EC-AFM - American Chemical Society

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MATERIALS AND INTERFACES Use of in Situ Electrochemical Atomic Force Microscopy (EC-AFM) to Monitor Cathode Surface Reaction in Organic Electrolyte Ruxandra Vidu, Forest T. Quinlan, and Pieter Stroeve* Department of Chemical Engineering and Materials Science, University of California-Davis, Davis, California 95616

Surface reactions are responsible for the cycling performance of Li-ion rechargeable batteries based on LiMn2O4. We report structural and electrochemical studies of the LiMn2O4 cathode at room temperature in LiPF6 electrolyte using in situ electrochemical atomic force microscopy (EC-AFM) and lateral force microscopy (LFM). Surface dynamics were monitored in situ in organic electrolyte under potentiostatic conditions. After the first charge/discharge cycle, surface features were clearly visible. During the second charging process at 4.3 V vs Li/Li+, dissolution of surface particles was observed. The surface topography was quantitatively analyzed by height distribution functions. The dissolution rate (calculated from the change of particle volume with time) was found to follow the square root of time. During the second discharge process at 3.8 V vs Li/Li+, new particles were formed on the surface. However, there were no further changes in the surface topography with time during polarization at 3.8 V vs Li/Li+. The surface dynamics monitored for various charge/discharge conditions showed that a complex dissolution/precipitation reaction of manganese and lithium compounds is involved in the charge/discharge process of spinel LiMn2O4-based cathode material. Introduction The development of surface-structure-sensitive techniques in recent years has led to in situ electrochemical atomic force microscopy (EC-AFM), which has made possible the performance of detailed studies of the surface chemistry, morphology, 3D structure, electrochemical response, and performance of battery electrodes.1,2 Important aspects revealed by EC-AFM studies on Li-ion battery electrodes are especially useful in explaining surface modifications in relation to the other surface-sensitive techniques. The relevance of using atomic force microscopy (AFM) and EC-AFM in surface-reaction studies has been demonstrated by several studies focused on lithium or graphite anode materials. Such investigations have been performed primarily because the performance of Li metal and Li-carbon anodes in secondary batteries depends on the nature of the surface films that cover them. Morigaki et al.3,4 studied the surface of lithium in a dry-air atmosphere and in 1 M LiClO4/propylene carbonate (PC) by in situ AFM. High-resolution AFM disclosed the nanostructure of the lithium surface and the morphological change due to lithium electrodeposition that occurred mainly by bulk diffusion and not by surface diffusion. Dynamic morphological changes on the graphite surface during the Li intercalation/deintercalation process were also studied by AFM.5,6 It was * To whom correspondence should be addressed: Pieter Stroeve, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616. E-mail: [email protected]. Telephone/Fax: (530) 7528778.

found that the formation of a solid electrolyte interface (SEI) film starts at about 2 V vs Li/Li+. Formation of a stable film with an islandlike morphology was observed at 0.9 V vs Li/Li+.5 A different reduction mechanism of the solvent was observed in ethylene carbonate (EC) + diethyl carbonate (DEC) and in EC + ethyl methyl carbonate (EMC), each containing LiPF6 salt.7,8 AFM studies on graphite anodes confirmed that the morphological changes of graphite particles occur at 1.0 V vs Li/Li+ in the first cathodic polarization observation. A larger expansion of graphite particles was observed in EC + DEC than in EC + EMC. This expansion of graphite is related to the decomposition of ternary solvated lithium-graphite intercalation compounds. Solid electrolyte interface (SEI) film formation on graphite electrodes was also studied on highly oriented pyrolytic graphite (HOPG) in nonaqueous electrolyte by in situ AFM.9 For potentials negative to 0.7 V vs Li/ Li+, an SEI film was formed on the HOPG electrode surface. After the first cycle, the film was rough and covered the surface of the HOPG electrode only partially, but after the second cycle, the HOPG surface was fully covered by a compact film. The thickness of the SEI film was measured by increasing the pressure of the AFM tip, thus scraping a part of the electrode surface. A thickness of about 25 nm was found for the SEI film formed after two scan cycles between 3 and 0.01 V vs Li/Li+. Stabilization and capacity fading mechanisms of graphite electrodes represent an important problem that has received special attention from Aurbach et al.10-12 They reviewed the results obtained in various electrolyte solutions (e.g., ethylene carbonate solutions, propylene

