Characterization of thermally aged AlPO4-coated LiCoO2 thin films.

0.76 MB·English

by  Jung, Eunhye

JungandParkNanoscaleResearchLetters2012,7:12
http://www.nanoscalereslett.com/content/7/1/12
NANO EXPRESS Open Access
Characterization of thermally aged AlPO -coated
4
LiCoO thin films
2
Eunhye Jung and Yong Joon Park*
Abstract
The electrochemical properties and stability during storage of pristine and AlPO -coated LiCoO thin films were
4 2
characterized. The wide and smooth surface of the thin film electrode might provide an opportunity for one to
observe surface reactions with an electrolyte. The rate capability and cyclic performance of the LiCoO thin film
2
were enhanced by AlPO surface coating. Based on secondary ion mass spectrometry analysis and scanning
4
electron microscopy images of the surface, it was confirmed that the coating layer was successfully protected from
the reactive electrolyte during storage at 90°C. In contrast, the surface of the pristine sample was severely
damaged after storage.
Introduction The LiCoO thin film electrode was introduced as a
2
Since Sony first developed lithium-ion batteries, exten- pristine cathode to investigate the intrinsic effect of
sive research has been carried out to obtain enhanced AlPO coating on the electrochemical property. The
4
specific capacity, cyclic performance, and stability [1-4]. smooth and wide surface of the thin film electrode
Surface coating of cathode powder is one of the most proved to be good for characterizing surface reaction. In
effective methods for improving the electrochemical particular, this article focused on the effects of coating
property of lithium-ion batteries. As coating materials, on rate capability, cyclic performance, and stability of
oxides [5-8] and phosphates [8-11] have been employed the surface of the cathode film.
to suppress unwanted interface reactions and enhance
the electrochemical property of the cathode. However, Experimental details
the property of a coated cathode was highly dependent The pristine LiCoO thin film was supplied by GS
2
upon the coating material, coating thickness, and coat- NanoTech Co., Ltd (Gangdong-gu, Seoul, South Korea).
ing shape [12-15]. Hence, careful characterization of the To prepare the coating solution, aluminum nitrate non-
coating layer of the cathode has been demanded. How- ahydrate (Al(NO ) 9H O; Sigma-Aldrich, St. Louis, MO,
3 3 2
ever, pristine cathode powders possess a circular shape USA) and ammonium dihydrogen phosphate ((NH )
4
with a diameter of several micrometers in general. After H PO ; Sigma-Aldrich, St. Louis, MO, USA) were dis-
2 4
surface modification by coating, a number of nanosized solved separately in 10 mL of a mixed solvent consisting
particles attach to the surface of the pristine cathode of distilled water, 1-butanol, and acetic acid. Then, the
powder. Thus, it has been difficult to characterize the solution was stirred continuously for 1 h at 25°C, after
coating layer directly because of the small particle size which the solution was coated onto a LiCoO thin film
2
of the pristine powder, the very low thickness of the substrate using a spin coater (K-359 model S-1; supplied
coating layer, and the rough surface of the positive elec- by Kyowa Riken Co., Ltd., Tokyo, Japan). The coated
trode. Moreover, most techniques used to analyze the LiCoO thin films were heat-treated in a rapid thermal
2
interface layer are seriously hampered by other compo- annealing system at 400°C for 30 min. The microstruc-
nents of the electrode, such as the binder and carbon. tures of the films were observed by field emission scan-
Herein, the authors prepared an AlPO -coated LiCoO ning electron microscopy (JEOL-JSM 6500F; JEOL Ltd.,
4 2
thin film and investigated its electrochemical property. Tokyo, Japan). The electrochemical characterization of
the coated LiCoO films was performed in nonaqueous
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*Correspondence:[email protected] half-cells. The cells were subjected to galvanostatic
DepartmentofAdvancedMaterialsEngineering,KyonggiUniversity,Suwon,
cycling using a WonAtech system (WonAtech Co., Ltd.,
Gyeonggi-do,443-760,RepublicofKorea
©2012JungandPark;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductionin
anymedium,providedtheoriginalworkisproperlycited.
JungandParkNanoscaleResearchLetters2012,7:12 Page2of4
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Seoul, South Korea). Secondary ion mass spectrometry
[SIMS] analysis was used to characterize both coated (a)250 (b)
0.2mAcm-20.4mAAcm-20.6mAcm-2 4.2
aipSBnnIeafMrdsofirocSpmr;rSmaCictseAiitoediMnnnuc.eEesTCfinihIlAngemssaCetsitocCtumohAtaeporMaab(ncBEttyauCe,isrnAPiaznaatIrhtMiciesoe,SnncF-to6mrenafrens)(a.tcmsieAtua)urgeCaenmtnset+ttehinecpeltressKimemwocearetonerreyart -2Capacity(Ahcm)11205050000 PArlPisOtin4e Voltage(V)2333334.......8024680 0.6mAAcm-20.4mAAcm-20.2mAcm-2
ion beam with a beam energy of 5.0 kV was used. The 00 10 20 30 40 50 60 2.60 50 100 150 200
Cycle (times) capacity(Ahcm-2)
beam current was 30 nA, and the raster size was 200

