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Local electronic structure of LiFePO4 nanoparticles in agedLi-ion batteries

Shrikant C. Nagpure a, S.S. Babu b, Bharat Bhushan a,⇑, Ashutosh Kumar b,Rohan Mishra b, Wolfgang Windl b, L. Kovarik b, Michael Mills b

a Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics, The Ohio State University, 201 West 19th Avenue, Columbus, OH 43210, USAb Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA

Received 7 December 2010; received in revised form 23 June 2011; accepted 21 July 2011Available online 31 August 2011

Abstract

The advanced LiFePO4 cathode based Li-ion batteries have shown potential for applications in electrification of automobiles. Therate capabilities of these batteries have been improved by the development of LiFePO4 nanoparticles. Here the effect of aging of Li-ion batteries on the local electronic structure of these LiFePO4 nanoparticles has been investigated with high resolution scanning trans-mission electron microscopy imaging. The local compositional changes within particles from unaged and aged batteries have been mea-sured from electron energy loss spectroscopy (EELS) on these LiFePO4 nanoparticles. Further, the Z + 1 approximation within thedensity functional theory (DFT) framework has been employed to theoretically compute the energy loss near edge structure of LiFePO4.The measured EELS spectra are compared with the computed O K edge and Fe L2,3 edge to understand the loss of active Li from the hostLiFePO4 structure in the aged battery, which corresponds to loss of capacity of the battery.� 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Li-ion battery; Aging; Electron energy loss spectroscopy; Vienna Ab-initio Simulation Package; Density of states

1. Introduction

There is a great deal of urgency to develop electric vehi-cles as it will help fuel US economic growth, reduce thedependency on foreign energy sources, and provide a clea-ner environment for future generations. Among otherchoices for electrification electrochemical energy storagesystems such as Li-ion technology have a very importantrole to play in the immediate future. While the operationof Li-ion cells is well established, the aging mechanisms stillneed to be investigated as the life of the Li-ion cell is stillthe weak link in the development of long-lasting hybridvehicles. Since its introduction in 1990 Li-ion battery tech-nology has been growing and gaining popularity at a rapidpace. Advanced Li-ion batteries with high energy andpower density are fast approaching compatibility withautomotive demands. According to the US Advanced Bat-

tery Consortium (USABC) a 42 V battery in a hybrid elec-tric vehicle (HEV) should have a calendar life of 15 years[1]. Electric vehicles (EV) should have a battery system thatcan last for 10 years [2]. In terms of cycles, 1000 cycles at80% depth of discharge are expected for EV [2], and300,000 cycles at 50 W h are expected for a plug-in HEV[3]. However, the capacity (range) and power (perfor-mance) capabilities of these batteries decrease when sub-jected to the harsh duty cycles (current levels, low stateof charge (SOC), and temperature profiles) of automobiles.Aging of the cell results in an increase in the thermal resis-tance (power fade) and a decrease in capacity (chargeacceptance) [4]. This degradation or “aging” is a result ofseveral simultaneous physico-chemical processes that occurwithin the electrode, electrode–electrolyte interface, andwithin the electrolyte [5].

The first generation Li-ion battery cathode material,LiCoO2, was replaced by layered structure oxides such asLiNi1/3Mn1/3Co1/3O2 to minimize the use of expensiveand toxic cobalt metal. These types of cathode had a very

1359-6454/$36.00 � 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.actamat.2011.07.043

⇑ Corresponding author. Tel.: +1 614 292 0651.E-mail address: bhushan.2@osu.edu (B. Bhushan).

