Probing the Reaction Interface in LiâOxygen Batteries Using Dynamic

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Probing the Reaction Interface in Li–Oxygen Batteries Using Dynamic Electrochemical Impedance Spectroscopy: DischargeCharge Asymmetry in Reaction Sites and Electronic Conductivity Jun Huang, Bo Tong, Zhe Li, Tao Zhou, Jianbo Zhang, and Zhangquan Peng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01351 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry Letters

Probing the Reaction Interface in Li–Oxygen Batteries using Dynamic Electrochemical Impedance Spectroscopy: Discharge-Charge Asymmetry in Reaction Sites and Electronic Conductivity Jun Huanga,b,c, Bo Tongb, Zhe Lic,Tao Zhoua,*, Jianbo Zhangc,*, Zhangquan Pengb,* a College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China. E-mail: [email protected] b State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, China E-mail: [email protected] c Department of Automotive Engineering, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China E-mail: [email protected] * corresponding authors

Fundamental questions concerning the reaction interface in Li-O2 batteries, including where reactions occur and discharge-charge asymmetries come from, have stimulated a flurry of investigations; nevertheless, heated debates still prevail. Herein, dynamic electrochemical impedance spectroscopy (EIS) is employed to probe the reaction interface in a Li-O2 battery under potentiostatic and galvanostatic modes. Two impedance semicircles are identified during discharge with the high and the low frequency one related with the Li2O2 film and the oxygen reduction reaction (ORR), respectively. However, upon triggering the oxygen evolution reaction (OER), only one semicircle is observed, implying that the reaction interface changes. Combining qualitative analysis on the EIS structure and quantitative information obtained from model fitting reveals that the ORR occurs on the Li2O2-electrolyte interface during discharge and the OER on the electrode surface during charge. In addition, it is found that the electronic conductivity of Li2O2 is higher at oxidative potentials (charge) than reductive potentials (discharge). Discharge-charge differences in the reaction interface and the electronic conductivity reported here expand the scope of the discharge-charge asymmetry of Li-O2 batteries. 1

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Understanding the reaction interface is indispensable for tackling two major challenges facing the development of Li-oxygen (O2) batteries: the unsatisfactory trade-off between specific energy and power, and the unacceptable high overpotential upon recharge.1-4 However, the reaction interface in Li-O2 batteries is fairly complicated because it is not static, but rather evolves strongly during discharge and charge processes. Moreover, unlike its siblings in other electrochemical energy devices, e.g., fuel cells, the reaction interface in Li-O2 batteries has dual roles: it assumes interfacial charge transfer reactions on one hand, and, peculiarly, accommodates the discharge product, Li2O2, on the other. The existence of Li2O2, maybe amorphous and porous, on the electrode surface gives rise to three types of electrochemical interface in Li-O2 batteries,

namely

the

electrode-electrolyte

interface,

the

Li2O2-electrolyte interface (the exterior case in Figure S1), and the electrode-Li2O2 interface (the interior case in Figure S1). Natural questions

include

which

interface

assumes

the

oxygen

reduction/evolution reaction (ORR/OER) upon discharge/charge, respectively, what they are and where they come from that make discharge and charge asymmetrical. Forays into these questions are not only for intellectual interests, but also of practical implications. For instance, if the ORR/OER occurs on the Li2O2-electrolyte interface, the discharge/charge kinetics is unlikely to be affected by nanoparticle catalysts that are covered by Li2O2.5 In contrary with the growing consensus on the discharge mechanism,1-4,6,7 the reaction interface in Li-O2 batteries is highly controversial.

Most

first-principles

hypothesize,

tacitly,

that

the

studies

ORR/OER

in take

Li-O2 place

batteries on

the

Li2O2-electrolyte interface.8,9 This scenario means that electrons must transport across the Li2O2 film before participating in the ORR/OER. However, the electronic conductivity of crystalline Li2O2 is staggeringly low (~10-19 S cm-1) at room temperature.10,11 Although amorphous Li2O2 has a much higher electronic conductivity (10-9~10-13 S cm-1),12,13 estimates

indicate

that

electron

transport

cannot

sustain

the

ORR/OER rate observed in experiments (although uncertainties in the 3



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electrochemically active area may weaken the credence to this assertion).14 The situation leads some researchers to other possibilities. Radin and Siegel suggest that the ORR/OER both occur at the electrode surface (either the electrode-Li2O2 or electrode-electrolyte interface), which is made possible when the Li2O2 film is porous or the electrode surface is partially covered.10 Recently, Wang et al. conceived an elegant way to test this hypothesis.14 An Au electrode was passivated using 18O2 and then subjected to discharge under

16O2.

