Effect of composition on the structure of lithium

12 Pages·2017·6.14 MB·English

by  Alpesh Khushalchand Shukla

Showcasing research from Dr Alpesh Khushalchand Shukla and As featured in:
collaborators at Berkeley Lab, SuperSTEM and Envia Systems.
Effect of composition on the structure of lithium- and
manganese-rich transition metal oxides
The crystal structure of lithium- and manganese-rich transition
metal oxides, cathode materials for lithium-ion batteries, has
been widely debated. In this work, we establish a definitive
structural model for these materials and demonstrate the effect
of composition on their bulk as well as the surface structure.
The results demonstrate the importance of a thorough
characterization of the pristine material’s bulk and surface to
avoid misinterpretations and ambiguities before turning to
transformation studies of cycled battery materials. See Alpesh Khushalchand Shukla
et al., Energy Environ. Sci.,
2018, 11, 830.
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Energy &
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Effect of composition on the structure of lithium-
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Ac Broadercontext
en Modernsociety’sincreasingdemandsforcleaner,safer,cheaper,andlonger-lastingenergystoragesolutionsaredrivingmanyindustriestodevelopnewand
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O improvedbatterymaterials.Thisisparticularlytrueintheautomotivesector,wheretheelectrochemicalshortcomingsthatcurrentlyplaguetheimplementation
ofhighcapacitymaterialssuchaslithium-andmanganese-richtransitionmetaloxides(LMRTMOs)asaviablelong-termchoiceoflithium-ionbatterycathode
mustbeaddressedurgently.Onepromisingavenueconsistsindeviatingfromthewidelystudiedhighlithium/transitionmetal(Li/TM)ratiochemistries.
However,almostnoinformationisavailableontheeffectofcompositiononthecomplexstructureofthepristine,uncycledLMRTMOs.Thisworkaddressesthis
long-standingissueandidentifiesthebulkstructureasasinglemonoclinicphasethroughtheentirecompositionrange,withcomplexchangestolocalordering
astheLi/TMratiodecreases.Crucially,wealsopresentstrongevidenceforthepresenceofaspinellayeronthesurfaceofpristineLMRTMOsirrespectiveof
composition, and emphasize that its presence in the pristine state cannot be ignored when studying cycled materials. These findings demonstrate the
importanceofathoroughcharacterizationofthepristinematerial’sbulkandsurfacetoavoidmisinterpretationsandambiguitiesbeforeturningtophase
transformationstudies.
1 Introduction
Advances in the electrification of vehicles and the successful
implementationofadecentralizedelectricitygridthatefficiently
aEnergyStorageandDistributedResourcesDivision,LawrenceBerkeleyNational
utilizesrenewableenergysourcesarelargelydependentonthe
Laboratory,Berkeley,California,USA.E-mail:[email protected]
bSuperSTEM,Daresbury,UK development of materials that would enable energy storage
cNationalCenterforElectronMicroscopy,MolecularFoundry, systems with substantially superior energy density, cycle life,
LawrenceBerkeleyNationalLaboratory,Berkeley,California,USA and safety compared to those provided by the state-of-the-art
dErichSchmidInstituteofMaterialsScience,AustrianAcademyofSciences,Leoben,
batteries. Applications such as electric cars specifically require
Austria
eEnviaSystems,Newark,California,USA batteries with high capacity to alleviate range anxiety, enable
†Electronicsupplementaryinformation(ESI)available.SeeDOI:10.1039/c7ee02443f longer lifetimes, and provide higher safety standards. It is
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knownthatincreasingthelithiumcontentinlayeredtransition Table1 CompositionofLMRTMOsstudiedasmeasuredusingICP-OES
metal oxides (Li M O , where x 4 0 and M refers to a
1+x 1(cid:2)x 2 Sample Li/TMratio Chemicalformula
combination of transition metals such as nickel, manganese,
andcobalt) can lead tocapacities greater than 250 mA h g(cid:2)1.1 HighLi:TM 1.353 Li1.15Ni0.1748Mn0.496Co0.18O2
MediumLi:TM 1.222 Li Ni Mn Co O
However, in spite of the high capacities found in lithium- and LowLi:TM 1.17 Li1.1 N0.i227 M0n.438 C0o.235 2O
1.079 0.248 0.411 0.263 2
manganese-richtransitionmetaloxides(referredtoasLMRTMO
in general, and LMR NMC where the transition metals are
