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Cathodically Stable Li-O2 Battery Operations Using Water-in-Salt Electrolyte

Dong, Qi; Yao, Xiahui; Zhao, Yanyan; Qi, Miao; Zhang, Xizi; Sun, Hongyu; He, Yumin; Wang, Dunwei

Published in:Chem

Link to article, DOI:10.1016/j.chempr.2018.02.015

Publication date:2018

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Dong, Q., Yao, X., Zhao, Y., Qi, M., Zhang, X., Sun, H., He, Y., & Wang, D. (2018). Cathodically Stable Li-O2Battery Operations Using Water-in-Salt Electrolyte. Chem, 4(6), 1345-1358.https://doi.org/10.1016/j.chempr.2018.02.015

Article

Cathodically Stable Li-O2 Battery OperationsUsing Water-in-Salt Electrolyte

Qi Dong, Xiahui Yao, Yanyan

Zhao, ..., Hongyu Sun, Yumin

He, Dunwei Wang

dunwei.wang@bc.edu

HIGHLIGHTS

Water-in-salt electrolyte exhibits

superior stability for Li-O2 battery

operations

Li2O2 was identified as the

discharge product in water-in-salt

electrolyte

Li-O2 battery operation for up to

300 stable cycles has been

obtained

Development of the Li-O2 battery as a practical energy storage technology has

been underpinned by the lack of a stable electrolyte to enable reversible

conversion between O2 and Li2O2, given that previous applied organic

electrolytes all show reactivity toward reactive oxygen species. Wang and

colleagues show that this issue can be solved with a water-in-salt electrolyte that

contains no organic solvent molecules. The net result is a highly effective

electrolyte that enables stable Li-O2 battery operations up to 300 cycles.

Dong et al., Chem 4, 1345–1358

June 14, 2018 ª 2018 Elsevier Inc.

https://doi.org/10.1016/j.chempr.2018.02.015

Article

Cathodically Stable Li-O2 BatteryOperations Using Water-in-Salt ElectrolyteQi Dong,1 Xiahui Yao,1 Yanyan Zhao,1 Miao Qi,1 Xizi Zhang,1 Hongyu Sun,2 Yumin He,1

and Dunwei Wang1,3,*

The Bigger Picture

As an electrochemical energy

storage technology that holds the

highest theoretical energy

density, the Li-O2 battery has

been studied for over two

decades. Its reported

performance, however, remains

much lower than expected. An

important reason for the slow

progress is the lack of stable

electrolytes. Here, we present a

solution to such a challenge. We

SUMMARY

Development of the Li-O2 battery into a practical technology hinges on the

availability of a stable electrolyte. Because of the high reactivity of oxygen

species in the system, no known organic electrolytes meet the stability require-

ments. The search for a suitable electrolyte system remains an outstanding chal-

lenge in Li-O2 battery research. Here, we show that the issue can be solved with

the use of a water-in-salt electrolyte system that involves no organic solvents.

In essence, the electrolyte consists of super-concentrated LiTFSI (lithium

bis(trifluoromethanesulfonyl)imide), in which H2O molecules are locked to the

ions and exhibit little reactivity toward Li2O2 or other oxygen species. The net

result is a highly effective electrolyte that permits stable Li-O2 battery opera-

tions on the cathode with superior cycle lifetimes. A new door to practical

Li-O2 batteries with high performance is opened up.

show that the ‘‘water-in-salt’’

electrolyte, which is essentially a

super-concentrated aqueous

solution, enables stable operation

of a Li-O2 battery on the basis of

reversible formation and

decomposition of Li2O2 for

superior long cycle lifetimes.

More importantly, it presents a

platform for future optimizations

to realize the full potential of the

Li-O2 battery as a stable, high-

capacity electrochemical energy

storage technology.

INTRODUCTION

Compared with other electrochemical energy storage technologies (e.g., Li-S and

Li-ion), the Li-O2 battery promises the highest theoretical energy density and has

therefore attracted significant research attention.1–11 Development of the Li-O2 bat-

tery into a practical technology, however, has been undermined by severe issues

regarding parasitic chemical reactions.12 In particular, the ubiquitous presence of

reactive oxygen species has proven an outstanding challenge because various cell

components, including the cathode, the electrolyte, and the Li anode, all show reac-

tivity toward oxygen species.12,13 Numerous approaches have been proposed to

address the issue, and varying degrees of success have been demonstrated. For

example, it has been shown that replacing the carbon cathode with non-carbon

ones greatly improves the cyclability.14–19 The issues regarding Li anode reactivity

could be addressed by covering the Li metal with artificial solid-electrolyte-interface

layers or by replacing the Li metal anode with other materials.20–25 The latter

approach (replacing Li metal) would reduce the achievable capacities.26 The most

significant issue regarding parasitic chemical reactions with the electrolyte, how-

ever, remains outstanding (Figure 1A).12,27,28 Some ideas have been proposed

and tested, such as modifying the molecular structure of the existing organic sol-

vents or using solid electrolytes to replace the liquid ones.29–32 Although promising

preliminary results have been reported, further research is needed to fully evaluate

the effect of these approaches in stabilizing practical Li-O2 battery operations. To

date, Li-O2 batteries with long cycle lifetime and stable electrolyte as supported

by rigorous product detection are still rare.