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good operating voltage of � 4.2 V but lacked the structuralstability. Upon removal of Li the structure became highlydisordered [6]. The next generation spinel type, Li[Mn2]O4

cathode material showed a very good structural stabilitybut the capacity was very poor [6]. Also Mn dissolved inthe electrolyte at high operating voltage (between 3 V and4 V) and poisoned the anode surface. The dissolution ofMn was minimized by using Li[Mn1.8Li0.1Ni0.1]O3.8F0.2

type spinel framework. But the capacity remained disap-

pointingly low [6]. Most recently, olivine structured LiFe-PO4 cathode material has become very popular. LiFePO4

has an operating voltage of � 3.4 V, has very good struc-tural stability and are stable at high temperatures. But inLiFePO4 Li transfer occurs only in 1D and it also has verypoor electronic conductivity. These drawbacks of LiFePO4

are overcome by reducing the size of the particles to nano-meter scale and by coating the particles with carbon [6].Thus, LiFePO4 is suitable for large scale applications as

Fig. 1. (a) Schematic of a proposed mechanism explaining the increase in the thermal diffusivity of a LiFePO4 cathode due to aging [7]. (b) Schematic of aproposed mechanism explaining the increase in the surface resistance of a LiFePO4 cathode due to aging [12]. (c) Surface potential map of the unaged (leftcolumn) and aged (middle column) LiFePO4 cathode samples with an external voltage of +3.3 V. The right column shows the normal probability densitydistribution of the surface potential values obtained for the unaged and aged samples. The mean value of the surface potential decreases after aging [15].

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well such as HEV and EV. Due to possible application ofthese batteries in automobile industry, the study of capac-ity loss and the degradation of a LiFePO4 cathode havebecome scientifically important [30].

LiFePO4 cathode degradation has been studied and amultiscale characterization plan has been developed tounderstand the mechanism behind physical damage to thecathode. In previous studies [7] thermal diffusivity analysisby the flash method [8] was shown to be an effective tech-nique to map damage to the cathode in Li-ion batteries atmillimeter length scales. Based on the thermal diffusivityexperimental results a first order approximate heat conduc-tion mechanism as the cathode ages due to cycling was pro-posed. Fig. 1a shows the schematic of a proposed mechanismexplaining the increase in the thermal diffusivity of a LiFe-PO4 cathode due to aging. As a first order approximationthe cathode can be considered as a porous medium. Also,it can be assumed that heat diffuses through the cathode onlydue to conduction. As the cathode ages the nanoparticlestend to coarsen by sintering. Due to this sintering of thenanoparticles the effective surface area per unit volumedecreases, with an associated decrease in the porosity ofthe cathode [9]. In general, as the porosity of the mediumdecreases the thermal conductivity increases, and aged sam-ples show an increased thermal diffusivity [10,11].

In another study using atomic force microscopy (AFM)Nagpure et al. [12] found that the surface resistanceincreases after aging of the cathode. Fig. 1b shows a sche-matic of the mechanism that leads to an increase in the sur-face resistance of a LiFePO4 cathode sample. When the tipscans over the surface of the sample a circular contact isformed between the LiFePO4 nanoparticles and the tip.As mentioned earlier, LiFePO4 nanoparticles have verypoor conductivity (r = 2 � 10�9 S cm�1), and so toincrease their conductivity, they are coated with carbon[13,14]. In the case of an unaged sample the total surfaceresistance measured is the resistance of the carbon-coatedLiFePO4 nanoparticles Ru, while for the aged sample it isRa. The resistance of the LiFePO4 nanoparticles increasessuch that Ra > Ru as the cells are cycled because of coars-ening of the particles. This coarsening also causes loss ofthe carbon coating, which leads to a further increase inthe resistance of these particles.

Nagpure et al. [15] studied the surface potential ofunaged and aged samples using AFM Kelvin probe micros-copy (KPM). Fig. 1c shows surface potential maps and sta-tistical analysis of the surface potential values measured forthe unaged and aged LiFePO4 cathode samples with anexternally applied +3.3 V DC. voltage. No contrast withina sample was observed, but the mean surface potential ofthe aged sample was lower than that of the unaged sample.As KPM is highly sensitive to structural and/or chemicalchanges, the change in the surface potential seen inFig. 1c can be attributed to phase changes, nanoparticlecoarsening, and/or loss of the carbon coating.