10.1021/ie020519z CCC: $22.00 © 2002 American Chemical Society Published on Web 11/12/2002

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carbonate solutions, and ether-based systems) and recent works on Li metal and Li-graphite anodes and on LixMOy cathodes (where M represents a transition metal such as Ni, Co, or Mn), with an emphasis on the importance of in situ AFM studies of surface phenomena along with other in situ and ex situ surface-sensitive techniques such as IR spectroscopy, impedance spectroscopy, and electrochemical quartz crystal microgravimetry. In a Li-ion battery, the behaviors of both the anode and cathode are important in ensuring the stability and reversibility of these batteries. However, unlike the anode, the cathode of Li-ion rechargeable batteries is not well understood in terms of surface chemistry and morphology. When scanning probe techniques such as AFM and scanning tunneling microscopy (STM) were first applied in this field, they were only used in air to characterize the surface quality of new cathode materials obtained by different methods (i.e., sol-gel, spin coating, pulse laser deposition).13-16 For example, AFM, along with IR spectroscopy, cycling voltammetry (CV), and in situ ultraviolet and visible spectroscopy, was used to study the influence of added cerium precursors on the electrochemical, optical, and structural properties of vanadium oxide and new mixed vanadium/cerium oxide thin films as intercalation compounds for lithium ions (at 55 and 38 at. % of V).15 In addition, studies of the morphology and roughness of the cathode surface before and after electrolyte exposure have been performed by scanning electron microscopy and AFM.16,17 Ex situ current-sensing atomic force microscopy (CSAFM) was also used to characterize interfacial processes and film formation on a thin-film spinel LiMn2O418 and Li2Mn4O913 electrode (prepared by spin coating onto a Pt substrate) after exposure to 1.0 M LiPF6, EC/DMC ) 1:1 electrolyte. Although the scanning probe microscopy (SPM) studies were performed in air and not in electrolyte, the results have suggested that a thin passive surface layer is formed on the surface. There are controversial opinions about the possibility that surface films are formed on the cathode and affect their behavior. However, recent work12,17,19-25 seems to indicate that there is a surface film that affects the electrochemical behavior of the cathodes made of LiMn2O4 as active material. Interesting results have been reported by Inaba et al.,26 who conducted electrochemical STM observations on LiMn2O4 thin films. Although the study was performed on LiMn2O4 thin films on Pt or Au as sample electrodes and not on actual cathodes (i.e., made of active material, black carbon, and Teflon), the results showed that the cathode surface changed after repeated cycling. The LiMn2O4 thin film in discharged state was covered by small particles that increased in number and decreased in size with repeated cycling.26 Among the surface techniques applied for electrode investigations in organic electrolytes, electrochemical scanning probe microscopes are the only instruments that can be used for in situ monitoring of the surface reactions, if any, involved in capacity fading. In our previous work, we have used several scanning modes for imaging the cathode material surface, such as contact AFM, tapping-mode AFM (TM-AFM), and lateral force microscopy (LFM), in addition to other surfacesensitive techniques [e.g., energy-dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS)].17 We used these imaging techniques to provide

Figure 1. Schematic of the electrochemical atomic force microscopy setup.