specifically nickel, manganese, and cobalt), their commercial NMC materials under study, as measured using inductively-
e. applicationhasbeenhinderedbyshortcomingsinthematerials coupled plasma optical emission spectrometer (ICP-OES) are
nc includingalargefirstcycleirreversiblecapacityloss,2voltageand showninTable1.Fig.1ashowstheratecapabilityandcycling
AM. Lice capacity fade,3 high DC resistance at low states of charge, and stabilityexhibitedbythesematerialsacrossaLi–metalanodeat
25:20 ported ctroamnspiotiuonndsmesttauldideidssohlauvteionh.a4dSoafraerl,atmivoeslyt ohfigthheliLthMiuRmTMtOo hFiigg.hSv1olitnagEeS.IT.†heUncodrerresstpanondidninggthveolrtaagteecparpoafibleilsityaroefsdhioffwenrenint
3 12:0 Un transition metal (Li/TM) ratio of 1.5, at which the previously compositionsisextremelycriticaltodecidethepracticalutilization
202n 3. describedshortcomingsaremorepronounced.Recentelectron ofthematerialforvehicularapplications.Theratioof2C/0.1Cfor
ed on 1/16/s Attributio mtmhioecinrrooscbcluionlpkicyssvttaruurdicaitenustrseo,5n–c7othnwisshiscitllesasotshfoefdmsoumartfaeaircnieasslscoohfrarvceeesrpstohaniondwinnfagtchetatost twchlieetahrthllyoriewne,ddmicifaefdteeirsuemnthtaactontmdhephcoigoshmitiLopino/sTsiMtairoerna8tw6ioi,tsh8,3rloeawsnpedercL7tii8v%aenlyd.foTMrhsniassmshtpoulwdesys
adon consistofaspinelstructure.6Itislogicaltoassumedecreasing higherratecapability.Ratecapabilityhasalwaysadirectimpacton
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wnlom theLi/TMratiocouldmitigatetheaforementionedissuesalbeit the DC-resistance behavior of the respective material and in
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nuarer a structurewasperformedonLMRTMOcontainingonlymanganese materials comes at the cost of increased voltage fade during
s Article. Published on 10 Ja This article is licensed und saLssinattttvnMosuruaduteddiRclfyyianfTteeeuboistMcclrfectkoeOmlenoeollnaiafcLtirtehntteMlhrhyeraetieRmutahsulnreeiNsersdemstMtwerruiairadCletsntthestiesnaohrrilwogiatasaidvniomlte.swghnaibwintmthehiegteirehgnLietahaililr/otlsTehwL’cpMeieoo/ralTmerrntM‘ac‘edpsttdritomooo.rnsaciOecehtihdnivseoeiitmwsuo,r8,amma,i9lcnblea,autLtlintrtinhp/idTcgear’.Mo’nmHpsoLyereaisardnMjttoteeciireOomteiast,2aiyatalnhteooriddeecrf cIvu2Fmtoy.it2cgiialsltl.tiaizedngar2Xegetisai-oirraslraasnnhayn.bdoigAdlnweeasisffnationsrbthiaknhaocoetcertewtridepoenerXanyDrs-ireaCtnaopyirpnFoelsbidiglictosia.afftwtia2inrnoabvncch,oet.ilitgtlohaohngeweeriapnapcactroettowitnsveesisrtrinytthaysdenuodoerfnfihntpiitgregheahdekoesisprseceefhrontaahuetrirrgnnegedgey.
ces In this work, we analyze several LMR NMC compositions between 201 and 301 decreases with a lower Li/TM ratio. The
c
n A withvaryingLi/TMratiosandsystematicallystudytherelationship XRD pattern of the material with the lowest Li/TM ratio has a
pe betweenthebulkandsurfacestructuresandtheelectrochemical closer resemblance to that of layered oxides with a trigonal
O
performanceofthesematerials.Theapplicationsgoalofthisstudy structure. However, additional peaks between 201 and 301 are
is to identify an optimum composition that could maximize the still visible. It has been argued that XRD patterns cannot be
LMR NMC’senergy while reducing the voltage fade over cycling. used to confirm whether the LMR NMC materials consist of
Thiseffortmustbeginwithclearlyunderstandingthestructureof eithertwophases,onetrigonalandonemonoclinic,orasingle
thematerialsatthepristinestage(priortoelectrochemicaltesting). monoclinic phase.10 Moreover, small peaks in this range of
Samples with three different Li/TM ratios were studied using apnglespcan also arise from a P3112 structure that consists of
electronmicroscopytechniquessuchashighangleannulardark ( ffi3ffiffi(cid:3) ffi3ffiffi)R301superlatticeorderingofthemanganese,nickel,
field (HAADF) imaging in the scanning transmission electron and cobalt in the transition metal layer.11 Therefore, a closer
microscope (STEM), electron energy loss spectroscopy (EELS), examinationusingtechniquesthatprovidehigherspatialresolution,
X-rayenergydispersivespectroscopy(XEDS),andvirtualimages such as aberration-corrected STEM, is warranted. In the following
formedfromSTEMdiffractionimagesrecordedatmanyprobe sections, results from various electron microscopy studies on
positions. LMR NMC materials with high, medium, and low Li/TM ratio
willbepresented.