Coarsening of the LiFePO4 nanoparticles has been fur-ther investigated here by high resolution scanning trans-

mission electron microscopy (STEM). Electron energyloss spectroscopy (EELS) has been used to determine theeffect of coarsening, and the local electronic structure ofthe LiFePO4 nanoparticles in the unaged and aged sampleshas been identified. The EELS analysis suggests variationsin the lithium content of LiFePO4 nanoparticles in theunaged and aged samples. An ab-initio simulation of theEELS spectra has been carried out to analyze the experi-mental data.

2. Experimental details

2.1. Experimental cells

The two commercial cylindrical Li-ion cells used in theseexperiments have a graphite anode and a cathode of LiFe-PO4 nanoparticles (40–50 nm). The graphite is bondedonto a copper substrate, and the LiFePO4 nanoparticlesare bonded onto an aluminum substrate using a polyvinyl-idene difluoride (PVDF) binder. The anode and cathodestrips, with a separator in between, are rolled and thenpacked into a can to form a cylindrical cell. The electrolyteused in this cell is a lithium hexafluorophosphate (LiPF6)salt in 1:1 ethylene carbonate and dimethyl carbonate.The cell has an operating voltage of 3.3 V and a nominaldischarge capacity of 2.3 Ah. As stated earlier, LiFePO4

has poor ionic and electronic conductivity [6] and so toimprove the conductivity the nanoparticles are coated withcarbon [6].

One cell was charged and discharged completely withthe C-rate of 1 C (1 C = 2.3 Ah) to verify its rated capacity.This cell was labeled as the unaged cell. The other cell wascycled with the C-rate of 16 C from 45% to 55% SOC untilthe capacity dropped by 20%. This cell was labeled as theaged cell. This protocol is consistent with the automotiveindustry standard, which considers a cell to be dead whenits capacity drops below 80% of the original rating [3].

2.2. TEM sample preparation using FIB

The TEM samples were prepared in a FEI Helios 600e

dual beam focused ion beam system (FIB). The TEM sam-ples were extracted from the surface through the thicknessof the sample using a lift-out technique. The lift-out tech-nique used here has been discussed in Giannuzzi et al.[16] and Giannuzzi and Stevie [17]. The steps followed inthe lift-out technique is shown in Fig. 2 and briefly dis-cussed here. A clean area was found on the sample andimaged with the ion beam (1). Then Pt was deposited onthe surface using a very small current of 2.8 nA (2). Thesample was turned by 20� in one direction and the areaaround the Pt deposit was milled away using a milling cur-rent of 9.3 nA. This step was repeated to mill the area onthe other side of the Pt deposit (3). The sample was thenturned by 7� and a trench cut along the depth of the sampleto extract the TEM foil (4). Then an omniprobe wasattached to the foil and the foil was pulled from the rest

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of the sample (5). The foil was then welded onto a TEM Cugrid (GRD-0001.01.01) using Pt (6) and detached from theomniprobe. Finally, the foil was further thinned to�200 nm using the ion beam.

2.3. TEM and EELS experimentation

The TEM imaging and EELS analysis at high energyresolution were performed using a FEI Tecnai F20 S/TEM System at the Center for Electron Optics Facility,Ohio State University. The instrument is equipped with aSchottky field emission gun and a Gatan imaging filter,which enables spectroscopy measurements at a resolution

of 0.8 eV. The EELS data was collected with a dispersionof 0.5 eV per channel. The energy range was chosen so asto include the oxygen K edge and the iron L2,3 edge. Thedata was background subtracted using the power lawmethod as implemented in the DigitalMicrograph analysissoftware (Gatan).

3. First-principles simulation method

The electronic structure calculations were performed onsupercells containing six formula units of LixFePO4 withx = 0, 0.25, 0.5, 0.75, and 1 for detailed analysis and inter-pretation of the experimental results. The structural data

Fig. 2. Steps showing TEM sample preparation using a FEI Helios 600e dual beam focused ion beam system (FIB).

