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Mapping Irreversible Electrochemical Processes on the Nanoscale: Ionic Phenomena in Li Ion Conductive Glass Ceramics Thomas M. Arruda, Amit Kumar, Sergei V. Kalinin,* and Stephen Jesse* The Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37922, United States ABSTRACT: A scanning probe microscopy approach for mapping local irreversible electrochemical processes based on detection of biasinduced frequency shifts of cantilevers in contact with the electrochemically active surface is demonstrated. Using Li ion conductive glass ceramic as a model, we demonstrate near unity transference numbers for ionic transport and establish detection limits for currentbased and strain-based detection. The tip-induced electrochemical process is shown to be a first-order transformation and nucleation potential is close to the Limetal reduction potential. Spatial variability of the nucleation bias is explored and linked to the local phase composition. These studies both provide insight into nanoscale ionic phenomena in practical Li-ion electrolyte and also open pathways for probing irreversible electrochemical, bias-induced, and thermal transformations in nanoscale systems. KEYWORDS: Electrochemical strain microscopy, lithium ion conductor, batteries, lithium ion conducting glass ceramic, band excitation, irreversible
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lectrochemical phenomena in solids including interfacial reactions and ionic transport underpin a broad range of energy technologies ranging from metalion1 and metalair2,3 batteries to fuel cells.4,5 Functionality of these systems is often enabled or controlled by the phenomena at solidgas and solidsolid interfaces, with examples ranging from electrocatalysis at triple-phase junctions5 to grain-boundary and interfacemediated phenomena that control ionic transport and local polarization.6 Recently, it was shown that nanoscale confinement can strongly affect ionic transport at interfaces,7 giving rise to phenomena such as ionic superconductors.8 Beyond energy-related applications, much attention has recently been attracted to logic and memory devices enabled by ionic phenomena, ranging from atomic switches9 to memristive10 and electroresistive11 memories. Finally, open is the issue of ionic effects in classical physical studies of systems such as correlated oxides, especially in the presence of large electric fields12 or exposed to ambient environment.13,14 Deciphering the interplay of ionic and electronic transport in ionic devices as well as understanding nanoscale mechanisms underpinning functionality of energy systems necessitates probing electrochemical processes in solids on the nanometer scale level of single structural elements such as grains, grain boundaries, or extended defects. The approach based on local microelectrodes pioneered by Fleig et al.15 and now used extensively in the electrochemical community16 allows extending classical electrochemical methods such as impedance spectroscopy to the ∼10 μm level. However, strong dependence of ionic impedances on contact radius precludes similar studies on the nanometer scale of individual structural defects or microstructural r 2011 American Chemical Society
elements. Furthermore, the fabricated electrode pattern is fixed, excluding spatially resolved studies. At the same time, the use of current and force-based scanning probe microscopy (SPM) techniques such as conductive atomic force microscopy (AFM),1722 nanoscale impedance microscopy,2325 or Kelvin probe force microscopy26 provide only indirect information on electrochemical functionality.27 Recently, we have introduced an SPM technique, electrochemical strain microscopy (ESM), based on the detection of electrochemical strains generated in solids in response to local bias applied to the tip.2830 Simple estimates suggest that the probing volume in ESM is 68 orders of magnitude below those accessible by classical current-based electrochemical methods.31,32 ESM was demonstrated for a variety of Li ion systems including layered oxides cathodes and Si anodes, as well as several oxide electrolytes (yttria stabilized zirconia)33 and mixed ionic electronic conductors. However, until now ESM measurements were performed only for the cases of fully reversible electrochemical systems, in which electrochemical processes can be cycled multiple times at the tipsurface junction. Measured hysteresis loops or time-dependent relaxation signal then provides information on the kinetics and thermodynamics of local reversible tip-induced electrochemical processes. At the same time, many electrochemical reactions are partially or fully irreversible and lead to the formation of insoluble or inactive products (i.e., Liair batteries). These phenomena are broadly explored in the SPM community in Received: June 17, 2011 Revised: August 12, 2011 Published: August 24, 2011 4161
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Figure 1. (a) IV curve for a single point measurement on the LICGC surface (inset is applied waveform). (b) Linear relationship illustrating the number of transferred electrons (via integration of IV curve) vs the number of Li atoms extracted (from volume of deposited particle). (c) AFM topography map (collected after IV measurement grid) with overlaid grid of maximum cathodic current measured at each fixed location, and (d) size distribution histogram for the particles formed in (c).
