Electrochemical Reactivities - The

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LiO2: Cryosynthesis and Chemical/Electrochemical Reactivities Xinmin Zhang, Limin Guo, Linfeng Gan, Yantao Zhang, Jing Wang, Lee Johnson, Peter G. Bruce, and Zhangquan Peng J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 08 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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The Journal of Physical Chemistry Letters

LiO2: Cryosynthesis and Chemical/Electrochemical Reactivities Xinmin Zhang, a,+ Limin Guo, a,+ Linfeng Gan, a Yantao Zhang, a Jing Wang, a Lee R. Johnson, b Peter G. Bruce,b and Zhangquan Peng a,* a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China

b

Departments of Materials and Chemistry, University of Oxford, Parks Road, Oxford OX1 3PH, UK

ABSTRACT: The reduction of O2 to solid Li2O2, via the intermediates O2- and LiO2, is the desired discharge reaction at the positive electrode of the aprotic Li-O2 batteries. In practice, a plethora of byproducts are identified together with Li2O2 and have been assigned to the side reactions between the reduced oxygen species (O2-, LiO2 and Li2O2) and the battery components (the cathode and electrolyte). Understanding the reactivity of these reduced oxygen species is critical for the development of stable battery components and thus high cycle life. O2- and Li2O2 are readily available and their reactivities have been studied in depth both experimentally and theoretically. However, little is known about LiO2, which readily decomposes to Li2O2 and is thus unavailable under usual laboratory conditions. Here, we report the synthesis and reactivity

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of LiO2 in liquid NH3 at cryogenic temperatures and conclude that LiO2 is the most reactive oxygen species in Li-O2 batteries.

TOC GRAPHICS

Because of its high theoretical specific energy that is far beyond what the current and even future Li-ion technology can achieve, the Li-O2 battery has attracted a great deal of interest in the past few years.1-10 However, realization of a practical Li-O2 battery is a major challenge because the overwhelming majority of current Li-O2 batteries suffer from limited chemical reversibility. 3,4,6

For instance, a plethora of byproducts (such as Li2CO3, LiOH, HCOOLi, CH3COOLi, etc.)

are identified together with Li2O2 at the end of discharge of Li-O2 cells, where the specific byproducts depend on the electrolyte solvents used.11-21 This limited electrochemical reversibility has been assigned to parasitic side reactions between the reduced oxygen species formed at the positive electrode of the Li-O2 cell and the cell components such as the cathode material and electrolyte.3,4,11-21 So far three reduced oxygen species, O2-, LiO2 and Li2O2, have been unequivocally identified in Li-O2 batteries7,8 and each of them has been invoked to account for


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the unwanted side reactions.19-21 However, it is still unclear as to which are strong nucleophiles and destructive to cell components. Towards eliminating (or significantly alleviating) these side reactions (i.e., building Li-O2 batteries with better reversibility), it is crucial to have a fundamental understanding of the reactivity of the reduced oxygen species O2-, LiO2 and Li2O2 formed in Li-O2 batteries. O2- and Li2O2 are readily available and their reactivities have been studied in depth both experimentally and theoretically.11-21 However, little is known about LiO2 because of its instability and therefore unavailability under usual laboratory conditions. In this work, we report a study of the synthesis and chemical/electrochemical reactivities of LiO2 species (not O2-…Li+ ion pair separated by solvents) in liquid NH3 at cryogenic temperatures (typically -78 oC), and compare its chemical reactivity with those of O2- and Li2O2 under the same conditions. It was found that O2- and Li2O2 are stable in liquid NH3, while LiO2 can react with liquid NH3 forming LiOH and LiOH.H2O, suggesting it is the most reactive oxygen species in Li-O2 batteries. The mechanism of the reaction of LiO2 and NH3 has been investigated using density functional theory calculations. Moreover, we also provide direct evidence that LiO2, while highly reactive towards its surroundings, can be electro-reduced to more stable Li2O2, a key step in O2 reduction to Li2O2 that has been proposed in many fundamental studies of Li-O2 electrochemistry but never before proved experimentally. Probing the reactivity of O2- was conducted by stirring tetramethylammonium superoxide (TMA+O2-) in liquid NH3 at -78 oC. TMA+O2- was cryosynthesized according to literatures22, 23 (see also Supporting Information for details) and used as O2- source because it contains almost unperturbed O2- and has good solubility in liquid NH3. After 12 hours of reaction, no chemical change or mass loss or gain was observed by FTIR, Raman, iodimetric and gravimetric analyses,


