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Synergetic effect of conductive additives on the performance of high power lithium ion batteries
NEW CARBON MATERIALS Volume 27, Issue 6, Dec 2012 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2012, 27(6):416–420.
Synergetic
effect
of
RESEARCH PAPER
conductive
additives
on
the
performance of high power lithium ion batteries WANG Qi1,2 , SU Fang-yuan3,*, TANG Zhi-yuan1 ,LING Guo-wei1 ,YANG Quan-hong1,3,* 1
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China;
2
Eighteen Research Institute of China Electronics Technology Group Corporation, Tianjin 300381, China;
3
Engineering Laboratory for Functionalized Carbon Materials,Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China.
Abstract: Two commercial conductive additives, carbon black (super P, SP) and vapor grown carbon fibers (VGCFs), were used to construct an effective conducting network in the cathode of commercial LiFePO4 lithium ion batteries (LIBs). Results suggest that the LIB with SP possesses a higher discharge capacity than that with VGCFs with the same mass fraction of the additives. The high-rate capacity of LIB with SP is much higher than that with VGCFs. Furthermore, the LIB with a mixture of these two additives has an apparently improved performance in low and high rate discharge capacity compared with the LIBs with a single component additive with the same mass fraction due to the synergetic effect. The same conclusion can be reached for the larger-capacity batteries (10 Ah or 50 Ah packs). Therefore, the use of two different fillers is important for the high power LIBs used in the electric vehicle and mass energy-storage industry. Key Words: Conductive additive; Synergetic effect; Lithium ion battery; High rate performance; Large capacity
1
Introduction
In order to improve the power capability of lithium ion batteries (LIBs), conductive additive, which are mainly sp2 carbon materials[1], are required in the cathode electrode to enhance the electronic conductivity of the active material. Meanwhile, the surface area, porosity and pore structure of the electrode are also strongly affected by the as-introduced additives[2-5]. This means that, although the mass fraction of additive is much lower than that of the active material, it plays a very important role in LIBs, especially for the higher-power LIBs[6]. In order to improve the electrochemical performance of the LIBs, an effective electron transport network must be constructed. However, too much conductive additive will cause an adverse effect on the LIBs, because it will block the ion transport path and decrease the energy density of LIBs [7-8]. Therefore, the choice of conductive additive is of significant importance for the LIBs. Based on the morphological structure, the commercial conductive additives up to now are normally divided into two types: particle-like and fiber like. The former one consists of acetylene black, carbon black and conducting graphite, while the latter are mainly metal fibers, vapor grown carbon fibers (VGCFs) and carbon nanotubes. Due to the morphological difference, a unique conducting ability can be found for the
two types of additives. It may raise a question as to, when the two types of additives are used together, is there any synergetic effect for the battery performance? In this study, VGCFs and carbon black (super P, SP) are chosen as the representatives of the two types of conductive additive mentioned above. Commercial LiFePO4 LIBs with moderate and high capacity are used to investigate the effect of the two types of additives[9-11]. Furthermore, a novel type of hybrid additive, which is composed of VGCFs and SP, is tested. It is confirmed that the two types of additives in the hybrid show a synergetic effect, manifested by a capacity and rate capability increase of LIBs as compared with the single component additive with the same mass fraction.
2 2.1
Experimental Pre-treatment of conductive additives
SP and VGCFs with diameter and length around 150 nm and 10-20 μm supplied by SHOWA DENKO, Japan, were firstly washed with de-ionized H2O until the pH= 7. Then they were centrifuged and heated at 60 qC to remove adsorbed water. SP and VGCFs are denoted as C1 and C2, respectively. The composite additive composed of C1 and C2 with a mass ratio of 1:1 is denoted as C3.
