Consequences of Air Exposure on the Lithiated Graphite SEI

Electrochimica Acta 105 (2013) 83–91

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Electrochimica Acta

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Consequences of air exposure on the lithiated graphite SEI

a a b b

Sara Malmgren , Katarzyna Ciosek , Rebecka Lindblad , Stefan Plogmaker ,

a,b b a a,∗

Julius Kühn , Håkan Rensmo , Kristina Edström , Maria Hahlin

Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden

Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden

a r t i c l e i n f o a b s t r a c t

Article history: In the present work, consequences of air exposure on the surface composition of one of the most reactive

Received 15 February 2013

lithium-ion battery components, the lithiated graphite, was investigated using 280–835 eV soft X-ray

Received in revised form 15 April 2013

photoelectron spectroscopy (SOXPES) as well as 1486.7 eV X-ray photoelectron spectroscopy (XPS) (∼2

Accepted 16 April 2013

and ∼10 nm probing depth, respectively). Different depth regions of the solid electrolyte interphase

Available online 29 April 2013

(SEI) of graphite cycled vs. LiFePO4 were thereby examined. Furthermore, the air sensitivity of samples

subject to four different combinations of pre-treatments (washed/unwashed and exposed to air before

Keywords:

or after vacuum treatment) was explored. The samples showed important changes after exposure to

XPS

SEI air, which were found to be largely dependent on sample pre-treatment. Changes after exposure of

Graphite unwashed samples exposed before vacuum treatment were attributed to reactions involving volatile

Air species. On washed, air exposed samples, as well as unwashed samples exposed after vacuum treatment,

LiPF6 effects attributed to lithium hydroxide formation in the innermost SEI were observed and suggested to

be associated with partial delithiation of the surface region of the lithiated graphite electrode. Moreover,

effects that can be attributed to LiPF6 decomposition were observed. However, these effects were less

pronounced than those attributed to reactions involving solvent species and the lithiated graphite.

1. Introduction are still exposed to air prior to analysis. To gain improved insight

into how to interpret such results, further studies on how different

Lithium-ion batteries are widely used in portable electronics components are affected by exposure to air would be beneﬁcial.

equipment. They are also highly interesting for larger scale energy The solid electrolyte interphase (SEI) [3] is a vital component in

storage in, e.g., electric vehicles, and may hence play an important lithium-ion batteries [4]. The SEI is situated at the interface between

role in decreasing the use of fossil fuels to mitigate global warm- the anode and the electrolyte. In lithiated state, the anode is highly

ing. The higher energy densities of lithium-ion batteries compared reductive and commonly used electrolytes are stable only if a sta-

to many other rechargeable battery technologies present a major ble passivating layer (SEI) can be formed at the anode/electrolyte

advantage in mobile applications. These higher energy densities are interface. The SEI not only lowers the rate of further electrolyte

associated with, e.g., the relatively higher voltage (∼4 V) of lithium- reductive decomposition, but is also known to decrease graphite

ion batteries compared to many alternative battery chemistries. exfoliation [4]. Anode ageing has been largely attributed to changes

At these voltages, non-aqueous electrolytes are required to avoid, at the anode/electrolyte interface [5], and insufﬁcient battery life-

e.g., hydrogen evolution at the anode, and even low levels of water time is considered as a major issue impeding the large scale use

contamination are well-known to be associated with accelerated of lithium-ion batteries in, e.g., electric vehicles. It is thus highly

ageing. Also on post mortem characterization, exposure to air has important to improve the anode/electrolyte interfaces.

been shown to be associated with signiﬁcant changes to, e.g., the Due to the thinness of SEI layers, which as an example on lithi-

anode/electrolyte interfaces [1,2]. However, in some cases, samples ated graphite electrodes has been estimated to ∼20 nm [6], post

mortem measurements using surface sensitive techniques are fre-

quently used to characterize the SEI. However, the experimental

challenges in post mortem characterization of the SEI are signif-

icant [7]. The air sensitive nature of the SEI presents one out of

many obstacles, casting doubt upon results of measurements where

air exposure is not avoided – for example during sample transfer

∗ from argon atmosphere to the measurement equipment. To facili-

Corresponding author. Tel.: +46 184713736; fax: +46 18513548.

E-mail address: [email protected] (M. Hahlin). tate experimental design and interpretation, improved insight into

84 S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91

the consequences of exposing the lithiated graphite SEI to air under

different conditions is helpful and the motivation for this study.

However, the results are more general than this. There is also a

need to understand how detrimental short-time exposure to air

is to the interiors of a lithium-ion battery, especially for the most

reactive component of the battery – the lithiated graphite.

