Anomalous Discharge Product Distribution in Lithium-Air CathodesJagjit Nanda,*,† Hassina Bilheux,*,‡ Sophie Voisin,‡ Gabriel M. Veith,† Richard Archibald,§
Lakeisha Walker,‡ Srikanth Allu,§ Nancy J. Dudney,† and Sreekanth Pannala*,§
†Materials Science and Technology Division, ‡Neutron Scattering Science Division, and §Computer Science and MathematicsDivision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
*S Supporting Information
ABSTRACT: Using neutron tomographic imaging, we report for thefirst time the three-dimensional spatial distribution of lithium productsin electrochemically discharged lithium-air cathodes. Neutron imagingfinds a nonuniform lithium product distribution across the electrodethickness, with the lithium species concentration being higher near theedges of the Li-air electrode and relatively uniform in the center of theelectrode. The experimental neutron images were analyzed in context ofresults obtained from 3D modeling that maps the spatiotemporalvariation of the lithium product distribution using a kinetically coupleddiffusion based transport model. The origin of such anomalous behavioris due to the competition between the transport of lithium and oxygenand the accompanying electrochemical kinetics. Quantitative under-standing of these effects is a critical step toward rechargeability of Li-airelectrochemical systems.
1. INTRODUCTIONLithium-air chemistry1,2 potentially offers a promise for veryhigh energy density (3450 Wh/kg, calculated on the basis ofLi2O2 weight) that if successful would revolutionize the worldof electric vehicles by extending their range to 500 miles orbeyond. The high energy density is due to two main reasons:(a) the cathodic component, in this case, oxygen, is not storedin the cell unlike the lithium-ion intercalated compounds; and(b) it uses Li-metal as the anode, which has close to an order ofmagnitude higher specific capacity as compared to commonlyused carbon anodes (372 mAh/g). However, to make thishappen, major fundamental scientific breakthroughs3 areneeded that could address various bottlenecks associated withits poor cycle life and power performance.2 One major issue hasbeen deposition of various insulating lithium decompositionproducts on the surface of the porous carbon foam (normallyreferred to as the air-electrode) during the discharge process,commonly referred to as the oxygen reduction reaction,ORR.4,5 Recent studies have found that the discharge reactionis strongly affected by electrolyte and solvent composition anddriven by complex reaction kinetics under chemical andelectrochemical condition, resulting in more complicateddischarge products than Li2O2.
5−8 The buildup of the dischargeproducts over time could lead to a drastic reduction of theelectronic conductivity due the insulating nature of ORRproducts impeding the electrochemical process at the interfacewith a concomitant decrease in the overall porosity of the glassycarbon electrode that could affect the ion transport. Under-standing the origin of the electronic passivation on the reactionsurface (in this case, graphite carbon foam) due to the complexelectrochemical and chemical decomposition of electrolyte−
solvent system and the resulting charge transfer kinetics atvarious current densities is therefore critical.9
While recent studies have focused on the temporal aspects ofthe discharge profiles as a function of various electrochemicaltransport parameters and have provided a guide for the overallperformance,9,10 there has been no attempt to understand thedischarge (or charge) mechanism spatially in terms of locallithium species concentration across the thickness of theelectrode. A three-dimensional imaging and tomographicmethod11 that could spatially map the lithium dischargeproduct distribution inside the 3D pores across the entireelectrode thickness during the discharge step could provideimportant insights into the ionic mass transport as well as thelimiting electronic conduction occurring across these thickelectrodes. This study reports for the first time neutronimaging12 and 3D computed tomography (CT) mapping of thelithium discharge products in Li-air cathodes to obtain asemiquantitative estimate of the spatial lithium concentrationacross the 3D volume element of discharged Li-air cathodes,which then directly relates to the distribution of the reactionproducts during the ORR.
