JES FOCUS ISSUE ON INTERCALATION COMPOUNDS FOR RECHARGEABLE BATTERIES
A Perspective on Coatings to Stabilize High-Voltage Cathodes:LiMn1.5Ni0.5O4 with Sub-Nanometer Lipon Cycled with LiPF6ElectrolyteYoongu Kim,a,∗,c,z Nancy J. Dudney,a,∗,z Miaofang Chi,a Surendra K. Martha,a,∗Jagjit Nanda,a,∗ Gabriel M. Veith,a,∗ and Chengdu Liangb,∗
aMaterials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USAbCenter for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
Manuscript submitted January 3, 2013; revised manuscript received March 6, 2013. Published March 21, 2013. This was Paper698 presented at the Honolulu, Hawaii, Meeting of the Society, October 7–12, 2012. This paper is part of the JES Focus Issue onIntercalation Compounds for Rechargeable Batteries.
In recent years a variety of coatings have been shown to enhancethe stability, particularly the capacity retention, of high voltage Li-ioncathodes where the advanced battery technology is pushing the limitsof the electrochemical and mechanical stability of both active andinactive materials as well as the electrolytes. However the engineeringof an ideal coating that extends performance of these cathodes tothousands of cycles with excursions to higher temperature, requires anunderstanding that is still lacking. A number of possible mechanismscan be proposed for coating-enhanced performance, and in fact theremay well be multiple coating properties that can be harnessed for morestable and high rate cycling. Achieving the maximum stability forhigh-voltage cathodes is critical to reducing excess cost, expense, andweight associated with the overcapacity built in to batteries requiringlong performance life.
The high-voltage LiMn1.5Ni0.5O4 spinel cathode (also noted asLMNO) is a promising candidate for rechargeable Li-ion batteriesbecause the high manganese (Mn) content in the cathode provides fora safer and less expensive cathode while the nickel (Ni) provides for ahigh voltage redox reaction. Performance of this spinel is highly de-pendent on details of cation ordering as well as stoichiometry,1–4 andexperimental trends are not fully settled. Many LiMn1.5Ni0.5O4 cath-odes suffer from rapid capacity degradations, particularly at elevatedtemperatures. Studies have attributed the capacity degradation to sev-eral possible mechanisms including: dissolution of Mn,5–7 electrolytedecomposition at high voltage,8 loss of crystallinity upon cycling,9
and if discharged to low voltages, a Jahn-Teller distortion at voltageslower than 3 V.10,11 Meanwhile, there are several reports of excel-lent cycling performance of LMNO spinel cathode powders, althoughthese seem to be the exceptions, rather than the norm. Shaju et al.have reported that nano-LiMn1.5Nn0.5O4 particles (∼15 m2/g) can becycled with excellent capacity retention at 50◦C.12 Two earlier reportsalso demonstrate excellent capacity retention and high coulomb ef-ficiency (>99%) for at least 50 cycles;13,14 these studies use coarserpowders of highly faceted 1–5 μm particles synthesized via spray py-rolysis and ionothermal synthesis. As each of these studies reported
∗Electrochemical Society Active Member.cPresent address: Infinite Power Solutions, Littleton, Colorado 80127, USA.zE-mail: [email protected]; [email protected]
no deliberate effort to modify the surface, the important question be-comes whether coating such high performance cathode powders canlead to further enhanced cycling stability and efficiency to meet longlifetime goals. To our knowledge, the required stability for the mostdemanding applications has not been demonstrated for either coatedor uncoated LiMn1.5Nn0.5O4 cathodes.
Examples of effective coatings for LiMn1.5Ni0.5O4 cathodes aregiven in Table I. Many coatings of inorganic compounds (insulators,semiconductors, electrolytes) were prepared by solution-based meth-ods followed by heat-treatment. While these bulk compounds arewell-known, the physical properties of thin coatings are not. Filmsprepared by atomic layer deposition (ALD)29 or by the physical vapordeposition, as described here, are exceptions in providing homoge-neous films that have been well studied independent of the cathodeapplication. In addition, processing that results in surface segregationis also an effective “coating”.30–33 Given these wide ranging reports,it is not surprising that a consensus toward the important interfaceor coating properties is lacking. Progress to understand the interface-related degradation and its amelioration requires systematic studywhere properties of the coating and cathode can be varied indepen-dently and widely, and both electronic wiring and ion transport can beensured. It is also important to appreciate that other inactive batterycomponents, such as the packaging, binder, separator, etc., may be im-plicated in degradation at high voltage and elevated temperatures.34,35
Eliminating such complication can be achieved with study of thin filmcathodes.35–39
We report here the effect of ultrathin Lipon coatings on high sur-face area LMNO powders. The comparison to earlier and ongoingstudies of Lipon coatings deposited onto layered, spinel, and Li-richcathode materials, and onto cathodes of different structures, includ-ing particles, agglomerates, thin films, and sintered pellets, providesvaluable insight.27,37,40,41 Lipon films have also been coated on com-posite cathodes of active particles with the usual binder and carbonadditives.26,42 Lipon, an amorphous lithium phosphorus oxynitride, isa well-studied thin film Li-ion electrolyte. Notably, Lipon has a Li-ionconductivity of 10−6 S · cm−1, a very low electronic conductivity of∼10−14 S · cm−1, and a ∼5.5 V stability window with lithium metal.43
These values were determined for films on the order of 0.1–1 μm-thick, however for Lipon of only ∼1 nm thickness, properties may
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A3114 Journal of The Electrochemical Society, 160 (5) A3113-A3125 (2013)
Table I. Lists of coating materials reported to stabilize 4-5 V cathodes, proposed degradation mechanisms and relevant coating properties. Eachlist is organized with regard to the conductivity; the relevant physical, chemical or mechanical property; or the operative scale being lattice,surface, or microstructure.
deviate. As illustrated in Fig. 1, a ∼1 nm film is only two PO4 groupsacross. Because the Lipon is deposited by magnetron sputtering atambient temperature, films 10 nm to 10 μm are dense, stronglyadhered, hard, and free of grain or columnar boundaries.43–45 Nopost-deposition processing is needed and films are routinely exposedto air for short periods of time. These new results show that com-pared to uncoated powders, cathodes with ∼1 nm Lipon coatingsalleviated cycling instability in LiMn1.5Ni0.5O4 cells reducing capac-ity losses at room temperature and at 60◦C. Further, the Lipon-coatedLiMn1.5Ni0.5O4 cells retained good capacities up to the 5C dischargerate. But more importantly, these results will contribute to our un-derstanding of the performance of a thin, solid electrolyte, single-ionconductive coating for high voltage Li-ion cathodes.