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see supplementary Figure S1 and Table S1. To probe the reactivity of Li2O2, freshly ball-milled commercial Li2O2 was stirred in liquid NH3 at -78 oC for 12 hours. Similar to the case of TMA+O2-, Li2O2 does not show any observable reactivity towards liquid NH3, see Figure S2 and Table S2 for the analytical results of the Li2O2 before and after treatment with liquid NH3. Unlike TMA+O2- and Li2O2 that are stable at room temperature and available with high purity, LiO2 can only be stable at temperatures below -35 oC and has never been successfully isolated before.25-27 However, in the context of Li-O2 batteries operated at room temperature, LiO2 has been observed as a transiently stable intermediate during the oxygen reduction reaction8, 23 or as a stable discharge phase in an Ir-catalyzed graphene-based Li-O2 cell.28 To obtain LiO2 species and study its reactivity, a cryochemistry based strategy has been developed, in which LiO2 was generated by oxygenation of liquid NH3 containing dissolved lithium metal at -78 oC (see Figure S3a for the details of experimental setup). It is known that liquid NH3 has the ability to dissociate lithium metal to solvated electrons and Li+ ions, and that the obtained Li-NH3 solution has considerable stability at cryogenic temperatures.24 By oxygenation of the Li-NH3 solution, the solvated electrons are expected to reduce O2 to O2- and combine with Li+ ions forming LiO2.29 Specifically, lithium metal was first dissolved in a strictly dry liquid NH3 at -78 oC, typically with a mole fraction of 0.04 (Li:NH3, mol/mol), under which conditions lithium was known to dissociate to Li(NH3)4+ and e(NH3)4-, and a fine blue color rapidly evolved once the dissolution initialized (Figure S3b). This fine blue color is a characteristic of solvated electrons.24 Purging dry O2 gas through the Li-NH3 solution gradually changed the blue color to bright lemon-yellow, which is the characteristic color of alkali metal superoxide, 30 see Figure S3c. This is consistent with the reaction O2 + e- + Li+ = LiO2 occurring in the Li-NH3 solution when purged with O2 gas and is the homogeneous analogue of the heterogeneous electrochemical O2 reduction in an


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aprotic Li+ electrolyte.3,4,8,10,23 The formation of LiO2 was further supported by a Raman spectroscopic study of a concentrated reaction solution at -78 oC, using a fiber optic probe that was interfaced with a portable Raman spectrometer (Figure S3a), in which a 1139 cm-1 band (Figure 1a) was identified and assigned to the O-O stretching vibration of LiO2 according to the frequencies reported for adsorbed LiO2, 8,28 bulk NaO2 31 and KO2.32 However, the bright lemon-yellow solution did not persist and gradually transformed to and stayed as a white suspension (Figure S3d). Coupled with this color-change process, gas release was observed in the bubbler linked to the reaction vessel. The released gas was collected at the end of reaction, and was identified as N2 (Figure 1b) by mass spectrometry.7, 23 See Supporting Information for the details of gas collection. To isolate the product formed in liquid NH3, Ar gas flow was used to assist the evaporation of liquid NH3 and a white solid powder was obtained at the bottom of the reaction vessel (Figure S3e). The solid product was collected and subjected to a combined analysis using FTIR, Raman and PXRD techniques. It was found that the final product of oxygenation of the Li-NH3 solution is a mixture of LiOH and LiOH.H2O, Figure 1c-e.


