GROWTH OF THE THIN SUFACE FILMS ON LITHIUM IN
ALKYL CARBONAT SOLUTION
Liraz Larush, Ella Zinigrad, Yossi Goffer and Doron Aurbach*
Department of Chemistry, Bar Ilan University,
Ramat Gan, Israel
Abstract. 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 (EC-DMC) 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 ºC 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 which grows upon storage. Scanning electron
microscopy (SEM) 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 ºC 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 ºC. The kinetics of growth of the surface films on Li
shows 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 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 non-metallic 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, what 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 protecting 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 Various surface sensitive techniques have been used in conjunction
with electrochemical methods for the intensive studies of the surface phenomena of
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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 status of the surface films at ambient temperatures
(at which they grow) and their thermal properties at elevated temperatures.
Comparing the relevant heat of reactions 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).
Experimental
Li foils (0.3 mm thick) were mechanically cleaned in an argon-filled glove box in
order to remove native surface films resulting from Li reactions with the atmospheric
components, and were then smoothed by rolling on them a glass rod. 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 in
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 solutions, and then sealed. The samples were
stored for a while and were then studied by calorimetry at a 40-350 ºC temperature
range at a heating/cooling rate 1 ºC/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 for empirically characterize the shape of the DSC
data. 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 obtain in all 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.
2-2
The samples for XPS measurements were 1cm2 square Li foil pieces that were
stored at room temperature or stored and heated to 100 or 140 ºC 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 Kα source. Survey scans were run at 75 W at 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 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 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 prechamber 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 prechamber pumped down to 1x10-6 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 measurements. Typical
DSC curves of Li samples at a heating/cooling rate of 1ºC/min (Fig. 1) show two
consequent exothermic peaks in the 805-14035 ºC 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 1, the exothermic peaks are preceded by small
endothermic processes in the range 80-90 ºC (which also increase upon storage), that
may relate to processes of the surface films. Since these peaks 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 increase in the reactions'
heat as a function of storage time at RT before the calorimetric measurements, as
demonstrated in Fig. 1.
The dependence of the heat of the exothermic reactions, (QV,1 and QV,2), on
storage time is presented in Fig. 2.
It is assumed that the two exothermic processes between 80 and 140 ºC 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 the moderately elevated temperatures, are further reduction of
meta-stable surface species (e.g. ROCO2Li and ROLi compounds) by Li (due to an
increase in the electronic conductivity of the surface films as the temperature arises)
and their partial decomposition to form stable species such as LiF and Li2O.
2-3
EXO
5 mW/cm2 Li
Heat flow
4
3
2
1
85
95
105
115
125
135
Temperature (ºC)
0.9
8
0.7
6
0.5
4
`
0.3
2
0.1
0
15
30
45
Heat of the second exothermic process (J/cm2 Li)
Heat of the first exothermic process (J/cm2 Li)
Fig. 1. 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 ºC /min.
60
Storage time (h)
Fig. 2. The dependence of the heat of the first exothermic reaction of the Li samples
stored in EC:DMC 1:1 /1M LiPF6 solutions as a function of storage time at RT.
The decomposition of solution components should be assigned to the thermal
processes observed at temperatures higher than 150 ºC. Inorganic end-product surface
species such as LiF and Li2O should be stable at high temperatures. The meta-stable
2-4
surface species accumulate in the outer part of the surface films, since in the inner part
of the surface films, close to the Li metal, all 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 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 (Fig. 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 ºC, the second of which is very
broad, should be attributed to the multi-component structure of the surface films
formed on Li.
Surface studies. Fig. 3 shows a typical SEM micrograph of Li a surface stored, and
then heated, in EC-DMC LiPF6 solution to 140 C. A 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 (Fig. 3b) 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 Lisolution reactions (that produce very rough surfaces).
All the XPS spectra of Li surfaces stored in solutions show Li, F, O, C peaks. XPS
spectra show typical C1s and F1s peaks of Li surfaces after storage in ECDMC/LiPF6 solution at RT and after storage at RT (same period of time) and heating
the sample in solution to 100 ºC. It is important to note that all the samples studied
were nearly identical. Thereby, the comparison of the peaks' area in the XPS charts
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 ºC
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
amount 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 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 data base, 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 BE
higher then ca. 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.
2-5
(b)
(a)
Fig. 3. 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 4b, show cracks in the surface films, resulted from heating. A scale
appears in each micrograph.
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 4 peaks, except for
the sample stored at RT (no further treatment), which C 1s spectra fitting needed a
minimum of five peaks. All these C 1s spectra contained a peak at ca. 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 weren’t 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. Since the oxygen peaks are poorly resolved, we did not attempt to fit them. In
addition, since 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 surface with compounds such as LiF and Li2O at the
expense of reductions in the presence of species with carbon-oxygen bonds.
2-6
DISCUSSION
The DSC curves of Li samples stored in the electrolyte solutions shows 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 solution species, as the passivation is broken down at
high enough temperatures (specially intensive as the lithium melts around 180 ºC).
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 (100-1000Å) 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 metal-oxide 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 ºC23 and at 90 ºK24 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 of 195-473 ºK.26 The kinetics of nitrogen reaction with calcium
films between 23 and 200 ºC is 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 ºC 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 represents well the increase in the
thickness of the surface films on Li upon storage in solutions. The further discussion
relates only to QV,1 since the second exotherm is complicated and cannot be described
well by a single peak (Fig. 1). The limited reproducibility of the second exotherm
(Fig. 2) is due to the complexity of the competing thermal processes that may occur
simultaneously and thereby could not be well resolved.
As can be seen in Fig. 4, 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-1/X1 lnt,
where X is the film thickness at a time t, estimated in this study by the reaction heat
QV,1. A and X1 are constants which represent the initial situation, namely the fact that
at t=0, there are already native surface films at a thickness of 1/A.
2-7
1/QV,1(cm2/J)
4
3
2
1
0
2.9
3.2
3.5
3.8
4.1
Ln t
Fig. 4. The 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
It is logical to assume that when the surface films are thin enough (up to several
tens of nanometers), the tunneling of electrons through then 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 ºC 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 reactions heats measured by DSC, which relate to the surface
films only, can serve as a probe 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 Mott-Cabrera 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 behinds this theory is the fact that for thin
solid electrolyte films electron tunneling from the active metal to 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 films formation on active
substrates using DSC.
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