2007 Kinetics investigation of a preferential (104) plane oriented LiCoO2 thin film prepared by RF

Kinetics investigation of a preferential (104)plane oriented LiCoO 2thin filmprepared by RF magnetron sputteringJ.Xie,N.Imanishi,A.Hirano,M.Matsumura,Y .Takeda ⁎,O.YamamotoDepartment of Chemistry,Faculty of Engineering,Mie University,1577Kurimamachiya-ch,Tsu,Mie 514-8507,JapanReceived 9May 2007;received in revised form 13June 2007;accepted 18June 2007AbstractLiCoO 2thin films with a preferential (104)orientation were prepared on Au substrates by radio frequency magnetron sputtering.The Li –ionchemical diffusion coefficients D˜Li were measured by cyclic voltammetry (CV),galvanostatic intermittent titration technique (GITT),potentiostatic intermittent titration technique (PITT),and electrochemical impedance spectroscopy (EIS).The D˜Li values depended on the content of Li in Li 1−δCoO 2.The D˜Li values by GITT and PITT were in the range of 10−10–10−12cm 2s −1and 10−11–10−12cm 2s −1,respectively,and those by EIS varied over a more wide rang of from 10−9to 10−12cm 2s −1.It was found that the D˜Li values from different methods showed thickness independent.The D˜Li values from different methods were compared with those reported previously.©2007Elsevier B.V .All rights reserved.Keywords:LiCoO 2;Thin film batteries;Chemical diffusion coefficient;GITT;PITT;EIS1.IntroductionThe high electrode potential vs.Li and the excellent cycling stability of LiCoO 2make it an attractive intercalation material for Li –ion batteries.LiCoO 2now is the most widely used cathode material in commercial Li –ion batteries.The kinetics of lithium transport through LiCoO 2is a critical factor that determines the charge and discharge rate.Thus,the better understanding of the Li –ion chemical diffusion coefficient is extremely important for the practical application of this material in lithium-ion batteries.The Li –ion chemical diffusioncoefficient D˜Li of LiCoO 2has been determined by the different methods such as cyclic voltammetry (CV)[1,2],galvanostatic intermittent titration technique (GITT)[3–5],potentiostatic intermittent titration technique (PITT)[2,5–7],and electro-chemical impedance spectroscopy (EIS)[1,2,5,7,8]using thin films.The values of Li –ion chemical diffusion coefficient determined by these methods vary greatly dependent on the measurement techniques,thin film preparation methods,thin film orientation,and the electrolyte.For example,Bouwmanet al.[5]found that the LiCoO 2thin film with a (104)preferred orientation showed a larger diffusion coefficient than that with a (003)preferred orientation.The (003)planes have the diffusion planes parallel to the substrate surface.Bates et al.[9]reported that the (104)oriented LiCoO 2thin film exhibited better electrochemical performance than (003)oriented LiCoO 2.Up to now,the reported literature on the chemical diffusion coefficient of LiCoO 2thin films is mainly on the (003)oriented thin film,and few are available on (104)oriented thin film.In our present work,(104)oriented LiCoO 2thin films were prepared by radio frequency (RF)magnetron sputtering and the Li –ion chemical diffusion coefficients of the thin films were measured by the different measuring techniques such as CV ,GITT,PITT,and EIS.The dependence of the chemical diffusion coefficients on the thickness of the thin film was also investigated.2.ExperimentalThe LiCoO 2thin films were deposited on Au substrates (10mm in diameter)by RF magnetron sputtering.The target (50mm in diameter)for sputtering was made by cold pressing the commercial LiCoO 2powder (Aldrich,99.8%).The film deposition was carried out for 30to 120min in pure Ar withaSolid State Ionics 178(2007)1218–1224/locate/ssi⁎Corresponding author.Tel.:+81592319419;fax:+81592319478.E-mail address:takeda@chem.mie-u.ac.jp (Y .Takeda).0167-2738/$-see front matter ©2007Elsevier B.V .All rights reserved.doi:10.1016/j.ssi.2007.06.007working pressure of 2Pa.The target power is 100W and the distance between the substrate and the target is 10cm.Prior to LiCoO 2sputtering,the target was pre-sputtered for 15min under the same conditions to eliminate the impurities.The as-prepared films were then annealed at 700°C for 30min in air to improve the crystallization of the LiCoO 2thin films.The crystalline structure of the films were characterized by X-ray diffraction (XRD)using a RINT2000/PC diffractometerequipped with Cu –K αradiation (λ=1.5406Å´).Electrochemical measurements of LiCoO 2thin films were performed on 2025coin type cells.The cells were assembled in an Ar filled glove box using Li foil as the