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Electrochimica Acta
j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta
ycling effects on surface morphology, nanomechanical and interfacial reliabilityf LiMn2O4 cathode in thin film lithium ion batteries
ing Zhu, Kaiyang Zeng ∗, Li Luepartment of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore
r t i c l e i n f o
rticle history:eceived 31 August 2011eceived in revised form 7 February 2012ccepted 7 February 2012vailable online 19 February 2012
eywords:ithium ion battery
a b s t r a c t
In this study, the effects of Li+ intercalation/deintercalation processes on surface morphology, nanome-chanical and interfacial properties of sputtered LiMn2O4 cathode film are studied using varioustechniques, including ex situ field emission scanning electron microscope (FESEM), atomic force micro-scope (AFM), nanoindentation tests combining with focused ion beam (FIB) sectioning. The results revealthat the spinel LiMn2O4 cathode film shows good cycling performance and morphology stability, accom-panying with a slight increase of surface roughness mainly due to the agglomeration of nano-grainscaused by phase transformations. The cathode film mechanically fails due to the stress induced by the
iMn2O4 Thin film cathodeanoindentationFM
nterfacial reliability
lattice parameter change upon Li+ intercalation/deintercalation and lattice mismatch related to inhomo-geneous phase transformation as well as the Jahn–Teller distortion in the 4 V region. The induced stressin the film causes the fragmentation of grains and the initiation/propagation of micro-cracks, leading tothe mechanical degradation. The non-uniformity, micro-cracks and nanomechanical degradation haveharmful effects on the electrical contact of LiMn2O4 cathode film, resulting in the capacity fading of thelithium ion battery.
. Introduction
Due to the increasing demands for the power sources forortable and mobile applications, rechargeable lithium ion bat-ery has attracted much attention since its emergence. Due tohe high energy density, long cycle life and little memory effect,ithium ion batteries have various commercial applications foroday’s information-rich society, such as mobile phones, laptops,nd digital cameras [1–3]. In recent years, the reduction in thecale, current density and power requirement of electronic deviceas promoted the development of all-solid-state thin film lithium
on microbatteries, which have great potential to be used in manyrecise electronic devices, such as smart cards, implanted med-
cal devices, semiconductor memory chips, and integrated partsn micro-electromechanical systems (MEMS) [4–6]. Among variousositive electrode materials proposed, LiMn2O4 with spinel struc-ure is one of the most favored cathodes for lithium ion batteriesue to its non-toxicity, low cost and relatively high voltage [7,8].owever, the commercial application of LiMn2O4 is still limited
ue to its poor cyclability. Over the last decade, several possibleechanisms such as the decomposition of electrolyte in the high
otential, manganese dissolution in the electrolyte, and structural
distortion due to the Jahn–Teller effect, have been proposed byseveral research groups [9,10].
The recent developments in various electronic applicationsrequire long battery life, promoting the studies on aging mecha-nism of the lithium ion batteries. The aging study is a significantand challenge issue since the capacity fading originates from var-ious interrelated processes occurring at the same time [11–13].Therefore, aging mechanisms of LiMn2O4 cathode are still notvery clear although they have been extensively studied for manyyears [9,14,15]. Apart from the electrochemical degradation duringcycling, such as the increase of impedance, aging phenomenon canalso be related to the mechanical failure due to the volume changeand lattice distortion induced by Li+ insertion/extraction as wellas the related generation/relaxation of intercalation-induced stress[16–19]. Since the fracture and delamination of active electrodematerial can result in the loss of electrical contact and capacityfading, the mechanical properties and adhesion strength of theelectrodes at nanoscale are critical issues to maintain the electro-chemical performance of lithium ion batteries. In recent years, a fewstudies have revealed the mechanical failure in active electrodesvia different experimental methodologies, including X-ray diffrac-tion (XRD) [16], laser probe beam deflection (LPBD) [14,15], optical
cantilever method [17], microscopy techniques [20,21], and finiteelement numerical simulations [18,19]. Despite the observationssuch as volume change, material fracture and stress transitions,the interfacial reliability of the active electrode has not been
orrelated to the capacity fading of lithium ion batteries. For thinlm microbatteries, the interfacial reliability of the electrode filmas become one of the most critical issues due to its significantole in both structural integrity and cycling performance. However,uantification of interfacial reliability is not simple and straightfor-ard, requiring not only practical experimental method but also
heoretical analysis on solid adhesion mechanics and sometimesumerical simulations [22–26].
