Journal of Electroanalytical Chemistry 531 (2002) 95 /99
www.elsevier.com/locate/jelechem
Short Communication
Dithiocarbonic anhydride (CS2)* a new additive in Li-ion battery
electrolytes
/
Yair Ein-Eli *
Covalent Associates, Incorporated, Woburn, MA 01801, USA
Received 16 May 2002; accepted 28 June 2002
Abstract
Dithiocarbonic anhydride (CS2) was evaluated as an additive to electrolytes in Li-ion rechargeable batteries. Graphite electrodes,
polarized versus Li metal, can undergo reversible intercalation /deintercalation processes in diethyl carbonate (DEC)/1 M LiPF6
electrolytes containing CS2 as an additive. It was found that the use of CS2 as a passive film formation agent on the graphite anode
surface is more powerful than the use of carbon dioxide (CO2) as an additive in DEC electrolytes. On the other hand, we found that
CS2 is less dominant, as a surface film formation agent, than SO2. Cyclic voltammetry and chronopotentiometric methods were used
in order to understand the reaction mechanism better. A methodology employing DEC is proposed to evaluate new electrolyte
formulations for Li and Li-ion batteries quickly and conveniently. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: CS2; Additive; Electrolyte; DEC; Passive film; Methodology
1. Introduction
The composition of the electrolyte solutions used in
Li-ion batteries is a key factor which influences the
overall performance of the cell. The quality of the
surface films developed on the carbon electrodes at
low potentials (below open circuit potentials) is highly
important in order to maintain a good cycle life of the
anode [1 /9]. Most solvents and salts are reduced on the
carbon anode to form passive layers, which are lithium
ion conductors, known as the solid electrolyte interphase (SEI) [10].
Previous work has demonstrated the feasibility of
using sulfur compounds, such as sulfur dioxide (SO2)
[8,9], S ,S ,-dimethyl dithiocarbonates (CH3 /S/CO /S /
CH3) [11], S2
[12,13] and ethylene sulfite (C2H4SO3)
y
[14,15] as passivating agents for the graphite anode. We
have found that SO2 can serve as an excellent surface
modification agent to produce a fully developed passive
film. Two important factors are responsible for the
* Present address: Department of Materials Engineering, TechnionIsrael Institute of Technology, Technion City, Haifa 32000, Israel.
Tel.: /972-4-829-4588; fax: /972-4-832-1978
E-mail address: [email protected] (Y. Ein-Eli).
remarkable behavior of the graphite electrodes cycled in
SO2 containing electrolytes:
(i) The reduction of SO2 takes place prior to any
reduction of the organic solvent or the anion of the
Li salt [8,9].
(ii) All the various reduction products of SO2 are
strictly inorganic, lacking any loose hydrocarboncontaining tail, and therefore complying with the
‘‘Sticky Fingers’’ model [16]. This model assumes the
existence of un-solvated organic and inorganic
lithium salts, as insoluble reduction products of the
electrolyte, near the electrode surface in a form of
electrochemical capacitor.
Thorough investigations have been conducted in the
past decade on the influence of CO2 added to organic
electrolytes on the performance of Li metal and graphite
electrodes [1 /3]. Li2CO3 was detected to be the prime
species responsible for the excellent behavior of the
graphite electrodes polarized in CO2 containing electrolytes. The excellent behavior of graphite electrodes
cycled in CO2 and SO2 containing electrolytes has
driven us to look for more excellent passivating agents.
Dithiocarbonic anhydride (CS2) was a natural prefer-
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 0 4 6 - X
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Y. Ein-Eli / Journal of Electroanalytical Chemistry 531 (2002) 95 /99
ence, since it is structurally similar to CO2, but contains
the desired sulfur atoms instead of the oxygen atoms.
Diethyl carbonate electrolytes were chosen to be a test
case for our studies, since lithium metal corrosion
cannot be inhibited, and rapid dissolution of the metal
takes place without the build-up of a stable passive film
on the lithium surface in solutions based on this solvent
[17]. This process results in a complete oxidation of the
metallic lithium forming dark brown precipitants. Consequently, graphite electrodes and lithium metal cannot
be electrochemically evaluated nor cycled in a single
solvent electrolyte based on DEC [18]. Therefore, we
believe that any surface additive agent should be tested
and evaluated in this extremely corrosive environment in
order to understand the extent of the surface modification that takes place at both the lithium metal and the
graphite anode in the presence of the new additive.
