Monitoring the Electrochemical Processes in the Lithium–Air Battery by Solid State NMR Spectroscopy (2024)

A multi-nuclearsolid-state NMR approach is employed to investigatethe lithium–air battery, to monitor the evolution of the electrochemicalproducts formed during cycling, and to gain insight into processesaffecting capacity fading. While lithium peroxide is identified by ¹⁷O solid state NMR (ssNMR) as the predominant product in thefirst discharge in 1,2-dimethoxyethane (DME) based electrolytes, itreacts with the carbon cathode surface to form carbonate during thecharging process. ¹³C ssNMR provides evidence for carbonateformation on the surface of the carbon cathode, the carbonate beingremoved at high charging voltages in the first cycle, but accumulatingin later cycles. Small amounts of lithium hydroxide and formate arealso detected in discharged cathodes and while the hydroxide formationis reversible, the formate persists and accumulates in the cathodeupon further cycling. The results indicate that the rechargeabilityof the battery is limited by both the electrolyte and the carbon cathodestability. The utility of ssNMR spectroscopy in directly detectingproduct formation and decomposition within the battery is demonstrated,a necessary step in the assessment of new electrolytes, catalysts,and cathode materials for the development of a viable lithium–oxygenbattery.

a. Electrode Fabrication

Oxygen electrodeswere prepared from a mixture of 24 wt % superP Li carbon (Timcal),38 wt % polyvinylidene fluoride (PVDF) binder, and 38 wt % dibutylphthalate(DBP, Sigma-Aldrich) in acetone. The slurry was then spread into aself-supporting film of thickness 150 μm, cut into disc shapewith 1/2″ diameter, and washed with diethyl ether to removethe DBP. The final film contained 40 wt % carbon. The electrodes werethen vacuum-dried at 115 °C and taken into an argon filled gloveboxwithout exposure to ambient atmosphere.

¹³C-enrichedelectrodes were made following the same procedure using ¹³C enriched (99%) amorphous carbon powder from Cambridge Isotope Laboratories.

b. Cell Assembly and Electrochemistry

Lithium–oxygencells were assembled in an argon-filled gloveboxusing standard 3205 coin cells (Hohsen Corp.) that were modified with1 mm diameter holes punched to their top case. Cells were assembledby stacking a disc of lithium foil (0.38 mm thickness, Sigma-Aldrich)on top of a stainless steel current collector, followed by a borosilicateglass fiber separator (Whatman). Electrolyte made of 1 M lithium bis(trifluoromethanesulfonyl)imide(LiTFSI, Sigma Aldrich, vacuum-dried at 250 °C prior to use for12 h) in 1,2-dimethoxyethane (DME, Sigma Aldrich, vacuum distilledand stored over molecular sieves in an argon glovebox) was added tothe separator. The self-supporting carbon electrode was then placedon top, followed by a stainless steel mesh (Advert Materials) andthe punched coin cell top. The cell was pressed closed, excess electrolyteremoved, and sealed in a glass chamber with two Young valves. Followingassembly in the glovebox, cells were flushed with oxygen gas for 30min through the valves. After adding the oxygen, the cells were restedfor 10–20 h and then cycled on an Arbin battery cycler.

Following cycling, cells were disassembled in the glovebox and thecathodes were extracted, washed with anhydrous acetonitrile, and vacuum-driedovernight in the glovebox’s prechamber and packed into theNMR rotors without exposure to ambient atmosphere.

For ⁶Li measurements, ⁶Li-enriched lithiumfoil (Cambridge Isotope Laboratories) was used as the anode.

c. ¹⁷O Isotope Enrichment

For ¹⁷O NMR measurements the cell assembly was performed as describedabove. Following the addition of natural abundance oxygen, the cellwas connected to a vacuum line. ¹⁷O enriched oxygen gas(60–70% enrichment, Isotec and Cambridge Isotope Laboratories)was connected to the line. The pressure in the cell was reduced toabout 0.8 atm and refilled back to 1 atm with the ¹⁷O enrichedoxygen gas resulting in about 20–25% ¹⁷O enrichedoxygen gas. The cell was then rested for 10–20 h and cycledas above.