Received date: 20 July 2012; Revised date: 08 December 2012 *Corresponding author. E-mail: [email protected]; [email protected] Copyright©2012, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(12)60026-2
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
2.2
LIB fabrication
In order to provide more convincing results, coin cells used for most of the previous research and commercial 18650 (1 200 mAh), 10 Ah and 50 Ah cylinder LIBs were prepared (50 cells paralleled for every type of the LIBs). Firstly, conductive additives (C1, C2 or C3) and LiFePO4 (Shanghai, battery level) were pre-mixed. Polyvinylidene fluoride (PVDF) and N-Methyl pyrrolidone (NMP) were then added, and as-obtained slurry was coated on Al foil, followed by a heat treatment to remove NMP. The addition mass fraction of C1, C2 and C3 was exactly the same. The anode sheet was composed of man-made graphite, conductive additive and binder. The cathode sheet, anode sheet and the separator (PE/PP, 30 μm in thickness) were rolled together to make the cell core. Electrolyte, LiPF6 dissolved in EC+DMC+EMC (1:1:1 in volume) with a concentration of 1mol/L, was injected into the battery core in a glove box to obtain a commercial battery pack. 2.3
Characterization
SEM and X-ray diffraction (XRD) were conducted using a S-4800 (Hitachi, Japan) and a TTR III (Rigaku, Japan) apparatus, respectively. ARBIN battery test system (ARBIN, America) was used for the electrochemical performance characterization of as-prepared LIBs.
3
Results and discussion
The XRD patterns of as-obtained SP and VGCFs are shown in Fig. 1. It can be clearly seen that in VGCFs, the (002) peak for graphitic structure is very sharp, while the same peak of SP is comparatively low and broad. The results suggest that the VGCFs are stacked much more regularly by graphene sheets than the SP[12]. Therefore, the crystalline degree of the VGCFs is much higher than that of SP, indicating a much higher conducting ability of VGCFs than that of SP. The micro-structure comparison of different conductive additives and as-obtained electrode sheets with 4 mass% additive can be found in Fig. 2. From Fig. 2a and 2b, it can be clearly seen that SP is formed by small agglomerated particles
while VGCFs are fibrous with a high aspect ratio. Fig. 2c, 2d and 2e show the electrode sheet images with C1, C2 and C3 additive, respectively. It can be seen that the conductive additives are dispersed well in all the three types of electrode sheets. It seems that the LIB fabrication process is appropriate for these additives. The effect of different conductive additives on battery performance is investigated using the 18 650 commercial LIBs (cylinder battery with 65 mm in height and 18 mm in diameter). The electrochemical performance comparison of the all three types of the LIBs is shown in Fig. 3. It can be seen that the specific capacity of LiFePO4 with C1 additive is 113.3 mAKgg-1, which is much higher than that with C2 additive (106.8 mAKgg-1). This means that, as for the LIB, the conductivity of the SP is much better than that of the VGCFs, although the electronic conducting ability of the VGCFs seems to be much better than that of the SP based on the XRD results. However, when it comes to the LIB using C3 additive, an interesting result is obtained. Although the addition fraction of SP is only 50 mass% of the C1, the LiFePO4 LIB with C3 additive shows the highest specific capacity, which is as high as 125.3 mAKgg-1. The result gives a primary hint that SP and VGCF may not just be mixed mechanically, and some kind of synergetic effect can be found in C3 additive. In order to confirm the synergetic effect for SP and VGCFs, the rate performance of the LIBs with the three types of additives is investigated and the results is shown in Fig .4. It can be seen that when the batteries with C1 and C2 additives are discharged at 3C, the capacity retention rate is 88.7% and 86.4%, respectively. The rate performance of the LIB with C1 is better than that of the LIB with C2, which corresponds well with the capacity performance shown in Fig. 3. When it comes to the C3 additive, the capacity retention rate is 91.4%, which is also better than the two former cases. Considering the results presented above, the synergetic effect between VGCFs and SP in C3 additive can be reasonably proposed here. As an additive, its enhancement for the LIB not only depends on its intrinsic electronic conducting ability, but also the connecting model for additives and active
Fig. 1 XRD patterns of (a) SP and (b)VGCFs
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
Fig. 2 SEM images of (a) SP, (b) VGCFs and (c, d, e) as-obtained electrode sheets with C1, C2 and C3 additives respectively
electron transport in the whole electrode, due to their large aspect ratio, VGCFs can perform much better than the particle-like SP additive. When the two types of additive are used together, the C3 possesses the advantage of SP and VGCFs. Since it is distributed on the surface of the LiFePO4 particles, SP just can provide the electrons a “short-range” highway. And for the VGCFs, due to their large aspect ratio, the electrons can transport in a “long-range”. Thereafter, C3 additive can transport the electrons more effectively simultaneously in “short-range” and “long-range”. Therefore, the LIB with C3 additive shows the best capacity performance and rate performance than the LIB with the single component of additive. Fig. 3 Electrochemical performance of LiFePO4 with different conductive additives