Careful examination of the issues associated with electrolyte stability reveals that

reactivity toward organic solvent molecules lies at the heart of the problem.33–36

Chem 4, 1345–1358, June 14, 2018 ª 2018 Elsevier Inc. 1345

Figure 1. Schematic Illustrations of the Key Advantages Offered by the WiS System and Its

Electrochemical Behaviors

(A) All known organic electrolytes exhibit a certain degree of reactivity toward the various oxygen

species in a Li-O2 battery system, resulting in the formation of by-products (represented as brown

blocks and bubbles) other than Li2O2 (represented as light blue toroids) and O2 (represented as

light blue bubbles). The parasitic chemical reactions are an important reason for the poor

cyclability of Li-O2 batteries reported to date.

(B) WiS solves the problem by replacing organic solvents. The desired functionality of high ionic

concentration and conductivity is achieved by mixing super-concentrated LiTFSI (21 m) with H2O.

Only Li2O2 is formed during discharge. High efficiency is observed during recharge as well. Stable

Li-O2 battery operation is realized.

(C) Cyclic voltammograms measured on a glassy carbon working electrode in WiS with O2 (solid line)

and N2 (broken line). Inset: CVs measured in DMA (top) and DME (bottom), respectively, with O2.

(D) The voltage profiles of the Vulcan carbon electrode during cycling under constant current

(50 mA/gcarbon) with a cutoff capacity of 250 mAh/gcarbon in WiS, DME, and DMA. LFP was used as a

pseudo anode. The test cells were cycled 8, 16, and 70 times in DMA, DME, and WiS, respectively,

before failing.

1Department of Chemistry, Boston College,Chestnut Hill, MA 02467, USA

2Department of Micro and Nanotechnology,Technical University of Denmark, KongensLyngby 2800, Denmark

3Lead Contact

*Correspondence: dunwei.wang@bc.edu

https://doi.org/10.1016/j.chempr.2018.02.015

These molecules are necessary because they provide the desired physical proper-

ties, such as ionic mobility and O2 solubility, to support Li-O2 battery operations.

It is, therefore, conceivable to address the issue by replacing organic solvent mole-

cules. The challenge would be how to maintain the high ionic mobility without the

supporting organic solvent. This goal became possible recently with a report on a

unique mixture of water and salt, which has proven effective in improving the perfor-

mance of Li-ion batteries.37–39 The electrolyte has been referred to as water-in-salt

(WiS).38,40–43 In essence, WiS takes advantage of the strong solvation effect of su-

per-concentrated LiTFSI (lithium bis(trifluoromethanesulfonyl)imide). Li+ is known

to be strongly solvated by H2O molecules. At high concentrations (21 mol/1 kg

of H2O, or 21 M), TFSI� was also found to exist in the solvation sheath. The net result

is that all H2O molecules in WiS strongly solvate both ions (Li+ and TFSI�), leavingfew free H2O molecules to behave like bulk H2O. As such, the chemical reactivity

characteristic of bulk H2O is greatly suppressed, whereas good ionic conductivity

(�10 mS/cm) is maintained to serve as an electrolyte.38 Indeed, previous research

has shown that the operation potential window of WiS can be significantly increased

1346 Chem 4, 1345–1358, June 14, 2018

in comparison with typical aqueous electrolytes (from 2.63–3.86 V in aqueous solu-

tion to 1.9–4.9 V in WiS; unless noted, all potentials reported in this article are rela-

tive to Li+/Li).38 More importantly, the limited protons in the WiS electrolyte have

been shown to be strongly solvated and thus exhibit little reactivity toward nucleo-

philes such as polysulfide species, by which a high performance aqueous Li-ion/S

battery has been demonstrated.43 Inspired by these results, in this work we explore

whether such an electrolyte system is suitable for Li-O2 battery operations. Our idea

is to take advantage of the fact that there are no organic solvents in WiS electrolyte.

We expect that the known electrolyte decomposition pathways in typical Li-O2 bat-

teries would be inaccessible (Figure 1B).12 Consequently, by-product formation is

expected to be greatly suppressed. Indeed, our results support that the WiS system

is stable under Li-O2 battery operation conditions. Most importantly, we prove that

the battery discharges and recharges through reversible conversion between O2

and Li2O2, but not reactions with H2O. As a result, significantly longer cycle lifetime

(over 70 cycles) can be achieved with a carbon cathode including WiS electrolyte

than with that including organic electrolytes (e.g., 16 cycles in 1,2-dimethoxyethane

[DME]). When the carbon cathode is replaced by a stable non-carbon one, over 300

cycles of reversible Li-O2 battery operations can be measured.

RESULTS

Electrochemical Performance of Li-O2 Cells Using Water-in-Salt Electrolyte

The first and most important concern we addressed was whether H2O in the

WiS electrolyte dominated the electrochemical behavior. That is, it is of critical

importance to understand whether the WiS electrolyte enables reversible aprotic

conversion between O2 and Li2O2. For this purpose, we carried out electrochemical

experiments in the WiS electrolyte and compared the cyclic voltammograms (CVs)

with those obtained in two organic electrolytes (Figure 1C). Specifically, we chose

to compare the WiS electrolyte with 0.1 M LiTFSI in N,N-dimethylacetamide

(DMA), which was predicted to be stable against nucleophilic attack by

superoxide,44,45 and 0.1 M LiTFSI in DME, which exhibits one of the best stabilities

and therefore has been the most commonly used electrolyte for Li-O2

research.2,15,18,46 As shown in Figure 1C, the reduction wave is turned on at ca.