The lithiated graphite SEI contains a range of different compo-

nents that could react with air on exposure. The LiPF6 salt may

hydrolyze and form toxic HF gas. HF has been proposed to convert

commonly suggested SEI species like lithium carbonate, lithium

alkyl carbonate, lithium oxide and lithium alkoxide into lithium

ﬂuoride [8], and also to attack ether linkages to form ﬂuorinated

organic species [2]. POF3 gas formed during such hydrolysis reac-

tions has been suggested to further hydrolyze to form solid OPF2OH

and OPF(OH)2 species [9]. However, the kinetics of LiPF6 hydrolysis

is reported to be relatively slow; the reaction can take days to equili-

brate in a mixed carbonate solution at ambient temperature [9–13].

Water and oxygen contamination in ethylene carbonate (EC) and

diethylene carbonate (DEC) electrolyte solvents subjected to low Fig. 1. Potato-shaped graphite from Toyo Tanso.

potentials has been found to be associated with lithium hydrox-

ide and lithium oxide formation in the DEC based electrolyte, and

The different probing depths of these measurements (∼2 and

with conversion of lithium alkyl carbonate into lithium carbonate

∼10 nm, respectively) enabled comparison of how air exposure

in the EC based electrolyte [14]. Also several different SEI compo-

affects the outer and the inner parts of the SEI. Furthermore, the

nents formed during cell cycling could react with air. For example,

consequences of air exposure on samples subject to different

spontaneous combustion of lithium alkoxides reacting with oxy-

pre-treatments (washed/unwashed and exposed before or after

gen gas has been proposed [2]. Lithium oxide conversion to lithium

vacuum treatment) were compared. Thereby, improved insight

hydroxide and/or lithium carbonate has also been suggested [15].

into how different electrode/electrolyte interface components

Finally, the lithium in the lithiated graphite surface is also highly

react on exposure to air could be obtained.

reactive on exposure to air.

X-ray photoelectron spectroscopy (XPS) is a surface sensi-

2. Experimental

tive technique with a probing depth (∼10 nm) suitable for the

characterization of Li-ion battery SEI layers. Previously, XPS

2.1. Sample preparation

measurements have been performed on air-exposed and cycled

delithiated graphite electrodes from cells with LiPF6-containing

Graphite/LiFePO pouch cell (polymer coated aluminium) bat-

electrolytes that had been subjected to air for 3.5 h [1]. These studies 4

teries were prepared inside an argon ﬁlled glovebox (≤1 ppm H O)

showed an increased oxygen signal, an increased P–O compound 2

as described in detail elsewhere [6]. Anodes (85 wt% graphite,

contribution and an increased graphite signal after exposure to

3 wt% KS6, 2 wt% Super P amorphous carbon and 10 wt% binder

air. The latter effect was interpreted as formation of volatile com-

(Kynarﬂex)) and cathodes (75 wt% carbon coated LiFePO , 10 wt%

pounds during exposure consuming the SEI layer and making it 4

carbon black and 15 wt% binder (Kynarﬂex 2801-00)) were

thinner. Some binding energy shifts were observed in the oxy-

matched to obtain a graphite overcapacity of 20%. 1 M LiPF (Ferro)

gen spectrum, which were interpreted as formation of lithium 6

in EC:DEC mixed in volume ratio 2:1 (Novolyte technologies) and

carbonate upon exposure. Considering the increased reactivity

Solupor separator were used. Cells were galvanostatically cycled

of lithiated compared to delithiated graphite, and the possible

between 2.7 and 4.2 V at C/10 for 3.5 cycles.

kinetic limitations of different processes associated with expo-

It is known that the SEI is inﬂuenced by a number of different

sure to air, it is also interesting to study the consequences of

properties of the graphite active material [7]. The graphite particles

exposing lithiated graphite samples to air for shorter periods of

selected here have smooth surface and the particle size is about

time more similar to practical air exposure times during, e.g.,

20 ␮m, as illustrated in Fig. 1. The anode ﬁlm thickness was in the

transfer of samples between different lab equipments. Shorter

range 25–40 mm. The BET surface area was 3.2 m /g, with majority

air exposure time (10 min) has previously been used in a study

of pores above 50 nm pore width.

of washed and soft-baked lithiated silicon SEI layers [2]. The

Batteries were opened inside an argon ﬁlled glovebox (≤1 ppm

exposed silicon samples showed decreased carbon and phospho-

H O). The battery anode was cut in four pieces subject to four dif-

rous intensities, while the lithium signal increased. It was shown 2

ferent combinations of sample pre-treatments (washed/unwashed

that the changes in SEI composition, seen on exposure to air, were

and exposed to air before or after vacuum treatment (uAV, uVAV,

highly dependent on the electrochemical histories of the sam-

wAV, and wVAV)), see schematic representation in Fig. 2. Washed

ples, which were prepared through, e.g., cyclic voltammetry and

chronoamperometry. samples were obtained by letting 1 ml of DMC (>99.9%) run over the