materials, and the electronic short-range and long-range transport coordination also plays a important role. Due to its particle character, SP can be coated on the LiFePO4 particles and be well distributed on its surface. Therefore, electrons can be effectively transported to the whole surface of the particles, and the electrochemical reaction can occur on the whole particles. However, the VGCFs can only contact the LiFePO4 particles at some certain points and electrons can only arrive at these points, not all the surface of active particles. Therefore, the electrochemical reaction zone is relatively small and the reaction rate is slow. So the LIB with VGCFs shows a worse capacity and rate performance than that with SP. However, when it comes to the
In order to optimize the process, the addition fraction of the C3 additive is also investigated. Fig. 5 shows the effect of addition fraction on the capacity and internal resistance performance of the LIBs with C3 additive. From Fig. 5a, it can be seen that when the conductive additive fraction becomes 4.5 mass%, the discharge capacity at 1C begin to decrease apparently. This fact can be interpreted as follows. As for the 18 650 LIB, its capacity is affected by the amount of the LiFePO4 in the battery. With the increase of the addition fraction of conductive additive, the amount of LiFePO4 becomes low. Nevertheless, the additive can enhance the LiFePO4 capacity performance to some extent. Hence the whole capacity of the 18 650 LIB keeps nearly the same with the additive fraction increases when the fraction is comparatively small. However, when the fraction becomes too
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
650 will decrease at this situation. From this point of view, the additive fraction should be not too large. At the same time, the internal resistance is also affected strongly by the addition fraction of the additive. In this work, two types of internal resistance of the LIBs are tested[13]. The first one is obtained after the LIB underwent 1 time of charge/discharge cycle, and the second one is obtained after 200 charge/discharge cycles. It can be seen that the two internal resistances both decrease with the addition fraction at the range of 2.5-3.5 mass%. It can be attributed to the fact the more are additives, the more effective conducting network constructed. However, when the addition fraction increases up to 4.5 mass%, the internal resistance decreases apparently. This may be caused by the bad dispersion of the conductive additives in the slurry[14-15] at high addition fractions. In this case, the effective conducting network cannot be constructed and the internal resistance increases sharply. In order to further confirm the synergetic effect of SP and VGCF conductive additive, much larger capacity LIBs with C3 additive are also fabricated. Fig. 6 shows the rate performance of as-obtained 10 Ah and 50 Ah LIBs. For the 10 Ah LIB, the discharge capacity at 3C is 91.1% of the capacity at 1C. And for the 50 Ah case, the retention rate is even higher 99.4% under the same measurment condition. It can be seen that due to the introduction of C3, the rate performance of the LIBs is enhanced to a large extent.
4
Fig. 4 Rate performance comparison of the LIBs with different types of conductive additive (C1: SP; C2:VGCF; C3: 50% of SP+50% of VGCF)
large (for example, larger than 4.5 mass%), the amount of LiFePO4 becomes even less, and then the capacity of the 18
Fig. 5
Conclusions
An effective hybrid conductive additive, composed of VGCFs and SP, is proposed due to their synergetic effect. The battery with such a hybrid additive shows much more excellent capacity and rate performance simultaneously than the ones with the sinlge component additive. Through addition fraction optimization, the hybrid additive can enhance the discharge capacity retention rate to 91.4% when it is discharged at 3C. The performance enhancement due to the synergetic effect of the hybrid additive can be further confirmed in high capacity commercial LIBs with 10 Ah and 50 Ah.
Effect of C3 additive fraction on the performance of LIBs: (a): Capacity at 1C; (b): Internal resistivity variation
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
Fig. 6
Electrochemical performance of as-fabricated power LIB with C3 additive : (a): 10Ah, (b): 50Ah content balance in Li-ion battery cathodes: Commercial carbon blacks
Acknowledgements
vs. in situ carbon from LiFePO4/C composites [J]. J Power Sources,
We appreciate the support from the Program for the integration of production-teaching-research of Ministry of Education Guangdong (No. 2011B090400342), Shenzhen special funding for the development of biology, internet, new energy and new material (No. JC201104210152A).