Experimental
LiMn1.5Ni0.5O4 (LMNO) powders were obtained from nGimat Co.(GA, USA). During the course of this study, powder was obtainedfrom two different batches. The powder was dried in a 120◦C oven forseveral hours before Lipon coating or slurry preparation. The Liponelectrolyte layers were prepared by RF-magnetron sputtering (80 W)
Figure 1. Unit structure of γ-Li3PO4.
of a Li3PO4 target in a reactive N2 atmosphere (20 mTorr). The pow-der was evacuated for several hours before initiating the deposition.During Lipon deposition, the 1.0-g batch of LiMn1.5Ni0.5O4 powderwas contained in a 4.4-cm Al foil pan and mechanically agitated byvibration of an audio speaker diaphragm to expose all the surfacesto the Lipon vapor flux. More detailed experimental procedures canbe found elsewhere.27,46 The Li3PO4 target was prepared by pressingthe Li3PO4 powder (99.99%, Litho, Division of FMC Corporation)and sintering it as 2.0-inch diameter × 1/8th inch disks with >85%density. This was bonded to a 2.0-inch diameter Cu disk.
The crystal and bond structures of LiMn1.5Ni0.5O4 particles wererecorded with X-ray diffraction (XRD) and Fourier transform in-frared spectroscopy (FTIR). The XRD pattern was scanned with the2θ angle between 10◦−70◦ using a Scintag theta-theta diffractome-ter having Cu Kα radiation with 1.5418 Å. The FTIR spectrum wasmeasured in the 400–4000 cm−1 range with 8-cm−1 resolutions in anitrogen-purged Fourier transform spectra, FTS-575C by Varian Bio-Rad. The infrared sample was prepared as a pressed pellet of KBrmixed with 0.3–0.5 mg of LiMn1.5Ni0.5O4 powder. The Mn/Ni ratio inLiMn1.5Ni0.5O4 particles was analyzed with energy dispersive X-rayspectroscopy (EDS). This EDS technique was also used to analyzesurface elements of the rinsed/cycled cathode samples. The surfacemorphologies of uncoated and Lipon-coated LiMn1.5Ni0.5O4 particleswere observed using JSM 840 scanning electron microscopy (SEM).The N2 Brynauer-Emmett-Teller (BET) surface area was measuredusing a Quantachrome Instruments Autosorb 1C instrument. Trans-mission electron microscopy (TEM) experiments were carried out onan aberration corrected FEI Titan 60/300-kV microscope. High reso-lution TEM imaging was employed to observe the Lipon thicknessesof Lipon-coated LiMn1.5Ni0.5O4 particles that were dry-dispersed onlacey carbon TEM grids.
For electrochemical measurements, uncoated and Lipon-coatedLiMn1.5Ni0.5O4 cathodes were fabricated on an Al foil from a slurryusing a doctor blade and dried at 90◦C in a vacuum oven for 10 hr.The slurry consisted of active materials (∼80 wt%), conductive Super-S carbon black (∼10 wt%) and poly(vinylidene difluoride) (PVDF)binder (∼10 wt%) in N-methylpyrrolidinone (NMP). To avoid damageof the coated/uncoated LMNO agglomerates, high-energy ballmillingand calendaring processes were avoided. The dry powders were firstshaken with a zirconia ball and Vortex mixer for 10 minutes, thenthe NMP slurry was thoroughly mixed overnight by stirring witha magnetic stir bar. The coated Al foil was punched into a 1.0-cm diameter circular disk. The loadings of active materials werelow at 1.7−3 mg · cm−2 compared to 5–12 mg · cm−2 for typical
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cathodes.1,15,18,21 Coin cells were assembled in an Ar-filled glove boxusing a LiMn1.5Ni0.5O4 / 2325 Celgard separator / Li metal stacksaturated with 1.2M LiPF6 + ethylene carbonate + ethyl methyl car-bonate (EC:EMC 3:7 v/v; 3.5 ppm H2O; 2.0 ppm HF) electrolyte(Ferro Corp.). A Maccor battery test system (Series 4000) was used tocharge the cells to 4.9 V at a constant C/5 rate, continuing the chargeby holding it for 60 min at 4.9 V. Following a 10 min rest period,the cells were discharged to 3.5 V with a constant C/5 rate. Un-coated and Lipon-coated LiMn1.5Ni0.5O4 cells were also dischargedby various C rates in the 0.1−5C range with the same C/5 con-stant current + constant voltage (CCCV) charging described above.Electrochemical impedance spectroscopy (EIS) of uncoated andLipon-coated LiMn1.5Ni0.5O4 samples was monitored at 3.5 V, 4.7 V,and 4.9 V periodically during cycles, targeting single phase compo-sitions at 0, 50, and 100% state of charge. EIS spectra were collectedusing a Solartron (1470E/1455A) cell test system. The EIS spectrawere measured over 0.05−106 Hz with applied AC signal of 10 mV.Three electrodes with a Li reference were used to measure the EISspectra.
Mn dissolutions were measured using a Thermo Jarrell Ash IRISinductively coupled plasma optical emission spectroscopy (ICP-OES)spectrometer. Calibration curves were generated with serial dilutionof a standard solution of Mn in 5-wt% HNO3 from Alfa Aesar (WardHill, MA). Approximately 80-mg of uncoated and Lipon-coatedLiMn1.5Ni0.5O4 powders were mixed with a 3-ml LiPF6 EC/EMCelectrolyte solution and then stirred with a magnetic stirring bar at60◦C for 10 days. For ICP-OES measurement, 0.2-ml of the stirredelectrolyte solutions were diluted with 10-ml 5-wt% HNO3.
