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discharged lithium-rich thin film cathode by scanning probe microscopy techniques
Journal of Power Sources 352 (2017) 9e17
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Nanoscale characterization of charged/discharged lithium-rich thin film cathode by scanning probe microscopy techniques Shan Yang, Jiaxiong Wu, Binggong Yan, Liu Li, Yao Sun, Li Lu, Kaiyang Zeng* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, 117576, Singapore
h i g h l i g h t s Li-rich cathode material under one cycle of charge/discharge. Characterization of Li-rich cathode by SPM based techniques. Nanostructure and properties changes after the charge/discharge cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2016 Received in revised form 21 February 2017 Accepted 18 March 2017
Lithium ion intercalation and de-intercalation at electrodes are critically issues for development of the high performance Li-ion batteries. However, the changes of the surface morphologies, electrical and mechanical properties during the charge/discharge cycles are still not well understood. In this study, by using Atomic Force Microscopy (AFM), Electrochemical Strain Microscopy (ESM), AM-FM (Amplitude Modulation - Frequency Modulation), and conductive AFM (c-AFM) techniques, we have studied the significant changes, with nanoscale resolution, on the surface topography, electrochemical deformation, stiffness/elastic modulus, and electronic conductivity of Li-rich layered oxide cathode under the first cycle of galvonostatic charging/discharging process. This study has established the relationship between the nanoscale morphology and properties variations and the macroscopic charge/discharge processes for the Li-rich layered oxide cathode material, and the results also prove the feasibility of combining various Scanning Probe Microscopy (SPM) based techniques on the studies of lithium ion battery materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries AFM ESM AM-FM Conductive AFM
1. Introduction Lithium-ion batteries (LIBs) play a crucial role for portable electronic devices, electric vehicles and energy storage systems. Generally-speaking, LIBs show several advantages over other types of batteries, such as absence of memory effect, long cycle life, good portability and high energy conversion efficiency [1e3]. Recent studies on LIBs have focused on nanoscale studies on the biasinduced electrochemical phenomenon and local structure and properties variations in a full battery or individual components [4e9]. These studies have offered great opportunities to understand the detailed phenomena associated with Li-ions movement in the electrode materials at the nanoscale levels. These studies also greatly promote the understanding of the functions and properties of LIB materials and therefore benefit to their development and
* Corresponding author. E-mail address: [email protected] (K. Zeng). http://dx.doi.org/10.1016/j.jpowsour.2017.03.082 0378-7753/© 2017 Elsevier B.V. All rights reserved.
applications with improved performance in rechargeable LIBs. The degradation or aging of LIBs can cause impedance increase, capacity decay and power fading. While the operations of LIB are well-studied, the aging mechanisms are still to be examined in detail. The surface structure and properties need to be evaluated in detail to understand the aging mechanisms of the battery materials. Generally speaking, various macroscopic characterization techniques have been widely used to characterize the structure and properties of LIB materials or battery systems. These techniques include X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge/discharge measurement, galvanostatic/potentiostatic intermittent titration techniques (PITT/GITT), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and so on. However, all of these techniques are conducted at macroscopic and device levels. The understanding of the structure and properties changes during the electrochemical cycles for the individual components in a battery is therefore necessary. In these aspects, Scanning Probe Microscopy (SPM) based techniques are
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capable to study the localized information at micro- to nano-scales. In this work, we therefore applied the SPM techniques to study the changes of surface structure and properties of the cathode materials during the charge/discharge cycling processes. In particular, we studied the surface morphology changes by high-solution AFM (Atomic Force Microscope), surface deformation by ESM (Electrochemical Strain Microscope), surface stiffness and elastic modulus changes by AM-FM technique (Amplitude Modulation - Frequency Modulation), and the conductivity variations by c-AFM technique (Conductive AFM). With the capability of characterizing the multiple surface properties, the combination of the SPM techniques is therefore powerful and ideal method to investigate the relationship between the aging mechanisms with the surface and properties variations for battery materials. There are many studies on using SPM-based techniques to study the properties of battery materials. For example, bias-induced deformation generated on the individual grains of the LIB electrode has been recently studied by using ESM technique [5,7e11]. This technique allows the application of a high frequency AC bias through the ESM tip on the surface of electrochemically active materials. This electrical bias can induce the local deformation (electrochemical strain) which can be directly probed by a highly sensitive photodetector in the ESM and can be correlated with the local concentration and diffusivity of Li-ions [10]. Accompanied with the ESM technique, band excitation (BE) is also used to detect the tipesample contact resonance within a predefined frequency band. Thus, the resonance frequency and amplitude can be accurately tracked with minimizing the crosstalk of surface morphology and maximizing signal detection sensitivity. The raw data at each pixel point can be fitted by post-data-analysis processes, e.g., simple harmonic oscillator (SHO) model [12]. In BE-ESM measurements, resonance amplitude is defined as electrochemical strain, i.e., bias-induced surface deformation [10,11,13,14]. Resonance frequency is associated with the contact stiffness of the sample surface and resonance Q-factor is a measurement of the relative mechanical dissipative energy of the tip-sample interaction. ESM has been emerging as a powerful technique to investigate the electrochemical phenomena associated with nanoscale Li-ion movement in electrode materials, such as Si anode, LiCoO2, LiNi1/ 3Co1/3Mn1/3O2 and Li-rich cathode and so on [5,7,8,15]. It was known that the mechanical properties of the LIB materials were changed upon Li-ion intercalation/de-intercalation processes [16]. Traditionally, nanoindentation technique was used to study the changes of elastic modulus of the electrode during the charge/ discharge processes [17e21]. With the development of the AFM based method, such as AM-FM method, the variation of the mechanical properties can be characterized at the nanoscale level [22,23]. AM-FM operates by exciting the cantilever at two resonance frequencies simultaneously: the fundamental resonance frequency and usually the second but also possibly the third or fourth resonance frequency. The lower cantilever resonance frequency is used for standard tapping mode imaging, also known as amplitude modulation (AM), and this provides non-invasive topography image with high resolution. The higher cantilever resonance frequency is operated in frequency modulation (FM) mode, which is sensitive to the elastic tip-sample interaction and hence can provide the information related to the sample elasticity. During the AM-FM measurement, the frequency shift can be related to the tip-sample stiffness kts by using Equation (1): [24]
kts ¼
2k2 Df2 f0;2
(1)
where f0.2 is the second resonant frequency measured at free vibration; Df2 is the frequency shift of the second resonant mode as
the tip interacts with the surface; k2 is the force constant of the second resonant mode. Hence, the frequency shift of the second mode can be converted into tip-sample contact stiffness (N/m) with suitable calibration of the cantilever. The tip-sample contact stiffness kts can be used to calculate the elastic modulus of the sample using a mechanical model such as a Hertz contact model, which approximates the shape of the AFM tip as a punch indenter, as shown in Equation (2): [19]
kts ¼ 2Eeff rc
(2)