a basis for composite electrode imaging, which is important in determining the imaging mode that maximizes the information on surface topography and composition. Results from EDAX have shown that compounds containing phosphorus and fluorine are deposited on the surface of the cathode, and the presence of MnF2 and MnO were identified by XPS. In this work, we studied, in situ, the surface of the spinel cathode material in both charge and discharge states with ECAFM. We also investigated the relationship between the micromorphological changes and the electrochemical characteristics. Experimental Section The cathode material was prepared by mixing 75 wt % of LiMn2O4 (synthesized by Mitsubishi Chemical Corp.), 20 wt % of acetylene black (Denki Kagaku Kogyo Co., Ltd.), and 5 wt % of Teflon (Mitsui Dupont Fluoro Chemical Co., Ltd.). The LiMn2O4 powder was analyzed by X-ray diffraction to confirm the structure as being spinel LiMn2O4 (data not shown). These components were ground and pressed into a flat sheet with a mortar and pestle. An 8-mm-diameter pellet was punched from the sheet and pressed onto an aluminum mesh. This was used as the working electrode in the electrochemical measurements. Before assembling the electrochemical cell, the working electrode was dried in a vacuum oven at 120 °C for 2 h. Two similar electrochemical cells were used. For the electrochemical studies (cyclic voltammetry and charge/ discharge experiments), a beaker-type, three-electrode cell kept inside an argon-atmosphere glovebox was used. The working electrode was placed in the cell with Li metal (Alfa Aesar) used as the counter and reference electrodes. The electrolyte used for electrochemical measurements consisted of 1 M LiPF6 in an organic solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (2:2:1 by volume; Mitsubishi Chemical Corp.). Cyclic voltammetry (CV) was carried out using a bipotentiostat (Pine Instrument Company) with a computerized interface. Figure 1 shows the electrochemical cell used in the EC-AFM experiments. It is a modified version of the beaker-type, three-electrode cell used in typical electrochemical measurements. The cathode was mounted with double-side conductive tape onto the sample holder and placed under a quartz electrochemical AFM cell. The rest of assembly was performed inside an argon atmo-

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measurements were performed using a multimode AFM (NanoScope IIIA, Digital Instruments). Commercially available triangular gold-coated cantilevers with pyramidal tips of 0.05 N/m force constant were used. To correct for tilt and bow, images were plane processed using the image-processing software of the instrument.

Figure 2. Cyclic voltammogram of LiMn2O4-based cathode material cycled between 3.3 and 4.4 V at 0.1 mV/s in 1 M LiPF6/EC + DMC + DEC.

sphere glovebox. All preparation and measurements were conducted at room temperature (25 °C). The in situ EC-AFM imaging of the cathode surface was performed under potentiostatic conditions for both contact and lateral-force mode. In these experiments, the electrochemical cell was connected to a potentiostat, and the system was allowed to equilibrate until a constant opencircuit voltage (OCV) of 3.5 V vs Li/Li+ was reached (approximately 30 min). With EC-AFM, one has the ability to monitor changes of the cathode surface as the potential changes. The LFM and in situ EC-AFM

Results and Discussions Cyclic Voltammetry. The electrochemical behavior of the cathode was checked by cyclic voltammetry, with the potential being cycled between 3.3 and 4.4 V. A typical cyclic voltammogram for LiMn2O4/EC + DMC + DEC-LiPF6/Li at 10 mV/min is shown in Figure 2. The peaks in the cyclic voltammogram are at 4.09 and 4.22 V for charge and at 3.88 and 4.02 V for discharge. As reported in the literature, the two peaks correspond to the two-step reversible (de)intercalation of lithium between LiMn2O4 and R-MnO2.27-30 Atomic Force Microscopy. Figure 3 shows typical scans of the cathode material, both in air and in electrolyte. The left-hand side of each scan is the either a height image (constant force) or a deflection image (variable force), and the right-hand side is the force image. Figure 3a shows a clear distinction between the active material and the binding/conducting material. In the height image, the lighter areas are the active LiMn2O4 material, and the darker areas are carbon black and Teflon. This conclusion is derived from the corresponding force image, where the lighter areas

Figure 3. LFM image of cathode surface in (a) air and (b) LiPF6 electrolyte at room temperature. In the friction-force image of the cathode surface in air, contrasting binder and active material areas are easily identified (the LiMn2O4 crystallites produce large contrast in the phase imaging relative to the remaining binder and carbon black phase). The LFM image in organic electrolyte presents the image of the cathode surface at the open-circuit voltage, OCV ) 3.54 V vs Li/Li+.