2010, 195(22): 7661-7668. [8] Su F-Y, He Y-B, Li B, et al. Could graphene construct an effective conducting network in a high-power lithiumion battery? [J]. Nano Energy, 2012, 1: 429-439. [9] Yamada A, Chung S C, Hinokuma K. Optimized LiFePO4 for Lithium Battery Cathodes [J]. Journal of The Electrochemical Society,
References [1]
YANG
2001, 148(3): A224-A229.
Quang-Hong.
micrographite-based
Two
nanoporous
types
of
carbon
and
porous
carbons:
graphene-based
nanoporous carbon [J]. New Carbon Materials, 2007, 22(4): 289-294. [2] Doyle M, Fuller T F, Newman J. Modeling of galvanostatic charge and discharge of the lithium/polymer/insertion cell [J]. Journal of The Electrochemical Society, 1993, 140(6): 1526-1533. [3] Fuller T F, Doyle M, Newman J. Simulation and optimization of the dual lithium ion insertion cell [J]. Journal of The Electrochemical Society, 1994, 141(1): 1-10. [4] Newman J. Optimization of porosity and thickness of a battery electrode by means of a reaction-zone model [J]. Journal of The Electrochemical Society, 1995, 142(1): 97-101. [5] Ramadesigan V, Methekar R N, Latinwo F, et al. Optimal porosity distribution for minimized ohmic drop across a porouse electrode [J]. Journal
of
The
Electrochemical
Society,
2010,
157(12):
A1328-A1334. [6] Spahr M E, Goers D, Leone A, et al. Development of carbon conductive additives for advanced lithium ion batteries [J]. Journal of Power Sources, 2011, 196(7): 3404-3413. [7] Palomares V, Goni A, de Muro I G, et al. Conductive additive
[10] Su F-Y, You C, He Y-B, et al. Flexible and planar graphene conductive additives for lithium-ion batteries [J]. Journal of Materials Chemistry, 2010, 20(43): 9644-9650. [11] Padhi A K, Nanjundaswamy K S, Goodenough J B. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries [J]. Journal of the electrochemical society, 1997, 144(4): 1188-1194. [12] YANG Quan-hong, LV W, YANG Yong-gang, et al. Free two-dimensional carbon crystal—single-layer graphene [J]. New Carbon Materials, 2008, 23(2): 97-103. [13] Schweiger H G, Obeidi O, Komesker O, et al. Comparison of several methods for determining the internal resistance of lithium ion cells [J]. Sensors, 2010, 10(6): 5604-5625. [14] Dominko R, Gaberscek M, Drofenik J, et al. The role of carbon black distribution in cathodes for Li ion batteries [J]. J Power Sources, 2003, 119-121: 770-773. [15] Dominko R, Gaberscek M, Drofenik J, et al. Influence of carbon black distribution on performance of oxide cathodes for Li ion batteries [J]. Electrochimica Acta, 2003, 48(24): 3709-3716.
Synergetic
effect
of
RESEARCH PAPER
conductive
additives
on
the
performance of high power lithium ion batteries WANG Qi1,2 , SU Fang-yuan3,*, TANG Zhi-yuan1 ,LING Guo-wei1 ,YANG Quan-hong1,3,* 1
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China;
2
Eighteen Research Institute of China Electronics Technology Group Corporation, Tianjin 300381, China;
3
Engineering Laboratory for Functionalized Carbon Materials,Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China.
