Scientific paper
Surface Passivation of Natural Graphite Electrode
for Lithium Ion Battery by Chlorine Gas
1 2 v 2 1 Satoshi Suzuki,1 Zoran Mazej,2 Boris @emva,2 Yoshimi Ohzawa1
and Tsuyoshi Nakajima1*
1 Department of Applied Chemistry, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan
2 Jozef Stefan Institute, 39 Jamova, 1000 Ljubljana, Slovenia
* Corresponding author: E-mail: nakajima-san@aitech.ac.jp Tel.: +81 565 488121; fax: +81 565 480076.
Received: 28-06-2012
Dedicated to Prof. Dr. Boris Zemva on the occasion of receiving the Zois' award for lifetime achievements.
Abstract
Surface lattice defects would act as active sites for electrochemical reduction of propylene carbonate (PC) as a solvent for lithium ion battery. Effect of surface chlorination of natural graphite powder has been investigated to improve charge/discharge characteristics of natural graphite electrode in PC-containing electrolyte solution. Chlorination of natural graphite increases not only surface chlorine but also surface oxygen, both of which would contribute to the decrease in surface lattice defects. It has been found that surface-chlorinated natural graphite samples with surface chlorine concentrations of 0.5-2.3 at% effectively suppress the electrochemical decomposition of PC, highly reducing irreversible capacities, i.e. increasing first coulombic efficiencies by 20-30% in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.). In 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.), the effect of surface chlorination is observed at a higher current density. This would be attributed to decrease in surface lattice defects of natural graphite powder by the formation of covalent C-Cl and C=O bonds.
Keywords: Surface modification, chlorine gas, chlorination, natural graphite electrode, lithium ion battery
1. Introduction
Lithium ion batteries using ethylene carbonate (EC)-based solvents have a disadvantage on the low temperature operation because EC has a high melting point of 36 °C. For natural graphite anode with high crystallinity, EC should be used for the quick formation of surface film (Solid Electrolyte Interphase: SEI) by decomposition of a small amount of solvent. To improve the low temperature operation of lithium ion batteries, it is desirable to use propylene carbonate (PC) with a low melting point, -55 °C. However, it is difficult to use PC for natural graphite with high crystallinity because electrochemical reduction continues on natural graphite surface without forming surface film, which gives a large irreversible capacity. Natural graphite powder is prepared by mechanical pulverizing of large particles. Therefore many lattice defects would exist at the surface, functioning as active sites for electrochemical reduction of the solvents. Various methods of sur-
face modification have been applied to improve electrode characteristics of carbonaceous anodes for lithium ion batteries. They are carbon coating,1-14 metal or metal oxide coating,15-25 surface oxidation,26-33 surface fluorina-tion34-50 and polymer, Si or Sb coating.51-58 These methods of surface modification improved the electrochemical properties of carbonaceous electrodes for lithium ion batteries. Surface fluorination using fluorinating gases such as F2 and ClF3 and plasma fluorination are effective for improving charge/discharge properties of natural and synthetic graphites. Surface fluorination of graphitized petroleum cokes opens closed edge surface and enhances surface disorder, which facilitates the formation of surface film on graphite, leading to increase in first coulombic efficiencies (decrease in irreversible capacities).38-40' 43-46 Surface fluorination of natural graphite powder samples with relatively small surface areas (< 5 m2g-1) increases the capacities by increasing surface areas and surface disorder.34-3741 On the other hand, main effect of surface
fluorination of those having large surface areas (7-14 m2g-1) is the surface passivation by forming covalent C-F bonds at the surface, which suppresses electrochemical reduction of PC, increasing first coulombic efficiencies of natural graphite in PC-containing solvents.42,47-50 Since F2 has a small dissociation energy (155 kJmol-1) and highest electronegativity, F2 has high reactivity with other simple substances and compounds. Even light fluorination using F2 causes the increase in surface area and surface disordering of graphite with formation of covalent C-F bonds. However, Cl2 gas has lower reactivity than F2 because of its larger dissociation energy (239 kJmol-1) and lower electronegativity. Therefore it is expected that chlorina-tion of natural graphite powder by Cl2 gas yields covalent C-Cl bonds at the surface without increase in surface area and surface disorder. In the present study, surface passivation of natural graphite powder samples has been performed using Cl2 gas and electrochemical behavior of surface-chlorinated samples has been investigated in PC-containing electrolyte solutions.