Figure 4. In situ EC-AFM image in height (left) and deflection (right) modes of surface particle dissolution in LiPF6 electrolyte at 4.3 V vs Li/Li+ for t ) (a) 0, (b) 3, and (c) 13 min. The P in part a labels the particle on which the observation and measurements were performed, as the first image (at larger scan size) shows the presence of other particles and particle agglomeration.

(softer material) are Teflon and carbon black, whereas the darker areas (harder material) are LiMn2O4. Figure 3b shows the LFM images of the cathode surface at the open-circuit voltage after equilibrium was reached. We expected to see a similar composite surface with the electrolyte as we had with air. Instead, we observed that the surface was covered with a homogeneous film. The contrast seen in Figure 3b, which could perhaps be interpreted as compositional variation, is more likely due to surface roughness. Because this

contrast is not very distinct, we attribute the features in the force-mode image to morphological changes in the sample. In our previous work,17 lateral-force-mode imaging of the cathode material in air, after storage at 70 °C, did not show a distinct contrast between phases. This observation, along with the LFM results shown here, suggests the spontaneous formation of a film on the surface when the cathode is brought into contact with the electrolyte. On the other hand, Figure 3b shows that the surface topography of the cathode consists of

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Figure 5. Time dependence of the height distribution function for the particle dissolution process shown in Figure 4.

two regions: granular areas surrounded by columns, and smoother columnar regions. These features have not previously been observed on the cathode surface during AFM or LFM observations in air, suggesting that the cathode surface relaxes when the electrolyte penetrates, thus increasing the roughness. Electrochemical Atomic Force Microscopy. In situ EC-AFM imaging of the cathode surface under controlled potential was performed by sweeping the potential between 3.3 and 4.4 V, i.e., following the electrochemical behavior as seen by cyclic voltammogram. The scanning area was restricted to the smooth regions of the electrode (e.g., regions that were similar to the bottom-right areas in Figure 3). This was done to isolate any potential particle formation and to allow for clear imaging of particle-formation behavior without other sample morphologies interfering in the scan. The cathode was charged and discharged twice at a sweep rate of 10 mV/min. During the first discharge, particles were formed on the cathode surface (not shown). The dissolution of these particles was monitored during continuous scanning at a constant potential of 4.3 V vs Li/Li+. Figure 4 shows an example of the dissolution of a particle (labeled P in Figure 4a) as observed by AFM in height (constant-force) and deflection (variable-force) modes. In the height-mode AFM images, the bright areas correspond to higher topographic features of the surface. The particles present morphologies similar to those of crystalline products. Figure 4b,c presents images of the particle at two time intervals. The particle gradually decreases and finally disappears after about 23 min. The qualitative aspects of the topographic surface changes presented in Figure 4 were quantitatively characterized by a height distribution function, n(H), which is the frequency histograms of the measured heights, for each scan of the AFM image. Data were taken at various time intervals. We used the height data of each point taken during scanning to create plots for the time intervals. Figure 5 shows the time dependence of the height distribution function for the dissolving particle. At time zero, the particle is fairly uniform in its height distribution. Obviously, height distribution functions derived for successive time intervals contain a smaller number of points at peak heights, as the surface area of the particle was significantly reduced during dissolution in comparison with its initial size. However, because the height distribution function quantifies the particle morphology changes presented in the sequence of EC-AFM images (Figure 4), its shape provides additional details about these changes during dissolution. For instance, as time increases, the singular Gaussian peak splits into multiple peaks that are more loosely distributed, indicating nonuniform dissolution. The width of each peak might also be caused by a combination of corrugations on the substrate surface

Figure 6. Dissolution rate as the change in particle volume with time. The solid line is a fit to the experimental data, and the dotted line is a fit to the experimental data using the solubility-limited dissolution model (film theory).