d. Synthesis of Model Compounds

¹⁷O-enriched LiOH was synthesized following the procedure of Abys etal.³⁷ from the reaction between n-butyllithium in hexane (52 mL) and ¹⁷O enriched (10%) water(1.5 mL) in 100 mL dry tetrahydrofuran, resulting in 1.5 g of LiOH. ¹⁷O-enriched Li₂O was obtained by heating the aboveLiOH to 700 °C under vacuum, raising the temperature in severalsteps, again following the procedure by Abys et al.³⁷¹⁷O enriched Li₂CO₃ wasmade by placing the above LiOH in a tube furnace heated to 100 °Cand flowing CO₂ gas at a rate of 15 cm³/minfor 12 h. The formation of the phases and their purity was confirmedby X-ray powder diffraction collected on a Panalytical X’PertPro diffractometer.

e. Solid State NMR Spectroscopy

¹H ssNMR measurements were performed on a Bruker 700MHz Avance IIIspectrometer using a 1.3 mm double resonance probe. A rotor synchronizedHahn echo sequence was used with a nutation frequency of 120 kHz anda spinning frequency of 60 kHz. The relaxation delay was optimizedfor each sample with optimal values in the range of 8–12 s.Spectra were referenced to adamantane set at 1.8 ppm. Spectral analysiswas performed using the DMFIT software.³⁸

¹³C ssNMR spectra of ¹³C-enriched cathodeswere acquired on a Bruker 300 MHz Avance I spectrometer using a 2.5mm double resonance probe. A rotor synchronized Hahn echo sequencewas used with a nutation frequency of 100 kHz, a spinning frequencyof 25 kHz, and a relaxation delay of 20 s. Spectra were referencedto the adamantane tertiary group set at 38.5 ppm. Two dimensional(2D) hom*onuclear correlation experiments were acquired on a Bruker400 MHz Avance I spectrometer using a 2.5 mm probe spinning with aspinning frequency of 10 kHz. Radio Frequency Driven Recoupling (RFDR)³⁹ was used to recouple the dipolar hom*onuclearinteractions using a nutation frequency of 105 kHz for a durationof 200 rotor cycles (20 ms).

¹H–⁶Li heteronuclear correlation experimentswere performed on a Bruker 700 MHz Avance III spectrometer using a1.3 mm double resonance probe and a spinning frequency of 60 kHz.Cross-polarization (CP) was used to correlate ¹H–⁶Li dipolar coupled nuclei with a 5 ms contact time.

¹⁷O spectra of discharged cathodes were acquired ona Bruker 850 and 700 MHz Avance III spectrometers using a 1.3 mm probeand a spinning frequency of 60 kHz. A rotor synchronized Hahn echosequence was used with the highest radio frequency (RF) nutation frequencyachieved equal to 91 and 96 kHz on the 700 and 850 MHz spectrometers,respectively). A relaxation delay of 1–1.5 s was used, andthe experiment times varied between 20 and 48 h on the 850 MHz spectrometer.The spectra of LiOH and Li₂O were acquired on a Bruker700 MHz Avance III with a single pulse excitation (nutation frequencyof 132 kHz) and a relaxation delay of 5 s. The spectrum of Li₂CO₃ was acquired on a Bruker 850 MHz Avance IIIspectrometer using a 4 mm double resonance probe using a single pulseexcitation (nutation frequency of 42 kHz) with a relaxation delayof 15 s. Natural abundance ¹⁷O spectra of anhydrous lithiumacetate (CH₃O₂Li) and lithium formate (HCO₂Li) were acquired on a Bruker 900 MHz Avance II spectrometer,using a 4 mm double resonance probe with a rotor synchronized Hahnecho (nutation frequency 42 kHz). A natural abundance ¹⁷O spectrum of Li₂O₂ was acquired on a Bruker850 MHz Avance III spectrometer using a static probe with a solidecho pulse excitation (62 kHz nutation frequency; 30 s relaxationdelay). MAS spinning frequencies are specified in the figure captions.Spectra were fit using either SPINEVOLUTION⁴⁰ spin dynamics simulation program or the line shape analysis toolin the Bruker software Topspin.

a. ¹⁷O NMR SpectralLibrary of PossibleReaction Products

In order to identify the electrochemicalproducts formed in the cathode, we first obtain ¹⁷O spectra(Figure ​(Figure1a–f)1a–f) and relevant NMR parameters(from fits to the spectra; Table 1) from anassembled library of possible reaction products. For lithium peroxideand carbonate, the fits to the ¹⁷O spectra were supportedby density functional theory calculations of the NMR parameters, asdescribed in ref (33). As the natural abundance of the ¹⁷O isotope is only0.034%, isotope enrichment was used where possible, and high fieldmeasurements were employed for increased sensitivity.

