Chapter 4 Results and Discussion
4.4. Dispersion of CNTs and Its Application to LiCoO 2 Electrode in LIBs…103
4.4.8. C-rate Performance of LIB
C-rate behavior of LIB is mainly controlled by Li ion diffusion in LiCoO2
cathode, but the cathode’s conductivity also plays a part. Figure 4-52 shows the discharge curves of two test batteries (cathodes with and without PEI addition) under different discharge rates of 0.2 to 8.0 C current densities. In 0.2 C discharge, the two discharge curves almost overlap each other with a capacity value of 140 mAh/g,
2.8 3.2 3.6 4.0 4.4
Figure 4-50. Cycle voltammograms of LIB test cells without or with 2 wt. PEI in LiCoO2 electrode.
Figure 4-51. The cycle performance at 1 C rate of test battery cells without or with 2
indicating a negligible effect of the CNTs addition. But at a discharge current as high as 8.0 C, the two capacity values differ. The battery without PEI has a much lower capacity of 36 mAh/g, which is only 25% of its 0.2 C-discharge capacity. On the other hand, the battery in which composite cathode contains 2 wt.% PEI has a discharge capacity of 101 mAh/g, equals to 74% of its 0.2 C-discharge capacity. Since the compositions in both batteries are the same (same weight percent of LiCoO2, binder, and conducting additive), diffusion behaviors in both cathodes are expected to be similar. Since the C-rate performance is controlled by the diffusion behavior and conductivity of cathode, as the diffusion behaviors being similar, the difference is hence lies on the conductivity. The LIB with the cathodes containing well dispersed CNT exhibit a higher discharge capacity because of the efficient charge carrier transport through the electrode. The reason that the dispersion shows no obvious improvement on 0.2-C discharge capacity is due to the relatively small discharge current flow. In such a case, the increase of conducting paths does not mean much for electron transport and thus the benefit resulted from CNT dispersion is obscure. On the contrary, in 8C discharge test, it requires more charge carrier transport paths since the current flow is large. The CNT network might act as a parallel circuit and benefit of resulted from the dispersion became obvious. It reduced the possible current crowding and allowed a fluent electron flow in the modified electrode. Hence an
enhancement of 8-C discharge capacity was observed. The results of high rate capability and long cycle life evidences the promising applications of such a LIB to power sources, e.g., power tools, electric bicycle, electric motorcycle, etc.
0 20 40 60 80 100 120 140 160
Vo lta ge ( V)
Charge 0. 2 C without PEIWithout PEI 0.2 C
Figure 4-52. Discharge curves of LIB test cells without or with 2 wt. PEI in LiCoO2
electrode. The two nearly overlapped profiles on the top of the figure are
the charging curves of test cells at the rate of 0.2-C current density.
Chapter 5
Conclusions
This thesis studies the dispersion techniques and its applications to the fabrication nano-scale BTO-relating ceramics and LiCoO2 electrode in LIBs. The major conclusions of this work are summarized in below:
(1) Nano-scale powder materials must be dispersed in order to eliminate the agglomeration caused by van der Waals attraction. This study demonstrated that adding appropriate amount of chemical dispersants (e.g., PMAA-Na or PDAAE) into the ceramics followed by a mechanical grinding/mixing process may effectively reduce the aggregation of nano-scale particles.
(2) In nano-BTO, it was found that distinct room-temperature dielectric properties (dielectric constant = 8000; dielectric loss = 5×10−3) could be achieved in the sample sintered at 1100°C for 6 hrs which possesses relatively small grain sizes about 140 nm and high density (about 95% T.D.). This clearly demonstrated that good dielectric properties could be obtained by a relatively low temperature sintering when nano-scale ceramic powders were adopted. Experimental analyses revealed that due to the large SSA of nano-scale powder, interface-driven mass
transport processes shifts the grain growth to the early stage of nano-BaTiO3
sintering in comparison with conventional process.
(3) In nano-BST, the best dielectric properties were obtained in the samples sintered at 1200°C for 6 hrs (grain size ≈ 200nm; dielectric constant = 9700; dielectric loss = 4×10−2) or at 1300°C for 1 hr (grain size ≈ 220nm; dielectric constant = 9800; dielectric loss = 7.5×10−2). The improvement of dielectric properties was attributed to the addition of Sr element as well as the fine grain structure in nano-BST samples. In addition, the Tc of nano BST was lower than conventional BST system. The TC is about 40°C for the sample sintered at 1200°C while the TC
for the sample sintered at 1300°C is about 20°C.
(4) In La-doped BST, addition of La2O3 retards both densification and grain growth and the inhibition effects were satisfactorily explained on the basis of La segregate in grain boundaries. The best dielectric properties were obtained in the 1.0LBST sample sintered at 1400°C for 1 hr (grain size ≈ 200nm; dielectric constant = 13800; dielectric loss =2.8E-4). The improvement of dielectric properties was attributed to the emergence of Ba2+/Sr2+ vacancies, lattice distortion and fine grain microstructure. The 1.0LBST sample possesses lower TC’s in comparison with conventional BST and BTO samples. The lower Tc was attributed to the fine grain structure of nano-LBST samples.