2. Experimental
2. 1. Surface Chlorination and Analyses of Natural Graphite Samples
Natural graphite powder samples with average particle sizes of 10 and 15 pm (abbreviated to NG10 pm and NG15 pm; d002 = 0.335 and 0.336 nm; surface area:47 9.2 and 6.9 m2g-1, respectively; purity: >99.95%), supplied by SEC Carbon Co., Ltd., were chlorinated by Cl2 gas (3 x 104 or 1 x 105 Pa) at 200, 300 and 400 °C for 3, 10, 20 or 30 min. The amount of natural graphite was 300 mg for one batch reaction. Surface composition of surface-chlorinated samples was determined by X-ray photoelectron spectroscopy (XPS) (SHIMADZU, ESCA-3400 with Mg Ka radiation). Surface disorder and its effect to the bulk structure were evaluated by Raman spectroscopy (Ranis-haw inVia Raman Microscope, 532 nm) and X-ray dif-fractometry (SHIMADZU, XRD-6100), respectively.
2. 2. Electrochemical Measurements for Surface-chlorinated Natural Graphite Samples
Beaker type three electrode-cell with natural graphite sample as a working electrode and metallic lithium as counter and reference electrodes was used for galvanosta-tic charge/discharge experiments. Electrolyte solutions were 1 molL-1 LiClO4 - EC/DEC/PC (1:1:1 vol.) (Kishida Chemicals, Co. Ltd., H2O: 2-10 ppm) and 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.) (Kishida Chemicals, Co. Ltd., H2O: < 3 ppm). Natural graphite electrode was prepared as follows. Natural graphite powder sample was dispersed in N-methyl-2-pyrrolidone (NMP) containing 12
wt% poly(vinylidene fluoride) (PVdF) and slurry was pasted on a copper current collector. The electrode was dried at 120 °C under vacuum attained by a rotary pump for half a day. After drying, the electrode contained 80 wt% natural graphite sample and 20 wt% PVdF. Charge/discharge experiments were performed at a current density of 60 m-Ag-1 or 300 mAg-1 between 0 and 3 V relative to Li/Li+ reference electrode at 25 °C.
3. Results and Discussion
3. 1. Surface Composition and Structure of Chlorinated Natural Graphite Samples
Surface composition and XPS spectra of chlorinated natural graphite samples are shown in Table 1 and Figs. 1 and 2, respectively. Small amounts of surface chlorine were detected for NG10 pm chlorinated by Cl2 of 1 x 105 Pa at 400 °C for 10-30 min and NG15 pm chlorinated by Cl2 of 1 x 105 Pa at 400 °C for 20 and 30 min while only a trace of chlorine was detected for all other samples as shown in Table 1. The surface-chlorinated samples are stable in air and under high vacuum during XPS measurement. Chlorination of active carbon by Cl2 at 550-600 °C gives hydrophobic chlorinated material.59 This means that high temperature reaction of Cl2 with carbon materials gives covalent C-Cl bonds. However, only the surface is chlorinated in the case of graphite with high crystallinity. It is known that chlorine is not intercalated in graphite. The Cl 2p3/2 and Cl 2p1/2 peaks are observed at 198.6 and 200.1 eV, respectively. These binding energies are not large values. The reason may be that surface-chlorinated sample is electro-conductive because covalent C-Cl and C=O bonds partly cover the graphite surface as discussed later. The surface chlorine was obviously lower than surface fluorine detected for NG10 pm and NG15 pm fluorinated under mild conditions (F2: 3 x 104 Pa, temp.: 200 and 300 °C, time: 2 min).47,49 Surface chlorine concentrations were only 0.1-0.2 at% when NG10 pm and NG15 pm were chlorinated with 3 x 104 Pa Cl2, at 200 and 300 °C and for 3 min. They were 0.2-0.3 at% even under the conditions of 1 x 105 Pa Cl2, 300 °C and 10 min. To obtain 0.5 at% or higher surface chlorine concentrations, chlorination conditions such as 1 x 105 Pa Cl2, 400 °C and 10-30 min are necessary as given in Table 1. Surface fluorine concentrations of the same NG10 pm and NG15 pm were 11-20 at% under the fluorination conditions, 3 x 104 Pa F2, 200 and 300 °C, and 2 min,47,49 which are much weaker reaction conditions than in the case of surface chlorination with Cl2. Nevertheless the surface fluorine concentrations are much larger than surface chlorine concentrations obtained in the present study. Main reasons are the difference in the reactivity between F2 and Cl2 gases and electronegativities of fluorine and chlorine. On the other hand, surface oxygen concentrations slightly increased compa-
Table 1 Surface composition of chlorinated natural graphite samples.