2.3 StructureofLMRNMCwithhighLi/TMratio
2 Results and discussion
ThestructureoftheLMRNMCwithahighLi/TMratioof1.35is
2.1 Electrochemicalmeasurements
shown in Fig. 3. The structure is similar to that observed in a
Inordertostudytheeffectofcompositiononthestructureand recentlypublishedstudyonLMRNMCwithhighLi/TMratio,6,7
electrochemical performance of LMR NMC materials, three andalsotothatofotherLMRTMOssuchasLi Ni Mn O ,5
1.2 0.2 0.6 2
materialswithdifferentLi/TMratioswerepreparedandelectro- whoseLi/TMratioisalso1.5.Therefore,onlyabriefdescription
chemically characterized. The compositions of the three LMR isprovidedhere.Fig.3bshowsatypicalHAADFimagetakenin
Thisjournalis©TheRoyalSocietyofChemistry2018 EnergyEnviron.Sci.,2018,11,830--840 | 831
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%
namely [100], [110], and [110], shown using orange, blue, and
greencolors,respectively.Thesevariantsgiverisetostreaksina
diffractogram because the individual reflections from these
variants are very close to each other as shown in the inset
of Fig. 3a. The intensity of the shared column is relatively
consistent throughout the primary particle except for the
variation caused by the change in particle thickness since the
image was taken from a tapered region near the edge of the
e. particleasshowninFig.3a.Anynon-uniformityinthispattern
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5:20 orted as Li1.2Ni0.13Mn0.54Co0.13.6 EELS and XEDS maps taken with
2p sub-nanometer resolution over several areas in different
3 12:0 Un particlesexhibitedrelativelyuniformdistributionoftransition
202n 3. metals except at certain facets where higher concentration of
1/16/butio nickelwasobserved,asshowninFig.4.
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nuarer a contrasttothehighLi/TMsample,theratioofLitoTMinthis
0 Jaund shared column appears random in some of the rows. These
d on 1ensed einffeFcitgs.c5acn.Ibteshoobusledrvbeednmooterdetchleaatrtlhyeininttheenlsiintyeopfrothfielecsosluhmownns
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s showsthetwostreaksbetweentherowsoffundamentalspots
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ce which are characteristic of the structure consisting of three
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n monoclinic domains. The domains corresponding to [100],
pe [11%0], and [110] can still be identified, although not as clearly
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as with the LMR NMC with a high Li/TM ratio. This structure
was observed over the entire primary particle. It can thus be
saidthatthegeneralstructureconsistsofa(disordered)mono-
clinic phase, which additionally exhibits local compositional
Fig.1 A comparison of electrochemical data for LMR NMC with low, variations.TheLi/TMratioofthesharedcolumnsisnotconstant
medium,andhighLi/TMratios:(a)specificcapacityinhalfcellcoincells throughout the particle, resulting in a degree of disorder in the
over various discharge rates from C/10 up to 2C, then 50 subsequent
structure.SimilartothehighLi/TMratioLMRNMC,EELSmaps
cycles atarateof C/3.Thefirstchargewas to4.6V,thenallfollowing
cycleswerebetween2.0–4.5Vvs.Li/Li+.(b)Normalizedaveragevoltage takenwithsub-nanometerresolutionoverseveralareasindifferent
overthe50cyclesinthehalfcells,(c)differentialcapacityplotforthefirst particles showed a relativelyuniform distributionoftransition
cycleinthehalfcells. metals,exceptforcertainfacetsasshowninFig.6.
2.5 StructureofLMRNMCwithlowLi/TMratio
[100] direction,wherethesubscriptsupercellreferstoa ThesamplewithalowLi/TMratiowasstudiedindetailusing
supercell
cell made by randomly stacking three variants of monoclinic HAADF imaging performed at various zone axes, giving a
structure fitted in an orthorhombic unit cell, as described in complete, three-dimensional characterization of its structure.
ref.6.TheHAADFimageexhibitsthepresenceofdoubletsthat HAADF images of the low Li/TM ratio material taken using
correspondtotransitionmetalcolumnsseparatedbyacolumn [100] zone axis, as shown in Fig. 7a, show a further
supercell
with lower intensity that consists of both transition metals increaseintherandomnessofLi/TMratiointhesharedcolumn
and lithium (called the ‘‘shared column’’ henceforth). The describedintheprevioussection.Thisisshownmoreclearlyin
HAADFimagealsodemonstratesthepresenceofdomainsthat the image with higher magnification (Fig. 7b) and the line
correspondtothreedifferentvariantsofmonoclinicstructure, profilesshowninFig.7cthatcorrespondtotherowsofatomic
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nuarer a facetsthatexhibitedhigherconcentrationsofnickel.
