enhancing high-rate and elevated-temperature...

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Copyright copy 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 12 7113ndash7120 2012

Enhancing High-Rate and Elevated-TemperaturePerformances of Nano-Sized and Micron-Sized LiMn2O4

in Lithium-Ion Batteries with Ultrathin Surface Coatings

Xinning Luan Dongsheng Guan and Ying Wanglowast

Department of Mechanical Engineering Louisiana State University Baton Rouge LA 70803 USA

LiMn2O4 suffers from severe capacity degradation when used as a cathode material in rechargeablelithium-ion batteries especially when cycled at high rates and elevated temperatures To enhanceits high-rate electrochemical performance at elevated temperature (55 C) we use atomic layerdeposition (ALD) to deposit ultrathin and highly conformal Al2O3 coatings (as thin as 072 nm)onto micron-sized and nano-sized LiMn2O4 with precise thickness-control at atomic scale All ALD-modified electrodes exhibit significantly improved capacities and cycling stability compared to bareelectrodes In particular the effect of ALD coating to improve electrochemical performance ofLiMn2O4 is more distinct for nano-sized LiMn2O4 than for micron-sized LiMn2O4 and more distinctfor electrochemical cycling at higher chargedischarge rates For example nano-LiMn2O4 electrodecoated with 6 Al2O3 ALD layers delivers higher initial capacity (1247 mA hg) and final capacity(1067 mA hg) after 100 cycles than bare electrode with an initial capacity of 1123 mA hg and afinal capacity of only 955 mA hg when cycled at a very high rate of 5 C at 55 C In addition nano-LiMn2O4 electrodes show much better rate performance than micron-LiMn2O4 electrodes at 5 CThe enhanced electrochemical performance of ALD-modified LiMn2O4 is ascribed to high-qualityALD oxide coatings that are highly conformal dense complete and thus protect active materialfrom severe dissolution and to a formed robust glass layer on the surface of LiMn2O4 that suppressits crystallographic transformation during electrochemical cycling Surface modifications of LiMn2O4

are also carried out by either ALD coating directly onto the entire LiMn2O4carbonPVDF compositeelectrode or coating only on LiMn2O4 particles in electrode The former results in more significantlyimproved electrochemical performance of cathode possibly because ALD coating onto entire elec-trode provides better mechanical integrity and preserves contact between LiMn2O4 particles andcarbonpoly-vinylidenefluoride network

Keywords Atomic Layer Deposition LiMn2O4 Rate Performance Particle Size Al2O3Lithium-Ion Battery

1 INTRODUCTION

In the past decade lithium-ion batteries (LIBs) have beenused as a major power source for portable electronics12

Spinel lithium manganese oxide (LiMn2O4 is one of mostpopular lithium-ion (Li-ion) battery cathode materials dueto its unique advantages such as high working voltageseasy production low cost and non-toxicity34 HoweverLiMn2O4 suffers rapid capacity fading during long termcycling especially at elevated temperature It is mostlydue to the three factors (1) Jahn-Teller distortion effectof Mn3+ ions due to the crystallographic transformation ofLixMn2O4 (1 lt x lt 2) from spinel structure to tetragonal

lowastAuthor to whom correspondence should be addressed

phase with poor Li-intercalation properties5 (2) electrolytedecomposition6 at high potentials caused by strong oxida-tion ability of Mn4+ at the end of charge reaction on thesurface of LiMn2O4 cathode The formed solid electrolyteinterphase (SEI) causes increased charge transition resis-tance and distinct electrochemical polarization on the elec-trode7 and (3) manganese dissolution into electrolytes89

during electrochemical cycling which is caused by theattack from acidic hydrofluoric acid (HF) resulted fromreactions between residual H2O and hexafluorophosphate(LiPF6 in the electrolyte Along the manganese dissolu-tion inactive phases such as rock salt phase (Li2MnO3and tetragonal phase (Li2Mn2O4 are yielded

10 Among thethree factors the third one is believed to be mostly respon-sible for the fast capacity degradation of LiMn2O4 but all

J Nanosci Nanotechnol 2012 Vol 12 No 9 1533-48802012127113008 doi101166jnn20126577 7113


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Enhancing High-Rate and Elevated-Temperature Performances of Nano-Sized and Micron-Sized LiMn2O4 Luan et al

of them would become more severe when LiMn2O4-basedLIBs are used for high power applications at elevated tem-perature conditionsTremendous efforts have been made to enhance high-

rate cycleability of LiMn2O4 especially at elevatedtemperature condition It has been reported that surfacemodification with oxides such as LiCoO2

1112 Co3O413

ZnO10 Al2O31415 CeO2

16 Cr2O317 and TiO2

18 can be usedto alleviate the above problems of LiMn2O4 The oxidecoatings scavenge trace hydrogen fluoride acids (HF) inLIBs and thus slow down dissolution of manganese ionsand decomposition of organic electrolytes at cathoderesulting in much better electrochemical performance ofLiMn2O4 However surface coating methods for LiMn2O4

are mostly limited to solndashgel processing14 emulsion dry-ing19 and melting impregnation20 techniques Coatingsresulted from these methods are usually 50ndash100 nm thick21

and lack conformality uniformity and completeness Thethick oxide coating on LiMn2O4 can hinder Li-ion trans-fer between active material and electrolyte Besides suchcoatings are too thick to be produced onto nanostructuredbattery electrodes which have attracted increasing researchattention recently Moreover surface coatings for nano-sized electrodes are more important because these elec-trodes have large surface area exposed to electrolytes anddissolution of active materials become even faster There-fore it is highly necessary to explore novel surface mod-ification techniques for achieving much thinner and moreconformal coatings for new-generation LIBs technologyCompared to conventional deposition methods of oxide

coatings mentioned above atomic layer deposition (ALD)can be used to deposit ultrathin and highly conformalsurface coatings with precise thickness control at atomicscale Additionally ALD requires only a minimal amountof precursors22 ALD is unique in the sense that filmgrowth by ALD is surface controlled via sequential expo-sures separating the (usually binary) reaction between pre-cursor compounds into two half-reactions During eachhalf-reaction only one monolayer of the reactant ischemisorbed (or is chemically bound) on the surface ofsubstrate Further layers which are only physisorbed areremoved by an inert gas purge before the other reac-tant is introduced As a result the process proceeds step-wise in self-limiting surface reactions separated by purgesteps Hence films grown using ALD are typically uni-form dense homogenous pinhole-free and extremelyconformal to the underlying substrate Moreover ALD hasexcellent step coverage (sim100) and refilling ability onparticles and porous structures2223