Sample
Chlorination
NG10 |m
NG15 |m
	Cl2 (Pa)	Temp. (°C)	Time (min)	C	Cl	O	C (at%)	Cl	O
Original	-	-	-	92.8	-	7.2	93.3	-	6.7
A, a	3 x 104	200	3	86.3	0.1	13.6	90.4	0.2	9.4
B, b	3 x 104	300	3	89.8	0.1	10.1	88.5	0.2	11.3
C, c	1 x 105	300	10	90.4	0.2	9.3	90.0	0.3	9.7
D, d	1 x 105	400	10	88.6	0.5	10.9	90.4	0.3	9.3
E, e	1 x 105	400	20	91.1	0.9	8.0	90.6	0.7	8.7
F, f	1 x 105	400	30	90.1	1.4	8.5	85.8	2.3	11.9
A-F: surface-chlorinated NG10 |im; a-f: surface-chlorinated NG15 |im.
Fig. 1. XPS spectra of original and surface-chlorinated NG10 |im samples. (Chlorination conditions of samples A-F are given in Table 1.)
Fig. 2. XPS spectra of original and surface-chlorinated NG15 |im samples. (Chlorination conditions of samples a-f are given in Table 1.)
red with those of original NG10 pm and NG15 pm particularly under mild chlorination conditions (3 x 104 Pa Cl2, 200 and 300 °C, 3 min and 1 x 105 Pa Cl2, 300 °C, 10 min). They were reduced in the case of surface fluorina-tion with F2,47'49 which is probably because some amount of surface oxygen is removed as COF2 gas by breaking of C-C bond. When Cl2 gas is introduced into Ni reactor, it reacts with adsorbed water molecules, yielding unstable HClO (Cl2 + H2O ^ HClO + HCl). Complete removal of adsorbed water is normally difficult because the temperature in the vicinity of flange of the reactor is lower than the chlorination temperatures. The reactions of lattice defects with HClO and HClO+Cl2 may give >C=O and -C=O(Cl) groups, respectively (>C + HClO ^ >C=O + HCl and -C + HClO + 1/2Cl2 ^ -C=O(Cl) + HCl). This would be the reason why surface oxygen concentrations were increased by chlorination. It is also inferred that unstable HClO causes the formation of surface lattice defects by breaking C-C bonds and releasing oxygen as CO
Fig. 3. Raman spectra of original and surface-chlorinated natural graphite samples. (Chlorination conditions of samples A-F (NG10 pm) and a-f (NG15 pm) are given in Table 1.)
gas. Under stronger chlorination conditions (1 x 105 Pa Cl2, 400 °C, 10-30 min), surface chlorine concentrations increased to 0.5-2.3 at% and surface oxygen slightly decreased, which suggest that formation of -CCl3 and >CCl2 groups preferentially takes place by the reactions of Cl2 with surface lattice defects (-C + 3/2Cl2 ^ -CCl3, >C + Cl2 ^ >CCl2).
