Achieving Low Overpotential LiâO2 Battery Operations by Li2O2

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Achieving Low Overpotential Li-O2 Battery Operations by Li2O2 Decomposition through One-electron Processes Jin Xie, Qi Dong, Ian P. Madden, Xiahui Yao, Qingmei Cheng, Paul Dornath, Wei Fan, and Dunwei Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04097 • Publication Date (Web): 19 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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Achieving Low Overpotential Li-O2 Battery Operations by Li2O2 Decomposition through Oneelectron Processes Jin Xie1, Qi Dong1, Ian Madden1, Xiahui Yao1, Qingmei Cheng1, Paul Dornath2, Wei Fan2, and Dunwei Wang1,* 1

Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon St.,

Chestnut Hill, MA, 02467 USA 2

Department of Chemical Engineering, University of Massachusetts, 686 North Pleasant St.,

Amherst, MA, 01003 USA

ACS Paragon Plus Environment

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ABSTRACT: As a promising high-capacity energy storage technology Li-O2 batteries face two critical challenges – poor cycle lifetime and low round-trip efficiencies, both of which are connected to the high overpotentials. The problem is particularly acute during recharge, where the reactions typically follow two-electron mechanisms that are inherently slow.

Here we

present a strategy that can significantly reduce recharge overpotentials. Our approach seeks to promote Li2O2 decomposition by one-electron processes, and the key is to stabilize the important intermediate of superoxide species. With the introduction of a highly polarizing electrolyte, we observe the recharge processes are successfully switched from a two-electron pathway to a single-electron one.

While similar one-electron route has been reported for the discharge

processes, it has rarely been described for recharge except for the initial stage due to the poor mobilities of surface bound superoxide ions (O2-), a necessary intermediate for the mechanism. Key to our observation is the solvation of O2- by an ionic liquid electrolyte (PYR14TFSI). Recharge overpotentials as low as 0.19 V at 100 mA/gcarbon are measured.

KEYWORDS: Energy Storage, Li-O2 Battery, Electrochemistry, Oxygen Evolution Reactions, Ionic Liquids


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Successful operations of Li-O2 batteries involve oxygen reduction reactions (ORR) during discharge and oxygen evolution reactions (OER) during recharge, both featuring significant overpotentials even at low current densities.1-7 The overpotentials present challenges that limit the development of this otherwise highly promising energy storage technology. On the one hand, the high overpotentials reduce the round-trip efficiencies by requiring significantly greater power to recharge than can be recovered during discharge.

On the other hand, the high

overpotentials induce parasitic chemistries that are the main reason for the short cycle lifetimes. The latter problem is particularly cumbersome for the recharge process, where potentials >4.0 V (vs. Li+/Li; the thermodynamic equilibrium potential for Li2O2 – 2e- ↔ 2Li+ + O2 is 2.96 V under standard conditions) are often necessary to fully decompose Li2O2.8-12 Collectively, the overpotential requirement poses a fundamental challenge and must be minimized in order to develop Li-O2 batteries into a practical energy storage technology.

The origin of the

overpotentials is intimately connected to the detailed chemical processes involved in Li-O2 battery operations, which are summarized in a simplified form in Scheme 1. The discharge process is initiated by the transfer of one electron from the cathode support to O2(g), producing O2-,13-15 followed by either disproportionation16,17 (Routes 1 & 3), where two LiO2 (or two solvated O2-) yield one Li2O2 and one O2(g), or a second electron transfer (Routes 2 & 4). Of the four pathways, Route 1 is a single electron process and is expected to feature the lowest overpotentials. Route 4 is the most difficult, especially when the second electron-transfer step is also slow, and is expected to feature the highest overpotentials. An important conjecture may be formed by examining Scheme 1 that solvation of O2- should help reduce discharge overpotentials by promoting reactions following Route 1. The hypothesis has indeed been confirmed by several recent independent studies.18,19


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Scheme 1. Proposed reaction pathways for Li2O2 formation and decomposition

The recharge pathways are comparatively much less studied. It is generally understood to initiate by a one-electron transfer process to form LiO2* species that are surface bound.20-23 If LiO2* underwent disproportionation decomposition (Route 1’ in Scheme 1), the overall decomposition would be a single-electron process that may be regarded as a pseudo-reverse reaction of Route 1. This route (Route 1’) is expected to feature low overpotentials. Due to the poor mobilities of surface bound LiO2* species, however, Route 1’ has only been observed during the initial recharge stage20,21. By and large, Route 2’, where a second electron transfer (often slow) is involved, is considered the most plausible route, and the overall reactions are more commonly regarded as a two-electron process. The two-electron transfer nature is a fundamental reason why high overpotentials are characteristic for the recharge reactions. Inspired by recent success in reducing discharge overpotentials by solvating O2- ions, we postulated that highly polarizing electrolytes may help solvate LiO2, enabling disproportionation pathways such as Route 3’. Low overpotentials are expected when the overall reaction is changed from a two-electron process to a single-electron one. We show here that the singleelectron mechanism can be enabled by the application of an ionic liquid electrolyte (N-butyl-N-