Although most Li-ion battery electrolytes contain a
binary mixture of ethylene carbonate (EC) with an open
chain alkyl carbonate [diethyl carbonate (DEC), or
dimethyl carbonate (DMC) or ethyl methyl carbonate
(EMC)], the preliminary results presented in this paper
are significant, since any proposed surface modification
agent at the graphite anode interface (or even at the
lithium metal interface) could be initially examined
under the extreme conditions of DEC solutions as
previously reported with the evaluation of sulfur dioxide
addition to organic electrolytes [8,9].
2. Experimental
Carbon electrodes, with a theoretical capacity of 1.8 /
2 mA h, were prepared from Timrex SFG44 synthetic
graphite powder (Timcal America) and tested as previously described [8,9]. The solvents used in this study
were DEC (Mitsubishi Chemicals) and dithiocarbonic
anhydrous (CS2, ]/99.95% (GC), Aldrich) dried over 4
Å molecular sieves (Fisher). CS2 was added in a
concentration of 5% (v/o) to DEC, forming a clear
binary mixture. Carbon dioxide (CO2) and sulfur
dioxide (SO2) (99.9%, Aldrich) were separately bubbled
into DEC solutions in a concentration of 5% (w/o, as
previously described [9]). LiPF6 (Hashimoto) was used
as received with a final concentrations of 1 mol l1. The
water content of the prepared electrolyte was determined with the use of a model DL18 Mettler Karl /
Fischer apparatus and was typically less than 30 ppm.
Cyclic voltammetry studies of the electrolytes were
performed using an EG&G Princeton Applied Research
potentiostat/galvanostat model 263A as previously described [8,9].
3. Results and discussion
3.1. Chemical stability of Li metal in DEC /CS2, DEC /
CO2 and DEC /SO2 electrolytes
Prior to any evaluation of any carbonaceous anode
material we recommend evaluating the chemical stability of lithium metal in the proposed electrolyte and the
compatibility of the passive films produced at the
lithium metal electrolyte interface with the electrolyte
components. Moreover, since the evaluation of the
anode and cathode materials is initially performed in
half-cells versus lithium metal, it is important to conduct
the chemical stability tests prior to any further electrochemical test. The instability of the lithium metal in a
given electrolyte system is indicative of poor stability of
the reduction products and consequently, an inability to
form a protective SEI on the lithium metal surface.
Our experimental data reveal that lithium metal is
highly stable when stored in a DEC/CS2 binary
mixture, indicating that the lithium is fully passivated
in this solution. The immersed polished Li metal was
still shiny even after a prolonged storage of several
weeks. On the contrary, Li metal immersed in DEC
electrolyte was found to be unstable, resulting in a fast
Li metal corrosion process observed with a rapid
dissolution, taking place within a few hours subsequent
to the immersion, in agreement with earlier findings [18].
Li metal immersed in DEC electrolytes containing SO2
was found to be stable also, in agreement with earlier
findings [8,9]. Fresh Li metal immersed in DEC electrolyte containing dissolved CO2 was found to be unstable
and within a few days dense dark-brown pitting started
to appear at the lithium metal surface. It is important to
note that CS2 is highly unstable at elevated temperatures
or upon a prolonged storage time with exposure to air,
and decomposes to CO2 and SO2 [19], which are
reported as excellent surface modifying agents explored
at the negative graphite electrode [1 /3,8,9].
3.2. The voltammetric behavior of DEC/CS2 binary
mixture
Cyclic voltammetry of DEC electrolyte versus a
lithium reference electrode cannot be obtained as
lithium metal is unstable in DEC based electrolytes
[18]. However, previous work indicated that the reduction peak of alkyl carbonate electrolytes is positioned at
potentials below 1.5 V (vs. Li ½ Li ), with the use of Pt
as the working electrode [9]. Fig. 1 presents the cyclic
voltammograms obtained from the binary mixture of
DEC/CS2, at a scan rate of 20 mV s 1. A Pt electrode
served as the working electrode and Li metal served
both as the counter and reference electrodes. In order to
understand the impact of the CS2 addition to DECbased electrolyte, the potential was scanned negatively
Y. Ein-Eli / Journal of Electroanalytical Chemistry 531 (2002) 95 /99
97
Fig. 2. The potential (V) /capacity (mA h g 1) profile obtained from a
graphite electrode polarized galvanostatically (j/0.1 mA cm 2,
charge rate of /C/20) against a Li metal counter electrode.