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Figure 1

¹⁷O experimental NMR spectraof the various model compounds(black) and simulated best fits (gray) (b–f); the simulatedspectrum shown in (a) was calculated using the NMR parameters obtainedfrom ¹⁷O-enriched Li₂O₂ obtainedelectrochemically:³³ (a) Static spectrumof natural abundance Li₂O₂, (b) MAS (12.5 kHz)spectrum of 10% ¹⁷O enriched Li₂CO₃, (c) MAS (12.5 kHz) of natural abundance CH₃CO₂Li, (d) MAS (60 kHz) spectrum of 10% ¹⁷O enriched LiOH,(e) MAS (12.5 kHz) of natural abundance HCO₂Li, and (f)MAS (60 kHz) spectrum of 10% ¹⁷O enriched Li₂O.

The naturalabundance spectrum of lithium peroxide (Figure ​(Figure1a)1a) was acquired without MAS due to the large C_q value expected from ab initio calculations of the NMR parameters.³³ While the low signal-to-noise (S/N) ratio doesnot allow accurate values of the isotropic chemical shift and quadrupolarcoupling to be extracted, the width of this spectrum of more than2000 ppm in a 20 T field is consistent with the quadrupole singularitiescalculated with NMR parameters determined from the electrochemicallyformed lithium peroxide, as shown in our previous work.³³ The large coupling constant (18 MHz), whichis more than twice that of most ¹⁷O species measured inthe solid state,⁴¹ makes it easy to distinguishLi₂O₂ from the other possible products. Therelatively sharp resonance between 0 and 200 ppm seen in the naturalabundance Li₂O₂ spectrum is due to a lithiumcarbonate impurity. The two crystallographically distinct oxygen sitesof lithium carbonate, the most common electrolyte decomposition product,are well resolved in its ¹⁷O MAS spectrum (Figure ​(Figure1b),1b), and can be fit with second order quadrupoleline shapes with C_q values of the orderof 7.2 and 7.4 MHz. The C_q of anotherpossible decomposition product LiOH (Figure ​(Figure1d)1d) is similar (7.05 MHz) but its chemical shift is very different,−15 ppm vs 154 and 174 ppm for the carbonate (note that theobserved center of masses of these resonances are shifted to higherfrequencies from their isotropic chemical shift values due to thesecond order quadrupolar shift). The natural abundance spectra oflithium acetate and formate (Figure ​(Figure1c,e)1c,e) donot have sufficient S/N, even at the high field strength of 21.2 T,to allow accurate determination of their quadrupolar coupling constants.Nevertheless, a C_q value of the orderof 7.5 MHz can be used to fit the line width with a chemical shiftof the order of 265 ppm, both values lying within the respective rangesreported for carboxyl functional groups.⁴¹ Finally, lithium oxide has been suggested as a discharge productformed in TEGDME electrolytes based on XRD data.⁴² No quadrupolar broadening is expected due to the cubicenvironment of the oxygen sites, and a narrow and well-resolved ¹⁷O resonance of Li₂O at 35.6 ppm (Figure ​(Figure1f)1f) is observed. Thus, if present, its ¹⁷O signature can be easily used to identify it in the discharge products.Products containing ether functional groups were not measured, buttheir ¹⁷O resonances are expected to have isotropic shiftsin the range of 0–100 ppm with C_q values of the order of 11 MHz.⁴¹

As the measurements of possible electrochemical productswere performedat several different field strengths and with various MAS frequencies,it is not straightforward to compare them with spectra obtained ofproducts formed in an operating battery. Therefore we have simulatedthe spectra using the parameters listed in Table 1, for all compounds at a single field, 20 T, and spinningfrequency, 60 kHz (Figure ​(Figure2).2). The simulatedspectra demonstrate that high-field ¹⁷O MAS spectroscopyis an effective tool for identifying and distinguishing the variousoxygen functionalities. Separating between lithium carboxylate groupssuch as formate and acetate, for example, will, however, require additional ¹H and/or ¹³C measurements.