Results and Discussion
Morphologies of Lipon-coated LiMn1.5Ni0.5O4 particles.— Thebare and Lipon-coated LiMn1.5Ni0.5O4 particles are shown inFig. 2; because the Lipon is very thin, the powders appear identical.The LiMn1.5Ni0.5O4 powder is composed of secondary agglomerates,∼15-μm diameter, formed of primary particles with ∼100-nm di-ameters (Fig. 2c). The BET surface area measured for our powderswas approximately 8.7 m2 · g−1, which is large (with one exception12)compared to most other LMNO spinel powders reported in literature.Based on this value, the calculated average diameter of the primaryparticles is approximately 150 nm assuming near spherical particlesand a bulk density of ∼4.47 g · cm−3.47 This result is close to thedimensions observed by TEM (Fig. 2c) and suggests that the agglom-erates are porous throughout.
The Lipon layers were confirmed to be approximately a nanome-ter thick by ICP analysis and TEM. Lipon dissolved from theLiMn1.5Ni0.5O4 particles and analyzed by ICP-OES show that theP concentration is proportional to the deposition time (Fig. 3a). TEMimage of Fig. 3b shows that the thickest film, deposited for 6 hours, isapproximately 1.5-nm thick with good uniformity. However, it is en-tirely reasonable that only the outer-most surface of the agglomerates,shown in Fig. 2, are actually coated by a 1.5-nm film, leaving muchof the total BET surface area uncoated. Estimating that the dissolvedP is from just ∼10% of the BET surface area, brings the ICP anal-ysis and TEM observation for the 6-hr sample into good agreement.For the shorter 2 hr deposition, TEM images show a proportionallythinner sub-nanometer Lipon coating (Fig. 3c), the Lipon films ap-pearing fairly uniform, but perhaps not entirely continuous. For thethinnest films from a 0.5-hr coating, the Lipon layer was almost invis-ible by directly observing the edge-on amorphous layer on the grains(Fig. 3d). Figure 3e shows the inversed FFT image formed by filter-ing out crystalline signal from Fig. 3d. The amorphous contrast thusindicates the existence of an amorphous layer on the grain, whichis most likely the Lipon layer. If the interfaces between Lipon andLiMn1.5Ni0.5O4 are assumed to be the edges of the LiMn1.5Ni0.5O4
lattices, the Lipon layer is approximately 0.6 nm thick or less.Compared to various surface coatings prepared in other laborato-
ries, these Lipon films are noted by good uniformity and adhesion,although limited to the outer surfaces of the agglomerates. The sput-
Figure 2. SEM images (a, b) of uncoated and Lipon-coated LiMn1.5Ni0.5O4particles and TEM images (c) of pristine LiMn1.5Ni0.5O4 particles. The scalebars in the inset figures indicate 1 μm.
tered vapor species are unlikely to penetrate the porous agglomeratebefore condensing onto a surface. We expect that solution-based coat-ings will similarly be limited to the most accessible surfaces becausethe initial coating materials are in gel states or have a high viscosityof ∼10 Pa · s.18,48 Further, solution processing often leaves a roughcoating morphology observed by SEM of the outer surfaces of theagglomerated powders.18,49 Further examination of Figure 3 showsthat the Lipon layers prepared here form strong pore-free adhesionon the surfaces of the LiMn1.5Ni0.5O4 particles. In addition, our re-cent TEM results have also shown that Lipon prepared on non-porousmicron-sized LiCoO2 particles have pore-free dense layers.27 Liponlayers with strong adhesion and uniform coating are likely to have amore pronounced influence on the cathode properties. Arumugam etal. indicated that weak adhesion of coating materials was a problemfor retaining capacity during prolonged cycles and attempted to im-prove adhesion of coating materials.50 Recently, Wu et al. report thatthe porous ZrP2O7 coatings generate poor cycling performances at55◦C because their interfacial pores do not protect cathode surfacesin contact with a liquid electrolyte.18
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Figure 3. Lipon coating as a function of the sputter coating time. The amounts of dissolved phosphorus determined by ICP-OES technique is shown in (a);high-resolution TEM cross-sections in (b-d). The inversed FFT image (e) was formed by filtering out the crystalline signal from (d).
Crystal structures of uncoated LiMn1.5Ni0.5O4 particles.— Fig-ure 4a shows the XRD pattern of as-received LiMn1.5Ni0.5O4 powder.The lattice parameter of the particles was 8.1709 Å, which falls be-tween values reported for ordered (P4332) and disordered (Fd3m)spinels.1,12,51 The star symbols of the XRD pattern indicate presenceof trace amounts of the LixNiyOz phase; this has been correlatedwith the disordered phase.3 As will be shown below, the shape ofthe discharge curves for our LMNO powder is also consistent with adisordered spinel.
As shown in Figure 4b, the FTIR spectrum of the as-receivedLiMn1.5Ni0.5O4 particles is characteristic of the disordered spinel inthe 400–700 cm−1 range, as bands are broad, poorly resolved, andMn-O A1g is the most intense.1 Our FTIR also shows a very weak OHpeak at around 3200 cm−1
, suggesting only a trace of water, muchlower compared to similar measurements.1 The sample was preparedwith less than 0.5 mg of LMNO powder in the KBr, so we cannot bemore quantitative. Adsorbed water in the current study is likely furtherreduced when the composite cathode electrodes were heated at 90◦Covernight in a vacuum oven, and also during the vacuum coatingwith Lipon. While water contamination can have a strong effect onelectrolyte stability, see section 3.7, we believe residual water fromour cathode powder is small with or without the Lipon coating.
We note that cathodes prepared with the second batch of the LMNOpowder indicate that the 2nd batch has a higher degree of orderedspinel structure. As shown in the next section, the voltage profilesdiffer slightly in that there is no distinct step in the 4.7 V range volt-
age, the cell voltage is slightly higher, and the 4 V plateau slightlysmaller. Regardless of the cause, cathodes from the second batch wereobserved to provide a more stable discharge capacity with extendedcycling, even without the Lipon coating. Consequently, the influenceof the Lipon coating was not dramatic and requires more precise ex-perimental analysis. Such variation among LiMn1.5Ni0.5O4 materialsis consistent with reports in the literature, but there is still conflictinginformation with regard to surface morphology, ordering of the Mnand Ni, and cycling performance.3,4,52 Because of such complications,we will not present results from batch 2 at this time.