itself and tilting of the surface with respect to the x-y plane as defined by the AFM. One can analyze the changes in the peaks at different dissolution times in terms of particle geometry, and a useful parameter is the full width at half-maximum (fwhm). During dissolution, the height distribution function shows broad peaks with a more complicated curvature at the tip, and the fwhm value increases from 12.8 ( 0.05 at 0 min to 20.5 ( 0.05 nm at 30 min. The dissolution process dictates the shape of the peaks, i.e., dissolution leads to a rough and tilted top surface, and the rate of dissolution depends on the orientation of the atomic plane. The dissolution rate was calculated from the change in particle volume with time and is presented in Figure 6. The dissolution rate presents a linear variation with the square root of time. The process of dissolution with escape of solute atoms/molecules from a solid surface into the bulk solvent has been described by various models.31 The dissolution of particles can be controlled either by mass transfer or by reaction at the surface of particles. In the case of solubility-limited dissolution models (film theory),32 the mass transfer of dissolved solids away from the solid-solution interface controls the rate of solid dissolution. If the mass transfer through the film controls the process, the time evolution of the average radius can be expressed as32

a2 ) ao2 - 2Dt/F where a is the particle radius, ao is the initial particle radius, D is the diffusion coefficient, t is the time, and F is the density of the particle. In this case, the time dependence of the volume of the particle gives the linear dependence V ∼ t3/2. For comparison, Figure 6 shows a fit to the experimental data using the solubility-limited dissolution model. The dissolution process can be fitted better with a linear function of V ∼ t1/2 than of V ∼ t3/2, showing that the particle dissolution rate is different from that predicted by the film model. Therefore, the results suggest that the dissolution rate is controlled kinetically by surface reactions. During the second discharge process, at 3.8 V vs Li/ Li+, new particles were formed on the surface. Figure 7 presents the process of particle formation. However, after 3 min of imaging of the cathode surface topography at 3.8 V, no additional changes were observed in the surface topography for another 20 min. The newly


Figure 7. In situ EC-AFM image in height (left) and deflection (right) modes of the cathode surface in LiPF6 electrolyte at E ) 3.8 V vs Li/Li+ after (a) 0, (b) 3, (c) 6, (d) 9, (e) 15, and (f) 20 min.

formed particles stopped growing, suggesting a concentration-limited reaction. Reactions Associated with Film Formation. The electrochemical behavior of the cathode in alkyl carbonate mixtures containing LiPF6 as a salt (and H2O and HF as species unavoidably present in solution33-35) is determined by a complex mechanism in which reactions that occur at, or on both sides of, the cathode/electrolyte interface can interfere with and influence each other. Atomic force microscopy, as well as other scanning probe techniques, can follow the surface dynamics of an electrode under various electrochemical conditions but cannot identify the surface-reaction products; additional techniques are necessary to understand the surface chemistry of the cathode. Therefore, the main reactions that determine or indirectly affect the electrochemical behavior of the cathode surface in contact with the electrolyte under potential control are briefly discussed below.

(a) Lithium Intercalation/Deintercalation Process. Particle formation and dissolution can be associated with the intercalation/deintercalation process, which is the main electrode reaction during electrochemical cycling within the potential range from 3.2 to 4.3 V vs Li. The lithium intercalation and extraction process can be characterized by the value of open-circuit voltage, i.e., the variation of the open-circuit voltage with composition. Before a charge/discharge cycle was run, the open-circuit voltage was 3.52 V, and the cycle was started from there. After the first charge/discharge cycle, the open-circuit voltage relaxed from the stopping point of 3.52 to 3.38 V, suggesting an incomplete Li intercalation process.36 In this context, it is widely recognized that manganese dissolution might increase the resistance for the Li intercalation/deintercalation process and causes capacity losses by both material loss in the loaded spinel and polarization loss due to the increase in resistance.36-39 Also, Mn dissolution is