anode.One mole LiClO 4in a mixture of ethylene carbonate (EC)/diethylene carbonate (DEC)(1:1in volume)was used as the electrolyte,and polypropylene (PP)microporous film as the separator.Galvanostatic cycling of the cells was carried out at a current density of 50μA cm −2between 3and 4.2V .The CV measurements were performed between 3.5and 4.3V at a scanning rate ranging from 0.05to 8mV s −1using a Solartron 1287electrochemical interface.For the GITT measurements,the cells were charged at a current density of 10μA cm −2for 10min followed by an open circuit relaxation for 2h.The procedure was continued until the voltage of the cells reaches a given value.For the PITT measurements,a potential step of 10mV was applied and the current was recorded as a function of time.The potential was stepped to the next level when the current decreases to below 0.1μA cm −2.The procedure was repeated between 3.85and 4.25V .The EIS measurements were conducted at various electrode potentials by applying an AC signal of 10mVamplitude over the frequency range of 1MHz to 1mHz using a Solartron 1287electrochemical interface combined with a Solartron 1260frequency response analyzer.All the electrochemical measurements were performed at room temperature.3.Results and discussionFig.1shows the XRD patterns of the LiCoO 2film sputtered for different sputtering periods,where Samples 1,2and 3arethe LiCoO 2films sputtered for 30,60and 120min,respectively.The thicknesses of these films are about 0.25,0.63and 1.46μm,respectively,which were calculated from the weight gains of the sputtered samples using the theoretical density of 5.06g cm −3for LiCoO 2.Note that for all the three samples,only a strong (104)diffraction peak and a weak (101)diffraction peak can be seen.Another main diffraction peak for the LiCoO 2powder,namely,the (003)diffraction peak that should appear at 2θ=19°,cannot be seen for all the three samples,even though for the 0.25μm thick sample.Bates et al.[9]found that the (003)peak evolves in the RF sputtered LiCoO 2thin films with the thickness of below 1μm on the alumina substrate with 0.3μm thick Au.However,on the Au substrate,all the LiCoO 2thin films can be considered to have a preferential (104)orientation and the (101)diffraction peak is rather weaker compared to (104)peak.The charge and discharge performance of cell with Sample 2was examined between 3.0and 4.2V under a 50μA cm −2constant current A typical charge –discharge and the incremen-tal capacity (ΔQ /ΔV)curves are shown in Fig.2.These curves are quite similar to those reported previously [6,7].The charge (Li –extraction)curves are characterized by a long potential plateau at about 3.9V and two successive quasi-plateaus at about 4.1and 4.2V ,respectively.These plateaus correspond to the peaks in the ΔQ /ΔV vs.V curve as clearly seen in Fig.2(b).Fig.1.XRD patterns of the LiCoO 2thin films prepared by RF magnetron sputtering.Sputtering period:Sample 1,30min;Sample 2,60min;Sample 3,120min.Fig.2.Charge –discharge (a)and incremental capacity (b)curves of Sample 2.1219J.Xie et al./Solid State Ionics 178(2007)1218–1224The peak at near 3.9V is attributed to the two-phase domain for Li 1−δCoO 2(0.07b δb 0.25)and a minimum at near 4.15V indicates the presence of a monoclinic distortion at δ=0.5where a metastable Li vacancy ordering occurs.The phase change related at near 4.1V is unidentified [10].The discharge (Li-insertion)capacity of the LiCoO 2thin film reaches 131mA h g −1,close to its theoretical value of 137mA h g −1when charged to the composition Li 0.5CoO2,indicating a high material utilization of the film.In addition,the charge –discharge coulombic efficiency of the thin film reaches 93%,indicating an excellent reversibility of lithium intercalation into the film.The high reversibility of the LiCoO 2thin film is also confirmed by the almost symmetrical incremental capacity plot.Based on the above electrochemical characteristics of the thin films,it is clear that the obtained LiCoO 2thin film is suitable for the characterization of its diffusion kinetics.Fig.3shows the CV curves of the LiCoO 2thin film at a low scanning rate of 0.05mV s −1and at various scanning rates.Note that at a low scanning rate,three pairs of well-defined current peaks,which correspond to the charge and discharge plateaus in Fig.2(a),can be seen.The first pair (marked by A and A ′)of current peaks is related to the transition between the two hexagonal phases,while the second pair (marked by B and B ′)and third pair (marked by C and C ′)between 4and 4.2Vcorrespond to the two-phase regions on the both sides of the