Lee et al have presented several reports about usingoxide ALD coatings to improve cycling performances ofboth cathodes and anodes712 23ndash31 And their studies onALD-coated cathodes focus on LiCoO2 To the best ofour knowledge there is no report about the effect of ALDcoatings on electrochemical properties of LiMn2O4 so farexcept a communication paper we published recently25

In the present work we use ALD to deposit ultrathinand highly conformal Al2O3 coatings onto micron-sizedand nano-sized LiMn2O4 particles or electrodes composedof these particles to explore their capacities and cyclingperformances at different chargedischarge rates at ele-vated temperature (55 C) and to study the effect ofALD surface coatings on electrode materials of differ-ent sizes Al2O3 is chosen as the coating material herebecause it is one of the most effective coatings producedvia wet chemistry methods for LiMn2O4 with respect toother oxides32 It should be noted that ALD offers flexi-bility of coating onto either individual LiMn2O4 particlesor whole composite electrode composed of LiMn2O4 par-ticles and carbonPVDF network Thus we fabricate twotypes of ALD-coated cathodes one is electrode directlycoated with Al2O3 ALD coating (ie ALD is coated ontothe entire LiMn2O4 composite electrode) and the other iselectrode composed of Al2O3 ALD-coated LiMn2O4 parti-cles and uncoated carbonPVDF network to further inves-tigate effects of ALD surface coatings on electrochemicalproperties of LiMn2O4 cathode material

2 EXPERIMENTAL DETAILS

21 Atomic Layer Deposition

Atomic layer deposition of Al2O3 surface coatings ontoLiMn2O4 particles or LiMn2O4 composite electrodes wascarried out in a Savannah 100 Atomic Layer Deposi-tion system (Cambridge NanoTech Inc) Precursors usedfor Al2O3 ALD were trimethylaluminum (TMA SAFCHitech) and deionized water Vapors of the two precur-sors were alternately carried by N2 gas into a reactionchamber where the temperature was set at 120 C forAl2O3 ALD Self-terminating chemical reactions involvedin Al2O3 ALD growth are as follows33ndash35

AlOHlowast +AlCH33 rarr AlOndashAlCH3lowast2+CH4 (1)

AlCHlowast3 +H2Orarr AlndashOHlowast +CH4 (2)

22 Characterizations

Al2O3 ALD coatings on LiMn2O4 particles were exam-ined using a high-resolution transmission electron micro-scope (HR-TEM JEM-2010 JEOL Ltd) at an accelerationvoltage of 200 kV Crystallographic structure of uncoatedand ALD-coated LiMn2O4 powders were examined usinga Rigaku MiniFlex X-ray diffractometer (XRD) with CuK radiation at a scan rate of 4 min Surface morphol-ogy and particle size of LiMn2O4 particles were observedusing a FEI Quanta 3D FEG scanning electron micro-scope (SEM) Surface composition of LiMn2O4 electrodescoated with 6 Al2O3 ALD layers were analyzed via anX-ray photoelectron spectroscope (XPS) on the AXIS 165spectrometer with a twin-anode Al K (14866 eV) X-raysource All the XPS spectra were calibrated according tothe binding energy of the C1s peak at 2848 eV

7114 J Nanosci Nanotechnol 12 7113ndash7120 2012


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Luan et al Enhancing High-Rate and Elevated-Temperature Performances of Nano-Sized and Micron-Sized LiMn2O4

23 Electrode Fabrication

LiMn2O4 powders of different sizes micron-sized(sim 2 m Alfa Aesar 995 m-LiMn2O4 and nano-sized (sim200 nm MTI Corporation 98 nm-LiMn2O4particles were used to prepare LiMn2O4 cathodes respec-tively LiMn2O4 powders well mixed with acetylene black(AB Alfa Aesar 995) and poly-vinylidenefluoride(PVDF Alfa Aesar) with a weight ratio of 801010 wereadded into N -methyl-2-pyrrolidone (NMP) solvent Theresultant viscous slurry was coated onto the aluminum cur-rent collector using an AFA-III automatic film applicator(MTI Corporation) with a thickness setting of 500 mand then dried at 120 C overnight under vacuum Thedried electrodes were pressed to an effective thickness of50 m by using an EQ-MR100A rolling press machine(MTI Corporation) Approximately 10 mg active materialwas loaded in the circular working electrode with a diam-eter of 189 mm

24 Electrochemical Measurements

LiMn2O4 composite electrodes or ALD-coated electrodeswere assembled into CR2032-type coin cells Lithiumfoil (Sigma-Aldrich 999) was used as the counterand reference electrode The two electrodes were sepa-rated by a porous Celgard-2320 separatormdasha 20-m thickpolypropylene (PP)polyethylenePP trilayer film A com-mercial electrolyte composed of 1 M lithium hexaflu-orophosphate (LiPF6 dissolved in a mixed solvent ofethylene carbonate dimethyl carbonate and diethyl car-bonate (111 volume ratio) (MTI Corporation) was usedAll the coin cells were assembled in an argon-filled glovebox (OMNI-Lab system Vacuum Atmosphere Co) TheLiMn2O4Li coin cells were cycled in a voltage rangeof 34ndash45 V using an 8-channel battery analyzer (MTICorporation) at 55 C

3 RESULTS AND DISCUSSION

Morphology and structure of uncoated LiMn2O4 andAl2O3-ALD-coated LiMn2O4 particles are investigated viaTEM To obtain detailed information about ALD coatingsLiMn2O4 particles are coated with thicker Al2O3 ALD film(50 ALD layers using 50 ALD growth cycles) Figure 1presents TEM images of uncoated m-LiMn2O4 particleand m-LiMn2O4 particle coated with 50 Al2O3 ALD lay-ers respectively Uncoated m-LiMn2O4 particle is crys-talline as lattice fringes are observed in Figure 1(a) whilean amorphous Al2O3 coating conformally coated on theLiMn2O4 particle is shown in Figure 1(b) This is in goodagreement with our previous findings by TEM36 In com-parison with non-uniform and incomplete Al2O3 coatingsmade by wet chemical methods usually with a thicknessfrom 100 nm to 1 m1137ndash39 Al2O3 ALD film on the sur-face of m-LiMn2O4 particles is homogeneous complete