The d002 values were not changed by chlorination, being 0.335-0.336 nm for original and surface-chlorinated samples. Half widths of (002) X-ray diffraction lines only slightly broadened by chlorination. Raman spectros-copy also revealed almost no change of surface structural disorder by chlorination as shown in Fig. 3. G-band and D-band appear at 1580 and 1360 cm-1, indicating graphitic and disordered structures of carbon materials. The ratios of D-band to G-band intensity (R=ID/IG) showing the degree of surface disorder were 0.35 and 0.39 for original NG10 pm and NG15 pm, respectively. The R values were 0.33-0.39 and 0.35-0.37 for surface-chlorinated NG10 pm and NG15 pm samples, respectively, which indicates that surface disorder of NG10 pm and NG15 pm is nearly the same before and after chlorination. This is another difference from the fluorination accompanying the increase in surface disorder.47, 49
3. 2. Charge/discharge Behavior of Surface-chlorinated Natural Graphite Samples
Fig. 4 shows charge/discharge potential curves at 1st cycle, obtained in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.) at 60 mAg-1. NG10 pm gave the higher first coulom-bic efficiency (58.9%) than NG15 pm (49.5%). The surface areas of NG10 pm and NG15 pm are 9.2 and 6.9 m2g-1, respectively.47 Since charge/discharge experiments were made by constant current method at 60 mAg-1, actual current density is the higher in NG15 pm than NG10 pm. This would be the reason why NG10 pm has a higher first coulombic efficiency than NG15 pm. The potential plateaus at 0.8 V vs Li/Li+ indicate reductive decomposition of PC. The surface >C=O and -C=O(Cl) groups which would be formed by chlorination should contribute to the decrease in surface lattice defects together with -CCl3 and >CCl2 groups. However, first coulombic efficiencies were low for the samples, A-B and a-d chlorinated under mild conditions. This may be due to that unstable HClO simultaneously yields surface lattice defects by C-C bond breaking. In addition, surface oxygen concentrations increased under mild chlorination conditions and slightly decreased under stronger conditions (1 x 105 Pa Cl2, 400 °C, 10-30 min), which suggests that formation of surface lattice defects by the reaction of HClO with natural graphite decreases and covalent C-Cl bonds are preferentially formed under the stronger chlorination conditions. First cou-lombic efficiency for NG10 pm increased from 58.9% to 81.0% with increasing surface chlorine (Table 2). Particu-
Table 2. Charge/discharge capacities and coulombic efficiencies for original and surface-chlorinated NG10 ||m samples at 1st cycle, obtained in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.) at 60 mAg-1. (Chlorination conditions of samples A-F are given in Table 1.)
Fig. 4. First charge/discharge potential curves of original and surface-chlorinated natural graphite samples at 60 mAg-1 in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.). (Chlorination conditions of samples A-F (NG10 |im) and a-f (NG15 |im) are given in Table 1.)
larly samples D, E and F with relatively larger surface chlorine concentrations of 0.5-1.4 at% gave high first coulombic efficiencies (79.5-81.0%). In the case of NG15 |im, first coulombic efficiency of original sample was 49.5% which is lower than that of NG10 |im by 10%. The samples e and f with surface chlorine concentrations of 0.7 and 2.3 at%, respectively, exhibited high first coulombic efficiencies of 82.4% (Table 3). First coulombic efficiencies thus increased with increasing surface chlorine and also oxygen concentrations. This result suggests that surface lattice defects acting as active sites for PC decomposition are reduced by the formation of covalent C-Cl bonds (-CCl3 and >CCl2 groups) and also C=O bonds (>C=O and -C=O(Cl) groups). First charge capacities of all NG10 |im and NG15 |im samples were 350-360 mAhg-1 as given in Tables 2 and 3, and cycleability was also good.
On the other hand, the results are somewhat different in 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.). First
Graphite	Discharge	Charge	Coulombic
sample	capacity	capacity	efficiency
	(mAhg-1)	(mAhg-1)	(%)
Original	605	357	58.9
A	526	359	68.2
B	585	365	62.4
C	472	356	75.4
D	447	356	79.5
E	449	359	79.8
F	444	359	81.0
Table 3. Charge/discharge capacities and coulombic efficiencies for original and surface-chlorinated NG15 |m samples at 1st cycle, obtained in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.) at 60 mAg-1. (Chlorination conditions of samples a-f are given in Table 1.)
