9138 | Phys. Chem. Chem. Phys., 2014, 16, 9138--9147 This journal is© the Owner Societies 2014

Cite this:Phys.Chem.Chem.Phys.,

2014, 16, 9138

In situ profiling of lithium/Ag2VP2O8 primarybatteries using energy dispersive X-raydiffraction†

Kevin C. Kirshenbaum,a David C. Bock,b Zhong Zhong,c Amy C. Marschilok,*bd

Kenneth J. Takeuchi*bd and Esther S. Takeuchi*abd

In situ, in operando characterization of electrochemical cells can provide insight into the complex dis-

charge chemistries of batteries which may not be available with destructive methods. In this study, in situ

energy-dispersive X-ray spectroscopy (EDXRD) measurements are conducted for the first time on active

lithium/silver vanadium diphosphate, Li/Ag2VP2O8, electrochemical cells at several depths of discharge

allowing depth profiling analysis of the reduction process. This technique enables non-destructive, in

operando imaging of the reduction process within a battery electrode over a millimeter scale interrogation

area with micron scale resolution. The discharge of Ag2VP2O8 progresses via a reduction displacement

reaction forming conductive silver metal as a discharge product, a high Z material which can be readily

detected by diffraction-based methods. The high energy X-ray capabilities of NSLS beamline X17B1 allowed

spatially resolved detection of the reduction products forming conductive pathways providing insight into

the discharge mechanism of Ag2VP2O8.

Introduction

Batteries are critical for the successful deployment of a numberof devices, including portable electronics, implantable medicaldevices, power tools and electric vehicles.1 In any batteryapplication, there is a gap between the theoretical energycontent and the realizable energy delivered by the battery ofinterest. Increased understanding of the factors that limitdischarge in electrochemical cells is an important step towardfurther improving the discharge characteristics of batteries.Internal cell resistance is an important contributor to this gap,in particular at high rates of discharge where inhomogeneousutilization of electrode material capacities can further contributeto low capacity.

While the chemical and electrochemical stability of phos-phate based materials such as LiFePO4 are viewed as significant

assets, their inherently low electrical conductivity must be over-come for practical deployment in batteries.2,3 Typical approachesto reduce cathode resistance have included solid solution dop-ing,4 carbon coating,5,6 co-synthesis with carbon,7,8 and additionof conductive metal powders to achieve improved cathode con-ductivity.9,10 Unfortunately, such strategies require additionalprocessing steps and can significantly reduce energy density.

We have reported an alternative strategy to address poorelectrical conduction within a battery via the in situ formationof an electrically conductive network upon electrochemicalreduction. The in situ formation of a conductive network insemiconducting oxide based materials has been observed in ahighly successful high power commercial battery.11–18 Further,we hypothesized that in situ formation of electrically conduct-ing metal particles could address conductivity limitations ofpoorly conducting materials, such as metal phosphates.19 Ourexplorations of silver phosphate based cathode materialsdemonstrated the feasibility of using this family of materialsas electrochemically active cathodes.20,21 Bimetallic systemsincorporating vanadium can exhibit multiple electron transfersper formula unit,17 yielding good reversibility for secondarybattery systems22–26 and high capacity on discharge for primarybattery systems.27 Consistent with our original hypothesis, adramatic increase in conductivity was demonstrated uponin situ formation of silver metal on initiation of discharge.28,29

Recently, we have successfully extended this materials con-cept to the diphosphate material family. Our recent report of

a Global and Regional Solutions, Brookhaven National Laboratory, Upton,

NY 11973, USAb Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA.

E-mail: esther.takeuchi@stonybrook.edu, amy.marschilok@stonybrook.edu,

kenneth.takeuchi.1@stonybrook.educ National Synchrotron Light Source, Brookhaven National Laboratory, Upton,

NY 11973, USAd Department of Materials Science and Engineering, Stony Brook University,

Stony Brook, NY 11794, USA

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp01220h

Received 12th February 2014,Accepted 31st March 2014

DOI: 10.1039/c4cp01220h

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the investigation of Ag2VP2O8 as a cathode material in Li basedbatteries showed that the material displays a reduction displa-cement mechanism similar to other silver vanadium phosphatematerials.30 The discharge process can be expressed as:

Ag2VP2O8 + xLi - yLixAg2�xV4+P2O8 + (1 � y)LixAg2�xV3+P2O8

+ xAg0

The presence of Ag0 as a reduction product in silver vana-dium phosphate cathodes has been verified ex situ using X-raydiffraction and scanning electron microscopy experiments.20,28,29,31

However, in situ measurements of the Ag0 provide the opportunityfor further insight into the electrochemical reduction mechanism,without modification of the electrode or exposure to ambient air,moisture, and other potentially deleterious substances. This reportencompasses the first in situ measurement of the discharge processof the Li/Ag2VP2O8 battery system.

