Following the Growth of Surface Films on Lithium and Their Thermal

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Following the Growth of Surface Films on Lithium and Their Thermal Behavior in Standard LiPF6 Solutions Using Differential Scanning Calorimetry Liraz Larush, Ella Zinigrad, Yossi Goffer, and Doron Aurbach* Department of Chemistry, Bar Ilan UniVersity, Ramat Gan 52900 Israel ReceiVed June 7, 2007. In Final Form: September 17, 2007 In this paper, we report on attempts to use differential scanning calorimetric measurements of aged Li electrodes for the study of the kinetics of the growth of surface films on the active metal. Standard, commonly used alkyl carbonate solutions such as ethylene and di-methyl carbonates with LiPF6 were explored. Heating Li samples in solutions after aging by DSC, resulted in well-resolved curves of reaction heats vs temperature. Exothermic reactions occurring at temperatures below 150 °C could be attributed to changes related to the surface films and their heat evolved, increased as a function of storage time, and hence these heats represent the thickness of the surface films that grow upon storage. Scanning electron microscopy of the Li surfaces as a function of storage and heating to different temperatures confirmed that the thermal reactions of Li surfaces in these solutions up to 150 °C relate to the surface films only. XPS studies revealed that these processes of the surface films change the metastable organic Li salts to more stable inorganic compounds such as LiF and Li2O. Massive red-ox reactions, between the salt anion and the solvents and between the solution species and the active metal, occur at temperatures higher than 150 °C. The kinetics of growth of the surface films on Li show an inverse logarithmic behavior, expected for thin surface films with which the rate-limiting step of their growth depends on ions transport across the film.

Introduction Lithium, lithiated carbon, and Li alloy electrodes react spontaneously with all relevant polar aprotic electrolyte solutions to form insoluble products that precipitate as surface films.1 The reduction of most of the commonly used polar aprotic solvents and salt anions by lithium or lithiated compounds (Li-C, Li alloys) forms Li salts (organic and inorganic), which are insoluble in the parent solutions.2 These surface films are usually electronic insulators but Li ion conductors, because of the property of most of Li salts, which behave like solid electrolytes as thin films. Their electronic/ionic behavior relates to the unique properties of the ionic bonds that Li forms with most of the nonmetallic elements.3 The basic scenario of active reducing surfaces in aprotic nonaqueous Li salt solutions is that the surface films formed initially allow electron tunneling through them until they reach a thickness of several tens of nanometers, which blocks further electron transport from the active metal to the solution through the surface film.4 Thus, a passivation is reached and the surface films stop growing. This process forms protective surface films that are called solid electrolyte interphase (SEI).5 The surface chemistry and physics of lithium and other reductive lithiated materials have been intensively explored over the last few decades because the surface film phenomena on Li, Li-C, and Li alloys (Li-Si, Li-Sn, etc.) are critical to the performance of most relevant anodes (negative electrodes) in Li and Li ion batteries.6-12 * To whom correspondence should be addressed. E-mail: aurbach@ mail.biu.ac.il. Phone: (+972) 3-531-8317. Fax: (+972) 3-738-4053. (1) Aurbach, D.; Cohen, Y. In Lithium-Ion Batteries: Solid-Electrolyte Interfacial Film; Balbuena, P., Ed.; Imperial College Press: Oxford, England, 2004; p 70. (2) Peled, E. In Lithium Batteries; Gabano, J. P., Ed.; Academic Press: New York, 1983; p 43. (3) Peled, E.; Golodnitsky, D.; Penciner, J. Anode/Electrolyte Interface. In Handbook of Battery Materials; Besenhard, J. O., Ed.; VCH: New York, 1999; p 410. (4) Aurbach, D.; Zinigrad, E.; Zaban, A. J. Phys. Chem. B 1996, 100, 3089. (5) Peled, E. J. Electrochem. Soc. 1979, 126, 2047-2051. (6) Aurbach, D.; Markovsky, B.; Gamolsky, K.; Levi, E.; Ein-Eli, Y. Electrochim. Acta 1999, 45, 67-86.