2. EXPERIMENTAL AND ELECTROCHEMICALMODELINGElectrode Preparation for Neutron Imaging and Li-Air
Cell Electrochemistry. The Li-air cathodes used in this work
Received: February 17, 2012Revised: March 19, 2012Published: March 22, 2012
were made from a graphitized carbon foam (0.035 g, 1 mmthick, surface area 2.2 m2/g as measured by N2 physisorptionon a Quantachrome Autosorb 1C) made at ORNL.13,14 Thegraphite foam was formed from Mitsubishi ARA24 pitch at1000 °C followed by graphitization in Ar at 2800 °C to givehighly conductive foam with open cells (0.4 mm diameter).Manganese oxide-coated electrodes were prepared by soakingthe carbon cathodes in an excess of a 0.1 M NaMnO4 (AlfaAesar) + 0.1 M Na2SO4 (Aldrich) solution for 5.5 h.15 Thecathodes were washed with 18 MΩ water until the washsolution was clear, and then soaked in 100 mL of water for 10min, followed by vacuum drying for 18 h. Swagelok cells wereconstructed in an argon-filled glovebox using 0.75 mm thick Lifoil (99.99%, Alfa), a Celgard 2500 separator, and 1.5 mL of 1.2M LiPF6 in 1:1 wt % ethylene carbonate/dimethyl carbonate(EC/DMC) (Ferro), which is a widely used electrolyte for Li-ion batteries and Li-air cells.16−23 The cells were assembled in avertical geometry with the cathode located at the top of the cellclosest to the oxygen/argon supply. Research grade oxygen (AirLiquide) was used, and the cells were operated with 20 PSI O2
with enough electrolyte to saturate the cathode. Cells weredischarged at 5 μA of current to 2.2 V on a Maccor batterycycler. For comparison, we also prepared samples coated withlithium peroxide on porous carbon foams. For this experiment,we used commercial glassy carbon foams obtained from DuocelCo. These foams have various pore sizes, described in terms ofpores per inch (PPI). The slurry-based coating of lithiumperoxide was performed inside an argon controlled atmosphere.In a typical batch, about 1 wt % colloidal mixture of Li2O2 wasprepared using N-methyl pyrolidone (NMP) as the solvent. Foreffective dispersion of Li2O2 in the solvent, the mixture wassonicated for about 10−15 min. The carbon foams were soakedin the slurry for 1 h, and then the excess slurry was wiped fromthe surface using soft tissue paper. The foams were then driedunder vacuum for about 12 h leaving a thin layer of Li2O2 insidethe pores of the foam. The excess dried Li2O2 comes out fromthe surface of the foams. For initial experiments, we used 45and 100 PPI carbon foams. Because the neutron imagingresolution was originally about 100 μm, then improved toapproximately 50 μm for the latest results, there was no need touse finer pores sizes. The typical dimensions of the foams usedfor imaging were 1 cm (length) × 0.5 cm (width). The sampleswere packed and sealed inside aluminum cylinders for theneutron experiments.Detailed information about neutron imaging and tomog-
raphy analysis of air cathodes is discussed in the SupportingInformation.3D Spatial Modeling of Li-Air Cathodes. The electro-
chemical transport across a three-phase boundary system suchas air cathode is driven by (i) the interplay between the reactionkinetics and rate, (ii) changes in the electronic conductivity dueto insulating nature of discharge product buildup, and (iii) masstransport of the dissolved oxygen species and lithium across theporous electrode. More importantly, these effects areinterrelated and affected by the dynamical change of porosityof the carbon foam as the discharge proceeds. Therefore, theequations to be solved are coupled reaction−diffusion of thefollowing form:
∂ ε∂
= ∇· ε ε ∇ +
∂ ε∂
= ∇· ε ε ∇ +
ct
D c
ct
D c
( )( ( ) )
( )( ( ) )
O2O2eff
O2 O2
XXeff
X X
Here, ε is the porosity in the air-cathode, CO2 and CX stand forthe mass fraction of the soluble O2 and reactants (such as Li+,lithium oxides, carbonates, lithium alkyl carbonates) concen-tration, Deff stands for the effective diffusion (here, we take it asdiffusion coefficient times ε0.5), and ℛ stands for the reactionterms leading to depletion of these species from the electrolyte.For lack of detailed knowledge about the exact productsdecomposition and the associated reaction steps and kinetics,24
the overa l l r eac t ion i s s impl i s t i ca l l y g iven as :
The reaction rate factor, k, was modified to include buildupof the decomposition product on the surface of the carbon bythe expression:
= *ε − *⎧⎨⎩
⎫⎬⎭tk
xd[Li CO ]d
1exp( )exp(1)
[Li] [O ] [EC]2 3s
Li 22
1.5
along with proper boundary conditions. The transport andreaction equations were solved on the basis of the rigorousvolume averaging approach typical of multiphase formulation.In this method, we have a unified, single-domain approachwhere complex geometries are naturally incorporated byincluding the local volume fraction of the different phasesand corresponding species concentration within each of thephases. The discharge product formation was analyzed undervarious limiting transport conditions, and specific results arediscussed in the Supporting Information.
3. RESULTS AND DISCUSSIONFigure 1 shows the discharge profiles of air cathodes at acurrent density of 5 μA/cm2 with and without the presence ofan active catalyst layer MnO2 on the surface of the graphitefoam. The samples were discharged to their full capacity toenable maximum discharge product formation for the neutronimaging experiments. Detailed analysis of the discharge productformation and their possible mechanism was recently reported
Figure 1. Discharge voltage profiles of Li-air cathodes without (topcurve) and with catalyst layer MnO2 on the carbon foam (bottomcurve). For neutron imaging and tomography experiments, theelectrodes were discharged at 5 μA/cm2.