where rc is the contact radius of the tip; Eeff is the effective elastic modulus. In general, higher resonance frequency means greater stiffness or elastic modulus. This technique can be used to characterize the variations of the mechanical properties of the LIB materials during the electrochemical charge/discharge processes [24]. In recent studies, we have applied this technique to characterize the structures and properties of the nanoparticles and thin film of the Li-rich cathode [15,22,23], the results show the AM-FM technique can correlate the elastic modulus with the detail of the surface structure of the materials, with much higher spatial resolution than that of the nanoindentation techniques. Conductive AFM (c-AFM) is a powerful current sensing technique for electrical characterization of conductivity or resistivity variations. The local conductivity of the sample is acquired by detecting the current passing through the conductive tip and the sample with a bias applied between tip and the electrode at the back of the sample. The c-AFM technique is capable of monitoring currents varying from ~0.5 pA to 10 nA, allowing the conductivity measurement in the materials with relatively high resistivity [25]. In addition, c-AFM can simultaneously map the topography and current of the sample surface, allowing the correlation between a topographical feature and its conductivity. This technique has been applied in many ion-conductive materials such as LiCoO2, LiMn2O4, LiNi0.8Co0.2O2, LiNi0.8Co0.15Al0.05O2, Li-ion conducting glass ceramics and so on [26e30]. The commonly-used cathode materials, such as layered LiCoO2, spinel LiMn2O4 and olive LiFePO4, have difficulties to meet the increased requirement of the high energy density due to the limited capacities (120-170 mAh/g) in those materials [31,32]. Recently, Li-rich layered oxide material Li2MnO3-LiMO2 (M ¼ Ni, Co, Mn) has attracted substantial attention due to its high energy density (280 mAh/g) [31,33,34]. However, this material suffers an irreversible capacity loss during the 1st charge/discharge cycle, it was found that, during the discharge process, the amount of reinserted Li-ions was only half of the extracted ones in Li2MnO3, therefore, an irreversible capacity loss existed in the first charge/ discharge cycle [35]. Up to now, most of studies on this Li-rich cathode material are focused on the structural characterization and macroscopic electrochemical properties at the battery level. Only limited studies are conducted on the nanoscale characterization of this material [15,22,23]. Therefore, in this work, in order to explore and understand what are exactly happened during the first charge/discharge cycle, the Li-rich cathode thin film, Li1.2Co0.13Ni0.13Mn0.54O2 (or written as 0.55Li2MnO3-0.45LiCo1/ 3Ni1/3Mn1/3O2 based on mass ratio), is studied by using the various SPM techniques. The structure characterization and macroscopic electrochemical properties of this material have been previously reported by Yan and colleagues [36]. The X-ray diffraction (XRD) pattern (Fig. S1-Supplementary Information) shows the film has clear (003) reflection peak, indicating a highly (003) oriented grain structure. The SEM image shows the film consists of densely packed nano-crystalline grains with grain size within the range of 100e600 nm (Fig. S2, Supplementary Information). The thin film cathode exhibits the first discharge capacity as high as 70 mAh/
S. Yang et al. / Journal of Power Sources 352 (2017) 9e17
cm2 mm (Fig. S3, Supplementary Information). In this study, a half-cell battery is first assembled with thin film Li1.2Co0.13Ni0.13Mn0.54O2 as cathode. To understand what are happened during the first charge/discharge cycle, three cathode samples are prepared in this study (Table 1). One is as-deposited cathode sample. The second one is the charged sample, which is fully charged for one time. The third one is discharged sample, which is firstly charged fully for one time and then discharged fully for one time. In theory, the fully charged sample, the Li-ions are extracted whereas for the fully charged/discharged sample, the Liions are re-inserted into the cathode. In such a way, we can therefore distinguish the changes during charge process or discharge process. In this work, AFM, BE-ESM, AM-FM and c-AFM techniques are used to observe the surface morphology, surface deformation, mechanical property (stiffness or elasticity) and conductivity of the as-deposited, charged, and discharged cathode films. In this combination, we can therefore understand what are happened during the first charge/discharge cycle, and what the effects to the cathode material are after the first charge/discharge cycle. Although SPM-based techniques, such as c-AFM or ESM, can be used to directly assess the structural and/or conductance changes in the cathode film, and there are many reported works on such kind of characterization on many certain electrode material, it should note that the deformation and conductance under SPM applied biases may be different from what happened during the macroscopic electrochemical processes in real battery systems. From this point-of-view, the combination of the various SPM methods with macroscopic charge/discharge processes to characterize what happened in cathode materials is rarely reported, especially for Li-rich cathode materials.
2. Results and discussion 2.1. Topography change During the charge/discharge processes, Li-ions move between the two electrodes, resulting in the great morphology changes in both electrodes. Fig. 1 shows the AFM surface height/deflection maps of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode thin film samples, respectively. The height images show clusters of grains in all three samples (Fig. 1(a), (c) and (e)). The as-deposited thin film has flat, smooth and uniform surface morphology. Compared with those in charged and discharged samples, the grains of the as-deposited thin film have a flat surface with well-defined polygonal geometry (hexagon-like mostly) (Fig. 1(b)). After charging, the thin film becomes non-uniform and less smooth, and it no longer has the distinct facet planes (Fig. 1(d)). Compared to that of the as-deposited sample, the grains become less distinct. After discharging (Fig. 1(f)), the film recovers to its original grain size with a slight increase in the gaps between the grains. However, the shape of the grains becomes round, and sharp boundaries become ambiguous. It is noted that the film cannot return to the original volume after the first cycle. Part of the film peels off from the substrate because of the extensive topography changes and induced stress. Examination of the topographic image
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reveals substantial changes in the electrode surface morphology and structural damage in the film. It was known that, during the charge process above 4.4 V, the Li-ions in Li2MnO3 are extracted, accompanied by oxygen evolution, while the amount of reinserted Li-ions during discharging is only half of the extracted ones in Li2MnO3 [31,37]. During the first cycle, the Li-rich layered structure might be quickly deteriorated to spinel-like domained structures [36], and this can deteriorate the crystalline structure and the atomic ordering in the cathode film samples. The large volume change induced by such phase transformation leads to not only surface topography changes but also fast mechanical degradation of the thin film samples. This will be discussed in more detail in the next section. In addition, surface topography changes lead to the changes in surface roughness of the cathode film, which can be quantitatively measured by the root-mean-squared (RMS) roughness values determined from AFM images. The surface roughness increases from ~51 nm for as-deposited film to ~63 nm for charged film (in which the Li-ions are extracted out), and then decreases to ~56 nm for the discharged films (in which the Li-ions are re-inserted to the cathode film), respectively. The overall increment of the surface roughness after cycling is also in agreement with previous studies on LiMn2O4 cathode films [18,38], i.e., the insertion of the Li-ions can reduce the surface roughness of the cathode surface.
2.2. Electrochemical activity loss It was known that electrochemical activity and diffusivity of the electrode material gradually changed during the charge/discharge cycles, which could lead to the capacity fading and electrochemical degradation [5]. Therefore, in this study, BE-ESM measurements are conducted on as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 thin film samples, respectively. Fig. 2 shows the BEESM (with 3Vac) amplitude images, frequency images, as well as the line section profiles in the corresponding amplitude images of asdeposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode samples. The line sections are drawn from one grain to another one across the boundary between them. For the as-deposited sample, the electrochemical strain response (detected as resonance amplitude) is not homogeneous across the sample surface and increases abruptly at boundaries, indicating enhanced ionic mobility and electrochemical activity at the grain boundaries area (Fig. 2(a)). Compared with the BE-ESM mapping of the as-deposited cathode film, the variations of electrochemical strain after the charge/ discharge cycles can be clearly observed. Firstly, the charged sample has evidently lower electrochemical strain response (~130 fm) compared to that of the as-deposited samples (~1.10 p.m.) (Fig. 2(a) and (d)). This indicates that, compared with that in the asdeposited one, the charged sample has decreased the electrochemical activity due to the extraction of the Li-ions from the cathode structure. In addition, by comparing the line section profiles in the amplitude images (Fig. 2(c) and (f)), the contrast of electrochemical strain between the grain interiors and grain boundaries in the charged sample become much smaller (~38 fm) in comparison with that in the as-deposited one (~0.87 p.m.),
Table 1 The detail of the samples used in this study. Sample Name
Description
As-deposited Charged Discharged
PLD deposited film sample Fully charged for one time and disassembled film sample Firstly charged fully for one time and then discharged fully for one time and disassembled film sample
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Fig. 1. AFM height/deflection images of Li1.2Co0.13Ni0.13Mn0.54O2 cathode films taken at different stages of galvanostatic charge/discharge processes: (a)/(b) as-deposited; (c)/(d) charged; and (e)/(f) discharged. The AFM images are obtained with a pixel density of 256 256 and a scan size of 5 5 mm2.