Various surface-sensitive techniques have been used in conjunction with electrochemical methods for the intensive studies of the surface phenomena of these reactive electrodes. These included FTIR,13 Raman,14 and AFM.15 The thermal behavior of surface films on graphite electrodes has also been explored in recent years.16,17 Despite the variety of tools that can be employed in the study of surface phenomena on electrodes, a kinetic study of the formation and growth of surface films is very difficult because in situ techniques, such as FTIR or Raman, suffer from the masking effect of the solution. In situ AFM senses only the outer surface of the electrodes. Ex situ techniques are limited in their ability to provide authentic information on dynamic processes. Electrochemical tools such as impedance spectroscopy can probe the electrodes’ surface dynamics. However, the information obtained is not unambiguous, and the interpretation of the data is never straightforward when very active electrodes (such as Li metal or its alloys) are involved. In this work, we attempted to develop a tool for the dynamic study of surface film formation and growth on Li (and lithiated electrodes) based on calorimetry. Storing active electrodes in solution, allowing surface films to grow during certain periods of time, and then applying an in situ calorimetric tool, such as DSC, can provide useful information about both the relative (7) Leroy, S.; Martinez, H.; Dedryvere, R.; Lemordant, D.; Gonbeau, D. Appl. Surf. Sci. 2007, 4895-4905. (8) Zhang, S. S. J. Power Sources 2007, 163, 713-718. (9) Chen, Z. H.; Liu. J.; Amine, K. Electrochem. Solid State Lett. 2007, 10, A45-A47. (10) Cheng, Y.; Wang, G.; Yan, M. M.; Jiang, Z. Y. J. Solid State Electrochem. 2007, 11, 310-316. (11) Zhao, L. W.; Watanabe, I.; Doi, T.; Okada, S.; Yamaki, J. J. Power Sources 2006, 161, 1275-1280. (12) Zhang, H. L.; Liu, S. H.; Li, F.; Bai, S.; Liu, C.; Tan, J.; Cheng, H. M. Carbon 2006, 44, 2212-2218. (13) Aurbach, D.; Chusid, O. J. Power Sources, 1997, 68, 463-470. (14) Kostecki, R.; McLarnon, F. J. Power Sources 2003, 119, 550-554. (15) Aurbach, D.; Cohen, Y. J. Phys. Chem. B 2000, 104, 12282-12291. (16) Richard, M. N.; Dahn, J. R. J. Electrochem. Soc. 1999, 146, 2068-2077. (17) Wang, Y.; Dahn, J. R. J. Electrochem Soc. 2006, 153, A2188-A2191.

10.1021/la701674x CCC: $37.00 © 2007 American Chemical Society Published on Web 11/15/2007

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Figure 1. Example of fitting the DSC curves obtained in the course of test #6 (Table 1): (a) the measured curve, (b) the simulated curve by the fitting, (c, d, e, f) the separate, simulated peaks.

status of the surface films at ambient temperatures (at which they grow) and their thermal properties at elevated temperatures. Comparing the relevant heats of reaction measured during temperature scanning after different periods of storage may provide significant dynamic information. In the present study, Li metal samples storied in commonly used standard EC-DMC/ LiPF6 solutions were studied by differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM).

Figure 2. Typical DSC measurements of Li samples that were stored during different periods of time at room temperature before the measurements in closed crucibles filled by 1M LiPF6 solutions in EC:DMC 1:1: (1) 24.5 h, (2) 33.0 h, (3) 43.4 h, (4) 54.8 h. Heating/cooling rate was 1 °C /min.