Abstract: Two commercial conductive additives, carbon black (super P, SP) and vapor grown carbon fibers (VGCFs), were used to construct an effective conducting network in the cathode of commercial LiFePO4 lithium ion batteries (LIBs). Results suggest that the LIB with SP possesses a higher discharge capacity than that with VGCFs with the same mass fraction of the additives. The high-rate capacity of LIB with SP is much higher than that with VGCFs. Furthermore, the LIB with a mixture of these two additives has an apparently improved performance in low and high rate discharge capacity compared with the LIBs with a single component additive with the same mass fraction due to the synergetic effect. The same conclusion can be reached for the larger-capacity batteries (10 Ah or 50 Ah packs). Therefore, the use of two different fillers is important for the high power LIBs used in the electric vehicle and mass energy-storage industry. Key Words: Conductive additive; Synergetic effect; Lithium ion battery; High rate performance; Large capacity
1
Introduction
In order to improve the power capability of lithium ion batteries (LIBs), conductive additive, which are mainly sp2 carbon materials[1], are required in the cathode electrode to enhance the electronic conductivity of the active material. Meanwhile, the surface area, porosity and pore structure of the electrode are also strongly affected by the as-introduced additives[2-5]. This means that, although the mass fraction of additive is much lower than that of the active material, it plays a very important role in LIBs, especially for the higher-power LIBs[6]. In order to improve the electrochemical performance of the LIBs, an effective electron transport network must be constructed. However, too much conductive additive will cause an adverse effect on the LIBs, because it will block the ion transport path and decrease the energy density of LIBs [7-8]. Therefore, the choice of conductive additive is of significant importance for the LIBs. Based on the morphological structure, the commercial conductive additives up to now are normally divided into two types: particle-like and fiber like. The former one consists of acetylene black, carbon black and conducting graphite, while the latter are mainly metal fibers, vapor grown carbon fibers (VGCFs) and carbon nanotubes. Due to the morphological difference, a unique conducting ability can be found for the
two types of additives. It may raise a question as to, when the two types of additives are used together, is there any synergetic effect for the battery performance? In this study, VGCFs and carbon black (super P, SP) are chosen as the representatives of the two types of conductive additive mentioned above. Commercial LiFePO4 LIBs with moderate and high capacity are used to investigate the effect of the two types of additives[9-11]. Furthermore, a novel type of hybrid additive, which is composed of VGCFs and SP, is tested. It is confirmed that the two types of additives in the hybrid show a synergetic effect, manifested by a capacity and rate capability increase of LIBs as compared with the single component additive with the same mass fraction.
2 2.1
Experimental Pre-treatment of conductive additives
SP and VGCFs with diameter and length around 150 nm and 10-20 μm supplied by SHOWA DENKO, Japan, were firstly washed with de-ionized H2O until the pH= 7. Then they were centrifuged and heated at 60 qC to remove adsorbed water. SP and VGCFs are denoted as C1 and C2, respectively. The composite additive composed of C1 and C2 with a mass ratio of 1:1 is denoted as C3.
Received date: 20 July 2012; Revised date: 08 December 2012 *Corresponding author. E-mail: [email protected]; [email protected] Copyright©2012, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(12)60026-2
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
2.2
LIB fabrication
In order to provide more convincing results, coin cells used for most of the previous research and commercial 18650 (1 200 mAh), 10 Ah and 50 Ah cylinder LIBs were prepared (50 cells paralleled for every type of the LIBs). Firstly, conductive additives (C1, C2 or C3) and LiFePO4 (Shanghai, battery level) were pre-mixed. Polyvinylidene fluoride (PVDF) and N-Methyl pyrrolidone (NMP) were then added, and as-obtained slurry was coated on Al foil, followed by a heat treatment to remove NMP. The addition mass fraction of C1, C2 and C3 was exactly the same. The anode sheet was composed of man-made graphite, conductive additive and binder. The cathode sheet, anode sheet and the separator (PE/PP, 30 μm in thickness) were rolled together to make the cell core. Electrolyte, LiPF6 dissolved in EC+DMC+EMC (1:1:1 in volume) with a concentration of 1mol/L, was injected into the battery core in a glove box to obtain a commercial battery pack. 2.3
Characterization
SEM and X-ray diffraction (XRD) were conducted using a S-4800 (Hitachi, Japan) and a TTR III (Rigaku, Japan) apparatus, respectively. ARBIN battery test system (ARBIN, America) was used for the electrochemical performance characterization of as-prepared LIBs.