Fig 1 High-resolution TEM images of (a) uncoated m-LiMn2O4 par-ticle and (b) m-LiMn2O4 particle coated with 50 Al2O3 ALD layers

and highly conformal even along the curved edges ofLiMn2O4 particle (Fig 1(b)) Thickness of this Al2O3

ALD coating (50 ALD layers) is estimated to 607 nmcorresponding to an ALD growth rate of 12 Aringcycle Thisgrowth rate is slightly higher than sim11 Aring in thickness perlayer as reported in literature34 In the present work theAl2O3 ALD film is deposited on particles and there is lesshindrance from neighboring molecules in the adsorptionstep during the film growth on curved surface of parti-cles Thus the ALD growth on particles is faster than ALDgrowth on flat substrates leading to higher growth rate40

Figure 2 displays XRD spectra of m- and nm-LiMn2O4 powders before and after being coated with6 Al2O3 ALD layers (072 nm thick) All the diffrac-tion peaks can be indexed to a face-centered cubic spinelstructure with an Fd3m space group (JCPDS 35-0782)No diffraction peaks of impurities or other phases arefound in the XRD pattern of ALD-coated LiMn2O4 parti-cles suggesting formation of amorphous Al2O3 ALD coat-ings We also perform XRD analysis on m-LiMn2O4

powders coated with 412 Al2O3 ALD layers (50 nmthick) which confirms that Al2O3 ALD is amorphousThese results are consistent with TEM characterization inFigure 1(b) In addition it can also be demonstrated thatthere is no phase change of bulk LiMn2O4 in ALD coatingprocessTo further investigate the conformality of ALD coat-

ings morphological features of uncoated and Al2O3-ALD-coated LiMn2O4 particles are characterized by SEMFigure 3(a) shows SEM image of uncoated m-LiMn2O4

particles They have an average size of sim2 m and exhibit

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20 40 60 80

(551

)(4

44)

(622

)(5

33)

(531

)(440

)

(511

)

(331

)

(400

)

(311

)(2

22)

(111

)(d)

(c)

(b)

(a)

2 ThetaDegree

Inte

nsity

au

Fig 2 X-ray diffraction patterns of (a) uncoated m-LiMn2O4 par-ticles (b) m-LiMn2O4 particles coated with 6 Al2O3 ALD layers(c) uncoated nm-LiMn2O4 particles and (d) nm-LiMn2O4 particlescoated with 6 Al2O3 ALD layers

a smooth polyhedron appearance No discernible change isfound in the morphology of m-LiMn2O4 particles aftercoating with 6 Al2O3 ALD layers suggesting ALD coat-ing is ultrathin and highly conformal and thus preservesthe morphology of m-LiMn2O4 particles Likewise nm-LiMn2O4 particles with a size range of 20ndash50 nm alsoremain their morphological features after being coatedwith 6 Al2O3 (Fig 3(b)) It is observed that gaps betweennm-LiMn2O4 primary particles are at nanometer scalewhich makes it difficult to deposit complete and conformalcoatings onto primary particles via commonly used wetchemical methods On the other hand this can be easilyand effectively achieved using ALDTo confirm formation of Al2O3 coatings on the surface

of LiMn2O4 XPS is employed to study surface elementalcomposition of LiMn2O4 electrodes coated with 6 Al2O3

ALD layers XPS spectrum taken from ALD-coated-m-LiMn2O4 electrode shows the Al2p peak from Al2O3 asshown in Figure 4(a) Figure 4(b) displays the Al2p peakat a binding energy of 752 eV which corresponds toAl O chemical bond of Al2O3 Similarly the Al2p peakat a binding energy of 751 eV is also observed in the XPSspectrum captured from ALD-coated-nm-LiMn2O4 elec-trode (Figs 4(c) and (d)) The binding energies of Al2p

Fig 3 SEM images of (a) uncoated m-LiMn2O4 particles and(b) uncoated nm-LiMn2O4 particles

peaks captured from both ALD-coated m-LiMn2O4 andnm-LiMn2O4 are consistent with reported data of Al2O3

41

which clearly prove the existence of Al2O3 on the surfaceof LiMn2O4We then investigate effects of the Al2O3 ALD coating

on electrochemical properties of LiMn2O4 of differentsizes We have tested cycling behaviors of uncoated nm-LiMn2O4 electrode and nm-LiMn2O4 electrodes coatedwith 4 6 and 8 Al2O3 ALD layers cycled at a specificcurrent of 300 mAg (25 C) at room temperature Allthree Al2O3-coated electrodes show enhanced cycling per-formances and deliver higher final capacities than uncoatedcathode over 100 electrochemical cycles Among them theelectrode coated with 6 Al2O3 ALD layers demonstratesthe best cycleability and delivers the highest final capac-ity of 965 mA hg after 100 cycles significantly higherthan final capacity of uncoated electrode (786 mA hg)The final capacity of the cathode coated with 6 Al2O3

ALD layers remains 951 of its initial capacity Elec-trode coated with a thinner coating (4 Al2O3 ALD lay-ers) exhibits less satisfying cycleability delivering a finalcapacity of 856 mA hg at the 100th cycle because thecoating is too thin to effectively protect the active materialover long electrochemical cycling For the electrode coatedwith a thicker coating (8 Al2O3 ALD layers) the initialcapacities are significantly lower than uncoated electrodedue to an overly thick insulating Al2O3 ALD coating willreduce the electronic conductivity of cathode resultingin lower capacities of cathode In summary Al2O3 coat-ing composed of 6 ALD layers is the optimal coating forimproving the electrochemical performance of LiMn2O4Thus m-sized and nm-sized LiMn2O4 electrodes will bemodified with 6 ALD layers for electrochemical measure-ments in this paperFigure 5(a) compares cycling performances of

