can aid in promoting Li2O2 decomposition.

The lithium–oxygen (Li–O2) electrochemistry provides an opportunity to develop high-energy-density batteries (~3.5 −1 kWh kg ) owing to the lightweight oxygen and multielectron transfer. However, the solid discharge product of lithium peroxide (Li2O2) has the poor ionic and electronic −19 −20 −1 o conductivities (10 –10 S cm at 25 C for both), which results in lower-than-theoretical capacities during discharge 1,2 and sluggish decomposition during recharge. The limitations in charge transport causes the premature termination + − of oxygen reduction reaction (ORR, 2Li + O2(g) + 2e →

0

+ 3

Li2O2(s), E = 2.96 V vs. Li/Li ). However, this propensity can be mitigated by enhancing the solubility of the superoxide – 4 (O2 ) intermediate to form Li2O2 by disproportionation. The more significant challenge is the large overpotentials required to decompose Li2O2 during oxygen evolution reaction (OER). Typically, the stifling effect of the limitations in charge transport causes the pronounced rise in recharge po+ tential to > 4 V (vs Li/Li ), and leads to the poor cyclability in 5 Li–O2 cells. On the other hand, we have demonstrated the significant suppression of recharge potential at < 3.7 V by 4c, 4d, 6 forming small sized and amorphous Li2O2. The important observation of the diverse range of OER potentials indicates differences in the preferential charge-transport route in Li2O2, which has not yet been thoroughly under7 stood from the studies using a single in situ analytical tool. Here, we uncover the decomposition route of conformal Li2O2 films by using multiple tools. In addition, by correlating all in situ results, including morphology change of Li2O2 from electrochemical atomic force microscopy (EC-AFM) and analyses of gas and soluble product, we suggest the strategies that can ameliorate charge transport and the OER rate. The Li–O2 model cell used for the EC-AFM experiments consists of highly oriented pyrolytic graphite (HOPG) as the working electrode (WE) and metallic lithium (Li) as the counter and reference electrodes (CE and RE). The cyclic voltammogram (CV, Figure S1) performed in O2-saturated LiTFSI/tetraglyme shows a pronounced cathodic peak at 2.1 V and the subsequent multiple anodic peaks are overlaid

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Figure 1. Deposition and decomposition (< 3.7 V) process of Li2O2 film on HOPG in 0.5 M LiTFSI/tetraglyme. (a) Representative cathodic profile at the constant potential of 2.2 V and (b–d) the corresponding in situ images of film deposition. (e) Subsequent −1 anodic potential sweep at 0.5 mV s and (f–h) the corresponding decomposition images up to ~3.7 V. All scale bars denote 500 nm and the scale of color code indicates height. The letters in (a) and (e) indicate the corresponding AFM images. The black arrows in AFM images indicate the scan direction. with the broad potential range of 3–4.5 V, which are compa8 rable to the typical ORR and OER responses in Li–O2 cells. Figure 1a–d shows the deposition process of Li2O2 on bare HOPG surface (Figure S2). In the beginning, nanoparticulate products appear at the HOPG step edge (red arrow), which then immediately coats the basal plane to form the conformal film of Li2O2 (Figure S3–S4). These in situ AFM images also reveal the inhomogeneous deposition rate of Li2O2 over the HOPG, as shown in the several local spots deposited later (cyan arrows, Figure 1c). Notably, this difference in deposition rate causes deviations in the film thickness. The typical film thickness is restricted to ~3 nm, which is attributed to 4a the electron-tunneling distance of insulating Li2O2. In

Figure 2. In situ analyses and proposed OER process scheme. −1 (a) Gas analysis with anodic potential sweep at 0.05 mV s using OEMS. The O2 and CO2 evolution rates are indicated by the solid squares and empty triangles, respectively. (b) RRDE current profiles with sweeping disk potential at 5 −1 mVdisk s and constant ring potential at 3.4 Vring under rotating at 900 rpm. The cathodic sweep was carried out with graphitized carbon nanotube (G-CNT) electrodes prior to OER for both (a) and (b). (c–f) Schematic illustration of the Li2O2 decomposition process. The gray parts indicate the active Li2O2 area for OER.

