Very Important Paper

Raman Spectroscopy in Lithium–Oxygen Battery SystemsForrest S. Gittleson,[a] Koffi P. C. Yao,[b] David G. Kwabi,[b] Sayed Youssef Sayed,[c] Won-Hee Ryu,[a] Yang Shao-Horn,[b] and Andr¦ D. Taylor*[a]

1. Introduction

The lithium–oxygen (Li–O2 or Li–air) battery is one of the most

promising next-generation rechargeable energy storage tech-nologies, with a practical energy density 2–3 times greater

than the current state-of-the-art Li-ion cell.[1, 2] Despite its po-tential, Li–O2 systems have not yet demonstrated reversible cy-

cling with high energy densities. Significant additional devel-opment is needed, both in understanding cell chemistry and

designing practical cell architectures.

During ideal Li–O2 cell discharge, Li+ and O2 combine ata porous oxygen electrode to form lithium oxides, and the pro-

cess is reversed upon charge.[3] Unlike in rechargeable Li-ioncells which operate by well-known Li+ intercalation/deinterca-

lation mechanisms, reactions at the oxygen electrode of Li–O2

cells involve the formation and evolution of products withvarying composition, conductivity and morphology. In non-

aqueous Li–O2 cells, Li2O2 is the desired discharge product dueto its two-electron capacity and proven reversibility (2 Li +

O2ÐLi2O2 ; E0 = 2.96 V vs. Li/Li+).[3–5] Electrode materials and the

cell environment (i.e. electrolyte, impurities) have significantimpacts on cell efficiency, cyclability and capacity.[6–12] Irreversi-

ble Li2O and side products (i.e. Li2CO3, LiOH) due to electrolyteor electrode decomposition, for example, may impede cell

function.[13]

Until recently, little was known about the reaction mecha-

nisms, intermediates and products in Li–O2 systems: informa-

tion that is critical to the demonstration of a practical re-chargeable battery. At present, there is still debate concerning

the nature of product compositions and structures.[3, 14–16] Totackle these questions, techniques including X-ray diffraction

(XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorp-tion spectroscopy (XAS), Fourier transform infrared spectrosco-py (FTIR) and Raman spectroscopy have been used for the

physicochemical characterization of electrochemically generat-ed products. Each of these techniques presents its own chal-lenges and benefits. In this review, we highlight Raman spec-troscopy as one of the most powerful and practical, yet un-

derutilized techniques in Li–O2 research.Most Li–O2 applications of Raman spectroscopy focus on de-

termining the chemistry of discharge products on the oxygenelectrode. In non-aqueous cells, studies of these productsusing XRD,[17–21] XPS,[22, 23] XAS[20, 24] and transmission electron

microscopy[25] have largely reported lithium peroxide (Li2O2).However, recent literature has suggested the presence of inter-

mediate species such as O2¢ , LiO2 and possibly Li3O4.[14, 26, 27] Su-

peroxides (O2¢ , LiO2) have only been detected in discharge

products using Raman spectroscopy, and in single instances

using XAS[28] and XRD.[29] Where supporting evidence is limited,Raman offers a simple and effective method to discern product

chemistry for low concentration and/or non-crystalline specieslike superoxides. In this review, we critically consider the exist-

ing Raman evidence supporting the presence of Li2O2 and su-peroxide species in Li–O2 cells. The use of Raman spectroscopy

Electrochemical processes in lithium–oxygen (Li–O2 or Li–air)batteries are complex, with chemistry depending on cycling

conditions, electrode materials and electrolytes. In non-aque-

ous Li–O2 cells, reversible lithium peroxide (Li2O2) and irreversi-ble parasitic products (i.e. , LiOH, Li2CO3, Li2O) are common. Su-peroxide intermediates (O2

¢ , LiO2) contribute to the formationof these species and are transiently stable in their own right.While characterization techniques like XRD, XPS and FTIR havebeen used to observe many Li–O2 species, these methods are

poorly suited to superoxide detection. Raman spectroscopy,however, may uniquely identify superoxides from O¢O vibra-

tions. The ability to fingerprint Li–O2 products in situ or ex situ,

even at very low concentrations, makes Raman an essentialtool for the physicochemical characterization of these systems.

This review contextualizes the application of Raman spectros-copy and advocates for its wider adoption in the study of Li–

O2 batteries.

[a] Dr. F. S. Gittleson, Dr. W.-H. Ryu, Prof. A. D. TaylorDepartment of Chemical and Environmental EngineeringYale University9 Hillhouse Ave., New Haven, CT 06511 (USA)E-mail : andre.taylor@yale.edu

[b] K. P. C. Yao, D. G. Kwabi, Prof. Y. Shao-HornDepartment of Mechanical EngineeringMassachusetts Institute of Technology77 Massachusetts Ave. , Cambridge, MA 02139 (USA)

[c] Dr. S. Y. Sayed+

The Research Laboratory of ElectronicsMassachusetts Institute of Technology77 Massachusetts Ave. , Cambridge, MA 02139 (USA)

[++] Current address:Department of ChemistryFaculty of Science, Cairo UniversityGiza 12613 (Egypt)

An invited contribution to a Special Issue on In Situ Monitoring of FuelCell and Battery Processes

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to analyze electrolyte stability and observe redox active mole-cules for lithium-oxygen systems is also reviewed.

2. Raman Spectroscopy

Experimentally verified in 1928,[30] Raman spectroscopy utilizesthe inelastic scattering of monochromatic light to detect mo-

lecular vibrations in polarizable species. With this techniquea laser pulse is applied to a sample, exciting molecules to virtu-

al energy states. Molecules then relax to the original vibration-al state or different states and emit photons which can be de-

tected (Rayleigh or Raman scattering, respectively).[31] The fre-quency shift of the emitted photon away from that of the exci-

tation frequency is considered a Stokes shift if the emittedphoton is lower in frequency (the final vibrational state ishigher in energy) or an anti-Stokes shift if the photon is higherin frequency (the final vibrational state is lower in energy).Since Stokes scattering is more intense than anti-Stokes scat-

tering, Raman spectra are usually presented in Stokes shifts.Typical Raman spectra represent the frequency shift (in cm¢1)

and intensity of vibrational modes, creating a profile that is

suitable for molecular fingerprinting.Detection with Raman spectroscopy is limited by the polariz-

ability of the species, its orientation and the surface area beingprobed.[32] For example, molecules with a strong permanent

dipole (i.e. O2¢) produce strong Raman scattering while non-

polar crystals exhibit little or no Raman scattering.[33] Pure

metals are generally Raman silent. The spatial resolution ofRaman spectroscopy is dictated by the excitation wavelength

and the aperture of the microscope objective where shorter

wavelengths and higher magnification objectives producesmaller spot sizes.[31, 33] The depth of Raman probing is a func-

tion of the excitation wavelength and corresponding sampleabsorption. The Raman probed depth is roughly half of the at-

tenuation length: �30–300 nm for opaque samples.[34]

A number of Raman studies have been conducted to probe

electrode and electrolyte changes during and after cycling in

Li-ion cells. For example, some of the most compelling data onthe formation of solid electrolyte interface/interphases (SEI)come from surface-enhanced Raman spectroscopy experi-ments.[35–37] Two recent reviews on the application of Ramanspectroscopy in Li-ion battery research, including in situ experi-ments, summarize the area rather well.[34, 38] This article, howev-

er, focuses on Raman applications in Li–O2 systems, whichpresent separate challenges and opportunities.