In-situ surface-enhanced

Raman spectroscopy (SERS) shows that Li218O2 is gradually replaced with Li216O2, indicating that ORR takes place at the Au-Li2O2 interface; the same conclusion holds for OER as well. McCloskey et al. is the first to

employ

isotope-labeled

differential

electrochemical

mass

spectrometry (DEMS) to study the reaction interface in Li-O2 batteries.15 They discharged the battery under observed 16O2 evolution prior to

18O2

18O2

and then

16O2,

and

evolution upon recharge, implying

that ORR and OER occur on the same interface. A modeling study by Lau and Archer argues that ORR occurs on free carbon surface, hence, the ‘sudden death’ on the end of discharge is caused by the elimination of free carbon surface.16

In-situ morphology techniques can provide visualized evidences of the reaction interface; however, sharply different views prevail in the literature. In 2013, Zhong et al. visualized the oxidation of a Li2O2 particle (electrochemically formed in a separate experiment), supported on a carbon nanotube (CNT) in a solid state Li-O2 battery, using in-situ transmission electron microscopy (TEM).17 It is revealed that Li2O2 is decomposed on the CNT-Li2O2 interface, indicating that Li2O2 oxidation is limited by electron transport. Later on, the formation and decomposition of spherical Li2O2 particles (~1.5 µm) during cycling in a solid-state Li−O2 battery were observed using in-situ environmental scanning electron microscope (ESEM).18 Two interesting phenomena were observed: a new Li2O2 particle, remote from the electrode surface, can be formed on a large Li2O2 particle, and the collapse of spherical Li2O2 particles occurs on the bare surface that is not in contact with the CNT; both phenomena indicate that ORR/OER occurs on the Li2O2 4


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surface, rather than the CNT surface. In 2017, Luo et al. arrived at the same conclusion for a solid-state Li−O2 battery by employing in-situ TEM.19 Using liquid-cell in-situ TEM, a better emulation to real liquid state Li-O2 batteries, Kushima et al. observed that ORR occurs on the Li2O2-electrolyte interface, while OER occurs on the electrode-Li2O2 interface, namely the reaction interface may be asymmetrical during discharge and charge.20 Discharge-charge asymmetry is remarkable in Li-O2 batteries, while the underlying causes are elusive. In a word, the reaction interface in Li-O2 batteries still remains a baffling puzzle and a pressing fundamental issue, awaiting more insights from a variety of approaches. In this regard, electrochemical impedance spectroscopy (EIS) is a valuable in-situ tool because it is particularly useful in discriminating different processes in a wide frequency range.21-24 In a previous study,23 we measured EIS at stationary

state

only,

i.e.,

the

battery

is

not

undergoing

discharging/charging. Comparing EIS measured on a pristine electrode and that on a fully discharged electrode, we found a new semicircle in the high-frequency range corresponding to the Li2O2 film formed during discharge. Combing EIS data with physics-based impedance models, we have also concluded that ORR occurs on the Li2O2-electrolyte interface upon discharge. Moreover, the exchange current density and the activation energy of ORR were determined from EIS data, revealing that the ORR is more facile on the Li2O2 surface compared with the pristine electrode surface.23 In this work, we collect EIS data while discharging/charging Li-O2 batteries in potentiostatic and galvanostatic modes; we term this measurement

approach

as

dynamic

EIS.25-27

Dynamic

EIS

measurement, complemented by mathematical modeling and other advanced characterization tools including in-situ SERS, in-situ DEMS, X-ray diffraction (XRD), and nuclear magnetic resonance (NMR), allows us to gain new insights into the reaction interface and the discharge-charge asymmetry of Li-O2 batteries. A three-electrode glass cell, with a planar glass carbon disc as the 5