Lipon thickness and capacities of LiMn1.5Ni0.5O4 cells.— Figure 5shows the discharge capacities for LiMn1.5Ni0.5O4 with differentLipon thickness. The thinner Lipon layers provided the expectedcapacity of ∼120 mAh · g−1 that is comparable to the uncoatedLiMn1.5Ni0.5O4 cathodes. However, when the Lipon deposition timewas increased from 0.5-hr to 2.0-hr, the discharge capacity decreasedto ∼98 mAh · g−1. Further increasing the Lipon deposition time re-sulted in a capacity of just 20 mAh · g−1. This can be attributed to thethicker Lipon, which has a very low electron conductivity of ∼10−14
S · cm−1.43 When Lipon layers are thinner than 1-nm, electrons maypass along pores or tunnel through Lipon itself. Supporting this vieware similar results for conformal Al2O3 layers prepared by atomiclayer deposition (ALD) on LiCoO2 particle surfaces16 and studiesof electron tunneling in the ADL films.53 The Al2O3 coatings withgreater than 1-nm thickness effectively blocked electron currents and
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Figure 4. XRD pattern and FTIR spectrum of uncoated LiMn1.5Ni0.5O4 par-ticles. Absorbance scale is the same for both sections of FTIR scan.
decreased the capacities of LiCoO2 cathode cells. Recently, we alsoreported that thicker Lipon layers reduce capacities of LiCoO2 pow-der cells27 where the Lipon uniformly coats the micron-scale particles.Efforts are underway to assess tunneling and breakdown propertiesof ultra-thin Lipon films. Results published in this regard for fiberconstructed batteries were misinterpreted.54 All cycling performanceresults below are for powders with the thinnest, 0.5-hr, coating ofLipon.
Figure 5. Discharge curves of bare and Lipon-coated LiMn1.5Ni0.5O4 cathodecells along Lipon deposition times. Discharge rate was C/5; all curves are the1st cycle. The dashed curve is for batch #2 of cathode powder.
Figure 6. Discharge capacities of uncoated and Lipon-coated LiMn1.5Ni0.5O4during cycling at room temperature (a) with selected discharge curves (b, c).All cells were cycled at C/5, except for results shown as the inset of the (a)where additional cathodes were cycled at C/3. Lipon-coated shown in red andfilled symbols. Initial charge shown as dashed curve.
Cell performances of uncoated and Lipon-coatedLiMn1.5Ni0.5O4.— Figure 6 and 7 show collected cycling per-formances for several cells with coated and uncoated LiMn1.5Ni0.5O4
cells at room temperature and 60◦C. Although there is cell-to-cell andLMNO batch-to-batch variation, the effect of the thin Lipon coatingto improve the cycling stability is clear. Figure 5a shows collectedresults for a number of cells cycled at C/5 with additional cells
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Figure 7. Discharge capacities of uncoated and Lipon-coated LiMn1.5Ni0.5O4at 60◦C. See caption of Fig. 6.
cycled at C/3 as the inset. Selected voltage profiles for the better cellsare shown in Figs. 5b and 5c where it is clear that the cathodecomposed of the Lipon coated powders maintained a higher voltageand capacity for the first 50 cycles at C/5. Similar voltage profiles
Table II. Average coulomb efficiency recorded for cycles 5 to 50(or 100) cycles.
Coulomb efficiency*
Temperature Cycle rate Uncoated Lipon coated (0.5-hr)
*Charging included a constant voltage 1-hour hold at 4.9 V. No CVholds upon discharge.
were observed for C/3 to 100 cycles. The extended cycles clearlyshow that the ultra-thin Lipon improved the cycling stability in the3.5−4.9 V range. When the LiMn1.5Ni0.5O4 cells were cycled at 60◦C,the Lipon-coated cell again showed superior cycling performancescompared to the bare powders (Fig. 6). The uncoated LiMn1.5Ni0.5O4
cell cycled at 60◦C degraded rapidly; on the other hand, the capacityand voltage shape of the Lipon-coated LiMn1.5Ni0.5O4 cell weremaintained reasonably well at 60◦C to 50 cycles (Fig. 6b, 6c).Although improvement from the Lipon is striking, the performancestill degrades more than desired for demanding applications andshows no indication of slowing due to passivation.
The coulombic efficiency is another measure of the stability. Theaverage coulomb efficiencies, calculated as discharge/charge capacityfor cycles 10 to 50 or 100, are tabulated in Table II for representativecells from Figs 6 and 7. These efficiencies are steady during cycling,but are surprisingly low. The discharge capacity retention with cycling(Fig. 6) of the bare and Lipon-coated cathodes is far better than what-ever process(es) accounts for the coulomb inefficiency. Excess chargeis clearly consumed for each charge cycle, which at this time is at-tributed to side or shuttle reactions occurring at the electrodes or othercell components, such as the metal can, carbon, and Al foil whichare also charged to 4.9 V. The electrochemical voltage window of the1.2M LiPF6 EC/EMC electrolyte used here is the highest commonlyavailable55,56 without special additives, but charging to 4.9 V exceedsthe normal operation window. In general, reactions occur when elec-trode voltages are outside of electrolyte reduction-oxidation voltagewindow.57 A recent publication demonstrated this to be the case for thestainless coin cell components.34 Others observe that the electrolytedecomposition correlates with the presence/absence of the organicbinder and carbon black in the cathode.35 As will be noted below, ourcells suffer an increase of the ohmic cathode resistance with cyclingthat suggests possible corrosion of the current collector. Recent re-sults investigating the coulomb efficiency in much more detail will bepublished separately.58
C-rate dependences of uncoated and Lipon-coated LiMn1.5Ni0.5O4
cells.— As shown in Figure 8, the capacities realized at high C-rateswere far improved for Lipon-coated LMNO at C-rates to 5C. It is notunusual to observe an enhanced high rate performance due to a coating.A similar improvement in rate performance (to 2C) was reported forthe perovskite lithium lanthanum titanate (LLTO) coatings on NCAcathodes,28 for Bi2O3 and Al2O3 coatings of NMC spinel to 10C;59
and we see a similar effect for Lipon coated Li-rich NMC.41 In thiswork the effect of the Lipon coating is the most dramatic for 1C to5C, where more than ∼75% of capacity is utilized at these high rates.This high C-rate performance of the Lipon-coated cathodes persistedwith extended cycling, even at higher temperatures.