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Figure 2

Simulationof the ¹⁷O central transition line shapeof the various products at 20 T and 60 kHz MAS with the NMR parametersgiven in Table 1.

b. Electrochemistry

In order to identifythe products formed in working lithium–oxygen batteries, cellswere assembled as described in the Experimental Section. The discharge process was limited by either the discharge capacity(1000 mAh/g; referred to as partial discharge) or by a voltage limitationof 2 V allowing the cell to fully discharge. Representative electrochemicalplots for cells discharged to 1000 mAh/g and with a discharge voltagelimit of 2 V are shown in Figure ​Figure3.3. For characterizationby ¹⁷O NMR, the cells were cycled under an ¹⁷O enriched oxygen atmosphere and spectra were obtained followingpartial and full discharge and on charging to 4.5 V after dischargeto a voltage of 2 V. For ¹H studies, additional stateswere investigated as indicated in Figure ​Figure33.

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Figure 3

Electrochemical profilesof two representative Li–oxygencells, one that was limited to a discharge capacity of 1000 mAh/g(dashed gray) and a second cell that was fully discharged to 2 V (black). ¹⁷O NMR spectra were acquired for cathodes at the state ofcharge indicated by black circles and ¹H NMR spectra wereacquired for those indicated by both black and gray circles.

c. ¹⁷O NMR of Products Formed in CycledCathodes

The ¹⁷O spectra of the partially andfully discharged and partially charged cathodes are shown in Figure ​Figure4a–c4a–c along with simulations (fits) of theline shapes using the parameters determined for the possible productsformed in the cathode (from Table 1). The maindischarge product seen on full discharge (corresponding to a capacityof 2000 mAh/g) is lithium peroxide (Figure ​(Figure4b),4b), corresponding to approximately 80% of the spectral intensity,on the basis of the spectral simulation. However, the spectra wereacquired with a relatively short relaxation delay that is shorterthan the longitudinal relaxation time, T₁, of the various possible expected products, and thus the actualamount of lithium peroxide is expected to deviate slightly from thisvalue. A detailed analysis of this effect as well as a discussionof the effect of the RF excitation bandwidth and variation in C_q on the relative phase fractions is providedin the Supporting Information. Taking the T₁ relaxation into consideration, the relativeamount of lithium peroxide is estimated as 83% with an error of 17%(mainly due to the error in determining the T₁ constant).In addition to the peroxide, a measurable amount of lithium carbonate(5.0 (0.4)%) of the total signal intensity (number in brackets representsthe error), lithium hydroxide (10 (5)%) and lithium formate (2.0 (0.2)%)are seen. The resonance at around 260 ppm was assigned to lithiumformate and not acetate based on the ¹H spectra presentedin a later section. The ¹H spectra also allow for moreaccurate quantification of the LiOH concentration. Similar measurementsperformed at a lower field of 16.4 T could be fit with similar ratiosof products (within a few percent) supporting the fits of the 20 Tspectra. All the products detected in this ¹⁷O measurementmust be a result of a reaction between one of the battery components(lithium ions, cathode and electrolyte) with the ¹⁷O enrichedoxygen gas or one of its ¹⁷O-enriched products. It is worthnoting that the lithium carbonate and hydroxide resonances do notshow the well-defined second order quadrupole line shapes expectedfrom crystalline solids. In the case of the carbonate, similar ¹⁷O line shapes were observed in spectra of crystalline carbonateat elevated temperatures due to rotation of the CO₃^2– on the microsecond time scale,⁴³ suggesting that the carbonate ions may be mobile, and possiblyindicating the formation of a disordered/amorphous carbonate phase.

The spectrum acquired from a partiallydischarged cathode (limitedto a discharge capacity of 1000 mAh/g) reveals a similar distributionof electrochemical products to that detected at full discharge withLi₂O₂ (80 (17)%), Li₂CO₃ (6.0 (0.5)%), LiOH (12 (6)%), and LiO₂CH (2.0 (0.2)%).Only small differences are seen in the relative concentrations ofspecies, which suggests that these four products are formed simultaneouslythroughout the discharge process rather than forming at differenttimes during the discharge. Thus, despite the improved cycling performanceachieved when limiting the depth of discharge,^4,44 asimilar extent of electrolyte decomposition is observed (relativeto peroxide formation) for partial and full discharge.