uncoated m-LiMn2O4 electrodes and m-LiMn2O4 elec-trodes coated with 6 Al2O3 ALD layers (072 nm thick)at different chargedischarge rates (1 C 2 C and 5 C) at55 C It can be seen that ALD-coated electrode showshigher capacities and better cycleability than uncoatedelectrode at all these different chargedischarge ratesAt 1 C uncoated m-LiMn2O4 electrode delivers an ini-tial discharge capacity of 794 mA hg and a final capacityof 368 mA hg after 100 electrochemical cycles whileALD-coated electrode delivers a higher initial dischargecapacity of 1023 mA hg and a higher final capacity of515 mA hg At 2 C ALD-coated electrode exhibits afinal capacity of 440 mA hg after electrochemical cyclingat 55 C also higher than the final capacity of 312 mA hgshown by uncoated electrode at 55 C When cycled at5 C both uncoated and ALD-coated electrodes show sig-nificant drop in capacity but the capacity of ALD-coatedelectrode is still higher than that of uncoated electrodeSuch enhanced electrochemical performances of LiMn2O4

electrodes are ascribed to the protection of ultrathin and



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85 80 75 70 651200 1000 800 600 400 200 0

1200 1000 800 600 400 200 0 85 80 75 70 65

(d)

Binding EnergyeV Binding EnergyeV

Inte

nsity

au

Al 2p(c)

Li 1s

Al 2p

C 1s

O 1sMn 2p

F 1s

F(kLL )

(a)

Inte

nsity

au

Binding EnergyeV

F(KLL) Mn 2p Li 1sAl 2p

C 1s

F 1s

O 1s

(b)

Inte

nsity

au

Al 2p

Binding EnergyeV

Inte

nsity

au

Fig 4 XPS spectra of (a) m-LiMn2O4 electrode coated with 6 Al2O3 ALD layers (b) Al2p XPS peak taken from m-LiMn2O4 electrode coatedwith 6 Al2O3 ALD layers (c) nm-LiMn2O4 electrode coated with 6 Al2O3 ALD layers and (d) Al2p XPS peak

highly conformal Al2O3 ALD surface coatings whichisolate the electrode from electrolyte and reduce Mn2+

dissolution It is also noted that capacity retention isimproved (though capacity decreases) for both uncoatedand ALD-coated electrode when chargedischarge rate isincreased from 1 C to 5 C The capacity drop is attributedto a smaller portion (surface part) of active material thatfunctions at a higher chargedischarge rate but the surfaceportion can absorb and release Li+ ions at necessaryspeed yielding a reduced capacity loss of LiMn2O4 dur-ing cycling Another reason for the capacity drop withincreasing rates (1 C to 5 C) is more severe corrosionof HF resulted from reaction between fluorinated anionsand residual H2O

89 Dissolution of Mn2+ becomes muchfaster when LiMn2O4 is cycled at a high chargedischargerate in a high-temperature environmentFigure 5(b) summarizes cycling behaviors of uncoated

nm-LiMn2O4 electrode and nm-LiMn2O4 electrode coatedwith 6 Al2O3 ALD layers (072 nm thick) when cycledat chargedischarge rates of 1 C 2 C and 5 C at 55 CLikewise Al2O3 ALD coatings are able to enhance capac-ity and cycleability of nm-LiMn2O4 electrodes at all dif-ferent chargedischarge rates In particular the effect ofALD coating to enhance electrochemical performance ismore significant for nm-LiMn2O4 electrode when it iscycled at high chargedischarge rate such as 5 C At 1 Cuncoated nm-LiMn2O4 electrode delivers an initial dis-charge capacity of 1384 mA hg and a final capacity of

1079 mA hg after 100 electrochemical cycles whereasthe coated electrode shows a higher initial discharge capac-ity of 1425 mA hg and a higher final capacity of1120 mA hg with a better capacity retention rate of 78At 2 C the final capacity increases from 1063 mA hgto 1100 mA hg after ALD coating At 5 C ALD coatedelectrode delivers an initial discharge capacity of 1247mA hg and a final capacity of 1067 mA hg after100 electrochemical cycles which is higher than the finalcapacity of uncoated electrode (955 mA hg) In addi-tion Al2O3-coated nm-LiMn2O4 electrode cycled at 5 Cexhibits the best cycling performance with a capacityretention rate of 85 and the effect of ultrathin ALD coat-ing to improve capacity and cycleability of nm-LiMn2O4

electrode is more distinct at high chargedischarge rateIf we compare Figures 5(a) and (b) nm-LiMn2O4 elec-

trode shows higher capacities and better rate capabilitiesthan m-LiMn2O4 electrode due to the larger surface areaand shorter diffusion distance provided by nanostructuredelectrodes4243 It can also be observed that the effect ofALD surface coatings to improve capacity and cyclingstability is more distinct for nm-LiMn2O4 electrode thanfor m-LiMn2O4 electrode because nano-sized electrodematerial is more active and easier subject to attack fromthe acidic HF formed by the residual H2O and LiPF6 in theelectrolyte3544ndash46 and thus surface passivation is even moreimportant to nanostructured electrodes Coatings synthe-sized via commonly-used wet chemical methods are too



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(a) Micro-sized LiMn2O4

Cycle Number

Cap

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Al2O3-coated electrode at 1 C

Uncoated electrode at 1 CAl2O3-coated electrode at 2 C

Uncoated electrode at 2 C








Fig 5 Cycling behaviors of (a) uncoated m-LiMn2O4 electrodesand electrodes coated with 6 Al2O3 ALD layers (b) uncoated nm-LiMn2O4 electrodes and electrodes coated with 6 Al2O3 ALD layers atchargedischarge rates of 1 C 2 C and 5 C at 55 C

thick to be deposited onto nano-sized electrodes to pre-vent manganese dissolution while ultrathin and highlyconformal ALD coatings are necessary for nanostructuredelectrodesTo further study the effect of ALD modification on

LiMn2O4 two types of ALD-coated electrodes are fabri-cated one is electrode composed of ALD-coated LiMn2O4

particles and uncoated carbonPVDF network (Al2O3-P)the other is ALD-coated electrode ALD film with ALDfilm coated onto the entire electrode (Al2O3-E) Structuresof these two electrodes in comparison with bare elec-trode are illustrated in Figure 6 Since ALD allows forthe growth of conformal films even on substrates withcomplex surface geometries3334 ALD film will penetratethe porous electrode and be coated onto both LiMn2O4

particles and mesoporous framework bridged by carbonand PVDF in the case of ldquoAl2O3-Erdquo as illustrated in theright picture of Figure 6 In Al2O3-E LiMn2O4 particlesare partially covered by ALD coatings because LiMn2O4

particles are tightly enwrapped by carbon and PVDF net-work On the other hand in Al2O3-P LiMn2O4 particles