ordered Li 0.5CoO 2phase as discussed above.Note that between the second and third current peaks,a minimum can be seen at about 4.15V (corresponds to δ=0.5in Li 1−δCoO 2)where a lattice distortion from hexagonal to monoclinic symmetry is considered to occur during charge and discharge [11].It is obvious that before 3.8V ,neither the cathodic nor the anodic current appears indicating that no electrode reactions occur before this potential.Setting δ=0.5at V =4.15V allows to derive the voltage with the composition δin Li 1−δCoO 2.As shown in Fig.3(b),with increasing the scanning rate the peak currents also increases with cathodic peaks shifting to the lower potentials and anodic peaks shifting to the higher potential.In addition,the well-separated current peaks become overlapped with increasing the scanning rate which is typical for LiCoO 2thin film [1–3].Note that at high scanning rate (over 1mV s −1),the peak current (I p )exhibits a linear relation with the square root of scanning rate (ν1/2)which is expected for the diffusion-controlled process as shown in the inset of Fig.3(b).Similar phenomenon was also found by Tang et al.[2].This is known as classical Randles –Sevcik relation [2,12],which can be expressed as:I p ¼0:4463n 3=2F 3=2C Li SR À1=2T À1=2˜DLi m 1=2ð1Þwhere I p is the peak current (A),n the charge transfer number,F the Faraday's constant (96486C mol −1),C Li the Li –ion concentration (0.052mol cm −3),S the surface area of the electrode (cm 2),R the gas constant (8.314J mol −1K −1),T theabsolute temperature (K),D˜Li the Li –ion chemical diffusion coefficient (cm 2s −1),and v the scanning rate (V s −1).The Li –ion apparent chemical diffusion coefficient calculated by Eq.(1)is 2.9×10−12cm 2s −1that is higher than that of 2×10−13cm 2s −1from the (003)oriented LiCoO 2thin film measured also by CV technique [2].GITT is a useful tool for the calculation of the chemical diffusion coefficient which was first reported by Weppner etal.Fig.3.CV curves of Sample 2at a scanning rate of (a)0.05mV s −1and(b)ranging from 0.05to 8mV s −1.Inset is the peak current I p vs.square root of the scanning rate v 1/2.Fig.4.Galvanostatic intermittent titration curve of Sample 2.The inset is the transient voltage change as a function of the square root of the time during a single titration process at 4.03V .1220J.Xie et al./Solid State Ionics 178(2007)1218–1224[13].The formula that obtained by solving the Fick's equation can be written as:˜D Li ¼4k I 0V m FS 2d E d d =d E d t 1=22;t b b L 2=˜D Li ð2Þwhere D˜Li (cm 2s −1),V m (cm 3mol −1),F ,I 0(A),S (cm 2),and L (cm )denote the Li –ion chemical diffusion coefficient,the molar volume of LiCoO 2(19.34cm 3mol −1),the Faraday's constant,the applied current,the surface area of the electrode,and the thickness of the electrode,respectively.d E /d δis the slope of the titration curve at each composition δ.Fig.4shows a typical GITT curve measured by charging the cell for 10min at a current density of 10μA cm −2followed by a relaxation period of 2h,where the cell was cycled between 3and 4.2V for three times before the GITT measurement.The inset of Fig.4is the variation of the voltage for a single titration at 4.03V .Note that the voltage exhibits a linear relation with the t 1/2in a short time.The D˜Li values calculated using Eq.(2)are in the range of 8×10−13–2×10−11cm 2s −1in the composition δbetween 0and 0.55.There are some data available on the Li –ion chemical diffusion coefficients for the LiCoO 2thin film measured by GITT [3–5,14],which vary in a wide range of10−14–10−10cm 2s −1.However,the published D˜Li values are in a given composition or only in a limited composition range.Itis obvious that the D˜Li values have a remarkable composition dependence.The dependences of D˜Li on δin Li 1−δCoO 2with different thicknesses are shown in a wide composition range inFig.5.As shown in this figure,the D˜Li vs.δcurves show similarity for the LiCoO 2thin film with different thicknesses.Amaximum and two minima of D˜Li in the composition region (0.45b δb 0.55)can be seen for all the samples.They correspond to the phase change at δ=0.5as shown above.Thedisordered phase may show a rapid lithium diffusion.The D˜Li values for Sample 2and Sample 3are almost in the same order ofmagnitude.However,for the thinner film,the D˜Li values are one order of magnitude lower than that of the other thicker samples.To obtain accurate D˜Li value using GITT,the current pulse should be as possible as small and short.However,in thepractical experiment,a somewhat larger and longer current pulse is necessary to complete the whole titration experiment.For the thinner film,rapider potential increase is often seen during the each titration compared to the thicker film due to the largercurrent density (mA g −1)leading to smaller D˜Li values.In addition,it is difficult to obtain an absolute equilibrium po-tential during a limited relaxation time.Taking into consideration of other errors coming from a number of assumptions andapproximations [13,15],it is reasonable to conclude that the D˜Li values of (104)oriented LiCoO 2thin films is thickness independence.As shown by Bouwman et al.