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* = NH3

(a) -1

Counts

LiO2 (1139 cm )

Oxygenated

*

*

Li-NH3 Liquid NH3

Gas concentration (%)

600

120

800

1000

1200 1400 1600 -1 Raman shift (cm )

(b)

1800

2000

Ar carrier

80 40 N2 released

0 20

30

40

50

60

Time (min) The final product LiOH. H2O

Transmittance (%)

(c)

LiOH

4000

3000

2000 -1 Wavenumber (cm )

1000

The final product LiOH.H2O

(d) Counts

LiOH

1000

2000 -1 Raman shift (cm )

(e)

3000

4000

The final product LiOH.H2O (PDF#24-0619) LiOH (PDF#32-0564)

Intensity (a.u)

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20

30

40 2θ (degree)

50

60

Figure 1. (a) Raman spectra of (black) liquid NH3 and (red) oxygenated Li-NH3 solution at 78oC, * marks the bands from liquid NH3. (b) Mass spectrometric study of the gas released during oxygenation of Li-NH3 solution. (c) FTIR, (d) Raman and (e) PXRD results of the solid product of oxygenation of the Li-NH3 solution.


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It is unexpected that oxygenation of the anhydrous Li-NH3 solution produces a mixture of LiOH and LiOH.H2O instead of Li2O2, since it has been reported that transformation of LiO2 to Li2O2 in an oxygen matrix can occur by increasing the matrix temperature from 4.2 to 30 K,26 and that at room temperature O2- and Li+ can react, via LiO2, forming Li2O2 in aprotic solvents.20,23 Moreover, oxygenation of liquid NH3 containing other dissolved alkali metals (e.g., Na and K) under the same conditions produces alkali superoxides (e.g., NaO2 and KO2) instead of hydroxide, see Figures S4 and S5. However, when LiClO4 was added to the suspensions of NaO2 (or KO2) in liquid NH3, LiOH and LiOH.H2O were once again obtained, see Figures S6 and S7. Li2O and LiOH have also been stirred in liquid NH3 at -78 oC, and no reactivity has been observed, see Figures S8 and S9. Therefore, the formation of LiOH and LiOH.H2O upon oxygenation of Li-NH3 solution is exclusively related with the reaction between LiO2 and liquid NH3. It shall be noted that previously the only work of oxygenation of a Li-NH3 solution was reported in 1951,29 and the isolated products at room temperature were claimed to be LiO2 based on UV-Vis spectrophotometry, which is proved to be incorrect by the results presented in this study (See Figure 1c-e). To gain insight into the molecular mechanism of the reaction between LiO2 and NH3, theoretical calculations were conducted to identify the reaction pathways and the corresponding activation energies. To identify the reaction precursor, various geometries of Li+, O2- and molecular LiO2 in liquid NH3 were calculated (Figure S10), among which, LiO2·NH3 (Figure S10c) was selected as the optimized reactant. A reaction mechanism, in which one H of NH3 shifts to the O of LiO2 as the first step (i.e., H abstraction reaction) followed with several elementary steps including O-O scission and structural reorganization, was identified