2.7 V, corresponding to O2 / Li2O2. On the reverse scan, the oxidation wave is

turned on at ca. 3.0 V, corresponding to Li2O2 / O2. These redox features closely

resemble those in DME and DMA (Figure 1C, inset) but are missing in the CV where

O2 is absent. Moreover, these electrochemical features are distinctly different

from those obtained in 1 m LiTFSI in H2O (1 mol/1 kg, denoted as salt-in-water

[SiW]),38 where the Li2O2 oxidation wave is missing (Figure S1). This is because the

O2 reduction product in a normal aqueous electrolyte is expected to interact

strongly with H2O (especially H+ in H2O) to undergo disproportionation.

The results shown in Figure 1C suggest that the electrochemical processes in WiS

within the measurement potential window are dominated by Li2O24O2 conversion

but not the electrolyte decomposition (such as H2O splitting).

Next, we performed galvanostatic characterization of the WiS electrolyte, and the

purpose was to study whether the electrolyte supports repeated Li2O24O2 conver-

sion under pseudo-operation conditions. For these experiments, we used Vulcan

carbon as the cathode (see Supplemental Information for preparation details). Li is

not suitable as an anode for these tests because it would react with H2O in WiS

(the stability window of WiS is 1.9–4.9 V versus Li+/Li).38 To eliminate possible com-

plications due to parasitic chemical reactions on the Li anode,47 we used LiFePO4

(LFP) as a pseudo anode for this portion of the study. LFP has been used as a pseudo

Chem 4, 1345–1358, June 14, 2018 1347

anode material for Li-O2 battery operations and has proven reliable;14,34,44,48 it has

also been tested in a WiS system previously.40 It is, nonetheless, noted that this

approach might not be suitable for practical Li-O2 battery implementations because

the utilization of LFP as the anode would not only decrease the overall cell voltage

but also undermine the gravimetric energy density. The true value of doing so lies

in the knowledge generated by presenting a study platform where confounding fac-

tors such as parasitic chemical reactions at the Li anode are eliminated.12,47 In addi-

tion to serving as the pseudo anode, a second piece of delithiated LFP was used as a

reference electrode (Figures S2–S4).40 All potentials have been converted to be rela-

tive to Li+/Li. Important to our purposes, no measurable parasitic chemical reactions

or catalytic effects were observed in this part of the study when the LFP pseudo

anode and reference electrode were used.

As shown in Figure 1D, significantly longer cycling numbers were achieved in WiS

(70 cycles) than in DME (16 cycles) before the discharge potential reached the

2.3 V cutoff. DMA (8 cycles) exhibited slightly worse performance than DME. In

all three electrolyte systems, an obvious decrease in the discharge potentials

was observed toward the end of the cycling experiments. The phenomenon

has been previously ascribed to the degrading catalytic activities of the carbon

cathode as a result of parasitic chemical reactions on the surface.2,12,13 The

voltage-capacity curves of selective cycles (Figure S5) support this understanding.

The relatively stable recharge potential of the WiS system toward the end of the

cycling test, however, is distinct from that in DME or DMA, where a rapid increase

of the recharge potential was also observed (Figure 1D). Previously, such an

increase has been ascribed to the presence of by-products (e.g., organic carbon-

ates due to electrolyte decomposition or carbon cathode decomposition or

both).2,12,13,49 That we do not observe this increase in WiS suggests the decompo-

sition of the WiS electrolyte is minimal. Lastly, we note that high capacity was also

measured in the WiS electrolyte, and the discharge capacity ranged from ca. 3,500

to 1,500 mAh/gcarbon for current densities of 50–400 mA/gcarbon, respectively (Fig-

ures S6 and S7). The values are comparable with those reported in the literature

under similar test conditions.50,51

Detection of Li2O2 as the Discharge Product

We then used X-ray diffraction (XRD) to examine the discharge products. Upon deep

discharge (with the capacity reaching 2,500 mAh/gcarbon), the most prominent

diffraction peaks at 2q = 32.8�, 34.9�, and 40.6� were unambiguously assigned

to Li2O2 (Figure 2A).17 The low-intensity peaks at 2q = 31.7� and 37.1� are close to

the standard peaks of Li2CO3, whose presence is commonly observed as a result

of the reactivity of the carbon cathode under deep discharge conditions.12,49 As dis-

cussed below, Li2CO3 is unlikely a result of electrolyte decomposition. The

morphology of the deep discharge products was studied by scanning electron mi-

croscopy (SEM; Figure 2E) and transmission electron microscopy (TEM; Figure 2F).