3
Results and discussion
The XRD patterns of as-obtained SP and VGCFs are shown in Fig. 1. It can be clearly seen that in VGCFs, the (002) peak for graphitic structure is very sharp, while the same peak of SP is comparatively low and broad. The results suggest that the VGCFs are stacked much more regularly by graphene sheets than the SP[12]. Therefore, the crystalline degree of the VGCFs is much higher than that of SP, indicating a much higher conducting ability of VGCFs than that of SP. The micro-structure comparison of different conductive additives and as-obtained electrode sheets with 4 mass% additive can be found in Fig. 2. From Fig. 2a and 2b, it can be clearly seen that SP is formed by small agglomerated particles
while VGCFs are fibrous with a high aspect ratio. Fig. 2c, 2d and 2e show the electrode sheet images with C1, C2 and C3 additive, respectively. It can be seen that the conductive additives are dispersed well in all the three types of electrode sheets. It seems that the LIB fabrication process is appropriate for these additives. The effect of different conductive additives on battery performance is investigated using the 18 650 commercial LIBs (cylinder battery with 65 mm in height and 18 mm in diameter). The electrochemical performance comparison of the all three types of the LIBs is shown in Fig. 3. It can be seen that the specific capacity of LiFePO4 with C1 additive is 113.3 mAKgg-1, which is much higher than that with C2 additive (106.8 mAKgg-1). This means that, as for the LIB, the conductivity of the SP is much better than that of the VGCFs, although the electronic conducting ability of the VGCFs seems to be much better than that of the SP based on the XRD results. However, when it comes to the LIB using C3 additive, an interesting result is obtained. Although the addition fraction of SP is only 50 mass% of the C1, the LiFePO4 LIB with C3 additive shows the highest specific capacity, which is as high as 125.3 mAKgg-1. The result gives a primary hint that SP and VGCF may not just be mixed mechanically, and some kind of synergetic effect can be found in C3 additive. In order to confirm the synergetic effect for SP and VGCFs, the rate performance of the LIBs with the three types of additives is investigated and the results is shown in Fig .4. It can be seen that when the batteries with C1 and C2 additives are discharged at 3C, the capacity retention rate is 88.7% and 86.4%, respectively. The rate performance of the LIB with C1 is better than that of the LIB with C2, which corresponds well with the capacity performance shown in Fig. 3. When it comes to the C3 additive, the capacity retention rate is 91.4%, which is also better than the two former cases. Considering the results presented above, the synergetic effect between VGCFs and SP in C3 additive can be reasonably proposed here. As an additive, its enhancement for the LIB not only depends on its intrinsic electronic conducting ability, but also the connecting model for additives and active
Fig. 1 XRD patterns of (a) SP and (b)VGCFs
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
Fig. 2 SEM images of (a) SP, (b) VGCFs and (c, d, e) as-obtained electrode sheets with C1, C2 and C3 additives respectively
electron transport in the whole electrode, due to their large aspect ratio, VGCFs can perform much better than the particle-like SP additive. When the two types of additive are used together, the C3 possesses the advantage of SP and VGCFs. Since it is distributed on the surface of the LiFePO4 particles, SP just can provide the electrons a “short-range” highway. And for the VGCFs, due to their large aspect ratio, the electrons can transport in a “long-range”. Thereafter, C3 additive can transport the electrons more effectively simultaneously in “short-range” and “long-range”. Therefore, the LIB with C3 additive shows the best capacity performance and rate performance than the LIB with the single component of additive. Fig. 3 Electrochemical performance of LiFePO4 with different conductive additives
materials, and the electronic short-range and long-range transport coordination also plays a important role. Due to its particle character, SP can be coated on the LiFePO4 particles and be well distributed on its surface. Therefore, electrons can be effectively transported to the whole surface of the particles, and the electrochemical reaction can occur on the whole particles. However, the VGCFs can only contact the LiFePO4 particles at some certain points and electrons can only arrive at these points, not all the surface of active particles. Therefore, the electrochemical reaction zone is relatively small and the reaction rate is slow. So the LIB with VGCFs shows a worse capacity and rate performance than that with SP. However, when it comes to the