sample, leaving the sample to dry and then repeating the process a

In the present work, consequences of short-time air exposure

second time. Samples exposed to air before insertion into vacuum

(in this case 4 min, estimated to be in the order of time for sample

(uAV and wAV) were measured ﬁrst to decrease solvent evapo-

transfers between different instrumentations used for investi-

ration prior to air exposure. Meanwhile, the electrode pieces to be

gations of such battery surfaces) on the surface composition of

vacuum treated prior to air exposure (uVAV and wVAV) were stored

one of the most reactive components of a lithium-ion battery,

for a few hours inside the glovebox, still within the opened battery

the lithiated graphite, were examined using photoelectron spec-

and with the separator remaining on the sample. Vacuum treat-

troscopy. The examined cells were prepared through galvanostatic

−8

ment was performed through storage in high vacuum (∼10 mbar)

cycling at C/10. Both synchrotron based soft X-ray photoelectron

while reference measurements on the sample in unexposed state

spectroscopy (SOXPES) and more bulk sensitive in-house X-ray

were performed. The air exposure lasted for 4 min.

photoelectron spectroscopy (XPS) measurements were performed.

S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91 85

Fig. 2. The four different combinations of sample pre-treatments selected for the air exposure study. One electrode was cut into four pieces where two pieces were washed

and two pieces left unwashed. The electrodes were then exposed to air either after vacuum treatment (during which a reference spectrum of the sample in unexposed state

was recorded) or before insertion into vacuum. The samples are in the text referred to as uRef, uVAV, uAV, wRef, wVAV, and wAV, according to the ﬁgure.

2.2. Transfer from glovebox to spectrometer – a new transfer different instruments without having to ﬁnd a new sample transfer

system solution for each instrument, versatile equipment for sample trans-

fer is desirable. In the present work, we have developed a ﬂexible

Samples to be exposed before insertion into vacuum, uAV and transfer system and used it for transfer of samples to the SOXPES

wAV, were transferred through air (1–25 mbar partial pressure of system. The transfer system can be used for moving samples from

H2O) from the glovebox to the XPS/SOXPES instrument. Samples an argon ﬁlled glovebox to any vacuum instrument via a C35 ﬂange

to be exposed to air after vacuum treatment, uVAV and wVAV, without exposing the samples to air. It is also ﬂexible in the sense

were transferred from the argon ﬁlled glovebox to the measure- that it either can incorporate the commonly used omicron sam-

ment system using air-tight transfer units attached directly to the ple transfer system or be used with an instrument speciﬁc sample

XPS/SOXPES instrument. holder.

To enable sample transfer from a glovebox to a spectrometer The transfer system developed in the present work is built

without exposure to air, we have previously used several different up by two parts (see Fig. 3). The ﬁrst part is an o-ring sealed

specially built or commercially available systems (e.g., the system adapter that can be plugged into the load lock of the glovebox.

used in the present work to transfer samples to XPS measure- The adapter has a C35 ﬂange, onto which the second, portable

ments). However, these systems have not been easily adaptable to part of the transfer system can be mounted. The second part is a

ﬁt many different instruments. To enable easy access to a range of portable unit and consists of a magnetic rod mounted on a small

Fig. 3. Photograph of the transfer chamber used in this study. The adapter (a) can be plugged into the load lock of a glovebox and either sealed using a ﬂange (b) or attached

to the portable unit (c–g). The portable unit consist of either an omicron fork (c) or an instrument speciﬁc sample transfer magazine (d) mounted onto a magnetic rod (g)

long enough to allow easy access to the fork or sample magazine inside the glovebox. Also a small vacuum chamber (f) with a valve (e) is attached to the magnetic rod.

86 S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91

vacuum chamber with a valve. On the magnetic rod, either a sample 3. Results and discussion

magazine or the commonly used omicron sample transfer system

can be mounted to match the existing experimental instrumen- 3.1. Synchrotron based SOXPES measurements

tation. The long magnetic rod allows easy access to the sample

magazine/omicron sample-carrying fork inside the glovebox. The Fig. 4a and b shows the results of the most surface sensitive

∼

portable unit can be attached to any C35 ﬂange, and can thus apart air exposure experiments ( 2 nm probing depth) on unwashed

from being mounted onto the adapter also be attached to the load (abbreviated uRef, uAV, and uVAV, see Fig. 2), and washed (abbrevi-

lock of a range of different experimental instruments. The versatile ated wRef, wAV, and wVAV, see Fig. 2), lithiated graphite electrodes,

system thus provides easy access to a range of different experimen- respectively. For unwashed samples, the ﬁgure shows both spec-

tal techniques without exposing samples to air during the transfer tra of electrodes exposed after vacuum treatment (uVAV) (red) and

from the glovebox to the measurement equipment. of electrodes exposed before insertion into vacuum (uAV) (light

green). The results are compared to spectra of unexposed unwashed

(uRef) (Fig. 4a) and washed (wRef) (Fig. 4b) reference electrodes

2.3. Photoelectron spectroscopy

(black, dotted), which were recorded during the vacuum treat-

ment of the uVAV and wVAV samples, respectively. In general,

Soft X-ray photoelectron spectroscopy (SOXPES) was performed

only smaller differences were observed between unexposed and

at the I411 beamline [16] at the MAX IV laboratory (Lund, Sweden).