working electrode and Pt as the reference and counter electrode, was used in this work. The electrolyte solvent is DMSO. Experimental details can be found in the supporting information (SI). In a Li+-free electrolyte, the ORR and the OER form two symmetrical peaks, signing a quasi-reversible redox process, in the cyclic voltammetry (CV) curve (Figure S2). However, the symmetry is apparently broken, signing an irreversible redox process, when a conducting lithium salt, LiClO4, is added. Where does the discharge-charge asymmetry come from, then? In-situ SERS and ex-situ XRD (see Figure S3-4 in the SI) provide compelling evidence that a Li2O2 film (with minimal other compounds) is formed during discharge through a two-step pathway, Li++O2+e→LiO2, and Li++ LiO2+e → Li2O2 (and/or 2LiO2 → Li2O2+O2). The asymmetry between ORR and OER involving Li+ may originate from four sources: asymmetry in the reaction pathway, asymmetry in the reaction interface, asymmetry in the reaction kinetics, and asymmetry in the charge transport across Li2O2. Theoretical calculations have elucidated that oxidation of Li2O2 proceeds via the formation of substoichiometric Li2−xO2,28 which is observed by operando X-ray diffraction (XRD),29 suggesting asymmetry in the reaction pathway. Moreover, Viswanathan et al. found that Tafel-plots for ORR and OER in Li-O2 batteries are, by and large, symmetrical, namely the asymmetry in the reaction kinetics is minor.30 Asymmetries in the reaction interface and the charge transport across Li2O2 remain elusive. We apply potentiostatic dynamic EIS (PDEIS), namely EIS measurement at consecutive potential steps, to probe the reaction interface and study the discharge-charge asymmetry. The potential steps and current response are shown in Figure 1 (a). The ORR current initiates at ca. -0.5 V (vs. the Pt reference electrode, or 2.67 V vs Li+/Li, the Pt reference potential scale is used hereinafter, unless otherwise noted), peaks at ca. -0.7 V, and decreases till -1.0 V. Upon recharge from 0 V to 1.5 V, the OER current is rather low until 1.2 V. Afterwards, a peak is seen at ca. 1.35 V. The significantly deferred onset potential of OER in the PDEIS experiment, compared with the onset potential of -0.4 V in the CV experiment shown in Figure S2 (a), is due to the 6


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significantly lengthened discharge time taken in measuring PDEIS. To prove this explanation, we measured CV curves with different scanning rates during discharge, namely varying discharge durations, and an identical scanning rate during charge. As shown in Figure S2(b), the onset potential of OER shifts toward more positive values when the scanning rate during discharge is lower. The dynamic EIS data measured at each potential stair are depicted in Nyquist plots in Figure 1 (b). Upon potentiostatic discharge, the impedance curve shrinks from 0 V to -0.6 V, and grows from -0.6 V to -1.0 V. In the subsequent potentiostatic recharge, the impedance curves are almost unaltered from 0 V to 0.5 V, and experience a drop-off region from 0.5 V to 1.35 V, followed with a growing tail from 1.35 V to 1.5 V.

7



Figure 1 (a) The cell voltage, current, and (b) dynamic EIS during stepwise potentiostatic discharge and charge. The cell potential is with reference to the Pt electrode, which can be transformed to the Li+/Li potential scale by a positive shift of 3.17 V. (c) shows PDEIS at several representative potentials in Nyquist plot (different scales are used for the sake of clarity). Model fitting results are shown in solid lines.

At the initial stage of discharge (e.g., 0 V), as shown in Figure 1 (c), a single semicircle is seen in the Nyquist plot; while a new semicircle emerges in the high-frequency range after discharge to -0.6 V. This transformation to a two-semicircle structure along with discharge is fully consistent with our previous work,23 and we have related this new semicircle with charge transport across the Li2O2 film. Upon recharge, two semicircles can be discerned at 0 V. However, the impedance spectrum is dominated by one semicircle above 0.7 V, and the high-frequency semicircle is nearly absent. Two causes are speculated: the high-frequency semicircle severely shrinks due to improved electronic conductivity of Li2O2 , or it entirely disappears due to transformation of the reaction interface from the Li2O2 interface to the electrode surface.23 To obtain quantitative information, we further fit the PDEIS data using electrical equivalent circuit (EEC) models, which have the theoretical ground on physics-based impedance models developed previously; 23 model details and fitting algorithm are provided in the SI. Figure 1 and S5 exhibit the comparison between model fitting and experimental data in Nyquist and Bode plot, respectively; fairly good agreement is obtained. We extract the film resistance Rflim from model fitting, and plot Rflim in a time sequence (Figure S6). It is clearly seen that Rflim grows during discharge and diminishes during recharge, moreover, Rflim grows synchronously with the discharge capacity. This is a new piece of evidence for the argument that the high-frequency semicircle is related with the Li2O2 film. In terms of the low-frequency semicircle, the charge transfer resistance Rct of ORR/OER is also determined and plotted together with 8