In this study, the interfacial reliability of LiMn2O4 cathode film isharacterized using wedge indentation experiment and the relatednalysis method, which overcomes the shortcomings of conven-ional mechanical testing techniques (e.g. mechanical flexure test),uch as complexity and time-consuming in sample preparation andata analysis [27,28]. This experimental and analysis method, pre-iously developed for determining the interfacial reliability of filmsn semiconductor devices [27,28], has been extended to RuO2 thinlm anode and proven to be feasible and reliable [29,30]. However,esides RuO2 thin film anode studied previously [30], the interfa-ial reliability of cathode film has not been characterized. It is wellnown that the cathode film has very different aging mechanismompared to that of the anode film. Therefore, the objective of thistudy is to investigate the effects of charge/discharge cycling on sur-ace morphology, mechanical properties and interfacial reliabilityf LiMn2O4 cathode film using various characterization techniques.he optimal goal is to have a comprehensive insight into the agingechanism of the cathode film, leading to a new perspective to
nderstand the aging of the thin film lithium ion batteries.
. Experimental
.1. Sample preparation and structural characterization
LiMn2O4 films were deposited on polished pure Ti substratessing radio frequency (rf) magnetron sputtering. A target withimension of 50 mm (2 in.) diameter and 3 mm thickness was fabri-ated using commercial LiMn2O4 powder (99.9%). The powder wasrst cold-pressed into a pellet under the pressure of 10 MPa, fol-
owed by sintering in air at 900 ◦C for 15 h to obtain a robust target.he deposition was carried out with a power of 100 W at a total gasressure of 1.4 Pa after pre-sputtering. The total gas flow rate waset as 14 standard cubic centimeters per minute (sccm) with theatio of Ar to O2 of 13:1. The substrate was kept at 650 ◦C duringhe deposition. The film thickness was found to be about 250 nm,hich was measured at a masked step using a surface profilometer.
The crystal structure of LiMn2O4 film was characterized byRD technique (Model XRD-7000, Shimadzu, Japan). To study theurface morphology changes of the cathode film after differentharge/discharge cycles, ex situ observations were carried out usingoth field emission scanning electron microscopy (FESEM) (Model-4300 FESEM, Hitachi, Japan) and atomic force microscope (AFM)Model MFP-3D, Asylum Research, USA). Both two- and three-imensional AFM topography images were obtained in tappingode using a silicon tip with spring constant of 2 N m−1 (Elec-
riLever, Olympus, Japan). The surface roughness of the thin film cane quantitatively measured by the root-mean-squared roughnessRMS) values determined from AFM images.
.2. Electrochemical characterization
The electrochemical half-cell used in this study is home-madewagelok cell, which was assembled in an Ar-filled glove box with
oth H2O and O2 levels less than 0.1 ppm. The counter electrodeas lithium foil, and the prepared LiMn2O4 film with an active area
f approximately 1.0 cm2 was served as the working electrode. Thelectrolyte was 1 M LiPF5 in 1:1 ethylene carbonate (EC) and diethyl
Acta 68 (2012) 52– 59 53
carbonate (DEC) solution. Galvanostatic charge/discharge cyclingwas carried out using a battery test system (Neware, Neware Tech-nology, China), in the voltage range between 3.5 and 4.3 V at aconstant current density of 10 �Ah cm−2. To investigate the prop-erty changes due to charge/discharge cycles, the battery cells werefirst performed various charge/discharge cycles and then dissem-bled; the dissembled LiMn2O4 cathode films were rinsed by acetonefor various ex situ characterizations.
