Design of Lithium-ion Battery for Electric Vehicles
The Hong Kong University of Science and Technology
Department of Chemical and Biomolecular Engineering
Supervisor: Prof. Chen Guohua
Group Members: Lu Siwan, Swan; Luo Weicheng, Ray; Zhang Leiting, Simon; Zhong Yuan, Zoe
Introduction
As automobiles are criticized for generating severe urban air pollution and consuming
depleting energy resources, greener cars with cleaner emission and less energy demand are
to be the new blue on the road, especially in modern metropolis like Hong Kong. An electric
vehicle (EV) recharged from the existing US grid electricity emits 115 g CO
2/km driven,
whereas a conventional US-market gasoline powered car emits 250 g CO
2/km.
EVs are generally powered by plug-in batteries and built-in electric motors. Electrochemical reaction inside each battery is reversible, and electricity is used and stored repeatedly. In terms of battery performance, lithium-ion
batteries are considered a promising candidate because of high energy density and efficiency in energy conversion.
Figure 1: Edison’s electric vehicle in 1913 Figure 2: Tesla electric vehicle in 2010
Synthesis of silica template
SBA-15
• SBA-15 served as a base material with 3D ordered structure
Carbon infiltration and
pyrolysis
• Sucrose as carbon precursor and pyrolyzed in vacuum at 950oC
Dissolution of SBA-15
• Silica removed by NaOH solution, producing CMK-3
ZnMn2O4 was synthesized via two distinctive
methods, namely the solvothermal (ST) method and the single source precursor (SSP) method .
ST method
Raw materials were added into anhydrous ethanol, stirred for 3 hours and then transferred into a
Teflon-lined stainless-steel autoclave for
solvothermal treatment at 160oC for 48 hours. The
mixture was then separated, dried under vacuum overnight. The as-synthesized ZnMn2O4 underwent a calcination treatment to further reinforce the
crystal structure. SSP method
Raw materials were added into water and stirred for 1 hour, followed by adding ZnO into the solution and stirred for another hour. The precipitate was
separated and dried under vacuum overnight. The resulting precursor was sintered in air at 700 oC for
2 hours to obtain the as-synthesized ZnMn2O4.
Synthesis of LiFePO
4precursor
• Under nitrogen atmosphere
Solvothermal digestion
• In autoclave bomb reactor under 180˚C for 12 hours with ethanol medium
Sintering
• 550 ˚C for 5 hours, obtained refined crystals
CVD carbon coating
• Deposit a nano carbon layer
Objectives
The current challenges in lithium-ion batteries remain in three areas, namely manufacturing costs, safety, and battery capacity. Our project focuses on the enhancement of lithium-ion battery
capacity since EVs generally require high power within a short duration, such as start-up, and limited capacities of conventional cathode and anode remain a major obstacle in the
commercialization of EVs. Quantitative studies of carbon coating modification on cathode material and syntheses of novel anode materials are illustrated in the following session.
Current
Challenges
Reduction of
Manufacturing
Costs
Enhancement
of Battery
Capacity
Novel Anode
Materials
Ordered
Mesoporous
Carbon (CMK-3)
Transition
Metal Oxide
(ZnMn
2O
4)
Cathode
Modification
LiFePO
4with
carbon
coating (CVD)
Improvement
of Battery
Safety
Methodology
Market and Plant Economy
-15 -10 -5 0 5 10 15 20 0 1 2 3 4 5 6 7 8 9 10 C um ula tiv e ca sh f lo w (i n M $ ) Year
Figure 4: Cash Flow Diagram
1) Environment of Battery Industry in China
The China Chemical News estimates China has more than 60 producers of lithium ferric phosphate (LFP), a basic cathode material in 2010. Nearly 20 of them have achieved large-scale production but none has emerged as a leader due to the lack of innovative technology and poor quality control. China is capable of producing 6.8 million kWh of lithium-ion batteries energy in 2010, and will reach 13.3 million kWh in 2015. The output of commercial EV/Plug-in hybrid EV in China is predicted to consume around 3.1 million kWh of energy capacity, only 23% of the total
output.