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d on 1ensed soefpealreamteemntoanloscelginreicgaatniodnt,riwgohnearelbpyhamseasnbgaasneedseo-nricohbsaerrevaastioanres
Article. PublisheThis article is lic Fig.3 HAADF images for LMR NMC with high Li/M ratio: (a) low wiaanssesstiiohggbnneseeebddruvtleotkoaaotrafreislgamaotmniovapnellloeypcshluifannosiircefo.a1rp2lml,h13tahdIsteiesstchrwoiobmhuuilpltdeioobsnrieteionmofonattsirenadininntsvhgietasitotaianrgletaamhtseoedutaagirlnhes
s magnification HAADF STEM image and (b) HAADF STEM image showing thisstudy,observationofanynon-uniformityinelementalcom-
s
ce domainscorrespondingtothreemonoclinicvariants.InsetinashowsFFTofb. positiondoesnotnecessarilyimplythepresenceofatwo-phase
c
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n compositestructureconsistingofseparateLi2MnO3(monoclinic)
e
p and LiNiMnCoO (trigonal) phases. Moreover, the homogeneity
O 2
columns shown in Fig. 7b. Certain rows show doublets like reported here refers to the results obtained from single grains
those observed in the LMR NMC with a high Li/TM ratio (for (excluding any surfaces) instead of over secondary particles as
example,lines2,4,5,10,and12),becausetheLi/TMratiointhe describedinthesereportsthateitherinvolvedgrainboundaries
shared column is high. A lower Li/TM ratio in the shared betweenprimaryparticlesornickel-richsurfaces.Ithasalsobeen
column leads to some rows that do not show the doublet shownrecentlythatevenparticlesthatappeartobemadeupofa
characterasclearly,asshowninlines3,6,8,and11.Inmany singlegraincan,infact,consistofseveralgrains.14Theoriginof
cases,thispatternchangesevenwithinthesamerow,withafew the presence of these grains within a primary particle can be
columns showing higher Li/TM ratio in the shared column attributed to the presence of several equivalent {111} planes in
(giving a doublet character) followed by several columns with theparentNaClstructureasdescribedindetailbyJarvisetal.15
lower Li/TM ratio, as shown in rows 1 and 6. The stacking of for Li MnNiO . The presence of these grains with different
1+x 2
rows with higher and lower Li/TM ratio appears to have no orientationsandtheresultinghiddensurfaceswithinaprimary
ordering attheobservedlengthscales.Thus, thereisa lackof particle further complicates structure determination of LMR
longrangeorderwherebyareaswithhighLi/TMratioandthose NMC. Indeed, high resolution HAADF STEM imaging and
withlowLi/TMratiointhesharedcolumnarenotextendedover spectroscopy results obtained over discrete grains have shown
several nanometers. It should be noted that this apparent that these elemental segregations are limited to surfaces, in
difference in structure, showing discontinuous rows of atomic agreementwithourresults.16Thepresenceofgrainboundaries
columns with and without doublets, is simply due to local within a primary particle is more common in LMRTMOs
compositional variation, whereas the symmetry remains the preparedusingco-precipitation,amethodusedmorecommonly
same for both types of regions. Both regions, with or without incommercialcathodematerials,ascomparedtothoseprepared
the appearance of doublets, are monoclinic. Furthermore, bymoltensaltmethod,whichmostlyresultsindiscreteparticles
althoughthereisavariationofcompositionattheatomicscale consistingofsinglegrains.6,17
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wnlom particle.Bothmapsshowrelativelyuniformdistributionoftransitionmetalsexceptatcertainfacets,wherehighconcentrationofnickelwasobservedas
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Fig.5 HAADF images for LMR NMC with medium Li/M ratio: (a) low magnification HAADF STEM image, (b) HAADF STEM image of the area
correspondingtotheredsquareina.InsetinashowsFFTof(b,andc)linescanscorrespondingtoeightrowsinb.
HAADF images taken using the [103] zone axis, intensitiesarenotuniformacrossthefieldofview,displayinga
monoclinic
which is orthogonal to the previously-discussed [100] level of randomness at this length scale. Furthermore, the
monoclinic
direction, further help in understanding the structure of LMR diffractogram clearly shows extra spots present at 1/3 and
NMC. A HAADF image taken from a single variant of a LMR 2/3rd distance from the fundamental spot, confirming the
NMCwithhighLi/TMratiowouldconsistoftworowsofatomic presenceofallthreemonoclinicvariants.Imagingthe sample
columns consisting of transition metals separated by a row of along the [001] direction, on the other hand, gives a
monoclinic
atoms with mixed lithium and transition metals, giving a HAADFimage thatapparently looksveryuniformasshown in
stripedpattern.Threevariantsthatarerotated1201fromeach Fig.7e.Thisapparentabsenceofdisorderinthisprojectionis