the context of tip-induced nanofabrication34 for applications such as nano-oxidation of metals and semiconductors,35,36 or deposition of carbon,37 semiconductor,38 or metals.39 In these cases, the onset and progression of reaction is established based on topographic changes due to formation of visible (in AFM topographic imaging mode, optical microscopy, or microRaman) reaction product, and local studies of reaction process as a function of probe bias, time, and surface location are essentially impossible or extremely time-consuming. Here, we develop an approach for local probing of irreversible electrochemical processes by ESM based on detection of resonant frequency shifts of the cantilever, and exemplify this approach using Li ion electrolytes used in Liair batteries. Further extensions of this method for spatially resolved mapping are discussed. Electrochemical Processes on Li Ion Conductive Glass Ceramics. As a model system, we have chosen commercially available Li ion conductive glass ceramics (LICGC) of the general composition Li2OAl2O3SiO2P2O5TiO2GeO2.40 The electrochemical properties of this material are well understood,41 and in particular this material offers very high Li ion conductivity at room temperature (104103 S cm1)42 and negligibly small electronic conductance.43 In addition, the chemically heterogeneous but topographically smooth nature of LICGC surface allows systematic studies of electrochemical reactivity as controlled by local composition and morphology. Furthermore, LICGC applications are targeted at Liwater and Liair batteries, hence requiring material that is stable in contact with the ambient environment (thus obviating the need for ESM imaging in controlled atmosphere) and rendering results directly relevant for applications. To explore the electrochemical activity on the LICGC surface, we perform direct measurements of the IV curves on the surface using the bipolar triangular voltage sweep (Figure 1a inset) by adopting strategies demonstrated earlier for mapping polarization-transport coupling in ferroelectric materials.44 The current was found to be below the detection limit for positive
voltage polarities, while for negative polarities nonzero currents were detected when sufficiently high (∼58 V) biases were achieved. Invariably, the current flow is associated with the formation of the reaction product and that can be detected as a topographic protrusion on sample surface. In conjunction with known ionic properties of LICGC, the combination of these results indicates that the tip induces extraction of Li ions to the surface (with likely formation of Li metal) followed immediately by precipitation of poorly soluble Li hydroxides and carbonates. To gain insight into the spatial variability of electrochemical reactivity of LICGC surface, the measurements were performed over a spatially resolved grid. Shown in Figure 1a is an example of an IV curve obtained at a single surface location. Such a curve resembles that of a Levich-type reduction sweep (with a lack of diffusion limited current) commonly employed to measure electrokinetic parameters of fuel cell and battery materials.45,46 Note that current exceeds detection limit (∼10 pA) for voltages as high as ∼10 V (range of current onset is 710 V) and rapidly increases with bias. Figure 1c reveals the three-dimensional AFM topographic image collected after the 100 point IV measurement and overlays the maximum measured current per location. Notice the universal correlation between magnitude of current observed and the presence of extracted particles. Comparison of the area under the IV curve (transferred charge) and the volume of deposited product allows the Coulombic efficiency of the process to be estimated. For the average particle volume of ∼3.4 108 nm3 the amount of deposited material is 1.6 1010 Li atoms (assuming that the deposit density is that of metallic Li). By integrating the cathodic region of the IV curve, a total charge of 1.84 nC was obtained, corresponding to 1.1 1010 electrons. Within the uncertainty of the experiment, this number is very close to the total transferred charge, consistent with almost unity transference number for Li ions (i.e., no electronic conduction) in LICGC. This behavior is further illustrated in Figure 1b which shows a linear relationship between the number of Li atoms deposited 4162
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Figure 2. (a) 1 μm 1 μm height AFM topography image of LICGC with scale units in nanometers. (b) ESM imaging amplitude (arbitrary units). (c) Frequency of ESM amplitude maximum with scale units in kilohertz. (d) Q-factor for SHO fitting of amplitude and phase.