Fig 6 Schematic representations of uncoated LiMn2O4 composite elec-trode (left) electrode composed of ALD-coated LiMn2O4 particles anduncoated carbonPVDF (center) and ALD-coated LiMn2O4 compositeelectrode (right)

are fully covered by ALD films and are separated fromcarbonPVDF network by surface ALD coatings There-fore in these three different electrodes there are variousinterfaces between LiMn2O4 particles ALD coatings car-bon and PVDF composites and surrounding electrolyteto affect electron transport and Li-ion diffusion duringdelithiationlithiation reactions resulting in different elec-trochemical behaviors of these electrodesFigure 7 presents high rate cycling performances of

ALD-modified m- and nm-LiMn2O4 electrodes at 55C

For both m- and nm-LiMn2O4 we compare Al2O3-Pelectrode and Al2O3-E electrode with bare electrode asillustrated in Figure 6 For nano-sized LiMn2O4 bothAl2O3-E and Al2O3-P are cycled at 5 C On the otherhand for micro-sized LiMn2O4 Al2O3-E and Al2O3-Pare cycled at 2 C because 5 C is too high for micron-sized LiMn2O4 which deliver very low capacities at sucha high rate Figure 7(a) compares cycling performancesof micron-sized Al2O3-P and Al2O3-E with that of baremicron-sized electrode at 2 C at 55 C It can be seenthat both ALD-modified electrodes show improved electro-chemical performances than bare electrode and Al2O3-Edelivers the highest capacities and the best cyclingstability Figure 7(b) summarizes cycling behaviors ofAl2O3-E Al2O3-P and uncoated electrode for nano-sizedLiMn2O4 at 5 C Similarly both ALD-modified elec-trodes exhibit enhanced electrochemical performances thanbare electrode and Al2O3-E delivers the highest capac-ities and the best cycleability with the final capacityof Al2O3-E (1067 mA hg) slightly higher than that ofAl2O3-P (1019 mA hg)Our results confirm that ultrathin Al2O3 ALD coat-

ings can improve the electrochemical performance ofboth micron-sized and nano-sized LiMn2O4 at differentchargedischarge rate at 55 C Furthermore the effect ofALD coating is more distinct for nano-sized LiMn2O4and more distinct for cycling at high chargedischargerates It is known that insulating Al2O3 is a conventionalcoating material for cathode materials in Li-ion batter-ies and most of research focuses on thick Al2O3 coatingsformed by traditional chemistry methods (eg solutionor solndashgel routes) Therefore the mechanism of ultrathin



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(a) Micro-sized LiMn2O4 at 2 C

0 20 40 60 80 10080

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120

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(b) Nano-sized LiMn2O4at 5 C

Cap

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Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode

Fig 7 High-rate cycling performances of uncoated LiMn2O4 elec-trodes LiMn2O4 electrodes coated with 6 Al2O3 ALD layers (Al2O3-E)and electrodes composed of LiMn2O4 particles coated with 6 Al2O3 ALDlayers and uncoated carbonPVDF network (Al2O3-P) at 55

C (a) m-LiMn2O4 (b) nm-LiMn2O4

Al2O3 ALD coatings (less than 1 nm) on LiMn2O4 hasnot been fully understood Based on a limited number ofexplorations several mechanisms are considered to playimportant roles in enhancing Li-ion intercalation behav-iors of LiMn2O4 electrodes by coating them with Al2O3

ALD The first one is HF scavenging effects of metal oxidecoatings on cathode materials Al2O3 ALD layer proba-bly reacts with a trace amount of HF from LiPF6 elec-trolytes and alleviates the severe dissolution of manganeseions resulting in higher initial capacity and better capacityretention of LiMn2O4 The second is the protective effectof Al2O3 in a solid electrolyte or glass state which isformed on the surface of LiMn2O4 during Li-ion intercala-tion process747 Liu et al48 used in-situ TEM technique toobserve the conversion of Al2O3 layer (4ndash5 nm thick) onAl nanowires into a LindashAlndashO glass layer with high ionicconductivity and low electronic conductivity during lithia-tion process33 This glass layer can prevent the direct con-tact between active material and electrolyte and therebyreduces decomposition of electrolyte components and cor-rosion from HF yielding better cycling performance ofLi-ion batteries4849 The mechanical robustness of this

glass layer also involves here It serves as a solid frame-work to restrain the phase transformation of LiMn2O4

from cubic to tetragonal structure and suppress harmfulJahn-Teller effects and favors long-term cycling stabilityof the material4850 However it should be noted that Al2O3

is an electronic insulator material which could be the rea-son for different performances of Al2O3-E and Al2O3-PIn Al2O3-P Al2O3 is only coated fully on LiMn2O4

particles while in Al2O3-E Al2O3 is coated onto bothLiMn2O4 particles and carbonPVDF network It is demon-strated that a direct Al2O3 ALD coating on particles canslow down electron transport between LiMn2O4 particlesand carbon network and Li-ion diffusion through theactive material in chargedischarge processes is hindered aswell24 In contrast Al2O3 ALD coating on electrode pre-serves the contact between LiMn2O4 particles and carbonnetwork which act as electronic pathways to allow muchhigher electrical conductivity24 As a result Al2O3-E deliv-ers higher capacities than Al2O3-P during cycling mea-surements in our work In summary ultrathin Al2O3 ALDcoatings can significantly improve electrochemical prop-erties of LiMn2O4 cathode material and such improve-ment becomes even greater for the coatings directly onelectrodes