2) Plant Economy of a Battery Assembly Plant
Capacity: 72,000 kWh per year Unit battery pack for EV: 16 kW
Price: US$ 8,500 per unit battery pack Variable cost estimation: LFP consumption
300 ton per year
Fixed capital investment estimation: using benchmark plant, GM Brownstown
assembly plant Breakeven Year: 5 IRR: 0.126
2) Anode: CMK-3
1) Cathode Synthesis
3) Anode: ZnMn
2O
4There are two major parts of this project – cathode and anode solutions. While the cathode capacity is expected to be improved by innovative synthesis method (LiFePO4 [LFP] synthesis) and surface modification (nano carbon coating), anode is expected to be improved by novel anode material (transition metal oxide) and structure (ordered mesoporous carbon CMK-3).
4) Battery Assembly
Conclusion
The results of this project have demonstrated that
Nano carbon coating formed by CVD method has significantly increased the
performance (specific capacity, efficiency, cyclability, and rate capability) of LFP; With increasing deposition time (up to 14 minutes), the capacity, cyclability, and
efficiency of LFP increased significantly;
A first-of-its-kind phenomenon of LFP encapsulated carbon tube was observed; With the two routes proposed, ZnMn2O4 were synthesized with 44% higher
specific capacity compared with conventional graphite anode;
the ordered mesoporous carbon (CMK-3) novel anode structure demonstrated 34% specific capacity improvement.
Highlight
The LFP encapsulated carbon tube structure is the first of its kind. Although numerous reseach effort has been spent on growing carbon
nanotube, with the LFP encapsulated inside the tube, it is expected to further enhance LFP
cathode performance by reducing LFP-LFP contact and subsequently reduce battery degradation.
The results demonstrate that nano carbon coating layer by CVD has significantly increased performance (specific capacity, efficiency, cyclability, and rate capability) of LFP.
Function as a protective layer to cover the active sites and reduce the electrolyte decomposition so that its structural stability can be enhanced.
Enhance the conductivity to contribute to its high rate performance, as shown by its higher rate capability.
It is observed that there is a general growing trend for overall carbon content, which could explain the increasing specific capacity of
batteries (as shown in Figure 8), as carbon has a high electron conductivity compared to the battery substrate (LFP). Therefore, we conclude that a longer deposition time is beneficial to increase the overall carbon content, which then
contributes to a higher specific capacity. Figure 6: TEM images of blank LFP vs. 14 min CVD treated LFP
Figure 7: Zoom-in view of 14 min CVD
treated LFP, and special morphology After a certain coating time (12 minutes) in this
experiment set, it is observed that carbon tubes would grow from the LFP substrate with LFP
encapsulated inside. This LFP encapsulated carbon tube structure provides a superior
structure for Li-ion transportation and storage, which is supported by the high capacity
stability of the 14 minutes sample.
Results and Discussion
1) Cathode: Efficacy of Carbon Coating
Comparison of results Specific Capacity under 0.1C (mAhg-1) Efficiency after 10 cycles Cyclability Rate capability Blank LiFePO4 77 98% 98.9% 74% LiFePO4 with CVD (14 min) 120 100% 100% 81%
2) Cathode: Study of CVD Deposition Time
Deposition time (min) EA (carbon %, of overall weight) XPS (carbon %, within 5nm under surface)
TEM (morphology) – See Figure 6,7
Thickness Overall Morphology
8 3.18% 86% Not clear Carbon layer not measurable
10 3.04% 84% ~9nm Uniform, distinguished carbon layer
12 4.47% 89% ~10nm Uniform, distinguished carbon layer
14 4.58% 87% ~10nm Uniform, distinguished carbon layer
with growing quantity and length of carbon tube.