μm × 200 μm. (c)4.2 (d)250 45oC, 4.25~3.0V, 0.4m Acm-2
Results and discussion 34..80 -2m)200 PAlrPisOtin4e
cTthohiacetkecndreoLsssisCo-osfeOtch2teiothLniianClofiimOlm2asgfeiilsmopfrwtehasesenaptperpdisrtoiinnxeiFmiaganutderleyA14l.PTμOmh4e-. Voltage(V)23333.....80246 0.60m.4AAmcAmc-2m-20.2mAcm-2 capacity(Ahc11505000
The coating layer of the coated film was not clearly 2.60 50 100 150 200 00 10 20 30 40 50
capacity(Ahcm-2) Cycle (times)
observed in the scanning electron microscopy [SEM]
Figure2TheelectrochemicalpropertiesofpristineandAlPO -
image, which may have been due to the thin coating 4
coatedLiCoO thinfilms.Thefilmsweremeasuredinthevoltage
2
thickness. However, the Al and P elements were con- rangeof4.25to3.0V.(a)Dischargecapacitiesofthefilmsatcurrent
firmed using energy dispersive spectroscopy [EDS] ana- densitiesof0.2,0.4,and0.6mA·cm-2.Initialdischargeprofilesof(b)
lysis, which implied the existence of an AlPO4 coating pristineand(c)AlPO4-coatedfilms(thefirstcycleofFigure2aat
eachcurrentdensity).(d)Cyclicperformancesofthefilmsat45°C
layer. To confirm the coating effect on the electrochemi-
(currentdensitywas0.4mA·cm-2).
cal property, the discharge capacity and cyclic perfor-
mance were measured at current densities of 0.2, 0.4,
and 0.6 mA·cm-2 in a voltage range of 4.25 to 3.0 V. a superior discharge capacity and cyclic performance.
The current densities could be converted to approxi- Figures 2b and 2c present the voltage profiles of the
mately 1-C, 2-C, and 3-C rates, considering that the pristine and coated samples at current densities of 0.2,
average capacity of the LiCoO thin film is approxi- 0.4, and 0.6 mA·cm-2 as a function of the capacity (the
2
mately 50 μAh·cm-2·μm-1. The film electrodes do not first cycle of Figure 2a at each current density). The dis-
contain conducting agents, such as carbon, and they are charge capacity of the pristine film dropped rapidly at a
vulnerable at the rate capability. As shown in Figure 2a, high current density. However, the coated film showed
the initial discharge capacities of the pristine and much improved capacity retention under the identical
AlPO -coated samples were similar. However, with an condition.
4
increase in the current density, the coated film showed Thecoatingeffectontheelectrochemicalpropertywas
veryclearlyobservedduringcyclingathightemperature
(45°C).Thedischargecapacityofthepristinefilmsignifi-
cantly decreased to nearly zero during 20 cycles (Figure
(a)
2d). Cyclic performance of the coated film also deterio-
rated compared with Figure 2a. However, the coated
sampleshowedmuchenhancedcyclicperformanceover
the pristine sample. The improved rate capability and
cyclic performance of the coated sample have been
(cid:24)(cid:745)(cid:80) explainedbytheprotectiveeffect ofthecoating layer.In
general, the surface of the cathode forms an interface
(b) layer during cycling due to a reaction with the acidic
electrolyte. The transition metals (Co, Ni, Mn) of the
cathode could easily dissolve into the electrolyte so that
(cid:36)(cid:79)(cid:15)(cid:3)(cid:51)
the unwanted interface layer could be formed, which
interruptsdiffusionofthelithiumionsandmovementof
electrons during cycling. A stable coating layer (AlPO )
(cid:24)(cid:745)(cid:80) 4
could protect the surface of the cathode electrode from
Figure 1 SEM images of (a) Pristine and (b) AlPO4-coated an attack by the acidic electrolyte and suppress the for-
LiCoO thinfilms.TheEDSpeakofthecoatinglayerisshownon
2 mation of an unwanted surface layer, leading to the
theright.
enhancedratecapabilityandcyclicperformance.
JungandParkNanoscaleResearchLetters2012,7:12 Page3of4
http://www.nanoscalereslett.com/content/7/1/12
As a follow up test, the pristine and coated samples
(a) Before storage
were charged to 4.25 V and stored at 90°C for 1 week
105
to observe the protective effect of the AlPO coating
l(avyeerrsuusnddeeprtha)hoafrsthhecopnridsittiinoen.anTdhecocaotnecdenfitlmratsio4wnasprmoefiale- unts/sec)110034
o
sured by SIMS of the pristine sample. Co-containing ion y(c102
(133Cs59Co+), O-containing ion (133Cs16O+), Li-contain- nsit
ing ion (133Cs7Li+), Al-containing ion (133Cs27Al+), and Inte101 PLCi o OAl (cid:24)(cid:19)(cid:19)(cid:81)(cid:80)
P-containing ion (133Cs31P+) were recorded for the pris- 1000 20 40 60 80 100
Sputter Depth(nm)
tine and coated samples. As shown in Figure 3a, the