(calculated using volume of particles and physical constants for Li) and the transferred charge for all 64 locations (partial particles on scan edges were removed from calculation) on the sample surface. Note the (i) excellent linear relationship between the two and (ii) relatively broad distribution of particle size (Figure 1d), illustrating the electrochemical reactivity across the sample surface. A significant fraction of locations do not form product under the action of the applied 13 V bias, whereas in active regions the volume of deposited material can differ by a factor of ∼3. Notice the linear relationship proceeds with a slope of ∼0.75 atoms/electron rather than the theoretical value of unity for the reaction Liþ þ e f LiðsÞ
ð1Þ
This indicates either (i) the presence of some charge transfer inefficiencies (i.e., stray surface currents possibly as a result of H2O meniscus) or (ii) the direct reaction of the deposited Li metal with ambient moieties (O2, N2, CO2, H2O, etc.) or, more likely, a combination of (i) and (ii). It is worth noting that true reproducibility error for this type of measurement is not possible to determine because of sample inhomogeneity, tip changes, etc. We further compare the sensitivity of current and topographic detection methods. The current measurements contain the information of particle growth as a function of bias (and potentially time), providing detailed kinetics information. However, the current detection limit of ∼10 pA with a 60 Hz noise floor over 7 kHz bandwidth suggests that a minimal amount of detectable material is ∼1.4 105 nm3 (assuming unity transference for eq 1). At the same time, measuring topographic changes allows particles as small as ∼102104 nm3 to be detected, several orders of magnitude below current detection. However, due to significant spatial variability of electrochemical activity across LICGC, surface measurements at dissimilar locations cannot be compared directly, whereas systematic measurements at a single location are time-consuming (e.g., necessitating multiple positioning of the tip on the same particle, bias application, and imaging of resultant changes). Below, we explore the
dynamic (ESM) strain detection for spectroscopic mapping of the electrochemical activity of LICGC surface at a single location. ESM Imaging of Li Ion Conductive Glass Ceramics. Shown in Figure 2a is the topographic image of the LICGC surface illustrating the presence of multiple grains and pores and closely resembling scanning electron microscopy (SEM) images40 of similar materials. The ESM images at 0 dc bias are shown in Figure 2bd. Briefly, the tip is excited using an electric bias of 12 Vac using a band excitation waveform.47 The measured response—comprising cantilever amplitude and phase versus frequency in the frequency band centered at the (preliminary determined) contact cantilever resonance—is analyzed using a simple harmonic oscillator (SHO) model and electromechanical amplitude at the resonance, resonance frequency, and quality factor are plotted as 2D maps. The details of data analysis in band excitation (BE) imaging and voltage spectroscopy are discussed in depth in several recent publications on ferroelectric materials.4851 The ESM frequency map in Figure 2c shows relatively strong (∼10 kHz) variations of the contact resonant frequency along the sample surface and is primarily related to variations in surface topography (topographic cross-talk). This strong variability of contact resonance frequency with position (and often with voltage) is ubiquitous at all practically relevant surfaces and is a primary factor limiting application of single-frequency resonance-enhanced SPM methods for ESM and piezoresponse force microscopy (PFM). The surface of LICGC shows remarkably low ESM amplitude (Figure 2b), except the presence of several well-localized spots with high electromechanical response. The comparison with the extant SEM data on similar materials allows us to identify the latter as impurity AlPO4 phase (berlinite) which has a crystal structure similar to quartz52 and is piezoelectric (d11 = 1.4 1012 C N1)53 but not ionically conductive.54 At the same time, the ESM response of the ionically conductive glass phase is significantly lower, and the signal cannot be unambiguously identified as due to ionic response or residual electrostatic interactions. We further note that for such systems band excitation and equivalent methods such as fast lock-in sweeps55 ring-down, response to pulse excitation,56,57 or rapid multifrequency imaging58 enable