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for the starting materials in these calculations were adaptedfrom Tang and Holzwarth [18]. LiFePO4 forms an ortho-rhombic olivine structure with a slightly distorted hexago-nal close packed oxygen array belonging to the symmetrygroup Pnma [19], listed as no. 62 in the International Tables

for Crystallography [20]. Fig. 3 shows the crystal structureof this material with one unit cell constructed with Materi-als Studio. The O atoms are located at the tetrahedral sitesaround each P atom. The divalent Fe ions form an octahe-dral arrangement with the O atoms. The Li atoms arelocated in channels along the b axis of the orthorhombicstructure [18].

Density functional theory (DFT), as implemented in theVienna Ab-initio Simulation Package (VASP) [21,22] wasemployed to perform the electronic structure calculationswithin the generalized gradient approximation (GGA) withPW91 [23] type projector augmented wave (PAW) poten-tials using a cutoff energy of 500 eV, enhanced by a cou-lombic term U [24] to include strong correlation effects(GGA + U) for the Fe d orbitals. Within this frameworkthe exchange term J is combined with the coulomb termU and (U–J), an effective value of U referred to as Ueff, isused. A value of 4.3 eV was chosen for Ueff, following Miaoet al. [25]. All the calculations here have been performedfor the ferro-magnetic configuration for consistency. Anenergy convergence of better than 10�6 eV was achievedwhen relaxing the geometry. The initial and relaxed latticeparameters for structures with varying lithium contents cal-

culated with the GGA + U approximation are shown inTable 1.

Following Duscher et al. [26], the Z + 1 approximationmethod has been applied to simulate the electron energyloss near edge structure (ELNES). The ELNES onset wasset up at the Fermi energy of the different structures. Thisapproximation has been shown to model core hole excita-tion spectra in oxides well if dipole selection rules are used(change in orbital angular momentum quantum number byone) [26,27]. Integration for the angular momentumresolved conduction band density of states (DOS) has beenperformed using the tetrahedron method with Blochl cor-rections for 6 formula unit supercells created from the geo-metrically relaxed unit cells, with a 4 � 4 � 4 Monkhorst–Pack k-point mesh for Brillouin zone sampling for thesupercells [28].

4. Results and discussion

Morphologies and nanostructures of the LiFePO4 sam-ples are shown in the TEM images (Fig. 4). They have thesame global morphologies. Particles of the aged samplewere bigger than the particles of the unaged samples. Thesamples have several layers of nanoparticles and, as such,contrast exists among the overlapping particles. Thismakes particle size analysis difficult, but the coarseningphenomenon is clearly visible in Fig. 4b. Good statisticalanalysis of the coarsening and the particle size variationcan only be achieved with a more subtle TEM sampleextraction process that would allow quantitative measure-ment of the relative fractions of agglomeration and sinter-ing, without destroying the configurations observed inFig. 4. Shearing et al. [29] have used X-ray tomographyto evaluate the spatial morphology of graphite negativeelectrodes from Li-ion batteries. In principle, even thoughsuch techniques can be used to evaluate coarsening,agglomeration and densification, these techniques lackthe necessary spatial resolution observed in TEM images.

Fig. 5 shows the microstructure and the correspondingEELS measured for the unaged and aged samples. Theareas were chosen for a single particle analysis. Due tothe uncertainty in establishing the absolute value for theenergy loss scale all the spectra were aligned at the O K

edge (532 eV). All the spectra were normalized to the inten-sity of the O K edge peak. As shown in Fig. 5b, a significantdifference in the shape of the O K edge peak is observed.The peaks in the data were fitted to a Gaussian function.The O K edge peak is at 532 eV, with a pre-peak at�528 eV found in the aged sample which is almost negligi-ble in the unaged sample. Fig. 5c shows changes in the FeL2,3 edge with aging. It can be seen in this figure that themain difference between the aged and unaged sample isthe position of the Fe L2,3 edge. The onset of the Fe L2,3

edge for the aged sample is �2 eV higher than for theunaged sample. The EELS measurements were conductedfrom the periphery to the core of the LiFePO4 particle inboth, the unaged and aged samples, but note that in

Fig. 3. Crystal structure of LiFePO4 showing one unit cell constructedwith Materials Studio.