contrast, the parts that are deposited later are ~1 nm thinner than the majority of the film as evidenced by the darker color contrast (cyan arrows in Figure 1d) and the corresponding height profiles (Figure S5–S6 and Supporting Note in Supporting Information). The subsequent OER process was performed alongside linear sweep voltammetry (LSV, Figure 1e). The low potential region below 3.7 V shows the first anodic peak and the shoulder appearing at 3.2–3.4 V, which is in good agreement 3, 8 with the CV profile (Figure S1) and in previous reports. However, there is the absence of any changes on the film surface as observed in the corresponding AFM image (Figure 1f). The visual evidence of Li2O2 decomposition appears at 3.4–3.7 V when the anodic current is diminishing. The emergence of small pits is exclusive to the thinner areas in Li2O2 (cyan arrows in Figure 1g–h), where the electrical resistance is lower. This result indicates that the slow charge-transport 4a is a critical hurdle for the decomposition of Li2O2. To understand the lack of correlation between the current response and the appearance of pits in Li2O2, in situ gas and soluble product analyses were performed using highly graphitized multi-walled carbon nanotubes (G-CNT, see Supporting Information) as the WE electrode. On-line electrochemical mass spectrometry (OEMS) in Figure 2a shows that the overall O2 evolution profile coincides well with the anodic LSV curve. In particular, the apparent OER, which is in line with the first anodic peak, corroborates the occurrence of Li2O2 decomposition. Whereas insignificant change is shown at the Li2O2 film surface in the AFM image, it can be inferred that decomposition starts away from the surface, namely, in the vicinity of electrode surface where the swift electron transfer can occur (Figure 2c–d and Figure S7a–b). This + process is accompanied by the dissociation of Li2O2 to Li ions and O2, which diffuse through the film and dissolve at the film surface. Computational reports have shown that fast + Li diffusion can occur along defects and grain boundaries of 2, 9 the Li2O2, and we anticipate to be in high concentration within the thinner Li2O2 deposits and expedites the OER. However, charge transport may become slow following the


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Figure 3. Decomposition process of Li2O2 film > 3.7 V. (a) Potentiostatic profile and the corresponding AFM images at (b) 3.7 V, (c) 4.10 and (d–f) 4.15 V. The alphabet in (a) indicates the corresponding AFM images. All scale bars denote 500 nm. decomposition of the thin Li2O2 deposits close to the HOPG surface (Figure S7c–d). Therefore, for the appearance of pits and the accompaniment of surface decomposition of Li2O2, another key process occurs, which can be probed by rotating ring disk electrode (RRDE) tests. Figure 2b shows the noticeable ring current (Iring, orange curve) at ~3.55 Vdisk, occurring after the OER peak at ~3.35 Vdisk (green curve). This Iring response may indicate the presence of the superoxide species in the electrolyte solution (Figure S8), and its emergence at ~200 mV positive than the OER peak can be correlated to the appearance of pits in the Li2O2 film (Figure 1e–h). The release of soluble superoxide species has been predicted as the + originating from Li dissolution on the Li2O2 surface at 3.26– 9d 3.36 V in a previous computational report. Additionally, our in situ probes demonstrate that the facile decomposition of the thinner Li2O2 is achieved resulting in the production of O2 and superoxide (Figure 2e and Figure S7d). Turning our attention to the higher-potential region ≥ 3.7 V. The OEMS result shows an increase in O2 evolution while

the negligible release of superoxide from RRDE test reveals the distinguishable OER process from the lower-potential region (Figure 2a–b). The decomposition process of the thicker Li2O2 surfaces is observed using AFM after holding at constant anodic potentials (Figure 3a). The pits formed at 3.7 V for 27 min (Figure 3b) become cracks which then splits the Li2O2 film into small islands at 4.1 V (Figure 3c and Figure S9). At 4.15 V, the continued decomposition reduces the lateral size of the Li2O2 islands (Figure 3d), which is completely eliminated with elongated reaction time (Figure 3e–f and Figure S10). These series of snapshots depict the preferential lateral decomposition and indicates the role of the Li2O2 sidewalls. The Li2O2 sidewalls of the small pits enlarge by the development of cracks (Figure 2f), which essentially serve as the interface with the electrolyte solution and also + the HOPG electrode. The accessibility of Li to the electrolyte solution and electrons to the electrode at the sidewalls of + − Li2O2 establish a new equilibrium (2Li + O2(g) + 2e Li2O2(s)), making this interface distinct from the Li2O2 in the interior. In comparison to the top surface of the Li2O2 film, the sidewalls are in contact with the electrode and likely contains a lower degree of side products as this interface area is continuously changing during OER, which aids in the preferential charge transport. On the other hand, the constant film thickness during OER reflects the difficulty in charge transport with respect to the vertical direction in the Li2O2. The requirement of high OER potentials is met with unin5b, 10 tended side reactions (Figure S3–S4) and CO2 evolution (Figure 2a). Therefore, this in situ study suggests that the considerable suppression in anodic overpotential can occur provided that the interfacial area enlarges, which has been demonstrated from our previous report on nanometer sized 6 and one-dimensional Li2O2. Alternatively, the Li2O2 decomposition can be promoted by the rerouting of charge carriers. The soluble redox mediator transports charge via the electrolyte solution, thus bypasses the need of charge transport within the insulating Li2O2 (Figure 4a). The representative redox mediator of TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) has been demonstrated to suppress the recharge overpotential in Li– 11 O2 cells, but the decomposition process has not been visually observed. Figure 4b shows quasi-reversible redox behavior

Figure 4. Li2O2 decomposition with TEMPO redox mediator (RM). (a) Illustration of redox mediator’s role. Reduction and oxidation of redox mediator are denoted as red and ox, respectively. (b) Voltammograms with 10 mM TEMPO in O2-free (black, 5 −1 −1 mV s ) and O2-containing environment (red, 0.2 mV s ) after ORR at 2.2 V. (c–e) In situ AFM images (c) at the beginning and (d–e) during the anodic LSV. (f) Height profiles for the lines indicated by yellow arrows in (e).