Graphite	Discharge	Charge	Coulombic
sample	capacity	capacity	efficiency
	(mAhg-1)	(mAhg-1)	(%)
Original	731	361	49.5
a	798	364	45.6
b	807	361	44.7
c	703	371	52.7
d	681	363	53.3
e	427	352	82.4
f	430	354	82.4
coulombic efficiencies for non-chlorinated NG10 |m and NG15 |m were both higher in 1 molL-1 LiPF6-EC/EMC/ PC (1:1:1 vol.) than in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.), being 75.0 and 63.8% at 60 mAg-1, and 66.4 and 58.3% at 300 mAg-1, respectively (Figs. 5 and 6, and Table 4). The first coulombic efficiencies for original NG10 |m and NG15 |m are higher by 14-16% in 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.) than in 1 mol-L-1 LiClO4-EC/DEC/PC (1:1:1 vol.). This is probably because a small amount of LiF generated by the reaction of LiPF6 with Li facilitates the formation of surface film on graphite. The electrode potentials more quicky decreased in surface-chlorinated samples than original graphite in the case of NG15 |m as shown in Figs. 5 and 6. However, potential profiles are similar to each other for original and surface-chlorinated NG10 |m samples. Table 4 shows that the larger increase in the first coulombic efficiencies is observed for NG15 |m having the smaller surface area than NG10 |m, and at the higher current density of 300 mAg-1 than 60 mAg-1. The increments of first coulombic efficiencies for NG15 |m reached ~15% and ~18% at 60 and 300 mAg-1, respectively. In the case of NG10 |m, the increments of first coulombic efficiencies were ~8% even at 300 mAg-1. The results show that the
Fig. 5. First charge/discharge potential curves of original and surface-chlorinated natural graphite samples at 60 mAg-1 in 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.). (Chlorination conditions of samples C-F (NG10 |m) and c-f (NG15 ||m) are given in Table 1.)
Fig. 6. First charge/discharge potential curves of original and surface-chlorinated natural graphite samples at 300 mAg-1 in 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.). (Chlorination conditions of samples D-F (NG10 |im) and d-f (NG15 |im) are given in Table 1.)
Table 4 First coulombic efficiencies for original and surface-chlorinated NG10 |im and NG15 |im samples in 1 molL-1 LiPF6-EC/EMC/PC (1:1:1 vol.) at 60 and 300 mAg-1. (Chlorination conditions of samples C-F and c-f are given in Table 1.)
Sample	60 mAg 1 300 mAg1
NG10 pm 75.0	66.4 (%)
C 76.8	-
D 78.0	71.3
E 74.0	74.6
F 77.0	74.2
Sample	60 mAg 1 300 mAg1
NG15 pm	63.8	58.3 (%)
c	64.9	-
d	70.1	64.7
e	77.2	76.7
f	78.9	76.3
effect of surface chlorination on first coulombic efficiency appears at a high current density in LiPF6-contai-ning electrolyte solution. As mentioned in the introduction, the reactivity and electronegativity of Cl2 are lower than those of F2. Therefore the stronger reaction conditions are necessary for the formation of covalent C-Cl bonds. Chlorination also causes the formation of covalent C=O bonds which would contribute to the decrease in surface lattice defects. In conclusion, surface chlorination is a good method for surface passivation of natural graphite powder to use natural graphite electrode in PC-containing electrolyte solution.