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by our group.6 A plausible reaction mechanism for carbonateelectrolyte decomposition was proposed recently by Bruce andco-workers. Briefly, in the presence of LiPF6−carbonateelectrolyte system, the discharge product is dominated by theelectrolyte decomposition product such as lithium alkylcarbonates and lithium carbonates instead of the ideal peroxideor superoxides for lithium-air chemistry. The electrodes weredischarged for >500 h to allow a significant deposition ofdischarge products for neutron contrast imaging (seeSupporting Information).Neutron Imaging and Tomography. Neutron imaging or
radiography is a powerful imaging method based on the relativeattenuation contrast between the atoms determined by theirrespective scattering and absorption cross-sections as governedby the Beer−Lambert law, I(λ) = I0(λ)e
−μ(λ)Δx, where I0 and Iare, respectively, the incident and transmitted neutron beamintensities for a given wavelength λ, μ is the attenuationcoefficient, and Δx is the thickness of the sample. Theattenuation coefficient μ is given by μ(λ) = σt(λ)(ρNA)/M,where σt(λ) is the material’s total cross-section for neutrons, ρis its density, NA is Avogadro’s number, and M is the molarmass. Because of the fundamental nature of the interaction ofneutrons with matter, some light nuclei such as hydrogen andits isotope deuterium greatly scatter neutrons, whereas someheavier elements such as Cu or Pb are not strong scatterers orabsorbers of neutrons and can therefore be easily penetrated. Incase of this study, that is, Li distribution in C foam electrodes,the contrast obtained in the neutron radiograph and theneutron tomography set is strongly dominated by the Licontent, as illustrated in Table 1. This table displays the
scattering (coherent and incoherent) and absorption cross-sections for the selected isotopes of Li and C. The total cross-section, which is necessary to calculate the transmissionthrough a sample, is the sum of all cross-sections at 1.5 Å.Clearly, the major isotope contributing to a strong attenuationof neutrons is Li6, with an attenuation cross-section of 940barns. Figure 2 shows a schematic diagram of the neutronimaging experimental setup currently operated at the High FluxIsotope Reactor (HIFR) at Oak Ridge National Laboratory.Briefly, a collimated neutron beam is incident on the samplemounted on a 3D rotatable stage for collecting image snapshotsacross the entire sample cross-sections.As mentioned above, with an objective toward understanding
the underlying electrochemical transport and the spatialdistribution of the discharge products on the pores of the Li-air cathodes, we performed imaging and neutron tomographicreconstruction of discharged air cathodes shown in Figure 3.Figure 3A and B shows 2D image slices of discharged Li-aircathodes labeled as Li-Air-1 and Li-Air-2 (same as cathode-1except with a MnO2 catalyst layer coating). It is noteworthythat in the context of the current experiment, the neutronattenuation contrast is only sensitive to atomic lithium rather
than the exact chemistry and composition of the dischargeproducts.25 The color contrast shows the lithium densitydistribution with brighter color representing higher lithiumcontent across the porous carbon surface. It is expected that theamount of lithium concentration should be a direct measure ofthe discharge products. The experimental Li-images of thedischarged air cathodes were compared to results from coupled3D transport model (see Supporting Information) shown inFigure 3C and D. The essential elements in the 3D modelinclude (i) a reaction rate dependence of the charge transferkinetics to account for the gradual buildup of discharge productformation on the surface of the porous carbon, (ii) Li-ion anddissolved oxygen transport in the electrolyte across the porouselectrodes, and (iii) coupled spatiotemporal changes in theoverall porosity of the electrode during the discharge period.Similar to the experimental discharge conditions, the 3D modelcalculations were carried for about 23 000 s with a startingporosity of 70% at a moderate kinetic rate factor of 5.0 × 104
(cm3/mol)3.5/s. The reaction rate factor is related to thecurrent density, a parameter that determines the rate underwhich the air cathodes are discharged. The calculated spatialproduct distribution and porosity distribution at 23 000 s (closeto the discharge time of the air cathodes) are shown in Figure3C and D with very good qualitative agreement withexperiment in terms of spatial lithium distribution. Mostnotably, the reaction rate expression9 is modified to include thedischarge product layer that is formed on the carbon surface.We model the rate expression as:
= *ε − *⎧⎨⎩
⎫⎬⎭tk
xd[Li CO ]d
1exp( )exp(1)
[Li] [O ] [EC]2 3s
Li 22
1.5
where xLi is the mass fraction of the deposited lithium product,and εs is the starting porosity of the matrix. The reaction rateorder was taken to be 4.5 on the basis of approximate globalreaction corresponding to the detailed reaction mechanismproposed by Bruce and workers for carbonate electrolytedecomposition in the presence of reduced oxygen species andlithium salt.24 The dissolved oxygen concentration value [O2]was obtained from measurement reported by Read and co-workers26 for similar electrolyte composition and was keptconstant. The coupled transport equations described in theSupporting Information (eqs 1,2) were solved self-consistentlyto calculate the spatial discharge products and the spatiotem-poral changes of porosity as shown in Figure 3C and D. In theabsence of a quantitative estimation of the reaction rate
Table 1. Total Neutron Scattering Cross-Section (in barns)of Isotopes of Lithium and Carbon
Figure 2. Schematic diagram of the neutron imaging setup at theHIFR neutron imaging prototype facility at Oak Ridge NationalLaboratory. The incident neutrons have a wavelength range between1.8 and 6 Å.