pristine film consisted of a homogeneous concentration
(b) After storage
of Li, Co, and O, except for a small deviation in the sur- 106
face prior to storage. However, it clearly shows that the
c)105
CTohewSaEsMdisismolavgeedodnurtihnegrsitgohrtagaels,oascosnhofiwrmnsinthFeigduarmea3gbe. unts/se104
foiflmtheshsouwrfaecdecdleuarirngposltyohraegder.aTl hgerasiunrsfabceefoorfeasLtoiCraogOe2. nsity(co 110023 Li O
However, after storage, the surface of the grains was Inte101 PCo Al
covered with small particles due potentially to a reaction 1000 20 40 60 80 100 (cid:24)(cid:19)(cid:19)(cid:81)(cid:80)
Sputter Depth(nm)
with the electrolyte. In contrast, the coated film pre-
sented a very stable concentration profile and SEM Figure 4 Depth profile of the constituent elements and SEM
surfaceimageoftheAlPO -coatedLiCoO film.(a)Before
images during storage (Figure 4). The AlPO coating 4 2
4 storage;(b)afterstorage.
layer was not clearly presented in the profile due to the
high background intensity of the Al- and P-containing
ions. However, it clearly shows that the Co concentra- noticeable point was that the surface grains of the
tion of the coated film was maintained stable after sto- coated sample were maintained without serious damage
rage (Figure 4b), which confirms the protective effect of during storage. The stable concentration profile and
the coating layer from the reactive electrolyte. The SEM SEM image of the AlPO4-coated film directly confirm
image also presents the stable surface of the AlPO - the protective effect of a coating layer, which is asso-
4
coated film during storage. As shown in the right side ciated with the enhanced electrochemical property of
of Figure 4a, the coated film was observed to possess a the coated electrode.
somewhat rough surface due to the coating layer. The
Conclusions
An AlPO -coated LiCoO thin film was characterized to
4 2
(a) Before storage investigate the surface coating effect. The AlPO -coated
4
105 film showed a superior rate capacity and cyclic perfor-
unts/sec)110034 mfilaen(cveerosvuesrdtheeptphr)isotifntehfeilmpr.isItninteheancodnccoeantterdatifoilnmpsr,oi-t
o
ntensity(c 110012 LCOio wpraesssceldeabrytahpaptlyCinogdainssAollPuOtio4ncowataisngsulacyceerssdfuurlilnygssutpo--
I rage at 90°C. The SEM image of the surface also shows
100 (cid:71) (cid:24)(cid:19)(cid:19)(cid:81)(cid:80)
0 20 40 60 80 100 the coated film to be stable from an attack of the reac-
Sputter Depth(nm)
tive electrolyte during storage.
(b) After storage

105
unts/sec) 110034 AESDIbMSbS:r:eesnveeicarogtniyodndasirsypieornsivmeasspsescptreocstcroompye;tSryE.M:scanningelectronmicroscopy;
o
y(c 102
nsit LOi Acknowledgements
nte 101 Co ThisresearchwassupportedbytheBasicScienceResearchProgramthrough
I 1000 20 Sputt4e0r Dept6h0(nm) 80 100 (cid:71) (cid:24)(cid:19)(cid:19)(cid:81)(cid:80) tEhdeucNaatitoionn,aSlcRieensceearachndFoTuecnhdnaotiloongyof(2K0o0r9e-a00(N71R0F7)3fu).ndedbytheMinistryof
Figure 3 Depth profile of the constituent elements and SEM Authors’contributions
surfaceimageofthepristineLiCoO film.(a)Beforestorage;(b)
2 EHdidthesyntheticandcharacteristicjobinthisjournal.YJgavetheadvice
afterstorage. andguidedtheexperiment.
JungandParkNanoscaleResearchLetters2012,7:12 Page4of4
http://www.nanoscalereslett.com/content/7/1/12
Competinginterests
Theauthorsdeclarethattheyhavenocompetinginterests.
Received:8September2011 Accepted:5January2012
Published:5January2012
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doi:10.1186/1556-276X-7-12
Citethisarticleas:JungandPark:Characterizationofthermallyaged
AlPO-coatedLiCoO thinfilms.NanoscaleResearchLetters20127:12.
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