In order to optimize the process, the addition fraction of the C3 additive is also investigated. Fig. 5 shows the effect of addition fraction on the capacity and internal resistance performance of the LIBs with C3 additive. From Fig. 5a, it can be seen that when the conductive additive fraction becomes 4.5 mass%, the discharge capacity at 1C begin to decrease apparently. This fact can be interpreted as follows. As for the 18 650 LIB, its capacity is affected by the amount of the LiFePO4 in the battery. With the increase of the addition fraction of conductive additive, the amount of LiFePO4 becomes low. Nevertheless, the additive can enhance the LiFePO4 capacity performance to some extent. Hence the whole capacity of the 18 650 LIB keeps nearly the same with the additive fraction increases when the fraction is comparatively small. However, when the fraction becomes too
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
650 will decrease at this situation. From this point of view, the additive fraction should be not too large. At the same time, the internal resistance is also affected strongly by the addition fraction of the additive. In this work, two types of internal resistance of the LIBs are tested[13]. The first one is obtained after the LIB underwent 1 time of charge/discharge cycle, and the second one is obtained after 200 charge/discharge cycles. It can be seen that the two internal resistances both decrease with the addition fraction at the range of 2.5-3.5 mass%. It can be attributed to the fact the more are additives, the more effective conducting network constructed. However, when the addition fraction increases up to 4.5 mass%, the internal resistance decreases apparently. This may be caused by the bad dispersion of the conductive additives in the slurry[14-15] at high addition fractions. In this case, the effective conducting network cannot be constructed and the internal resistance increases sharply. In order to further confirm the synergetic effect of SP and VGCF conductive additive, much larger capacity LIBs with C3 additive are also fabricated. Fig. 6 shows the rate performance of as-obtained 10 Ah and 50 Ah LIBs. For the 10 Ah LIB, the discharge capacity at 3C is 91.1% of the capacity at 1C. And for the 50 Ah case, the retention rate is even higher 99.4% under the same measurment condition. It can be seen that due to the introduction of C3, the rate performance of the LIBs is enhanced to a large extent.
4
Fig. 4 Rate performance comparison of the LIBs with different types of conductive additive (C1: SP; C2:VGCF; C3: 50% of SP+50% of VGCF)
large (for example, larger than 4.5 mass%), the amount of LiFePO4 becomes even less, and then the capacity of the 18
Fig. 5
Conclusions
An effective hybrid conductive additive, composed of VGCFs and SP, is proposed due to their synergetic effect. The battery with such a hybrid additive shows much more excellent capacity and rate performance simultaneously than the ones with the sinlge component additive. Through addition fraction optimization, the hybrid additive can enhance the discharge capacity retention rate to 91.4% when it is discharged at 3C. The performance enhancement due to the synergetic effect of the hybrid additive can be further confirmed in high capacity commercial LIBs with 10 Ah and 50 Ah.
Effect of C3 additive fraction on the performance of LIBs: (a): Capacity at 1C; (b): Internal resistivity variation
WANG Qi et al. / New Carbon Materials, 2012, 27(6): 427–432
Fig. 6
Electrochemical performance of as-fabricated power LIB with C3 additive : (a): 10Ah, (b): 50Ah content balance in Li-ion battery cathodes: Commercial carbon blacks
Acknowledgements
vs. in situ carbon from LiFePO4/C composites [J]. J Power Sources,
We appreciate the support from the Program for the integration of production-teaching-research of Ministry of Education Guangdong (No. 2011B090400342), Shenzhen special funding for the development of biology, internet, new energy and new material (No. JC201104210152A).
2010, 195(22): 7661-7668. [8] Su F-Y, He Y-B, Li B, et al. Could graphene construct an effective conducting network in a high-power lithiumion battery? [J]. Nano Energy, 2012, 1: 429-439. [9] Yamada A, Chung S C, Hinokuma K. Optimized LiFePO4 for Lithium Battery Cathodes [J]. Journal of The Electrochemical Society,
References [1]
YANG
2001, 148(3): A224-A229.
Quang-Hong.
micrographite-based
Two
nanoporous
types
of
carbon
and
porous
carbons:
graphene-based
nanoporous carbon [J]. New Carbon Materials, 2007, 22(4): 289-294. [2] Doyle M, Fuller T F, Newman J. Modeling of galvanostatic charge and discharge of the lithium/polymer/insertion cell [J]. Journal of The Electrochemical Society, 1993, 140(6): 1526-1533. [3] Fuller T F, Doyle M, Newman J. Simulation and optimization of the dual lithium ion insertion cell [J]. Journal of The Electrochemical Society, 1994, 141(1): 1-10. [4] Newman J. Optimization of porosity and thickness of a battery electrode by means of a reaction-zone model [J]. Journal of The Electrochemical Society, 1995, 142(1): 97-101. [5] Ramadesigan V, Methekar R N, Latinwo F, et al. Optimal porosity distribution for minimized ohmic drop across a porouse electrode [J]. Journal
of
The
Electrochemical
Society,
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