2.1. Surface-Enhanced Raman Spectroscopy(SERS)

With an in situ or operando[39] methodology, Raman spectros-

copy may be applied in practical Li–O2 cells to study chemical

mechanisms. This type of experiment is particularly valuablebecause it eliminates effects from air contamination and allows

the observation of short-lived reaction intermediates. BecauseRaman scattering is typically weak (�1 in 107 photons), it may

be necessary to employ surface-enhanced Raman spectroscopy(SERS).[33] With detection sensitivity down to a few monolayers

of material or even a single molecule, SERS is ideal for studiesof surface species with very low concentrations.[40, 41] For in situ

applications in Li–O2 cells, we focus on the SERS technique.Surface-enhanced Raman spectroscopy utilizes a phenomen-

on in which certain substrates selectively enhance Raman scat-tering for materials in close proximity. This effect can produce

an enhancement to Raman scattering of �10 orders of magni-tude (more or less depending on the nanostructure of the sub-

strate).[42, 43] Two mechanisms are commonly cited to explain

the SERS effect. The plasmonic theory holds that the enhance-ment in Raman scattering derives from an increase in the in-tensity of local fields by surface plasmon resonance.[44] Thiseffect may produce a local field intensity enhancement of 3–6

orders of magnitude translating to roughly 6–12 orders ofmagnitude enhancement to Raman scattering.[44, 45] Studies of

surface plasmon resonance have suggested the presence of

“hot spots”—specific locations on the substrate surface wherelarge Raman enhancements are produced.[43, 46] These correlate

well with nanometer-scale crevices between particles, support-ing the need for a roughened substrate.[44] The chemical en-

hancement theory holds that chemical interactions betweenmolecules and a SERS-active surface produce a resonance

Raman-like effect where the excitation photon causes an elec-

tronic transition in the molecule.[47] The chemical effect is oftenconsidered weaker than the plasmonic effect and only pro-

vides 1–2 orders of magnitude enhancement to Ramanscattering.[48]

Au, Ag, and Cu are among the few substrate materials effec-tive for SERS, although the effect has been predicted and/or

observed for alkali metals, Al, Ga, Fe, Pt and Pd.[43] Roughened

surfaces (of typically 20–100 nm diameter particles) typicallyproduce the strongest enhancement.[42, 46]

2.2. Application to Li–O2 Systems

In Li–O2 research, in situ Raman spectroscopy benefits from

the SERS technique to detect low-concentration components(Li2O2 and LiO2) in close proximity to the electrode. The relative

amounts of species can be discerned qualitatively from the in-tensity of Raman features. The orientation of bonds relative tothe excitation laser, the position of species within a sampleand bond resonance may all influence the intensity of features,

making it difficult to compare the Raman intensity of one spe-cies to that of another. Such evaluations are most useful within situ data where the same focus conditions are used and thesame region of the sample is observed for each spectrum.

The electrode of choice for in situ Raman studies consists of

roughened gold prepared by etching gold foil in acid,[5, 49–52]

electroless deposition on Ni foam,[53] electrodeposition with

a hierarchical template[54] or sputtering.[55] Gold is preferred

due to its wide potential range of stability in contrast to Agand Cu, which may dissolve and redeposit during typical Li–O2

operation, according to their standard potentials (E0 = 3.84 and3.56 V vs. Li/Li+ , respectively). In situ cells typically use glass,

quartz or sapphire optical windows, though quartz and sap-phire yield superior clarity to the resulting spectrum. One ex-

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ample of a two-electrode in situ Raman spectroscopic cell is

provided from the literature (Figure 1).[53]

Raman excitation wavelengths of 633 or 785 nm[49, 50, 53, 54] are

standard for Li–O2 studies, but these selections are largely arbi-trary. Most Li–O2 electrolyte solvents exhibit absorption cutoffs

lower than l= 300 nm (i.e. dimethoxyethane: 220 nm, dimeth-yl sulfoxide: 265 nm), such that absorption is negligible in the

visible range[56] and electrolyte resonance is avoided. Altering

the excitation wavelength may be of interest in future studiesto enhance the detection of low concentration components

such as LiO2 by resonance Raman spectroscopy. For example,superoxide species have shown enhanced signals with excita-

tion wavelengths of 458 nm for KO2 and CsO2 powder.[57]

Perturbations to Li–O2 chemistry by Raman spectroscopy areminimal when the irradiation power is restricted to a reasona-

ble range. Sample heating during a measurement is a functionof the excitation wavelength, irradiation power (itself a functionof spot size), absorption of the sample, grating etc.[58, 59]

Though Li2O2 may transform to Li2O at elevated temperatures(>300 8C),[60] this and similar effects are not expected undernormal Raman operating conditions. While no Li–O2 reports

have estimated the sample temperature during irradiation,some offer an estimate of the power delivered to the samplesurface (�2.5 mW)[49, 52] for future comparison.

2.3. Other Physicochemical Characterization

Several methods other than Raman spectroscopy have been

employed in the literature to probe product composition in Li–

O2 cells. A summary of the most prevalent are found in Table 1along with their potential to detect important Li–O2 species.

Many products may be identified by XRD,[17, 18] XPS[22, 23] andXAS,[24, 28, 61] yet some are undetectable except by analyses of

molecular vibration (i.e. Raman and FTIR). Several recent re-ports have suggested the presence of amorphous or thin film

Li–O2 products which cannot bedetected by diffraction meth-

ods,[14, 21, 24, 62] but may be studiedby Raman spectroscopy.

X-ray diffraction is the mostprevalent technique in Li–O2 lit-

erature, used to demonstratethe reversibility of Li2O2. XRD,which utilizes the elastic scatter-

ing of X-rays to discern crystalstructures, is simple to executebut requires a large samplevolume and is only suitable forcrystalline samples (i.e. , Li2O2,LiOH, Li2CO3). The use of XRD as

a stand-alone characterization

technique is insufficient, asrecent work has demonstrated

the presence of non-crystallineproducts in Li–O2 cells.[21, 62] As

an in situ tool, XRD only allows for a narrow window of fixed-angle scattering.[17, 18, 63]

X-ray photoelectron spectroscopy is a surface-sensitive tech-

nique in which samples are irradiated by an X-ray sourceunder ultra-high vacuum (UHV) and outer-shell electrons areejected and detected. Detection of electron binding energieswith XPS gives a measure of elemental composition from the

top �10 nm of the sample surface, making this technique val-uable for analyzing interfacial Li–O2 species.[32] Since vacuum

conditions may cause changes in the composition of Li–O2

samples,[29] XPS is most useful as an ex situ technique in sup-port of other characterization. In situ Li–O2 experiments have

been demonstrated with “ambient XPS,” but require nonstan-dard cell structures (i.e. with solid-state electrolytes).[22, 23] Addi-

tionally, deconvoluting XPS data requires complex overlappingfittings which make explicit conclusions difficult.

X-ray absorption spectroscopy, typically X-ray absorption

near-edge structure (XANES) in Li–O2 literature, involves the ex-citation of a ground-state sample by X-ray photons followed

by a reorganization of the electronic structure resulting in elec-tron and fluorescent photon emission. Either the sum of Auger

and secondary electrons, called total electron yield (TEY), and/or the total fluorescent yield (FY), expressing a joint density of

Figure 1. In situ Li–O2 cell schematic for Raman spectroscopy. (Adapted with permission of ACS Publishing fromRef. [53] .)

Table 1. Comparison of detection methods for common products. X indi-cates evidence of detection in Li–O2 battery literature to date.

Method In situ Discharge products Side productsdifficulty Li2O2 O2

¢ , LiO2 LiOH Li2CO3 Li2O

XRD moderate X – X X XXPS hard X – X X XXAS hard X – X X XFTIR moderate X – X X XRaman easy X X X X X

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states, is reported.[64] The near-edge structure can be used toeffectively fingerprint Li–O2 species as a function of valence

and environment (i.e. O or Li K-edge), similar to XPS, but de-convolution can be challenging.[24, 28, 61] A synchrotron-based

X-ray source is necessary for both ex situ and potential in situXAS studies.

Fourier-transform infrared spectroscopy (FTIR) is similar toRaman spectroscopy in its detection of molecular vibrations.

However, FTIR only involves the excitation of molecules to

their exact vibrational state and detection of the output versusinput frequency and intensity [via attenuated total reflectance

(ATR)] .[65] It is common for weak Raman bands to be intensewith FTIR and vice versa, depending on the symmetry of the

molecule. FTIR and Raman are thus complementary tools fordiscerning molecular bonding, but FTIR is more difficult to

apply with an in situ methodology. With the common ATR

technique, samples must be pressed against a single crystalmedium, necessitating a confined cell environment. A number

of Li–O2 species can be detected with this method, includingLi2O2, LiOH and Li2CO3. While superoxides may theoretically be

detected by FTIR,[66] no report has been able to confirm theirpresence in Li–O2 products by this method.