(5) In comparison with conventional ceramics, the nano-ceramics possess advantages including: (i) lower sintering temperature; (ii) better dielectric properties and (iii) shift the Tc to lower temperature.
(6) A hyperbranched polymer, PEI, was synthesized as the dispersing agent for CNTs slurry containing NMP solvent. The shear-thinning behaviors observed by rheological measurements as well as the minimum sedimentation volume measurement indicated that optimum amount of PEI for the dispersion of CNTs in slurry is about 2 wt.%. Dispersion mechanism for CNTs was attributed to the steric effect resulted from the highly branched structure of PEI. For the electrode with 2 wt.% optimal dispersant content, the ac resistance decreased from 287 Ω for cell without PEI to 161Ω for the cell at containing 2 wt.% of PEI. This is ascribed to the CNTs network that promotes the charge carrier migration in electrode. The high-rate discharge test indicated that the LIB with modified electrode possesses 8 C-discharge capacity of 101 mAh/g, which is about 74% of its 0.2 C-discharge capacity. The results of high rate capability and long cycle life evidences the promising applications of such a LIB to power sources, e.g., power tools, electric bicycle, electric motorcycle, etc.
Chapter 6
Future Works
6.1. Nano-ceramics:
In this research, though the best dielectric properties is obtained 1.0LBST sample, its sintering temperature is still too high. In the future, if the initial particle size can be reduced to less than 10 nm, a sintering temperature less than 1000°C and further improvement on dielectric properties can be expected.
At present, the explanations relating to the dielectric properties of ferroelectric materials are in essential for the conventional micrometer-scale samples. For nano-ceramics, there exist many ambiguities requiring further in-depth study, in particular, the fundamental studies on the relationships of microstructure and dielectric properties. For instance, the effects of internal stress on dielectric properties, the domain structure in nano-ceramics, the phase transition behaviors at TC, etc. must to be clarified so that the origin of dielectric properties of nano-ceramics could be understood.
6.2. CNT-relating Researches:
We successfully synthesized the chemical dispersant that is compatible to the
study clearly demonstrates its feasibility to high-power LIBs, however, in order to make this research complete, further researches including life time test, high power electrical measurements and applicability to other electrode materials have to be carried out. Besides, CNT has a relatively high mechanical strength and electrical conductivity. So, one should extends the applications of the finely dispersed CNTs to various occasions such as reinforcement of composites or conductive plastics, chemical or force sensors, electromechanical devices, etc.
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Curriculum Vitae
Kuo-Liang Ying (應國良)
PERSONAL DATA
Born: May 07, 1977, Kaohsiung City, Taiwan Nationality: Taiwan, the Republic of China
Office Address: Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-Hseuh Road, Hsinchu, Taiwan, ROC.
Tel: 886-3-5712121 ext.55338 E-mail: hsin_i_0507@yahoo.com.tw
EDUCATION
Ph. D. of, Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC. October 2007
Master of, Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan, July 2003
Bachelor of, Department of Chemistry, Soochow University, Taipei, Taiwan, July 2000
Journal Papers
1. Kuo-Liang Ying and Tsung-Eong Hsieh, “Dispersion of Nano-scale BaTiO3
Suspensions by the Combination of Chemical and Mechanical Grinding/Mixing Processes”, J. Appl. Polym. Sci., (2005)
2. Kuo-Liang Ying and Tsung-Eong Hsieh, “Sintering Behaviors and Dielectric Properties of Nanocrystalline Barium Titanate”, Materials Science and Engineering B, (2006)
3. Kuo-Liang Ying, Tsung-Eong Hsieh and Yi-Feng Hsieh, “Colloidal Dispersion of Nano-scale ZnO Powders Using Amphibious and Anionic Polyelectrolytes”, Cearm. International, (2007)
4. Kuo-Liang Ying, Jung-Cheng Lin, Tsung-Eong Hsieh, “A Novel Chemical Dispersant for the Dispersion of Carbon Nanotube and Its Applications to High-Power Lithium-ion Battery”, Electrochemistry Comm., (2007)
5. Kuo-Liang Ying, Jung-Cheng Lin, Tsung-Eong Hsieh, “A New Chemical Dispersant for the Dispersion of Carbon Nanotube: Synthesis, Characterizations and Its Application to LiCoO2 Electrode in Lithium Ion Battery”, J. Appl. Polym.
Sci., (2007)
6. Kuo-Liang Ying and Tsung-Eong Hsieh, “Sintering Behaviors, Microstructure and Dielectric Properties of Nano-Ba0.7Sr0.3TiO3 Ceramic”, J. J. Appl. Physics,
(2007)
7. Kuo-Liang Ying and Tsung-Eong Hsieh, “Sintering Behaviors and Dielectric Properties of Nano La2O3-doped (Ba,Sr)TiO3”, Preparedness (2007)