air-exposed sample spectra. Therefore, these spectra were plotted

The excitation energies (P2p: 280 eV, C1s: 430 eV, O1s: 680 eV, F1s:

on top of each other to better illustrate the changes seen upon expo-

835 eV) were chosen so that the kinetic energies of emitted elec-

sure to air. The core level spectra were normalized with respect to

trons were ∼140 eV. Since the probing depth depends on the kinetic

the background.

energy of emitted electrons, the probing depths of the different core

The most important differences between the unexposed refer-

level measurements were thus very similar. The emission angle was

◦

60 . ence samples and the exposed samples can be found in the O1s and

C1s spectra of unwashed samples exposed to air before insertion

More depth sensitive X-ray photoelectron spectroscopy (XPS)

into vacuum (uAV) shown in Fig. 4a. In these spectra, much more

measurements utilizing 1486.7 eV AlK␣ radiation were performed

pronounced 531.8 and 532.9 eV O1s features as well as 286.2 and

at an in house Perkin Elmer PHI 5500 system. The emission angle

◦

289.8 eV C1s features are observed. These features are attributed

was 45 . The probing depth of the SOXPES and XPS measurements

to ether and carbonate environments. Since no similar changes are

is estimated to ∼2 and ∼10 nm, respectively, deﬁning the probing

observed on exposure of vacuum treated (Figs. 4a and b) (uVAV)

depth as the emission depth including 95% of the elastically emitted

and washed (Fig. 4b) samples (wVAV and wAV) these changes

electrons and approximating the electron inelastic mean free path

are attributed to reactions involving air and volatile components

(IMFP) by the IMFP in polyethylene [17].

present in the sample. On exposure, volatile species, such as, e.g.,

XPS spectra were energy scale calibrated with respect to the

electrolyte solvent residues, can react to form less volatile compo-

C–H/C–C (hydrocarbon) feature set to 284.4 eV based on results

nents, which to a higher degree remain on the surface of the sample

of comparisons of anode and cathode spectra [6]. SOXPES spec-

at the low pressures inside the SOXPES system.

tra were energy calibrated by setting the peak maxima of the most

All exposed sample spectra (Fig. 4) show some decrease in the

intense feature in the respective core level spectrum to 284.4 eV

P2p signals. Furthermore, a more pronounced lower binding energy

(C1s), 687 eV (F1s), 531.8 eV (O1s) and 136.5 eV (P2p). These bind-

F1s component, attributed to LiF, with respect to the higher bind-

ing energies were obtained from previous results on similarly

ing energy component, assigned to P–F containing compounds, is

prepared unexposed, unwashed samples energy calibrated with

observed. These changes may be attributed to LiPF decomposition

respect to the C–H/C–C feature set to 284.4 eV [6]. In the present 6

to form P–F containing gas (e.g., PF , POF , or both) and LiF. More-

work, the number of measurement scans was kept to a minimum 5 3

over, a slightly more pronounced lower binding energy feature in

in order to reduce possible radiation damage.

the P2p spectra was generally also seen in air exposed sample mea-

Curve ﬁts were performed using 70% Gaussian 30% Lorentzian

surements. This feature is assigned to P–O containing compounds,

Voigt peak shapes. The lithiated graphite peak was also convoluted

which have been suggested to form on, e.g., LiPF hydrolysis [9].

with an asymmetric Gelius peak with peak shape parame- 6

However, the amounts are not large and one reason could be the

ters in agreement with the CASA reference library [22], ﬁtting

slow kinetics reported for the LiPF hydrolysis in carbonate solu-

well to XPS spectra of pure graphite powder acquired in our 6

lab. tions at ambient temperature [9–13].

In general, the fact that only smaller changes are seen in the

XPS spectra were normalized to show the relative amounts of

outermost region of the SEI on exposing unwashed and washed

the different elements, i.e., spectra were divided by spectrum area

samples after vacuum treatment (uVAV and wVAV) indicate that

and multiplied by the detected relative amount of the correspond-

volatile species play an important role in the chemical reactions

ing element. The relative amounts are here deﬁned as the relative

occurring in the outermost SEI during exposure to air.