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the current in Figure S7. Rct drops by one-to-two orders in the initial discharge stage ([0, −0.5] V), and further by two-to-three orders when the discharge current peak occurs (ca. -0.7 V). The initial drop in Rct is attributed to the transformation of the reaction interface from the electrode surface to the Li2O2 surface on which we have revealed ORR is much more facile.23 The second drop in Rct is caused by a lower area-specific charge transfer resistance (Ω m ) at a larger overpotential (a corollary from Butler-Volmer equation),25 and the increase in the electrochemical active surface (new interface emerges during Li2O2 formation). Upon recharge, Rct keeps almost constant until 0.9 V, followed by a quick drop. The film conductivity, , is an intrinsic property independent on the electrochemically active surface which is varying during discharge and charge and thus difficult to determine with accuracy. can be calculated in the following form,

=

(1)

where is the vacuum permittivity, the relative permittivity of Li2O2, and

the coefficient and exponent of the constant

phase element (reduced to a pure capacitance when

= 1) used in

the EEC ( is shown in Figure S8). The deduction of Eq.(1) is provided in the SI. The case of

= 1 has been formulated by Kaiser

et al.24 Given , and

extracted from model fitting and

= 19 taken from Kaiser et al.,24 we are equipped to calculate using Eq.(1); the result is shown in Figure 2. Kaiser et al. reported in-situ of Li2O2 during discharge only,24 while that during charge has never been reported before. In consistency with Kaiser et al.,24 decreases from 2 × 10%& S m% to 1 × 10%( S m% as the discharge proceeds. The decrease in is attributed to growth in the film thickness,

an

essential

characteristic

of

electron

tunneling.31

Intriguingly, it is found that increases from 2 × 10%( S m% to 9



1 × 10%) S m% upon charging from 0 V to 0.7 V, wherein the OER current are both nearly zero (Figure 1(a)). It is thus indicated that the increasing upon charge is not due to the thinning of the Li2O2 film, but due to decrease in the activation barrier of electron tunneling and increase in the hole polaron concentration.31 It is difficult to determine

in the potential range of [0.7, 1.5] V because the high-frequency semicircle is almost absent. As a result, no reliable conclusion can be drawn in this potential range.

Figure 2 Conductivity of the Li2O2 film calculated using Eq.(1) from PDEIS data. Note that conductivity data above -0.5 V during discharge is unavailable as the electrode surface is almost free of Li2O2.

In a latter experiment, galvanostatic dynamic EIS (GDEIS) was measured while the Li-O2 cell was under constant current discharge and charge. Note that PDEIS is more suitable to gain fundamental insights into the reaction interface because potential is the driving force of reactions and transport, while GDEIS is also valuable because batteries are usually operated in the galvanostatic mode. Compared with PDEIS, GDEIS is measured in a narrower frequency range, 104-100 Hz, to avoid violation of the stability criterion for EIS measurement.26 10


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Figure 3 (a) shows GDEIS during discharge and charge. An EEC model with

only

one

resistance-CPE

component

is

used

to

fit

the

high-frequency impedance data; no attempts are made to fit the low-frequency data because of large uncertainties thereof. The resistance determined from model fitting is shown in Figure 3 (b), together with the discharge-charge curve.

Figure 3 Galvanostatic dynamic EIS measured when the Li-O2 cell is under galvanostatic discharge and charge at 3 μA (the geometrical surface area of the glassy carbon electrode is 3.14 mm2). The cell voltage and resistance extracted from EIS data are shown in panel (b).