Energy dispersive X-ray diffraction (EDXRD) presents aspecial opportunity to achieve spatial resolution of the electro-chemical reduction process within the electrode inside a func-tioning electrochemical cell, providing the opportunity fortomographic-like data analysis.32 The study of Ag2VP2O8 as amodel system can provide insights into the electrochemicalreduction process for inherently poorly conducting phosphateand diphosphate based cathode materials. The Ag0 metalproduct formed upon discharge of Li/Ag2VP2O8 is a strongX-ray scatterer, and as such its presence can be detected insmall amounts.33 Thus, our system is particularly well suitedfor EDXRD in situ study.

While various strategies have been used to address batteryresistance in total, this study provides mechanistic insight intothe limitations of discharge due to cell resistance, particularlyat the mesoscale level. In this study, in situ EDXRD measure-ments are conducted on active lithium/silver vanadium diphos-phate cells, Li/Ag2VP2O8, at 0, 0.1, and 0.5 electron equivalentsof discharge. These levels of electrochemical reduction wereselected to deliberately probe the early stage reduction wherethe large change in resistance has been noted.30 The use ofEDXRD enables spatial visualization of Ag0 formation and lossof parent Ag2VP2O8, including relative estimation of concen-tration as a function of position. Impedance is measured ateach level of discharge and the EDXRD data are used to rationalizechanges in battery resistance. This study provides unique insightinto the evolution of conduction pathways formed via a reduction-displacement reaction in a poorly conducting cathode matrix.

ResultsElectrochemistry

A typical galvanostatic discharge curve for a lithium/Ag2VP2O8

cell is shown, involving reduction to three electron equivalents(176 mA h g�1), Fig. 1. Three discharge levels (0, 0.1, and0.5 electron equivalents) were deliberately selected to studythe cathode material in early stages of the reduction, as ourprior study of the Ag2VP2O8 cathode material showed signifi-cant changes in conductivity on the initiation of discharge.30

After the discharge step, the cells were allowed to rest atopen circuit potential for several days and the electrochemicalimpedance spectroscopy (EIS) response of each cell was deter-mined, as shown in Fig. 2. To our knowledge, this is the firstreport of the impedance of Li/Ag2VP2O8 cells. The EIS responsewas measured for the cells prior to discharge, where similarimpedance response was observed (see ESI†). The cells werethen discharged to different depths (0.1 and 0.5 electronequivalents). The EIS response of the cells was measured beforeand after EDXRD measurements to verify that the X-ray beamdid not affect the status of the cells; there was no discernabledifference in either the open circuit voltage or the EIS responseof the cells after EDXRD measurements.

The impedance spectra were fit to the equivalent circuitmodel shown in the lower inset of Fig. 2, where the sameequivalent circuit model was used for the cells in both theundischarged state and partially reduced state. The selected

Fig. 1 Galvanostatic discharge curve of Li/Ag2VP2O8 electrochemical cell.

Fig. 2 Electrochemical impedance spectroscopy (EIS) measurements ofcoin cells with Ag2VP2O8 cathodes at various states of electrochemicalreduction (0, 0.1 and 0.5 electron equivalents). Upper insets: magnifica-tions of the low-resistance portions of the main graph. Lower inset:equivalent electrical circuit used to fit the impedance spectra.

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equivalent circuit is a modified version of the Randles circuitwith Voigt-type RC elements in series.34,35 The fitting of thedata to an equivalent circuit enables more quantitative compar-isons of the values obtained for the RC elements as a function ofstate of reduction of the cells.36,37 The two Voigt-type RC elementsused in the equivalent circuit accurately fit the two semicircularfeatures seen in the impedance measurement and are tabulated,Table 1. Note that the values for the Warburg impedance (ZW)were fixed in the non-discharged state.