corresponding to the indiscernible grain features in the amplitude images (Fig. 2(d)). These changes are due to the lack of Li-ions in the charged cathode. Secondly, the overall electrochemical strain (~530 fm) of the discharged Li1.2Co0.13Ni0.13Mn0.54O2 sample (Fig. 2(g)) becomes higher than that of the charged one (Fig. 2(d)). This can be understood from the increased amount of Li-ions after discharging since Li-ions reinsert into the cathode structure during the discharging process. However, the average electrochemical strain is still lower than that of the as-deposited one (Fig. 2(a)). This shows the capacity loss after the first cycle of charging and discharging cycle. It was reported that the amount of reinserted Liions during the first discharge process was approximately half of the extracted ones in the Li2MnO3 component, and led to the decreased amount of Li-ions in the cycled Li1.2Co0.13Ni0.13Mn0.54O2 cathode [31]. In addition, the variations of electrochemical strain between the grain boundaries and interiors (0.41 p.m.) have become larger than that of the charged sample but still less than that of the as-deposited one. These changes show that the degradation of the electrochemical activity is more significant at the boundary regions upon Li-ion intercalation/de-intercalation processes in the cycled Li1.2Co0.13Ni0.13Mn0.54O2 cathode. In the ESM images, the resonance frequency image is a measurement of conservative tipesurface interaction and provides information on the contact stiffness. The dark-spots in the resonance
frequency images (Fig. 2(b), (e) and (h)) indicate that the resonance frequency is not tracked at those spots. As shown in those figures, the frequency images fully represent the surface topographical features. The average frequency decreases from ~300 kHz to ~229 kHz, and then increases to ~265 kHz for the three samples. Large resonance frequency indicates higher contact stiffness. Therefore, the contact stiffness is largest for as-deposited sample, and decreases for charged sample and increases again for the discharged sample. The contact stiffness of the discharged sample is still less than that of the as-deposited sample. This result corresponds well to the results obtained from AM-FM technique, which will be discussed in the next section. 2.3. Stiffness/elastic modulus degradation Mechanical degradation of LIB electrodes has been correlated with capacity fade and impedance growth over repeated charging and discharging processes. Therefore, the knowledge of how the mechanical properties of electrode materials in LIB are affected by Li-ion intercalation/de-intercalation processes can provide insight into battery design that alleviate mechanical damage and extend device lifetime. It was known that the mechanical properties of electrode materials were Li concentration-dependent: some electrode materials were stiffened by Li ions (e.g. graphite) [39], while
S. Yang et al. / Journal of Power Sources 352 (2017) 9e17
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Fig. 2. The typical BE-ESM resonance amplitude/frequency images of the thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode taken at different stages of galvanostatic charge/discharge processes: (a)/(b) as-deposited, (d)/(e) charged; and (g)/(h) discharged. (c), (f) and (i) are the line section profiles in amplitude image, corresponding to the lines in (a), (d) and (g). The BE-ESM images are obtained with 3 Vac bias with a pixel density of 100 100 and a scan size of 1 1 mm2.
others were softened by Li ions (e.g. Si) [40]. In this study, the changes of the elasticity during the charge/discharge processes is studied by applying the AM-FM technique. Fig. 3(a)e(c) show AMFM contact stiffness images of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode films, respectively. In addition, as abovementioned, elastic modulus of the sample can be calculated by using the mechanical model. For the unknown tip geometry, a reference material with known elastic modulus is required to calculate the elastic modulus of the tested samples. Assuming the same tip-sample contact conditions for both reference material and tested samples: [41]
E* ¼ Er*
kts ktr
32 (3)
where Er* and ktr are the effective elastic modulus and the contact stiffness of the reference sample, respectively. Finally, the elastic modulus of the sample can be calculated through below equation: [42]
1 1 1 ¼ þ Es E* Et
(4)
where Es and Et are the elastic modulus of the sample and tip, respectively. In this work, the Si wafer (E~136 GPa from nanoindentation experiments) is used as a reference material for
calculating the elastic modulus of Li1.2Co0.13Ni0.13Mn0.54O2 cathode film. The same AM-FM measurement is conducted on the Si sample (reference material) with the same tip and the same parameters. Thus the elastic modulus of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode films can be calculated and mapped (Fig. 3(d)e(f)). After charging, the thin film shows significantly lower contact stiffness and elastic modulus (Fig. 3(b) and (e)). The contact stiffness and elastic modulus decrease by ~50% and ~70%, respectively. This change is associated with the microstructural changes occurring upon lithium content reduction, it can also be related to phase changes from the layered structure to spinel one [34], which occurs concurrently in the crystalline samples. The decrease of stiffness/ elastic modulus agrees well with the observations in the previous study of nanoindentation, which showed that the charged LiCoO2 cathode has significant lower Young's modulus E, and this reduction was related to severe depletion of Li at the grain surface [18,19]. After discharging, the contact stiffness and elastic modulus of the thin film increase again (Fig. 3(c) and (f)). It is known that the change from the weak van der Waals interlayer interaction to ionic interaction during Li-ion intercalation was responsible for the increase of elasticity in the materials with layered structure [16]. The increase of stiffness/elastic modulus after discharging is in agreement with the results by Qi et al., who have shown that Young's modulus E was positively dependent on Li concentration in layered structures such as graphite and LixCoO2 using the density
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Fig. 3. The AM-FM contact stiffness (Kts) and elastic modulus maps taken at different stages of galvanostatic charge/discharge processes in thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode: (a)/(d) as-deposited; (b)/(e) charged; and (c)/(f) discharged. The images are generated by a matrix of 256 256 data points with a scan size of 1.5 1.5 mm2.
functional theory (DFT) [16]. It is also noted that the contact stiffness and elastic modulus of the discharged film are lower than that of the as-deposited one. After one full charge/discharge cycle, the contact stiffness and elastic modulus recover but they are still lower than that of the as-deposited sample by ~25% and ~35%. This indicates the degradation of the mechanical properties during the first cycle is very significant. This result agrees well with the previous nanoindentation studies that the Young's modulus (E) decreased gradually during the charge/discharge cycles in LiNi1/ 3Co1/3Mn1/3O2 and LiMn2O4 cathode films, as well as in RuO2, and TiO2 anode films [5,18e20]. Generally-speaking, the mechanical degradation can reduce the interfacial adhesion and the contact between the film and the substrate, hence may cause cracking in the electrodes and reduce the battery life. It should be also noted that these significant changes in mechanical properties occur within the normal electrochemical cycling window of a cathode, and therefore the design of mechanically robust electrodes should consider that the mechanical properties of the active compound can deviate significantly from those reported for uncharged electrode materials, even within a single electrochemical charge and discharge cycle. 2.4. Current variation As shown in the previous study [23], during the c-AFM measurements, the conductivity or conductance is proportional to the current. Fig. 4 shows current image (at a sample voltage of 3 V), as well as the line section profiles in the corresponding current images of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode film samples, respectively. In these current images, the brighter region represents higher current and hence the higher electronic conductivity, while the darker region represents lower electronic conductivity and thus higher resistivity. The strong variability in conductivity between grains and grain boundaries can be clearly observed (Fig. 4). In as-deposited sample, the current response varies substantially from the grain interiors to the boundaries (Fig. 4(a)). Characteristically, the area at grain interiors exhibit very poor conductivity compared to the areas at grain boundaries where the conductivity is substantially higher.