Table 1Initial lattice parameters and relaxed lattice parameters for structures withvarying lithium content obtained by generalized gradient approximation(GGA) with PW91 type projector augmented wave (PAW) potentialsusing a cutoff energy of 500 eV, enhanced by a coulombic (U) term toinclude strong correlation effects (GGA + U).

Compound a Lattice (A) b Lattice (A) c Lattice (A)

Initial Relaxed Initial Relaxed Initial Relaxed

FePO4 9.81 9.946 5.79 5.891 4.78 4.866Li0.25FePO4 10.33 10.041 6.008 5.947 4.69 4.861Li0.5FePO4 10.33 10.203 6.008 5.993 4.69 4.806Li0.75FePO4 10.33 10.324 6.008 6.026 4.69 4.787LiFePO4 10.33 10.418 6.008 6.067 4.69 4.745

The structural coordinates were adapted from Tang and Holzwarth [18]for FePO4 and LiFePO4, while the intermediate structures were created bysubsequently removing lithium from the LiFePO4 structure.

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Fig. 5b and c not all spectra are shown, and only a repre-sentative of the phenomenon is presented. As can be seen inFig. 5b, the intensity of the O K edge pre-peak is higher atthe core than at the periphery of the large particle in theaged sample. Thus the ratio of the pre-peak to the mainfeature of the O K edge peak was less at the core of the lar-ger particle in the aged sample than at the periphery. It isalso interesting to note that if the EELS spectra areobtained at the core and the periphery of a large particlein the aged sample there is a shift in the Fe L2,3 edge to ahigher energy.

The differences in the shape of the O K edge and theposition of the Fe L2,3 edge have been explained in previ-ous spectroscopy work by Laffont et al. [30] and Miaoet al. [25]. The pre-peak at the O K edge and the shift tohigher energy of the Fe L2,3 edge observed in the aged sam-ple is characteristic of a highly delithiated LiFePO4 (acharged cell), while the spectra observed in the unagedsample is characteristic of a well lithiated LiFePO4 (a dis-charged cell). Since in this study both samples were dis-charged before disassembly, it is believed thephenomenon observed here is due to aging of the cells.The measurements suggest that the core of the larger,coarsened LiFePO4 particle in the aged sample has a differ-

ent lithium composition than the periphery. It is thusbelieved that during aging the LiFePO4 nanoparticles sintertogether resulting in coarsening of the particles. This coars-ening is expected to increase the Li ion diffusion lengththrough the particles and thus transformation of the nano-particles during discharge of the cell from FePO4 back toLiFePO4 is expected to be incomplete, leaving a FePO4

(or low Li concentration) core and a LiFePO4 shell aroundthe core in large particles. As the cell ages the FePO4 (lowLi concentration) core increases and the LiFePO4 shelldecreases. This leads to the conclusion that aging hascaused significant compositional changes in the LiFePO4

nanoparticles. Active lithium is lost from the host cathodematerial, thus reducing its capacity. The diminished levelsof Li in the cathode can be expected to lead to a compro-mised capability of the overall cell to recharge.