The most striking aspect of the impedance data is the largesize of the semicircle in the undischarged state and thedramatic decrease on initiation of reduction. The proposedphysical interpretation of the data is that the high frequencyfeatures are associated with short relaxation time processes andconversely the low frequency features are associated with longtime processes consistent with prior literature reports.38 Thus,the first resistor (Rs) is attributed to series resistance, the firstparallel resistor–capacitor circuit component (Rm, Cdl,1) isattributed to the anode, while the second resistor (Rct) andWarburg element (ZW) in parallel with capacitor (Cdl,2) circuitcomponent is attributed to the cathode. Notably, only smallchanges in the values for the majority of the circuit elementswere observed among the group of cells, with the one notableexception, Rct. Upon initiation of discharge, the value of Rct

decreases from B1 MO to B1500 O and B500 O on reductionto 0.1 and 0.5 electron equivalents, respectively. Thus, theinitial decrease in Rct is observed to be B700 times, with afurther decrease by an additional 3 times upon reduction from0.1 to 0.5 electron equivalents. The dramatic change in the Rct

resistance is assigned to the cathode where the significant changesupon reduction are taking place with the formation of Ag0 onreduction. This assignment for Rct resistance is consistent withprior reports where the Rct resistance was assigned to the chargetransfer layer for nonporous35,39 and porous40,41 electrodes.

Energy dispersive X-ray diffraction

Energy dispersive X-ray diffraction (EDXRD) allows determina-tion of the reaction progress in situ, within a sealed electro-chemical cell. EDXRD data were obtained for each of the threecells where the geometry of the experiment is illustrated inFig. 3. The size of the gauge volume was approximately 2 � 2 �0.02 mm3. In this experiment, gauge volume was centered

radially (x,y position) on the cell cathode and the axial (z)position of the cell was varied over the series of measurements.The cell was moved for sequential measurements at differentheights or z-positions with the same x,y-position.

Diffraction spectra from EDXRD measurements of the elec-trochemical cells are shown in Fig. 4. On the left side the figureare five characteristic spectra for each of the cells discharged to(a) 0, (b) 0.1, and (c) 0.5 electron equivalents. The five scanswere taken at points: (1) near the edge of the cathode where ittouches the stainless steel cell housing, (2) 40–60 mm inside thecathode on the stainless steel side, (3) the center of the cathode,(4) 40–60 mm inside the cathode on the side facing the Li anode,and (5) the edge of the cathode on the side closest to the Lianode. The vertical dashed lines indicate the positions ofAg2VP2O8 Bragg peaks while the solid vertical line indicatesthe position of the Ag(111) peak. The peak positions for Ag2VP2O8

match previously reported results.30,42

On the right side of Fig. 4 are intensity contour plots of56 EDXRD spectra for cells discharged to (d) 0, (e) 0.1 and (f) 0.5electron equivalents. Spectra were measured at 20 mm intervals,with the axial (z) position of the gauge volume expressed on thevertical axis. The intensity is plotted on a log scale, with whitepoints representing the highest intensities and black being thelowest intensity points. The middle section of the figures withthe highest density of peaks is the Ag2VP2O8 cathode, with thesteel cell housing visible at the top, and the Li anode visible atthe bottom. The Li peaks are only intermittently visible; ingeneral, the Li(110) peak is observed in scans taken closer to

Table 1 Fit parameters for ACI circuit elements of Li/Ag2VP2O8 cells at various stages of electrochemical reduction

DOD (elec. equiv.) 0 0.1 0.5

w2 (Z-view fit) 0.0005007 0.0001179 0.0000434Rs (O) 8.30 � 0.10 14.63 � 0.06 6.90 � 0.04Cdl,1 (F) (3.07 � 0.28) � 10�5 (3.19 � 0.13) � 10�5 (3.07 � 0.10) � 10�4

Cdl,1power 0.706 � 0.009 0.713 � 0.004 0.493 � 0.003Rm (O) 27.22 � 0.42 30.05 � 0.19 28.20 � 0.18Cdl,2 (F) (3.587 � 0.007) � 10�5 (1.625 � 0.010) � 10�4 (4.237 � 0.016) � 10�4

Cdl,2power 0.851 � 0.001 0.869 � 0.002 0.855 � 0.001Rct (O) 1 058 300 � 5400 1448 � 29 508 � 2Zw,R (O) 4100a 4266 � 152 4140 � 154Zw,T (O) 2000a 1174 � 94 3663 � 341Zw,P 0.35a 0.281 � 0.008 0.439 � 0.002

a See experimental section for details.

Fig. 3 The experimental configuration used to measure in situ energydispersive X-ray diffraction (EDXRD) on coin cells.