Experimental Section Li foils (0.3-mm thick) were mechanically cleaned in an argonfilled glove box to remove native surface films resulting from Li reactions with the atmospheric components, and were then smoothed by rolling a glass rod on them. A differential scanning calorimeter (DSC) from Mettler Toledo Inc., Model DSC 822, was used for the thermal analysis. For the storage/thermal studies, lithium disks of 3-mm diameter, with a total surface area of 0.17 cm2, were cut and placed in standard high-pressure, gold-plated, stainless steel crucibles, 30 µL in volume (Mettler Toledo Inc.) in a glove box (VAC, Inc.) under highly pure argon atmosphere. The crucibles were filled with 1M LiPF6/EC:DMC 1:1 solution, and then sealed. The samples were stored for awhile and then studied by calorimetry at a 40-350 °C temperature range and a heating/cooling rate of 1 °C/min. The PeakFit Version 4 software (SPSS Inc.) was used for the peaks deconvolution in the thermograms thus obtained. The Pearson Type IV distribution18 was applied to empirically characterize the shape of the DSC data (Figure 1). This data treatment is especially well-suited to model pull distributions, which are asymmetric. The overall thermal effect (i.e., heat evolution) was estimated by the STARe Software, Version 8.01. The first exothermic peak obtained in all of the experiments was described well by the Pearson Type IV distribution. In this respect, the first exothermic peak was distinguished from the second one (which appears in all of the thermograms obtained). The shape of the second exotherm did not permit its fitting by only one peak. Therefore, the thermal effect of the second exothermic processes was calculated as the difference between the overall heat evolution and the response of the first exotherm. Pristine Li foil and Li surfaces after storage and heating were characterized by scanning electron microscopy (SEM, JEOL, JSM 840) and XPS. For the microscopic studies, Li samples taken from the crucibles after the processes could be used. The samples for XPS measurements were 1 cm2 Li foil pieces that were stored at room temperature or stored and heated to 100 or 140 °C in a 1M LiPF6/ EC:DMC 1:1 solution in stainless steel vessels that filled and sealed in a glove box. XPS analysis was performed by a Kratos AXIS-HS spectrometer, using a monochromatized Al KR source. Survey scans were run at 75 W at a PE of 80 eV. High-resolution spectra were acquired at the same power source in a hybrid mode analyzer at detection pass energies of 20 (18) Stuart, A.; Ord, J. K. Kendall’s AdVanced Theory of Statistics; 6th ed.; Oxford University Press: Oxford, Vol. 1; 1994.

eV. All measurements were carried out with an electron flood gun with low-energy electrons for charging neutralization. Prolonged measurements did not reveal any susceptibility to X-ray radiation by the Li samples. All of the spectra were calibrated by referencing the binding energy of the lowest C 1s peak to 285 eV. The air-sensitive Li samples were transferred from the glove box to the XPS pre-chamber with no exposure to any atmospheric contaminants, by a homemade transfer device that includes a magnetic manipulator and a gate valve, loaded in the glove box, and then it is attached by a Conflat flange sealed with Viton O-rings, to the XPS pre-chamber pumped down to 1 × 10-6 Torr. Data processing was carried out with the VISION 2.1 software (“Kratos”). Curve fitting was performed using a 80/20 Gaussian/Lorentzian line shape.

Results DSC Measurements. Li samples were stored at room temperature in 1M LiPF6/ EC:DMC solutions during various time periods before measurement. Typical DSC curves of Li samples at a heating/cooling rate of 1 °C/min (Figure 2) show two consequent exothermic peaks in the 85-135 °C range. The peaks are irreversible, as proven by DSC measurements during cooling at the end of the heating process. As can be seen in the curves of Figure 2, the exothermic peaks are preceded by small endothermic processes in the 80-90 °C range (which also increase upon storage) that may relate to processes of the surface films. As these peaks are too small and hence close to the noise level, we could not treat them in the framework of this study. Both the first and the second exotherms demonstrate increases in the reactions’ heat as a function of storage time at RT before the calorimetric measurements, as demonstrated in Figure 2. The dependence of the heat of the exothermic reactions, (QV,1 and QV,2), on storage time is presented in Figure 3. It is assumed that the two exothermic processes between 80 and 140 °C relate only to the surface films. They do not correspond to the same surface compounds but rather reflect a serial conversion of the meta-stable surface species formed at RT to more stable compounds upon heating. The most probable assignment for these processes, observed at moderately elevated temperatures, are further reduction of meta-stable surface species

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Figure 3. Dependence of the heat of the first exothermic reaction of the Li samples stored in EC:DMC 1:1 /1M LiPF6 solution as a function of storage time at RT.