4. Conclusions
It is difficult to use PC-containing electrolyte solution for natural graphite powder electrode since PC is easily reduced on natural graphite, which largely increases irreversible capacity, i.e. decreasing first coulombic efficiency. This would be because surface lattice defects of natural graphite powder act as active sites for electrochemical reduction of PC. To improve charge/discharge characteristics of natural graphite powder electrode in PC-containing electrolyte solution, surface chlorination of natural graphite powder has been performed to passivate the surface active sites. Chlorination of natural graphite pow-
der increases both surface chlorine and oxygen concentrations. Both surface chlorine and oxygen would contribute to the decrease in surface lattice defects by the formation of covalent C-Cl and C=O bonds. Surface-chlorinated natural graphite samples having 0.5-2.3 at% Cl well suppress the electrochemical decomposition of PC, increasing first coulombic efficiencies by 20-30% in 1 molL-1 LiClO4-EC/DEC/PC (1:1:1 vol.). In 1 molL-1 LiPF6-EC/ EMC/PC (1:1:1 vol.), the effect of surface chlorination is observed at a higher current density. The increments of first coulombic efficiencies for NG15 pm with smaller surface reached ~15% and ~18% at 60 and 300 mAg-1, respectively. These results were obtained under the chlori-nation conditions of 1 x 105 Pa CL, 400°C and 10-30 min.
5. Acknowledgements
The present study was partly supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT) Private University Project Grant under Contract # S1001033. Natural graphite used in the study was kindly supplied by SEC Carbon Co., Ltd. The authors gratefully acknowledge them.
6. References
1.	H. Wang, M. Yoshio, J. Power Sources 2001, 93, 123-129.
2.	S. Soon, H. Kim, S. M. Oh, J. Power Sources 2001, 94, 68-73.
3.	M. Yoshio, H. Wang, K. Fukuda, Y. Hara, Y. Adachi, J. Elec-trochem. Soc. 2000, 147, 1245-1250.
4.	H. Wang, M. Yoshio, T. Abe, Z. Ogumi, J. Electrochem. Soc. 2002, 149, A499-A503.
5.	M. Yoshio, H. Wang, K. Fukuda, T. Umeno, N. Dimov, Z. Ogumi, J. Electrochem. Soc. 2002, 149, A1598-A1603.
6.	Y.-S. Han, J.-Y. Lee, Electrochim. Acta 2003, 48, 10731079.
7.	Y. Ohzawa, M. Mitani, T. Suzuki, V. Gupta, T. Nakajima, J. Power Sources 2003, 122, 153-161.
8.	Y. Ohzawa, Y. Yamanaka, K. Naga, T. Nakajima, J. Power Sources 2005, 146, 125-128.
9.	Y. Ohzawa, T. Suzuki, T. Achiha, T. Nakajima, J. Phys. Chem. Solids 2010, 71, 654-657.
10.	H.-Y. Lee, J.-K. Baek, S.-M. Lee, H.-K. Park, K.-Y. Lee, M.-H. kim, J. Power Sources 2004, 128, 61-66.
11.	W.-H. Zhang, I. Fang, M. Yue, Z.-L. Yu, J. Power Sources 2007, 174, 766-769.
12.	J.-H. Lee, H.-Y. Lee, S.-M. Oh, S.-J. Lee, K.-Y. Lee, S.-M. lee, J. Power Sources 2007, 166, 250-254.
13.	Y.-S. Park, H. J. Bang, S.-M. Oh, Y.-K. Sun, S.-M. Lee, J. Power Sources 2009, 190, 553-557.
14.	G. Park, N. Gunawardhana, H. Nakamura, Y.-S. Lee, M. Yoshio, J. Power Sources 2011, 196, 9820-9824.
15.	R. Takagi, T. Okubo, K. Sekine, T. Takamura, Electrochemistry 1997, 65, 333-334.
16.	T. Takamura, K. Sumiya, J. Suzuki, C. Yamada, K. Sekine, J. Power Sources 1999, 81/82, 368-372.
17.	Y. Wu, C. Jiang, C. Wan, E. Tsuchida, Electrochem. Commun. 2000, 2, 626-629.
18.	S.-S. Kim, Y. Kadoma, H. Ikuta, Y. Uchimoto, M. Wakihara, Electrochem. Solid-State Lett. 2001, 4, A109-A112.
19.	J. K. Lee, D. H. Ryu, J. B. Ju, Y. G. Shul, B. W. Cho, D. Park, J. Power Sources 2002, 107, 90-97.
20.	I. R. M. Kottegoda, Y. Kadoma, H. Ikuta, Y. Uchimoto, M. Wakihara, Electrochem. Solid-State Lett. 2002, 5, A275-A278.