In addition to surface coatings, a number of factors are known tocorrelate with the rate performance of this high voltage spinel cathodemaking it difficult to compare our results with published performance.These include the electronic conductivities and the amounts of carbonadditives,2 the Mn/Ni ratio and the oxygen stoichiometry,60 and latticeparameters related to extent of disorder of Ni-Mn on the lattice.2,61
Most materials may be mixtures of the ordered (P4332) and disordered
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Figure 8. C-rate tests of uncoated (black) and Lipon-coated (red, filled) LiMn1.5Ni0.5O4 particles at room temperature and 60◦C. Results are shown as cyclenumber (a), and also as C-rate (b, c) where lines connect capacities cycles 1–12 and 40–52. Selected room temperature discharge curves are shown (d).
(Fd3m) phases which are indistinguishable by powder XRD.3 For ourmaterials, the lattice parameter is close to that of the ordered phasewhich may limit the electronic conductivities and C-rate performancesof our cells. Also our cathode formulation has only 10% carbon black,which is small compared to other examples. Overall rate performanceof our Lipon-coated cathode is good, but a number of uncoated LMNOcathodes are also reported to cycle well at 5C and 10C.
EIS spectra of uncoated and Lipon-coated LiMn1.5Ni0.5O4.—With strong influence of Lipon-coating shown in last sections, theimpedance analysis serves to resolve contributions of surface andbulk to this performance. All EIS measurements were carried out in a3-electrode configuration using an additional lithium reference elec-trode. Here we only studied the changes in EIS on the cathode. InFigure 9 we show the EIS spectra measured at 4.7 V; this is nearthe midpoint of the state of charge where the LiMn1.5Ni0.5O4 is ex-pected to be single phase and homogeneous.1 Regardless of the Liponpresence, Figures 9a, 9b show one semicircle at mid-frequency whichgrows larger with continued cycling. These semicircles correspondto resistances (RSEI) of the solid-electrolyte interphase (SEI) due toreaction layers formed at the cathode interface and also the Liponcoating.62 The circuit model used to fit the EIS spectra is given inFig. 9c, with selected results tabulated in Table III. The low-frequencytails correspond to lattice diffusion, represented by the Warburg el-ement, and the high-frequency resistance (RHF) corresponds to thecombined resistances of a liquid electrolyte, Al foil current collector,and other cell connections.
Examining first the SEI component, the presence of the sub-nanometer Lipon coating apparently adds ∼180 � to the initial RSEI.However upon cycling, the RSEI resistance of the uncoated cell grewrapidly and surpassed that of the Lipon coated cathode. Fig. 9d showsthis increase in resistance normalized to the initial value at the firstcharge. The growth of the interphase or reconstruction of the inter-face that forms the resistive SEI layer is effectively impeded by thepresence of the Lipon coating. The capacitances if normalized by theBET surface area are on the order of 0.1 μF/cm2. The RHF resistancesof 4−6.5 �, which represents a far smaller contribution to the overallcell resistance, also increased with cycling suggesting either a highercontact resistance of the Al or metal pieces in the coin cell, or perhapsan increase in the resistivity of the liquid electrolyte due to dryingor a change in composition. A Bode plot of the Real component,Re(Z*), Fig. 9e, reveals these changes more clearly than the Nyquistplots.
The Warburg component is assessed with the last plot, Fig. 9f.Here the lower frequency Real and Imaginary components of theimpedance for 1st and 50th cycles of the Lipon coated and uncoatedcathode are plotted as reciprocal of the radial frequency. The fact thatthe Re(Z) and Im(Z) are both linear and nearly parallel for each dataset confirms that the charge-discharge process is diffusion limited atthis voltage. With proper account of the thermodynamic and geometricparameters, the chemical diffusion can be determined from the slopes(diffusivity goes as 1/slope2). Although such determinations presentlarge uncertainty,63 here it suffices to see that the lithium diffusivityfor LMNO probably changes little with either the Lipon coating or
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Figure 9. EIS results determined at 4.7 V androom temperature. Uncoated and Lipon-coatcathode cells are shown in (a) and (b) with thecorresponding Bode and Warburg type plots in(e) and (f). The normalized values of the SEIresistance are plotted in (c). The equivalentcircuit used to fit results is shown in (d).
with extended cycling. This is important as it supports our implicitassertion that the sputtering process to form the Lipon coating doesnot change the bulk of the LMNO powder. Further, our results areconsistent with a study by PITT study of the Li chemical diffusivity inLMNO thin films cathodes reporting values of 10−12 to 10−11 cm2/swith a strong dependence on the voltage 4.6–4.8 V.36
The EIS scans for cells equilibrated at 3.5 V and 4.9 V are shown forjust the initial tests in Figs. 10a and 10b. The mid- to high-frequencyresponses are independent of the cell voltage, confirming our assign-ment of this behavior to the RLipon and RSEI. The low-frequency regionsof the plots are more complex. Warburg plots constructed for samplesequilibrated at 3.5 and 4.9 V do not exhibit the nice linear dependenceover a wide frequency range, as for Fig. 9e, so charge transfer ratherthan diffusion processes control the response under full charge and
Table III. Resistance and capacitance determined for RC elementsof SEI(+Lipon) as determined by Zview fits to the circuit modelshown in Figure 8d.
RSEI (�); CSEI (μF)
Uncoated Lipon (0.5-hr) coated
1st cycle, 25◦C 41 μF; 163 � 25 μF; 344 �
50 cycles, 25◦C 14 μF; 756 � 13 μF; 684 �
full discharge conditions. The second semicircle for 4.9 V reflectsthe charge transfer process at the cathode, which again is made moreresistive by the Lipon coating. For 3.5 V, results are complicated bybeing at the steep shoulder of the voltage profile, as charge transferresistance increases with filling of the lattice sites.
It is at first surprising that the Lipon coating adds significant in-terfacial resistance, as measured by the EIS, yet the Lipon-coatedcathodes exhibit a much better high power performance and more sta-ble cycling even at the 1st cycle. This is understood as the differencein DC cycling and the AC response with the non-ohmic propertiesof the interface. The applied AC signal is only 10 mV for the EIS,resulting in currents for the equilibrated cathode that are far less thanthose for the battery cycling with polarization voltages of 50–100 mV.The comparison of EIS and cell cycling behavior is also complicatedby the rapid aging of the interface properties for the uncoated LMNOcathodes.