(e.g., ROCO2Li and ROLi compounds) by Li (due to an increase in the electronic conductivity of the surface films as the temperature rises) and their partial decomposition to form stable species such as LiF and Li2O. The decomposition of solution components should be assigned to the thermal processes observed at temperatures higher than 150 °C (Figure 1). Inorganic end-product surface species such as LiF and Li2O should be stable at high temperatures. The metastable surface species accumulate in the outer part of the surface films, because in the inner part of the surface films, close to the Li metal, all of the species are expected to be in their final, low oxidation state, due to their contact with lithium. Hence, the logical scenario that seems to be well-reflected by the DSC curves described herein is that as the temperature rises and the electron tunneling through the surface films is facilitated, all of the metastable (organic) compounds of the surface films are reduced. These reduction processes are reflected by the mild exothermic reactions below 150 °C. At temperatures above 150 °C, pronounced exothermic processes are detected, which should be attributed to both intrinsic red-ox processes of the electrolyte solution19 and the massive reaction between the solution and the lithium, as the passivation is broken at elevated temperatures (especially after Li melting >180 °C). The DSC curves (Figure 1) also reflect the fact that the surface films on Li are multicomponents, as was concluded from previous spectroscopic studies.20,21 Hence, the fact that two exothermic peaks are detected below 150 °C, the second of which is very broad, should be attributed to the multicomponent structure of the surface films formed on Li. Surface Studies. Figure 4 shows a typical SEM micrograph of a Li surface stored and then heated, in EC-DMC LiPF6 solution to 140 °C. An SEM micrograph of a Li surface after storage in solution at RT is also presented in this figure for comparison. The Li surface morphology becomes irregularly cracked (Figure 4b) due to both storage and heating, but the basic morphology remains somewhat similar. These SEM micrographs show that heating the Li samples in solutions up to 140 °C related to processes of the surface films that change their morphology and not to pronounced Li-solution reactions (that produce very rough surfaces). All of the XPS spectra of Li surfaces stored in solutions show Li, F, O, C peaks. Figure 5 shows typical C1s and F1s peaks of (19) Gnanaraj, J. S.; Zinigrad, E.; Levi, M. D.; Aurbach, D.; Schmidt, M. J. Power Sources 2003, 119, 799-804. (20) Aurbach, D.; Zaban, A.; Chusid, O.; Weismann, I. Electrochim. Acta 1994, 39, 51-71. (21) Aurbach, D.; Markovsky, B.; Schechter, A.; Ein-Eli, Y.; Cohen, H. J. Electrochem. Soc. 1996, 143, 3809-3820.

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Li surfaces after storage in EC-DMC/LiPF6 solution at RT and after storage at RT (same period of time) and heating the sample in solution to 100 °C. It is important to note that all of the samples studied were nearly identical. Thereby, the comparison of the peak areas in the charts of Figure 5 reflects true quantitative differences in the content of the element under heating. It is clear that carbon species disappear from the Li surface upon heating while fluorine species become more dominant. These results further show that heating below 140 °C leads to processes related to changes/decomposion of the surface films and not to massive reactions between the Li and solution species (which should form high amounts of carbon-containing surface species). The broad F 1s peaks that characterize the X-ray photoelectron spectra of all the Li samples is an evidence of the presence of at least two F-containing surface species in varying proportions. Hence, the F 1s spectra of all the samples were fitted using two peaks. The fitted peaks correlated well with the experimental data. Basically, all of the spectra contain one peak at high binding energy (BE), around 686.5-687.7 eV, and one at low BE, around 684.8-685.9 eV. Judging from the NIST database, one can suggest that the low BE peak belongs to a simple ionic fluoride salt, namely LiF. The high BE F 1s peak can originate from more complex compounds such as LixPFy, LixPOFy, or organo-fluorine compounds. The latter possibility is less plausible, as the C 1s spectra did not reveal features at BEs higher than ∼291 eV, which can be attributed to organo-fluorine compounds. No substantial trend was found regarding the correlation between the sample history and the concentration ratio between the high and low BE F 1s peaks. However, it should be mentioned that the heated samples contained the higher concentration of the low BE F 1s species than that of the high BE species, which suggests that the heated samples surfaces are rich in LiF. The C 1s spectra from all the samples are complex, and reflect the existence of several carbon-containing species on the Li sample surfaces. The C 1s spectra were all fitted to the smallest number of peaks that gave a good fit , namely four peaks, with the exception of the sample stored at RT (no further treatment), which C 1s spectra fitting needed a minimum of five peaks. All of these C 1s spectra contained a peak at ∼290.2-290.5 eV that can be assigned to oxalate or to organic carbonate. The most interesting point that can be learned from these spectra is that the heated samples showed significantly more low BE C 1s features than the samples that were not heated. This suggests that the original carbon-based species in the surface films formed at ambient temperatures, such as carbonates and alkoxides (the peaks at around 290 and 286-287 eV, respectively), transform at elevated temperatures to carbon species at lower oxidation states. As the oxygen peaks are poorly resolved, we did not attempt to fit them. In addition, because the positions of the lithium peaks in the leading data bases (XI SDP V2.3 XPS International, Kawasaki, Japan, 2001 and the NIST Standard Reference Database 20, Version 3.4) are very scattered, we decided not to try to assign the Li peak to specific compounds. The most important insight from the XPS data was gained by following the relative concentration of the various elements in the surface films on lithium (Li, O, C, F) as a function of storage and heating. As can be seen in Table 2, there is a dramatic difference between the heated and unheated samples. The most striking change is that the heated samples become very rich in F and Li, compared to the unheated samples, which are richer in carbon. Hence, these surface studies confirm the above-mentioned assumption that heating Li samples leads to a further reduction and decomposition of their surface species, which enrich the Li