This agrees with the observation in the earlier study [23]. The charged thin film cathode exhibits excellent electronic conductivity in most grain area (Fig. 4(c)). The insulating regions on the cathode surface are usually associated with the presence of solid state interphase (SEI) products and intergranular deep cavities. The conductivity difference between the grain boundaries and interiors is no longer significant (Fig. 4(b) and (d)). It was reported that the drastic change in the electronic conductivity of the layered LiCoO2 at an early stage of Li-ion de-intercalation was caused by the insulator-metal transition [43,44]. Hence, the increased current at grain interior indicates a metallic character of charged thin film. This agrees well with the study of layered Li1-xNi0.8Co0.15Al0.05O2 that the electronic conductivity increased with decreasing Licontent over the range of x ¼ 0.0 to 0.6 using direct current polarization technique as well as the electrochemical impedance spectroscopy [45]. As shown in Fig. 4(e) and (f), after discharging, the conductivity contrast between the grain interiors and boundaries becomes more distinct compared with that of the charged film, but still less than that of the as-deposited one, which indicates the changes of the conductivity due to the surface modification after the charge/discharge cycle. In addition, Fig. 4(f) shows that the current level in the discharged thin film is lower than that of both charged and as-deposited samples. This indicates the noticeable decline of the electronic conductivity after cycling. This result is consistent with the results by Qui et al. and Shibuya et al. on layered LiNi1/3Co1/3Mn1/3O2 and LiCoO2 cathodes, in which the electronic conductivity increased during the charge process and decreased during the discharge cycles [46e48]. The loss of the electronic conductivity suggests the morphological and structural changes of the cathode, and this corresponds well with the topographic image. The electronic conductivity loss after cycling was also observed in other layered cathode materials, such as LiNi1/3Co1/3Mn1/3O2, LiNi0.8Co0.2O2, and LiNi0.8Co0.15Al0.05O2 [5,26,28]. The surface conductivity affects the rate at which Li-ions diffuse out of cathode into anode during the charging process, thus an ionic resistance would increase if the surface conductivity was reduced [49]. This conductivity degradation can further lead to impedance increase, capacity fading, and power decay of the battery, and ultimately reduce the cycle life of the battery. Thus, the conductivity changes
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Fig. 4. The current maps of thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode with 3 V DC bias over the scanning area of 1 1 mm2 taken at different stages of galvonostatic charge/ discharge processes: (a) as-deposited; (c) charged; and (e) discharged. (b), (d) and (f) are the line section profiles in current images, corresponding to the lines in (a), (c) and (e). The current images above are generated by a matrix of 256 256 data points.
should be taken into account while characterizing the performance of the electrode materials. 3. Conclusion In summary, this paper reports the changes of the nanoscale structure and properties in thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode during the first charge/discharge cycle, characterized by the combination of various SPM-based techniques and macroscopic electrochemical charge/discharge measurements. These changes include surface topography, electrochemical strain, electrical conductivity and stiffness/elastic modulus. First, the changes of the surface topography in the thin film cathode are probed with nanometer resolution, and a clear correlation between the charging/discharging processes and local morphology is established. The cathode film contracts with Li-ion extraction and expands with Li-ion insertion. After the first cycle, the changes on the surface morphology are obvious, with an increase in surface roughness. Secondly, local variations of the electrochemical strain response in the thin film cathode after the charge/discharge processes have been investigated by BE-ESM technique. The results have shown that the electrochemical strain response decreases
significantly after the charging, with certain amount of recovery after discharging. The results allow us to establish a direct relationship between the changes in electrochemical strain responses and the Li-ion concentration as well as the electrochemical activity at the nanoscale. Thirdly, the stiffness/elastic modulus has been mapped by AM-FM technique. It is found that the stiffness/elastic modulus of the cycled thin film is lower than that of the asdeposited film. In addition, conductance mapping has been examined upon first cycle by using c-AFM, which reveals that the electrical conductivity of the thin film cathode reduces after the first cycle. All these results can be correlated well with the macroscopic capacity decay during the first cycle in Li-rich cathode that is observed at macroscopic battery level. These results also provide the new perspective for understanding the battery aging from the combination of the materials properties at the micro to nanoscales. This study also demonstrates that combining various SPM techniques is very effective to study the morphology, electrochemical strain (deformation), electrical and mechanical properties of the cathode materials, particularly their dependence on Li-ion interaction and extraction at the nanoscale for any cathode materials. Thus, these factors need to be considered carefully in the design and development of new battery materials.
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As SPM technique can sensitively probe the surface properties of the LIB materials, it can be used to study the damage mechanisms of the cathode materials. Thus, SPM study of a LIB electrode which is extracted from batteries and aged to different degrees of life would provide important information about the onset of damage, further to determine the perdurability and reliability of the electrode. 4. Experimental section 4.1. Sample preparation The target material Li1.2Mn0.54Ni0.13Co0.13O2 was first made by using raw materials Li2CO3 (Sigma-Aldrich, 99.9%), MnO2 (SigmaAldrich, 99.9%), Co3O4 (Sigma-Aldrich, 99.9%), and NiO (SigmaAldrich, 99.9%) as the precursors. The molar ratio of Ni:Co:Mn was set to be 0.13:0.13:0.54 with 10% excess Li source to compensate the Li loss at high sintering temperature. The starting precursors were wet ball-milled for 2 h in a steel container. After drying in air at 80 C for 12 h, the resultant mixture was cold pressed into a pellet with the dimension of 26 mm in diameter and 4 mm in height. The cold compact was then sintered at 900 C for 24 h to obtain a solid target for the subsequent film deposition by pulsed laser deposition (PLD). During the PLD process, a KrF excimer laser beam (248 nm, 180 mJ) (Lambda Physik, USA) was used at a repetition frequency of 10 Hz. Thin films were deposited on Au substrates at 650 C with an oxygen partial pressure of 350 mTorr. The target-substrate distance was kept at 20 mm during the film deposition. The as-deposited thin films were post-annealed at 800 C with oxygen flow for 40 min.
conducted at room temperature (~25 C) under the ambient air condition (with average humidity of ~60%). During the SPM experiments, a conductive cantilever with silicon tip coated with Ti/Pt layer (AC240TM, Olympus, Japan) was used. The cantilever has an average spring constant of 2 N/m and tip radius of 28 nm. The same tip is used for each measurement. 4.4.2. Electrochemical strain measurement by BE-ESM BE-ESM mapping was performed with 3Vac bias signal and a bandwidth of 20 kHz with the BE control software (Asylum Research/Oak Ridge National Laboratory, USA) associated with the SPM system. The BE-ESM images are formed by fitting the response curves at each point using the simple harmonic oscillator (SHO) method after the measurements. 4.4.3. Stiffness measurement by AM-FM AM-FM measurement was conducted with the special cantilever holder (Asylum Research, CA, USA) attached to the SPM system. The drive voltage is set at 2 V for the first eigenmode (AM), and 10 mV for the third eigenmode (FM). This small amplitude of FM mode prevents the perturbation to the first eigenmode cantilever dynamics during the measurements. 4.4.4. Current measurement by c-AFM Current measurement was performed by using a commercial conductive module (ORCA™, Asylum Research, CA, USA) with the special cantilever holder and sample stage attached to the SPM system. In the c-AFM measurements, bias was applied via the substrate, and the cantilever tip was grounded. The thin film was in contact with the tip firmly by using about 90 nN loading force.
4.2. Material characterization
Acknowledgment
The crystal structure of the thin film and the substrate were characterized using XRD equipment (Model XRD-7000, Shimadzu Corporation, Kyoto, Japan) with Cu Ka radiation (l ¼ 1.5418 Å). Surface morphology of the thin film was characterized using a field emission scanning electron microscopy (FESEM, Hitachi S-4100, Japan).
This work is supported by Ministry of Education (Singapore) through National University of Singapore under Academic Research Fund (R-265-000-406-112). One of the author (YS) would also like to thank the support of scholarship by Ministry of Education (Singapore) under the Academic Research Fund (R-265-100-406112) and National University of Singapore.