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methylpyrrolidinium

bis(trifluoromethylsulfonyl)

imide,

PYR14TFSI,

mixed

with

dimethoxyethane, DME). Average recharge overpotential of as low as 0.19 V was measured. While PYR14TFSI has been previously introduced as an electrolyte for low-overpotential Li-O2 battery operations,24 the mechanism presented by this work is fundamentally different. Our results show that while PYR14TFSI exhibits marginal advantage over other electrolytes such as DME in terms of stabilities, it reduces the recharge overpotentials mainly through the switch of mechanisms.

Figure 1. Discharge/recharge behaviors of a-3DOm carbon in PYR14TFSI/DME electrolytes during the first five cycles. The electrolyte compositions are: (A) 100% PYR14TFSI; (B) 75% PYR14TFSI, 25% DME; (C) 50% PYR14TFSI, 50% DME; (D) 25% PYR14TFSI, 75% DME; (E) 100% DME. (F) Average discharge and recharge potentials during the first 5 cycles for data presented in panels A through E.

The dotted horizontal line marks the thermodynamic

equilibrium potential of 2.96V vs. Li+/Li. Discharge/charge rate is 100mA/gcarbon.


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The cathode support used for the present study was three-dimensionally ordered mesoporous (3DOm) carbon25,26 with a loading density of 0.20-0.25 mg/cm2. 3DOm carbon with an average pore size of 35 nm has been selected for this body of work as the deposition of Li2O2 is expected to benefit from its large pores. To preserve the inherent properties of carbon and minimize influences by other components, 3DOm carbon were used as-synthesized with only a postpreparation annealing treatment (denoted as a-3DOm carbon). No other surface modifications were performed. When PYR14TFSI was used, a mean recharge potential of 3.15 V was obtained after averaging the first 5 cycles (Figure 1A), corresponding to an overpotential of 0.19 V (denoted as ηc henceforth), one of the lowest reported in the literature.27-32 Surprisingly, the average discharge potential (2.51 V, ηd=0.45 V) was much lower than can be obtained when DME was used (ηd=0.18 V, Figure 1E). There are two possible reasons for the high ηd by 3DOm carbon, namely poor ORR properties of a-3DOm carbon or slow O2 diffusion in PYR14TFSI. The first possibility can be ruled out by control experiments where a-3DOm carbon was replaced by Vulcan carbon and similarly high discharge overpotentials were measured (Figure S6 in the SI). The result suggests that the high ηd is inherent to the electrolyte but not the electrode. Further supporting that a-3DOm is a good ORR catalyst was the low ηd measured (0.18 V) when the electrolyte was replaced by DME (Figure 1E). It is, therefore, concluded that poor O2 diffusion in PYR14TFSI leads to high ηd. How O2 diffusion could influence Li-O2 operations has been recently discussed by us within the context of 3DOm carbon.26 Given the relatively low diffusion coefficient of O2 in PYR14TFSI (1.8×10-6 cm2/s),33 when compared with that in DME (1.22×10-5 cm2/s),34 polarizations within the solution due to the O2 concentration gradient are expected. To better take advantage of the low ηc offered by PYR14TFSI and the low


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ηd by DME, we next mixed the two electrolytes and observed systematic reduction in ηd when DME concentration was increased (Figure 1B to D). Similarly, an increase in PYR14TFSI concentration corresponded to a decrease in ηc. The results are summarized in Figure 1F, where we see that 75% PYR14TFSI and 25% DME enabled the highest round-trip efficiencies at 83%.

Figure 2. Schematic illusion of the decomposition pathways of Li2O2 when PYR14TFSI is present. The key hypothesis is that the solvation effect by PYR14TFSI promotes the singleelectron pathway (the upward branch in the illustration) that feature low recharge overpotentials.

It has been recently reported that by-products such as carbonates formed during discharge are an important reason for the high recharge overpotentials.10,35-37 These by-products contribute to the overpotentials in several ways. For instance, their relatively poor conductivity may increase electronic polarizations, and their decompositions are difficult and require high potentials, especially toward the end of the recharge process. Researchers have thus hypothesized that replacing the popularly used electrolytes (e.g., dimethyl sulfoxide or DME) with more stable


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ones such as PYR14TFSI24 or 2,3-dimethyl-2,3-dimethoxybutane38 may help mitigate the problem. While PYR14TFSI does exhibit greater resistance against reactions with Li metal39 and superoxide ions40,41, its decomposition has been reported11. So long as the OER processes follow a 2-electron pathway, a minimum overpotential of 0.32 V would be expected, as predicted by computations.1,22 The purported stability alone could only be employed to explain why no high overpotentials are needed to decompose solid-phase by-products, but fail to fully account for the low overpotential measured here.