Fig. 6 shows ELNES for the O K edge and Fe L2,3 edgeobtained by the first-principles calculations. A small pre-peak for the O K edge is found for FePO4, which is notfound in structures with LixFePO4 (x > 0). This is in agree-ment with the observed experimental trend presented inFig. 5b. For the Fe L2,3 edge (Fig. 6b) a peak shift towardshigher energies can be observed with decreasing lithiumcontent, which has also been observed in our experimental

Fig. 4. (a) TEM images of unaged and aged LiFePO4 cathode samples at lower magnification and (b) TEM images of unaged and aged LiFePO4 cathodesamples at higher magnification. The images are from the central area in (a).

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results (Fig. 5c). For FePO4 with no lithium the calculatedEELS shows a reverse shift in the Fe L2,3 edge, in agree-ment with Miao et al. [25], which our band structure calcu-lations suggest could potentially be due to the fact that Liacts as a donor in FePO4, causing a shift in the Fermi level.

Fig. 7 shows O p states, Fe d states, and P s and p statesand their hybridization based on band structure calcula-

tions. In a perfectly ionic state the O ions in FePO4 couldbe expected to have a valence of �2 with fully filled 2p

states. Covalent bonding, however, leads to hybridizationbetween the Fe 3d, O 2p, and P 3s and 3p states, giving riseto empty states (above the Fermi level) with some O 2pstates. This can be seen from Fig. 7, where the atomresolved DOS, obtained from first-principles calculations,

Fig. 5. (a) TEM images of unaged and aged LiFePO4 cathode samples showing the location of the EELS spectrum (b and c). (b) O K edge of the unagedand aged LiFePO4 cathode samples. A pre-peak in the case of the aged sample exists, suggesting a change in the density of states of O. (c) Fe L2,3 edge ofthe unaged and aged LiFePO4 cathode samples. The edges shift to the right in case of the aged sample. The EELS data was normalized to the O K edgepeak. The individual spectra are offset vertically in order to present the details.

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has been plotted for three compositions: FePO4,Li0.5FePO4, and LiFePO4. In all three compounds thevalence band just below the Fermi level (region 1 inFig. 7) has major contributions from the O 2p states withsome Fe 3d states. The conduction band just above theFermi level (region 2 in Fig. 7), on the other hand, is dom-inated by Fe 3d states with small contributions from O 2p

states. Moving to higher energy bands (>10 eV, region 3 inFig. 7), antibonding states are found with O 2p, 3s, and 3p

states. In FePO4 the contribution of O 2p states in region 2is significant and gives rise to the pre-peak A observed inthe simulated O K edge EELS spectra, shown in Fig. 6a,due to transitions from the 1s core orbital of oxygen. TheO 2p states present in region 3 gives rise to peaks B and

C seen in Fig. 6a, after core hole corrections which shiftthese states towards lower energies by approximately 3 eV.

While Li addition does not affect the positions of peaksB and C, it does lead to a continuous decrease in the inten-sity of pre-peak A, as observed in both the experimentaland simulated O K edge EELS spectra (Figs. 5b and 6).This can be attributed to the following two causes. Firstly,on addition of Li the Fe 3d states shift to higher energies(from �3–5 eV in FePO4 to �6–8 eV in LiFePO4) becauseof the reduction in the oxidation state of Fe from Fe3+ toFe2+ (region 2 in Fig. 7). Secondly, it leads to a continuousdecrease in the extent of hybridization between Fe 3d andO 2p states, such that the contribution of O 2p states inregion 2 becomes negligible in LiFePO4. This analysis is

Fig. 6. (a) The O K edge electron energy loss near edge structure (ELNES) calculated within the Z+1 approximation and dipole selection rules using theVienna ab-initio simulation package (VASP). (b) Analogous calculation of the Fe L2,3 edge ELNES. The energies are set at the Fermi energy, which is 0 eVon the x-axis. The ELNES intensities were normalized to the O K edge peak. The individual spectra were offset vertically in order to present the details.