resonance-enhanced electromechanical characterization, whereas applicability of amplitude feedback based approaches such as dual ac frequency tracking (DART)59 can be expected to be limited due to very small response in some locations. ESM Voltage Spectroscopy in Reversible Regime. To explore the presence of ionic motion in nonzero fields, ESM spectroscopy measurements were performed. Briefly, the bias applied to the tip is modulated as shown in Figure 3a, with the envelope of the sequence of square pulses following a slow (∼0.110 s) triangular wave. The on-field pulses induce ion redistribution and electrochemical processes in materials, whereas the ESM response measured in the off-field state provides the information on bias-induced transformations. This approach is chosen to minimize the contribution of electrostatic forces inevitable for relatively soft cantilevers (k = 3 N m1) and large tipsurface potential differences.60 Similar to ESM imaging, the spectroscopy was performed in the band excitation mode. Illustrated in panels c and e of Figure 3 are the 2D response spectrograms at a single location, representing the evolution of cantilever amplitude and phase as a function of frequency and time for triangular bias sweeps. The amplitude and phase of the response at the resonance can be extracted, and 4163
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Figure 4. AFM topography (a) before 100 point ESM voltage spectroscopy grid (10 μm 10 μm, maximum direct current amplitude of 5.0 V) and (b) 11 μm 11 μm topography measured after ESM voltage spectroscopy measurements. Note the voltage spectroscopy waveform applied here was that of the first-order reversal curve described in Figure 6c.
Figure 3. (a) ESM spectroscopy waveform (inset illustrates multifrequency band excitation wave packet). (b) Hysteresis loops calculated by extracting amplitude and phase from the regions shown in (d). (c) Single point 2D amplitude spectrogram. (d) 600 600 nm ESM map of hysteresis loop area. (e) Single point 2D phase spectrogram showing the ESM envelope waveform and (f) 600 600 nm AFM topography with scale units in nanometers.
plotted as a function of voltage, giving rise to hysteresis loops as shown in Figure 3b. Finally, in the ESM spectroscopic imaging mode the hysteresis loops are acquired at each location on the square grid, and resultant loop array is analyzed to extract relevant parameters such as area under the loop, positive and negative nucleation bias, etc.61 The interpretation of these parameters in terms of relevant materials property is obviously specific to the type of phenomena explored; e.g., for ferroelectric materials, nucleation biases correspond to the nucleation of ferroelectric domains, while in electrochemical systems they correspond to the onset of electrochemical processes. For LICGC, the hysteresis loop opening and topographic maps (Figures 3d,f) in the 3.5 to 3.5 V bias widow are presented. It can be seen clearly that the hysteresis loop is open in most locations, evidencing the ionic activity. Interestingly, the AlPO4 grains with high piezoelectric response exhibit closed loops, in agreement with piezoelectric but non-ion conductive nature of the material. No topographic changes in the surface can be detected after imaging, indicative of the reversibility of observed ionic dynamics for the bias range used. However, we further note that ESM contrast is somewhat unstable (streaky lines in fast scan direction) and hysteresis loops often show regions with negative susceptibility, indicative of lack of saturation. At the same time, ESM imaging with higher bias windows (5 V, Figure 4) leads to the rapid loss of contrast in the image; i.e., hysteresis loops will be open in the first several locations and will
be closed on the remainder of the scan. The examination of surface topography before and after spectroscopic measurements clearly illustrates the formation of the particles of reaction product. Note that at these early stages of the process, the inhomogeneity of the surface is significantly larger as compared to data in Figure 1 (where reaction proceeded to a higher degree and the volume of deposited product is much larger). The attempts to reverse the electrochemical process by applying large positive biases to the tip were unsuccessful, indicating strong irreversibility of electrochemical process. Finally, we note that for small pulse magnitudes (Figure 4), the particle sizes are highly nonuniform and in some locations particles do not form, evidencing highly nonuniform electrochemical properties