A representative toroidal structure was observed, consistent with literature reports

where fast kinetics favors toroid formation (e.g., when H2O impurities were present

in the organic electrolytes).51 TEM micrographs and electron energy loss spectros-

copy (EELS) spectra (Figure S8) prove that the toroid is indeed Li2O2, consistent

with previous studies.52,53 We next characterized the test cell ex situ under

pseudo-operation conditions (cycled at 500 mAh/gcarbon) to study the reversibility

of the product formation and decomposition. The XRD patterns shown in Figure 2B

were consistent with those of the deep discharge, albeit at lower intensities. No

other major diffraction peaks were observed. The Raman spectra shown in Figure 2C

provide additional support that Li2O2 is the product of discharge,54 which can be

1348 Chem 4, 1345–1358, June 14, 2018

Figure 2. Qualitative Product Analysis

(A) XRD pattern of a deeply discharged cathode (2,500 mAh/gcarbon). The standard diffraction patterns of various possible products are shown below the

experimental pattern. The peaks (labeled by ✶) can be unambiguously assigned to Li2O2.

(B) XRD patterns of pristine, discharged, and recharged carbon cathodes under pseudo-operation conditions (500 mAh/gcarbon). Peaks are

marked by ✶.

(C) Raman spectra of pristine, discharged, and recharged carbon cathodes. The two distinguishing peaks (labeled by ✶) are from Li2O2.

(D) XPS spectra of pristine, discharged, and recharged carbon cathodes showing the binding energies of Li 1s electrons. The component in the

deconvoluted data assigned to Li2CO3 is most likely due to the brief sample exposure to ambient air during sample transfer.

(E) SEM image of discharged cathode. Scale bar: 2 mm.

(F) TEM image of a typical Li2O2 toroid. Scale bar: 500 nm.

reversibly decomposed upon recharge. Similar to the XRD studies, the Raman

spectra were collected without sample exposure to ambient air. We used X-ray

photoelectron spectroscopy (XPS) to study the Li 1s electron-binding energies,

and the spectra are plotted in Figure 2D. Note that for the XPS experiment, the

samples were briefly exposed to ambient air during sample transfer because of

instrumentation limitations. That is why we see contributions from Li2CO3 in the

deconvoluted peak compositions. Nevertheless, consistent with other literature re-

ports,18 the main composition of the discharge product was indeed Li2O2. Impor-

tantly, we saw no measurable Li2CO3 contribution in the recharged sample (Figures

2D and S9), further supporting that the observed Li2CO3 by XPS in the discharged

state is due to exposure to ambient air but not inherent to the discharge process.

This behavior (Figure 2D) needs to be distinguished from that of deep discharge

(Figure 2A), where carbon cathode degradation may become measurable because

of the high capacities and the low cutoff potentials.12,48,55 We applied Fourier-trans-

form infrared spectroscopy (FTIR) to confirm that Li2O2 was the discharge product

and could be fully decomposed (Figure S10).15 Importantly, no LiOH

was detected by FTIR. Additional SEM micrographs also revealed that Li2O2 formed

upon discharge could be completely removed after recharge (Figure S11).

The toroidal morphology of Li2O2 was consistently observed for a discharge

Chem 4, 1345–1358, June 14, 2018 1349

Figure 3. Quantitative Product Analysis

(A) The yield of Li2O2 as measured by iodometric titration in different electrolytes. The yield is

presented as the percentage of the expected theoretical amount on the basis of the charges

passed. Data are represented as mean G standard deviation.

(B) Comparison of Li2O2 measured with the WiS electrolyte (same as in A) and without the WiS

electrolyte, proving that no significant contribution to the capacity comes from soluble by-products

such as H2O2. Data are represented as mean G standard deviation.

(C) Detection of isotope labeled 36O2 evolution by GC-MS with H218O in the WiS electrolyte

(right axis). The electro-oxidation of the electrolyte was only observed at high potentials (ca. 4.6 V

versus Li+/Li). No measurable CO2 evolution was detected. The voltage profile (left axis) is plotted

together for easy viewing.

(D) Normalized 1H NMR spectra of cycled WiS, DME, and DMA electrolytes. The peak centered at

4.78 ppm is from HDO. The characteristic by-product of LiCO2H (at 8.46 ppm) can be observed in

the magnified view shown on the right. Trace amount of LiCO2H observed in the WiS is most likely a

result of the decomposition of the carbon cathode.

rate % 200 mA/gcarbon, above which less regularly shaped products started to

appear (Figure S12). No LiOH was observed in this part of the product

analysis. The result provides strong support that the WiS electrolyte enables

aprotic O2 4 Li2O2 conversion but not O2 4 LiOH, which would be expected

from common aqueous Li-O2 systems and is undesired for the present study.4,56

The selectivity is most likely due to limited H2O reactivity and mobility toward nucle-

ophiles such as Li2O2. Similarly low reactivity toward polysulfides in the WiS electro-

lyte has been reported by both experimental and theoretical studies.43

Quantitative Product Analysis

Quantitative analysis of the product was conducted next, and the purpose was to

establish that the measured charges in electrochemical characterization corre-

sponded to Li2O2 formation and decomposition. In other words, our goal was to

prove that the faradic efficiency was high. We first explored the iodometric titra-

tion technique developed by McCloskey et al.55 For the WiS electrolyte, we ob-

tained a yield of 85.0%. The yield for DME was 79.4%, and that for DMA was

79.1% (Figures 3A and S13). As has been discussed by McCloskey et al., the titra-

tion yield is sensitive to several factors such as the type of cathode materials used,