2.3. Nanomechanical and interfacial characterization
Two different nanoindentation systems were used to assessthe nanomechanical and interfacial properties of LiMn2O4 cath-ode film. The hardness (H) and elastic modulus (E) of LiMn2O4cathode film were determined by nanoindentation with the contin-uous stiffness measurement (CSM) option and a standard Berkovichindenter (inclusion angle: 131◦; estimated tip radius: 50 nm)under the strain-rate control (0.05 s−1) (Nano Indenter XP®, MTSNano-instruments, MTS Corporation, USA). To study the interfa-cial reliability, wedge indentation was conducted using anothernanoindenter (UMIS-2000H®, CSIRO, Australia) equipped with a90◦ diamond wedge indenter, the tip length was measured to be4.055 �m [29]. The wedge indentation consisted of three steps: (a)loading to the maximum load in 20 s; (b) holding at the maximumload for 5 s; (c) unloading to 30% of the maximum load in 20 s. Thepenetration depths were within the range from 50% to more than150% of the film thickness. The indentation tests were conductedin the ranges from 4 to 18 mN with the load-control. To determinethe indentation plastic depth, at least 20 indentations were madeon each set of conditions. The details of experiments can be foundin the previous studies and will not be repeated here [29,30].
To reveal the interfacial delamination and measure the cracklength, Focused Ion Beam (FIB) (Quanta 200 3D, FEI Company, USA)was used to make cross-sectional cuttings which were perpendicu-lar to the wedge indentation impression. In addition, to characterizeinterfacial crack profile, FIB and FESEM were used to capturethe cross-sectional view and plane view images of the interfacialdelamination, respectively. To ensure the calculation accuracy, atleast eight FIB cuttings were conducted for each set of indentations.Through these observations, the relationship between the indenta-tion load–displacement (P–h) curves and initiation/propagation ofinterfacial cracks can be established.
3. Results and discussion
3.1. Structural and electrochemical characterization
Fig. 1 shows the XRD pattern of sputtered LiMn2O4 thin film. Forcomparison, the XRD pattern of the commercial LiMn2O4 powderis also included. Excluding the strong peaks from the Ti substrate,it can be observed a relatively stronger diffraction peak of (1 1 1)accompanying with two peaks of (3 1 1) and (4 0 0). The peak (2 2 2)at 37.9◦ is overlapped with that of the substrate and thereforecannot be distinguished. It is noticed that both (3 1 1) and (4 0 0)reflections match with those of LiMn2O4 sample powder very well;however, (1 1 1) reflection shifts slightly by about 0.8◦ to higher2� position (from 18.7◦ to 19.5◦), indicating that the manganeseand oxygen atoms may not occupy their ideal lattice positions inthe spinel structure completely. In other words, the shift of (1 1 1)deflection may be attributed to the presence of impurities, such asMnO2 [31], and the low crystallization of spinel structure [32]. Nev-
ertheless, the main X-ray diffraction features of the deposited filmare in agreement with regular spinel LiMn2O4 phase.
Fig. 2(a) and (b), respectively, shows the Galvanostaticcharge/discharge curves during the first cycle and the cycling
54 J. Zhu et al. / Electrochimica A
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ig. 1. XRD patterns of the as-deposited LiMn2O4 thin film prepared by RF mag-etron sputtering.
erformance of LiMn2O4 cathode film between 3.5 and 4.3 V at constant current density of 10 �Ah cm−2. It is clear that theoltage profile during the first cycle exhibits two typical chargend discharge plateaus at about 4.0 V and 4.15 V obviously. Thisoltage profile agrees well with what reported in the literatures8,31]. The charge/discharge plateaus correspond to the reversiblelectrochemical reactions of the spinel LiMn2O4 cathode on Li+eintercalation/intercalation process, which can be written as fol-
owing two-step reactions [31]:
iMn2O4 ↔ Li0.5Mn2O4 + 0.5Li+ + 0.5e− (1)
i0.5Mn2O4 ↔ �-MnO2 + 0.5Li+ + 0.5e− (2)
The two plateaus during charge process correspond to thewo successive steps of Li+ deintercalation from LiMn2O4 toorm �-MnO2, whereas the discharge plateaus correspond to Li+ntercalation into �-MnO2 to form LiMn2O4 [Fig. 2(a)]. Fig. 2(b)hows that the initial discharge capacity of LiMn2O4 cathode films about 53.33 �Ah cm−2 �m. Taking the theoretical density ofiMn2O4 (4.3 g cm−3), the discharge capacity is found to be about24 mAh g−1 and it decays at an average rate of about 0.25% per
ycle up to 100 cycles. However, a direct comparison of the capac-ty value between thin film and bulk electrode is not appropriate,ince about 10% calculation error is expected due to the estimationrror of film thickness and area. It can be observed that about 75%
Fig. 2. (a) The first Galvanostatic charge–discharge curves and (b) c
cta 68 (2012) 52– 59
of its initial discharge capacity (40 �Ah cm−2) is still maintainedeven after 100 cycles, indicating that the sputtered nano-crystallineLiMn2O4 cathode film exhibit good cycling stability and capacityretention. Besides the capacity fading, the charge plateaus havebecome higher and the discharge plateau has become lower as thecycling number increases, indicating the increased cell polarizationand the degradation of spinel LiMn2O4 structure.