of the surface. These observations prompt us to develop the spectroscopic ESM modes that allow single-point probing of the early stages of local irreversible electrochemical processes and, eventually, spatially resolved mapping of corresponding critical potential. Mapping of Irreversible Phenomena by ESM. The irreversible nature of electrochemical reaction on the LICGC surface, combined with pronounced surface inhomogeneity, brings forth the obvious challenge of probing irreversible processes to differentiate electrochemical reactivity of dissimilar grains, grain boundaries, and heterophase interfaces. Application of bias pulses sufficiently large to induce electrochemical process everywhere on the surface with subsequent AFM imaging of surface topography is limited by fact that particles grow rapidly once critical bias is exceeded. Thus, large particles formed in the active region will overshadow the adjacent locations. At the same time, applications of a series of increasing bias pulses at each location and subsequent topographic imaging after each pulse and at each location are clearly impractical; e.g., a 30 30 pixel map sampled at 10 voltage levels will require ∼104 topographic images (unless the SPM platform is completely free of drift). Here, we develop a spectroscopic strategy for probing irreversible electrochemical processes (and other bias-induced transformations) based on detection of resonant frequency shift during application of unipolar or bipolar first-order reversal curve (FORC) waveforms. We note that contact resonant frequency is very sensitive to the geometry of the tipsurface contact, as analyzed in detail in the context of atomic force acoustic microscopy.6267 Combined with very high precision with which resonant frequency can be measured (to date, ∼100 Hz for band excitation used here as limited by the resonance peak width and pulse length), this allows detection of changes in tipsurface 4164
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Figure 5. (a) Single point 2D amplitude spectrogram collected using negative unipolar FORC waveform for a location where no particle formed. (b) Single point 2D amplitude spectrogram in a location where a particle formed. (c) The applied negative dc FORC envelope waveform. (d) AFM topography image collected before the 100 point FORC grid. (e) AFM topography after FORC grid showing newly formed particles with scan beginning at bottom left side of image.
Figure 6. (a) Single point 2D amplitude spectrogram for FORC measurements at a location which exhibited a single frequency shift. (b) Single point spectrogram for a pixel which exhibited two frequency shifts. (c) Applied bipolar FORC DC envelope waveform.
contact area, on the level of several tens of percent. Correspondingly, particles as small as several nanometers—equivalent to ∼103 transferred electrons—should be detectable. Note however that particle shape measured by AFM represents a convolution of the particle and tip shape; hence volume measurements on small particles are highly unreliable and, while the presence of the particle can be established, its volume cannot unless the particle shape is determined by another method, e.g., electron microscopy, etc. The evolution of the ESM signal as a function of time during the application of unipolar negative triangular wave with increasing amplitude is shown in Figure 5. Note that for small biases (here, we refer to the bias as the one of the envelope of the waveform) the response amplitude increases with bias, but the resonant frequency remains relatively constant. AFM imaging illustrates that in this regime there is no particle formation on the surface, and hence the observed signal behavior is consistent with Li ion accumulation in the material below the tip. At the same time, for bias of 5.43 V the resonance frequency exhibits rapid jump by 20.4 kHz in a very narrow (1.7 V) interval, indicative of sharp first-order-like transition. Subsequent imaging of the surface correlates this jump with formation of nanoparticles of reaction product, i.e., the onset of the irreversible electrochemical process. Subsequent evolution of the signal is highly irregular, with both amplitude and frequency changes during biasing, as illustrated in Figure 5. We attribute this behavior to the growth of the particle at the tipsurface junction and intermittent loss of contact between tip and ionic conductor. This is reflected in resonant frequency shift (changes in contact area) and fluctuations in electromechanical response amplitude (contact with ionic conductor). Notably, the use of the negative unipolar waveform leads to rapid degradation of tip surface contact. The frequency jumps were observed for only 18 locations, whereas at the remaining 82 locations the signal was almost bias independent, with small increase in amplitude and decrease in resonant frequency. Subsequent topographic images illustrate that particle formation is observed only for locations exhibiting strong frequency shifts (similar to Figure 5b), and no particle formation is observed in locations with no frequency shifts (similar to Figure 5a). This