other would give rise to a criss-cross pattern as described in observed because the variants of monoclinic are stacked in
ref.6.InthecaseofNMCwithpuretrigonalstructurewithout suchawaythatcolumnscorrespondingtoamixtureoftransi-
any ordering of the transition metals, the intensity of all the tionmetalsandlithiumsitdirectlyaboveacolumnconsisting
columns (in the equivalent [001] direction) would be purely of transition metals, thus giving an averaged intensity
trigonal
equal. In Fig. 7d, which shows the HAADF image taken foreachcolumnthatappearsuniformthroughouttheparticle,
onLMRNMCwithlowLi/TMratiousinga[103] zone as one would observe in a [001] direction of a pure trigonal
monoclinic
axis, a criss-cross pattern is clearly observed, but the column phase,butwithalargercolumnspacing.Faint{110}and{120}
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3 0 Fig.6 EELSforLMRNMCwithmediumLi/Mratio:(a)HAADFimageand
202n 3. EELS maps showing relatively uniform distribution of transition metals Fig.8 EELSforLMRNMCwithlowLi/Mratio:(a)HAADFimageandEELS
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nuarer a exemplifying how FFTs do not necessarily provide the same imagingexperiments,wherea2DimageofthediffractedSTEM
0 Jaund informationasthatobtainedfromelectrondiffractionpatterns. probewasrecordedovera2Dgridofprobepositions,forminga
d on 1ensed Ttehcehsneiqrueseusltasndalszoonheigahxelisghfotrthsoelviimngpocrotamnpcleexofsturuscintugrems:uwlthipillee f‘‘o4uDr--SdTimEMenseixopnearlimdaetnats’’e.tTthhaetsewe4Dw-iSlTlEhMereeaxfpteerrirmefeenrtstoaallsowa
Article. PublisheThis article is lic daelmiercebtcirtgoiuonnitydtifhffraornmactHHioAAnAADDisFFmiimmoaareggiinnaggm,fboitirgucthaoneus[b00efo1ru]mstoehnfueoclli[n1fio0cr3d]aimrveoonciotdicoliinnnig.c pdspiaffattrteiaarcnltiortenhsaomtluawtpiaposinn.gs18uo–m2n0mpFaeirdgt.icu9lesasinswghito5hw6ls2a5ragneinfdeielielvdcidstruooafnlviddeiiwffffrraaatcchttiiioognnh
s
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Fig.7 HAADFimagesforLMRNMCwithlowLi/Mratio:(a)HAADFimagetakenusing[100]/[11%0]/[110] zoneaxisshowinglossoforderwithinthe
monoclinic
domainscorrespondingtothemonoclinicvariants,(b)HAADFimagewithhighermagnificationofasectionin(aandc)linescanscorrespondingtothe
rowsofatomiccolumnsin(bandd)HAADFSTEMtakenusing[103] zoneaxis,(e)HAADFSTEMimagetakenusing[001] zoneaxis.
monoclinic monoclinic
Thisjournalis©TheRoyalSocietyofChemistry2018 EnergyEnviron.Sci.,2018,11,830--840 | 835
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e.
c
n
M. ce
ALi
5:20 orted
2p
12:Un
3 0
202n 3.
1/16/butio
ed on s Attri
dn
ao
om
nlm
wo
oC
ary 2018. Da Creative Fdstiagrre.ka9kfiselc4doDirm-reSaTsgpEeoMnsdhaionnwagliyntsogistvhoaefritaLhnMrteR1eN(m[1Mo0Cn0o])wc(ildtinh)icvliorvtwauraiLalin/dMtasrk(rbaf)tiieiomld:a(igam)eassguhemoswmhieondgwipdnoigffsrivataicortiniaosnntof2pma(t[1tae1%sr0kns]),cta(oekr)erevnisrputuosainnldgdina[1grk0t0ofi]e/tl[hd1e1%i0mt]h/a[r1ge1ee0ms]mhooonnwoocinclingliincviczaorvinaaenritaan3xti,(s[(1cs1h)0ov])iwratiunnadgl
nuer (f)virtualdarkfieldimageoftheLMRNMCparticle.
ad
0 Jun
d on 1ensed patternsobtainedusingaprobesizeofapproximately2nmand duetotheprojectionproblem,weperformed4D-STEMexperiments
Article. PublisheThis article is lic abn[1res1istg0seh]potvefsarirdzifieaufnfnortfdasca2otminfoemtnnh.tseaTpmlhorteoesnfplcoeaocicrrtlriioenosfnipcssotpranehrdaeaiksnosebg,bsateeostrwvtdeheedeesncd[r1uti0hbe0eet]d,oro[it1wnh1%se0Fo]ci,glfoa.tsnhSed3e- mor[3en0sou2nt]lhmotseco.lnsionFacmiliigcn.iecpSpzh5aoarnstiieenclewaExiuSitsIhs.†inEagslhesscoietnwrvogeslnreatlhdvzeiaoffrnirraaeencstuatixoltiensnsoastihnbmtdeauif[nl3oae0utdi2no]ndusdsidiifrneoegnrcttiittcohhaneel
s inESI.†Fig.9bshowsthevirtualaperturesusedforthesethree andasupercellmadeusingthreemonoclinicvariantsareshown
s
ce variants. The fundamental reflections, shown in gray, are inFig.S6inESI,†alongwithanelectrondiffractionpatternfora
c
n A common for all three variants. As shown in Fig. 9c–e, the trigonalstructureinitsequivalentdirection([77%2]).Althoughthe
e
Op individual variants are not resolved as rows of atoms except slabs of monoclinic domains would overlap each other in this
forthinareasintheedgesduetotheaforementionedrandom- orientation, thespots corresponding tothe monoclinicvariants
ness in this structure, unlike in the case of LMR NMC with a
arewell-separatedandthe4D-STEMdiffractionmapsshowthat
highLi/TMratioshowninapreviouslypublishedstudy20where
theentireparticleismadeupofasinglemonoclinicphase.
thevariantsareclearlyresolvedusingthistechnique.However,
Although it is clear from these results that over the entire
itcanbenoticedthattheintensityfromthesethreemonoclinic
composition range studied here LMR NMC is not a ‘‘composite’’
variants are present roughly equally throughout the crystal.