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the surface of the anode, while the Li(200) peak is typicallyfound through the bulk of the anode but less frequently at thesurface. The intensity of the different peaks changes with beamposition and between cells. This variation in intensity is likelydue to two factors: the low scattering of Li leading to weakerBragg peaks,43 and preferential orientation of Li due to proces-sing factors.44,45

The presence of Ag0 metal can be observed measuring theintensity of the Ag(111) peak (at 1/d = 0.4239 �1). In the not-discharged cell (a), the Ag(111) peak is visible only in thespectrum taken at the surface of the Li anode. In the spectra

for the discharged cells (Fig. 4(b and c)), Ag0 metal is visible onthe surface of the cathode closer to the steel coin cell top,consistent with the formation of Ag0 on discharge. In the 0.1electron equivalent discharged cell (Fig. 4(b)), only the patternrecorded closest to the steel coin cell surface and the patternsclosest to the anode clearly show the presence of Ag0. In the 0.5electron equivalent discharged cell, however, Ag0 is detectablethroughout the cathode.

Since the detection limits of the EDXRD tomographymethod are dictated largely by peak resolution capability, it iscritical to carefully assess the specific X-ray diffraction spectra

Fig. 4 Energy-dispersive X-ray diffraction spectra for Li/Ag2VP2O8 electrochemical cells. Five characteristic spectra for cells discharged to (a) 0, (b) 0.1,and (c) 0.5 electron equivalents. Contour plots for cells discharged to (d) 0, (e) 0.1, and (f) 0.5 electron equivalents, with white representing the highestintensity and black the lowest.

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to appropriately interpret the data. In this case, the Ag2VP2O8

(132) and (151) peaks are positioned closely enough that theycannot be resolved separately and will be referred to as theAg2VP2O8(C) peak for brevity. When Ag0 is present, the Ag(111)and Ag2VP2O8(C), 1/d = 0.4239 and 0.4280 �1, respectively, areconvoluted and appear as one peak with a position between thetheoretical positions of the two peaks. Thus, when both peaksare present small amounts of Ag0 appear as a shift in theAg2VP2O8(C) peak position. This effect is illustrated in Fig. 5 inwhich we show the reference patterns for Ag2VP2O8 and Agmetal (vertical black lines at top and bottom, respectively) andcompare them to EDXRD spectra for (a) pure Ag2VP2O8 and (e)Ag powdered standards measured at the beam line and EDXRDspectra measured in the electrochemical cell discharged to 0.5electron equivalents at beam positions of (b) 2.3, (c) 2.2, and (d)2.1 mm, respectively. At position 2.3 mm, the position of theAg2VP2O8(C) peak is at approximately the same position asthe powder standard, indicating the absence of Ag metal.At 2.2 mm, however, there is a single broad peak appearingbetween the Ag(111) and Ag2VP2O8(C) positions caused by thepresence of a small amount of Ag0. Finally, at a beam positionof 2.1 mm, the peak matches the position of the Ag metalpowder standard.

The peaks in the spectra were fit and the positions of thesepeaks (circles) alongside the position of peaks for the referencematerials (small diamonds) are plotted in Fig. 6(a), (b), and (c)for the 0, 0.1, and 0.5 discharged cells, respectively. Theintensity of Ag2VP2O8 and Ag were normalized to the samevalue for all 3 figures to make comparisons of intensity possi-ble. The color of the circles represents the intensity of the peaks

on a log scale, with blue circles representing the lowest inten-sity and red circles representing the highest. The black dia-monds are peak positions for Ag2VP2O8, the red diamonds arethe positions of the Ag(111) and Ag(200) peaks, and the greydiamonds are the position of the Li(110) peak. The placementof the diamonds with respect to beam position is a guide to theeye. These data show the convolution of the Ag2VP2O8(C) andAg(111) peaks causing an apparent shift in the position of theAg2VP2O8(C) peak. Within the bulk of the cathode and near thestainless steel interface of the cathode in the nondischargedcell, Fig. 6(a), the position of the Ag2VP2O8 peaks do not differfrom their theoretical positions, indicating the absence of Ag0

through the bulk of the cathode. However, in the partiallydischarged cells, Fig. 6(b and c), the position of the Ag2VP2O8

peaks do differ from their theoretical positions in some regionsof the cathode, indicating the presence of Ag0.