4 CONCLUSIONS

We have modified the surface of micro-sized and nano-sized LiMn2O4 with ultrathin Al2O3 coatings (as thin as072 nm) via atomic layer deposition for enhanced elec-trochemical performance of LiMn2O4 at elevated temper-ature (55 C) The effect of ALD coating to improve thecapacity and cycleability of LiMn2O4 is even more dis-tinct for nano-sized LiMn2O4 than micro-sized LiMn2O4and more distinct for electrochemical cycling at higherdischargecharge rates The surface modifications are car-ried out by either ALD coating only on LiMn2O4 particlesor coating onto the entire LiMn2O4carbonPVDF com-posite electrode Though both ALD-modified LiMn2O4

exhibit improved electrochemical performances than bareelectrode ALD coating directly on LiMn2O4 compos-ite electrode shows better effect than coating directly onLiMn2O4 particles possibly because the former has bet-ter mechanical integrity and the ALD coating preservesthe contact between LiMn2O4 and carbonPVDF networkfor better electronic conducting pathways Several mech-anisms are proposed to be responsible for such positivecoating effects The Al2O3 ALD coating on LiMn2O4

effectively alleviates dissolution of manganese ions intoelectrolyte by scavenging HF and retards electrolytedecomposition by isolating LiMn2O4 from electrolyte Theconversion of Al2O3 ALD coating to a LindashAlndashO glass layeralso enhances the structural stability of active LiMn2O4

during cycling Thus the ultrathin and highly confor-mal ALD surface coatings can improve electrochemical



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performance of Li-ion battery cathodes particularly fornanostructured cathodes cycled at high chargedischargerates in an elevated-temperature environment

Acknowledgment This work is supported by the RalphE Powe Junior Faculty Enhancement Award sponsored byOak Ridge Associated Universities (ORAU) DART2 Fundsponsored by NASA-LABOR and PFund sponsored byNSF-LABOR The authors would like to thank JianqingZhao for helpful discussions The authors also acknowl-edge Materials Characterization Center at LSU for usingXRD XPS TEM and SEM D S Guan acknowledges LSUGraduate School Supplementary Award and X N Luanacknowledges LSU Graduate School Enrichment Award

References and Notes

1 M S Whittingham Chem Rev 104 4271 (2004)2 M Winter J O Besenhard M E Spahr and P Novak Adv Mater

10 725 (1998)3 M Hirayama H Ido K Kim W Cho K Tamura J I Mizuki and

R Kanno J Am Chem Soc 132 15268 (2010)4 A Manthiram J Phys Chem Lett 2 373 (2011)5 M M Thackeray Y Shao-Horn A J Kahaian K D Kepler J T

Vaughey and S A Hackney Electrochem Solid-State Lett 1 7(1998)

6 G G Amatucci C N Schmutz A Blyr C Sigala A S GozdzD Larcher and J M Tarascon J Power Sources 69 11 (1997)

7 S W Lee K S Kim H S Moon H J Kim B W Cho W ICho J B Ju and J W Park J Power Sources 126 150 (2004)

8 A Blyr C Sigala G Amatucci D Guyomard Y Chabre and J MTarascon J Electrochem Soc 145 194 (1998)

9 R J Gummow A Dekock and M M Thackeray Solid State Ionics69 59 (1994)

10 Y K Sun K J Hong and J Prakash J Electrochem Soc 150A970 (2003)

11 J Cho G B Kim H S Lim C S Kim and S I Yoo ElectrochemSolid-State Lett 2 607 (1999)

12 Y S Jung A S Cavanagh A C Dillon M D Groner S MGeorge and S-H Lee J Electrochem Soc 157 A75 (2010)

13 J P Cho T J Kim Y J Kim and B Park Chem Commun 1074(2001) DOI 101039b101677f

14 J P Cho Y J Kim T J Kim and B Park Chem Mater 13 18(2001)

15 D Guan J A Jeevarajan and Y Wang Nanoscale 3 1465 (2011)16 H W Ha N J Yun and K Kim Electrochim Acta 52 3236 (2007)17 H Sahan H Goktepe S Patat and A Ulgen Solid State Ionics

181 1437 (2010)18 C Lai W Ye H Liu and W Wang Ionics 15 389 (2009)19 H Sahan H Goktepe S Patat and A Ulgen J Alloys Compd 509

4235 (2011)20 J Tu X B Zhao G S Cao D G Zhuang T J Zhu and J P Tu

Electrochim Acta 51 6456 (2006)21 R Beetstra U Lafont J Nijenhuis E M Kelder and J R van

Ommen Chem Vap Deposition 15 227 (2009)22 M Kemell V Pore J Tupala M Ritala and M Leskela Chem

Mater 19 1816 (2007)

23 I D Scott Y S Jung A S Cavanagh Y An A C Dillon S MGeorge and S-H Lee Nano Lett 11 414 (2011)

24 Y S Jung A S Cavanagh L A Riley S-H Kang A C DillonM D Groner S M George and S-H Lee Adv Mater 22 2172(2010)

25 K Leung Y Qi K R Zavadil Y S Jung A C Dillon A SCavanagh S-H Lee and S M George J Am Chem Soc 13314741 (2011)

26 A C Dillon L A Riley Y S Jung C Ban D Molina A HMahan A S Cavanagh S M George and S H Lee Thin SolidFilms 519 4495 (2011)

27 L A Riley S Van Ana A S Cavanagh Y Yan S M GeorgeP Liu A C Dillon and S-H Lee J Power Sources 196 3317(2011)

28 L A Riley A S Cavanagh S M George S-H Lee and A CDillon Electrochem Solid-State Lett 14 A29 (2011)

29 L A Riley A S Cavanagh S M George Y S Jung Y Yan S-HLee and A C Dillon Chem Phys Chem 11 2124 (2010)

30 L A Riley S-H Lee L Gedvilias and A C Dillon J PowerSources 195 588 (2010)

31 A C Dillon A H Mahan R Deshpande R Parilla K M Jonesand S H Lee Thin Solid Films 516 794 (2008)

32 J S Kim C S Johnson J T Vaughey S A Hackney K A WalzW A Zeltner M A Anderson and M M Thackeray J Elec-trochem Soc 151 A1755 (2004)

33 A C Dillon M L Wise M B Robinson and S M George J VacSci Technol A-Vac Surf Films 13 1 (1995)

34 M D Groner F H Fabreguette J W Elam and S M GeorgeChem Mater 16 639 (2004)

35 A W Ott J W Klaus J M Johnson and S M George Thin SolidFilms 292 135 (1997)

36 D S Guan and Y Wang Ionics (2012) DOI 101007s11581-012-0717-9

37 Z H Chen and J R Dahn Electrochem Solid-State Lett 6 A221(2003)

38 Z H Chen and J R Dahn Electrochim Acta 49 1079 (2004)39 H Miyashiro Y Kobayashi S Seki Y Mita A Usami

M Nakayama and M Wakihara Chem Mater 17 5603 (2005)40 R L Puurunen J Appl Phys 97 121301 (2005)41 XPS International Mountain View California (2011) httpwww

xpsdatacomXI_BE_Lookup_tablepdf Accessed 4 November 201142 A S Arico P Bruce B Scrosati J M Tarascon and W Van