1350 Chem 4, 1345–1358, June 14, 2018

the loading amount, and the discharge capacity, as well as the extent to which im-

purities are removed.55 The yields we obtained are in line with the literature re-

ports, although slightly lower than the best reported values (see Table S1).57

That we measured the highest yields in WiS strongly supports that a lower amount

of by-product was formed in WiS than in DME or DMA. The unaccounted products

by the titration method (ca. 15%) could be dissolved species (e.g., H2O2 and LiOH)

and non-Li2O2 solid species (e.g., carbonates), as well as the result of system errors

inherent to the titration method. To study whether any products could be dis-

solved in the electrolyte as a result of a strong solvation effect or parasitic reac-

tions,58,59 we compared the titration results with and without the electrolytes in

Figure 3B and observed no significant differences. The result suggests that no

measurable contribution of the measured capacity comes from H2O2 as possible

by-products, further supporting the suitability of WiS as a stable electrolyte system

for Li-O2 battery operations. To characterize the recharge products, we used isoto-

pically labeled H218O in the WiS electrolyte. Only at high recharge potentials

(over ca. 4.6 V versus Li+/Li) did we observe detectable 36O2 (ca. 4%) as an indica-

tion of electrolyte oxidation (Figure 3C). Importantly, no measurable CO2 evolution

was detected throughout the recharge process, in stark contrast to when organic

electrolytes were used.17,57,60 The gas evolution result lends strong support that

WiS electrolyte is more stable against decomposition during recharge (Figures

S14 and S15). To further prove that the electrolyte was stable upon cycling, we

examined the electrolytes after five cycles of repeated discharge and recharge

(in the discharged state) by using nuclear magnetic resonance (NMR) spectros-

copy. The spectra (Figures 3D, S16, and S17) clearly show that a small amount

of LiCO2H as by-product was present with DME and DMA electrolytes but was

by and large absent with the WiS electrolyte. Lastly, we also monitored the pres-

sure during the first ten cycles of the test cell featuring the WiS electrolyte. The

calculated pressure recovery efficiency (�DPdischarge/DPrecharge) was ca. 100%

throughout the process (Figure S18). Taken as a whole, the quantitative analysis

presented in this section strongly supports that the WiS electrolyte enables revers-

ible Li2O2 4 O2 conversion with minimum decomposition reactions commonly

observed in other organic electrolyte systems.

Cycling Performance of Li-O2 Cells Using Carbon-free Cathode

It has been reported that the parasitic chemical reactions within the electrolyte and

on the carbon cathode exhibit a synergistic effect.12 For instance, the susceptibility

of carbon to reactive oxygen species leads to the formation of carbonates on carbon

cathode surfaces, which increases the need for applied potentials, especially during

recharge. The high applied potentials are an important reason for electrolyte

decomposition, which in turn increases carbonate deposition on the carbon support,

exacerbating the situation.12,49 Thus, the use of carbon as a cathodemakes it difficult

to fully actualize the promises enabled by the WiS electrolyte. Indeed, we attributed

the loss of yield for WiS electrolyte as shown in Figure 3A to possible parasitic chem-

ical reactions with the carbon cathode, which was supported by spectroscopic char-

acterization of the cycled carbon electrodes (Figures S19–S21). To better take

advantage of the stability offered by WiS electrolyte, we next investigated a car-

bon-free cathode material developed by us previously, TiSi2 nanonets decorated

with Ru nanoparticles.18 The idea was to understand the true potentials enabled

by WiS without confounding factors connected to either the Li metal anode (by re-

placing it with LFP) or the carbon cathode (by replacing it with Ru/TiSi2). Indeed,

remarkable performance in terms of cyclability was obtained. At a discharge depth

of 1,000mAh/gRu (or 500mAh/g(Ru+TiSi2)), over 300 cycles of repeated discharge and

recharge was measured (Figure 4A). An average round-trip energy efficiency (based

Chem 4, 1345–1358, June 14, 2018 1351

Figure 4. Cycling Performance of a Stable Li-O2 Battery with WiS as the Electrolyte, LFP as the

Pseudo Anode, and a Non-carbon Material (Ru/TiSi2) as the Cathode

(A) The voltage profiles of selected cycles.

(B) Average voltage and round-trip energy efficiencies (based on the measured voltage of the

cathode) as a function of cycling numbers. For clarity, only data for every 20 cycles are shown.

on the measured voltage of the cathode) of ca. 62% was achieved (Figure 4B).2,18

This cyclability is among the highest reported in the literature at that capacity.4,61

The product was further characterized by XPS, Raman spectroscopy, and SEM to

be only Li2O2 (Figures S22–S24),17,18 and the results are consistent with those shown

in Figure 2 (obtained on the carbon cathode).