4.3. Electrochemical measurement Half-battery cells were assembled in an argon-filled glove box using the deposited Li-rich thin film as the cathode, metal Li foil as the anode, two pieces of Celgard 2500 as separators and a few drops of electrolyte (1 M LiPF6 in EC:DEC ¼ 1:1 organic solutions). Swageloks were used for half-battery assembling. The gavolnostatic charge/discharge tests were conducted at a constant current density of 2 mA/cm2 between 2.0 V and 4.8 V at room temperature using Battery Test Station (Neware, China). To investigate the nanostructure and properties changes due to charge/discharge processes, one battery cell was first fully charged for one time followed by dissembling, and the second battery cell was fully charged and then fully discharged for one time followed by dissembling. The dissembled cathode films were rinsed by acetone for various SPM characterizations. In such way, there are three separate samples used in this study: i.e., one as-deposited, one charged, and one discharged thin film samples. 4.4. SPM characterizations 4.4.1. General SPM setup A commercial Scanning Probe Microscopy system (MFP-3D, Asylum Research, CA, USA) was used as the primary characterization tool for imaging and analysis. All SPM studies in this work were
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Nanoscale characterization of charged/discharged lithium-rich thin film cathode by scanning probe microscopy techniques Shan Yang, Jiaxiong Wu, Binggong Yan, Liu Li, Yao Sun, Li Lu, Kaiyang Zeng* Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, 117576, Singapore
h i g h l i g h t s Li-rich cathode material under one cycle of charge/discharge. Characterization of Li-rich cathode by SPM based techniques. Nanostructure and properties changes after the charge/discharge cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 November 2016 Received in revised form 21 February 2017 Accepted 18 March 2017
Lithium ion intercalation and de-intercalation at electrodes are critically issues for development of the high performance Li-ion batteries. However, the changes of the surface morphologies, electrical and mechanical properties during the charge/discharge cycles are still not well understood. In this study, by using Atomic Force Microscopy (AFM), Electrochemical Strain Microscopy (ESM), AM-FM (Amplitude Modulation - Frequency Modulation), and conductive AFM (c-AFM) techniques, we have studied the significant changes, with nanoscale resolution, on the surface topography, electrochemical deformation, stiffness/elastic modulus, and electronic conductivity of Li-rich layered oxide cathode under the first cycle of galvonostatic charging/discharging process. This study has established the relationship between the nanoscale morphology and properties variations and the macroscopic charge/discharge processes for the Li-rich layered oxide cathode material, and the results also prove the feasibility of combining various Scanning Probe Microscopy (SPM) based techniques on the studies of lithium ion battery materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium-ion batteries AFM ESM AM-FM Conductive AFM
1. Introduction Lithium-ion batteries (LIBs) play a crucial role for portable electronic devices, electric vehicles and energy storage systems. Generally-speaking, LIBs show several advantages over other types of batteries, such as absence of memory effect, long cycle life, good portability and high energy conversion efficiency [1e3]. Recent studies on LIBs have focused on nanoscale studies on the biasinduced electrochemical phenomenon and local structure and properties variations in a full battery or individual components [4e9]. These studies have offered great opportunities to understand the detailed phenomena associated with Li-ions movement in the electrode materials at the nanoscale levels. These studies also greatly promote the understanding of the functions and properties of LIB materials and therefore benefit to their development and
* Corresponding author. E-mail address: [email protected] (K. Zeng). http://dx.doi.org/10.1016/j.jpowsour.2017.03.082 0378-7753/© 2017 Elsevier B.V. All rights reserved.
applications with improved performance in rechargeable LIBs. The degradation or aging of LIBs can cause impedance increase, capacity decay and power fading. While the operations of LIB are well-studied, the aging mechanisms are still to be examined in detail. The surface structure and properties need to be evaluated in detail to understand the aging mechanisms of the battery materials. Generally speaking, various macroscopic characterization techniques have been widely used to characterize the structure and properties of LIB materials or battery systems. These techniques include X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge/discharge measurement, galvanostatic/potentiostatic intermittent titration techniques (PITT/GITT), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and so on. However, all of these techniques are conducted at macroscopic and device levels. The understanding of the structure and properties changes during the electrochemical cycles for the individual components in a battery is therefore necessary. In these aspects, Scanning Probe Microscopy (SPM) based techniques are
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capable to study the localized information at micro- to nano-scales. In this work, we therefore applied the SPM techniques to study the changes of surface structure and properties of the cathode materials during the charge/discharge cycling processes. In particular, we studied the surface morphology changes by high-solution AFM (Atomic Force Microscope), surface deformation by ESM (Electrochemical Strain Microscope), surface stiffness and elastic modulus changes by AM-FM technique (Amplitude Modulation - Frequency Modulation), and the conductivity variations by c-AFM technique (Conductive AFM). With the capability of characterizing the multiple surface properties, the combination of the SPM techniques is therefore powerful and ideal method to investigate the relationship between the aging mechanisms with the surface and properties variations for battery materials. There are many studies on using SPM-based techniques to study the properties of battery materials. For example, bias-induced deformation generated on the individual grains of the LIB electrode has been recently studied by using ESM technique [5,7e11]. This technique allows the application of a high frequency AC bias through the ESM tip on the surface of electrochemically active materials. This electrical bias can induce the local deformation (electrochemical strain) which can be directly probed by a highly sensitive photodetector in the ESM and can be correlated with the local concentration and diffusivity of Li-ions [10]. Accompanied with the ESM technique, band excitation (BE) is also used to detect the tipesample contact resonance within a predefined frequency band. Thus, the resonance frequency and amplitude can be accurately tracked with minimizing the crosstalk of surface morphology and maximizing signal detection sensitivity. The raw data at each pixel point can be fitted by post-data-analysis processes, e.g., simple harmonic oscillator (SHO) model [12]. In BE-ESM measurements, resonance amplitude is defined as electrochemical strain, i.e., bias-induced surface deformation [10,11,13,14]. Resonance frequency is associated with the contact stiffness of the sample surface and resonance Q-factor is a measurement of the relative mechanical dissipative energy of the tip-sample interaction. ESM has been emerging as a powerful technique to investigate the electrochemical phenomena associated with nanoscale Li-ion movement in electrode materials, such as Si anode, LiCoO2, LiNi1/ 3Co1/3Mn1/3O2 and Li-rich cathode and so on [5,7,8,15]. It was known that the mechanical properties of the LIB materials were changed upon Li-ion intercalation/de-intercalation processes [16]. Traditionally, nanoindentation technique was used to study the changes of elastic modulus of the electrode during the charge/ discharge processes [17e21]. With the development of the AFM based method, such as AM-FM method, the variation of the mechanical properties can be characterized at the nanoscale level [22,23]. AM-FM operates by exciting the cantilever at two resonance frequencies simultaneously: the fundamental resonance frequency and usually the second but also possibly the third or fourth resonance frequency. The lower cantilever resonance frequency is used for standard tapping mode imaging, also known as amplitude modulation (AM), and this provides non-invasive topography image with high resolution. The higher cantilever resonance frequency is operated in frequency modulation (FM) mode, which is sensitive to the elastic tip-sample interaction and hence can provide the information related to the sample elasticity. During the AM-FM measurement, the frequency shift can be related to the tip-sample stiffness kts by using Equation (1): [24]
kts ¼
2k2 Df2 f0;2
(1)
where f0.2 is the second resonant frequency measured at free vibration; Df2 is the frequency shift of the second resonant mode as
the tip interacts with the surface; k2 is the force constant of the second resonant mode. Hence, the frequency shift of the second mode can be converted into tip-sample contact stiffness (N/m) with suitable calibration of the cantilever. The tip-sample contact stiffness kts can be used to calculate the elastic modulus of the sample using a mechanical model such as a Hertz contact model, which approximates the shape of the AFM tip as a punch indenter, as shown in Equation (2): [19]
kts ¼ 2Eeff rc
(2)