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similar to the arguments put forward by Kinyanjui et al.[31], who studied only the end compounds. Based on thedecrease in intensity of pre-peak A with Li addition, thepre-peak is expected to be completely absent for Li concen-trations greater than �75%. Similarly, in the case of Fe L

edges the intensities of the sharp lines themselves scale withthe number of unoccupied 3d states of the Fe atom. In theaged sample the shift is �2 eV. According to the first prin-ciples simulations presented this would correspond to areduction in Li content of �80%. Thus, the FePO4 coreand LiFePO4 shell, co-exist within large particles, and asthe cell ages the FePO4 (low Li content) core expands whilethe LiFePO4 shell decreases.

The energy at the Fe L2,3 edge peak obtained from first-principles calculations is plotted against the Li concentra-tion in LixFePO4 in Fig. 8. The shift to higher energy inthe location of the Fe L2,3 edge peak follows a linear trend

with an approximate slope of �1.9 eV with increasing Liconcentration. Considering the experimental shift of�2 eV between the unaged and aged samples, this wouldindicate a Li loss of �80% in the aged sample. Since com-plete Li depletion from LiFePO4 would lead to a down-ward rather than the observed upward peak shift, thecalculation here indicates that the aged sample is not com-pletely Li depleted, but rather remains at a low Li contentof �20%.

5. Conclusion

TEM and EELS were used to study the effects of agingin the LiFeO4 cell. TEM revealed coarsening of the grainsas the cell ages. EELS showed that the density of states forO changes as the cell ages. This was evident in the presenceof the pre-peak in the case of the O K edge. The increase inthe ratio of the pre-peak to the O K edge peak in the EELSdata from the core to the surface of the large LiFePO4 par-ticle indicates different lithium composition within the par-ticle. There is also a shift of almost 2 eV for the L2,3 edge ofFe in the aged sample. At a certain age of the cell the par-ticles start to coarsen. The ion diffusion length is expectedto increase in the coarser particle, and this is expected tolead to an incomplete transformation between LiFePO4

and FePO4 while charging and discharging. The FePO4

core is expected to expand in subsequent cycles, thus reduc-ing the capacity of the host cathode material. The experi-mental results were analyzed with the help of simulatedELNES spectra for the olivine structure (space groupPnma) of LixFePO4 with x = 0, 0.25, 0.5, 0.75, and 1.The nature of the O pre-peak was confirmed by comparingthe position of the O 2p bands arising due to strong hybrid-ization among the O p states, Fe d states, and P s and p

states with the Fermi level position. The Li loss was

Fig. 7. O p states Fe d states, and P s and p states and their hybridization showing how the states move with Li concentration and thus give rise tovariations in the intensity of O K edge pre-peak. The black line is the Fermi level given by Vienna Ab-initio Simulation Package (VASP).

Fig. 8. Calculated energy at the Fe L2,3 edge peak vs. Li concentration inLixFePO4. The data fits a straight line with slope �1.9 eV. Based on thisplot it is estimated that the aged sample is Li0.2FePO4.

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quantified by these calculations as �80%, suggesting astrongly Li depleted, but not completely Li free, coreregion in the coarsened particles.

EELS has been successfully applied to study the effectsof phase transitions in the case of laboratory and experi-mental cells over an area of the order of a few nanometerssquare. However, as the studies reported here are based onreal size cells it was impractical to obtain several spectraover the large cathode sample to present evidence of thephenomenon on a more global scale.

Acknowledgements

The authors sincerely thank the Institute for MaterialsResearch (IMR) at The Ohio State University for provid-ing the financial support for this research. The authorsthank the Center for Emergent Materials at OSU, anNSF MRSEC (Award No. DMR-0820414), and DOE-NEUP (Award No. 09-842) for partial support. Computa-tions were performed at the Ohio Supercomputer Center(Grant No. PAS0072). The authors also thank Profs. YannGuezennec and Giorgio Rizzoni for providing the Li-ioncell samples and technical discussions. The authors wouldalso like to thank Ryan Dehoff for his support during theexperiments and Dr. Oscar Restrepo for helpful discussionsduring the theoretical analysis.

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