We note that the Ru/TiSi2 cathode is not yet optimized.62 For instance, the cata-

lytic activity of Ru in WiS for the oxygen evolution reactions (OER) may be different

from that in organic electrolyte systems, although the discharge and recharge

characteristics of Ru/TiSi2 in WiS (Figure 4A) are similar to those of Ru/TiS2 in

DME from our previous results.18 Such different catalytic activity is most likely

due to different solvation effects between H2O and DME. Moreover, the activities

of Ru/TiSi2 in catalyzing O2 reduction and evolution reactions in WiS are expected

to be different from that of carbon, which helps explain the slightly different

discharge and recharge characteristics as shown in Figures 4A and S5. Our goal

of introducing Ru was mainly to compensate the ORR activity when replacing

the unstable carbon with the stable carbon-free TiSi2 cathode (Figure S25). The tar-

geted ORR catalytic effect in WiS was indeed prominent, although the OER cata-

lytic effect was inferior to that in DME. For future optimizations, we envision that

replacing Ru with a more effective OER catalyst could readily improve the recharge

characteristics.61,63,64 Although additional research will be needed for fully under-

standing the behavior of Ru/TiSi2 as a cathode for Li-O2 battery operations, our

previous research has already established that it functions as expected in cata-

lyzing O2 reduction and evolution reactions without exhibiting the parasitic chem-

ical reactions that undermine carbon-based cathode materials.18 The results

reported here thus prove that when a stable electrolyte and a stable cathode

(both with minimum parasitic decomposition reactions) are used for Li-O2 battery

operations, long cycle lifetimes become attainable. The system studied here pro-

vides a platform for further optimization to better take advantage of this promising

electrolyte system for Li-O2 battery applications. For example, we envision that the

WiS electrolyte can be readily used for constructing practical devices with stable

anode materials (such as protected Li). To further demonstrate the value offered

by the WiS electrolyte, we have fabricated proof-of-concept Li-O2 batteries in

which protected Li is used in conjunction with WiS. Both carbon (Figure 5A) and

Ru/TiSi2 (Figure 5B) cathodes were tested. The results shown in Figure 5 are

encouraging. Nevertheless, the cyclability (ten cycles) remains relatively poor.

1352 Chem 4, 1345–1358, June 14, 2018

Figure 5. Cycling Performance of Li-O2 Batteries with WiS as the Electrolyte and Protected Li as

the Anode

(A) Li-O2 battery cycling performance using a carbon cathode.

(B) Li-O2 battery cycling performance using a Ru/TiSi2 cathode.

More engineering optimization, especially on the protected Li anode side, will

enable us to take full advantage of the WiS electrolyte system.

DISCUSSION

The reversible formation and decomposition of Li2O2 in WiS electrolyte as reported

here is unique. Although the formation of Li2O2 species in H2O has been previously

reported with a highly concentrated LiCl and LiOH mixture for aqueous Li-O2 bat-

tery applications, LiOH was found to be used for part of the recharge process.56

Compared with other commonly used organic solvents, H2O features a high

acceptor number (ca. 60).51 It is known to solvate intermediates of Li-O2 discharge

and recharge such as superoxide species.65,66 Its positive effects toward Li-O2 bat-

tery operations were not recognized until recently. For instance, McCloskey and

co-workers found that H2O (up to 4,000 ppm) is key to the formation of the charac-

teristic toroidal structures and contributes to the high discharge capacity.51 Wang

and colleagues reported that H2O in electrolyte helps reduce discharge overpoten-

tials through a proton-mediated mechanism.67 Zhou and co-workers reported

similar reduction in the overpotentials during recharge by adding H2O.59 The

Zhou group further suggested that the presence of H2O could improve the stability

of organic electrolytes by interacting with the superoxide species.59,68 On the other

hand, excess amount of free H2O will induce the formation of LiOH, which is

suggested to be detrimental for the cell operations.69,70 For the WiS

electrolyte, H2O replaces the organic solvent molecules that were popularly used

in previous Li-O2 research and enables the required functionalities of an electrolyte

with superior stability for Li-O2 battery operations. Notably, the characteristic

toroidal structure of Li2O2 formed in WiS suggests that the solvation effect

of H2O exists and mediates the discharge process toward higher capacities. The

good rate performance also lends strong support for the solvation effect by H2O.

The lack of LiOH detected in the product indicates that the proton in WiS exhibits

little reactivity toward the electrochemically formed Li2O2 during discharge.

In conclusion, we have shown that WiS, as an electrolyte system for Li-O2 battery

operations, is stable against parasitic chemical reactions with reactive oxygen spe-

cies. It provides the necessary functionalities to support aprotic Li-O2 operations

via reversible Li2O2 formation and decomposition. The lack of organic solvent mol-

ecules is a key advantage here. It eliminates the known reaction pathways that

lead to by-product formation from organic electrolyte systems. Both qualitative

Chem 4, 1345–1358, June 14, 2018 1353

and quantitative product analyses support that no measurable by-products form

in the WiS system. With a carbon cathode, greatly improved cyclability of over 70 cy-

cles can be obtained with WiS electrolyte in comparison with organic electrolytes.

When the carbon cathode is replaced with a carbon-free material, up to 300 cycles

of stable Li-O2 battery operations are obtained. This result sets a new benchmark in

Li-O2 battery performance with quantitative product detection. It sets the stage for

future optimizations to realize the full potential held by Li-O2 batteries as a stable,

high-capacity electrochemical energy storage technology.

EXPERIMENTAL PROCEDURES

Electrochemical Measurements

All electrochemical tests were carried out with a VMP3 potentiostat (BioLogic)

in either an electrochemical cell or a home-designed Swagelok-type cell.

