The world energy consumption, along with CO2 emission, has been increasing exponentially during the past 50 years or so. As we become more aware of greenhouse gas emissions (GHG, such as CO2 and CH4) and their detrimental effects on our planet, it has become more important than ever to develop clean and renewable energy systems, such as solar cells, fuel cells, batteries, and wind power generators. Being powered largely by burning fossil fuels, transportation vehicles, including automobiles, ships, airplanes, and spacecrafts, are among the primarily sources for the GHG 1. The inevitably increasing fuel shortage, along with the public awareness of ‘greenhouse’ effects, has made it highly desirable to develop electric vehicles (EVs, hybrid electric vehicles (HEVs) and/or plug-in hybrid electric vehicles (PHEVs), instead of fossil fuel vehicles, with a low GHG emission. However, commercial applications of EVs will not be realized if advanced energy storage systems with an efficient energy saving and emission reduction cannot be successfully developed 2.
With no moving parts or noise and virtually without any pollution, batteries can convert chemical energy directly into electricity. They require little upkeep for potential large-scale applications. Among the entire battery family, several battery types, including lead acid (LA) batteries, nickel metal hydride (NiMH) batteries, and lithium ion batteries (LIBs), have great potential for EV applications. Although the current vehicle market is dominated by LA and NiMH batteries, the LIBs have received intensive research and development focus because of their high energy density, long cycle life, and superior environmental friendliness 3. So far, LIBs have been widely used in various portable and smart devices (e.g., cell phones, MP3 devices, cameras, and laptops). For EV applications, however, the performance of LIBs, particularly their energy density, safety, and cost, still need to be significantly improved. Therefore, further development of the LIB technology is urgently needed.
Figure 1 shows the basic working principles for a LIB. As can be seen, a lithium ion battery
is composed of three essential components, namely the Li+ intercalation anode, cathode, and the electrolyte/separator. Li+ ions move from the cathode to the anode through the electrolyte/separator during charging and back when discharging, and simultaneously, the electrons flow out of the external circuit to provide the electrical power (Figure 1).
Figure 1 Schematic representation of a typical lithium ion battery.
Why Cathode material?
The key to success in the development of advanced LIBs to meet theemerging EV market demands is the electrode materials, especially the cathode. Indeed, the recently-released Argonne National Laboratory Battery Performance and CostModel (BatPac) 4 shows that the cost of electrode materials accounts for?44% (?30% for the Li1.05(Ni4/9Mn4/9Co1/9)0.95O2 cathode and ?14% for the graphite anode) in a typical batterydesign involving cathode and anode electrodes, current collector, electrolyte, binders, cell/battery modules, and battery jacket. The cathode costs nearly twice as much as the anode. This could be attributed to the fact that the working voltage, energy density, and rate capability of a LIBare mainly determined by the limited theoretical capacity and thermodynamics of the cathode material in the present LIB technology. Therefore, it is critical to develop promising cathode materials for the current LIB technology.
Future Cathode materials:
Figure 2 shows a suggested road map for the research and development of LIB electrode materials in terms of the achievable voltage and capacity. Most of the current andfuture promising cathode materials shown in Figure 2 can be classified into four groups: LiMn1.5Ni0.5O4, lithium-excess LiLi, Mn, Ni, CoO2, lithium metal polyoxyanion Li3V2PO4,LiMPO4 and LiMSiO4 (M=Mn, Fe, Co, and combinations of thereof), and (O2, S, Li2S). As can be
seen, there are rather limited number of cathode materials of significant promise.
Figure 2 Electrode materials and corresponding electrochemical performances in the current LIB technologies (adapted from Reference 16).
Major Pros and Cons for each material group:
1) In the past two decades, the layered oxide LiCoO2 cathode has been widely used in portable electronics 3. The high cost, toxicity, chemical instability in the deep charged state, safety concern, and limited capacity (only ?135 Ah/ kg) associated with LiCoO2, however, have prevented its large-scale applications in transportation and stationary storage. Having a similar capacity of ?140 Ah/kg as that of LiCoO2, but a relatively high working voltage of ?4.7 V (?4.1 V for LiCoO2), LiMn1.5Ni0.5O4 is becoming an attractive candidate for high-energy applications. Furthermore, the cycle life and rate capability of doped LiMn1.5Ni0.5O4 (spinel structure) could be enhanced significantly by cationic substitutions (Co, Cr, Fe, Ga, or Zn) 5 and surface modification (AlPO4, ZnO, Al2O3 and Bi2O3) 6. In order to obtain a uniform surface modification and/or strong cationic coating, however, a complicated and high cost post-chemical process is necessary 7. To make the matter even worse, the currently used standard electrolytes (LiFP6 in EC/DEC/DMC) are not appropriate for LiMn1.5Ni0.5O4 cathode, which requires the high working voltage (?4.7 V).