A comparison of the intensity of Ag0 and Ag2VP2O8 as afunction of beam position is plotted in Fig. 6(d–f) for the 0, 0.1,and 0.5 discharged cells. The intensity was determined bymeasuring the intensity of the EDXRD data at characteristicpeak positions and subtracting the background value. Theintensity of Ag2VP2O8 is denoted by the black diamonds,Ag(111) by the red diamonds, and Li(110) by the grey diamonds.The intensity of Ag2VP2O8 was determined using the averageintensity of three peaks (1/d = 0.5130, 0.5303, and 0.5506)chosen to be well-isolated from stainless steel, Ag, or Li peaks.In order to compare the relative intensity of Ag0 and Ag2VP2O8

among all three cells, the intensities were normalized to thesame value for all 3 figures.

In Fig. 6(d–f), simple labels are shown for general indicationof the cell housing, cathode, and anode regions. At the surfaceof the cathode closer to the Li metal low levels of Ag0 aredetectable; in Fig. 6(d) we can see that the intensity of theAg(111) peak shows a maximum where the Li peak ends, butdoes not completely disappear before the onset of Ag2VP2O8.However, three factors make precise determination of thelocations of the cathode and anode boundaries difficult: pos-sible swelling or roughening of the cathode surface duringdischarge, diminishing of the Ag2VP2O8 signal on dischargepreviously noted in ex situ studies, and cathode or cell tilt. Thecells were manually leveled with respect to the beam, howeverwith a gauge volume 2 mm wide, a tilt of 21 would create a70 mm difference in height between the two sides of the gaugevolume, broadening any surface features.

Discussion

In our previous study of Ag2VP2O8, the electrochemical reductionmechanism was probed via ex situ X-ray powder diffraction of thecathode material at various states of discharge.30 Up to 2.0electron equivalents of discharge, formation of silver metal wasevidenced by an increasing integral area of the peak at 38.11 two-theta, corresponding to (111) of Ag0. While the bulk ex situ XRDmeasurements in the prior report were useful to verify theformation of silver at early stages of reduction, it was not possible

Fig. 5 Intensity versus peak position (1/d) for EDXRD scans of (a) as-synthesized Ag2VP2O8; Ag2VP2O8 discharged to 0.5 electron equivalents ataxial (z) positions of (b) 2.3 mm, (c) 2.2 mm, and (d) 2.1 mm; and (e) Ag0

metal powder. Top line reference pattern is Ag2VP2O8 (PDF 01-088-0436),and bottom line reference pattern is Ag metal (PDF 01-073-6977).

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to determine the location of the Ag0, or the homogeneity of itsdistribution within the cathode. The EDXRD methodologydescribed here allowed the first visualization of a ‘‘tomographic’’profile of the Ag2VP2O8 cathode material as a function of bothdepth of discharge and position within an electrode. Further,in situ EDXRD presented the first opportunity for direct in situinterrogation of the Li/Ag2VP2O8 in its native environment, elim-inating the possibility of decomposition or structural degradationduring material removal from the cell.

Conducting Ag0 nanoparticles could have several extrinsiceffects on the functional characteristics of the cathode. Assynthesized, the Ag2VP2O8 is a highly resistive material andcan be considered as an insulator. Upon reduction of Ag2VP2O8,formation of conducting Ag0 may facilitate electrical contactamong the partially reduced AgxVP2O8 active material particlesin the bulk of the cathode, and between the cathode and theother cell components, including the current collector. With a

sufficient amount of Ag0 metal, the particles of Ag2VP2O8 activematerial can be considered as insulators surrounded by aconductor. Percolation theory models have previously dis-cussed such a scenario; indeed, some of the earliest experi-ments on percolation were performed on insulating glasses orplastics coated with metallic Al0, Cu0, or Ag0.46 In one suchstudy, 10 mm diameter glass spheres were coated in a 60 nmlayer of Ag0 to create highly conductive (Ag0-coated glass)particles.47 By mixing the conducting and insulating particles,conductivity was determined to sharply increase at a percola-tion threshold of 0.17, corresponding to B17% by volume ofconducting (Ag0-coated glass) with B83% by volume of insulat-ing (uncoated glass) particles. Based on this report, the volumepercentage of Ag0 required to achieve percolation is only 0.3%of the total volume of material.

In order to determine if sufficient Ag0 is present in ourpartially discharged Li/Ag2VP2O8 cells to account for an increase

Fig. 6 Li/Ag2VP2O8 electrochemical cells discharged to (a, d) 0, (b, e) 0.1, and (c, f) 0.5 electron equivalents. Beam position vs. 1/d for (a–c). Peakpositions are represented by colored circles, with highest intensity peaks colored red and lowest intensity colored blue. Small diamonds are thetheoretical peak positions for Ag2VP2O8 (black), Li (grey), and Ag (red). Intensity of Ag2VP2O8 and Ag0 as a function of beam position for (d– f). Symbolsrepresent the intensity of the EDXRD spectrum measured at the Steel(211) position (1/d = 0.8517 Å�1) – blue, the Li(110) position (1/d = 0.4030 Å�1) –grey, Ag(111) peak (1/d = 0.4239 Å�1) � red and the average of three Ag2VP2O8 peak positions (1/d = 0.5130, 0.5303, 0.5506 Å�1) – black.