The electrochemical data were collected in an Ar-filled/O2-tolerant glovebox

(MBRAUN, H2O < 0.1 ppm) at room temperature. CV measurements were carried

out in an electrochemical cell. WiS electrolyte was bubbled with either O2 or N2

to maintain an O2- or N2-saturated solution, respectively. It has been shown that

the solubility of O2 decreased from SiW (1 mol/1 kg, 1 m) to water-in-salt

(21 mol/1 kg, 21 M) conditions, but the decreased O2 solubility was not found to

be a limiting factor for the measurements in this study. The Pt wire and delithiated

LFP film were immersed in the electrolyte as the counter and pseudo reference

electrode, respectively. Glassy carbon (3 mm diameter) was applied as the working

electrode. The CV measurements were performed at a scan rate of 25 mV/s. The

home-designed Swagelok-type cell was equipped with three electrodes for galva-

nostatic measurements. Two delithiated LFP films were used as the pseudo anode

(counter electrode, in excess) and the pseudo reference electrode. The total capac-

ity of the delithiated LFP pseudo anode was at least three times higher than the

tested capacities of the cathode for all measurements. The Li-O2 cell with protected

Li anode was fabricated with Li-ion conducting glass-ceramics (LICGC, OHARA) as a

protection layer. Vulcan carbon or Ru/TiSi2 was used as the cathode. The current

density and capacity were normalized to the weight of Vulcan carbon or Ru or Ru

plus TiSi2. Two pieces of glass microfiber (GF/F, Whatman) were introduced to

separate each electrode. In a typical experiment, 200 mL of electrolyte was used.

After the cells were assembled, ultrahigh purity oxygen (Airgas) was purged

through the head space of the Swagelok-type cell at 20 sccm for 1 min. The cell

was then isolated from the gas line after the pressure equilibrated to 760 torr.

The galvanostatic cycling tests for understanding the behavior of the LFP counter

and reference electrodes were conducted at a current density of either 20 or

50 mA/cm2. The capacity was set to 40 and 100 mAh/cm2, respectively (operating

for 4 hr/cycle). The cycling tests to compare WiS, DME, and DMA electrolytes

with a Vulcan carbon cathode and LFP pseudo anode were conducted with a cur-

rent density of 50 mA/gcarbon at a cutoff capacity of 250 mAh/gcarbon (operating

for 10 hr/cycle). The galvanostatic cycling test with WiS as the electrolyte, carbon

as the cathode, and protected Li as the anode was conducted with a current density

of 50 mA/gcarbon at a cutoff capacity of 500 mAh/gcarbon (operating for 20 hr/cycle).

Ru/TiSi2 cathode was tested in the WiS electrolyte with the Swagelok-type cell with

either an LFP pseudo anode or a protected Li anode for the cycling test at a cutoff

capacity of 1,000 mAh/gRu or 500 mAh/gRu+TiSi2 (operating for 4 hr/cycle). The sam-

ples for spectroscopic and microscopic characterization with Vulcan carbon were

operated at 50 mA/gcarbon to reach 500 or 2,500 mAh/gcarbon. The samples

for spectroscopic and microscopic characterization with Ru/TiSi2 cathode were

operated at 1,000 mAh/gRu. To simultaneously monitor the anode and cathode

1354 Chem 4, 1345–1358, June 14, 2018

potentials against the reference electrode, we studied the Swagelok-type cells by

using the GCPL2 program (part of the Biologic software suite) to ensure voltage reli-

ability. The equilibrium potential of the LFP electrode in DMA and DME electrolytes

was measured against Li+/Li electrode to be 3.46 G 0.01 V. In WiS, the LFP elec-

trode potential was calibrated against a standard AgCl/Ag reference electrode (cali-

brated to be ca. 0.197 V versus normal hydrogen electrode) and converted to

3.46 G 0.02 V versus Li+/Li (see Figure S4 for more discussions). More details about

the materials can be found in the Supplemental Information.

Material Characterization

XRD measurements were conducted on a PANalytical X’Pert Pro diffractometer

with Cu Ka radiation. XPS was carried out on a K-Alpha + XPS (Thermo Scientific)

with an Al X-ray source (incident photon energy, 1,486.7 eV). The fitting of XPS

data was performed by XPS Peak 4.1 software. Raman spectra were acquired

with a micro-Raman system (XploRA, Horiba) with 532 nm laser excitation. FTIR

was performed with a Bruker ALPHA FTIR spectrometer in a N2-filled glovebox.

SEM was conducted with a JEOL 6340F microscope operated at a 10 kV acceler-

ating voltage. EELS was used as a qualitative fingerprint to verify the presence of

Li and the components in the toroid particles. The EELS spectrum was acquired

on a Tecnai T20 G2 TEM (FEI, 200 kV) with a Gatan imaging filter. The energy res-

olution was ca. 2.1 eV as measured from the full width at half maximum of the zero

loss peak. All EELS data were recorded with a pixel dwell time of 0.1 s, dispersion of

0.05 eV/channel, and a selected entrance aperture of 2 mm. Vulcan carbon cathode

samples were discharged and/or fully recharged to 500 mAh/gcarbon for XRD, XPS,

Raman spectroscopy, FTIR, SEM, TEM, and pressure monitoring measurements.

The deeply discharged samples (2,500 mAh/gcarbon) were also prepared for

XRD and SEM. Ru/TiSi2 cathode samples were operated at 1,000 mAh/gRu or

500 mAh/gRu+TiSi2 for XPS, SEM, and Raman spectroscopy. The electrode samples

were collected in an O2-tolerant Ar-filled glovebox (MBRAUN, H2O < 0.1 ppm),

washed in DME at least five times to remove residue salts, and then dried under

a vacuum at room temperature for 6 hr before characterizations. For XPS measure-

ments, an airtight Ar bag was used to transfer samples. Samples were then

mounted onto the stage with short exposure to the ambient air (typically for ca.