Schalkwijk Nat Mater 4 366 (2005)43 J Cabana T Valdes-Solis M R Palacin J Oro-Sole A Fuertes

G Marban and A B Fuertes J Power Sources 166 492 (2007)44 C J Curtis J X Wang and D L Schulz J Electrochem Soc 151

A590 (2004)45 S H Ye J Y Lv X P Gao F Wu and D Y Song Electrochim

Acta 49 1623 (2004)46 J H Choy D H Kim C W Kwon S J Hwang and Y I Kim

J Power Sources 77 1 (1999)47 C Li H P Zhang L J Fu H Liu Y P Wu E Ram R Holze

and H Q Wu Electrochim Acta 51 3872 (2006)48 Y Liu N S Hudak D L Huber S J Limmer J P Sullivan and

J Y Huang Nano Lett 11 4188 (2011)49 L J Fu H Liu C Li Y P Wu E Rahm R Holze and H Q Wu

Solid State Sci 8 113 (2006)50 L F Hakim D M King Y Zhou C J Gump S M George and

A W Weimer Adv Funct Mater 17 3175 (2007)

Received 15 March 2012 Accepted 11 June 2012



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of them would become more severe when LiMn2O4-basedLIBs are used for high power applications at elevated tem-perature conditionsTremendous efforts have been made to enhance high-

rate cycleability of LiMn2O4 especially at elevatedtemperature condition It has been reported that surfacemodification with oxides such as LiCoO2

1112 Co3O413

ZnO10 Al2O31415 CeO2

16 Cr2O317 and TiO2

18 can be usedto alleviate the above problems of LiMn2O4 The oxidecoatings scavenge trace hydrogen fluoride acids (HF) inLIBs and thus slow down dissolution of manganese ionsand decomposition of organic electrolytes at cathoderesulting in much better electrochemical performance ofLiMn2O4 However surface coating methods for LiMn2O4

are mostly limited to solndashgel processing14 emulsion dry-ing19 and melting impregnation20 techniques Coatingsresulted from these methods are usually 50ndash100 nm thick21

and lack conformality uniformity and completeness Thethick oxide coating on LiMn2O4 can hinder Li-ion trans-fer between active material and electrolyte Besides suchcoatings are too thick to be produced onto nanostructuredbattery electrodes which have attracted increasing researchattention recently Moreover surface coatings for nano-sized electrodes are more important because these elec-trodes have large surface area exposed to electrolytes anddissolution of active materials become even faster There-fore it is highly necessary to explore novel surface mod-ification techniques for achieving much thinner and moreconformal coatings for new-generation LIBs technologyCompared to conventional deposition methods of oxide

coatings mentioned above atomic layer deposition (ALD)can be used to deposit ultrathin and highly conformalsurface coatings with precise thickness control at atomicscale Additionally ALD requires only a minimal amountof precursors22 ALD is unique in the sense that filmgrowth by ALD is surface controlled via sequential expo-sures separating the (usually binary) reaction between pre-cursor compounds into two half-reactions During eachhalf-reaction only one monolayer of the reactant ischemisorbed (or is chemically bound) on the surface ofsubstrate Further layers which are only physisorbed areremoved by an inert gas purge before the other reac-tant is introduced As a result the process proceeds step-wise in self-limiting surface reactions separated by purgesteps Hence films grown using ALD are typically uni-form dense homogenous pinhole-free and extremelyconformal to the underlying substrate Moreover ALD hasexcellent step coverage (sim100) and refilling ability onparticles and porous structures2223

Lee et al have presented several reports about usingoxide ALD coatings to improve cycling performances ofboth cathodes and anodes712 23ndash31 And their studies onALD-coated cathodes focus on LiCoO2 To the best ofour knowledge there is no report about the effect of ALDcoatings on electrochemical properties of LiMn2O4 so farexcept a communication paper we published recently25

In the present work we use ALD to deposit ultrathinand highly conformal Al2O3 coatings onto micron-sizedand nano-sized LiMn2O4 particles or electrodes composedof these particles to explore their capacities and cyclingperformances at different chargedischarge rates at ele-vated temperature (55 C) and to study the effect ofALD surface coatings on electrode materials of differ-ent sizes Al2O3 is chosen as the coating material herebecause it is one of the most effective coatings producedvia wet chemistry methods for LiMn2O4 with respect toother oxides32 It should be noted that ALD offers flexi-bility of coating onto either individual LiMn2O4 particlesor whole composite electrode composed of LiMn2O4 par-ticles and carbonPVDF network Thus we fabricate twotypes of ALD-coated cathodes one is electrode directlycoated with Al2O3 ALD coating (ie ALD is coated ontothe entire LiMn2O4 composite electrode) and the other iselectrode composed of Al2O3 ALD-coated LiMn2O4 parti-cles and uncoated carbonPVDF network to further inves-tigate effects of ALD surface coatings on electrochemicalproperties of LiMn2O4 cathode material

2 EXPERIMENTAL DETAILS

21 Atomic Layer Deposition

Atomic layer deposition of Al2O3 surface coatings ontoLiMn2O4 particles or LiMn2O4 composite electrodes wascarried out in a Savannah 100 Atomic Layer Deposi-tion system (Cambridge NanoTech Inc) Precursors usedfor Al2O3 ALD were trimethylaluminum (TMA SAFCHitech) and deionized water Vapors of the two precur-sors were alternately carried by N2 gas into a reactionchamber where the temperature was set at 120 C forAl2O3 ALD Self-terminating chemical reactions involvedin Al2O3 ALD growth are as follows33ndash35

AlOHlowast +AlCH33 rarr AlOndashAlCH3lowast2+CH4 (1)

AlCHlowast3 +H2Orarr AlndashOHlowast +CH4 (2)