Scoﬁeld cross section [18], , and polyethylene mean free path [17],

, normalized signals, Aj, from the different elements in the sample,

3.2. XPS measurements

as described by Eq. (1). A

/( · )

j j j The more surface sensitive SOXPES measurements were com-

cj = (1)

plemented by more bulk sensitive XPS measurements, probing not

Ai/(i · i)

only the outermost SEI but also the SEI close to the surface of the

lithiated graphite and in some cases even in the active material

∼

No corrections were made for differences in transmission func- itself. The probing depth was here 10 nm.

tion in the analyzer, as the inﬂuence of differences in transmission XPS core level spectra of air-exposed unwashed and washed

are negligible at the energies used in the present investigation. lithiated graphite interface layers are shown in Figs. 5 and 6, respec-

No relative amounts of the different elements could be calcu- tively. Reference spectra of both unwashed and washed unexposed

lated from SOXPES spectra due to variations in the X-ray ﬂux. samples (uRef and wRef) are also included in the ﬁgures. To better

Instead, these spectra were normalized with respect to the back- illustrate the changes seen after exposure, deconvoluted spectra are

ground. shown. The intensities of the core level spectra show the relative

S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91 87

CO3 P-F F1s P-F/binder O1s C-O C1sC-H/C-C P2p

C-O/binder

LiF CO3/binder P-O/P=O Background normalized counts

692 687 682 537 532 527 292 287 282 142 137 132 Binding Energy / eV Binding Energy / eV Binding Energy / eV Binding Energy / eV F1s O1s C1sC-H/C-C P2p C-O CO3 P-F binder / P-F C-O/binder

WashedLiF Unwashed CO3/binder P-O/P=O

Background normalized counts 692 687 682 537 532 527 292 287 282 142 137 132

Binding Energy / eV Binding Energy / eV Binding Energy / eV Binding Energy / eV

Fig. 4. SOXPES spectra (∼2 nm probing depth) showing the effect of 4-min-long air exposure of unwashed (top) and washed (bottom) lithiated graphite. The unexposed

reference spectra (black, dotted) and the spectra of samples exposed after vacuum treatment (red line) are plotted on top of each other. In the unwashed sample case, also

spectra of samples exposed before insertion into vacuum (light green line) are included. Spectra were normalized vs. the background. (For interpretation of the references

to color in the text and legend, the reader is referred to the web version of this article.)

a) F1s O1s C1s P2p P-F/binder C-H/C-C P-F

C-O binder 2 10 / C-O

5 CO3 5 CO binder LiF 3/ lithiated P-O/P=O ROLi graphite

0 0 0 0 b) LiOH 2 10 5 5 Intensity

0 0 0 0

2 10 5 5

0 0 0 0

692 687 682 536 531 526 292 287 282 140 135 130

Binding Energy / eV Binding Energy / eV Binding Energy / eV Binding Energy / eV

Fig. 5. XPS spectra of unwashed lithiated graphite (a) unexposed, (b) exposed after vacuum treatment and (c) exposed before insertion into vacuum. The spectra were

normalized to show the relative amounts of the different elements, as described in the experimental section.

88 S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91

a) P-F/binder O1s C-H/C-C P-F F1s C1s P2p P-O/P=O C-O/binder CO3 CO3/binder 5 0.2 C-O ROLi 5 5 LiF lithiated graphite

Li2O 0

0 0 0 b) LiOH

5 0.2 5 Intensity

0 0 0 0 c)

5 5 0.2

0 0 0

692 687 682 536 531 526 292 287 282 140 135 130

Binding Energy / eV Binding Energy / eV Binding Energy / eV Binding Energy / eV

Fig. 6. XPS spectra of washed lithiated graphite (a) unexposed, (b) exposed after vacuum treatment and (c) exposed before insertion into vacuum. The spectra were normalized

to show the relative amounts of the different elements, as described in the experimental section.

amounts of the different elements, as described in the experimen- dominate in samples pre-treated in different ways. Unwashed

tal section. The relative amounts of the different elements are also samples exposed before insertion into vacuum (uAV) (Fig. 5c) show

summarized in Table 1. more pronounced C1s and O1s features attributed to carbonate

After exposure to air, an increased amount of material appears and ether environments. Similar effects were seen in the SOXPES

to cover the graphite active material, since the lithiated graphite measurements (Fig. 4), and could as described in Section 3.1 be

∼

( 282 eV) signal is almost fully attenuated in the exposed sample attributed to volatile components forming less volatile species

spectra (see Figs. 5 and 6). Previous longer term exposure studies on when reacting with air. The relative increase is similar in SOXPES

delithiated graphite electrodes have shown the opposite effect with and XPS measurement results, indicating a uniform distribution of

a more pronounced graphite contribution to C1s spectra after expo- these reaction products within the SEI. This would be in agreement

sure to air [19]. The differences in results might be associated with with assuming that volatile components residing within the pores

the different degrees of lithiation of the graphite in the two exper- of the unexposed SEI on exposure to air forms less volatile species,

iments, since delithiated graphite is expected to be less reactive which remain in pores throughout the SEI. After exposure of

than lithiated graphite. samples pre-treated in other ways (uVAV, wVAV, and wAV) these

The relative amount of oxygen in the anode/electrolyte interface reaction products are not observed. This can be related to the vapor

increased after exposure of both unwashed (Fig. 5) and washed pressures of DEC and EC, which both are at least ﬁve orders of mag-