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Figure 4. SEM micrographs of Li surfaces: (a) After mechanical stripping in dry argon atmosphere and then storage in solution at RT (24 h), (b) Li surface prepared as (a) stored in LiPF6/EC:DMC 1:1 solution for 24 h at room temperature and then was heated to 140 °C. The light lines of irregular shape appearing in the images were formed by the mechanical stripping and are clearly seen in both micrographs. The white arrows in b show cracks in the surface films, resulted from heating. A scale appears in each micrograph. Table 1. Results of DSC Measurements of Li Samples That Were Stored in 1 M LiPF6 EC:DMC 1:1 Solutions during Various Periods before the Thermal Measurementsa first exotherm test mass of storage peak temp QV,1 no. sample (mg) time (h) (°C) (mJ) 1 2 3 4 5 6 7

5.497 7.234 7.103 5.860 5.343 6.679 5.459 a

18.3 21.0 24.5 33.0 43.4 44.5 54.8

81.3 86.8 95.0 93.7 89.2 83.9 87.6

43.5 46.4 54.6 72.1 78.9 73.0 133.5

second exotherm QV,2 (mJ)

QV,2/QV,1

315.3 211.4 292.4 185.4 811.8 309.8 758.3

7.2 4.6 5.4 2.6 10.3 4.2 5.7

The heating rate was 1 °C/min.

Discussion Figure 5. Typical C1s (a) and F1s (b) XPS spectra of Li surfaces after storage in LiPF6/EC:DMC 1:1 solution for 24 h at room temperature, and after storage during 24 h and heating to 100 °C in solution, as indicated.

surface with compounds such as LiF and Li2O at the expense of reductions in the presence of species with carbon-oxygen bonds.

The DSC curves of Li samples stored in electrolyte solutions show that it may be possible to well separate between thermal processes that reflect changes in the surface films and processes in which massive reactions of the solution species take place. The latter include two type of reactions: red-ox reactions between the salt anion (an oxidizer) and solvent molecules (reducing agents)19 and bulk reactions between the active electrode and the

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Table 2. Atomic Concentration (%) of the Elements Found on the Li Surfaces after Storage during 24 h and after Storage and Heating storage conditions room temperature with following heating to element