22 Characterizations

Al2O3 ALD coatings on LiMn2O4 particles were exam-ined using a high-resolution transmission electron micro-scope (HR-TEM JEM-2010 JEOL Ltd) at an accelerationvoltage of 200 kV Crystallographic structure of uncoatedand ALD-coated LiMn2O4 powders were examined usinga Rigaku MiniFlex X-ray diffractometer (XRD) with CuK radiation at a scan rate of 4 min Surface morphol-ogy and particle size of LiMn2O4 particles were observedusing a FEI Quanta 3D FEG scanning electron micro-scope (SEM) Surface composition of LiMn2O4 electrodescoated with 6 Al2O3 ALD layers were analyzed via anX-ray photoelectron spectroscope (XPS) on the AXIS 165spectrometer with a twin-anode Al K (14866 eV) X-raysource All the XPS spectra were calibrated according tothe binding energy of the C1s peak at 2848 eV



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20 40 60 80

(551

)(4

44)

(622

)(5

33)

(531

)(440

)

(511

)

(331

)

(400

)

(311

)(2

22)

(111

)(d)

(c)

(b)

(a)

2 ThetaDegree

Inte

nsity

au







41








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85 80 75 70 651200 1000 800 600 400 200 0

1200 1000 800 600 400 200 0 85 80 75 70 65

(d)


Inte

nsity

au

Al 2p(c)

Li 1s

Al 2p

C 1s

O 1sMn 2p

F 1s

F(kLL )

(a)

Inte

nsity

au

Binding EnergyeV


C 1s

F 1s

O 1s

(b)

Inte

nsity

au

Al 2p

Binding EnergyeV

Inte

nsity

au











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0 20 40 60 80 10080

100

120

140


Cycle Number

Cap

acity

mA

hg

0 20 40 60 80 1000

20

40

60

80

100


Cycle Number

Cap

acity

mA

hg
























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0 20 40 60 80 1000

20

40

60

80

100

Cycle Number

Cap

acity

mA

hg



0 20 40 60 80 10080

100

120

140


Cap

acity

mA

hg

Cycle Number

Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode









4 CONCLUSIONS







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20 40 60 80

(551

)(4

44)

(622

)(5

33)

(531

)(440

)

(511

)

(331

)

(400

)

(311

)(2

22)

(111

)(d)

(c)

(b)

(a)

2 ThetaDegree

Inte

nsity

au







41








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85 80 75 70 651200 1000 800 600 400 200 0

1200 1000 800 600 400 200 0 85 80 75 70 65

(d)


Inte

nsity

au

Al 2p(c)

Li 1s

Al 2p

C 1s

O 1sMn 2p

F 1s

F(kLL )

(a)

Inte

nsity

au

Binding EnergyeV


C 1s

F 1s

O 1s

(b)

Inte

nsity

au

Al 2p

Binding EnergyeV

Inte

nsity

au











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0 20 40 60 80 10080

100

120

140


Cycle Number

Cap

acity

mA

hg

0 20 40 60 80 1000

20

40

60

80

100


Cycle Number

Cap

acity

mA

hg
























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0 20 40 60 80 1000

20

40

60

80

100

Cycle Number

Cap

acity

mA

hg



0 20 40 60 80 10080

100

120

140


Cap

acity

mA

hg

Cycle Number

Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode









4 CONCLUSIONS







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20 40 60 80

(551

)(4

44)

(622

)(5

33)

(531

)(440

)

(511

)

(331

)

(400

)

(311

)(2

22)

(111

)(d)

(c)

(b)

(a)

2 ThetaDegree

Inte

nsity

au







41








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85 80 75 70 651200 1000 800 600 400 200 0

1200 1000 800 600 400 200 0 85 80 75 70 65

(d)


Inte

nsity

au

Al 2p(c)

Li 1s

Al 2p

C 1s

O 1sMn 2p

F 1s

F(kLL )

(a)

Inte

nsity

au

Binding EnergyeV


C 1s

F 1s

O 1s

(b)

Inte

nsity

au

Al 2p

Binding EnergyeV

Inte

nsity

au











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0 20 40 60 80 10080

100

120

140


Cycle Number

Cap

acity

mA

hg

0 20 40 60 80 1000

20

40

60

80

100


Cycle Number

Cap

acity

mA

hg
























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0 20 40 60 80 1000

20

40

60

80

100

Cycle Number

Cap

acity

mA

hg



0 20 40 60 80 10080

100

120

140


Cap

acity

mA

hg

Cycle Number

Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode









4 CONCLUSIONS







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85 80 75 70 651200 1000 800 600 400 200 0

1200 1000 800 600 400 200 0 85 80 75 70 65

(d)


Inte

nsity

au

Al 2p(c)

Li 1s

Al 2p

C 1s

O 1sMn 2p

F 1s

F(kLL )

(a)

Inte

nsity

au

Binding EnergyeV


C 1s

F 1s

O 1s

(b)

Inte

nsity

au

Al 2p

Binding EnergyeV

Inte

nsity

au











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0 20 40 60 80 10080

100

120

140


Cycle Number

Cap

acity

mA

hg

0 20 40 60 80 1000

20

40

60

80

100


Cycle Number

Cap

acity

mA

hg
























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0 20 40 60 80 1000

20

40

60

80

100

Cycle Number

Cap

acity

mA

hg



0 20 40 60 80 10080

100

120

140


Cap

acity

mA

hg

Cycle Number

Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode









4 CONCLUSIONS







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0 20 40 60 80 10080

100

120

140


Cycle Number

Cap

acity

mA

hg

0 20 40 60 80 1000

20

40

60

80

100


Cycle Number

Cap

acity

mA

hg
























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0 20 40 60 80 1000

20

40

60

80

100

Cycle Number

Cap

acity

mA

hg



0 20 40 60 80 10080

100

120

140


Cap

acity

mA

hg

Cycle Number

Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode









4 CONCLUSIONS







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Mater 19 1816 (2007)































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0 20 40 60 80 1000

20

40

60

80

100

Cycle Number

Cap

acity

mA

hg



0 20 40 60 80 10080

100

120

140


Cap

acity

mA

hg

Cycle Number

Al2O3-E electrode

Al2O3-P electrode

Uncoated electrode









4 CONCLUSIONS







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enhancing high-rate and elevated-temperature...

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