3. Analysis of Reaction Products in Li–O2 Cells

Reversible processes in non-aqueous Li–O2 cells involve the

formation and evolution of mainly Li2O2 products, but superox-ides play an important role as intermediates. Reactions (1)–(5)[3]

present a general mechanism wherein the reduction reactionis rate-limited by a one-electron transfer to O2 to form a super-oxide (O2

¢ , LiO2) [Reactions (1) and (2)]:

O2 þ e¢ ! O2¢ ð1Þ

O2¢ þ Liþ ! LiO2 ð2Þ

The superoxide is further reduced to Li2O2 through dispro-portionation [Reaction (3a)] or direct electrochemical reduction

at the electrode [Reaction (3b)]:

2 LiO2 ! Li2O2 þ O2 ð3aÞ

LiO2 þ Liþ þ e¢ ! Li2O2 ð3bÞ

resulting in a net two-electron charge transfer. The oxidationprocesses of LiO2 and Li2O2, while incompletely understood,

are proposed to occur directly [Reactions (4) and (5)]:

LiO2 ! Liþ þ O2 þ e¢ ð4Þ

Li2O2 ! 2 Liþ þ O2 þ 2 e¢ ð5Þ

although there is some debate that Li2O2 oxidation may pro-

ceed through an intermediate superoxide-like phase.[24, 67] Su-peroxides themselves may participate as one-electron reversi-

ble products in addition to Li2O2 or react with electrode andelectrolyte materials to form irreversible products. Each aspect

of the non-aqueous reduction and oxidation mechanism iswell suited for study using Raman spectroscopy.

3.1. Lithium Peroxide (Li2O2)

The detection of Li2O2 by Raman spectroscopy is well estab-lished and presents an opportunity to complement techniques

like XRD. Peroxide-type O¢O bonding in Li2O2 is typically evi-denced by Raman bands at �250 and 790 cm¢1. Raman fea-

tures attributed to Li2O2 from the existing literature are sum-

marized in Table 2. In the first report of a non-aqueous Li–O2

battery by Abraham and Jiang,[68] Raman spectroscopy was

used to demonstrate the formation of Li2O2 (795 cm¢1) ona cobalt-catalyzed carbon electrode after discharging. The lack

of a Li2O signal (521 cm¢1) was used as evidence that Li2O2

could be formed and evolved reversibly. This report excluded

spectral evidence following the charge reaction and was con-

ducted with an unstable alkyl carbonate-based polymer elec-trolyte, so despite electrochemical indications, the reversibility

of products on subsequent cycles was not definitive. Morerecent and convincing evidence of Li2O2 formation and evolu-

tion was provided by Peng et al.[5] after discharge and charge(Figure 2), using a gold SERS electrode (Li2O2 at 790 cm¢1). The

flux of Li2O2 was also recently observed in situ during cell cy-

cling with a gold electrode.[55] Ex situ SERS results have similar-ly shown Li2O2 formation while XRD showed no evidence of

this species.[53] On discharged carbon electrodes, Li2O2 hasbeen confirmed by Raman spectroscopy, usually alongside fea-

tures attributed to LiO2-like products.[26, 29, 69–74]

One difficulty in using Raman spectroscopy to detect Li2O2 isthat feature intensities are strongly correlated to polarizability.

While modeling studies indicate that Li2O2 may present ther-modynamically stable polar surfaces,[75, 76] nearly all experimen-tal Raman features related to this species are low in intensityand broad, especially without surface enhancement. Since

Figure 2. SERS spectra of a nanoporous gold cathode at the end of dis-charge and charge in 0.1 m LiClO4/DMSO electrolyte. (Reproduced with per-mission of the AAAS from Ref. [5] .)

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Raman excitation wavelengths typically penetrate up to

300 nm,[34] minimal scattering from thin layers of dischargeproducts may account for the difficulty in detecting this mor-

phology of Li2O2. Large crystalline Li2O2 toroids (�3 mm diame-

ter), however, are easier to discern than thin product films,[70]

perhaps due to the greater concentration of O¢O peroxide

bonds. Recent literature has suggested the presence of amor-phous Li2O2 in addition to the toroidal morphology.[14, 21, 24, 62]

A recent modeling study noted that this amorphous species, ifpresent, would be slightly undercoordinated (vs. crystalline

Li2O2) and would include lithium vacancy and hole polaron de-

fects.[77] The O¢O peroxide bond may also be shorter. Thesefindings suggest that amorphous Li2O2 could be easier to

detect by Raman spectroscopy and may exhibit a higher O¢Ovibrational frequency than crystalline Li2O2, but further experi-mental studies are needed to analyze this product. Despite thechallenges, there is significant evidence to support the unam-

biguous identification of Li2O2 by Raman spectroscopy.

3.2. Superoxides (O2¢/LiO2)

The attribution of LiO2 features in Raman spectra can be chal-

lenging due to the transient nature of this species, in contrastto more stable alkali superoxides (i.e. NaO2, KO2, RbO2,

CsO2).[78–81] The basis of most lithium superoxide attributions is

from work using Raman and FTIR spectroscopy to study its vi-brational structure in solid matrices at low temperatures. With

this technique, lithium superoxide O¢O features have beennoted at 1093 and 1097 cm¢1 in oxygen[82] and argon[83, 84] ma-

trices, respectively. These studies suggest a probable ionicstructure of Li+O2

¢ , where there is little true bonding between

Li and O. Interestingly, features at �1068 and �1035 cm¢1 are

related to bonding between different oxygen isotopes O16¢O18

and O18¢O18, respectively. Raman features attributed to lithium

superoxide have also been reported at 1047 and 1080 cm¢1 in

molten carbonates[85, 86] and 1107 cm¢1 in molten fluorides,[87]

although all of these melts included multiple cations: Li, Na

and/or K. In fact, the former examples are possible misattribu-tions of carbonate features, noted in the same study at

�1065 cm¢1.The most common reference for lithium superoxide is crys-

talline potassium superoxide (�1145 cm¢1), which is thermo-

dynamically stable in pure form at room temperature and solu-ble in a number of organic solvents.[79, 88] This relation is obvi-

ously imperfect as the environment surrounding the superox-ide ion (cation, solvent, temperature etc.) affects O¢O bonding.

According to the ionic model of Rittner[89] and Andrews,[83]

a more polarizable cation (i.e. one with a greater dipole

moment) will more easily withdraw electrons from O¢O anti-bonding densities, yielding stronger O¢O bonding and in-creased vibrational frequencies. This model provides a rationale

for the lower vibrational frequency of LiO2 (1097 cm¢1) vs. KO2

(1108 cm¢1), RbO2 (1110 cm¢1) or CsO2 (1114 cm¢1) in argon ma-

trices at 4 K.[83] We may be tempted to use a similar heuristicto explain how cation–solvent or superoxide–solvent coordina-

tion affects the vibrational frequency of O2¢ , but the Li–O2

system, which could also include superoxide-like solid prod-ucts, is simply too complex to make these pronouncements.

Another concern for the superoxide attribution is a persistentbelief that O2

¢ and LiO2 are highly unstable. In fact, dissolved

superoxide has been shown to be transiently (kinetically)stable in certain organic solvents.[90] Goolsby and Sawyer[91]

Table 2. Li2O2 and superoxide attributions from Li–O2 literature.

Electrode Electrolyte Temperature Superoxide type [cm¢1] Li2O2 [cm¢1] In situ/ex situ Refs.