21.	I. R. M. Kottegoda, Y. Kadoma, H. Ikuta, Y. Uchimoto, M. Wakihara, J. Electrochem. Soc. 2005, 152, A1595-A1599.
22.	L. J. Fu, J. Gao, T. Zhang, Q. Cao, L. C. Yang, Y. P. Wu, R. Holze, J. Power Sources 2007, 171, 904-907.
23.	F. Nobili, S. Dsoke, M. Mancini, R. Tossici, R. Marassi, J. Power Sources 2008, 180, 845-851.
24.	M. Mancini, F. Nobili, S. Dsoke, F. D'Amico, R. Tossici, F. Croce, R. Marassi, J. Power Sources 2009, 190, 141-148.
25.	F. Nobili, M. Mancini, P. E. Stallworth, F. Croce, S. G. greenbaum, R. Marassi, J. Power Sources 2012, 198, 243250.
26.	E. Peled, C. Menachem, D. Bar-Tow, A. Melman, J. Elec-trochem. Soc. 1996, 143, L4-L7.
27.	J. S. Xue, J. R. Dahn, J. Electrochem. Soc. 1995, 142, 36683677.
28.	Y. Ein-Eli, V. R. Koch, J. Electrochem. Soc. 1997, 144, 2968-2973.
29.	Y. Wu, C. Jiang, C. Wan, E. Tsuchida, J. Mater. Chem. 2001,
II,	1233-1236.
30.	Y. P. Wu, C. Jiang, C. Wan, R. Holze, Electrochem. Commun. 2002, 4, 483-487.
31.	Y. Wu, C. Jiang, C. Wan, R. Holze, J. Power Sources 2002,
III,	329-334.
32.	Y. P. Wu, C. Jiang, C. Wan, R. Holze, J. Appl. Electrochem. 2002, 32, 1011-1017.
33.	J. Shim, K. A. Striebel, J. Power Sources 2007, 164, 862-867.
34.	T. Nakajima, M. Koh, R.N. Singh, M. Shimada, Electrochim. Acta 1999, 44, 2879-2888.
35.	V. Gupta, T. Nakajima, Y. Ohzawa, H. Iwata, J. Fluorine Chem. 2001, 112, 233-240.
36.	T. Nakajima, V. Gupta, Y. Ohzawa, H. Iwata, A. Tressaud, E. Durand, J. Fluorine Chem. 2002, 114, 209-214.
37.	T. Nakajima, V. Gupta, Y. Ohzawa, M. Koh, R.N. Singh, A. Tressaud, E. Durand, J. Power Sources 2002, 104, 108-114.
38.	T. Nakajima, J. Li, K. Naga, K. Yoneshima, T. Nakai, Y. Ohzawa, J. Power Sources 2004, 133, 243-251.
39.	J. Li, K. Naga, Y. Ohzawa, T. Nakajima, A. P. Shames, A. I. Panich, J. Fluorine Chem. 2005, 126, 265-273.
40.	J. Li, Y. Ohzawa, T. Nakajima, H. Iwata, J. Fluorine Chem. 2005, 126, 1028-1035.
41.	H. Groult, T. Nakajima, L. Perrigaud, Y. Ohzawa, H. Yashiro, S. Komaba, N. Kumagai, J. Fluorine Chem. 2005, 126, 1111-1116.
42.	K. Matsumoto, J. Li, Y. Ohzawa, T. Nakajima, Z. Mazej, B. Zemva, J. Fluorine Chem. 2006, 127, 1383-1389.
43.	K. Naga, T. Nakajima, Y. Ohzawa, B. Žemva, Z. Mazej, H. Groult, J. Electrochem. Soc. 2007, 154, A347-A352.
44.	K. Naga, T. Nakajima, S. Aimura, Y. Ohzawa, B. Žemva, Z. Mazej, H. Groult, A. Yoshida, J. Power Sources 2007, 167, 192-199.