5 min). For XRD and Raman characterization, an airtight sample holder was assem-

bled in the O2-tolerant Ar-filled glovebox and transferred out for direct measure-

ment through an observation window. FTIR was conducted in a N2-filled glovebox

(MBRAUN, O2, H2O < 0.1 ppm), and the samples were transferred in an airtight Ar

bag but with a short exposure to ambient air (typically for ca. 5 min). The SEM

samples were mounted onto a stage with a short exposure in the ambient air

(typically for ca. 5 min) before being transferred to the high-vacuum chamber.

The TEM samples were prepared by first sonicating the discharged samples in

DME for 2 min in an airtight vial and then drop casting onto the TEM grid in a

N2-filled glovebox. The samples were exposed in ambient air for ca. 10 min.

Gas chromatography mass spectrometry (GC-MS) measurements were performed

with a Shimadzu QP2010 Ultra and a Carboxen 1010 PLOT column. The 36O2 cali-

bration curve was constructed by manually injecting known amounts of air into the

injection port at 298 K. For in situ gas evolution detection, a Swagelok-type cell with

a Vulcan carbon cathode, an LFP anode, and the WiS electrolyte were connected to

the GC-MS sampling loop (500 mL in volume) with He as the carrier gas (ultrahigh

purity, 10 sccm flow rate). The total discharge and recharge capacity was limited

to 200 mAh/gcarbon for data acquisition. The current density was consistent with pre-

vious test conditions. The cell was allowed to rest for 1 hr to achieve a flat baseline

before applying potentials. After the recharge process was completed, the gaseous

Chem 4, 1345–1358, June 14, 2018 1355

products were further sampled for 1 hr to let the MS signals reach a stable baseline

again. The MS signals during recharge processes were recorded for gas evolution

and faradic efficiency calculations. For the titration experiments, Swagelok-type

cells were discharged to the cutoff voltage of 2.5 V versus Li+/Li and then

dissembled in an O2-tolerated Ar-filled glove box (H2O level < 0.1 ppm, MBRAUN)

immediately. For each individual cell, the cathode and electrolyte were collected

separately in glass vials for titration. The samples were soaked in 3.0 mL

deionized H2O and 1.0 mL HCl (1 M) for 15 min. Before titration, 2 mL of 2% KI so-

lution and 50 mL of Mo catalyst solution were added in sequence and then kept in

the dark. The reaction mixture was then titrated with a calibrated Na2S2O3 solution

(4.3 mM). Each condition was repeated at least four times. 1H NMR was performed

on a 600 MHz spectrometer. The electrodes and all electrolytes from each cell were

collected and soaked with same amount of D2O or CDCl3 to normalize the amount

of by-products. For a more accurate normalization, a trace amount of C6H6 (ben-

zene) was added as the internal standard for the first discharge. All 1H NMR chem-

ical shifts were reported in ppm in relation to a residual HDO peak at 4.78 ppm

(trimethylsilylpropanoic acid at 0 ppm). The loading of Ru for the carbon-free cath-

ode was quantified by inductively coupled plasma optical emission spectrometry.

The mass ratio of Ru and TiSi2 was found to be ca. 1:1, consistent with our previous

results. Pressure monitoring tests were carried out by connecting the Swagelok-

type cell to an airtight pressure gauge with a sensitivity of 0.1 torr (MKS, 902B

Piezo). The current density was set at 100 mA/gcarbon to maximize the signal/noise

ratio. The pressure for the first ten cycles was monitored in situ.

SUPPLEMENTAL INFORMATION

Supplemental Information includes Supplemental Experimental Procedures, 25 fig-

ures, and 1 table and can be found with this article online at https://doi.org/10.1016/

j.chempr.2018.02.015.

ACKNOWLEDGMENTS

This work was supported by Boston College. We thank Longtao Han and Predrag

Krstic for insightful discussions. We also thank Shasha Zhu for her assistance in

designing the schematic figures, Kaicheng Li for his help in analyzing the NMR re-

sults, and Jeffery Byers for granting access to the FTIR instrument. XPS was per-

formed at the Center for Nanoscale Systems at Harvard University.

AUTHOR CONTRIBUTIONS

Q.D. and D.W. conceived the idea. Q.D. carried out most of the experiments. X.Y.

prepared the TiSi2 cathode substrate and assisted with the XRD and GC-MS mea-

surements. Y.Z. and H.S performed the TEM and EELS experiments. M.Q. assisted

with the NMR and FTIR experiments. X.Z. prepared the carbon electrodes and

contributed to the schematic design. Y.H. participated in designing the Swagelok-

type cell. All authors contributed to the discussions. Q.D. and D.W. wrote the manu-

script with input from all authors. D.W. supervised the research.

DECLARATION OF INTERESTS

The authors declare no competing interests.

Received: December 22, 2017

Revised: January 9, 2018

Accepted: February 15, 2018

Published: April 12, 2018

1356 Chem 4, 1345–1358, June 14, 2018

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