Goldnanoporous Au 0.1 m nBu4NClO4/CH3CN ambient O2

¢ 1109 in situ/operating [49]nanoporous Au 0.1 m LiClO4/Me-Im ambient O2

¢ �1105 �790 in situ/operating [51]nanoporous Au 0.1 m LiClO4/DMSO ambient O2

¢ �1110 �795 in situ/operating [51]hierarchical Au 0.01 m LiTFSI/Pyr14TFSI ambient O2

¢ 1107 805 in situ/operating [54]hierarchical Au 0.01 m LiTFSI/C2mimTFSI ambient O2

¢ 1110 in situ/operating [54]nanoporous Au 0.1 m LiClO4/CH3CN ambient LiO2 1137 808 in situ/operating [49]nanoporous Au 0.1 m LiClO4/DME ambient LiO2 �1125 �790 in situ/operating [51]nanoporous Au 0.1 m LiClO4/CH3CN ambient LiO2 �1130 �805 in situ/operating [51]Au–Ni foam 0.1 m LiClO4/DMSO ambient LiO2 1138–1148

1131791 in situ/operating

ex situ[53]

sputtered Au 0.1 m LiClO4/DMSO ambient 788 in situ/operating [55]nanoporous Au 0.05 m LiTFSI/DMSO ambient complex 1160 �800 in situ/operating [52]nanoporous Au 0.1 m LiClO4/DMSO ambient �790 in situ [5]nanoporous Au 1 m LiClO4/DMSO + TTF ambient �790 in situ [50]

Carbonactivated carbon/PVDF 1 m LiCF3SO3/TEGDME ambient

80–160 KLiO2 1125

1121–1126788 ex situ [26]

activated carbon/PVDF 1 m LiCF3SO3/TEGDME ambient LiO2 1123 790 ex situ [29, 69, 70]CNT/PVDF 1 m LiClO4/TEGDME ambient LiO2 1123 790 ex situ [71]KB carbon/PTFE 0.5 m LiCF3SO3/DEGDME 83 K unknown 1130–1200 789 ex situ [72]CB + graphite + Co LiPF6/EC:PC PAN polymer ambient 795 ex situ [68]carbon cloth 0.1 m LiPF6/DMSO ambient �790 ex situ [73]glassy carbon 0.1 m LiClO4/DMSO ambient 783 ex situ [74]

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have noted that with water content minimized, superoxideanions (derived from KO2) exhibit a decomposition rate of as

little as 2.9 % per hour in dimethyl sulfoxide (DMSO). A morerecent study of superoxide stability in high-purity Li–O2 elec-

trolyte solvents (i.e. DMSO, monoglyme, diglyme, tetraglyme),revealed superoxide ions (also derived from KO2) to be quitestable over �16 days.[92] These findings provide some valida-tion for attempting to detect superoxide species on or near Li–O2 electrodes in situ or ex situ.

The general mechanism for oxygen reduction in non-aque-ous Li–O2 cells [Reactions (1)–(5)] was established by identify-ing superoxide intermediates with Raman spectroscopy. Manymechanisms have been proposed to explain the formation of

Li2O2 in aprotic media by direct electrochemical reduction ordisproportionation of intermediates,[8, 12, 93–96] but the most con-

vincing spectroscopic evidence comes from Peng et al.[49] Em-

ploying in situ SERS, Peng et al. showed the formation of su-peroxide (O2

¢) and adsorbed Au–O2¢ species during oxygen re-

duction on a nanoporous gold electrode in 0.1 m n-Bu4NClO4/CH3CN. With a Li+ electrolyte of 0.1 m LiClO4/CH3CN, the au-

thors claimed the formation of intermediate lithium superoxide(LiO2) followed by its conversion to Li2O2 (Figure 3).

A 1140 cm¢1 FTIR feature related to O2¢ in CH3CN[97] and

a 1108 cm¢1 Raman feature from KO2 in DMSO was used to jus-

tify the Raman attribution of LiO2 in CH3CN (1137 cm¢1), butthis relation is clearly imperfect. While Peng et al. successfully

established the oxygen reduction (and evolution) mechanism

shown in Reactions (1)–(5), there remains some debate aboutthe nature (i.e. cation and solvent coordination) of the super-

oxide species and its corresponding Raman feature.Studies by Johnson et al.[51] and Gittleson et al.[53] provide ad-

ditional and, in some cases conflicting, results regarding theposition of the superoxide Raman feature and how it relates to

product chemistry. Using in situ SERS experiments with nano-porous gold electrodes, Johnson et al. studied the effect of su-

peroxide solubility on the Li–O2 reduction mechanism(Figure 4). This study proposed an electrode surface-based re-

duction pathway with low donor number solvents [i.e. , CH3CN,dimethyoxyethane (DME)] or a solution-based pathway with

high donor number solvents [i.e. , DMSO, 1-methylimidazole(Me-Im)]. The authors suggest that the former yields adsorbed

LiO2 and the latter a combination of adsorbed O2¢ and dis-

solved O2¢/LiO2. Though the Raman shifts of superoxide fea-

tures are used by Johnson et al. as evidence of either O2¢

(�1105–1110 cm¢1) or LiO2 (�1125–1130 cm¢1) adsorbed onthe gold electrode, no support is provided for these attribu-

tions except for rotating ring disc experiments that measurethe current from dissolved species. At high overpotentials, the

reduction of superoxide species to Li2O2 (�790–805 cm¢1) sup-

ports the overall superoxide attribution, but not necessarilythe differentiation between adsorbed and/or dissolved species.

Superoxide solvation in electrolytes with different donor num-bers may alone account for differences in O¢O bond strength

and their corresponding Raman frequency shifts. A similarSERS study by Gittleson et al.[53] (Figure 5) claimed in situ LiO2

attributions of �1140 cm¢1, which contrasts with the

�1110 cm¢1 attribution of adsorbed O2¢ in the same DMSO

electrolyte by Johnson et al. A high current per surface area

(�12 mA cm¢2) in the former study provides a reasonable ex-planation, as increased rates have been suggested to favor ad-

sorbed superoxide species on carbon electrodes.[70] Superoxidefeatures were also observed to shift from �1138 to

�1148 cm¢1 during discharging, signaling a decrease in O¢O

bond distance.[53] While the change in O¢O bonding was notfurther explored, it suggests again that the vibrational frequen-

cies of superoxides are highly influenced by their environment.Further evidence of this environmental effect is provided by

in situ SERS studies with superoxide attributions of�1110 cm¢1 in two different ionic liquids[54] and �1160 cm¢1

for a superoxide complex in DMSO.[52] Controlled experiments

are clearly needed to disentangle the effect of solvent andcation on superoxide vibrational frequency as well as the dif-

ference between dissolved and adsorbed species.In addition to SERS studies with gold electrodes, a number

of ex situ reports have shown Raman evidence of superoxidespecies on carbon electrodes. The groups of Amine and Curtiss

first noted a Raman feature at 1125 cm¢1 related to superox-ide-like species when examining a discharged activated carbon(AC) surface.[26] A broad feature related to Li2O2 at 788 cm¢1

was also apparent, showing that both species may co-exist.Support for the 1125 cm¢1 superoxide attribution included the

study of Peng et al.[49] and a number of DFT calculations forsolid-phase LiO2 (�1103 cm¢1) and [Li2O2]n clusters which ex-

hibited superoxide-like O¢O stretching modes (1150–

1190 cm¢1). Subsequent studies by the same authors notedconnections between the superoxide Raman intensity (at

1123 cm¢1), discharge current, and charge voltage with an acti-vated carbon electrode.[69] At higher rates of discharge (up to

0.2 mA cm¢2), the intensity of the superoxide feature increased(Figures 6 a–c),[70] but after charging beyond 3.5 V the superox-

Figure 3. In situ SERS during O2 reduction and oxidation on Au in O2-saturat-ed 0.1 m LiClO4/CH3CN. Spectra collected at a series of times at the reducingpotential of 2.2 V vs. Li/Li+ followed by other spectra at the oxidation poten-tials shown. The peaks are assigned as follows: 1) C¢C stretch of CH3CN at918 cm¢1, 2) O¢O stretch of LiO2 at 1137 cm¢1, 3) O¢O stretch of Li2O2 at808 cm¢1, 4) Cl¢O stretch of ClO4

¢ at 931 cm¢1. (Reproduced with permissionof Wiley-VCH from Ref. [49] .)

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ide feature was diminished (Figures 6 d–f). These findings sup-

port a solid-state disproportionation mechanism [Reaction (3a)]

during discharging and lower-charge overpotentials for LiO2 vs.Li2O2 (also observed in other studies).[24, 98] Another study on an

activated carbon electrode examined the stability of superox-ide-like Li–O2 discharge products under different environmen-

tal conditions, concluding that superoxide is stabilized in thepresence of electrolyte solvent and salt molecules.[29] Once the

electrolyte was removed, superoxide Raman features de-

creased in intensity. Lending credence to these ex situ results,

a feature directly proportional to that at 1123 cm¢1 was seenat 1505 cm¢1 (Figure 6) and attributed by DFT simulation to

a C¢C stretching mode of the activated carbon electrode inter-face with a LiO2-like component.[70] This supporting peak is no

small matter, since superoxides typically exhibit only a singleRaman feature that may overlap with the vibrational modes of

Figure 4. Surface-enhanced Raman spectra demonstrating that at low cathodic overpotentials O2¢ and LiO2 species are observed on the electrode surface at

short times in high and low donor number solvents, respectively, to be replaced by Li2O2 over time. At low voltages (high overpotentials) Li2O2 is apparentfrom short times. Spectra collected at a gold electrode during O2 reduction in the presence of 0.1 m LiClO4 in various aprotic solvents, recorded at differenttimes at various constant potentials indicated by the matching colored markers in the CVs above each stack of spectra. Vertical dotted lines with grey shadingshow positions of O2

¢ , LiO2 and Li2O2. Insets : expanded areas of spectral regions outlined by the dashed circles. Spectra at the bottom were collected at theopen-circuit potential. (Reproduced with permission of the Nature Publishing Group from Ref. [51] .)