materialconsistingof‘‘structurallyintegrated’’domainsofLiMnO
2 3
None of the 5625 frames in the dataset exhibited diffraction and LiNiMnCoO as suggested by earlier reports,1,3,21,22 there is
2
patterns without the streaks. The experiment was repeated
another structural model that also warrants consideration. Extra
using a step size of 5 nm over the entire primary particle and
reflections in electron diffraction patterns of layered oxides, both
identical results were obtained, as shown in Fig. S5 in ESI.†
withstoichiometric(Li=1)andexcesslithiumcompositions,have
These observations show that there are no areas having long- p p
ffiffiffi ffiffiffi
been attributed to long-range ordering of the type ( 3(cid:3) 3)
range order with a trigonal structure throughout the crystal
R301.11,23,24Mengetal.23proposedthatinthecaseofLiNi Mn O
(which would give only fundamental reflections without the p 0p.5 0.5 2
streaks). and Li1.11Ni0.3Mn0.55O2, there is an in-plane ( ffi3ffiffi(cid:3) ffi3ffiffi) R301
It can be argued, however, that each diffraction pattern in ordering of lithium, manganese, and nickel ions such that the
the 4D-STEM dataset is an averaged diffraction pattern taken overallsymmetryofthecrystalisP3112withsomeC2/mstacking.
throughthethicknessofthesample,andthatthepresenceof Yabuuchi et al.11 and Weill et al.24 proposed similar in-plane
anypuretrigonalregionsinthedirectionoftheelectronbeam ordering based on electron diffraction studies for materials
will not be observed in these patterns. To confirm that the containing nickel, manganese, and cobalt. We also considered
entireparticlesindeedconsistedofonlythemonoclinicphase this possibility for the presence of faint reflections and streaks
throughouttheparticleandthattherewasnomisinterpretation in electron diffraction patterns, although they can be clearly
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attributed tothepresence ofthe threemonoclinic variants. We (vis-a`-vis that of the bulk) plays on the electrochemical short-
found that although the electron diffraction patterns simulated comingssuchaspoorcyclelife,voltagefade,andDCresistance
using a unit cell with a superstructure closely match the isnotyetclearlyunderstood.Inordertoinvestigatetheinfluence,
experimental diffraction patterns for certain zone axes (such ifany,oftheLi/TMratioonthissurfacelayerandultimatelyon
as [001] ), the model fails for other zone axes, even high the electrochemical performance of the cathode material, we
trigonal
%
symmetry ones such as [010] and [110] as shown in Fig. S8 in studied the surface of the LMR NMC with the aforementioned
ESI,†whichshowselectrondiffractionpatternssimulatedusing compositionsusingHAADFSTEMimaging,4D-STEM,EELS,and
theunitcelldescribedinref.11.Infact,inalaterpublication, XEDS.WefoundthattheLi/TMratiohardlyhadanyeffectonthe
e. Boulineauetal.25usedamodelexampleofmonoclinicLi2MnO3 thickness, structure, or composition of this surface layer. All
c
n to show how a superstructure was not needed for the inter- of the LMR NMC samples, irrespective of the composition,
M. ce
ALi pretation of these extra reflections and how these can be exhibitedaspinellayer.EELSandXEDSexperimentssuggested
5:20 orted described simplybythestacking ofvariantsofthe monoclinic that the spinel surfaces exhibited a higher concentration of
2p phase, as we suggest here. We agree with this interpretation, nickel (and some cobalt) at the expense of a lower content of
3 12:0 Un andourresultsshowthatthisistrueforLMRNMCoverawide manganeseatthesurfaceasshowninFig.4,6,8andFig.S8in
202n 3. range of compositions. This does not suggest that there is no ESI.† We note that the exact composition of this thin layer of
oaded on 1/16/mons Attributio smdohfiffaomkrraeto-cnritatoinoccngllieenpairaoctrtvtdheaerarrnitaisntnhgatesreorearfxtothcrralemurrsetetdheflarepinncrtigtinoo,cnlibopsunaatglln-ytrdhadnasutgtereteth(aopektsffi3ffiffihs(cid:3)iienmppuerlffi3leffieffia)sctReitonr3onc0nes1 stHuhpsAiiisnnAegDslptFmhinieimeg[l1ha0tgl3aeb]ysemetaroanfkfoaecetclnintheiocidgnzhboLeynrMetharReexsibNsou,MlluwkCthicoeownrmei.thapFnoimgsai.eptidp1oi0rnuoa.mxFiaminLga.id/t1Te0bMlyssh2rhoanotwmiwos
wnlom orderingoftransitionmetals.Thelatterwouldalsoleadtoextra thick spinel surface layer having a [112] zone axis is clearly