The ¹⁷O spectrum of the fully discharged and then chargedto 4.5 V cathode (Figure ​(Figure4c)4c) shows that a largeamount of the peroxide has decomposed, decreasing in relative contributionto 56 (13)%, with lithium carbonate now corresponding to 16 (2)% ofthe total spectral intensity, hydroxide to 15 (8)%, and formate toabout 13 (1)%. Thus, as the voltage increases to 4.5 V, most, butnot all, of the lithium peroxide has been decomposed. (A method toallow the spectral intensity of the three different spectra to becompared is discussed later.) Previous studies have shown that carboncathodes preloaded with lithium peroxide exhibited a charge voltageof 4.15 V, leading to almost full removal of the lithium peroxide.⁴⁵ Our results indicate that indeed some of theperoxide was removed at lower voltages but the formation of additionaldecomposition products must increase the overpotential of the chargeprocess beyond 4 V. This is consistent with DEMS measurements performedfollowing the discharge of lithium–oxygen cells where mostlyO₂ evolution was detected on charge below 4 V vs lithium,while significant CO₂ release was observed at higher chargingpotentials between 4 and 4.5 V.^22,23

In summary, ¹⁷O NMR measurements reveal that the mainelectrochemical product in the first discharge is lithium peroxide,but with a non-negligible contribution from the ¹⁷O-enricheddecomposition products lithium carbonate, hydroxide, and formate and/oracetate.

d. ¹H ssNMR of Cycled Cathodes

To monitor the hydrogen containingspecies, ¹H Hahn-echoNMR spectra were acquired from cathodes extracted from cells cycledto various stages (Figure ​(Figure3).3). The spectra aredominated by a resonance at 2.6 ppm, which corresponds to the PVDFbinder in the cathode. Examining first cells that were fully discharged(Figure ​(Figure5a),5a), we can identify two main products:a broad peak in the range −1.0 to −1.5 ppm assignedto LiOH, and a weaker resonance at 8 ppm assigned to lithium formate.Another resonance can be resolved at about 0.5 ppm, which will bediscussed shortly. As the cells are charged, the LiOH and 0.5 ppmpeak decrease in intensity until they almost completely disappearwhen the cell is half charged (charge capacity of 1000 mAh/g). Theformate peak intensity on the other hand does not change noticeablyas the cell is charged and is still present at midcharge. In the spectrumof the cell following the second discharge (top row; D2v-C4.65-D2v)the intensity of all the signals grow, indicating that even more significantdecomposition occurs with further cell cycling.

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Figure 5

¹H Hahn echo spectra of cathodes at different statesof charge following (a) full discharge to 2 V and (b) partial dischargeto 1000 mAh/g. Spectra are labeled according to the state of charge,where D and C stand for discharge and charge, respectively, and eitherthe voltage or capacity limit are specified. Spectra have been normalizedto the intensity of the PVDF resonance, which is assumed to be constant.

Spectra acquired from cells thatwere stopped at partial discharge(Figure ​(Figure5b)5b) display similar ¹H resonanceswith lithium hydroxide resonating in the range of −1.5 to 1ppm and lithium formate at 8 ppm. The main difference observed withdepth of discharge is the disappearance of the LiOH resonance at lowercharging voltages, (4.3 V for partially discharged cathodes as comparedwith 4.5–4.6 V for cathodes that were fully discharged). Thisdifference can be ascribed to the thinner layer of insulating productsformed on the cathode surface at partial discharge, which leads toa lower overpotential on charge.