We are therefore encouraged to propose an alternative

hypothesis as schematically shown in Figure 2 that involves the switch of recharge processes from a two-electron one to a single-electron one. Our key postulation is that, as a highly polarizing substance, PYR14TFSI interacts favorably with superoxide species according to the hard soft acid base (HSAB) theory, providing the necessary stabilization effect for the one-electron transfer pathway (Route 3’ in Scheme 1). The first piece of evidence that supports this hypothesis is that a significant portion (up to 18.7%) of discharge products exists in the PYR14TFSI electrolyte at the end of the discharge cycle as detected by iodometric titration (vide infra). It proves that PYR14TFSI indeed helps solvate discharge products. It is noted that the discharge products as titrated by iodometric methods mainly point to the composition of Li2O2. While Li2O2 can easily form by disproportionation from superoxide species, it is of great importance to directly observe the presence of superoxide in the electrolyte. For this purpose, we used chemical labeling by nitrotetrazolium blue chloride, which turns blue when superoxide ions are present13. For this experiment, electrolytes from three sets of samples were examined (Supplementary Information, Figure S3), namely (i) cell discharged in PYR14TFSI; (ii) cell discharged in DME; and (iii) pre-loaded commercial Li2O2 in PYR14TFSI.

Superoxide ions were only detected in sample (i).

The results prove that


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superoxide ion is an intermediate of the discharge process, and that PYR14TFSI helps stabilize it in the solution. A representative comparison between samples (i) and (ii) are shown in Figure 3A as the inset. More quantitative information about the dissolved superoxide ions was obtained by iodometric titration.42,43 In doing so, we expect the dissolved superoxide species undergo disproportionation to yield H2O2, which was then quantified. Because the reaction does not involve receiving (or giving) more electrons from (or to) the electrode, the quantification should report on the true Faradaic yield of the discharge process. As is seen in Figure 3A, when the products in the PYR14TFSI electrolyte were included, a total yield of 70.6% was obtained; when only the solid product accumulated on the a-3DOm cathode was considered, a yield of 57.4% was measured. In other words, 18.7% of the discharge products are dissolved (or dispersed) in the electrolyte. In stark contrast, when DME was used as the electrolyte, nearly identical yields (60.4% with DME vs. 59.2% without DME) were measured whether the electrolyte were included or not. Taken as a whole, these results support that up to 18.7% of discharge products dissolve (or disperse) in PYR14TFSI. Furthermore, nitrotetrazolium blue chloride detection proves that some of the products exist in the form of superoxide species.


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Figure 3. Product detection.

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(A) Iodometric titration results of samples discharged in

PYR14TFSI and DME after the 1st cycle. Filled bars are the yield with the electrolyte also titrated (represented as dotted products plus precipitate in the inset schematics), and open bars are the yield only from the cathode support (represented as the precipitate in the schematics). Inset in the dashed box: superoxide detection using nitrotetrazolium blue chloride. Top: in PYR14TFSI; bottom: in DME. (B) Iodometric titration results of samples at the end of the 1st, 2nd and 5th cycles (discharged state).

It has been previously shown experimentally20,21 and computationally22,44,45 that the oneelectron transformation from Li2O2 to LiO2* can take place at low overpotentials. However, direct electrochemical decomposition of LiO2* is much more difficult.

When no other

stabilization effect is present, the overall recharge process would feature a low initial overpotential, followed by mechanistic switch to high overpotentials. By comparison, if LiO2* could be dissolved in the electrolyte, a new disproportionation pathway (Route 3’ in Scheme 1) is enabled, and the overall recharge process would be of low overpotentials. Naturally, the real recharge processes are likely an intricate combination of all possible pathways as shown in Scheme 1. But the presence of super oxide species in PYR14TFSI provides strong evidence that the low overpotential single-electron pathways now become possible. These pathways provide an explanation to the electrochemical behaviors as shown in Figure 1. When the yields of Li2O2 as quantified by iodometric titration for the first 5 cycles of discharge/recharge are compared, PYR14TFSI appears to perform better in terms of promoting Li2O2 formation. As the cycle numbers were increased, the yield in PYR14TFSI was better maintained than DME (Figure 3B). The overall yield by PYR14TFSI, nevertheless, is still


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relatively low, 52.8% after five cycles. While there are a few reports on using ionic liquids for lithium oxygen batteries, their stabilities as determined by different experimental methods differ, and their possible decomposition routes were not discussed before. For instance, Elia et al. reported that no significant solid by-products have been observed using X-ray photoelectron spectroscopy (XPS) on cathode surface when PYR14TFSI was utilized as electrolyte.24 The authors therefore highlighted the stability of PYR14TFSI as a reason for the observation of low overpotentials.