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Figure 3. Normalized two-dimensional reconstructed slices of discharged Li-air cathodes showing spatial distribution of lithium concentration: (A)Li-Air-1, (B) Li-Air-2 (includes the catalyst layer MnO2). (C) Calculated Li-product spatial distribution at 23 000 s. The color code shows therelative lithium compound or discharge product formation. (D) Calculated porosity changes at 23 000 s as a result of the discharge product
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parameters for the intermediate steps during LiPF6−carbonatesalt decomposition,24 the 3D model simulations27,28 describedhere only provide a qualitative picture of spatial dischargeprofile and the transport limiting mechanism.One of the significant observations from modeling results is
that while we certainly see a gradient in lithium distributionacross the spatial dimension of the electrode, the overallchanges in the porosity were only of the order of 3%. This maynot be sufficient to limit the mass transfer effects due to poreclogging.9,29 To further substantiate our results, the individual2D slices are then reconstructed into 3D tomographicstructures (described in the Supporting Information) asshown in Figure 3E and F. These tomographic images containinformation about the bulk lithium product distribution (withinthe resolution) that can be then extracted along any internalcross-sectional planes of the electrode to obtain the lithiumdistribution. An example of this is shown in Figure 3G and H.Further along these directions, an analysis was carried out to
estimate the total integrated lithium volume expressed as %
lithium, along with the electrode thickness that shows theaverage variation of lithium concentration across the bulkthickness of the electrode. This can be obtained, in absoluteterms, by using a reference lithium standard along the beam inparallel with the discharge air-cathodes. In the absence of a sucha standard, here we calculated the Li content in the individual2D image slices at various lithium signal or count thresholds(T) (T can vary between 0 and 1) and then integrated alongthe individual volume dimensions to estimate the total lithiumvolume (in %) across the total thickness of the electrode asshown by the line profiles for the air cathode structures 1 and 2,in Figure 4A and B. Strikingly, these results clearly show adouble peak profile near the top and bottom region and arelatively flat profile in the central part, signifying a higherlithium product formation at the region of cathode that is closeto the lithium metal/separator and also near the oxygen/airfront. These are qualitatively in agreement with 3D modelingresults (Figure 3D) that essentially capture the varioustransport limiting phenomena that could drive uneven product
Figure 3. continued
formation. (E) Reconstructed 3D tomography image of Li-Air-1. (F) Reconstructed 3D tomography image of Li-Air-2. The 3D pictures areintentionally sliced at the edges to show the spatial Li-variation. (G) A typical cross-sectional lithium distribution derived from the bulk of Li-Air-1cathode. (H) A typical cross-sectional lithium distribution derived from the bulk of Li-Air-2 cathode. The scale bars for the 3D figures are the same asthose given in (A) and (B).
Figure 4. Calculated line profile showing the relative Li volume percent in Li-air cathodes. The line profiles are calculated at different thresholds (Li-signal in the voxel) for uncertainty quantification. See Supporting Information for details. (A) Li-concentration profile for Li-Air-1 and (B) Li-Air-2.(C) Results obtained from modeling of the Li-discharge products after 23 000 s.