45.	T. Nakajima, S. Shibata, K. Naga, Y. Ohzawa, A. Tressaud, E. Durand, H. Groult, F. Warmont, J. Powerr Sources 2007, 168, 265-271.
46.	H. Groult, T. Nakajima, A. Tressaud, S. Shibata, E. Durand, L. Perrigaud, F. Warmont, Electrochem. Solid-State Lett. 2007, 10, A212-A215.
47.	T. Achiha, T. Nakajima, Y. Ohzawa, J. Electrochem. Soc. 2007, 154, A827-A833.
48.	T. Achiha, S. Shibata, T. Nakajima, Y. Ohzawa, A. Tressaud, E. Durand, J. Power Sources 2007, 171, 932-937.
49.	T. Nakajima, T. Achiha, Y. Ohzawa, A. M. Panich, A. I. Shames, J. Phys. Chem. Solids 2008, 69, 1292-1295.
50.	X. Cheng, J. Li, T. Achiha, T. Nakajima, Y. Ohzawa, Z. Mazej, B. Žemva, J. Electrochem. Soc. 2008, 155, A405-A413.
51.	S. Kuwabata, N. Tsumura, S. Goda, C. R. Martin, H. Yone-yama, J. Electrochem. Soc. 1998, 145, 1415-1420.
52.	M. Gaberscek, M. Bele, J. Drofenik, R. Dominko, S. Pejov-nik, Electrochem. Solid State Lett. 2000, 3, 171-173.
53.	J. Drofenik, M. Gaberscek, R. Dominko, M. Bele, S. Pejov-nik, J. Power Sources 2001, 94, 97-101.
54.	M. Bele, M. Gaberscek, R. Dominko, J. Drofenik, K. Zupan, P. Komac, K. Kocevar, I. Musevic, S. Pejovnik, Carbon 2002, 40, 1117-1122.
55.	M. Gaberscek, M. Bele, J. Drofenik, R. Dominko, S. Pejovnik, J. Power Sources 2001, 97/98, 67-69.
56.	B. Veeraraghavan, J. Paul, B. Haran, B. Popov, J. Power Sources 2002, 109, 377-387.
57.	M. Holzapfel, H. Buqa, F. Krumeich, P. Novak, F. M. Petrat, C. Veit, Electrochem. Solid-State Lett. 2005, 8, A516-A520.
58.	C.-C. Chang, J. Power Sources 2008, 175, 874-880.
59.	Y. Kanaya, N. Watanabe, DENKI KAGAKU 1974, 42, 349-353.
Povzetek
Površinske nepravilnosti kristalinične snovi lahko delujejo kot aktivna mesta za elektrokemijsko redukcijo propilen karbonata (PC), ki se uporablja kot topilo v litij-ionskih baterijah. Učinek površinskega kloriranja naravnega grafita v prahu je bil predmet raziskave z namenom izboljšanja karakteristik grafitnih elektrod v elektrolitskih raztopinah, ki vsebujejo propilen karbonat. S kloriranjem naravnega grafita se poveča ne le vsebnost površinsko vezanega klora ampak tudi kisika. Oboje prispeva k zmanjšanju površinske nepravilnosti naravnega grafita. Pri vzorcih pripravljenih s površinskim kloriranjem naravnega grafita s koncentracijo klora med 0,5-2,3 % se je občutno zmanjšal elektrokemijski razpad propilen karbonata, zmanjša se ireverzibilna kapaciteta oziroma v 1 mol raztopini LiClO4 v EC/DEC/PC (1:1:1 vol. deleži) se poveča Coulombova učinkovitost pri prvem polnjenju za 20-30%. V primeru 1 mol raztopine LiPF6 v EC/EMC/PC (1:1:1 vol. deleži) opazimo efekt površinskega kloriranja pri večji gostoti toka. To lahko pripišemo zmanjšanju nepravilnosti na površini naravnega grafita zaradi nastanka kovalentnih C-Cl in C=O vezi.