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notably high in the charged state (at E > 4.1 V vs Li/ Li+), at which potential the electrochemical oxidation of the solvent molecules is also significant.36 Studies on the acid delithiation of LiMn2O4 have shown a reduction in lattice parameters but no change in space group.40-43 The Li+-extracted materials maintain the cubic symmetry of the starting material LiMn2O4, even though a contraction of the framework occurs during delithiation. The mechanism is proposed to occur by solid-state diffusion and involves the acid-assisted dissolution of lithium oxide and disproportionation of Mn3+ in sulfuric acid during delithiation.40,43 This reaction results in Mn2+ ions that dissolve into the acid solution and Mn4+ ions that remain in the spinel framework. Nonuniform dissolution of the Mn2+ ions from the surfaces of grains/ crystals has also been observed by transmission electron microscopy (TEM).44,45 Possible lithium ordering or manganese displacement could contribute to the modified electrochemical behavior of the acid-delithiated material in comparison with the standard LiMn2O4 phase. The differences observed in the initial electrochemical behavior of chemically and electrochemically delithiated LiMn2O446 suggest that the delithiation processes have different impacts on the spinel structure. Electrochemical extraction of Li+ ions from the LiMn2O4 structure leaves the manganese lattice undisturbed, whereas acid delithiation of LiMn2O4, with concomitant loss of manganese from the lattice sites, alters the microstructure of spinel grains. On the other hand, the conductivity of a mixed-valence compound such as Li1-xMn2O4 is governed by the amount of carrier (electron of Mn3+) and the hopping length (Mn-Mn interaction length). The amount of electron carrier decreases with the amount of coupled delithiation of the oxidation of Mn3+ to Mn4+, and the Mn-Mn distance in the spinel structure is contracted by delithiation. Measurements of the conductivity of Li1-xMn2O4 thin films47 have shown that the effect shortening the diffusion length is more prominent than the decrease in the amount of electron carrier. Moreover, a significant increase in conductivity was observed for the reaction at 4.15 V,47 where the lithium insertion/extraction process is a twophase process.28 Because of the coexistence of two cubic phases (ac ) 0.804 and 0.8142 nm) without any intermediate lattice constant, Li0.5Mn2O4 changes partially to λ-MnO2 according to the degree of lithium extraction during the course of delithiation. The λ-MnO2 phase, which is the denser phase in the system during delithiation, acts as a conduction path for the electron carriers originating in Li0.5Mn2O4. On the other hand, we found in our previous work that the charged state, which is mostly delithiated, has the manganese as MnO2.17 This result agrees with the Mn dissolution process that has been proposed in the literature, where Mn dissolution proceeds through a disproportionation reaction

2Mn3+ f Mn2+ + Mn4+ However, spinel oxides can be dissolved in two different ways: dissolution of Mn from the oxide lattice and dissolution of the spinel framework (both Mn and oxide ions),48 but evidence is not yet available on the dissolution mechanism. Because the change in open-circuit voltage shows that there is less lithium intercalated back into the spinel structure, the excess lithium available to the electrolyte can react and deposit on the electrode surfaces as reaction products.

(b) Electrolyte. Reactions in electrolyte solutions based on mixtures of cyclic and aliphatic alkyl carbonates containing LiPF6 as the electrolyte salt also occur that affect the film-formation process on both the anode and cathode surfaces, as well as the lithium intercalation/deintercalation kinetics and mechanism. For instance, electrolyte studies have provided evidence for ester exchange reaction among all of the carbonate esters49

DMC + DEC T 2EMC EC + DMC T DMDOHC EC + DEC T DEDOHC EC + EMC T EMDOHC where DMDOHC, DEDOHC, and EMDOHC are the mixed carbonate esters dimethyl-2,5-dioxahexane carboxylate [CH3(OOC-O-CH2-CH2-O-COO)CH3], diethyl-2,5-dioxahexane carboxylate [C2H5(OOC-O-CH2CH2-O-COO)C2H5], and ethylmethyl-2,5-dioxahexane carboxylate [C2H5(OOC-O-CH2-CH2-O-COO)CH3], respectively. These reactions, along with other surface reactions, can change the electrolyte resistance by changing the electrolyte composition. (c) Surface Reactions. The study of cathode surface chemistry must take into consideration the activity of the electrolyte at high oxidizing potentials, where reactions of the electrolyte for both direct oxidation and corrosion of the collector material for the positive electrode can occur. Generally, three problems are associated with the cathode in lithium and lithium-ion batteries:50-52 (i) reaction of electrolyte with the cathode at positive potentials, a reaction that depends on both solvents and solutes in the electrolyte; (ii) corrosion of the substrate metal foil that is used as the carrier for the active material; and (iii) changes in the composition of the solution resulting from surface reactions and reactions among the constituents of the electrolyte. The first type of cathodic instability at the cathode/electrolyte interface includes the anodic oxidation of the electrolyte. In this respect, cyclic and linear ethers are rather easily oxidized, but linear esters are stable over 4.5 V vs Li. The second type of instability is assumed not to be significant in our experiments because they were performed in a relative short time and the corrosion currents were low for the aluminum collector in LiPF6/EC/DEC.51 The third type of instability, i.e., changes in the composition of the solution due to surface reactions and reactions among the constituents of the electrolyte, is reflected in the increased resistance of the electrolyte in lithium-ion batteries upon storage at opencircuit conditions. The reactions change the composition of the solution through the consumption of solution components or dissolution of solution reaction products. Aurbach et al.50 reviewed the major reactions of Li electrodes in some alkyl carbonates commonly used salts and contaminants. However, the initial composition of the solution also changes through the reaction of lithium alkyl carbonate salts and contaminants such as HF and H2O. For instance, lithium alkyl carbonate can react with water according to