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theoretically. The detailed elementary steps and the corresponding activation energies are depicted in Figure S11. Based on the above experimental results, it can be concluded that LiO2, among the reduced oxygen species of O2-, LiO2 and Li2O2, has the highest chemical reactivity in liquid NH3. The reactivity trends of O2-, LiO2, Li2O2 in other organic solvents (e.g., DMSO, MeCN, DME and TEGDME) commonly used in Li-O2 batteries are likely the same as in liquid NH3 due to their similarities in dehydrogenation energies (Table S3), if H abstraction is the first step of parasitic side reactions.33 In addition, LiO2 also has higher reactivity than the reduced oxygen species O2-, NaO2 and KO2 involved in Na/K-O2 batteries,31,32 which sheds light on the underlying reason why they have better reversibility or less side reactions than their Li-O2 siblings.6 In Li-O2 batteries, LiO2 is a necessary intermediate formed from the reaction of O2 + e- + Li+ (or O2- + Li+) during discharge.8,10,23 Depending on the properties of the electrolyte solvents used, LiO2 can either rapidly transform to Li2O2 the final discharge product,34 or dissolve as ion pairs of [O2-…Li+] having relatively long lifetime and slowly transform to Li2O2.35 Faster reaction of O2- and Li+ producing Li2O2 result in a shorter lifetime of the LiO2 intermediate,23,36 and therefore less exposure of LiO2 to the electrolyte solvents and less side reactions. In addition, it is proposed in many fundamental studies of Li-O2 electrochemistry that LiO2 can be electroreduced to Li2O2 under large overpotentials (or high discharging rates),23 and in this case less exposure of LiO2 to the electrolytes is also expected. To verify these hypotheses, i.e., the effects of solvent properties and discharging rates on the lifetime of LiO2 and Li2O2 production, Li-O2 cells have been discharged in three different electrolyte solvents including liquid NH3, DMSO and MeCN at various current densities. We note here that in liquid NH3 at cryogenic temperatures LiO2 has a relatively long lifetime, while


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in DMSO and MeCN the rate constants of the reaction of O2- and Li+ forming Li2O2 at room temperature have been measured to be 24.6 and 560 M-1s-1,23,36 respectively, suggesting that the lifetime of LiO2 in DMSO is longer than in MeCN. Moreover, O2- and Li2O2 demonstrate considerable stability in dry DMSO and MeCN solvents, as proved by the UV-Vis spectroscopy (Figure S12) of the solutions containing dissolved TMA+O2- for one week aging, and by the NMR spectra (Figure S13) of the solvents after stirring with Li2O2 for 7 days.

Table 1: Li2O2 yields in Li-O2 cells containing different electrolyte solvents discharged with a fixed capacity of 500 mAh g-1carbon under various current densities. Current Densities (mAg-1carbon )

Electrolyte Y Li O (%) 2

2

MeCNa

DMSOb

NH3c

50

70.4 ± 8.1

65.6 ± 4.7

0

100

80.6 ± 5.5

73.8 ± 5.2

0

200

86.6 ± 7.7

80.3 ± 5.6

2.3 ± 0.7

500

91.8 ± 6.3

84.6 ± 4.0

8.0 ± 3.8

a

Li1-xFePO4 | 0.1 M LiClO4 MeCN | porous carbon (O2) at room temperature, bLi1-xFePO4 | 0.1 M LiClO4 DMSO | porous carbon (O2) at room temperature, cLi4+xTi5O12 | 0.1 M LiClO4 NH3 | porous carbon (O2) at -50 oC

At the end of discharge, the cathodes were rinsed with MeCN and vacuum-dried at room temperature, and the Li2O2 formed in the cathodes were quantified by iodimetric titration,37 see Table1 for the Li2O2 yields of the cathodes and the configuration details of the corresponding LiO2 cells. It was found that for the cathodes discharged at the same current densities but in different electrolyte solutions, the solvent identity plays a critical role in determining the Li2O2 yield. For instance, at a low current density of 50 mAg-1carbon, there is no Li2O2 formation in