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Figure 5. Normalized two-dimensional reconstructed slices of lithium peroxide-coated carbon foams: (A) 45 pores per inch (PPI) foam, (B) 100 PPIcarbon foam, (C) reconstructed 3D tomography image of 45 PPI Li2O2-coated carbon foam, (D) reconstructed 3D tomography image of 100 PPILi2O2-coated carbon foam, (E) cross-sectional lithium distribution of 45 PPI reconstructed carbon foam, and (F) cross-sectional lithium distributionof 100 PPI reconstructed carbon foam. (G) Calculated line profile showing the relative Li volume percent in 45 PPI Li2O2-coated carbon foam. (H)Calculated line profile showing the relative Li volume percent in 100 PPI Li2O2-coated carbon foam. The line profiles are calculated at differentthresholds (Li-signal in the voxel) for uncertainty quantification.
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distribution both at the Li front as well as the oxygen end. Thisis further validated by comparing the calculated lithiumdischarge product profile using an experimental conditionsimilar to that shown in Figure 4C for lithium carbonatedischarge product formation. Ignoring the experimentalartifacts such as edge effects and various factors that can leadto broadening of the experimental line profiles, we notice ahigher concentration of Li-product deposition at the edgesconfirming to the experimental trends. As was alluded tobefore, the discharge product profile is strongly driven bycompetition between the reaction rate (kinetics), diffusion, ormass transfer effects; these factors fundamentally determine thecurrent density or rate that the air cathodes could sustain toprovide a uniform spatial product distribution or a transportlimited scenario in which case there is an uneven distribution ofdischarge product. The Supporting Information (Figure 4A−C)describes the results based on transport limiting conditionswithin the limits of our 3D model. Briefly, under limited oxygenor lithium diffusion, the product distribution profile falls steeplyacross the cathode thickness, leading to a gradient in dischargeproduct concentration (Figure 4B). Further, by adjusting thediffusion coefficients (of Li and oxygen species) and the ratekinetic factor, one can also be able to sustain a uniform spatialdistribution, which could practically translate to lower dischargecurrents. In the present case, the air-cathodes were dischargedat 5 μA/cm2, which as per our previous report9 is a high enoughcurrent density to produce transport driven polarization,leading to buildup of discharge product at the electrode edges.To further ascertain that the observed anisotropy in the
discharge air-cathodes is a result of the electrochemistry driveneffects, we performed a control experiment in which lithiumperoxide (Li2O2) was chemically deposited on the carbon foamhaving two pore sizes, 45 and 100 pores per inch (PPI). Theneutron imaging and tomographic reconstruction results arebriefly discussed here in context of Figure 5. The analyzed 2Dreconstructed image slices showing the relative lithiumconcentrations for 45 and 100 PPI carbon foams are shownin Figure 5A and B, and the corresponding reconstructed 3Dtomography images are shown in Figure 5C and D. The Li-threshold calculated line profile (Figure 5E and F) showedrelatively uniform lithium concentration across the bulkthickness of the carbon foam for both 45 and 100 PPI Li2O2-coated foam. This behavior is expected because, in this case,lithium peroxide was uniformly coated from the solution(dipped and dried under vacuum) and was not subjected toelectrochemical cycling unlike the air cathodes.
■ CONCLUSIONThis study reports the first experimental evidence of the spatialvariation of discharge products across the bulk of the Li-airelectrode using neutron tomography imaging, virtually allowingone to “see through” inside the bulk electrode. The results wereanalyzed in context of the 3D transport model that includesreaction kinetics and mass transport across the air cathodethickness. Neutron imaging finds a nonuniform lithium productdistribution across the electrode thickness, the lithium speciesconcentration being higher near the edges of the Li-airelectrode and relatively uniform in the center of the electrode.The origin of such anomalous product distribution is related tothe polarization factors due to the kinetic and diffusion barriersthat could lead to a discharge product gradient. Our resultprovides key insights into the discharge mechanism in thick aircathodes at a 3D spatial scale that could lead to the design of air
cathodes ideal for Li-air chemistry. Efforts are underway toimprove the spatial resolution of the neutron imaging techniqueto few micrometers, further increasing the usefulness of thismethod to monitor lithium transport in electrodes at a morelocal level.30
■ ASSOCIATED CONTENT*S Supporting InformationAdditional discussion and figures. This material is available freeof charge via the Internet at http://pubs.acs.org.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSThis research is sponsored by the Laboratory DirectedResearch and Development Program of Oak Ridge NationalLaboratory, managed by UT-Battelle, LLC, for the U.S.Department of Energy. The user facility at the High FluxIsotope Reactor is sponsored by the U.S. Department of EnergyOffice of Science, Office of Basic Energy Sciences (BES).ORNL is managed by UT-Battelle, LLC, for the U.S. DOEunder contract DE-AC05-00OR22725. We sincerely thank Drs.Keely Willis, Jack Wells, Thomas Proffen, Gene Ice, and ParthaMukherjee for scientific discussions and support.
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