3.2. Surface morphology
Fig. 3 shows SEM images of the surface morphology of LiMn2O4films at different cycling states. The as-deposited LiMn2O4 filmhas flat, smooth and uniform surface morphology [Fig. 3(a)]. Thefilm consists of densely packed columnar nano-grains with fairlyhomogeneous grain size. The average grain size is estimated tobe about 80 nm, confirming the nanostructure of the as-depositedLiMn2O4 film [Fig. 3(c)]. After 100 charge/discharge cycles, thereis no distinct morphology change observed in the film surfacefrom SEM images [Fig. 3(a) and (b)]. The film maintains its origi-nal grain shape with a slight increase in the gaps between grains,only some stacking and agglomerations of irregular nano-grainscan be formed and observed, resulting in the non-uniform andnon-smooth cycled film after charge/discharge cycles [Fig. 3(b)and (d), indicated by white arrows]. On the other hand, fromthe three dimensional AFM images [Fig. 4], the surface topog-raphy revolution with the increase of cycling number can beobserved more evidently. In addition, surface topography changeslead to changes in surface roughness of LiMn2O4 cathode film,which is quantitatively determined by AFM image analysis interms of RMS (root mean square) by averaging from ten randomareas of 1 �m × 1 �m (Fig. 5). The surface roughness and standarddeviations increases from 5.54 ± 0.40 nm for as-deposited film to6.41 ± 0.43 nm, 8.03 ± 0.45 nm and 11.67 ± 2.52 nm for the filmsafter 10, 50 and 100 charge/discharge cycles, respectively. Theseoverall increments of the surface roughness and standard devia-tion are in agreement with previous studies on LiMn2O4 and LiCoO2cathode films prepared by pulse laser deposition (PLD) [7,33].
It is well known that several factors may contribute to thesurface topography change in the cathode film, including frag-mentation and agglomeration of nano-grains [33], corrosion effects[7], surface layer formation [34], coarsening of the substrate/thinfilm interface and many other reasons. Among of those, the frag-mentation and agglomeration of nano-grains can be identified inthis study [Fig. 3(d)]. Several large agglomerates of irregular nano-
grains are formed and observed on the surface after 100 cyclesfrom the high magnification SEM images [indicated in white arrowsin Fig. 3(d)]. It is generally believed that the most likely reasonfor the changes of the topography in LiMn2O4 cathode film is the
ycling performance of LiMn2O4 cathode film up to 100 cycles.
J. Zhu et al. / Electrochimica Acta 68 (2012) 52– 59 55
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ig. 3. Ex situ FESEM images of LiMn2O4 cathode films taken at (a) as-deposited; (b
nduced-stress, which is generated by the volume variations dueo reversible Li+ intercalation/deintercalation. With the increasef cycling number, the induced-stress accumulates and finallyurpasses the strength of cathode materials, leading to the frag-entation of nano-grains. Since the fragmentation/agglomeration
f nano-grains associated with “stress accumulation” is an irre-ersible process, there is a continuous change in morphology with
slight increase in surface roughness. However, it is still difficult toetermine whether other effects are responsible for the topographyhange, and this requires further studies in the future.
.3. Degradation of the nanomechanical properties
In this study, the elastic modulus and hardness of the LiMn2O4athode films are determined by analyzing the nanoindentation–h curves (with standard Berkovich indenter tip) using the well-nown Oliver and Pharr’s method [35]. Similar to the previoustudies [29,30], the method developed by Joslin and Oliver issed to determine the film-only properties (elastic modulus andardness) [36]. The elastic modulus and hardness of as-depositediMn2O4cathode film are found to be 202.98 ± 13.30 GPa and0.81 ± 1.79 GPa, respectively. Fig. 6 shows that the measured elas-ic modulus of cathode film decreases with the increase of theycling number. The decrement is more significant during the first0 cycles, and has become relatively small during the subsequentycles. The hardness only deceases after the initial 10 cycles, andemains nearly unchanged after that. In addition to the changesf the elastic modulus and hardness of the film, nanoindentation–h curves (Fig. 7) also show the degradation of the mechanical
roperties of the LiMn2O4 cathode films at different cycling stages.or example, at the same indentation load (8 mN), the displacementncreases from about 100 nm for as-deposited film to about 250 nmor 100th cycled film [Fig. 7(a) and (d)]. This is mainly due to the
r 100th cycles; enlargement view of (c) as-deposited film; (d) 100th cycled film.