At a high current density (~100 μA cm% ), Li2O2 formation proceeds via the surface-mediated pathway, leading to a conformal Li2O2 film covering the electrode surface, as indicated by in-situ SERS and ex-situ XRD data (see Figure S3-4 in the SI).14,32 Under this mechanism, the film thickness is a linear function of time during galvanostatic discharge. Viswanathan et al. developed an exponential relation between electronic conductivity of Li2O2 film and its thickness.33 As a result, an exponential relation between the film resistance and time is expected, which is the case as shown in Figure S9. This exponential relation further corroborates that the high-frequency semicircle is related with the Li2O2 film. Upon recharge, the cell voltage first increases quickly, arrives at a plateau, and then ascends up gradually. Intriguingly, we see an abrupt 11



Page 12 of 17

increase in the resistance upon making the switch from discharge to charge. Moreover, a two-stage behavior is seen for the resistance. Based on the results obtained from PDEIS, the resistance should decrease if it is related to the Li2O2 film because is higher for charge. As a result, we conclude that this resistance is not the film resistance, but rather the charge transfer resistance. In other words, OER occurs on the electrode surface (the interior case in Figure S1), rather than the Li2O2 surface (the exterior case in Figure S1). In this scenario, electron transport across the Li2O2 is not involved in OER, and only ionic transport of Li+ across the Li2O2 is present which can be safely regarded as a pure resistor,23 and thus EIS exhibits one semicircle corresponding to OER on the electrode surface, as predicted by physics-based models.23 In a study on the solid-electrolyte interphase (SEI) by Newman et al., it is also shown that Li+ transport across the outer porous layer of the SEI is not reflected as a semicircle in the EIS plot when the charge transfer reactions occur at the inner interphase.34 After the first phase of charge, most of Li2O2 has been oxidized, as indicated from ex-situ XRD (Figure S4), and the electrode comes into direct contact with the electrolyte. Therefore, electrolyte decomposition occurs, leading to a decrease in the resistance and an increase in the cell voltage, which is actually a mixed potential as pointed out by Højberg et al.21 Our speculation is supported by the observation of (CH3)2SO2 in the electrolyte harvested from a Li-O2 cell cycled between 2.0 V to 4.5 V vs. Li/Li+ for five cycles (Figure S10), and observation of CO2 generation in the latter stage of charge by DEMS (Figure S11). Due to technical limitations, NMR and DEMS were conducted on a coin cell using a KB carbon cathode, rather than the glass cell using a glassy carbon cathode

in impedance measurements; while

the

same

electrolyte was used in two cells. We summarize key findings in Figure 4. During discharge, the ORR occurs on the Li2O2-electrolyte interface, and the EIS structure has two semicircles with the high- and the low frequency one related with the Li2O2 film and the ORR, respectively. In addition, the electronic conductivity of the Li2O2 film decreases with increasing the film 12


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thickness, leading to an exponential increase in the film resistance along with discharge. In-situ electronic conductivity of the Li2O2 film during charge is reported for the first time, revealing that the electronic conductivity increases at oxidative potentials due to decrease in the tunneling barrier and increase in the polaron concentration. During galvanostatic

recharge,

the

reaction

interface

shifts

to

the

electrode-Li2O2 interface for the OER, hence, EIS shows one semicircle only (because electron transport across the Li2O2 film is no longer involved). As the charge proceeds, bare electrode surface comes into contact

with

the

electrolyte,

decomposition

and

rendering

incurring a

mixed

unwanted potential.

electrolyte This

study

demonstrates that the old electrochemical tool, EIS, can provide unique insights in studying emerging batteries, e.g., Li-O2 batteries in this case.

Figure 4 Reaction interface during galvanostatic discharge and recharge processes of Li-O2 batteries.

Acknowledgements J Huang appreciates financial support from the starting fund for new faculty members at Central South University (No.502045001). T. Zhou gratefully acknowledges financial support from the Hunan 13



Provincial Science and Technology Plan Project (No.2016TP1007). J. Zhang gratefully acknowledges financial support from National Natural Science Foundation of China under the grant number of U1664259. Z. Peng is indebted to the National Natural Science Foundation of China (No. 91545129).

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