Impedance spectroscopic study for the initiation of passive ®lm on
carbon electrodes in lithium ion batteries
C.R. YANG, J.Y. SONG, Y.Y. WANG* and C.C. WAN
Department of Chemical Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan
(*author for correspondence, e-mail: yywang@faculty.nthu.edu.tw)
Received 14 May 1997; accepted in revised form 9 December 1998
Key words: a.c.-impedance, lithium-ion battery, organic electrolyte, passive ®lm
Abstract
The formation of passive ®lm at the interface between the mesocarbon microbeads (MCMB) electrode and the
organic electrolyte in a lithium-ion battery during the initial period of intercalation was investigated by
a.c.-impedance spectroscopy. An equivalent-circuit model consisting of ®ve parallel RC-circuits in series
combination was adopted for the curve-®tting analysis of the obtained impedance spectra. The results indicated
that both the total interfacial resistance and the passive ®lm thickness increased with decreasing intercalation
potential in the ethylene carbonate (EC) or dimethyl carbonate (DMC) single-solvent system, whereas an opposite
trend was observed in the system containing diethyl carbonate (DEC) only. In addition, the total interfacial
resistance was clearly affected by the porous structure of the passive ®lm in a single-solvent system. In binary solvent
systems such as EC/DEC and EC/DMC, on the other hand, the effect of the porous structure on the total interfacial
resistance was negligible. The total interfacial resistance and the passive ®lm thickness were also smaller in these
systems than those in single-solvent systems. Finally, the variation of the total interfacial resistance and of the
passive ®lm thickness in the EC/DEC (or EC/DMC) system were also found to be similar to those in the parent
DEC (or DMC) system during intercalation.
1. Introduction
Carbonaceous materials are widely employed as the
anodes in lithium-ion batteries. Much effort has been
devoted to the improvement of the reversible perfor-
mance of these insertion materials [1±7]. Although the
structure of carbon itself plays a vital role in the lithium
intercalation/deintercalation process, the passive ®lm, or
the solid electrolyte interphase (SEI), formed on the
surface of the carbon electrode is equally important [8±
13]. The signi®cance of the passive ®lm in primary and
secondary batteries cannot be overestimated. It deter-
mines the safety, power capability, shelf life and cycle
life of a battery. The morphology (compact or porous),
thickness, ionic transference number and conductivity of
the passive ®lms are some of the most critical factors
affecting the performance of carbon electrodes.
When a carbon electrode is polarized to low poten-
tials, electrolyte species will undergo reduction at
potentials higher than that of the lithium insertion
process. Passive ®lms will consequently precipitate on
the carbon surface in a way resembling that of a lithium
electrode. If these passive ®lms are fully developed and
form compact and ion-conductive layers which com-
pletely isolate the carbon active materials from the
electrolyte solution before it attains the insertion
potential, the electrode can be, at least kinetically,
stabilized.
Impedance spectroscopy has been routinely utilized to
study the passive ®lms formed on the surface of an
electrode. Aurbach and Zaban [14, 15] used this
technique to investigate the SEI formation on lithium
anodes in contact with various organic electrolytes.
They suggested an SEI structure which consists of ®ve
different consecutive layers for re¯ecting the variations
in composition and morphology of the multilayered
SEI, and delineated this model by a series of four to ®ve
parallel RC circuits representing the capacitance and
resistance of each layer. The thickness of each layer of
the SEI could consequently be calculated with some
assumptions introduced. Besenhard [10] used a simpler
equivalent-circuit model to calculate the passive ®lm
resistance. Takami [16] also employed a similar model to
estimate the diffusion coef®cient of lithium ion h
Li
within the carbon electrode.
Most of the studies mentioned above were performed
in a single solvent system. A comparative study of the
passive ®lm in electrolyte systems using ethylene car-
bonate (EC), diethyl carbonate (DEC), or dimethyl
carbonate (DMC) separately, or in combination as the
solvent(s) is still rarely found. Furthermore, most
impedance studies were focused on the characteristics
Journal of Applied Electrochemistry 30: 29±34, 2000.
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2000 Kluwer Academic Publishers. Printed in the Netherlands.