y 2018. DoCreative C rFsehigfol.ewScn6ti,oiwnnhsFiciignh.tiSsh2eclienroarEwlSyoIn.f†oftuonbdsaemrveendtianlerexpfleercitmioennst,aalspasthtoerwnnsains vLiMsiARbsleNd.MiSsciCmuwsisliaetrhdlyte,haFerilgileo.rw1,e0tshcteaLnni/odTnMd-usnrhaiftooiwormianit[[y101o100f]]tsrpzaionnnesleistuaioxrnifsa.mceeotanl
nuarer a McCallaetal.studiedtheeffectofcoolingrateoncombinatorial distribution reported in recent studieson samples with multiple
0 Jaund samples prepared for a Li–Co–Mn oxide system.26 Although their grainscanbeattributedtothepresenceofgrainboundariesand
d on 1ensed stoystthemeredsidulntsootbctoanintaeidninnictkheel,psroemseentofsttuhdeiyr.fTinhdeyinfgosucnadntbheatreplhaatesde shuorwfatcheesstehantoanr-eunriicfhoermriintiensicckaenlabnedocbosbearvlte.dFiegv.e1n0dindtehmeocnassetraotfeas
Article. PublisheThis article is lic sripnaertpetepahroaearfcteicadoosnoeulsooiinfncgcmgu(iorBnsstt1efocr1morCmleamdmyieiaenrtree(cid:2)cdi1ca)oo,loxwciladihntehiglseordsiaytetnedmtsoha(eeBtsseirnz1ieo0adlts1Co.ucIstmciunwirgnasf(cid:2)aon1r)vo,estaaresymdusptlsholeeawdst single grain. It is well-known that crystals often contain growth
s at intermediate cooling rates, the system does not have sufficient
s
e
c time to make larger crystallites for each phase. They also
c
A
n observedthatXRDpatternsforthesampleswithcompositions
e
Op between LiCoO2 and Li2MnO3 that showed phase separation
exhibited peak splitting, especially at the peaks around 451
and 651 2y, a feature that was absent in our XRD patterns. It
shouldbenotedthatthephaseseparationforthecompositions
discussed by McCalla et al. do not refer to a coexistence of
LiCoO andLi MnO phases.
2 2 3
2.6 Effectofcompositiononsurfacestructure
SeveralstudieshaveshownthatthesurfaceofLMRNMChasa
different structure and composition than the bulk.27,28 In a
recently-published study on LMR NMC with higher Li/TM
ratio,6itwasdemonstratedthatthissurfaceisaspinel(typically
nickel-and/orcobalt-enriched,based onthecomposition)and
has an orientation relationship with the bulk given by
(001) 8(11%1) , [010] 8[110] , where subscripts M and S refer
M S M S
to the monoclinic and spinel phases, respectively. The spinel
phase is preferentially formed on {131} facets. The Fig.10 HAADF images showing surface spinel on LMR NMC particles:
monoclinic
(aandb)HAADFimagestakenonLMRNMCwithmediumLi/TMratiousing
results were confirmed in a detailed study by Johnston-Peck
[001] zoneaxis,(candd)HAADFimagetakenonLMRNMCwith
et al.7 published recently. In spite of several studies that have monoclinic
low Li/TM ratio using [010] zone axis. Note that the top most
monoclinic
discussed the role of phase transformations on the surface of atomiclayersofspinelsurfaceshowpartialdamageduetoelectronbeam
thecathodeparticles,29–32therolethatthisspinelsurfacelayer sputtering.
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ledges,33suchasoneshownusingaredarrowinFig.10d,which the composition range studied. In the bulk, although the
showsahiddensurfacelayercontainingthespinelstructure.Since structure across the different compositions consists of an
thespinelsurfaceisrichinnickel,anXEDSmaptakeninprojection aperiodic arrangement of monoclinic variants, there is an
would show an area with relatively higher manganese content obviousdecrease inorderaswe decrease theLi/TMratio. The
sandwichedbetweenareaswithrelativelyhighernickelcontent. transitionmetallayerinLMRNMC,whenexaminedalongthe
Forcomparison,wealsostudied thesurfaceofa‘‘stoichio- [100] direction, consists of pairs of columns of transition
metric’’NMCwithanominalcompositionofLiNi Mn Co O metalsseparatedbyamixed-contentcolumnconsistingofboth
1/3 1/3 1/3 2
and found that the sample also exhibited a spinel surface with lithiumandtransitionmetals.TheratioofLi/TMinthisshared
e. thesameorientationrelationshipwiththebulkasobservedwith columnismoreorlessuniforminLMRNMCwithmoreexcess
c
n theLMRNMCsstudiedinthiswork.Thefactthatthethickness, lithium and it becomes random at lower amounts of excess
M. ce
5:20 Aorted Li rdeoleastivneoctocmhpanosgietiowni,thanddiffoerrieennttaLtiio/TnMrelraattiioonsshsuipggweistths tthheatbuthlke pliuthrieumm.oWnoecldinidicnpohtafisnedcroengsioisntisnwgitohfpmuarengtraingeosnealo(nLliyMaOs2)thoer
2p formation of this spinel layer is common for a wide variety of transition metal (Li MnO ). This observation is particularly
3 12:0 Un layeredoxides.Thematerialsinthisstudywerepreparedusinga relevant in the conte2xt of a3 drive towards higher-performance
202n 3. co-precipitationmethoddescribedintheMethodssection,but batterycathodes,becauseitisacommonpracticetouseextra