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other species. (The secondary Li¢O stretch which could theo-retically confirm the presence of LiO2 is at least 100 times less

intense and thus rarely detected.[82, 84]) Other literature has sug-gested that the peaks observed as O¢O and C¢C vibrations oncarbon electrodes may in fact be related to C¢C and C=C vi-brations, respectively, either from alkyl carbonates[99] or the de-composition products of a PVDF binder.[14] Noting that thesefeatures have been observed with PVDF and not PTFE does

raise a significant concern, however none of the in situ studiesusing gold electrodes employed a PVDF (or any) binder materi-al and still exhibited superoxide-like features in the range1100–1140 cm¢1. Two other groups have separately detectedLiO2-like species ex situ on carbon electrodes under low tem-

perature (83 K) and ambient temperature conditions,[71, 72] sup-porting these attributions.

Because many Raman studies have been conducted with

DMSO-based electrolytes, it is necessary to note that electro-lyte decomposition products dimethyl sulfone (DMSO2) and

Li2SO4[100, 101] may yield features close to those associated with

the superoxide. The S=O stretch in both of these species corre-

sponds to a Raman shift around 1120–1130 cm¢1.[102, 103] DMSO2

and Li2SO4, however, exhibit a number of additional intense

peaks (i.e. , 498, 698 and 1004 cm¢1 for DMSO2, �504 and

�1010 cm¢1 for Li2SO4)[103] which should theoretically distin-guish them from the superoxide. In the absence of these fea-

tures, Raman spectroscopy has not been able to confirm thepresence of either DMSO2 or Li2SO4 in practical Li–O2 cells.

Figure 5. Operando Raman spectra of an Au–Ni foam electrode with a 0.1 mLiClO4/DMSO electrolyte: a) first discharge and charge cycle (front to back)and b) second discharge cycle (back to front). Red curves represent spectrataken at open-circuit potential. (Reproduced with permission of ACS Publish-ing from Ref. [53] .)

Figure 6. Raman spectra of toroids on the surface of discharged activatedcarbon (AC) electrodes at a discharge capacity of 1000 mAh g¢1 with differ-ent current densities of a) 0.2, b) 0.1 and c) 0.05 mA cm¢2. Raman spectra oftoroids on discharged AC electrodes after three cycles with a discharge ca-pacity of 1000 mAh g¢1, current density of 0.1 mA cm¢2 and upper cutoff vol-tages of d) 3.5, e) 3.8, and f) 4.0 V. The values of the peaks (in cm¢1) are 250(P1), 790 (P2), 1123 (S1), 1505 (S2), 1340 (D), 1600 (G). (Reproduced with per-mission of ACS Publishing from Ref. [53] .)

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Taken together, the existing literature with gold and carbonelectrodes presents a compelling picture that 1) superoxide

species are detectable in the products of non-aqueous Li–O2

cells using Raman spectroscopy and 2) the exact vibrational

frequency of the superoxide is dependent upon its environ-ment. Further studies are especially needed to examine the

latter point.

3.3. Side Products

One of the greatest challenges to Li–O2 development involvesdiscovering electrodes and electrolytes that do not degradeupon continued cycling. To this end, a number of reports have

demonstrated the instability of carbonate-based electro-lytes[104–109] and carbon-based electrodes.[110] The use of airrather than pure O2 has also been shown to yield irreversible

products,[111] and electrolyte salt anions have been noted fortheir tendency to form passivating surface films.[112–114] While

the composition of side products varies depending on thesystem, the most prevalent species are LiOH, Li2CO3 and Li2O.

Raman spectroscopy is particularly good at detecting these

materials, as noted in Table 1.While Raman spectroscopy has not been the primary

method of investigating Li–O2 side product formation, it hasbeen employed in several studies with increasing use recently.

McCloskey et al. used this technique on carbon electrodes todemonstrate the instability of carbonate-based electrolytes.[104]

The Raman spectra of electrodes discharged in ethylene car-

bonate/dimethyl carbonate (EC:DMC) and polypropylene car-bonate/dimethoxyethane (PC:DME) electrolytes, for example,

showed the presence of a Li2CO3 decomposition product,while an electrode discharged in a DME electrolyte showed

only the desired Li2O2. Another study similarly demonstratedthe stability of DME (vs. CH3CN) when contacted with Li2O2

crystals.[115] Decomposition products from several electrolyte

solvents (PC, DMSO, CH3CN, tetraglyme etc.) have also beenevaluated by Raman when mixed with a superoxide source

(KO2).[116] Though Raman features were not attributed to specif-ic species in this study, all solvents exhibited some level of de-composition, with a major product indicated as Li2CO3. A nota-ble drawback of these reports is the lack of thorough spectral

analysis when explaining electrolyte decomposition.Following studies by Peng et al.[5] and Laoire et al. ,[8] DMSO-

based electrolytes have become more prevalent in Li–O2 litera-ture. Despite being more stable than carbonate-based solventsand with kinetic benefits over ether-based solvents,[117] DMSO

has been the subject of several recent stability studies. Chenet al. , for example, showed Raman evidence of a Li2CO3 side

product on a gold electrode, despite achieving 100 completedischarge and charge cycles with a DMSO-based electrolyte

and a redox mediator.[50] Li2CO3 Raman features have also been

found after fixed potential discharging in DMSO solvents ona carbon cloth electrode[73] and a glassy carbon electrode.[74]

Carbonate Raman peaks are sometimes accompanied by Li2Ofeatures, suggesting that these two species form through the

same DMSO decomposition mechanism. Dilimon et al.[74] spe-cifically noted that Li2CO3 is a product of systems with DMSO

and LiClO4, but not LiPF6, which raises the specter of decompo-sition mediated by salt anions. LiOH, DMSO2 and Li2SO4 have

also been proposed as products of superoxide attack onDMSO[100] with support from FTIR[101, 118] and Raman spectrosco-

py (only LiOH for the latter).[101] A new report, however, sug-gests DMSO decomposition is unlikely in practical Li–O2

cells.[119] Despite the lack of clarity, we must accept that DMSOis not as stable an electrolyte solvent as once thought.

The most challenging aspect of Raman spectroscopy for Li–

O2 systems is the lack of reliable reference material for dis-charge and side products. To partially remedy this situation,we provide in Figure 7 standard spectra for commercially pur-chased powders of common products.

3.4. Electrode and Electrolyte Interaction

Another intriguing use of Raman spectroscopy is to study Li–

O2 electrode-electrolyte interaction. The most obvious realiza-tion of this concept is by using SERS to monitor changes inthe intensity of electrolyte features in situ. Such changes maybe due to the formation of discharge products or reorientation

of electrolyte molecules. By monitoring changes in electrolyteand electrode feature intensities, one study observed the for-

mation and evolution of product films on an Au–Ni foam elec-trode surface.[53] As products precipitated, electrolyte mole-cules were displaced from the Au surface and the related fea-

tures decreased in intensity. The opposite was observed uponcharging, when the active surface was freed of products. Simi-

lar results can be found in the literature,[50, 51] but have notbeen highlighted directly. An extension of this concept to

other electrolytes and electrodes is possible.

Probing the orientation of polarizable molecules is possiblewith the application of SERS and Raman surface selection

rules. Surface selection rules theorized by Moskovits et al.[120]

hold that Raman signals are more enhanced for bonds perpen-

dicular to the substrate than for those horizontally oriented.The intensity of signals from these molecules may also be

Figure 7. Raman spectra of commercial powders for reference. (Reproducedwith permission of ACS Publishing from Ref. [53] .)