Fig. 1. Cyclic voltammograms (scan rate of 20 mV s 1) obtained by
consecutive sweeps at a Pt working electrode in DEC/1 M LiPF6
solution containing CS2 (5 v/o). First cycle (1), second cycle (2) and
third cycle (3) are represented by dashed, dotted and solid lines,
respectively.
from open circuit potential ( /3 V vs. Li ½ Li ) to 50
mV (vs. Li ½ Li ), then positively to 4 V (Li ½ Li scale)
and back to the open circuit potential. Two pronounced
reduction peaks positioned at 1.9 and 1.1 V (vs.
Li ½ Li ) are detected in the first reduction step. Based
on our earlier studies with alkyl carbonate electrolytes
and the absence of this peak from the CS2-free electrolyte voltammograms [9], we believe that the first peak
observed at 1.9 V (vs. Li ½ Li ) is attributed to the
reduction of CS2, while the peak at 1.1 V is attributed to
the reduction of DEC electrolyte components [9].
During the second scan, the peak at 1.9 V disappeared,
while the peak amplitude at 1.1 V is reduced and shifted
negatively by /100 mV. Upon consecutive cycling this
peak decreases further and is shifted negatively by
another 100 mV to 0.9 V. The negative sweeps indicate
that the Pt working electrode is not fully passivated after
the first scan, although CS2 is reduced at the first stage
of the negative sweep. The Pt electrode is passivated
only during the next negative sweep as indicated by the
reduced peak at /1 V and the negative shift in its
position.
The substitution of the oxygen atoms in the CO2
molecular structure with sulfur atoms, resulting in the
formation of CS2, is not responsible for deterioration in
the anodic stability of the electrolyte, as was observed
with the use of thio-carbonates [11]. The addition of CS2
does not limit the positive potential window of the
electrolyte, up to 4 V (Li ½ Li scale). This result along
with the characteristic behavior of CS2 as a passivating
agent points out the potential use of CS2 as a surface
modifying agent in alkyl carbonate electrolytes.
Li metal dissolved extensively, in agreement with previous work [17,18]. However, the addition of 5% CS2 to
the DEC solution changes the results dramatically.
Fig. 3 presents the electrochemical behavior of a
graphite electrode cycled (C/20) in a binary mixture of
DEC/CS2. A long irreversible potential plateau,
equivalent to x /0.33 (in Lix C6), at 1.45 V is observed,
followed by another (but smaller) potential shoulder in
the potential region of 0.6 /0.4 V, leading to the Li-ion
intercalation stages observed in the potential range of
200 /10 mV (vs. Li ½ Li ). The plateau at 1.45 V may be
attributed to the reduction of the CS2 on the graphite
substrate, while the potential shoulder at 0.6 /0.4 may be
ascribed to the reduction of the DEC electrolyte
components. The overall capacity loss caused by irreversible film formation is extremely high and is calculated to be approximately 60%. It is interesting to note
that the reduction potentials of the electrolyte components are shifted negatively by 0.5 V once the working
electrode is switched from Pt (used in the cyclic
voltammetry) to graphite (cycled galvanostatically vs.
Li metal). A possible explanation for the potential shift
observed may be the charge/discharge characteristics of
Pt alloying and de-alloying with lithium. The formation
of this alloy may explain the difference in the reduction
potentials observed.
3.3. The electrochemical behavior of graphite electrodes
cycled in DEC/CS2 solution
Fig. 2 presents the poor electrochemical behavior of a
graphite electrode, cycled versus a Li metal counter
electrode, in DEC/1 M LiPF6 electrolyte at a charge
rate of C/20. No intercalation step is observed, while the
Fig. 3. The potential /time profile obtained from cycling graphite
electrode against a Li metal counter electrode (j/0.1 mA cm 2,
charge rate of /C/20) within the potential limits of 1.5 /0.01 V (vs.