behavior can be attributed to either the degradation of conductive coating of the tip or tip contamination by reaction product. The comparative analysis of the topographic image before and after imaging suggests that tip contamination is the primary effect. It is also worth noting that these observations are repeatable when the tip is replaced and a “fresh” region of the LICGC is selected. The first-order reversal curve (FORC) ESM mapping using a positive bias unipolar waveform reveals no resonant frequency shifts or particle formation, as expected. The signal amplitude for such waveforms produce resonance amplitude signals that are highly reproducible with bias, which we ascribe to Li ion depletion under the tip. Finally, we observed that the use of bipolar waveforms allows us to minimize tip contamination and in most cases observe frequency shifts. Shown in Figure 6 is the evolution of the ESM signal during the application of bipolar waveform. Note that the resonant frequency shifts are now much more abrupt, and fluctuations of the resonant frequency and response amplitude are much smaller than for the negative unipolar waveform. While the origins of this improved behavior are unclear, we argue that a small degree of reversibility precludes particle accumulation on the probe (tip cleaning) and results in much more regular behavior. This mode allows reliable ESM statistical analysis and mapping of irreversible electrochemical processes, as illustrated in Figure 7. Here, the bipolar first-order reversal curve mapping was performed over a 100 point grid. The traces of the amplitude and resonance frequency (panels a and b of Figure 7, respectively) are plotted for all locations along the 100 sample surface locations (vertical axes) and time during the bias sweep (horizontal sweep). This representation establishes the uniformity of the electrochemical behavior along the LICGC surface. The histogram of the first frequency shift, shown in Figure 7c reveals a strong variability for the ∼3.5 V to ∼5 V region. The second jump (e.g., as illustrated in Figure 6b) is observed for a much smaller number of points, and the corresponding histogram is shown in Figure 7d, which is centered at ∼4.5 V. Note that the bin size is limited by the type of waveform used (i.e., period of first-order reversal curve waveform). Under this biasing protocol, 81 of the 100 points revealed single frequency shifts (type illustrated in Figure 6a), while 25 of those points included a second frequency shift. While the evolution of the 4165
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Figure 7. (a) 2D map showing the emergence amplitude of the resonance across a 100 point grid as a function of time/voltage. (b) 2D plot showing the emergence resonance frequency across a 100 point grid as a function of time/voltage. (c) Histogram showing the distribution of single frequency shifts occurring as a function of bias for the 100 point grid. (d) Histogram showing the distribution of double frequency shifts occurring as a function of bias for the 100 point grid.
second frequency shift is unclear, we suggest that it may involve the formation of a second particle phase (i.e., Li2O2, Li2O, LiOH, etc.) or changes to the tip surface contact ascribed to tip cleaning. This analysis illustrates that nucleation bias for electrochemical Li ion extraction on the LICGC surface in active region varies, likely due to the presence of nucleation centers that facilitate the formation of metallic Li phase. Therefore, the average value of nucleation potential (4.5 V) is close to the standard Li ion reduction potential (3.04 V vs reversible hydrogen electrode, RHE), and the variability of the nucleation bias is relatively small for local SPM-based measurements. In comparison, the variation of ferroelectric switching biases can be as large as 50200%. This observation agrees with the amorphous nature of LICGC (i.e., lack of mesoscopic nucleation centers) and high Li conductivity of the material (i.e., small