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in conduction due to a percolation-type network, our results areconsidered in light of the previous studies on simpler systems.Based on consideration of our prior magnetic susceptibilitymeasurements in conjunction with our prior ex situ XRD datadescribed above, silver reduction is the preferred initial process,with little reduction of vanadium below 0.5 electron equivalentsof Ag2VP2O8 discharge.30 Assuming that the sole reductionproduct of Ag2VP2O8 at low levels of discharge results from Ag+

reduction to Ag0 (i.e., no V4+ reduction), the volume ratio of Ag0

metal to Ag2VP2O8 for the cells in this study can be calculatedusing the densities reported in the literature (4.632 g cm�3 forAg2VP2O8 from PDF 01-088-0436 and 10.503 g cm�3 for Ag fromPDF 01-073-6977). Based on our estimates, at 0.1 electronequivalents, B1.1% Ag0 is present by volume, exceeding theminimum percolation threshold of 0.3%. Thus, it is possible thatin situ reduction of Ag+ ions upon partial reduction of Ag2VP2O8

could create a conducting percolation network through thecathode causing or contributing to the observed decrease inimpedance providing that the spatial location of the silver wasappropriate.

Elucidating the spatial and functional relationship of con-ducting solids and less conductive active materials in a batteryelectrode is critical toward full understanding of compositeelectrodes. In this case, the position and homogeneity of theconducting silver nanoparticles provides the opportunity forkeen insight into these mechanisms, as various factors includ-ing limitation of local electronic conductivity may limit thedischarge rate in lithium based batteries, and the determina-tion of these factors is integral to improving performance. Fewtechniques enable non-destructive, in situ imaging of thereduction process within a battery electrode over a millimeterscale interrogation area with micron scale resolution. The highenergy X-ray capabilities of NSLS beamline X17B1 allowtomographic type detection with the needed specificationsfor assessment of the Ag0 distribution uniformity, providingunique insight into the discharge mechanism. The presence ofsilver formation is clearly detectable in these experimentsproviding the opportunity to track the influence of the con-ductive component of the cathode whereas some conductiveadditives such as carbon would be not be as clearly detectable.Ag0 is present on side of the cathode facing the Li anode in allthree cells, with lowest intensity in the not-discharged cell andhighest intensity in the 0.5 electron equivalent cell, as shown inFig. 6. This is most apparent in the intensity of the EDXRDspectrum at the Ag(111) peak position, shown as the red data inFig. 6(d–f).

In order to further probe the observation of Ag0 at thesurface of the cathode facing the Li anode in the non-discharged cell, we constructed two additional Li/Ag2VP2O8

cells. EIS was measured on one cell, while the other was leftunmeasured. Both cells were allowed to rest at open circuitpotential for 10 days, after which they were disassembled. Eachof the cells showed similar results, confirming no significanteffect of the EIS measurement. The cathodes were measured viaex situ XRD, and no Ag0 could be detected. The Li anodesshowed a change in appearance from metallic silver to a darker

color on the side facing the cathode. Each anode was digestedand quantitative inductively coupled plasma optical-emissionspectroscopy confirmed the presence of silver on the anodes, inan amount consistent with a thin layer of Ag0 0.1 mm thickassuming uniform distribution over the lithium surfaces.

Because of the large width and depth of the gauge volume(2 � 2 mm2) and the fact that Ag0 is a strong X-ray scatterer,EDXRD could measure a relatively large signal from such a thinlayer of Ag0. We propose that the Ag0 detected by EDXRD in thevicinity of the Li anode of the undischarged cell is likely a resultof dissolution or ion exchange of the Ag+ from the Ag2VP2O8

active material and reduction at the Li metal surface to formAg0. The dissolution and ion exchange of silver ions from arelated silver vanadium phosphate material, Ag2VO2PO4, hasbeen determined and previously reported.48,49