where rc is the contact radius of the tip; Eeff is the effective elastic modulus. In general, higher resonance frequency means greater stiffness or elastic modulus. This technique can be used to characterize the variations of the mechanical properties of the LIB materials during the electrochemical charge/discharge processes [24]. In recent studies, we have applied this technique to characterize the structures and properties of the nanoparticles and thin film of the Li-rich cathode [15,22,23], the results show the AM-FM technique can correlate the elastic modulus with the detail of the surface structure of the materials, with much higher spatial resolution than that of the nanoindentation techniques. Conductive AFM (c-AFM) is a powerful current sensing technique for electrical characterization of conductivity or resistivity variations. The local conductivity of the sample is acquired by detecting the current passing through the conductive tip and the sample with a bias applied between tip and the electrode at the back of the sample. The c-AFM technique is capable of monitoring currents varying from ~0.5 pA to 10 nA, allowing the conductivity measurement in the materials with relatively high resistivity [25]. In addition, c-AFM can simultaneously map the topography and current of the sample surface, allowing the correlation between a topographical feature and its conductivity. This technique has been applied in many ion-conductive materials such as LiCoO2, LiMn2O4, LiNi0.8Co0.2O2, LiNi0.8Co0.15Al0.05O2, Li-ion conducting glass ceramics and so on [26e30]. The commonly-used cathode materials, such as layered LiCoO2, spinel LiMn2O4 and olive LiFePO4, have difficulties to meet the increased requirement of the high energy density due to the limited capacities (120-170 mAh/g) in those materials [31,32]. Recently, Li-rich layered oxide material Li2MnO3-LiMO2 (M ¼ Ni, Co, Mn) has attracted substantial attention due to its high energy density (280 mAh/g) [31,33,34]. However, this material suffers an irreversible capacity loss during the 1st charge/discharge cycle, it was found that, during the discharge process, the amount of reinserted Li-ions was only half of the extracted ones in Li2MnO3, therefore, an irreversible capacity loss existed in the first charge/ discharge cycle [35]. Up to now, most of studies on this Li-rich cathode material are focused on the structural characterization and macroscopic electrochemical properties at the battery level. Only limited studies are conducted on the nanoscale characterization of this material [15,22,23]. Therefore, in this work, in order to explore and understand what are exactly happened during the first charge/discharge cycle, the Li-rich cathode thin film, Li1.2Co0.13Ni0.13Mn0.54O2 (or written as 0.55Li2MnO3-0.45LiCo1/ 3Ni1/3Mn1/3O2 based on mass ratio), is studied by using the various SPM techniques. The structure characterization and macroscopic electrochemical properties of this material have been previously reported by Yan and colleagues [36]. The X-ray diffraction (XRD) pattern (Fig. S1-Supplementary Information) shows the film has clear (003) reflection peak, indicating a highly (003) oriented grain structure. The SEM image shows the film consists of densely packed nano-crystalline grains with grain size within the range of 100e600 nm (Fig. S2, Supplementary Information). The thin film cathode exhibits the first discharge capacity as high as 70 mAh/
S. Yang et al. / Journal of Power Sources 352 (2017) 9e17
cm2 mm (Fig. S3, Supplementary Information). In this study, a half-cell battery is first assembled with thin film Li1.2Co0.13Ni0.13Mn0.54O2 as cathode. To understand what are happened during the first charge/discharge cycle, three cathode samples are prepared in this study (Table 1). One is as-deposited cathode sample. The second one is the charged sample, which is fully charged for one time. The third one is discharged sample, which is firstly charged fully for one time and then discharged fully for one time. In theory, the fully charged sample, the Li-ions are extracted whereas for the fully charged/discharged sample, the Liions are re-inserted into the cathode. In such a way, we can therefore distinguish the changes during charge process or discharge process. In this work, AFM, BE-ESM, AM-FM and c-AFM techniques are used to observe the surface morphology, surface deformation, mechanical property (stiffness or elasticity) and conductivity of the as-deposited, charged, and discharged cathode films. In this combination, we can therefore understand what are happened during the first charge/discharge cycle, and what the effects to the cathode material are after the first charge/discharge cycle. Although SPM-based techniques, such as c-AFM or ESM, can be used to directly assess the structural and/or conductance changes in the cathode film, and there are many reported works on such kind of characterization on many certain electrode material, it should note that the deformation and conductance under SPM applied biases may be different from what happened during the macroscopic electrochemical processes in real battery systems. From this point-of-view, the combination of the various SPM methods with macroscopic charge/discharge processes to characterize what happened in cathode materials is rarely reported, especially for Li-rich cathode materials.
2. Results and discussion 2.1. Topography change During the charge/discharge processes, Li-ions move between the two electrodes, resulting in the great morphology changes in both electrodes. Fig. 1 shows the AFM surface height/deflection maps of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode thin film samples, respectively. The height images show clusters of grains in all three samples (Fig. 1(a), (c) and (e)). The as-deposited thin film has flat, smooth and uniform surface morphology. Compared with those in charged and discharged samples, the grains of the as-deposited thin film have a flat surface with well-defined polygonal geometry (hexagon-like mostly) (Fig. 1(b)). After charging, the thin film becomes non-uniform and less smooth, and it no longer has the distinct facet planes (Fig. 1(d)). Compared to that of the as-deposited sample, the grains become less distinct. After discharging (Fig. 1(f)), the film recovers to its original grain size with a slight increase in the gaps between the grains. However, the shape of the grains becomes round, and sharp boundaries become ambiguous. It is noted that the film cannot return to the original volume after the first cycle. Part of the film peels off from the substrate because of the extensive topography changes and induced stress. Examination of the topographic image
11
reveals substantial changes in the electrode surface morphology and structural damage in the film. It was known that, during the charge process above 4.4 V, the Li-ions in Li2MnO3 are extracted, accompanied by oxygen evolution, while the amount of reinserted Li-ions during discharging is only half of the extracted ones in Li2MnO3 [31,37]. During the first cycle, the Li-rich layered structure might be quickly deteriorated to spinel-like domained structures [36], and this can deteriorate the crystalline structure and the atomic ordering in the cathode film samples. The large volume change induced by such phase transformation leads to not only surface topography changes but also fast mechanical degradation of the thin film samples. This will be discussed in more detail in the next section. In addition, surface topography changes lead to the changes in surface roughness of the cathode film, which can be quantitatively measured by the root-mean-squared (RMS) roughness values determined from AFM images. The surface roughness increases from ~51 nm for as-deposited film to ~63 nm for charged film (in which the Li-ions are extracted out), and then decreases to ~56 nm for the discharged films (in which the Li-ions are re-inserted to the cathode film), respectively. The overall increment of the surface roughness after cycling is also in agreement with previous studies on LiMn2O4 cathode films [18,38], i.e., the insertion of the Li-ions can reduce the surface roughness of the cathode surface.
2.2. Electrochemical activity loss It was known that electrochemical activity and diffusivity of the electrode material gradually changed during the charge/discharge cycles, which could lead to the capacity fading and electrochemical degradation [5]. Therefore, in this study, BE-ESM measurements are conducted on as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 thin film samples, respectively. Fig. 2 shows the BEESM (with 3Vac) amplitude images, frequency images, as well as the line section profiles in the corresponding amplitude images of asdeposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode samples. The line sections are drawn from one grain to another one across the boundary between them. For the as-deposited sample, the electrochemical strain response (detected as resonance amplitude) is not homogeneous across the sample surface and increases abruptly at boundaries, indicating enhanced ionic mobility and electrochemical activity at the grain boundaries area (Fig. 2(a)). Compared with the BE-ESM mapping of the as-deposited cathode film, the variations of electrochemical strain after the charge/ discharge cycles can be clearly observed. Firstly, the charged sample has evidently lower electrochemical strain response (~130 fm) compared to that of the as-deposited samples (~1.10 p.m.) (Fig. 2(a) and (d)). This indicates that, compared with that in the asdeposited one, the charged sample has decreased the electrochemical activity due to the extraction of the Li-ions from the cathode structure. In addition, by comparing the line section profiles in the amplitude images (Fig. 2(c) and (f)), the contrast of electrochemical strain between the grain interiors and grain boundaries in the charged sample become much smaller (~38 fm) in comparison with that in the as-deposited one (~0.87 p.m.),
Table 1 The detail of the samples used in this study. Sample Name
Description
As-deposited Charged Discharged
PLD deposited film sample Fully charged for one time and disassembled film sample Firstly charged fully for one time and then discharged fully for one time and disassembled film sample
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S. Yang et al. / Journal of Power Sources 352 (2017) 9e17
Fig. 1. AFM height/deflection images of Li1.2Co0.13Ni0.13Mn0.54O2 cathode films taken at different stages of galvanostatic charge/discharge processes: (a)/(b) as-deposited; (c)/(d) charged; and (e)/(f) discharged. The AFM images are obtained with a pixel density of 256 256 and a scan size of 5 5 mm2.