2ROCO2Li + H2O f Li2CO3 + CO + 2ROH These observations demonstrate the importance of examining all of the variables in the cell. The surface dynamics observed by in situ EC-AFM in this work


suggest that a complex dissolution/precipitation reaction of surface compounds is involved in the charge/discharge process of LiMn2O4-based cathodes, in agreement with the STM results on spinel LiMn2O4 thin films.26 Candidates for the surface products are manganese compounds and lithium compounds (e.g., Li2CO3 formed by a surface reaction similar to that observed on the Li anode surface in solutions containing alkyl carbonates and EC12,53,54). Compounds containing phosphorus and fluorine might be deposited on the surface of the cathode,17 as well as yet-to-be-determined compounds. The observed surface reactions might also be responsible for the dissolution of the Mn ions reported for this system, because capacity fading is influenced by many processes, and each is expected to manifest itself during operation of a battery. Conclusions Electrochemical atomic force microscopy in contact and lateral-force modes was used for the in situ study of surface reactions that occur on the LiMn2O4 cathode surface. Using in situ EC-AFM, we have studied the dynamic morphological changes on the cathode surface in LiPF6/EC + DMC + DEC solution under controlled potentials. At the open-circuit voltage, the surface topological changes suggested that the cathode material reacts as soon as it comes into contact with the electrolyte. This behavior requires detailed investigation to establish the nature of such structural surface modifications. After the first charge/discharge cycle, the surface dynamics of the cathode were monitored in situ in the organic electrolyte under potentiostatic conditions. During the second charging process at 4.3 V vs Li/Li+, the dissolution of surface particles was observed and analyzed quantitatively by height distribution functions. The time dependence of the dissolution rate (calculated from the change in particle volume with time) follows the relation V ∼ t1/2. Because the dissolution follows V ∼ t1/2, the particle dissolution rate is less than that expected from the film dissolution model (V ∼ t3/2). Therefore, the dissolution rate is kinetically controlled and is affected by surface reactions that slow the particle dissolution. At 3.8 V vs Li/Li+, new particles are quickly formed on the surface and stop growing after only a short time. Surface dynamics monitored for various charge/discharge conditions show that a complex dissolution/precipitation reaction of lithium and manganese compounds (possibly containing phosphorus and fluorine) is involved in the charge/discharge process of spinel LiMn2O4-based cathode materials. These results demonstrate the wealth of information brought by EC-AFM to the investigation of cathode materials in Li-ion batteries, in addition to the other characterization techniques. It offers a direct way to monitor the cathode surface reaction in organic electrolyte, which is a great advantage over other microscopes. This is especially important for Li-ion battery research, where the electrodes are highly sensitive to air and moisture. Acknowledgment The authors thank Dr. Y. Tsurita, Dr. K. Tasaki, Mr. K. Sano, and Dr. K. Okahara of Mitsubishi Chemical Company for their valuable discussions. This work was supported in part by Mitsubishi Chemical Company and

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Received for review July 15, 2002 Revised manuscript received September 30, 2002 Accepted September 30, 2002 IE020519Z

EC-AFM - American Chemical Society

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