of TEMPO. Anodic peaks appear at over 3.7 V in both Ar- and O2-contained environment, thus the oxidized TEMPO, indi+ cated as TEMPO , can thermodynamically accept electrons from Li2O2, leading to OER. Following the addition of TEMPO, in situ AFM images in Figure 4d–e show vigorous decomposition of the Li2O2 film that contains pits due to time-restricted ORR (Figure 4c and Figure S11). As TEMPO is actively oxidized, the entire Li2O2 is promptly eliminated at 3.73–3.75 V. The line-by-line height profiles in Figure 4f reveal the apparent propensity for pit broadening of Li2O2 in the beginning (from (1) to (3), purple arrows). This decomposition behavior is attributed to the short-distance + TEMPO/TEMPO shuttling in between the bare HOPG surface and the vicinity of Li2O2. As the potential rises, the in+ creased TEMPO concentration extends the accessibility of + TEMPO to the top surface of Li2O2 film, which leads to a decrease in the film height as shown in (4). This result shows the benefit of charge carrying vessels to surmount the sluggish charge-transport in the Li2O2. In summary, we have visualized the decomposition process of Li2O2 films using in situ probing systems and explored the preferential OER route within Li2O2. The thinner parts of the Li2O2 film decomposes at < 3.7 V with O2 evolution and dissolution of superoxide species, resulting in the appearance of the pits. The thicker parts of Li2O2 film decomposes at over 4.0 V in the lateral direction, indicating the critical limitation of sluggish charge transport towards the Li2O2 surface even at thicknesses of ~3 nm. By comparison, the enlargement of the sidewall area of Li2O2 acting as the interface to electrolyte and electrode aids the OER process. Alternatively, the addition of redox mediators enables charge transport to be bypassed and leads to the fast decomposition of Li2O2. This study reveals the OER process of insulating Li2O2 and gives insights into favorable decomposition route to suppress the potential rise.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, additional AFM images and electrochemical current profiles, qualitative characterization using XPS are included in Supporting Information (PDF file).

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]

2017R1A2B3010176), the Nano Material Technology Development Program through the NRF funded by the ministry of Science, ICT and Future Planning (Grant 2009-0082580), RIKEN, and JST-ALCA SPRING project. M. H. is thankful for support from POSCO TJ Park Science fellowship. R.A.W. is grateful for support from the IPA fellowship at RIKEN.

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The work is supported by the National Research Foundation (NRF) of Korea (Grant NRF-2016R1C1B2008690 and NRF-


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SYNOPSIS TOC.



Deposition and decomposition (< 3.7 V) process of Li2O2 film on HOPG in 0.5 M LiTFSI/tetraglyme. (a) Representative cathodic profile at the constant potential of 2.2 V and (b–d) the corresponding in situ images of film deposition. (e) Subsequent anodic potential sweep at 0.5 mV s−1 and (f–h) the corresponding decomposition images up to ~3.7 V. All scale bars denote 500 nm and the scale of color code indicates height. The letters in (a) and (e) indicate the corresponding AFM images. The black arrows in AFM images indicate the scan direction. 246x123mm (300 x 300 DPI)


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In situ analyses and proposed OER process scheme. (a) Gas analysis with anodic potential sweep at 0.05 mV s−1 using OEMS. The O2 and CO2 evolution rates are indicated by the solid squares and empty triangles, respectively. (b) RRDE current profiles with sweeping disk potential at 5 mVdisk s−1 and constant ring potential at 3.4 Vring under rotating at 900 rpm. The cathodic sweep was carried out with graphitized carbon nanotube (G-CNT) electrodes prior to OER for both (a) and (b). (c–f) Schematic illustration of the Li2O2 decomposition process. The gray parts indicate the active Li2O2 area for OER. 182x108mm (300 x 300 DPI)



Decomposition process of Li2O2 film > 3.7 V. (a) Potentiostatic profile and the corresponding AFM images at (b) 3.7 V, (c) 4.10 and (d–f) 4.15 V. The alphabet in (a) indicates the corresponding AFM images. All scale bars denote 500 nm. 187x139mm (300 x 300 DPI)


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Li2O2 decomposition with TEMPO redox mediator (RM). (a) Illustration of redox mediator’s role. Reduction and oxidation of redox mediator are denoted as red and ox, respectively. (b) Voltammograms with 10 mM TEMPO in O2-free (black, 5 mV s−1) and O2-containing environment (red, 0.2 mV s−1) after ORR at 2.2 V. (c– e) In situ AFM images (c) at the beginning and (d–e) during the anodic LSV. (f) Height profiles for the lines indicated by yellow arrows in (e). 300x123mm (300 x 300 DPI)


Determining the Facile Routes for Oxygen Evolution Reaction by

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