ed on 1/16/s Attributio ssstiymundtihilaeesrsirsseustgeugclethssnthitqahuvaeetsbthseueencshpsihansoewtlh,newihmniocLhlMtemRnoNssaMtltlCimkemeltyahdfooedrmw.6i,s1t7hdTuohtrhienseger lmdituehrtiiaunlmgosxdyinudtrehisnegsiinsth,eoanrsddyenritthitesosdicsioffiomfcupstletonitscoahatieoccmfuoerratrtitechlyelicthlooinsufsmirmotfraalnnitsheiitxuiaomcnt
dn
ao thesynthesisprocess,isacommonoccurrenceinawiderange stoichiometric composition (Li/TM ratio = 1) in these materials
om
wnlom of layered oxides, including both lithium-rich and stoichio- even with well-established techniques such as inductivelycoupled
y 2018. DoCreative C mfliatcehetirtuicm(wchodimicffhpuosissioitniao)lsnoas.ntdThheaefasfcepewitneculonfriortersmcpesollnsodniolnyfgosptnointthehleeaucpntasttahasbolaef pnloawsTmhthaeasotepwtsietcrahulacevtmeuraiaslssliooondbessmepreovcantrtsoitosrnacotsepday,.rienathlletchaesemoforceomimpopsoirtitoannst
nuarer a stabilizing layer. The surface of this spinel layer has a {111} with lower Li/TM ratio, a significant improvement in electro-
0 Jaund facet,whichisknowntobeaverystablesurface.34Itisnotclear chemical performance, especially with respect to voltage fade
d on 1ensed iitf,thbiuststphieneflalcatyethraftacpilriitsattinesetshaemdpiffleussi(opnrioorfltiotheiulemctroorchheinmdiecrasl acanpdacDitCy. rTehsiestfaanccte,thaaltbethiteacthtahraectceorsisttiocfs soofmtheedseucrrfeaacsee dino
Article. PublisheThis article is lic tcfaeoorsermtmimnpagoon)sscidtetiilod(irknenedlosyutucdegexugdheesivbttoosiltttatahhgneaeytcfcahthhdaaeenn,gcgcehaespaininancgtiehtthsyeefisanpbdiueenl,lekealcnswttdrrhouDeccnhCtuevrmraeersiyicosianftlagLnpMtceherRe)- ntmchaoiaptgtahscittightpnyelifaftaiyrcdaaaenn,lstvaflooryglrtcemahrgaaretnoifgolaeendiweni,nitathhnthedcehmDabCanungrlieekfsseisoistnftaatctnihoocemne.pcoWaoftshhiisitosliedouneethssmesuruaegctgheherasiavatessl
s NMCratherthantoeffectsrelatedtothissurfacelayer. been very detailed and systematic studies on the effect of
s
ce Wewouldliketoemphasizethatuseofhighspatialresolution, cycling on phase transformation at the surface of layered
c
n A electronprobe-basedimagingandspectroscopywasessentialin oxides,29,30itisoftenassumedinthesestudiesthatthesurface
e
p revealing the formation of surface spinel structure on specific has the same crystal structure as that of the bulk. The demon-
O
crystallographicplanes.TechniquessuchasX-rayphotoelectron strationofthepresenceofasurfacespinellayeronlayeredoxides
spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), with a wide range of compositions in this study therefore
although being surface-sensitive can only provide information warrants a closer and detailed examination of as-synthesized
averaged over all the facets of the particles and therefore do samples before studying any phase transformation that may
not provide a complete or accurate description of the surface occuruponcycling.
structureandcomposition.
3 Conclusion 4 Methods
Materialssynthesis
Insummary,thepresentstudyisthefirstindemonstratingthat
even small amounts of excess lithium (or Li/TM ratio 4 1) in Thelithium-richNMCmaterialsweresynthesizedbyco-precipitation
LMRNMC materials result ina material consistingsolely ofa reaction.35,36 Stoichiometricamountsof metal sulfates were slowly
monoclinic phase rather than one with the parent trigonal dripped using a peristatic pump into a continuously stirred tank
structureofLiMO ,whichisoftenassumedforlayeredoxides. reactor (10L CSTR), wherein sodium carbonate and ammonium
2
While decreasing the lithium content resulted in a significant hydroxidewerealsoaddedusingaperistaticpump.ThepHofthe
changeintermsoforderandatomic-levellocalcompositionin reaction was maintained at approximately 7.5 and the reaction
thebulkofLMRNMCprimaryparticles,nonoticeablechange temperature was maintained at 50 1C while the agitation was
in structure or thickness was observed for the surface spinel maintainedat500rpminorderenableacompleteprecipitation
layerthatwasconsistentlyobservedonspecificfacetsofthese with a well-defined particle morphology. The metal carbonate
materials.Thesurfacespinelexhibitedahigherconcentration precursor was then mixed appropriately withlithium carbonate
of nickel and cobalt compared to that in the bulk, across andcalcinedat5501Cfor10hand850deg.Cfor20h.
838 | EnergyEnviron.Sci.,2018,11,830--840 Thisjournalis©TheRoyalSocietyofChemistry2018