−7

(Fig. 6) samples to air. The increased oxygen content is associated nitude larger than the base pressure of the instrument (<10 mbar)

with two different types of spectral changes. Different effects [20,21], indicating that these components can be largely removed

during vacuum treatment. However, in spectra of samples exposed

after vacuum treatment and/or washing (uVAV, wVAV, and wAV) a

Table 1 new component appears at 530.8 eV in the O1s spectra. This feature

Relative amounts of different elements according to XPS measurements.

is attributed to lithium hydroxide. No 530.8 eV feature was seen

in the more surface sensitive measurement results, hence lithium

% C (%) F (%) Li (%) O (%) P (%)

hydroxide appears to form only in the deeper parts of the SEI. Fur-

Unwashed

thermore, the lithium hydroxide feature is more intense in exposed

Unexposed 29 33 23 8 7

Vacuum treated prior to exposure 29 25 24 16 5 washed samples (wVAV and wAV) than in exposed unwashed

Exposed 38 18 18 24 2 samples (uVAV and uAV) and also more intense in spectra of

Washed samples exposed after vacuum treatment (uVAV and wVAV) than

Unexposed 42 20 17 20 1 in samples exposed before insertion into vacuum (uAV and wAV).

Vacuum treated, exposed 32 15 30 23 0.9

That is, both unwashed and washed samples show more lithium

Exposed 35 11 29 24 0.4

hydroxide in samples where some SEI material was removed

S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91 89

Fig. 7. Schematic pictures of the outermost surface layer of lithiated graphite electrodes subject to different pre-treatments (see Fig. 1), and subsequently exposed to air. The

images were in general based on SOXPES and XPS measurements, however such measurements require vacuum and the images with dashed border were therefore deduced

from results on samples treated in other ways. The two most dramatic changes upon air exposure involved (1) volatile species (purple) forming less volatile components (red)

and (2) lithium hydroxide (green) formation at the graphite surface through partial delithiation of the surface region of the lithiated graphite electrode. (For interpretation

of the references to color in the text and legend, the reader is referred to the web version of this article.)

during the sample pre-treatment prior to exposure. Both possible O1s spectra. However, this goes also for unwashed samples exposed

differences in the amount of lithium hydroxide in the samples and prior to vacuum treatment (uAV) where no lithium hydroxide sig-

differences in the attenuation of the lithium hydroxide signal asso- nal was detected. Furthermore, considering the much less intense

ciated with the varying amount of electrolyte material deposited lithium alkoxide feature (530.3 eV) in O1s spectra of the unexposed

in the outer parts of the SEI could contribute to these variations. samples (Figs. 5a and 6a) compared to the lithium hydroxide fea-

In previous air exposure studies, lithium hydroxide has been ture (530.8 eV) in the exposed sample spectra (Figs. 5b, 6b and c),

proposed to form through, e.g., reactions between lithium alkoxide lithium alkoxide conversion is unlikely to give a major contribution

and oxygen gas [2]. to lithium hydroxide formation in these samples on exposure to air.

Lithium hydroxide could also form in a reaction between the

RCHOLi + O2 → CO2 + LiOH + R

lithiated graphite and water vapor in the air:

Indeed, in exposed sample spectra, unlike unexposed sample

−

+ + → +

spectra, no alkoxide feature (530.3 eV) is seen in the deconvoluted 2Li 2e 2H2O 2LiOH H2

90 S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91

In the samples where lithium hydroxide was detected after described the presence of depth gradients in the SEI [6]. These are

exposure to air, also an increased relative amount of lithium not shown here. Instead, the focus is on the major observed changes

was seen after exposure (see Table 1), which is in agreement in the SEI seen on exposure to air. The observed major changes on

with delithiation of the graphite during air exposure. Further- air exposure were attributed to:

more, the increased overall amount of material deposited onto

•

the active material after exposure to air indicates that air com- Reactions involving volatile species in unwashed, non-vacuum

ponents react with the sample to form species deposited at the treated samples forming less volatile carbonate and ether con-

electrode/electrolyte interface during exposure. That is, both these taining compounds on reaction with air. The relative changes

observations are agreement with lithium hydroxide formation in the carbonate and ether intensities in the C1s spectrum

involving partial delithiation of the surface region of the lithiated were similar in measurements with different depth sensitivity.

graphite electrode on exposure to air. Therefore, it is proposed that volatile species also in SEI pores

Also lithium oxide has been suggested to form when lithiated react to form less volatile species largely remaining in the pores

graphite reacts with air. However, none of the spectra of the also after insertion into vacuum. In Fig. 7, this effect is indicated

exposed samples indicate lithium oxide formation upon exposure. by a difference in the colour of electrolyte rests and products

On the contrary, the O1s feature at 527.3 eV attributed to lithium deposited in the SEI pores.