The PVDF signal, although usefulas a reference for quantifyingthe amounts of products formed, obscures any additional weaker resonanceswith chemical shifts in a similar shift range, for example, lithiumacetate (1.9 ppm), lithium methoxide (3.5 ppm), and other possibleether fragments (approximately, 3 ppm). These resonances can potentiallybe identified by subtracting the pristine cathode spectrum from thatof the cycled ones, allowing the signal at around 0.5 ppm to be resolvedmore clearly and an additional resonance at 3.5 ppm can be identifiedin the spectra of the partially discharged cells (Figure S5, Supporting Information). Further support forthe presence of these signals is provided by a ¹H–⁶Li correlation NMR spectra acquired from a cathode dischargedto 1000 mAh/g vs ⁶Li metal to form ⁶Li-enrichedelectrochemical products (Figure ​(Figure6).6). This experimentallows signals from protons and ⁶Li nuclei in close spatialproximity to be correlated by using cross-polarization (CP)⁴⁶ for magnetization transfer from ¹H to ⁶Li, and it therefore selects products that containboth nuclei in close proximity. Although the various species cannotbe resolved in the ⁶Li dimension, four species can be clearlyresolved in the ¹H projection: lithium formate at 8 ppm,lithium hydroxide at −1 ppm and the two additional environmentsat 3.5 ppm and 0.5 ppm. Based on solution NMR measurements of cathodeswashed with D₂O (Figure S6, SupportingInformation), we assign the 3.5 ppm peak to the dilithium saltformed from the central fragment of DME, LiOCH₂CH₂OLi. This signal is observed in significant amounts only in cathodesthat were partially discharged to 1000 mAh/g and for cathodes thatwere only partially charged after this partial discharge. It disappearson charging to 4.3 V (Figure S5, Supporting Information). Its absence from the 1D difference and 2D heteronuclear correlationspectra of cathodes that were fully discharged suggests that thisdilithium salt reacts further as the discharge process proceeds, possiblyoxidizing to lithium formate and lithium hydroxide (see below). The0.5 ppm environment is assigned to a disordered lithium hydroxidephase: while crystalline, stoichiometric lithium hydroxide gives riseto ¹H resonance at −1.4 ppm (Figure S7a, Supporting Information); the hydroxide signalsdetected from the cathode span −1 to 1 ppm. This shift in resonancefrequency is tentatively assigned to the disorder in the hydroxidephase. To confirm this assignment, we monitored the ⁷Lisignal build up in a cross-polarization experiment as a function ofthe cross-polarization time from surrounding protons (Figure S7c). The similar time scale of the buildup rate of the signals at −1 and 0.5 ppm confirms they belongto phases that are structurally similar at least on a short length-scale.

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Figure 6

¹H–⁶Li 2D heteronuclear correlationof a cathode discharged to 1000 mAh/g. The spectrum was acquired at16.4 T (¹H Larmor frequency 700 MHz) at 60 kHz MAS usinga 5 ms cross-polarization time to transfer magnetization from ¹H (vertical) to ⁶Li (horizontal) spectra nuclei.

The ¹H ssNMR measurements are consistent with the ¹⁷O spectra, showing the formation of lithium hydroxide uponcathode discharge and its full removal during the charging process.In addition, lithium formate, which has been detected in former studies,³¹ is shown here to accumulate in the cathode uponcycling. The 1,2-ethanediol lithium salt is observed at partial discharge,but it then decomposes on charging the cell to 4.3 V. It has almostcompletely disappeared in a cathode allowed to fully discharge.

e. ¹³C ssNMR of ¹³C EnrichedCathodes

In order to identify the sources of lithium carbonateformation, cells were cycled with cathodes that were prepared with ¹³C enriched amorphous carbon. While these cathodes may differin their performance from that of superP Li carbon (due to differencesin properties such as the nature of the surface groups, particle sizesand surface area), they are used here to investigate the carbon stabilityin the relevant electrochemical window. A representative electrochemicalprofile is shown in Figure S8 in the SupportingInformation.

The ¹³C MAS spectra (Figure ​(Figure7)7) all contain the signal of the bulk electrode at130 ppm, a typical shift position for an sp² hybridizedcarbon. As the cell is fully discharged, a new weak carbon resonanceappears at 168 ppm corresponding to lithium carbonate. Upon charge,the carbonate signal grows noticeably, but is completely removed atfull charge when a voltage of just over 4.5 V is reached. Significantlylarger quantities of carbonate are formed on the second dischargeand accumulate on the cathode by the end of two cycles. On the fifthdischarge, the amount of carbonate formed corresponds to about 9%of the total carbon signal detected.

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Figure 7

¹³C Hahn echo MAS NMR spectraacquired from cycled ¹³C enriched cathodes. The positionof the carbonate groupof Li₂CO₃ is indicated via gray dashes, alongwith its integrated intensity relative to the total carbon signal.

Since a lithium carbonate signalis not detected when using thenatural abundance (nonenriched) carbon electrode, for similar experimentalparameters (and direct excitation of the ¹³C resonances),the carbonate signal detected in these measurements must be enrichedin ¹³C and thus originate from the decomposition of thecarbon electrode rather than the aprotic solvent. This signal shouldbe contrasted with the carbonate signal detected in the ¹⁷O spectra that can originate from the electrolyte or carbon cathodeor both. One possibility, supported by prior DEMS, FTIR, and carbonatedissolution reactions also using ¹³C enriched electrodes,^16,24 is that the carbonate is formed from the reaction between lithiumperoxide and the carbon electrode, especially at the higher voltagesreached during charge.