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liquid NH3, and the Li2O2 yields in DMSO and MeCN are ~ 65.6% and ~ 70.4%, respectively, consistent with the reversal of the order of the lifetime of LiO2 in these solvents.23,36 For the cathodes discharged in the same electrolyte solutions but at different current densities, higher discharge current density increases the Li2O2 yield. For instance, in a MeCN electrolyte the Li2O2 yield in a cathode discharged at 50 mAg-1carbon is ~ 70.4%, and has increases to~ 91.8% when discharged at 500 mAg-1carbon. It is particularly interesting that at high current densities Li2O2 can even form in liquid NH3 based Li-O2 batteries, although it has a very low Li2O2 yield of ~ 2.3% at 200 mAg-1carbon and ~ 8.0% at 500 mAg-1carbon, see Table 1. The formation of Li2O2 in NH3-based Li-O2 cell at high current densities provides compelling evidence that LiO2, while highly reactive towards its surroundings, can be electro-reduced to more stable Li2O2, a key step that has been proposed in many fundamental studies of Li-O2 electrochemistry but never before proved experimentally. In summary, LiO2, the unusual and contentious intermediate in Li-O2 batteries, has been cryosynthesized in liquid NH3, and its chemical reactivity has been examined and compared with those of O2- and Li2O2. It was found that LiO2 has the highest reactivity among the reduced oxygen species involved in Li-O2 batteries. It was also found that the reduced oxygen species O2-, NaO2 and KO2 involved in aprotic Na/K-O2 batteries are less reactive than LiO2, explaining why Na/K-O2 batteries have less side reactions than their Li-O2 siblings. The identification of LiO2 as the most reactive oxygen species in Li-O2 batteries has important consequences for the development of Li-O2 cells with better reversibility. For instance, by selecting electrolyte solvents, in which LiO2 can rapidly disproportionate to Li2O2, and by optimizing cell operating conditions to promote the electro-reduction of LiO2 to Li2O2, Li-O2 cells having less side reactions could be realized.


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ASSOCIATED CONTENT Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: DOI: 10.1021/ Experimental procedures, FTIR and Raman spectra of the reaction of TMA+O2- and liquid NH3 (Figure S1); FTIR, Raman and PXRD analyses of the reaction of Li2O2 and liquid NH3 (Figure S2); photos of the cryosynthesis setup and the Li-NH3 solution at various stages of oxygenation (Figure S3); Raman and PXRD results of NaO2 obtained by oxygenation of Na-NH3 solution (Figure S4); Raman and PXRD results of KO2 obtained by oxygenation of K-NH3 solution (Figure S5); PXRD analysis of the non-soluble product of the reaction of NaO2 and LiClO4 in liquid NH3 (Figure S6); PXRD analysis of the non-soluble product of the reaction of KO2 and LiClO4 in liquid NH3 (Figure S7); FTIR and PXRD analyses of the reaction of Li2O and liquid NH3 (Figure S8); FTIR and PXRD analyses of the reaction of LiOH and liquid NH3 (Figure S9); Modeling of the Li+, O2- ionic species and solvated LiO2 neutral species in liquid NH3 (Figure S10); Calculated reaction pathway of LiO2 and NH3 (Figure S11); UV-Vis spectra of TMA+O2- in DMSO and MeCN solutions (Figure S12); 1H and 13C NMR spectra of the reaction of Li2O2 and solvents including CH3CN and DMSO (Figure S13); Mass and purity analyses of the reaction of TMA+O2- and liquid NH3 (Table S1); Mass and purity analyses of the reaction of Li2O2 and liquid NH3 (Table S2); Deprotonation energy and dehydrogenation energy of typical electrolyte solvents (Table S3). AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (Z.P) Author Contributions +

X.Z and L.G. contributed equally to this work.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Z.P. is indebted to the National Foundation of China (Grant No. 91545129 and 21575135), the “Strategic Priority Research Program” of the CAS (Grant No. XDA09010401), the National Key R&D Program of China (Grant No. 2016YBF0100100), and the Science and Technology Development Program of the Jilin Province (Grant No. 20150623002TC and 20160414034GH). Helpful assistance from and insightful discussion with Dr. Matthew T. J. Lodge and Prof. Peter P. Edwards (University of Oxford, U.K.) about the Li-NH3 system and the critical role of LiO2 in aprotic Li-O2 batteries are gratefully acknowledged. REFERENCES (1) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29. (3) Lu, J.; Li, L.; Park, J.-B.; Sun, Y.-K.; Wu, F.; Amine, K. Aprotic and Aqueous LiO2 Batteries. Chem. Rev. 2014, 114, 5611-5640. (4) Luntz, A. C.; McCloskey, B. D. Nonaqueous Li-Air Batteries: A Status Report. Chem. Rev. 2014, 114, 11721-11750.


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Electrochemical Reactivities - The

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