•

oxide disappears after exposure to air. The absence of lithium Lithium hydroxide formation at the graphite surface through

oxide could, however, be attributed to the thicker interface layer partial delithiation of the surface region of the lithiated graphite

on the exposed samples. Depth sensitive hard X-ray photoelectron electrode on reaction with water. The lithium hydroxide con-

spectroscopy (HAXPES) studies with a probing depth larger than tribution was only visible in washed and/or vacuum treated

10 nm have shown that lithium oxide resides close to the active samples. This can be attributed to removal of material from the

material [6]. An increased amount of material deposited in the SEI pores and/or differences in the amounts of lithium hydroxide

interface layer after exposure may thus fully attenuate the lithium in the different samples. In the case of unwashed samples

oxide signal. This is supported by the almost complete attenuation exposed to air before insertion into vacuum (uAV) no lithium

of the graphite signal. However, it is also possible that lithium hydroxide signal was seen. This may be due to full attenuation

oxide on exposure converts to lithium hydroxide and/or lithium of the signal from a possible lithium hydroxide layer related to

carbonate [15]. the larger amount of material in the SEI pores, and therefore the

Unlike the more surface sensitive SOXPES results, the XPS spec- presence of lithium hydroxide in this sample remains uncertain.

tra show no increase in the lower binding energy P2p feature

Finally, also effects that could be interpreted as decomposition

attributed to P–O containing compounds. However, less intense F1s

of LiPF rests and/or products on exposure to air were observed

and P2p signals are in many cases seen in air exposed samples com- 6

in washed and exposed sample spectra. However, the results in

pared to unexposed samples. This is especially true in the case of

general show that other effects dominate the changes seen in XPS

the unwashed sample exposed to air before insertion into vacuum

measurements of samples exposed to air.

(uAV) however in this case the decreased ﬂuorine and phospho-

rous content can be largely attributed to the increased amount of

4. Conclusions

products formed from volatile species in the SEI. Some loss of phos-

phorous and ﬂuorine due to decomposition of LiPF6 may occur,

The present study shows that short exposure to air inﬂuences

however the results in general show that other effects dominate

the composition of the lithiated graphite SEI. It also shows that the

the changes seen in XPS measurements of samples exposed to air.

sample pre-treatment inﬂuence the SEI’s sensitivity to exposure

to air.

Volatile components, e.g., electrolyte solvents, appears to react

3.3. A schematic picture of the air exposed graphite surface

on exposure to air and form less volatile compounds deposited on

the sample surfaces.

Fig. 7 illustrates a summary of our interpretation of the above

After removal of some volatile components through vacuum

presented SOXPES and XPS results, and shows the major conse-

treatment and/or washing, the air sensitivity of the lithiated

quences of air exposure observed in samples pretreated according

graphite is more pronounced than the air sensitivity of the SEI. The

to the four different sample preparation procedures used in the

lithiated graphite is suggested to react with water to form lithium

present study (illustrated in Fig. 2).

hydroxide in the deeper part of the SEI, and during this process

The ﬁgures were in general based on the spectra in Figs. 4–6,

partial delithiation occur.

however the images surrounded by a dashed border could not be

Changes that could be attributed to LiPF6 decomposition were

directly based on photoelectron spectroscopy results as such mea-

observed after exposure to air. However, these effects were less

surements require high vacuum. Instead, the images with dashed

pronounced than the changes that could be attributed to reactions

border were deduced from the observed differences between mea-

involving solvent species and the lithiated graphite.

surements on samples treated in other ways. In the unwashed case,

samples exposed after vacuum treatment, (uVAV) reacted very dif-

Acknowledgements

ferently from samples exposed before vacuum treatment (uAV).

This is attributed to removal of volatile species from the SEI pores,

The authors would like to thank Torbjörn Gustafsson for valu-

and is in Fig. 7 illustrated as a difference in composition (different

able discussions. We would also like to thank Bertrand Philippe

colours in the ﬁgure) and smaller amount of material in the SEI

for experimental assistance during the study. FFI and the VIN-

pores in the vacuum treated compared to the non-vacuum treated

NOVA Green-car project as well as StandUp for Energy, the

unexposed, unwashed sample spectrum. In the washed sample

Swedish Hybrid Vehicle Centre, and the Swedish Energy Agency

case, samples exposed to air before and after vacuum treatment

are acknowledged for ﬁnancial support.

(wVAV and wAV) were similar. It was therefore assumed that they

were similar also prior to air exposure.

References

The SEI is in the ﬁgure illustrated as a bulk SEI, shown in grey,

with a porous structure where some electrolyte rests remain in the

[1] M. Herstedt, D.P. Abraham, J.B. Kerr, K. Edström, X-ray photoelectron

pores (not shown to scale). Previous studies have focused on and spectroscopy of negative electrodes from high-power lithium-ion cells

S. Malmgren et al. / Electrochimica Acta 105 (2013) 83–91 91

showing various levels of power fade, Electrochimica Acta 49 (2004) [12] T. Kawamura, S. Okada, J. Yamaki, Decomposition reaction of LiPF6-based elec-

5097. trolytes for lithium ion cells, Journal of Power Sources 156 (2006) 547.