Figure 8: Trend of capacity variation
(under 0.1C) with different CVD time
3) Anode: CMK-3
Figure 11 reveals the x10000
magnitude SEM images of ZnMn2O4 synthesized with ST method (left) and SSP method (right). The hollow spherical ZnMn2O4 ST clusters consist of 100-200 nm ZnMn2O4 particles. The average
particle size of the flaky shaped
ZnMn2O4 SSP is around 200-300 nm. Sizes of both are within the range of sub-micron meter, which has been
proved a proper size range for higher anode capacities.
ST method
A significant phenomenon observed is a capacity recovery of ZnMn2O4 ST sample after undergoing a step discharge/charge pattern with varying current density (100-2000 mAg-1). After discharging back under
100 mAg-1 for another 40 cycles, the
capacity increased from 538 mAhg-1 to 680
mAhg-1, showing a 26% recovery. Similar
cases have been reported previously, but none of them is such significant. This may be attributed to the formation of a polymer/ gel-like film adjacent to the electrode,
showing an active electrochemical property of the as-synthesized ZnMn2O4.
SSP method
The most outstanding characteristic is that it requires less time and energy to produce high purity products. Conventional ZnMn2O4 synthesis requires hours of sintering
corresponding metal oxides at elevated temperature. In contrast, the precursor of the SSP method can be synthesized within two hours and the formation of the final
product is completed in another two hours’ sintering. Consequently, the synthesis is easy to be performed and the process can be scaled up more readily.
Since the quality of our target carbon material CMK-3 is greatly influenced by the quality of its
base template SBA-15, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were
deployed in characterizing synthesized SBA-15
samples, regarding to the pore size and uniformity. Shown in figures, results are in agreement with
literature data and qualified for further CMK-3 synthesis. The diffraction angle peaked at around 1.0 and two minor peaks at around 1.3 and 1.8 in XRD indicate our SBA-15 template has narrow
distribution of pore sizes and TEM image further validates the template quality.
Acknowledgment
The group would like to acknowledge the project supervisor, Prof. Guohua Chen, for the overall guiding in this research project. We also thank Dr. Yingshun Li for
coaching for LFP synthesis process, Dr. Chun He and Dr. Frank Lam for explaining details in CVD method and OMC synthesis, Dr. Yuanfu Deng for providing technical support in synthesizing ZnMn2O4. Dr. Chun He and Mr. Hongjie Xu’s assistance in
taking material characterizations are greatly appreciated.
Highlight
Compared with the present anode material graphite, both ZnMn2O4 and CMK-3 have shown an improved capacity. The theoretical capacity of graphite, the conventional anode material, is only 372mAhg-1, which is
130mAhg-1 and 166mAhg-1 less than our
synthesized CMK-3 and ZnMn2O4, respectively. Considering present challenges in lithium-ion
batteries, both materials are greatly favored because of their inexpensive raw materials and enhanced capacity.
The quality of synthesized CMK-3 samples was characterized by BET technique. Specific surface area of CMK-3 was calculated as 1279 m2g-1 by
adsorption and desorption of N2 gas which was close to literature.
The battery testing results regarding to cycle
numbers versus discharge capacity are shown in
Figure 13. The first discharge capacity reached
1291 mAhg-1 and stabilized at around 500 mAhg-1
after 20 cycles. Further testing cycles showed great stability of CMK-3, as the columbic efficiency
remains 100% as more cycles were repeated.
Figure 5: Rate capability comparison
Table 1: Comparison of blank LFP vs. coated LFP
Table 2: Impact of deposition time on LFP quality
Figure 9: XRD pattern of SBA-15
Figure 10: TEM image of SBA-15
Figure 11: SEM images of ZnMn2O4
Figure 12: Capacity recovery
of ZnMn2O4 ST
4) Anode: ZnMn
2O
4Figure 13: Comparison of