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a product of Raman spot size and electrode roughness up toseveral monolayers from the surface.[121] In practice, the orien-

tation of molecules closest to the substrate can be determined.Though not highlighted directly, results supporting this con-

cept are seen with redox mediator and complexing molecules.In the former, the SERS intensity of a tetrathiafulvalene (TTF)

feature was shown to be greater following charging than fol-lowing discharging, consistent with a closer proximity to the

electrode surface.[50] In the latter, SERS features related to a Di-

ImTFSI–O2¢ [Di-ImTFSI: o-xylene-a,a’-diylbis(1,2-dimethyl-1H-

imidazol-3-ium) Di(bis-(trifluoromethanesulfonylmidate))] com-plex were used to demonstrate the mechanism of oxygen re-duction with this complexing agent.[52] Comprehensive studies

using this strategy will be valuable in elucidating redoxmechanisms.

By analyzing the electrode and separator after cell disassem-

bly, it is possible to probe product composition as a functionof proximity to the electrode. For example, Ryu et al. used

Raman spectroscopy to show the composition of productsnear a carbon nanotube (CNT) electrode surface and those

products trapped in an electrospun catalytic membrane abovethat surface.[71] Surprisingly, this study demonstrated nearly

complete reversibility of Li2O2, even for products which are

several microns away from the electrode.Another intriguing possibility is the application of Raman

spectroscopy to monitor the surface of Li metal anodes. Manystudies have noted the instability of Li metal in various electro-

lytes and a few have analyzed these surfaces usingXPS,[112, 122, 123] though not by Raman spectroscopy. The ability

to detect and discern a number of decomposition species on

the anode is a natural, yet untapped, application of thistechnique.

4. Conclusions and Outlook

Achieving a reliable and comprehensive understanding of theelectrochemical processes in Li–O2 systems is critical to the fur-

ther development of this technology. Ongoing work mustfocus on discerning the chemistry of products and how com-position, morphology, proximity, and interaction affect cell per-formance. With creative applications, Raman spectroscopy can

address all of these complexities.The realization that superoxides play an important role in

oxygen reduction and evolution necessitates the use of toolsthat can specifically discern these species. To date, Ramanspectroscopy is the only reliable technique that accomplishes

this. The ability of Raman spectroscopy to also fingerprint themost relevant electrochemically generated products (Li2O2,

LiOH, Li2CO3 and Li2O) should encourage its use as an essentialcharacterization method for Li–O2 systems alongside already

common techniques such as XRD, XPS and FTIR.

In situ Raman studies are both simple and extremely valua-ble to support electrochemical mechanisms, but they must be

conducted with practical materials and cell structures (e.g.with an operando methodology) in order to truly be useful.

Cross-referencing Raman attributions and supporting themwith simulation makes for significantly more convincing results.

To that end, controlled studies are necessary to disentanglethe interactions between superoxides and/or Li2O2 in the pres-

ence of different electrolytes. Along with SERS, it may be possi-ble to apply resonance Raman spectroscopy to better discern

interfacial species with very low concentrations. Expandingin situ Raman beyond the typical Au SERS electrode is also

highly desirable to study the effect of different electrode surfa-ces and redox molecules on product composition.

While this review deals with the complexities of Li–O2 sys-

tems, it is instructive to note the similarities in application toother metal-oxygen cells including Na–O2 and K–O2. Much ofthe chemistry differs only in the cation since these systems in-volve comparable reduction and oxidation processes. Though

products may be slightly different, the application of Ramanspectroscopy is no less essential for these next-generation

energy storage technologies.

Acknowledgements

F.S.G. and A.D.T. acknowledge support from the National Science

Foundation NSF-CBET-0954985 CAREER Award and AFOSR(FA9550-11-1-0219). S.Y.S. is thankful for the support of a postdoc-

toral fellowship from the Natural Sciences and Engineering Re-search Council (NSERC) of Canada. W.-H.R. acknowledges support

from The NatureNet Program of the Nature Conservancy.

Keywords: Li–air batteries · lithium peroxide · metal–air

batteries · metal–oxygen batteries · superoxides

[1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke, J.Phys. Chem. Lett. 2010, 1, 2193 – 2203.

[2] R. Padbury, X. Zhang, J. Power Sources 2011, 196, 4436 – 4444.[3] N. Imanishi, A. C. Luntz, P. Bruce, The Lithium Air Battery: Fundamentals,

Springer, New York, 2014.[4] T. Ogasawara, A. D¦bart, M. Holzapfel, P. Nov�k, P. G. Bruce, J. Am.

Chem. Soc. 2006, 128, 1390 – 1393.[5] Z. Peng, S. A. Freunberger, Y. Chen, P. G. Bruce, Science 2012, 337,

563 – 566.[6] S. Meini, S. Solchenbach, M. Piana, H. A. Gasteiger, J. Electrochem. Soc.

2014, 161, A1306 – A1314.[7] K. U. Schwenke, M. Metzger, T. Restle, M. Piana, H. Gasteiger, J. Electro-

chem. Soc. 2015, 162, A573 – A584.[8] C. O. Laoire, S. Mukerjee, K. M. Abraham, E. J. Plichta, M. A. Hendrick-

son, J. Phys. Chem. C 2010, 114, 9178 – 9186.[9] S. R. Gowda, A. Brunet, G. M. Wallraff, B. D. McCloskey, J. Phys. Chem.

Lett. 2013, 4, 276 – 279.[10] Y.-C. Lu, H. A. Gasteiger, Y. Shao-Horn, J. Am. Chem. Soc. 2011, 133,

19048 – 19051.[11] Y.-C. Lu, H. A. Gasteiger, M. C. Parent, V. Chiloyan, Y. Shao-Horn, Electro-

chem. Solid-State Lett. 2010, 13, A69 – A72.[12] Y.-C. Lu, H. A. Gasteiger, E. Crumlin, R. McGuire, Y. Shao-Horn, J. Electro-

chem. Soc. 2010, 157, A1016 – A1025.[13] S. Meini, N. Tsiouvaras, K. U. Schwenke, M. Piana, H. Beyer, L. Lange,

H. A. Gasteiger, Phys. Chem. Chem. Phys. 2013, 15, 11478 – 11493.[14] A. C. Luntz, B. D. Mccloskey, Chem. Rev. 2014, 114, 11721 – 11750.[15] M. D. Bhatt, H. Geaney, M. Nolan, C. O’Dwyer, Phys. Chem. Chem. Phys.

2014, 16, 12093 – 12130.[16] Y.-C. Lu, B. M. Gallant, D. G. Kwabi, J. R. Harding, R. R. Mitchell, M. S.

Whittingham, Y. Shao-Horn, Energy Environ. Sci. 2013, 6, 750.[17] S. Ganapathy, B. D. Adams, G. Stenou, M. S. Anastasaki, K. Goubitz, X.-F.

Miao, L. F. Nazar, M. Wagemaker, J. Am. Chem. Soc. 2014, 136, 16335 –16344.

ChemElectroChem 2015, 2, 1446 – 1457 www.chemelectrochem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1455

Minireviews

[18] J.-L. Shui, J. S. Okasinski, C. Chen, J. D. Almer, D.-J. Liu, ChemSusChem2014, 7, 543 – 548.

[19] J.-J. Xu, Z.-L. Wang, D. Xu, L.-L. Zhang, X.-B. Zhang, Nat. Commun.2013, 4, 2438.

[20] Y.-C. Lu, D. G. Kwabi, K. P. C. Yao, J. R. Harding, J. Zhou, L. Zuin, Y. Shao-Horn, Energy Environ. Sci. 2011, 4, 2999 – 3007.

[21] B. D. Adams, C. Radtke, R. Black, M. L. Trudeau, K. Zaghib, L. F. Nazar,Energy Environ. Sci. 2013, 6, 1772 – 1778.

[22] Y.-C. Lu, E. J. Crumlin, G. M. Veith, J. R. Harding, E. Mutoro, L. Baggetto,N. J. Dudney, Z. Liu, Y. Shao-Horn, Sci. Rep. 2012, 2, 715.

[23] Y.-C. Lu, E. J. Crumlin, T. J. Carney, L. Baggetto, G. M. Veith, N. J.Dudney, Z. Liu, Y. Shao-Horn, J. Phys. Chem. C 2013, 117, 25948 –25954.

[24] B. M. Gallant, D. G. Kwabi, R. R. Mitchell, J. Zhou, C. V. Thompson, Y.Shao-Horn, Energy Environ. Sci. 2013, 6, 2518 – 2528.