[2] K.W. Schroder, H. Celio, L.J. Webb, K.J. Stevenson, Examining solid electrolyte [13] U. Heider, R. Oesten, M. Jungnitz, Challenge in manufacturing electrolyte solu-

interphase formation on crystalline silicon electrodes: inﬂuence of electro- tions for lithium and lithium ion batteries quality control and minimizing

chemical preparation and ambient exposure conditions, Journal of Physical contamination level, Journal of Power Sources 81–82 (1999) 119.

Chemistry C 116 (2012) 19737. [14] D. Aurbach, A. Zaban, Y. Gofer, Y.E. Ely, I. Weissman, O. Chusid, O. Abramson,

[3] E. Peled, The electrochemical behavior of alkali and alkaline earth metals in Recent studies of the lithium–liquid electrolyte interface Electrochemical, mor-

nonaqueous battery systems – the solid electrolyte interphase model, Journal phological and spectral studies of a few important systems, Journal of Power

of the Electrochemical Society 126 (1979) 2047. Sources 54 (1995) 76.

[4] R. Fong, U. von Sacken, J.R. Dahn, Studies of lithium intercalation into carbons [15] D. Aurbach, A. Zaban, Impedance spectroscopy of lithium electrodes: Part 1.

using nonaqueous electrochemical cells, Journal of the Electrochemical Society General behavior in propylene carbonate solutions and the correlation to sur-

137 (1990) 2009. face chemistry and cycling efﬁciency, Journal of Electroanalytical Chemistry

[5] J. Vetter, P. Novák, M.R. Wagner, C. Veit, K.-C. Möller, J.O. Besenhard, M. Win- 348 (1993) 155.

ter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, Ageing mechanisms in [16] M. Bässler, A. Ausmees, M. Jurvansuu, R. Feifel, J.-O. Forsell, P. de Tarso Fon-

lithium-ion batteries, Journal of Power Sources 147 (2005) 269. seca, A. Kivimäki, S. Sundin, S.L. Sorensen, R. Nyholm, O. Björneholm, S.

[6] S. Malmgren, K. Ciosek, T. Gustafsson, M. Gorgoi, M. Hahlin, H. Rensmo, K. Aksela, S. Svensson, Beam line I411 at MAX II – performance and ﬁrst results,

Edström, Comparing anode and cathode electrode/electrolyte interface compo- Nuclear Instruments and Methods in Physics Research Section A 469 (2001)

sition and morphology using soft and hard X-ray photoelectron spectroscopy, 382.

Electrochimica Acta 97 (2013) 23. [17] L.R. Painter, E.T. Arakawa, M.W. Williams, J.C. Ashley, Optical properties

[7] P. Verma, P. Maire, P. Novák, A review of the features and analyses of the of polyethylene: measurement and applications, Radiation Research 83

solid electrolyte interphase in Li-ion batteries, Electrochimica Acta 55 (2010) (1980) 1.

6332. [18] J.H. Scoﬁeld, Theoretical Photoionization Cross Sections from 1 to 1500 keV,

[8] K. Kanamura, S. Shiraishi, Z. Takehara, Electrochemical deposition of very Lawrence Livermore Report UCRL-51326, 1973, http://dx.doi.org/10.2172/

smooth lithium using nonaqueous electrolytes containing HF, Journal of the 4545040.

Electrochemical Society 143 (1996) 2187. [19] Sigma–Aldrich, http://www.sigmaaldrich.com/catalog/product/aldrich/

[9] S.F. Lux, I.T. Lucas, E. Pollak, S. Passerini, M. Winter, R. Kostecki, The mechanism d91551?lang=en®ion=SE (02.02.13.).

of HF formation in LiPF6 based organic carbonate electrolytes, Electrochemistry [20] Sigma–Aldrich, http://www.sigmaaldrich.com/catalog/product/aldrich/

Communications 14 (2012) 47. e26258?lang=en®ion=SE (02.02.13.).

[10] W. Li, B.L. Lucht, Inhibition of the detrimental effects of water impurities in [21] http://www.sigmaaldrich.com/catalog/product/aldrich/d91551?lang=en

lithium-ion batteries, Electrochemical and Solid-State Letters 10 (2007) A115. ®ion=SE (13-05-13).

[11] A.V. Plakhotnyk, L. Ernst, R. Schmutzler, Hydrolysis in the system LiPF6- [22] M.C. Biesinger, X-ray photoelectron Spectroscopy Reference Pages,

propylene carbonate-dimethyl carbonate-H2O, Journal of Fluorine Chemistry http://xpsﬁtting.blogspot.com/2008/12graphite.html (2012-12-09).

126 (2005) 27.