decrease of film hardness after charge/discharge cycles, since theindentation tests measure the resistance to the penetration of theindenter tip [37]. In addition, the mechanical responses of LiMn2O4cathode films after 50 and 100 cycles are nearly the same, whichalso agrees with the changes of the hardness.
The mechanical degradation of LiMn2O4 cathode film duringcharge/discharge cycling is closely related to the induced-stress,which may arise from several contribution factors, such asthe volume variation due to the lattice parameter change,inhomogeneous local structure, surface film formation, and sur-face tension change due to the adsorption of species, etc. [9,14,15].Firstly, the main factor responsible for the induced-stress isthe volume expansion/contraction of cathode film due to thelattice parameter change induced by the repeated Li+ intercala-tion/deintercalation. When LiMn2O4/Li half cell is charged, thespinel LiMn2O4 transforms to �-MnO2 upon Li+ extraction, which
has a smaller lattice parameter (8.04 ´̊A) than that of original
LiMn2O4 (8.24 ´̊A) [14]. This volume contraction of the cathode filmis constrained by the rigid substrate, resulting in tensile stress inthe film during the first charge cycle. When the half cell battery isfully discharged, Li+ is inserted into the host �-MnO2 frameworkwith 2.4% increase of lattice parameter and the volume expan-sion, resulting in the compressive stress in the film. Secondly, inthe high voltage region, the coexistence of two cubic phases with
different lattice parameters (8.04 ´̊A and 8.14 ´̊A) leads to the inho-mogeneous structural change and lattice mismatch [9], which isa cause for the stress generation. Thirdly, stress also arises fromthe surface property changes induced by the formation of a sur-face layer as well as the surface absorption of species from the
liquid electrolyte [15]. Since the mechanical degradation is moresignificant during the initial 10 cycles (Fig. 6), a so-called solid elec-trolyte interphase (SEI) layer, which is the product of electrolytedecomposition, accompanying with the absorbed species on the
56 J. Zhu et al. / Electrochimica Acta 68 (2012) 52– 59
taken
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Fs
Fig. 4. Ex situ AFM 3D topography images of LiMn2O4 cathode films
lm surface, may play significant roles in surface tension and stresstate changes. In addition to the above reasons, several researchersave suggested that Jahn–Teller distortion in the 4 V region is also
possible reason for the stress generation [14,15,38]. Although its generally believed that Jahn–Teller distortion is observed whenhe concentration of Jahn–Teller ions (Mn3+) exceeds 50 mol% inhe host structure at the end of discharge to 3 V, accompanyingith the phase transformation from spinel LiMn2O4 to tetragonal
i2Mn2O4 [9]. Thackeray et al., however, have observed the onsetf Jahn–Teller distortion even in the 4 V region using ex situ TEMechniques [39–41], which has been recently convinced by in situaser beam deflection method [14,38]. In this case, the mean Mn
alence at particle surface is below 3.5 V since the tetragonal phasenly forms on the surface, which is different from Jahn–Teller effectn the 3 V region where the entire particle transforms to tetrago-al phase. It is noted that Jahn–Teller distortion can lead to 5.6%
ig. 5. Surface roughness measured by AFM for LiMn2O4 cathode films at differenttages of cycling.
expansion of unit cell volume, and this may result in largestress inside the cathode film. Hence, although the mechanism ofJahn–Teller effect in 4 V LiMn2O4 is still not very clear, Jahn–Tellerdistortion can be considered as one of the possible reasons. Inaddition, compared with the continuous volume variation inducedby Li+ intercalation, which is proportional to the amounts of Li+insertion/extraction, the volume change induced by Jahn–Tellerdistortion is larger and more abrupt.