On the contrary, recent work focused on differential electrochemical mass

spectrometry (DEMS) by Das et al. suggested PYR14TFSI is not stable as evidenced by the poor recharge Coulombic efficiency (similar to that of DME).46 Our results on a-3DOm carbon with PYR14TFSI as the electrolyte agree with these reports that minimum solid by-products are formed (as shown in XPS characterization in Figure 4A and Figure S6 in Supporting Information).

Furthermore, gaseous by-products are also minimum (as shown in GC-MS

characterization in Figure 4B).

We next applied nuclear magnetic resonance spectroscopy

(NMR) as a tool to investigate the possible by-products formed or dissolved in the electrolyte itself rather than in solid or gas phase. NMR spectra (Figure 4D) revealed that PYR14TFSI is indeed decomposed via Hofmann elimination to give characteristic alkene peaks (see Figure S4 in the Supporting Information for more details). It is concluded that while PYR14TFSI is more stable during Li-O2 operations than DME, by-product formation is only reduced by a small margin. The key difference between PYR14TFSI and other electrolyte systems such as DME is that the by-products mainly reside in the solution phase. The dramatic decrease of the stable recharge overpotential thus cannot be fully explained by the stability argument. The mechanistic switch to a single-electron process from a two-electron one becomes a more plausible alternative, as discussed above.


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The discharge product of O2 was quantified by GC-MS in an on-line, real time configuration. The evolved O2 accounted for 83% of the total charges passed, which is higher than the yield obtained by iodometric titration characterizations. Due to the necessity to titrate the cathode and electrolyte separately, and the need to extract PYR14TFSI before titration, the relatively low yield by titration is expected as a system error35,42. Importantly, the yield (up to 83%) is among the highest reported in the literature1, supporting that the reported electrochemical behaviors in Figure 1 indeed correspond to Li2O2 formation and decomposition.

Figure 4. Spectroscopic studies. (A) X-ray photoelectron spectra of a-3DOm carbon cathode support after various experimental operations (pristine, 1st cycle discharged, 1st cycle charged and 5th cycle charged). The NiOx signal comes from the current collector used to support the cathode materials. (B) Online Gas-Chromatography Mass Spectrometry (GC-MS) of gaseous recharge products. Electrolyte used was PYR14TFSI, recharge current density was 500


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mA/gcarbon. (C) Accumulated oxygen evolution compared with the theoretical amount. (D) NMR spectra of PYR14TFSI electrolyte after cycling tests.

The high recharge overpotential is an important challenge for Li-O2 batteries. It is connected to the detailed chemical processes involved during the decomposition of Li2O2. While the low overpotential, one-electron processes are frequently observed for the discharge processes, a similar pathway for the recharge has been comparatively much more difficult. This is because such a process requires the mobilization of superoxide species during recharge, which tend to be surface bound and feature poor mobilities. Our results presented here prove that when the superoxide species are stabilized in the solution by the presence of PYR14TFSI, the one-electron pathway featuring low recharge overpotentials becomes possible. Furthermore, it is shown that discharge overpotentials may be inadvertently increased due to poor O2 diffusivity in the PYR14TFSI, which may be addressed by adding an electrolyte that favor O2 diffusion (e.g., DME). Together, high round-trip efficiencies (up to 83%) were obtained.

ASSOCIATED CONTENT Supporting Information: Experimental details and additional characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]


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Author Contributions J.X. carried out the overall experimental procedures; Q.D. and I.M. performed product detections including titration and GCMS with X.Y. and Q.C.’s assistance. P.D. and W.F. prepared 3DOm carbon. D.W. supervised the project. J.X. and D.W. wrote the manuscript. All authors read and commented the manuscript. Funding Sources The work is sponsored by Boston College (to J.X., Q.D., I.M., X.Y., Q.C. and D.W.). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank H. Wu, T. Jayasundera, L.-Y. Chou, and C.-K. Tsung for their technical assistance.

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Low recharge overpotentials are achieved by one-electron processes for the decomposition of Li2O2. The incorporation of PYR14TFSI, which interacts strongly with O2- species and helps solvate the necessary reaction intermediate, is key to this feat. The results open up doors toward high round-trip efficiency Li-O2 battery operations.


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Achieving Low Overpotential LiâO2 Battery Operations by Li2O2

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