concentration overpotentials). It is also important to note that even though a two electrode configuration was employed for these measurements, we contend that the average nucleation bias of 4.5 V is comparable to more commonly accepted values vs RHE by considering a voltage divider effect describe in detail elsewhere.33 Summary. Here, we demonstrate a novel ESM approach for mapping irreversible electrochemical processes with nanometer resolution. For a model LICGC system, we demonstrate near unity transference numbers for ionic transport and demonstrate detection limits for current and strain-based detection. We establish the limits for static AFM imaging of reaction product and demonstrate the spectroscopic imaging of a bias-induced resonant frequency shift, which allows detection of early stages of the process. The statistical properties of the Li ion extraction on LICGC surface are established and found to be close to Li ion reduction potential. We have demonstrated low-resolution mapping and propose that high resolution can be achieved if the feedback circuit is used to interrupt the first-order reversal curve waveform after the frequency shift is detected. The ESM imaging of LICGC has illustrated the ability to distinguish the piezoelectric AlPO4 phase from the amorphous
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Li ion conducting phases, which interestingly exhibited poor piezoelectric properties. ESM voltage spectroscopy confirms the inactivity of AlPO4 toward Li ion conduction. This suggests that LICGC formulations in absence of AlPO4 may provide higher Li ion conductivity as a result of having fewer Li ion conducting surface voids. The unipolar and bipolar first-order reversal curve biasing spectroscopies have illustrated the biasing protocols required to irreversibly extract Li ions and electroplate Li metal to the surface of the electrolyte. These consequences are often overlooked in favor of studying the chemical stability of the electrolyte. However, considering the large chargedischarge hysteresis for Liair batteries (often requiring voltage exceeding 4.5 V for recharge) it is worth understanding the stable electrochemical window of LICGC. These studies both provide insight into nanoscale ionic phenomena in a practical Li ion electrolyte and also open pathways for probing other irreversible electrochemical transformations in nanoscale systems. These potentially include electroforming in memristive and electroresistive systems, early stages of solidelectrolyte interphase (SEI) formation in Li ion batteries,68 fatigue and degradation in ferroelectric oxides,69,70 degradation of oxide and polymeric fuel cells and photovoltaic materials, and many others. Finally, we note that the proposed strategy can be extended beyond bias-induced transformations toward force-induced (e.g., in molecular unfolding spectroscopy7173 and nanoindentation74 measurements) and thermal induced transformations enabled by recently introduced thermal probes,7578 suggesting tremendous future potential for this family of techniques.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected],
[email protected].
’ ACKNOWLEDGMENT The authors acknowledge financial support by the Laboratory Directed Research and Development Program (LDRD). Experiments were conducted at the Center for Nanophase Material Sciences, which is sponsored at the Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The authors also thank Alexander Tselev for insightful discussions. ’ REFERENCES (1) Nazri, G.-A.; Pistoia, G. Lithium Batteries: Science and Technology, 1st ed.; Springer-Verlag: New York, 2009. (2) Abraham, K. M; Jiang, Z. J. Electrochem. Soc. 1996, 143 (1), 1–5. (3) Wang, D.; Xiao, J.; Xu., W.; Zhang, J. G. J. Electrochem. Soc. 2010, 157 (7), A760–A764. (4) Bagotsky, V. S. Fuel Cells: Problems and Solutions, John Wiley & Sons, Inc.: Hoboken, NJ, 2009. (5) O’Hayre, R.; Cha, S. W.; Colella, W.; Prinz, F. B. Fuel Cell Fundamentals; John Wiley & Sons, Inc.: Hoboken, NJ. 2006. (6) Adler, S. B. Chem. Rev. 2004, 104 (10), 4791–4843. (7) Tuller, H. L.; Litzelman, S. J.; Jung, W. C. Phys. Chem. Chem. Phys. 2009, 11, 3023–3034. (8) Silassen, M.; Eklund, P.; Pryds, N.; Johnson, E.; Helmersson, U.; Bottiger, J. Adv. Funct. Mater. 2010, 20, 1–6. (9) Tsuruoka, T.; Terabe, K.; Hasegawa, T.; Aono, M. Nanotechnology 2010, 21 (42), 425205. (10) Strukov, D. B.; Snider, G. S.; Stewart, D. R.; Williams, R. S. Nature 2008, 453 (7191), 80–83. (11) Sawa, A. Mater. Today 2008, 11 (6), 28–36. 4166
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