room temp

100 °C

140 °C

F O C Li

16.6 25.8 39.5 13.2

51.2 4.7 6.14 35.8

46.8 6.5 9.3 37.4

solution species, as the passivation is broken down at high enough temperatures (especially intensive as the lithium melts around 180 °C). The Li surface studies by SEM and XPS support the conclusion that we can really probe thermal reactions which relate mainly to the surface films alone. Thereby, we used the heat values of the exothermic transformation processes related to the surface films on Li up to 140 °C as a function of storage time, in order to estimate the rate of the film growth. The Mott-Cabrera theory predicts an inverse logarithmic law for the growth of very thin oxide films (100-1000 Å) on metals under oxygen atmosphere and for the anodic formation of oxide films in electrolytes containing oxide ions.22 According to this theory, ions and electrons move in the solid ionic films independently. At a low temperature, electron tunneling from the bulk metal to the oxidizing species occurs rapidly compared to the ionic motion across the surface film. The rate-limiting step for mass transport in the oxidation process that forms the films is at the metaloxide interface and involves the entry of the metal “ion” from the metal into the oxide lattice. The verification of the Mott-Cabrera theory assumption for growth rate has been carried out in various systems. The growth of a thin Al oxide film in the temperature range 50-400 °C23 and at 90 K24 was well described by this theory. The growth kinetics of the anodic film of solid carbon fluoride in molten 2HF-KF also fits the theory.25 This theory was also applied to carbon monoxide absorption on barium films in a temperature range from 195 to 473 K.26 The kinetics of nitrogen reaction with calcium films between 23 and 200 °C are also consistent with a modified Mott and Cabrera theory of thin-film formation.27 We applied this theory to the kinetics of thin film growth on lithium in a polar aprotic solution, assuming that the heats dissipated upon heating Li surfaces up to 140 °C belong to processes of the surface films only and hence are proportional to their mass and thickness (note that the surface area of all the Li samples measured herein was identical, 0.17 cm2). Hence, measuring QV,1 and QV,2 (Figures 1-3, Table 1) as a function of storage time well represents the increase in the thickness of the surface films on Li upon storage in solutions. The further discussion relates only to QV,1 because the second exotherm is complicated and cannot be described well by a single peak (Figure 1). The limited reproducibility of the second exotherm (Figure 3) is due to the complexity of the competing thermal processes that may occur simultaneously and thereby could not be well resolved. (22) Cabrera, N.; Mott, N. F. Rep. Prog. Phys. 1949, 12, 163-184. (23) Hunt, G. L.; Rithie, I. M. Surf. Sci. 1972, 30, 475-478. (24) Popova, I.; Zhukov, V.;Yates, J. T. Phys. ReV. Lett. 2002, 89, 276101/ 1-276101/4. (25) Lantelme, F.; Chemla, M. J. Chim. Phys. Phys.-Chim. Biol. 1983, 80, 267-274. (26) Ricca, F.; Della Porta, P. Vacuum 1960, 10, 215-222. (27) Roberts, M. W.; Tompkins, F. C. Proc. Roy. Soc. London. A, Math. Phys. Sci. 1959, A251, 369-377.

Figure 6. Linear correlation obtained between the inverse heat of reaction of the surface films (first exothermic reaction, QV,1), and the logarithm of storage time according to inverse logarithmic law of the growth of thin surface films, predicted by the Mott-Cabrera theory.22

As seen in Figure 6, the change in the thickness of the surface layer on Li represented by QV,1 vs t, obeys an inverse logarithmic law:

1/X ) A - B ln t where X is the film thickness at a time t, estimated in this study by the reaction heat QV,1. A and B are constants. It is logical to assume that when the surface films are thin enough (up to several tens of nanometers), the tunneling of electrons through them from the active metal to solution species is rapid, and hence the transport of Li ions across the surface films (+interfacial Li-ion transport) is the rate-determining step of the film growth as the above theory predicts.

Conclusion Differential scanning calorimetry of Li electrodes stored in electrolyte solutions seems to provide enough resolution in the separation of processes related to chemical transformations of surface films formed at low temperatures, from processes related to massive bulk reactions between the active electrode and the solution species and internal red-ox reactions of the electrolyte solutions that occur at high temperatures. For Li stored in standard EC-DMC/LiPF6 solutions, exothermic reactions measured by DSC up to 150 °C relates to transformation of metastable surface species (e.g., organic Li salts such as ROLi and ROCO2Li) to more stable species such as LiF and Li2O by further reduction and decomposition upon heating during the DCS measurement. Hence, the heats of reaction, measured by DSC, which relate to the surface films only, can serve as probes to follow their kinetics of growth upon storage. Following this logic, it was shown in this work that the growth of surface films on Li in the above solution behaves according to the MottCabrera theory, which means an inverse logarithmic behavior of the surface film’s thickness (represented by the heat of reaction) on time of storage. The physics behind this theory is the fact that for thin solid electrolyte films, electron tunneling from the active metal to the solution species (reduction processes that produce the surface species) may be fast and hence ionic transport across the film is the rate-limiting step for its growth. This work opens the door for further similar studies of the kinetics of surface film formation on active substrates using DSC. LA701674X