Abstract
Addition of silver hexafluorophosphate to propylene carbonate (PC)-based electrolyte may suppress the cointercalation and decomposition of PC in mesocarbon microbeads (MCMB). During the first charge/discharge cycle, silver ions are reduced at a potential (2.15 V vs. 
higher than that of the PC cointercalation potential (0.75 V vs. 
to form a protective film on the MCMB surface, thereby obstructing PC cointercalation. Results show an increasing trend for both reversible capacity and cycle life performance with increasing 
A satisfactory reversible capacity may be obtained when the silver addition is greater than 5 wt %. © 2004 The Electrochemical Society. All rights reserved.
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Irreversible capacity of a graphite electrode varies considerably with the electrolyte used, especially its solvent compositions. Although propylene carbonate (PC)-based electrolyte systems are known for their low-temperature behaviors as compared to the ethylene carbonate (EC)-based electrolyte systems,1 some irreversible reactions have worse effects because PC solvated lithium ions can cause severe degradation to the graphite structure.2 3 4 5 6 7 8 Generally, this degradation may be suppressed by a well-developed protecting film.
Several organic additives such as ethylene sulfite (ES),9 vinylene carbonate (VC),10 11 halogenated solvents,12 and chloroethylene carbonate13 14 enhance the formation of a protective film. With these additions, a less permeable film formed may effectively decrease the intercalation of PC solvated lithium ions through the surface of graphite.
Organic additives mentioned above are to induce a film electronically insulating but ionically conductive to protect the graphite from PC cointercalation. It would be more rewarding if the film may be conductive both electronically and ionically, and hence the idea of this article is to create a conductive porous metal film under the traditional solid electrolyte interphase (SEI) film. The role of the porous metal film is to suppress PC cointercalation into the graphite structure, while on top the traditional electronically insulating SEI film is to stop the anode from reacting with the electrolyte.
According to Suzuki et al. ,15 covering a carbon fiber anode with a silver film via vacuum deposition may help enhance the intercalation of lithium into anode materials. The reduction potential for silver ion is higher than that of the intercalation of solvated lithium ions, therefore silver ions are deposited before any intercalation of the solvated lithium ions into graphite. 
a silver salt, was chosen therefore as the additive for its high compatibility with the PC-based electrolyte system.
To create such a conductive porous metal film, a metal salt dissolved in the electrolyte would have its metal ions reduced on mesocarbon microbeads (MCMB) surface to form a metallic film during the first charging process. This convenient addition technique easily controls the precise amount of silver and achieves similar results as the complicated vacuum deposition technique.
Experimental
MCMB was the negative electrode material for the Li-ion test cells. 93 wt % MCMB (2528 Osaka Gas, 25 μm in diameter) and 7 wt % polyvinylidene fluoride (Aldrich) binder were mixed in a solvent of 
-methyl-2-pyrrolidone (Aldrich) to slurry. The slurry was coated on copper foil (15 μm thick) and dried at 140°C. The electrode was then pressed with a roller to a resultant density of 
Electrochemical experiments were carried out in a three-electrode cell. Lithium foil was used as the counter and reference electrodes. Separator (Celgard 2320) was placed between the MCMB electrode and the lithium counter electrode. Electrolyte was 1 M 
in a mixture of 60% PC (Merk, battery grade) and 40% diethyl carbonate (Merk, battery grade) by volume. Silver hexafluorophosphate 
was added to electrolyte at different weight percentages. Test cells were fabricated in a dry argon-atmosphere glove box. Electrochemical tests were performed on a charge/discharge unit (Maccor model series 4000). Cells were charged (intercalation) at a constant current (C/10 rate) to a cutoff potential of 10 mV vs. 
Discharge (deintercalation) was performed at the same rate to a cutoff potential of 1.8 V vs. 
Surface morphology of the MCMB electrodes was observed by a scanning electron microscope (LEO-1530). Before any observation, specimens were disassembled in a glove box, washed with diethyl carbonate, and dried in a vacuum oven at 100°C for 5 h. Crystal structures of the MCMB electrodes were identified by X-ray diffraction (XRD, XD-5) with a Cu Kα target.
Results and Discussion
Figure 1 shows the changes in potential of MCMB electrodes with different 
amounts in electrolyte during the first charge/discharge cycle. The potential curve without additive remains at about 0.75-0.8 V (vs. 
, no lithium ion can intercalate into the graphite layers of MCMB due to the destruction by PC solvated lithium ions. With nothing to stop PC solvated lithium ions from squeezing into the graphite layers, structures exfoliate and are destructed rapidly.2
3
4
5
6
7
8 Silver ions deposited on MCMB surface prevent effectively the electrode from PC destruction, shown from the curves in Fig. 1. Cointercalation and decomposition of PC are suppressed more with increasing 
and hence intercalation/deintercalation capacity increases accordingly. This effect is pronounced when 
addition is more than 2.5 wt %, where clearly the irreversible capacity around 0.75 V (vs. 
becomes small and the total reversible capacity lower than 0.2 V (vs. 
increases. The reversible capacity below 0.2 V is generally contributed from the formation of 
which occurs via several phase transitions reflected by the corresponding plateaus in a potential curve.