corresponding to the indiscernible grain features in the amplitude images (Fig. 2(d)). These changes are due to the lack of Li-ions in the charged cathode. Secondly, the overall electrochemical strain (~530 fm) of the discharged Li1.2Co0.13Ni0.13Mn0.54O2 sample (Fig. 2(g)) becomes higher than that of the charged one (Fig. 2(d)). This can be understood from the increased amount of Li-ions after discharging since Li-ions reinsert into the cathode structure during the discharging process. However, the average electrochemical strain is still lower than that of the as-deposited one (Fig. 2(a)). This shows the capacity loss after the first cycle of charging and discharging cycle. It was reported that the amount of reinserted Liions during the first discharge process was approximately half of the extracted ones in the Li2MnO3 component, and led to the decreased amount of Li-ions in the cycled Li1.2Co0.13Ni0.13Mn0.54O2 cathode [31]. In addition, the variations of electrochemical strain between the grain boundaries and interiors (0.41 p.m.) have become larger than that of the charged sample but still less than that of the as-deposited one. These changes show that the degradation of the electrochemical activity is more significant at the boundary regions upon Li-ion intercalation/de-intercalation processes in the cycled Li1.2Co0.13Ni0.13Mn0.54O2 cathode. In the ESM images, the resonance frequency image is a measurement of conservative tipesurface interaction and provides information on the contact stiffness. The dark-spots in the resonance
frequency images (Fig. 2(b), (e) and (h)) indicate that the resonance frequency is not tracked at those spots. As shown in those figures, the frequency images fully represent the surface topographical features. The average frequency decreases from ~300 kHz to ~229 kHz, and then increases to ~265 kHz for the three samples. Large resonance frequency indicates higher contact stiffness. Therefore, the contact stiffness is largest for as-deposited sample, and decreases for charged sample and increases again for the discharged sample. The contact stiffness of the discharged sample is still less than that of the as-deposited sample. This result corresponds well to the results obtained from AM-FM technique, which will be discussed in the next section. 2.3. Stiffness/elastic modulus degradation Mechanical degradation of LIB electrodes has been correlated with capacity fade and impedance growth over repeated charging and discharging processes. Therefore, the knowledge of how the mechanical properties of electrode materials in LIB are affected by Li-ion intercalation/de-intercalation processes can provide insight into battery design that alleviate mechanical damage and extend device lifetime. It was known that the mechanical properties of electrode materials were Li concentration-dependent: some electrode materials were stiffened by Li ions (e.g. graphite) [39], while
S. Yang et al. / Journal of Power Sources 352 (2017) 9e17
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Fig. 2. The typical BE-ESM resonance amplitude/frequency images of the thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode taken at different stages of galvanostatic charge/discharge processes: (a)/(b) as-deposited, (d)/(e) charged; and (g)/(h) discharged. (c), (f) and (i) are the line section profiles in amplitude image, corresponding to the lines in (a), (d) and (g). The BE-ESM images are obtained with 3 Vac bias with a pixel density of 100 100 and a scan size of 1 1 mm2.
others were softened by Li ions (e.g. Si) [40]. In this study, the changes of the elasticity during the charge/discharge processes is studied by applying the AM-FM technique. Fig. 3(a)e(c) show AMFM contact stiffness images of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode films, respectively. In addition, as abovementioned, elastic modulus of the sample can be calculated by using the mechanical model. For the unknown tip geometry, a reference material with known elastic modulus is required to calculate the elastic modulus of the tested samples. Assuming the same tip-sample contact conditions for both reference material and tested samples: [41]
E* ¼ Er*
kts ktr
32 (3)
where Er* and ktr are the effective elastic modulus and the contact stiffness of the reference sample, respectively. Finally, the elastic modulus of the sample can be calculated through below equation: [42]
1 1 1 ¼ þ Es E* Et
(4)
where Es and Et are the elastic modulus of the sample and tip, respectively. In this work, the Si wafer (E~136 GPa from nanoindentation experiments) is used as a reference material for
calculating the elastic modulus of Li1.2Co0.13Ni0.13Mn0.54O2 cathode film. The same AM-FM measurement is conducted on the Si sample (reference material) with the same tip and the same parameters. Thus the elastic modulus of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode films can be calculated and mapped (Fig. 3(d)e(f)). After charging, the thin film shows significantly lower contact stiffness and elastic modulus (Fig. 3(b) and (e)). The contact stiffness and elastic modulus decrease by ~50% and ~70%, respectively. This change is associated with the microstructural changes occurring upon lithium content reduction, it can also be related to phase changes from the layered structure to spinel one [34], which occurs concurrently in the crystalline samples. The decrease of stiffness/ elastic modulus agrees well with the observations in the previous study of nanoindentation, which showed that the charged LiCoO2 cathode has significant lower Young's modulus E, and this reduction was related to severe depletion of Li at the grain surface [18,19]. After discharging, the contact stiffness and elastic modulus of the thin film increase again (Fig. 3(c) and (f)). It is known that the change from the weak van der Waals interlayer interaction to ionic interaction during Li-ion intercalation was responsible for the increase of elasticity in the materials with layered structure [16]. The increase of stiffness/elastic modulus after discharging is in agreement with the results by Qi et al., who have shown that Young's modulus E was positively dependent on Li concentration in layered structures such as graphite and LixCoO2 using the density
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Fig. 3. The AM-FM contact stiffness (Kts) and elastic modulus maps taken at different stages of galvanostatic charge/discharge processes in thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode: (a)/(d) as-deposited; (b)/(e) charged; and (c)/(f) discharged. The images are generated by a matrix of 256 256 data points with a scan size of 1.5 1.5 mm2.
functional theory (DFT) [16]. It is also noted that the contact stiffness and elastic modulus of the discharged film are lower than that of the as-deposited one. After one full charge/discharge cycle, the contact stiffness and elastic modulus recover but they are still lower than that of the as-deposited sample by ~25% and ~35%. This indicates the degradation of the mechanical properties during the first cycle is very significant. This result agrees well with the previous nanoindentation studies that the Young's modulus (E) decreased gradually during the charge/discharge cycles in LiNi1/ 3Co1/3Mn1/3O2 and LiMn2O4 cathode films, as well as in RuO2, and TiO2 anode films [5,18e20]. Generally-speaking, the mechanical degradation can reduce the interfacial adhesion and the contact between the film and the substrate, hence may cause cracking in the electrodes and reduce the battery life. It should be also noted that these significant changes in mechanical properties occur within the normal electrochemical cycling window of a cathode, and therefore the design of mechanically robust electrodes should consider that the mechanical properties of the active compound can deviate significantly from those reported for uncharged electrode materials, even within a single electrochemical charge and discharge cycle. 2.4. Current variation As shown in the previous study [23], during the c-AFM measurements, the conductivity or conductance is proportional to the current. Fig. 4 shows current image (at a sample voltage of 3 V), as well as the line section profiles in the corresponding current images of the as-deposited, charged and discharged Li1.2Co0.13Ni0.13Mn0.54O2 cathode film samples, respectively. In these current images, the brighter region represents higher current and hence the higher electronic conductivity, while the darker region represents lower electronic conductivity and thus higher resistivity. The strong variability in conductivity between grains and grain boundaries can be clearly observed (Fig. 4). In as-deposited sample, the current response varies substantially from the grain interiors to the boundaries (Fig. 4(a)). Characteristically, the area at grain interiors exhibit very poor conductivity compared to the areas at grain boundaries where the conductivity is substantially higher.