Post cycling and ex-situ characterization.— From the EIS and cy-cling results it is clear that the sub-nanometer Lipon coating slows,but does not stop, reactions or structural changes which increase theresistance at the interface. Post cycling examination of the cathodeinterface can help separate these possibilities. As shown in Figure11, the EDS spectra of the cycled cathodes’ surfaces include highfluorine, phosphorus and oxygen, indicating that reaction productsmay consist of solid compounds such as LixOy, LiF or LixPFyOz. The
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Journal of The Electrochemical Society, 160 (5) A3113-A3125 (2013) A3121
Figure 10. Complex impedance for cells equilibrated at 3.5, 4.7, and 4.9 V.Uncoated LiMn1.5Ni0.5O4 (a); Lipon coated LiMn1.5Ni0.5O4 (b). Data wasrecorded for 1st cycle. For clarity, some data points are omitted.
Figure 11. EDS spectra of cycled uncoated (black curve) and Lipon-coated(gray shaded) LiMn1.5Ni0.5O4 cathodes. Microscope, detector and collectionparameters were identical for each scan. A typical spectra for fresh uncycledLMNO cathode is included (red curve) for comparison.
Figure 12. XRD patterns of LiMn1.5Ni0.5O4 cathodes comparing the freshuncoated cathode (black) with cycled uncoated (blue) and cycled Lipon-coated(red) cathodes. The cycled cathodes were charged/discharged to 100 times atroom temperature.
Lipon-coated cathode had a similar and perhaps slightly thinner reac-tion layer compared to the uncoated cathode. Compared to our earlierresults of LiCoO2 powders cycled to just 4.4 V,27 the reaction fromcycling LMNO to 4.9 V produces a thicker and more resistive reactionlayer. Postmortem analysis by EDS and XPS is much simpler withthin film cathodes which are free of binder and carbon additives.35,64,65
Selected X-ray reflections of cathodes following 100 cycles areshown in Fig. 12. While there is no evidence of secondary phases orserious change in crystallinity by XRD, there were small changes inpeak width and height. In particular, the peak intensities of the (400)and (311) increased relative to that of the (111) reflection, more sofor the uncoated than for the Lipon-coated cathode. Qualitatively thisindicates that Lipon may have reduced the cycling induced structuralchanges of the spinel lattice. The loss of oxygen, the increase of latticedefects, and the formation of a secondary rock-salt phase have beenproposed as factors of capacity degradation.4,9,66,67 Goodenough et al.have indicated that oxygen can be generated from cathode materialswhen energy bands of transition-metal redox couples are pinned onthe top of oxygen energy band.57 In case of LiMn1.5Ni0.5O4 cathode,the energy band of the Ni4+/Ni3+ redox couple is pinned on the en-ergy band of oxygen at ∼5.0 V,31 suggesting decomposition is likely.While layered intercalation cathodes are known to be unstable whenfully delithiated at high voltage, reports of oxygen loss and formationof lattice defects at the surface of the spinel cathodes charged to high
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A3122 Journal of The Electrochemical Society, 160 (5) A3113-A3125 (2013)
Figure 13. Selected discharge cycles during C/5 cycling over 4.9 to 2.0 V foruncoated (a) and Lipon coated (b) LMNO cathodes.
voltages are less evident. On the other end of the voltage window how-ever, cycling spinel cathodes, including the current LMNO cathodes,through the low (2–3 V) voltage range leads to rapid degradation ofboth the structure and the capacity due to the tetragonal phase transfor-mation. After 50 cycles to 2 V, about half of the capacity for the currentcathodes is lost for both Lipon-coated and uncoated LiMn1.5Ni0.5O4
cathodes, as shown in Fig. 13.An interesting ex-situ experiment is summarized in Table IV. Here
the standard LiPF6 (EC:EMC) electrolyte (∼2.0 ppm HF; ∼3.5 ppm
Table IV. Appearance of electrolyte + additive after 2 weeks at80◦C.
Figure 14. Mn dissolution of uncoated and Lipon-coated LiMn1.5Ni0.5O4powders stirred at 60◦C for 10 days. The dissolved Mn was normalized to theMn content of the untreated powder.
H2O) and samples of various coating materials were sealed in borosil-icate glass NMR tubes; no LMNO was added. After 2 weeks at 80◦C,the only electrolyte to remain colorless contained a Lipon film de-posited onto Al foil. The Li3PO4 sample also has less darkening.A number of studies have looked at the thermal stability of LiPF6
carbonate electrolytes at elevated temperatures.68–71 Although not en-tirely settled, decomposition of LiPF6 to LiF+PF5, then further theautocatalytic reaction of PF5 promoted by proton containing impu-rities (H2O and ethanol) are associated with thermal decomposition.Additives, which may include the cathode itself, are proposed to re-versibly sequester the PF5 or scavenge the proton impurity to stabilizethe electrolyte at temperatures comparable to 80◦C and higher. Thesepossible reactions of the Lipon with the electrolyte require furtherstudy, but this is clearly one mechanism for stabilization.
Many investigators have concluded that their coatings on varioushigh voltage cathodes act as a reactant to either getter acidic species(such as HF) from the electrolyte or scavenge the reaction productsto form a protective yet conductive SEI barrier. For layered cath-odes, this includes work of Lu with AlPO4-coated LiCoO2 cycled to4.7 V,20 recent work of Martha for the Li-rich layered cathode cy-cled to 4.9 V,72 and others. Lu included evidence that the AlPO4 doesnot need to be in direct contact with the LiCoO2 to aid in forminga protected interface. Typical reaction layers were reported to con-tain many different compounds, polymerized organic and inorganic.For 5 V spinel cathodes, Liu and Manthiram showed that Al2O3-,ZnO-, Bi2O3- and AlPO4-coating all slow the HF-forming electrolytedecomposition reaction leaving thinner and less resistive SEI layers.Previously, Sun et al. demonstrated that ZnO-coated LiMn1.5Ni0.5O4
powders decreased residual HF in 1M LiPF6 EC/DMC electrolyte.21
Sun’s group has also shown that BiOF coatings minimize the RSEI re-sistances and HF concentration in LiMn1.5Ni0.5O4 cells.24 They haveproposed that the BiOF coatings scavenge the HF and protect elec-trode surfaces. Recently, Oh et al. proposed that Al2O3 coatings canscavenge H2O and HF in 1M LiPF6 EC/DEC electrolytes when theAl2O3-coated LiCoO2 cells are cycled to 4.6 V.73 If coating materialsprimarily act as scavengers, more effective and efficient means towardgood performance may lie with further purification of the electrolyte,drying of components, or incorporation of getters in forms other thancathode coatings.