Figure 1. Changes in potential of MCMB electrodes with different 
amounts in electrolyte during the first charge/discharge cycle.
Scanning electron microscopy (SEM) morphology of MCMB electrodes after the first charge/discharge cycle with 0 and 5 wt % 
additions in electrolyte are shown in Fig. 2. The electrode without additive is covered with a thick surface film of significant amount of cracks. Such a film usually results from reactions between Li ions and the electrolyte during charging. In addition to the thick surface film, Fig. 2a also shows a seriously damaged MCMB electrode without the addition of 
to electrolyte. Destruction has resulted from PC cointercalation (PC solvated lithium ions) into the graphite layer structure during charging. Comparatively, the structure in Fig. 2b of the electrode with 5 wt % 
remains more original and complete.
Figure 2. SEM morphology of MCMB electrodes after the first charge/discharge cycle with 0 and 5 wt % 
additions in electrolyte.
Figure 3 is the XRD pattern for MCMB electrode with 10 wt % 
additive to electrolyte after charge/discharge cycle. Clearly the characteristic peaks of Ag(111), Ag(200), and Ag(311) indicate the presence of a silver protective layer over MCMB surface, which is reduced during the first charging process to suppress PC cointercalation and decomposition.
Figure 3. XRD pattern for MCMB electrode with 10 wt % 
additive to electrolyte after charge/discharge cycle.
Derivative capacity as a function of electrode potential for an electrolyte of 5 wt % 
is shown in Fig. 4. A silver ion reduction has occurred during the first charging process around 2.15 V (vs. 
and does not reappear afterward. This disappearance indicates that no further silver ion deposition is possible because the SEI stops electron transfer from the electrode to the electrolyte and inhibits silver ion reduction. A peak of PC cointercalation at 0.75 V only appears in the first cycle, and decreases significantly afterward. In the range below 0.2 V there exist three oxidation and reduction peaks (marked as 1, 2, 3) representing the formation and decomposition of lithiated carbons, respectively. According to other previous studies on lithiation of carbon fibers and graphites,16
17
18 these oxidation and reduction peaks correspond to the potentials of two-phase coexistence. Clearly, silver ions do not intercalate into MCMB, instead they are only deposited on the surface. With the property of silver ion being easily reduced at a potential above PC cointercalation and decomposition potential, 
in electrolyte induces the formation of a porous silver film over MCMB before PC destruction. Follow this trend, any metal ions of reduction potentials higher than PC cointercalation and decomposition potential are hypothesized to result similar effects in lithium-ion battery systems as the silver of this work.
Figure 4. Derivative capacity as a function of electrode potential for an electrolyte of 5 wt % 
Figure 5 shows the reversible capacities of MCMB electrodes with electrolyte of different 
additions. When the addition is more than 5 wt %, the highest capacity achievable is 325 mAh/g. Any capacity lower may be due to the uneven or inadequate distribution of the silver film over MCMB surface which allows PC solvated lithium ions to leak through and destroy sites available for lithium ions to intercalate/deintercalate, and the reversible capacity is therefore reduced.
Figure 5. Reversible capacities of MCMB electrodes with electrolyte of different 
additions.
Furthermore, from the cycle life performance in Fig. 6, the reversible discharge capacity decays drastically after a few cycles when the addition is less than 5 wt %. A MCMB electrode with insufficient silver protection, less than 5 wt % for example, after charge/discharge would crack significantly, and these cracks will be further cracked. In the case of enough additions 
the reversible capacities do not decrease upon cycling because the silver deposition films are very stable and permit no crack to form. A complete graphite layer of MCMB electrode is the key to a good cycle life performance. The silver film has more functions than merely the assurance to the completeness of the graphite layer in regard to the cycle life performance of an electrode. According to research of Suzuki et al.15 on carbon fiber anodes, a silver film may effectively help increase the intercalation of Li ion into carbon by vacuum deposition. No lithium ion is blocked by the film, instead the ions are believed to have migrated through the film, through the Ag/MCMB interface, through the carbon layer, and formed a Li-intercalated structure; reverse process in deintercalation.15 As a result, this silver film protects the graphite from PC destruction and allows lithium ions to move easily, giving rises to good cycle life performance.
Figure 6. Relationship between the discharge capacity and cycle number of MCMB electrode in electrolyte with different 
additions.
Conclusions
Addition of silver hexafluorophosphate to PC-based electrolytes may effectively restrain PC molecules from cointercalating into MCMB electrodes. As the amount of this additive increases, positive effects on properties such as reversible capacity and cycle life performance become pronounced. Best results may be obtained when the addition is more than 5 wt %. During charging, silver ions are reduced at 2.15 V vs. 
which is much higher than the potential for PC cointercalation and decomposition (0.75 V vs. 
. Reduced silver ions then form a porous silver film over MCMB, demonstrated from XRD results. This conductive metal film not only blocks the cointercalation and decomposition of PC molecules, but also enhances movements of lithium ions in intercalation/deintercalation to the MCMB electrode.
Acknowledgments
This work was supported by the Ministry of Economic Affairs of Taiwan under contract no. 92-EC-17-A-08-R7-0312. The authors also thank Dr. J. T. Lee and Y. W. Lin for assistance with sample preparation.
Industrial Technology Research Institute assisted in meeting the publication costs of this article.