In summary, for LiMn2O4 cathode film, in term of the stressesinduced by intercalation/deintercalation and phase transforma-tion, tensile stress arises during the charge process and compressivestress is due to the discharge process, and this can result in themechanical degradation of the LiMn2O4 cathode film. However,the cycling dependence of mechanical properties is different fromthat of the surface topography which only changes slightly and
continuously. Although the detail reasons for the major decreaseof elastic modulus and hardness during the first 10 cycles is stillnot clear, several possible factors should be considered. Besidesthe stress induced by Li+ intercalation and phase transformation,
Fig. 6. Elastic modulus and hardness of LiMn2O4 cathode films at different stagesof cycling.
J. Zhu et al. / Electrochimica Acta 68 (2012) 52– 59 57
e film
dtalbmfSocioa
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Fig. 7. Indentation load–displacement curves of LiMn2O4 cathod
uring the initial cycles, surface film formation and dissolution ofhe electrode in the electrolyte also play some roles. In addition,fter 10 charge/discharge cycles, the decrease of both elastic modu-us and hardness has become relatively small. This can be explainedy both relaxation and reduction of induced-stress. Firstly, it isainly due to the cracking/fragmentation of nano-grains after a
ew cycles, which is expected to relax larger part of induced-stress.econdly, to some extent, the intercalation induced stress dependsn the mechanical performance of electrode material [18]. Theycled film which has a lower elastic modulus can result in smallernduced-stress. Finally, capacity, which corresponds to the amountf Li+ intercalation, decays with the increase of cycling number,lso results in the reduction of induced-stress.
.4. Interfacial reliability
In this study, the interfacial reliability between the LiMn2O4athode film and Ti substrate has been quantified using the wedgendentation experiment and the related analysis method devel-ped previously [29,30]. Through the combination of the FESEMlain view and FIB cross-sectional view images of wedge indenta-ion impression [Fig. 8(a) and (b)], it is found that the interfacialelamination occurs when the indentation load is higher than aritical value (i.e. ∼8 mN for as-deposited film, and ∼6 mN for 10th,0th and 100th cycled films). Furthermore, at these critical loads,here are also so-called “pop-in” events observed in the inden-
ation P–h curves, i.e. a sudden increase of indentation depth atn approximately constant indentation load. Therefore, the “pop-n” events can be used to predict the interfacial delamination foriMn2O4 cathode film. For both the as-deposited and cycled films,
s at (a) as-deposited; (b) 10 cycles; (c) 50 cycles; (d) 100 cycles.
the interfacial delamination initiates at the indentation depth ofapproximately 150 nm, which corresponds to about 60% of thefilm thickness. With the increase of indentation load, the interfa-cial crack steadily extends in the direction normal to the wedgeindentation impression, finally forming a V-shaped morphologyfrom the cross-sectional view [Fig. 8(b)] and an elliptical shapedelamination area from the plain view [Fig. 8(a)]. The detailedinterfacial delamination process and related adhesion mechanismshave been discussed previously [29,30]. Using the critical load fordelamination, the interfacial crack length determined from FIBcross-sectional images, and the delamination area determined fromFESEM images, the interfacial toughness � i (in terms of strainenergy release rate) can be calculated similar to that used in theprevious studies [29,30]. Table 1 summarizes the results of inter-facial toughness and key parameters of interfacial delaminationfor LiMn2O4 cathode film at different cycling stages. These keyparameters are determined at the critical load to minimize the cal-culation error. At this load, the indentation depth is only 60% of thefilm thickness; the substrate deformation can be minimized eventhough the indentation plastic zone may not be completely con-strained within the film. In addition, amount of the strain energyreleased in terms of film crack is also minimized due to the lowindentation load.