[25] R. R. Mitchell, B. M. Gallant, Y. Shao-Horn, C. V. Thompson, J. Phys.Chem. Lett. 2013, 4, 1060 – 1064.

[26] J. Yang, D. Zhai, H.-H. Wang, K. C. Lau, J. A. Schlueter, P. Du, D. J. Myers,Y.-K. Sun, L. A. Curtiss, K. Amine, Phys. Chem. Chem. Phys. 2013, 15,3764 – 3771.

[27] G. Yang, Y. Wang, J. Phys. Chem. Lett. 2014, 5, 2516 – 2521.[28] K. P. C. Yao, Y.-C. Lu, C. V. Amanchukwu, D. G. Kwabi, M. Risch, J. Zhou,

A. Grimaud, P. T. Hammond, F. Bard¦, Y. Shao-Horn, Phys. Chem. Chem.Phys. 2014, 16, 2297 – 2304.

[29] D. Zhai, K. C. Lau, H. Wang, J. Wen, D. J. Miller, J. Lu, F. Kang, B. Li, W.Yang, J. Gao, E. Indacochea, L. A. Curtiss, K. Amine, Nano Lett. 2015, 15,1041 – 1046.

[30] C. V. Raman, K. S. Krishnan, Nature 1928, 121, 501 – 502.[31] R. L. McCreery, Raman Spectroscopy for Chemical Analysis, John Wiley

and Sons, New York, 2000.[32] C. R. Brundle, C. A. Evans Jr. , S. Wilson, Encyclopedia of Materials Char-

acterization, Butterworth-Heinemann, Stoneham MA, 1992.[33] D. J. Gardiner, P. R. Graves, Practical Raman Spectroscopy, Springer-

Verlag, Berlin, 1989.[34] R. Baddour-Hadjean, J.-P. Pereira-Ramos, Chem. Rev. 2010, 110, 1278 –

1319.[35] D. E. Irish, Z. Deng, M. Odziemkowski, J. Power Sources 1995, 54, 28 –

33.[36] G. Li, H. Li, Y. Mo, X. Huang, L. Chen, Chem. Phys. Lett. 2000, 330, 249 –

254.[37] G. Li, H. Li, Y. Mo, L. Chen, X. Huang, J. Power Sources 2002, 104, 190 –

194.[38] V. Stancovski, S. Badilescu, J. Appl. Electrochem. 2014, 44, 23 – 43.[39] M. A. BaÇares, Catal. Today 2005, 100, 71 – 77.[40] K. Kneipp, Y. Wang, H. Kneipp, L. Perelman, I. Itzkan, R. Dasari, M. Feld,

Phys. Rev. Lett. 1997, 78, 1667 – 1670.[41] K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari, M. S. Feld, Chem. Rev. 1999,

99, 2957 – 2975.[42] Z. Tian, B. Ren, D. Wu, J. Phys. Chem. B 2002, 106, 9463 – 9483.[43] M. Moskovits, Phys. Chem. Chem. Phys. 2013, 15, 5301 – 5311.[44] S. Schlìcker, Surface-Enhanced Raman Spectroscopy: Analytical, Bio-

physical and Life Science Applications, Wiley-VCH, Weinheim, 2011.[45] M. Moskovits, Rev. Mod. Phys. 1985, 57, 783 – 826.[46] A. Campion, P. Kambhampati, C. Harris, Chem. Soc. Rev. 1998, 27, 241 –

250.[47] A. M. Michaels, M. Nirmal, L. E. Brus, J. Am. Chem. Soc. 1999, 121,

9932 – 9939.[48] P. Kambhampati, C. M. Child, M. C. Foster, A. Campion, J. Chem. Phys.

1998, 108, 5013 – 5026.[49] Z. Peng, S. A. Freunberger, L. J. Hardwick, Y. Chen, V. Giordani, F. Bard¦,

P. Nov�k, D. Graham, J.-M. Tarascon, P. G. Bruce, Angew. Chem. Int. Ed.2011, 50, 6351 – 6355; Angew. Chem. 2011, 123, 6475 – 6479.

[50] Y. Chen, S. A. Freunberger, Z. Peng, O. Fontaine, P. G. Bruce, Nat. Chem.2013, 5, 489 – 494.

[51] L. Johnson, C. Li, Z. Liu, Y. Chen, S. A. Freunberger, J.-M. Tarascon, P. C.Ashok, B. B. Praveen, K. Dholakia, P. G. Bruce, Nat. Chem. 2014, 6,1091 – 1099.

[52] C. Li, O. Fontaine, S. A. Freunberger, L. Johnson, S. Grugeon, P. G.Bruce, M. Armand, J. Phys. Chem. C 2014, 118, 3393 – 3401.

[53] F. S. Gittleson, W. Ryu, A. D. Taylor, ACS Appl. Mater. Interfaces 2014, 6,19017 – 19025.

[54] J. T. Frith, A. E. Russell, N. Garcia-Araez, J. R. Owen, Electrochem.Commun. 2014, 46, 33 – 35.

[55] Y. Qiao, S. Ye, J. Phys. Chem. C 2015, 119, 12236 – 12250.[56] K. Izutsu, Electrochemistry in Nonaqueous Solutions, Wiley-VCH, Wein-

heim, 2002.[57] S. A. Hunter-Saphir, J. A. Creighton, J. Raman Spectrosc. 1998, 29, 417 –

419.[58] D. D. Archibald, P. Yager, Appl. Spectrosc. 1992, 46, 1613 – 1620.[59] B. J. Kip, R. J. Meier, Appl. Spectrosc. 1990, 44, 707 – 711.[60] K. P. C. Yao, D. G. Kwabi, R. A. Quinlan, A. N. Mansour, A. Grimaud, Y.-L.

Lee, Y.-C. Lu, Y. Shao-Horn, J. Electrochem. Soc. 2013, 160, A824 – A831.[61] B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thomp-

son, Y. Shao-Horn, J. Phys. Chem. C 2012, 116, 20800 – 20805.[62] J. Lu, L. Cheng, K. C. Lau, E. Tyo, X. Luo, J. Wen, D. Miller, R. S. Assary,

H.-H. Wang, P. Redfern, H. Wu, J.-B. Park, Y.-K. Sun, S. Vajda, K. Amine,L. A. Curtiss Nat. Commun. 2014, 5, 4895.

[63] H. Lim, E. Yilmaz, H. R. Byon, J. Phys. Chem. Lett. 2012, 3, 3210 – 3215.[64] D. C. Koningsberger, R. Prins, X-Ray Absorption : Principles, Applications,

Techniques of EXAFS, SEXAFS and XANES, John Wiley and Sons, NewYork, 1988.

[65] B. H. Stuart, Infrared Spectroscopy : Fundamentals and Applications,John Wiley and Sons, Chichester, 2004.

[66] L. Andrews, J. Chem. Phys. 1969, 50, 4288 – 4299.[67] Y. Lu, Y. Shao-Horn, J. Phys. Chem. Lett. 2013, 4, 93 – 99.[68] K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1 – 5.[69] D. Zhai, H.-H. Wang, J. Yang, K. C. Lau, K. Li, K. Amine, L. A. Curtiss, J.

Am. Chem. Soc. 2013, 135, 15364 – 15372.[70] D. Zhai, H. Wang, K. C. Lau, J. Gao, P. C. Redfern, F. Kang, B. Li, E. Inda-

cochea, U. Das, H.-H. Sun, H.-J. Sun, K. Amine. L. A. Curtiss, J. Phys.Chem. Lett. 2014, 5, 2705 – 2710.

[71] W.-H. Ryu, F. S. Gittleson, M. Schwab, T. Goh, A. D. Taylor, Nano Lett.2015, 15, 434 – 441.

[72] C. Xia, M. Waletzko, L. Chen, K. Peppler, P. J. Klar, J. Janek, ACS Appl.Mater. Interfaces 2014, 6, 12083 – 12092.

[73] I. Gunasekara, S. Mukerjee, E. J. Plichta, M. A. Hendrickson, K. M. Abra-ham, J. Electrochem. Soc. 2014, 161, A381 – A392.

[74] V. S. Dilimon, D.-G. Lee, S.-D. Yim, H.-K. Song, J. Phys. Chem. C 2015,119, 3472 – 3480.