[5],Li 1−δCoO 2has three phases as phase I,phase II and monoclinic Li 0.5CoO 2.Phase I and II denote two rhombohedral structures with different lithium concentration and both exist as in the two-phase region inthe range 0.07b δb 0.25.The D˜Li of phase II is about one order higher than that of phase I.The D˜Li of phase I,phase II and Li 0.5CoO 2are about 10−12,10−11and 10−10cm 2s −1.It is important to mention that the d E /d δis close to zero in the two-phase region.Thus,the chemical diffusion coefficients in this region are not reliable and can only be considered as apparent chemical diffusion coefficient.PITT is another useful method to measure the chemical diffusion coefficient in the ion and electron mixed conductors.The PITT method relies on solving the Fick's diffusionequationparisons of Li –ion chemical diffusion coefficients of the three LiCoO 2thin film samples usingGITT.Fig.6.Time dependence of the current when the voltage is increased from 4.06to 4.07V:(a)I t vs.t and (b)Ln (I t /A)vs.t of Sample 2.1221J.Xie et al./Solid State Ionics 178(2007)1218–1224like GITT.In PITT,a potential step is applied to change the equilibrium composition of the electrode and the current decay is measured as a function of time.The time (t )dependence of the transient current (I t )at each potential step can be expressed asI t ¼2FS ðC S ÀC 0Þ˜D Li L exp Àk 2˜D Li t4L !ð3Þwhere F is the Faraday's constant,S the surface area ofelectrode,C s −C 0concentration difference at the surface at time t and time t =0during each potential step,and L the thickness ofthe electrode [16].The D˜Li can be calculated from the slope of the linear region of the Ln (I t )vs.t plot using the following equation:˜DLi ¼dLn ðI t Þd t 4L 2k 2ð4ÞA typical current decay result with time is shown for Sample 2in Fig.6,where the voltage was increased abruptly from 4.06V to 4.07V .The chemical diffusion coefficients for the samples with different thickness calculated from Eq.(4)are shown in Fig.7.It can be seen from this figure that the curveshape of D˜Li vs.δusing PITT also bears similarity for the LiCoO 2thin film with different thicknesses.Among the threesamples,the D˜Li values for Sample 1and Sample 2are rather close to each other in the whole composition range.The higher D˜Li values of Sample 3may come from the overestimation of thickness of the film since the stacking compactness may varyfor LiCoO 2with different thicknesses.The difference of the D˜Li values between the three samples is acceptable considering the errors intrinsic to the PITT resulting from a series ofassumptions and approximations [15,16].Thus,the D˜Li values obtained using PITT is also considered to be thickness independent and are in the order of 10−11–10−12cm 2s −1which are close to the values obtained from GITT.EIS is considered to be a powerful measurement technique to identify the kinetics of Li insertion/extraction into/from theelectrode.In our present work,it was also used to evaluate the composition dependence of the Li –ion chemical diffusion coefficient.Fig.8(a)shows a typical Nyquist plot measured for Sample 2equilibrated at 4.05V .It is clearly that the plot consists of a semicircle at high frequency region and a straight line of 45°slope at medium frequency region and a steeper straight line at low frequency region.The high-frequency semicircle is attributed to the charge –transfer process,the straight line of 45°slope is related to the Warburg region associated with the Li –ion diffusion in the bulk Li 1−δCoO 2thin film,and the steeper straight line corresponds to the onset of the finite length diffusion.The chemical diffusion coefficient is estimated from Eq.(5)[17]:˜D Li ¼12V m FS r d E d d 2ð5Þwhere V m ,F ,S (cm 2),σ(ΩHz 1/2)and d E /d x (V)are the molar volume of LiCoO 2,the Faraday's constant,the surface area of the electrode,Warburg factor,and slope of the electrode potential E position δ,respectively.The Warburg factor is obtained from the slope of Z ′or −Z ″vs ω−1/2as shown inFig.8(b).The composition dependences of D˜Li for the samples with different thicknesses calculated from Eq.