There is clear evidence of Ag0 formation at the surface of thecathode facing the lithium anode in the partially dischargedcells, shown in Fig. 6(e and f). Notably, the EDXRD dataprovides conclusive evidence of non-uniform discharge of thematerial, as evidenced by non-uniform Ag0 and Ag2VP2O8

intensity distributions in Fig. 6. In a previous report we showedthat as Li/Ag2VP2O8 cells are discharged, the intensity of thediffracted X-rays coming from Ag2VP2O8 decreases, with com-plete loss of the parent Ag2VP2O8 pattern at a depth of dis-charge of 2 electron equivalents.30 Although the cells in thisstudy were discharged only to calculated bulk values of 0.1 and0.5 electron equivalents, due to the higher local level ofdischarge at the cathode surface adjacent to the Li anode, asevidenced by higher Ag0 intensity, concomitant diminishing ofthe local Ag2VP2O8 signal is expected, consistent with the datashown in Fig. 6.

Interestingly, in the 0.1 and 0.5 electron equivalent cells, thepresence of silver metal also can be observed on the side of thecathode facing the cell housing (not facing the anode), asshown in Fig. 6(e and f). The detectable Ag0 extends only ashort distance into the cathode, estimated at o60 mm. Oninitial discharge the active cathode material closest to the cellhousing also reduces, not just the cathode material facing thelithium anode as might be expected. Thus, these results affirmsignificant discharge of the active cathode material near thecell housing acting as a current collector. This is furtherevidence that the availability of electrons in addition to Li ionsis a significant factor in the discharge of poorly conductingmaterials.

The center of the cathode shows a lesser extent of localreduction relative to the edges of the cathode. As shown inFig. 5, due to the close proximity of the Ag(111) and Ag2VP2O8(C),the peak position of the main peak near 1/d = 0.420–0.430 can beused as an indication of the presence of Ag0, where lower 1/dvalue indicates higher concentration of Ag0 and higher 1/d valueindicated higher concentration of Ag2VP2O8. Notably, the notdischarged cell, Fig. 6(a), shows a 1/d value consistent withAg2VP2O8(C) across the main bulk of the cathode. Fig. 6(b) showsa small shift in peak position for the 0.1 electron equivalent cell,with more shift evident at the edges of the cathode and less shiftin the center of the cathode. Fig. 6(c) shows a clear shift away

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from the Ag2VP2O8(C) position, indicating the formation of Ag0

throughout the thickness of the cathode for the 0.5 electronequivalent discharged cell. Fig. 6(f) also illustrates the presenceof Ag0 through the thickness of the 0.5 electron cathode.

Experimental detailsMaterial synthesis and characterization

Ag2VP2O8 was prepared via a solid state reaction previouslyreported in the literature.42 Diffraction of the synthesizedmaterial was performed using a Rigaku XRD system (Cu Ka

radiation). Samples matched the reference pattern (PDF #01-088-0436) with no impurity phases noted. Ex situ X-ray diffrac-tion (XRD) data was collected using Cu-Ka radiation. XRD andEDXRD spectra were fit using PDXL software. Inductivelycoupled plasma-optical emission spectroscopy (ICP-OES) datawas collected using a Thermofisher ICap 6000.

Cell assembly and testing

After characterization, stainless steel coin type cells with lithiummetal anodes were fabricated in an argon atmosphere glove box.Cathodes of as synthesized Ag2VP2O8, separator material, lithiummetal anodes, and electrolyte consisting of 1 M LiPF6 in ethylenecarbonate–dimethylcarbonate were employed in all cells.

Electrochemical impedance spectroscopy (EIS) was measuredover the frequency range of 0.5 mHz to 100 kHz at a temperatureof 30 1C. For the undischarged cells, the impedance was suffi-ciently large that the diffusional tail was not observed over thisfrequency range. Equivalent electronic circuit analysis and errorassessment of EIS data was performed using Zview softwarepublished by Scribner Associates, using the equivalent circuitshown in Fig. 2. For the undischarged cells, the values associatedwith the Warburg element (Zw,R (O), Zw,T (O), Zw,P) were fixedto values of 4100, 2000, and 0.35, respectively. Cells weredischarged using a current density of 0.023 mA cm�2. One cellwas discharged to 0.1 electron equivalents or 3.3% depth ofdischarge and a second cell to 0.5 electron equivalents or 16.67%depth of discharge, while a third cell was left as prepared and notdischarged. Additional EIS data is shown in the ESI.†

In situ EDXRD

Energy dispersive X-ray (EDXRD) measurements were per-formed on the high energy X-ray superconducting wiggler beamX17B1 at the National Synchrotron Light Source at BrookhavenNational Laboratory. The wiggler produces a ‘‘white’’ beam ofX-ray radiation with an energy range up to approximately200 keV. The beam is passed through a Cu foil to filter outthe low energy end of the spectrum which does not contributesignificantly to diffraction. Standards of CeO2 and LaB6 wereused to calibrate the detector.