[75] M. D. Radin, D. J. Siegel, Energy Environ. Sci. 2013, 6, 2370 – 2379.[76] M. D. Radin, J. F. Rodriguez, F. Tian, D. J. Siegel, J. Am. Chem. Soc. 2012,

134, 1093 – 1103.[77] F. Tian, M. D. Radin, D. J. Siegel, Chem. Mater. 2014, 26, 2952 – 2959.[78] J. Sangster, A. Pelton, J. Phase Equilib. 1992, 13, 296 – 299.[79] X. Ren, Y. Wu, J. Am. Chem. Soc. 2013, 135, 2923 – 2926.[80] P. Hartmann, C. L. Bender, M. Vracar, A. K. Dìrr, A. Garsuch, J. Janek, P.

Adelhelm, Nat. Mater. 2013, 12, 228 – 32.[81] O. Gerbig, R. Merkle, J. Maier, Adv. Funct. Mater. 2015, 25, 2552 – 2563.[82] D. A. Hatzenbuhler, L. Andrews, J. Chem. Phys. 1972, 56, 3398 – 3403.[83] L. Andrews, R. R. Smardzewski, J. Chem. Phys. 1973, 58, 2258 – 2261.[84] H. Hìber, G. A. Ozin, J. Mol. Spectrosc. 1972, 41, 595 – 597.[85] T. Itoh, T. Maeda, A. Kasuya, Faraday Discuss. 2006, 132, 95 – 109.[86] T. Itoh, K. Abe, K. Dokko, M. Mohamedi, I. Uchida, A. Kasuya, J. Electro-

chem. Soc. 2004, 151, A2042.[87] F. L. Whiting, G. Mamantov, G. M. Begun, J. P. Young, Inorg. Chim. Acta

1971, 5, 260 – 262.[88] L. Andrews, J. Chem. Phys. 1971, 54, 4935 – 4943.[89] E. S. Rittner, J. Chem. Phys. 1951, 19, 1030 – 1035.[90] D. Sawyer, J. Valentine, Acc. Chem. Res. 1981, 14, 393 – 400.[91] A. D. Goolsby, D. T. Sawyer, Anal. Chem. 1968, 40, 83 – 86.[92] K. U. Schwenke, S. Meini, X. Wu, H. A. Gasteiger, M. Piana, Phys. Chem.

Chem. Phys. 2013, 15, 11830 – 11839.[93] T. Fujinaga, S. Sakura, Bull. Chem. Soc. Jpn. 1974, 47, 2781 – 2786.[94] D. T. Sawyer, G. Chlerlcato, C. T. Angells, E. J. Nannl, T. Tsuchlya, Anal.

Chem. 1982, 54, 1720 – 1724.[95] D. Aurbach, M. Daroux, P. Faguy, E. Yeager, J. Electroanal. Chem. Interfa-

cial Electrochem. 1991, 297, 225 – 244.[96] C. O. Laoire, S. Mukerjee, K. M. Abraham, E. J. Plichta, M. A. Hendrick-

son, J. Phys. Chem. C 2009, 113, 20127 – 20134.[97] D. Vasudevan, H. Wendt, J. Electroanal. Chem. 1995, 192, 69 – 74.[98] S. Kang, Y. Mo, S. P. Ong, G. Ceder, Chem. Mater. 2013, 25, 3328 – 3336.

ChemElectroChem 2015, 2, 1446 – 1457 www.chemelectrochem.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1456

Minireviews

[99] S. Y. Kim, H.-T. Lee, K.-B. Kim, Phys. Chem. Chem. Phys. 2013, 15,20262 – 20271.

[100] D. Sharon, M. Afri, M. Noked, A. Garsuch, A. A. Frimer, D. Aurbach, J.Phys. Chem. Lett. 2013, 4, 3115 – 3119.

[101] D. G. Kwabi, T. P. Batcho, C. V. Amanchukwu, N. Ortiz-Vitoriano, P. Ham-mond, C. V. Thompson, Y. Shao-Horn, J. Phys. Chem. Lett. 2014, 5,2850 – 2856.

[102] R. McLachlan, V. Carter, Spectrochim. Acta 1970, 26A, 1121 – 1127.[103] E. Cazzanelli, R. Frech, J. Chem. Phys. 1984, 81, 4729 – 4736.[104] B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar, A. C.

Luntz, J. Phys. Chem. Lett. 2011, 2, 1161 – 1166.[105] W. Xu, K. Xu, V. V. Viswanathan, S. A. Towne, J. S. Hardy, J. Xiao, Z. Nie,

D. Hu, D. Wang, J.-G. Zhang, J. Power Sources 2011, 196, 9631 – 9639.[106] S. A. Freunberger, Y. Chen, Z. Peng, J. M. Griffin, L. J. Hardwick, F. Bard¦,

P. Nov�k, P. G. Bruce, J. Am. Chem. Soc. 2011, 133, 8040 – 8047.[107] W. Xu, J. Hu, M. H. Engelhard, S. A. Towne, J. S. Hardy, J. Xiao, J. Feng,

M. Y. Hu, J. Zhang, F. Ding, M. E. Gross, J.-G. Zhang, J. Power Sources2012, 215, 240 – 247.

[108] J. Xiao, J. Hu, D. Wang, D. Hu, W. Xu, G. L. Graff, Z. Nie, J. Liu, J.-G.Zhang, J. Power Sources 2011, 196, 5674 – 5678.

[109] W. Xu, V. V. Viswanathan, D. Wang, S. A. Towne, J. Xiao, Z. Nie, D. Hu,J.-G. Zhang, J. Power Sources 2011, 196, 3894 – 3899.

[110] M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng, P. G. Bruce, J. Am.Chem. Soc. 2013, 135, 494 – 500.

[111] J. Christensen, P. Albertus, R. S. Sanchez-Carrera, T. Lohmann, B. Kozin-sky, R. Liedtke, J. Ahmed, A. Kojic, J. Electrochem. Soc. 2012, 159, R1 –R30.

[112] R. Younesi, M. Hahlin, K. Edstrçm, ACS Appl. Mater. Interfaces 2013, 5,1333 – 1341.

[113] P. Du, J. Lu, K. C. Lau, X. Luo, J. BareÇo, X. Zhang, Y. Ren, Z. Zhang, L. A.Curtiss, Y.-K. Sun, K. Amine, Phys. Chem. Chem. Phys. 2013, 15, 5572 –5581.

[114] I. Gunasekara, S. Mukerjee, E. J. Plichta, M. A. Hendrickson, K. M. Abra-ham, J. Electrochem. Soc. 2015, 162, A1055 – A1066.

[115] D. Chalasani, B. L. Lucht, ECS Electrochem. Lett. 2012, 1, A38 – A42.[116] K. Takechi, S. Higashi, F. Mizuno, H. Nishikoori, H. Iba, T. Shiga, ECS Elec-

trochem. Lett. 2012, 1, A27 – A29.[117] F. S. Gittleson, R. C. Sekol, G. Doubek, M. Linardi, A. D. Taylor, Phys.

Chem. Chem. Phys. 2014, 16, 3230 – 3237.[118] N. Mozhzhukhina, L. P. M¦ndez De Leo, E. J. Calvo, J. Phys. Chem. C

2013, 117, 18375 – 18380.[119] M. A. Schroeder, N. Kumar, A. J. Pearse, C. Liu, S. B. Lee, G. W. Rubloff,

K. Leung, M. Noked, ACS Appl. Mater. Interfaces 2015, 7, 11402 – 11411.[120] M. Moskovits, J. Chem. Phys. 1982, 77, 4408 – 4416.[121] J. C. Tsang, J. R. Kirtley, T. N. Theis, J. Chem. Phys. 1982, 77, 641 – 645.[122] H. Lee, D. J. Lee, J.-N. Lee, J. Song, Y. Lee, M.-H. Ryou, J.-K. Park, Y. M.

Lee, Electrochim. Acta 2014, 123, 419 – 425.[123] M. Roberts, R. Younesi, W. Richardson, J. Liu, J. Zhu, K. Edstrom, T. Gus-

tafsson, ECS Electrochem. Lett. 2014, 3, A62 – A65.

Manuscript received: May 17, 2015Accepted Article published: July 31, 2015Final Article published: August 20, 2015

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