Mn dissolution of uncoated and lipon-coated LiMn1.5Ni0.5O4.—Figure 14 shows the Mn dissolution from uncoated and Lipon-coatedLiMn1.5Ni0.5O4 powders when fresh powder was soaked in elec-trolyte. The Mn dissolves readily at higher temperature, 10–17 timesmore than at room temperature. Lipon coating appears to inhibit this
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Journal of The Electrochemical Society, 160 (5) A3113-A3125 (2013) A3123
process, although the correlation with Lipon thickness is weak. Giventhe agglomerated nature of our LMNO powders, it is surprising thatLipon covering only ∼10% of the BET area provides any sort ofeffective barrier, but the Lipon coating may also plug some of thepores and alter the electrolyte chemistry, as discussed above. Experi-ments using thin-film cathodes with a dense structure will help resolvesome of these issues. Our previous study using LiMn2O4 film cath-odes indicated that Lipon films prevented both the Mn dissolutions,as well as its migration to the anode when the Lipon is coated ontothe separator.40 Dissolved Mn may have a large detrimental effect onthe cell performance when it migrates and deposits at the lithium orgraphite anode.
Many reports have indicated that Mn dissolutions are one ofthe reasons causing capacity degradations in LiMn2O4-based spinelcathodes.5,6 The Mn dissolution has been explained by the dispro-portionation of Mn3+ to Mn4+ and the soluble Mn2+.74 Collectively,the studies show that Mn dissolution depends on the temperature, thestate of charge, and the spinel composition, as well as the presence/absence of a surface coating. Jang et al. concluded that Mn dissolvedmore readily from the charged than the discharged states in spinel-cathode materials due to electrolyte instability at high voltages.5 Choiet al. demonstrated that LiMn1.5Ni0.5O4 powders had much less Mndissolution than LiMn2O4 powders,6 however Talyosef et al. observedthat Mn and Ni dissolutions of LiMn1.5Ni0.5O4 proceeded upon 45-daystorage at 60◦C to the point of leaving surface phases of λ-MnO2.
7
Kang et al. showed that BiOF coatings reduced Mn and Ni dissolu-tions at the 5 V charged states of LiMn1.5Ni0.5O4 cells.24 As stated in3.7, we see no evidence of λ-MnO2 or other phases after prolongedcycling.
Summary of results.— � The untreated LMNO has a particularlyhigh surface area compared to most other research powders, and ishighly agglomerated. Only a trace of water or hydrogen contaminationis present on the surface. Some cathodes prepared of untreated LMNOpowders cycle better than others, but all show significant capacity fadebefore 100 cycles.
� Lipon coating is sub-nanometer thick, confined to outer areas ofLMNO agglomerates which constitute ∼10% of the BET surface area.The sub-nanometer Lipon does not block the electronic connections tothe current collector, but thicker Lipon coating leaves powder insulatedand inactive. (The carbon, binder, foil and cathode terminal are notLipon coated.)
� The Lipon coating doubles the resistance attributed to thecathode-electrolyte interface of a fresh cell.
� Lipon coating effectively slows the increase of resistance at thecathode-electrolyte interface, thereby improving both the rate perfor-mance and the capacity retention with cycling. Lipon is also effectiveat 60◦C where the increase of resistance for the uncoated material ismore rapid. Post cycling characterization of the interface suggests aslightly thinner reaction layer for the Lipon coated cathodes.
� Although the capacity fade is reduced by Lipon, the coulombicefficiency indicates that side reactions are still operable and may beconsuming capacity or electrolyte.
� Lipon coating slightly reduces the Mn dissolution upon soakingin the electrolyte. Mn dissolution is more rapid at 60◦C.
� Lipon does not slow degradation when the spinel is cycled tolow voltage (2 V) through the tetragonal phase transformation.
Perspectives on Mechanisms and Future Directions
It is intriguing that a minute coating or additive of the right materialcan have a dramatic effect on the cycling of a high voltage battery, yetalso frustrating that the performance of coated cathodes still fall shortof ensuring a long-lived 5 V battery. Without clear understanding ofhow coatings alter interface mechanism(s), it is difficult to projectperformance or even productive paths to optimize a synthetic surfacecoating. Although there are many studies addressing degradation inrechargeable Li-ion batteries with high voltage cathodes, it is actuallydifficult to pinpoint the process(es) leading to degradation that are
minimized by an interface coating. Charging to high voltages pushesthe limits for stability of both the cathode and the electrolyte, so thereare many possible mechanisms to unravel. Table I summarizes pro-posed mechanisms (bulk, interface, and micro-structural), that if al-lowed to occur, will degrade cell capacity, efficiency and performance.The table also lists coating properties with effects on transport, chem-istry, defects and mechanics. To design effective synthetic cathodecoatings, it is important to determine which of the coating propertiesare critical to ensure the best performance, also recognizing that bulkaging processes within the cathode are likely to be unimpeded by anyinterface coating.
The most dramatic and unambiguous result from this study, is thata very thin Lipon film can slow the growth of the resistance at the inter-face with the electrolyte. This is the clear conclusion from the voltageprofile with cycle number, the EIS, and the C-rate performance atboth room temperature and 60◦C. This is consistent with other reportsof increasing interface resistances with cycle numbers.8,75,76 How-ever, although the sub-nanometer Lipon coating slows degradation, itdoes not stop the increase of resistance which may involve thicken-ing and/or increasing resistivity of the interface reaction layer(s). Fullpassivation is needed for extended battery performance of many thou-sands of cycles. Generally passivation occurs when the scale becomesthick or dense, but may also be due to full consumption of a reactant orloss of active sites. Typically, studies of coated cathodes, including thecurrent work, are complex and fall short of identifying how the coat-ing slows increase of the interface resistance. Progress requires goodcontrol and characterization of the coating, particularly its thickness,composition, density, microstructure, and transport properties. Stud-ies using model dense thin-film cathodes, either polycrystalline filmsor epitaxial films as pioneered by Hirayama and Kanno,38,39 providea means where coating can be evaluated independent of contact to thecurrent collector and free of binders and additives. It is also essentialto assess the effects of coatings (if any) on cathode powders with thebest baseline performance.