As a result, there is an observable decrease of interfacial tough-ness of LiMn2O4 cathode film during cycling, which is mainlyattributed to the reduction of elastic modulus, since the indenta-
tion induced stress is proportional to the elastic modules [27,28]. Inaddition, micro-cracks induced by Li+ intercalation/deintercalationalso play significant roles; the as-deposited film is denser and henceit has higher interface toughness than that of cycled films. The
58 J. Zhu et al. / Electrochimica Acta 68 (2012) 52– 59
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Fig. 8. (a) FESEM plane view and (b) FIB cross-sectional view images of in
verage decrease rate of interfacial toughness is about 1.11% perycle during the first 10 cycles, and about 0.58% per cycle between0 and 50 cycles, whereas this rate decreases to only 0.18% perycle from 50 to 100 cycles. It is interesting to note that the capac-ty decay rate is about 4.5% per cycle during the first 10 cycles, andhereafter about 3.0% per cycle between 10 and 50 cycles, finallyecreases to only 2.2% per cycle from 50 to 100 cycles. This suggestshat there may be a possible relationship between the capacity fad-ng and the degradation in interfacial reliability. With the increasef cycling number, there is a slow decrease in the magnitude ofhe interface toughness, which is mainly due to the decrease ofhe induced-stress. The capacity decay corresponds to the decreasef the amount of Li+ insertion/extraction. In addition, higher elas-ic modulus may induce higher intercalation induced-stress dueo the rigidity of the host structure. Therefore, interfacial relia-ility has become stable with the decrease of induced stress overycles.
Furthermore, compared to the studies on RuO2 thin film anodeeported previously [30], the LiMn2O4 thin film cathode has betterycling and morphology stability and less significant degradation inoth mechanical and interfacial reliability. This is mainly due to theifferent electrochemical mechanisms during cycling in these twoaterials. The RuO2 anode film shows significant changes in sur-
ace morphology due to the large volume expansion/contractionnduced by phase transformation, inducing tensile stress afterischarge/charge cycles, enhancing the initiation/propagation oficro-cracks. Since the brittle oxide material is more sensitive to
ensile stress, the mechanical degradation of RuO2 anode film isore significant than that of the LiMn2O4 thin film cathode. For
iMn2O4 cathode film, the propagation of micro-cracks is moreifficult through the compressive stress induced by Li+ interca-
ation and phase transformation. The grain fracture only occurshen the induced compressive stress is larger than the critical
able 1esults of interfacial toughness and key parameters of the interfacial delamination forLiM
otes: The critical loads, Pmax, for as-deposited, 10th, 50th and 100th cycled films are 8 mNmpressions.
tion induced interfacial crack pattern at LiMn2O4/Ti substrate interface.
compressive strength of cathode material. Combining the previ-ous study [30] and this work, it is found that the thin film anodeshows much more degradation in film mechanical properties andinterfacial reliability than those of thin film cathode under the samecycling conditions. Therefore, in addition to the general considera-tion of the electrochemical performance of the electrode materials,the mechanical properties and interfacial reliabilities need to beaddressed in the battery design and development of new batterymaterials.
4. Conclusions
In this study, lithium manganese oxide film, used as cathodefor lithium ion battery, has been deposited by rf sputtering andits electrochemical properties are investigated. Ex situ FESEM,AFM, nanoindentation experiments combining with FIB sec-tioning are conducted to characterize the changes in surfacemorphology, mechanical and interfacial reliability during Li+ inter-calation/deintercalation cycling process. As a result, spinel LiMn2O4cathode film shows good cycling performance and surface mor-phology stability with a slight increase of surface roughness.Non-uniformity can be observed in the cycled film due to thefragmentation and agglomeration of nano-grains. The stressesinduced by Li+ intercalation/deintercalation and phase transfor-mation, as well as the changes of the surface properties causethe initiation/propagation of micro-cracks and fracture of grains.The induced-stress accumulates during the cycling, finally result-ing the degradation in both mechanical and interfacial reliability.In summary, the non-uniformity, micro-cracks and nanomechan-
ical degradation have harmful effects on the electrical contact ofthe LiMn2O4 cathode film, hence affecting the battery aging. Thisstudy provides a new perspective for understanding the mecha-nism of thin film battery aging. These factors need to be considered
, 6 mN, 6 mN and 6 mN, respectively, and the data are averaged from 10 indentation
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n the design and development of lithium ion batteries. This studylso demonstrates that the nanoindentation experiment and theelated analysis method are proved effective to quantify interfacialeliability of the thin film microbatteries.
cknowledgements
The authors would like to thank Institute of Materials Researchnd Engineering (IMRE), Singapore for the help of nanoindenta-ion experiments. This work was supported by Agency for Science,echnology and Research (A*STAR), Singapore on research project721340051 (R-265-000-292-305) and the Ministry of Education,ingapore through National University of Singapore under Aca-emic Research Funds (R265-000-305-112).
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