Li ½ Li ), in DEC/1 M LiPF6 solution containing CS2 (5 v/o).
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Y. Ein-Eli / Journal of Electroanalytical Chemistry 531 (2002) 95 /99
The intercalation steps observed in the potential range
of 200 /10 mV are not completed and the overall
capacity obtained in the deintercalation process is fixed
at x /0.35, in Lix C6. As can be seen in Fig. 2, the
second graphite discharge process (Li-ion intercalation)
also occurs, indicative of a repeated Li-ion intercalation
process. However, the second Li-ion intercalation process consumes 50% more capacity than the capacity
measured during the previous deintercalation. This
observation is in good agreement with the cyclic
voltammetry data (Fig. 1), indicating that the working
electrode is not fully passivated even after three consecutive cycles. A comparison of the graphite electrode
cycling behavior obtained from DEC/CS2 and DEC/
SO2 [8,9] electrolytes shows that the SO2 strongly affects
the graphite cycling behavior and high reversible
capacities for x /0.9 (in Lix C6) are obtained, while
the reversible capacity obtained from DEC/CS2 electrolyte holds for the lower value of x /0.35 (in Lix C6).
The outcome of the preliminary chemical and electrochemical evaluation and the comparison of the DEC/
CS2 electrolyte system with other related electrolyte
systems (addition of SO2 and CO2) allow us to rank the
additives in DEC solutions, from a highly efficient agent
to a poorly effective one, in the following order: SO2 /
CS2 /CO2. Lithium metal is unstable in DEC electrolytes containing CO2 and therefore, no electrochemical
evaluation of carbonaceous anode material is feasible.
However, incomplete intercalation/deintercalation of
lithium ions takes place in DEC/CS2 electrolyte, while
a complete intercalation/deintercalation of lithium ions
into graphite anode occurs in DEC/SO2 electrolyte
systems [8,9].
4. Conclusions
The preliminary results obtained and presented in this
communication reveal that CS2, which serves as a
surface passivating agent, may be regarded as an
efficient additive in DEC based electrolytes. Lithium
metal was found to be stable in DEC electrolyte
containing CS2, while a fast lithium metal corrosion
process takes place in DEC free CS2 electrolyte. Lithium
metal was also found to be unstable in DEC electrolytes
containing CO2 and therefore, no electrochemical evaluation of carbonaceous anode material is feasible in this
electrolyte. However, incomplete intercalation /deintercalation of lithium ions takes place in DEC/CS2
electrolytes, while a complete intercalation/deintercalation of lithium ions into a graphite anode takes place in
DEC/SO2 electrolyte systems.
This communication also proposes a methodology for
the evaluation of new electrolyte systems designed for
use in Li and Li-ion batteries. In order to understand the
extent and limitation of any new proposed additive, one
can initially evaluate the stability of Li metal in DEC
electrolyte containing this new additive. If indeed Li
metal is stable in the new electrolyte and no metal
corrosion occurs, the next step in the methodology
should be an extensive electrochemical evaluation of Li
metal and the carbonaceous anode materials in organic
electrolytes containing this new additive.
Additional work is needed in order to understand
fully the role of CS2 as an additive to organic electrolytes. Spectroscopic methods must be applied in order to
understand the function of CS2 as a surface modifying
agent. At this stage of the research we can postulate that
the resemblance in the molecular structure to CO2 may
indicate the formation of Li2CS3, the analogue of
Li2CO3, as illustrated in Scheme 1.
Questions regarding the optimal concentrations of
CS2, the nature of the surface chemistry developed on
the graphite, the influence of CS2 on the overall
electrolyte conductivity and cathode compatibility (the
extent of the anodic stability, especially versus a highly
oxidizing cathode material such as LiCoO2) in these
unique electrolytes will be answered in future communications.
Acknowledgements
This work was funded by the National Institutes of
Health [National Heart, Lung, and Blood Institute]
under Grant No. 2 R44 HL57679-02. The author would
like to thank Dr. V.R. Koch for stimulating and
enlightening discussions.
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Scheme 1. Proposed reduction pathway of CS2 in the presence of Li ions and a schematic representation of Li2CS3 on the graphite surface.
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