(5)are showninparisons of Li –ion chemical diffusion coefficients of the three LiCoO 2thin film samples usingPITT.Fig.8.Results of EIS of Sample 2at 4.05V:(a)Nyquist plot and (b)Z ′or −Z ″vs.ω−1/2in the Warburg region.1222J.Xie et al./Solid State Ionics 178(2007)1218–1224Fig.9.The D˜Li values are in the range of 3.2×10−12–5×10−10cm 2s −1in the composition of δ=0to 0.55.The D˜Li value at δ=0.5is around 6×10−10cm 2s −1.The D˜Li value is comparable with around 1×10−10cm 2s −1s for the (003)oriented Li 0.5CoO 2thin film prepared by pulsed laser deposition [2,7].As shown in Fig.9,no significant change with the thickness of the sample is observed,with only Sample 1exhibiting a somewhat higher D˜Li values in the phase II.The errors may come from the deviation of the slope of straight linein Warburg region from 45°.The D˜Li values from EIS are in the order of 10−9–10−12cm 2s −1.As mentioned above,d E /d δis close to zero in the two-phase region.In the present work,a pseudocapacitance C ϕis used instead of d E /d δin the two-phase region to calculate the D˜Li values using the method reported by Funabiki et al.[18].Fig.10summarizes the composition dependence of the Li –ion chemical diffusion coefficients of Sample 3using differenttechniques.The similar composition dependences of D˜Li are observed for the results by the different techniques.In themonoclinic phase,a maximum of D˜Li can be seen at δ=0.5(at 4.15V),which is associated with the order/disorder transition near the composition Li 0.5CoO 2where the degree of Li vacancy ordering is increased [6].Similar diffusion coefficient maximums were also observed in the literature [6,7].Compar-ing with D˜Li values by GITT and PITT,the values by EIS are about one order of magnitude higher in the whole compositionrange.As seen in Fig.10,the D˜Li of Li 0.5CoO 2by EIS is about 2×10−10cm 2s −1and those by GITT and PITT are about 6×10−11and 1×10−11cm 2s −1,respectively.Jang et al.[6]andWang et al.[19]reported that the D˜Li values of sputtered thin films measured by PITT and EIS were 3×10−10and 6(±1.3)×10−9cm 2s −1,respectively.Tang et al.[2]reported that the chemical diffusion coefficients of the highly (003)oriented Li 0.5CoO 2are estimated to be about 6×10−12cm 2s −1by PITT and 1.3×10−11cm 2s −1by EIS,which are lower compared with the (104)oriented Li 0.5CoO 2due to the fact that in the (104)oriented LiCoO 2thin film,the Li layers lie parallelto Li –ion diffusion direction which will facilitate its diffusion,while in the (003)oriented thin film,the Li layers lie perpendicular to Li –ion diffusion direction which will impedeits diffusion.Generally,the D˜Li values by EIS are higher than those by PITT or GITT [2,5,20].It is believed that the values from EIS may be overestimated since the slope of straight line at the Warburg region is not strictly 45°in many cases and sometimes the Warburg region was masked by the finite length diffusion region and/or the high-frequency semicircle.Inaddition,the D˜Li values from EIS at two-phase coexistence region (0.07b δb 0.25)are considered to be not reliable.4.ConclusionsLiCoO 2thin films with a (104)preferred orientation were prepared by RF magnetron sputtering.The Li –ion diffusion kinetics was investigated by CV ,GITT,PITT and EIS.Thechemical diffusion coefficients D ˜Li determined by above methods are in the order of 10−12,10−10–10−12,10−11–10−12,and 10−9–10−12cm 2s −1,respectively.The D˜Li values from GITTand PITTare close to each other and are considered tobe more reliable.The D˜Li values from EIS are higher than those from GITT and PITT as previously reported.The D˜Li vs.δcurves obtained by three methods show similarity.The highest D˜Li is observed at Li 0.5CoO 2and D ˜Li of the rhombohedral phase II is higher that of phase I.We can conclude that the chemical diffusion coefficient of the (104)oriented Li 0.5CoO 2is about one order of magnitude higher than that of the (003)oriented Li 0.5CoO 2because Li layers lie parallel to Li –ions diffusion direction in the (104)oriented LiCoO 2thin film,while for the (003)oriented thin film,Li layers lie perpendicular to Li –ion diffusion direction.AcknowledgementsThis research work was carried out under a collaboration program of Mie University and Genesis Research Institute,Nagoya,Japan.parisons of Li –ion chemical diffusion coefficients of the three LiCoO 2thin film samples usingEIS.parisons of Li –ion chemical diffusion coefficients D ˜Li vs.δof Sample 3using different techniques.1223J.Xie et al./Solid State Ionics 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