Powdered standards of Ag2VP2O8 and Ag0 as well as the partsof the coin cell were measured using the EDXRD setup atbeamline X17B1 at NSLS-I and compared to the reported values.The positions of the Bragg peaks showed good correspondenceto reported values and standards, however it should be noted

that the intensities of the peaks as measured by EDXRD are verydifferent than the intensities measured in angle-resolved XRDexperiments. Unlike angle-resolved measurements which useX-rays of a constant energy, the intensity profile of the X-rayscoming from the wiggler beam changes with energy. Addition-ally, the signal is attenuated by the cell parts, a Cu foil filterused to remove much of the intensity of the low energy portionof the beam, and the cathode outside of the gauge volume.

The intensities of the diffracted X-rays were measured by ahigh resolution germanium detector with resolution of approxi-mately 25 eV. The detector was set at 2y = 3.051, and the slit sizewas set so that the gauge volume had a length of approximately2 mm. As the beam was 20 mm high, moving the cell stepwisethrough the gauge volume provided a spatial profile of the cellwhere the discharge progress could be determined as a func-tion of height. The coin cells were placed on an automatedstage such that the entire height of the cathode could be probed(along the z direction in Fig. 3). In this experiment, gaugevolume was centered radially on the cathode and the axialposition of the cathode was varied.

Conclusion

In this study, in situ energy dispersive X-ray diffraction (EDXRD)and electrochemical impedance spectroscopy (EIS) data werecollected on lithium/silver vanadium diphosphate (Li/Ag2VP2O8)cells discharged to 0, 0.1 and 0.5 electron equivalents. Based onthe complementary nature of EDXRD and EIS, the relationshipsamong cathode reduction, electrochemical cell discharge, andimpedance can be conceptualized. In the as-prepared cell(nominally not discharged), a limited amount of Ag0 is formedon the lithium surface facing the cathode, as a result of Ag+

dissolution. The bulk Ag2VP2O8 cathode is an insulator with highresistance, manifesting in high measured impedance (Rct 41 MO). Upon initiation of discharge, Ag0 is formed on thecathode, on the side adjacent to the cell housing as well as theside facing the lithium anode, resulting in 4700� decrease inRct upon 0.1 electron equivalent of reduction. With increasingdischarge to 0.5 electron equivalents, 3� further decrease in Rct

is observed, consistent with formation of Ag0 throughout thethickness of the discharged cathode as is noted by the EDXRDresults.

As with other silver bimetallic phosphorous oxide com-pounds, a large decrease in impedance in discharged cells ascompared to the not-discharged cell coincides with the devel-opment of Ag0 nanoparticles. The data presented suggest twopossible mechanisms contributing to the decrease in impe-dance: formation of a percolation network that decreasesconductivity throughout the entire cathode, and a decrease inthe charge-transfer layer resistance by creating a conductinglayer around the particles of active material increasing theelectronically accessible surface area of the cathode. Once aparticle begins to discharge, the formation of Ag0 would make iteasier to continue discharging in that local region. Thus, moresurface area of the insulating Ag2VP2O8 active material particles

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could be accessed as a result of rapid charge transfer throughthe highly conducting silver nanowires. The EDXRD resultsindicate that a majority of the Ag0 formation occurs at the canand anode interfaces of the cathode, only moving throughoutthe cathode with higher levels of discharge. Although thiswould appear to suggest that a conducting layer around theparticles is the more likely scenario given that the volumefraction of Ag0 can be very small in a percolation network, bothmechanisms remain feasible.

Acknowledgements

E. Takeuchi, K. Takeuchi, A. Marschilok and D. Bock acknow-ledge funding from the Department of Energy, Office of BasicEnergy Sciences, under grant DE-SC0008512. Utilization of theNational Synchrotron Light Source (NSLS) beamline X17B1 wassupported by U.S. Department of Energy Contract DE-AC02-98CH10886. K. Kirshenbaum acknowledges Postdoctoral sup-port from Brookhaven National Laboratory and the Gertrudeand Maurice Goldhaber Distinguished Fellowship Program.The authors also acknowledge Mark C. Croft for helpfuldiscussions.

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