To investigate the location of the carbonatespecies, a ¹³C hom*onuclear correlation experiment was performed(Figure ​(Figure8),8), which allows us to detect spatialproximity between ¹³C sites. A hom*onuclear correlationwas detected between thelithium carbonate resonance at 168 ppm, and the carbon electrode signalat 130 ppm, which shows that at least some of the lithium carbonateforms directly (within a few angstroms) on the surface of the carbonelectrode and not on top of the layer of peroxide in contact withthe electrolyte. This result is consistent with the observed ¹³C enriched carbonate being a product of the reaction betweenthe electrode and lithium peroxide. Interestingly, the ¹³C signal of the carbonate signal nearby the carbon electrode thatgives rise to the cross-peaks is at a slightly higher frequency thanthe sharp carbonate signal. The shift is tentatively ascribed to electroniceffects due to the interaction with the carbon electrode and/or thedisorder of the carbonate at this interface.

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Figure 8

¹³C–¹³C 2D hom*onuclear correlationof a ¹³C carbon enriched cathode on its 5th discharge to2 V. The spectrum was acquired at 9.4 T (Larmor frequency 100 MHz),with 10 kHz MAS, and an RFDR mixing time of 20 ms.

Measurements were also performed using DME 20% ¹³C-enrichedon the methyl group. No ¹³C signal could be detected fromfully discharged cathodes in experiments using direct excitation of ¹³C resonances, and ¹H to ¹³C and ⁷Li to ¹³C CP experiments, indicating that the electrolytedecomposition products formed from the CH₃ group of DME(formate and carbonate) are present in too low concentrations to bedetected over the natural abundance carbon signal. A weak signal fromthe lithium formate could be detected in a ¹H to ¹³C CP spectrum at the end of the second discharge, consistent withthe ¹H NMR experiments, which show that this species accumulateson cycling. Further experiments are in progress using DME with higher ¹³C enrichment levels to obtain spectra with better signal-to-noise.

f. Summary of Electrochemical Reactions in theFirst Cycle in DME

We can now combine all of the informationcollected from various NMR measurements described above and estimatethe extent of formation and decomposition of the various detectedproducts (Figure ​(Figure9).9). The quantities of lithiumhydroxide, formate, and 1,2-ethanediol lithium salt are based on the ¹H spectra where the PVDF signal was used as internal reference.The relative lithium peroxide and carbonate amounts were extractedfrom the ¹⁷O data (correcting for the relaxation effectas discussed in the Supporting Information) and were determined by comparing their relative signal intensityto that of the ¹⁷O signals of the protonated species lithiumformate. Due to the relatively large error in determining the amountof lithium hydroxide from the ¹⁷O NMR data, only ¹H spectra were used to quantify its formation.

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Figure 9

Summary of the products formed in thefirst cycle of the cell inmoles per gram of carbon electrode: Li₂O₂ (blue),Li₂CO₃ (pink), LiOH (green), HCO₂Li (red), and (LiOCH₂)₂ (purple). Data collectedfrom cathodes with a discharge limit of 2 V is given by filled circlesand a limit of 1000 mAh/g in open circles. The whole range is shownin (a) and an enlargement of the gray area is plotted in (c). Representativevoltage profiles are shown in (b).

At partial discharge(limited to 1000 mAh/g), five products can be identified: lithiumperoxide as the major product, and four decomposition products, LiOH,lithium formate, lithium carbonate and the 1,2-ethanediol lithiumsalt. The decomposition pathway of ether solvents has been discussedin several publications.^10,47−49 While it is mostly agreed that lithium peroxide is formed on dischargeby a two-step process,⁵⁰