2) Lithium-excess layered oxides, LiLi, Mn, Ni, CoO2, such as (Li2MnO3)x(LiMO2 (M=Ni, Co, Mn))1-x, offer a ?4.0 V working voltage with much higher capacity values of
?250 Ah/kg than those of LiCoO2 and LiMn1.5Ni0.5O2 8. However, there is often a huge irreversible capacity loss associated with the oxygen and lithium loss from thehost structure of the lithium-excess layered oxides (LiLi, Mn, Ni, CoO2) at the end of the first charging process. Although the irreversible capacity loss can besignificantly reduced by coating with insulating materials (e.g., Al2O3, AlPO4, MgO, RuO2) 9, the high surface area associated with the nanostructured lithium-excesslayered oxides (LiLi, Mn, Ni, CoO2) could have a high surface reactivity to induce side reactions between the electrodes and the electrolyte. This could lead todestabilization of the active materials and an increase in impeding passivation. Therefore, the electrolyte safety, together with the relatively high cost of theelectrode materials, is the major concern for lithium excess layered oxides to be used as the cathode in LIBs.
3) Compared with the cathode materials described above, Li–O2, Li–S/C, and Li2S–Si can offer a much higher energy density (Figure 2) 13-15, which is a distinguished advantage that could make them the most promising cathode material. For the Li–O2 battery, however, the reversibility and compatibility are still big problems, as well as the catalysts for the O2 cathode. For the Li–S/C and Li2S–Si battery systems, the cycling life and polarization of the nanostructured sulfur also need to be further improved.
How to resolve above mentioned Limitations?
To overcome the above-mentioned limitations associated with cathode materials and to facilitate the commercialization of LIBs for the EV market, some further research efforts to be taken include:
1) Development of proper electrolytes with a wide electrochemical window,high anodic stability, low volatility, low flammability, and good environmental friendliness (e.g., the optimized organic ethylene carbonate/diethyl carbonate electrolyte (EC: DEC) and ionic liquids (ILs)) especially for high voltagecathode materials (44.5 V) 16-18.
2) Development of ideal binders (e.g., carboxymethyl cellulose (CMC) andalginate 19,20) to make an intimate adherence between the current collector and electrode materials in LIBs for improved electrochemical performance and enhanced stability during electrochemical cycling.
3) Establishment of a combined computational and experimental approach for
material screening to identify cathode materials with high capacity, high energy density, and low cost 21.
4) Construction of various prototype LIBs with different promising cathode materials (Figure 2) assisted by tradeoff analyses on the gravimetric energy density, volumetric
energy density, cost and environmental friendliness for different applications 22,23.
My current research work was on the electrochemical performance of V2O5/acetylene black (V2O5/C) nanocomposite, used as a cathode material in lithium-ion batteries, prepared by a facile chemical synthesis route. The nanocomposite shows an outstanding electrochemical performance with a discharge capacity of 361 mAh g-1 during 1st cycle and retains a capacity of 254 mAh g-1 after 100 cycles, at a step rate of 0.1 C. Further, electrochemical impedance spectroscopy was employed to examine the reaction kinetics before and after cycling and the Li+-diffusion coefficient was calculated to be 1.67×10-12 and 6.13×10-15 cm2 s-1, respectively.
I would like to have a detail research on the cathode materials for next generation Lithium-ion batteries.
PLAN OF WORK:
My project will proceed as follows:
Year 1: Detailed literature review of the subject to understand it and know about the latest developments in the field.
Year 2: Designing of specific compositions after consulting the research adviser/supervisor and their processing, training and learning of the required experimental techniques.
Year 3: Completion of the experimental work, analysis of the obtained data and organization of the whole project into the form of a thesis in accordance with host university regulations for the degree of Doctor of philosophy, preparation of manuscripts for possible publication in national and international journals of material science and nano-world.
Accomplishing my goal to become a Physicist will be one of my greatest achievements in life and I am determined and prepared to pay the price for success.
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