While passivation may kinetically stabilize the interface, additionof an auxiliary electrolyte may actually promote electrochemical sta-bility. This would certainly be the better solution and is yet to bedemonstrated for any high-voltage cathodes. Addition of a solid elec-trolyte in series with the liquid electrolyte could in principle decreasethe potential seen by the liquid electrolyte moving interfaces to withinits stability limits. This approach is successful for solid state systems;for example, a Lipon barrier electrolyte film creates a stable interfacebetween a Li metal anode and a Ti-containing glass ceramic electrolytethus preventing reduction of the electrolyte.77 For liquid electrolytecells, only very thin Lipon and LLTO28 (Table I) have been appliedas uniform Li electrolyte coatings to high voltage cathodes and thesefilms are likely too thin to act as electrolytes with negligible electronictransport. Space charge regions at the interfaces, as well as the relativebulk conductance of the electrolytes, determine the electrochemicalpotential distribution across layers of multiple electrolytes at differentcurrents. From the relative bulk Li ion conductivities,43,78 a simpleview of the cell potential predicts that during cycling, half of the Li-ion cell potential might fall across a 10–20 nm Lipon film greatlyreducing the high voltage potential for a 25 μm thick liquid elec-trolyte. Detailed evaluation of such a dual electrolyte requires furtherinvestigation where the thickness and composition of the electrolytescan be altered independently. Methods for in-situ investigation of theelectrochemical potential may include electron holography79,80 andscanning probe techniques. A potentially severe drawback is that theinterface between dual electrolytes may add a large resistance to thebattery. Such a high interface resistance has been reported81 for aceramic electrolyte – liquid electrolyte junction. Another concern islong term stability of a thicker inorganic coating at the cathode surfacewhich must withstand the volume changes with deep cycles.
Impurities from the electrolyte, the cathode surface, or other bat-tery components can complicate the high voltage performance andcontribute to instability. The variability noted from cell to cell withLMNO cathodes in this work, and indeed in the literature, suggeststhat small effects such as impurities may be influential. The possibility
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A3124 Journal of The Electrochemical Society, 160 (5) A3113-A3125 (2013)
that applying a coating may unintentionally ‘condition’ the cathodesurface by, for example, removing surface groups or plugging surfacepores, should not be overlooked. Further there is good evidence thatLipon and other coating materials may provide getters or barriers forimpurities, such as water and HF, as well as electrolyte decompositionproducts. If the coating is largely acting as a getter, then there are al-ternative and possibly more effective ways to incorporate the materialinto the cell.
While the spinel structure studied here is relatively stable, a numberof high-voltage cathodes suffer changes due to phase transformations,oxygen loss, and defects or disorder of the lattice upon cycling. Somepublished results suggest that coatings may impede these changes,although this may be difficult to separate from changes in the interfaceresistance. A recent example is the effect of Lipon coatings on Li-excess cathodes where a lower interface resistance may extend cyclecapacity and rate performance but complicate analysis of the so-calledvoltage-fade processes.41 Our work using LiCoO2 powders charged tohigh-voltage shows that Lipon coating on the dense ∼10 μm particlesgreatly reduces cracking at the surface, perhaps by filling preexistingflaws,27 but even for solid state batteries, phase changes driven by fullydelithiating the LiCoO2 cathodes persist.82,83 For layered cathodes ofMn and Ni, transformation to spinel regions may initiate at the particlesurface,84 but there are likely other locations where the transitionmetal cations can move to the conduction plane even if the surface isstabilized by a solid coating.
In summary, it is difficult to separate all the possible effects ofcoatings on the stability of high voltage cathodes, and there are fewdefinitive studies. Good control and systematic variation of the coat-ing structure, thickness, and composition will provide insight. Studiesusing model cathodes in the form of dense thin films will provide ameans where coating can be controlled independent of contact to thecurrent collector. Use of epitaxial cathode films allows surface recon-struction and reaction to be probed in greater detail. The conditionsfor passivation need to be clarified and distinguished from complicat-ing effects of chemical corrosion and cathode dissolution. A uniformcoating that acts as an auxiliary electrolyte may stabilize the liquidelectrolyte for use with high voltage cathodes.
Conclusions
This research demonstrates that a sub-nanometer amorphous Liponelectrolyte coating can stabilize the 3.5–4.9 V cycles of the LMNOcathode cells, reducing the rate of capacity degradation at room tem-perature and 60◦C compared to that observed for the uncoated LMNO.The ultra-thin Lipon layer was also effective in allowing the coatedcathodes to deliver more capacity at discharge rates above 1C. Al-though cell performances are affected by many factors that should befurther investigated, the cell stability observed here can be attributed tothe ultra-thin Lipon layers minimizing the SEI resistances and reduc-ing Mn dissolution. The Lipon coating used here is not thick enoughto eliminate electronic transport, so it cannot reduce the electrochem-ical potential of the LMNO cells seen by liquid electrolyte. SimilarlyLipon is not sufficiently thick or conformal to block Mn dissolutionfrom agglomerated powders; this needs to be tested with dense smoothpowders. Future work will investigate thicker Lipon coatings on thinfilm LMNO cathodes to elucidate these effects.
Acknowledgments
Research sponsored by the Division of Materials Sciences andEngineering, Office of Basic Energy Sciences, U.S. Department ofEnergy. This includes fabrication, electrochemistry, and characteriza-tion by YGK, NJD, MFC, GMV. Electron microscopy was conductedat the Shared Research Equipment (SHaRE) user facility, which issponsored by the Division of Scientific User Facilities, U.S. Depart-ment of Energy. Contributions of Raman spectroscopy by SKM andJN and assistance with cycle tests were supported by the Assistant Sec-retary for Energy Efficiency and Renewable Energy, Office of Vehicle
Technologies of the U.S. Department of Energy. The raw materialswere provided by nGimat Co. (GA, USA) for this research.
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