This agrees with the observation in the earlier study [23]. The charged thin film cathode exhibits excellent electronic conductivity in most grain area (Fig. 4(c)). The insulating regions on the cathode surface are usually associated with the presence of solid state interphase (SEI) products and intergranular deep cavities. The conductivity difference between the grain boundaries and interiors is no longer significant (Fig. 4(b) and (d)). It was reported that the drastic change in the electronic conductivity of the layered LiCoO2 at an early stage of Li-ion de-intercalation was caused by the insulator-metal transition [43,44]. Hence, the increased current at grain interior indicates a metallic character of charged thin film. This agrees well with the study of layered Li1-xNi0.8Co0.15Al0.05O2 that the electronic conductivity increased with decreasing Licontent over the range of x ¼ 0.0 to 0.6 using direct current polarization technique as well as the electrochemical impedance spectroscopy [45]. As shown in Fig. 4(e) and (f), after discharging, the conductivity contrast between the grain interiors and boundaries becomes more distinct compared with that of the charged film, but still less than that of the as-deposited one, which indicates the changes of the conductivity due to the surface modification after the charge/discharge cycle. In addition, Fig. 4(f) shows that the current level in the discharged thin film is lower than that of both charged and as-deposited samples. This indicates the noticeable decline of the electronic conductivity after cycling. This result is consistent with the results by Qui et al. and Shibuya et al. on layered LiNi1/3Co1/3Mn1/3O2 and LiCoO2 cathodes, in which the electronic conductivity increased during the charge process and decreased during the discharge cycles [46e48]. The loss of the electronic conductivity suggests the morphological and structural changes of the cathode, and this corresponds well with the topographic image. The electronic conductivity loss after cycling was also observed in other layered cathode materials, such as LiNi1/3Co1/3Mn1/3O2, LiNi0.8Co0.2O2, and LiNi0.8Co0.15Al0.05O2 [5,26,28]. The surface conductivity affects the rate at which Li-ions diffuse out of cathode into anode during the charging process, thus an ionic resistance would increase if the surface conductivity was reduced [49]. This conductivity degradation can further lead to impedance increase, capacity fading, and power decay of the battery, and ultimately reduce the cycle life of the battery. Thus, the conductivity changes
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Fig. 4. The current maps of thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode with 3 V DC bias over the scanning area of 1 1 mm2 taken at different stages of galvonostatic charge/ discharge processes: (a) as-deposited; (c) charged; and (e) discharged. (b), (d) and (f) are the line section profiles in current images, corresponding to the lines in (a), (c) and (e). The current images above are generated by a matrix of 256 256 data points.
should be taken into account while characterizing the performance of the electrode materials. 3. Conclusion In summary, this paper reports the changes of the nanoscale structure and properties in thin film Li1.2Co0.13Ni0.13Mn0.54O2 cathode during the first charge/discharge cycle, characterized by the combination of various SPM-based techniques and macroscopic electrochemical charge/discharge measurements. These changes include surface topography, electrochemical strain, electrical conductivity and stiffness/elastic modulus. First, the changes of the surface topography in the thin film cathode are probed with nanometer resolution, and a clear correlation between the charging/discharging processes and local morphology is established. The cathode film contracts with Li-ion extraction and expands with Li-ion insertion. After the first cycle, the changes on the surface morphology are obvious, with an increase in surface roughness. Secondly, local variations of the electrochemical strain response in the thin film cathode after the charge/discharge processes have been investigated by BE-ESM technique. The results have shown that the electrochemical strain response decreases
significantly after the charging, with certain amount of recovery after discharging. The results allow us to establish a direct relationship between the changes in electrochemical strain responses and the Li-ion concentration as well as the electrochemical activity at the nanoscale. Thirdly, the stiffness/elastic modulus has been mapped by AM-FM technique. It is found that the stiffness/elastic modulus of the cycled thin film is lower than that of the asdeposited film. In addition, conductance mapping has been examined upon first cycle by using c-AFM, which reveals that the electrical conductivity of the thin film cathode reduces after the first cycle. All these results can be correlated well with the macroscopic capacity decay during the first cycle in Li-rich cathode that is observed at macroscopic battery level. These results also provide the new perspective for understanding the battery aging from the combination of the materials properties at the micro to nanoscales. This study also demonstrates that combining various SPM techniques is very effective to study the morphology, electrochemical strain (deformation), electrical and mechanical properties of the cathode materials, particularly their dependence on Li-ion interaction and extraction at the nanoscale for any cathode materials. Thus, these factors need to be considered carefully in the design and development of new battery materials.
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As SPM technique can sensitively probe the surface properties of the LIB materials, it can be used to study the damage mechanisms of the cathode materials. Thus, SPM study of a LIB electrode which is extracted from batteries and aged to different degrees of life would provide important information about the onset of damage, further to determine the perdurability and reliability of the electrode. 4. Experimental section 4.1. Sample preparation The target material Li1.2Mn0.54Ni0.13Co0.13O2 was first made by using raw materials Li2CO3 (Sigma-Aldrich, 99.9%), MnO2 (SigmaAldrich, 99.9%), Co3O4 (Sigma-Aldrich, 99.9%), and NiO (SigmaAldrich, 99.9%) as the precursors. The molar ratio of Ni:Co:Mn was set to be 0.13:0.13:0.54 with 10% excess Li source to compensate the Li loss at high sintering temperature. The starting precursors were wet ball-milled for 2 h in a steel container. After drying in air at 80 C for 12 h, the resultant mixture was cold pressed into a pellet with the dimension of 26 mm in diameter and 4 mm in height. The cold compact was then sintered at 900 C for 24 h to obtain a solid target for the subsequent film deposition by pulsed laser deposition (PLD). During the PLD process, a KrF excimer laser beam (248 nm, 180 mJ) (Lambda Physik, USA) was used at a repetition frequency of 10 Hz. Thin films were deposited on Au substrates at 650 C with an oxygen partial pressure of 350 mTorr. The target-substrate distance was kept at 20 mm during the film deposition. The as-deposited thin films were post-annealed at 800 C with oxygen flow for 40 min.
conducted at room temperature (~25 C) under the ambient air condition (with average humidity of ~60%). During the SPM experiments, a conductive cantilever with silicon tip coated with Ti/Pt layer (AC240TM, Olympus, Japan) was used. The cantilever has an average spring constant of 2 N/m and tip radius of 28 nm. The same tip is used for each measurement. 4.4.2. Electrochemical strain measurement by BE-ESM BE-ESM mapping was performed with 3Vac bias signal and a bandwidth of 20 kHz with the BE control software (Asylum Research/Oak Ridge National Laboratory, USA) associated with the SPM system. The BE-ESM images are formed by fitting the response curves at each point using the simple harmonic oscillator (SHO) method after the measurements. 4.4.3. Stiffness measurement by AM-FM AM-FM measurement was conducted with the special cantilever holder (Asylum Research, CA, USA) attached to the SPM system. The drive voltage is set at 2 V for the first eigenmode (AM), and 10 mV for the third eigenmode (FM). This small amplitude of FM mode prevents the perturbation to the first eigenmode cantilever dynamics during the measurements. 4.4.4. Current measurement by c-AFM Current measurement was performed by using a commercial conductive module (ORCA™, Asylum Research, CA, USA) with the special cantilever holder and sample stage attached to the SPM system. In the c-AFM measurements, bias was applied via the substrate, and the cantilever tip was grounded. The thin film was in contact with the tip firmly by using about 90 nN loading force.
4.2. Material characterization
Acknowledgment
The crystal structure of the thin film and the substrate were characterized using XRD equipment (Model XRD-7000, Shimadzu Corporation, Kyoto, Japan) with Cu Ka radiation (l ¼ 1.5418 Å). Surface morphology of the thin film was characterized using a field emission scanning electron microscopy (FESEM, Hitachi S-4100, Japan).
This work is supported by Ministry of Education (Singapore) through National University of Singapore under Academic Research Fund (R-265-000-406-112). One of the author (YS) would also like to thank the support of scholarship by Ministry of Education (Singapore) under the Academic Research Fund (R-265-100-406112) and National University of Singapore.
4.3. Electrochemical measurement Half-battery cells were assembled in an argon-filled glove box using the deposited Li-rich thin film as the cathode, metal Li foil as the anode, two pieces of Celgard 2500 as separators and a few drops of electrolyte (1 M LiPF6 in EC:DEC ¼ 1:1 organic solutions). Swageloks were used for half-battery assembling. The gavolnostatic charge/discharge tests were conducted at a constant current density of 2 mA/cm2 between 2.0 V and 4.8 V at room temperature using Battery Test Station (Neware, China). To investigate the nanostructure and properties changes due to charge/discharge processes, one battery cell was first fully charged for one time followed by dissembling, and the second battery cell was fully charged and then fully discharged for one time followed by dissembling. The dissembled cathode films were rinsed by acetone for various SPM characterizations. In such way, there are three separate samples used in this study: i.e., one as-deposited, one charged, and one discharged thin film samples. 4.4. SPM characterizations 4.4.1. General SPM setup A commercial Scanning Probe Microscopy system (MFP-3D, Asylum Research, CA, USA) was used as the primary characterization tool for imaging and analysis. All SPM studies in this work were
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