several possibilities have been raisedforthe mechanism for the decomposition of the ether electrolyte, whichdiffer in the nature of the attacking group. Among the suggestionsare reactions involving the superoxide ion and molecular oxygen,^10,51 autoxidation by molecular oxygen,⁵² andreactions with the peroxide product.^48,53 DEMS and coulometrymeasurements have shown that the electron to oxygen ratio (e^–/O₂) consumed in the discharge reaction in DME based electrolytesis always larger than two.²⁴ Since twoelectrons are required for the formation of lithium peroxide, it wasconcluded that any parasitic reactions involving the electrolyte areprobably due to the reactivity of the strong nucleophile Li₂O₂ (and not the intermediate LiO₂).²⁴ DFT calculations performed by Bryantsev et al.have demonstrated that it is unlikely that DME decomposition is initiatedby nucleophilic attack of the superoxide ion, these calculations beingsupported by reactivity tests of DME with KO₂.⁵⁴ DME decomposition pathways by reaction withsolid lithium peroxide are supported by the high reactivity of theperoxide surface groups.⁵⁵ However, severalmechanistic pathways can occur, resulting in different decompositionproducts. Recent computational studies have shown that hydrogen abstractionfrom the methylene group of DME is energetically favorable, assumingreactivity of single oxygen bridging sites on the surface of the peroxidespecies.⁴⁹ An experimental study basedon in situ electrochemical quartz crystal microbalance (EQCM), solutionphase NMR, and matrix-assisted laser desorption/ionization time-of-flight(MALDI-TOF) measurements ascribed the reactivity in triglyme electrolytesto the peroxide anion (LiO₂^–) speciesformed prior to precipitation of the solid Li₂O₂.⁴⁸

Some plausible reactions for the formation of the observedproductsin the current study (with ¹⁷O enriched O₂)are

The formation of the diglyme lithiumsalt,LiOCH₂CH₂OLi, at partial discharge (eq iii), which is only detected by ¹H NMRand not by ¹⁷O NMR spectroscopy, supports the mechanismssuggested by Aurbach et al., involving a reaction between the peroxideand the DME α-carbon (reaction path b, Scheme 2 in ref (48)). Presumably, the methylperoxylithium product formed in reaction iii, CH₃OOLi, is not detected in our study, because it is readilyoxidized to lithium formate (reaction iv) viaformaldehyde as an intermediate. Reaction iv also forms water, which can react further to form LiOH (reaction v). Note that the decomposition of one molecule ofDME results, via these reactions, in two molecules of water and fourmolecules of LiOH, helping to explain why LiOH is a dominant producton discharge. On deeper discharge, the diglyme lithium salt has alower concentration, presumably due to its further reaction to formother lithium salts such as carbonate (reaction vi), hydroxide and formate. Formate is present throughout (in low concentrations)as it is both a product and a reactant in the decomposition pathwayssuggested above. The reaction of the carbon electrode to form lithiumcarbonate (reaction viii) is not significanton the first discharge, as very little carbonate is seen in the ¹³C spectra of ¹³C enriched electrodes at this stage,consistent with the DEMS and FTIR study of Thotiyl et al.¹⁶ Upon charge, the lithium hydroxide graduallydecomposes and is completely removed when the voltage increases upto 4.5 V. Lithium peroxide is decomposed, partially in a reversiblemanner to release O₂ and to a lesser extent by reactingwith the carbon electrode to form lithium carbonate (as detected by ¹³C NMR):

This reaction produces only two electronsfrom three peroxide molecules (as compared to six electrons for fulloxidation of the peroxide). While the lithium peroxide accounts forabout 80% of the discharge capacity (1600 mAh/g), on the basis ofthe total capacity and the Li₂O₂

:decompositionproducts ratio, its signal intensity observed in the ¹⁷O spectrum quickly drops by about 80% when the cell is charged from2 to 4.5 V with a charge capacity of only about 500 mAh/g. Oxidationof the carbon electrode can at least partially explain this fast decreasein peroxide intensity, the decomposition of Li₂CO₃ at 4.5 V, and above releasing the residual four electrons (fromthe original three peroxide anions):

Although essentially allthe carbonate decomposesat higher voltages in the first cycle, in subsequent cycles, thisis not complete (as observed by ¹³C NMR). Interestingly,the 2D

¹³C NMR spectrum of the ¹³C enricheddischarged electrode after five cycles shows two types of carbonatesignals: one nearby the carbon, and a sharper resonance that is furtherfrom the carbon (i.e., it does not give rise to a carbon–carbonatecross peak) and is less disordered. It is likely that it is this lattercarbon that is more difficult to decompose electrochemically on chargeand gradually accumulates on cycling. Finally, in contrast to thecarbonate, lithium formate, which is mainly formed upon discharge,maintains an essentially constant level and is not removed upon charge.The accumulation of the formate and at a later stage lithium carbonatemay be one cause of the capacity fading observed in operating cells.

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