A498
Journal of The Electrochemical Society, 158 (5) A498-A503 (2011)
0
013-4651/2011/158(5)/A498/6/$28.00 VC The Electrochemical Society
Passivating Ability of Surface Film Derived from Vinylene
Carbonate on Tin Negative Electrode
*,z
Sangjin Park, Ji Heon Ryu, and Seung M. Oh
Department of Chemical and Biological Engineering, WCU Program of C2E2, College of Engineering, Seoul National
University, Seoul 151-744, Republic of Korea
The passivating ability of surface film derived from vinylene carbonate (VC) is addressed on tin (Sn) negative electrode after a
comparative study on the thickness, film growth pattern, chemical composition, and mechanical flexibility of the surface films gen-
erated from VC-free and VC-added electrolytes. The surface film derived from the former electrolyte is enriched by inorganic fluo-
rinated and carbonate species, and shows a poor passivating ability to cause a continued electrolyte decomposition, film growth
and eventual electrode failure. In contrast, organic carbon-oxygen species are dominant in the film derived from the VC-added
electrolyte. Even if this film is thinner than the other, it passivates the Sn electrode surface more effectively. As a result, the film
growth and electrode polarization are less significant. The superior passivating ability of organic-rich surface film has been
ascribed to a uniform coverage and higher mechanical flexibility.
VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3561424] All rights reserved.
Manuscript submitted October 18, 2010; revised manuscript received February 2, 2011. Published March 18, 2011.
The Li-alloying materials (Si and Sn) have been developed as an
alternative to the present graphitic carbons for lithium-ion batteries
(
ference in passivating ability and mechanical flexibility for two
surface films, the chemical compositions are analyzed by using
depth-profiling x-ray photoelectron spectroscopy (XPS) and Fourier-
transform infrared spectroscopy (FT-IR), whereas the film growth
pattern and thickness are estimated from the cross-sectional scanning
electron microscope (SEM) images of the cycled electrodes.
1
LIBs). The practical use of these high-capacity negative electro-
–4
des is, however, still limited due to their poor cycling behavior,
which may be caused by: (i) cracking/crumbling of the active mate-
rial (Si and Sn), (ii) electrical contact loss by a massive volume
change during alloying/de-alloying, and (iii) continuous electrolyte
decomposition and surface film formation, named solid electrolyte
5
interphase (SEI).
Experimental
The electrolyte decomposition and concomitant film deposition are
inevitable in the present LIBs since they operate far beyond the ther-
modynamic stability window of the common electrolytes. The suc-
cessful commercialization of LIBs has, however, been possible since
this surface film passivates the electrode surface, once it forms, to pre-
vent additional electrolyte decomposition. In order for surface film to
accomplish its own passivating role, the SEI layer should thus be com-
pact and perfectly cover the electrode surface to prevent additional
electrolyte decomposition. In addition, SEI layer should be thin to
The Sn film was electro-deposited on a piece of copper foil in a plat-
ing bath composed of 0.14 M tin (II) sulfate (SnSO ), 0.34 M potassium
4
sulfate (K
2 4 6 11 7
SO ), and 0.42 M sodium d-gluconate (C H O Na). The
–2
electro-deposition was performed at a current density of 5 mA cm for
8
min at room temperature.
Electrochemical performances of the Sn film negative electrodes
were examined using a coin-type cell (2032-type), which was fabri-
cated by inserting a polypropylene separator (20 lm) between the Sn
film electrode and lithium foil (Cyprus Co.). The used electrolyte was
þ
allow a fast Li ion movement. The mechanical flexibility of surface
6
1.0 M lithium hexafluorophosphate (LiPF ) dissolved in ethylene car-
film seems to be another important requirement particularly for the Li-
alloying materials. Namely, inflexible surface film cannot effectively
passivate the electrode surface since it is easily broken or deformed by
the massive volume change evolved with the Li-alloying materials,
leading to an exposure of electrode surface to electrolyte solution.
The SEI formation mechanism on the Li-alloying materials is some-
what different to that on the carbonaceous negative electrodes. On
graphite surface, SEI layer forms within a few initial cycles, whereas
the film formation is continued on the Li-alloying materials since new
electrode surface is generated as a result of continued cracking/crum-
bling. The film growth patterns are much more complicated on the
Li-alloying materials. As a result, the SEI study on the Li-alloying mate-
bonate (EC) and dimethyl carbonate (DMC) (1:1, volume ratio), in
which 5 wt. % of VC was added. Galvanostatic charge/discharge cy-
cling was made at a current density of 400 mA g in the voltage
ꢀ
1
þ
range of 0.01–1.2 V (vs. Li/Li ).
For a post mortem field-emission SEM (Model JSM-6700F,
JEOL), XPS and FT-IR analyses, the cells were dismantled in a glove
box and the electrodes were washed with dimethyl carbonate. A her-
metic vessel was used to transfer the samples from the glove box to
the instrument chamber. The XPS data were collected in an ultra-high
vacuum multipurpose surface analysis system (Sigma probe, Thermo,
ꢀ
10
UK) that operates at a base pressure of <10
mbar. The photoelec-
6,7
trons were excited by Al Ka (1486.6 eV) radiation at a constant
power of 150 W (15 kV and 10 mA); the x-ray spot size was 400 lm.
During data acquisition, a constant-analyzer-energy mode was used at
a pass energy of 30 eV and a step of 0.1 eV. The depth-profiling XPS
rials is still limited, while those on graphite electrodes are numerous.
The prime concern in this work is to assess the passivating abil-
ity of surface film derived from vinylene carbonate (VC), which is
8
,9
one of the common SEI formers. To this end, a comparative study
is made on the surface films derived from the VC-added and VC-
free electrolyte, from which the passivating ability of VC-derived
surface film is addressed. In details, the passivating ability is
assessed by measuring the reduction current associated with the
electrolyte decomposition on an exposed Sn electrode surface. To
estimate the mechanical flexibility, a bending experiment is devised,
in which the Sn electrode is bent after surface film deposition. An
expectation here is that, if the surface film is not mechanically flexi-
bility, it is deformed by bending to generate new Sn surface, onto
which electrolyte decomposition is induced. To account for the dif-
þ
measurement was made by a continued Ar ion sputter etching and
the chemical composition was estimated by using the atomic sensitiv-
10,11
ity factor.
For the cross-sectional images, the electrode samples
þ
were crosscut by using an Ar ion beam polisher (SM-09010, JEOL)
at a constant power of 0.5 W (5 kV and 0.1 mA) under vacuum
–
4
(
<2.0 ꢁ 10 Pa). The infrared spectra were obtained using the atte-
nuated total reflection (ATR) mode (Nicolet 5700 FT-IR spectrome-
ter). The cycled electrodes were mounted on an internal reflection
crystal (Ge) facing the surface layer toward the crystal. For the bend-
ing experiment, a pouch-type cell was fabricated with the Sn film
electrode (3ꢁ 3 cm), polypropylene separator and lithium foil. In this
report, the lithiation (alloying) is expressed as discharging, whereas
the de-lithiation (de-alloying) charging on the basis of the standard
lithium-ion cell configuration.
*
z
Electrochemical Society Active Member.
E-mail: seungoh@snu.ac.kr