Subodh Mhaisalkar Nanyang Technological University
Krishna Shenai University of Toledo
Gehan Amaratunga University of Cambridge
Arokia Nathan University College London
F1: New Energy Systems I
Monday PM, November 28, 2011
Back Bay C (Sheraton)
11:00 AM - **F1.1
Solid State Paper Battery.
Rodrigo Martins 1 2 , Elvira Fortunato 1 2 , José Inácio Martins 3 , Isabel Maria Ferreira 1 2 Show Abstract
1 Materials science Department, FCTUNL, Caparica Portugal, 2 CEMOP, Uninova, Caparica Portugal, 3 Eng. Quimica, FEUP, Porto Portugal
R. Martins, E. Fortunato, B. Brás, J.I. Martins, I. FerreiraThe possibility of producing flexible, recyclable, non-toxic, and light weight electronic devices made with and on paper is highly appealing. Furthermore, the low-cost production of these devices renders them economically feasible and, since cellulose is one of the most abundant biopolymers on Earth, they are also ecologically disposable. Powering, in an integrated manner paper electronic devices is a challenging goal. This demand could be fulfilled by producing thin film batteries into the same paper device substrate. By doing so, self-sustained and energetic autonomy is achieved while keeping it mobile, light and flexible. Although cellulose based batteries have been reported, the need of liquid electrolytes, impregnation of salt solutions or CNTs paper functionalization, limits its integration into the substrate of the paper transistors. In this work we produce dry paper batteries where papers is simultaneously used as electrolyte and membrane. Moreover, the paper battery is rechargeable by expositing it to water vapor.Likely in most batteries, in this dry battery the voltage is dependent on the chosen pair of metal electrodes, while the current density depends on the thickness of the paper electrolyte, its porosity, bulk contents, density and composition of fibers. Thereafter, by actuating in any of these characteristics, the output of the device can be optimized to a specific purpose. All solid state batteries with Al/paper/LiAlF4/WO3/V2O5/GZO (zinc oxide doped with gallium oxide) structure show a voltage of 1.4 V. Moreover, paper reached in P2O5, SiO2, Al2O3 and CaO components contain higher concentration of ions (Na+, Cl-, SO4 (-2), PO4 (-3) , and Ca2+) leading to higher current density.
11:30 AM - F1.2
A Thin-Film Battery Architecture Generated by Rapid Layer-by-Layer Assembly.
Forrest Gittleson 1 , Gustave Macheras 1 , Andre Taylor 1 Show Abstract
1 Chemical and Environmental Engineering, Yale University, New Haven, Connecticut, United States
Lithium polymer technology represents the current energy density leader in commercially available lithium batteries. Despite the literal and figurative flexibility of this technology and its wide potential for application in cell phones, laptops, and other portable devices, the processes for design, assembly, and packaging of lithium-ion polymer cells are limited. We demonstrate a versatile technique for thin-film assembly that is used to produce layer-by-layer (LbL) films of active lithium battery materials. The spin-spray layer-by-layer (SSLbL) method allows rapid assembly of polymer-composite films with nano-level control and superior uniformity when compared to films produced by the common dip-coating method of LbL assembly. Thin films of several different polyelectrolyte systems with incorporated carbon nanotubes (CNT) were analyzed for charge/discharge capacity, cycling stability, ion conductivity, and electrical conductivity. We found that when the amount of active material (CNT) is controlled in these electrode films, the selection of the polymer matrix, as well as the LbL deposition parameters, has a significant effect on cycle stability and capacity. Tuning the ion conductivity of the polymer matrix by varying layer thickness and polymer blend content is an essential tool in optimizing electrode performance. Polymer electrolyte films were also assembled by the SSLbL method, demonstrating the effect of salt content, polymer choice, and polymer concentration on ion conductivity. These findings showcase a novel method of developing improved solid polymer electrolytes, which have been shown to exhibit a better safety profile than more common polymer-gel electrolytes. Patterning of functional battery materials using the SSLbL technique is also demonstrated and has particular relevance in the design of micro-scale or flexible batteries.
11:45 AM - F1.3
Highly Flexible Printed Batteries: Properties, Processing and Performance.
Abhinav Gaikwad 1 , Daniel Steingart 1 Show Abstract
1 CUNY Energy Institute, The City College of New York, New York, New York, United States
Direct write printing technologies can be used to print battery electrodes of desired capacity and form factor. Printed batteries have advantages such as low cost, substrate as packaging, and high material yield. Moreover the battery can be tailored specifically to niche device requirements in ways traditional battery cannot. However, even printed batteries, as they are now are not sufficient for fully flexible electronics. Existing battery electrodes, printed or otherwise, are formulated with stiff, non-compliant components and brittle composites, making these electrodes fundamentally at odds with the mechanical perturbations expected during the use cycle of a flexible device . Thus, a functional, flexible printed battery requires a wholesale reconsideration of the traditional components and optimization methods.To examine this coupling, we have studied the electrochemical and mechanical performance of a printed silver micro-battery in a microfluidic device. We observed that discharge of the battery decreased when the electrode was under a shear stress and different phases of silver oxide had different mechanical strength. We also demonstrated a technique to make highly flexible battery electrodes by embedded the electroactive material inside a mesh support. The mesh absorbed the stress during bending.In present work we extend these methods to secondary battery systems while also linking ink rheology and printing parameters to the final battery. From the rheological property of the ink and the desired electrode dimensions the necessary printing parameters are shown to be predictable through basic continuum fluid dynamics modeling. Beyond the mechanics of deposition, we examine the mechanical-electrochemical behavior of our composite through a combinatorial study of binder, solvent, and active materials. In this we relate the drop in discharge capacity of the battery during flexing with electrode cracking and also present techniques such as EIS to predict the drop in discharge capacity with bending radius.
12:00 PM - F1.4
Reverse Electrowetting – A New Approach to High-Power Harvesting of Mechanical Energy.
Tom Krupenkin 1 , J. Ashley Taylor 1 , Supone Manakasettharn 1 Show Abstract
1 , University of Wisconsin - Madison, Madison, Wisconsin, United States
Over the last decade electrical batteries have emerged as a critical bottleneck in portable electronics development. High-power mechanical energy harvesting can potentially provide a valuable alternative to the use of batteries, but until now, its adoption has been hampered by the lack of an efficient mechanical-to-electrical energy conversion technology. In this talk a novel mechanical-to-electrical energy conversion method is discussed. This method is uniquely suited for high-power energy harvesting from a wide variety of previously inaccessible environmental mechanical energy sources, including human locomotion. The method is based on reverse electrowetting (REWOD) – a novel microfluidic phenomenon. Electrical energy generation is achieved through the interaction of arrays of moving microscopic liquid droplets with novel nanometer-thick multilayer dielectric films. Advantages of this process include the production of very high power densities, up to 1 KW per sq. m; the ability to directly utilize a very broad range of mechanical forces and displacements; and the ability to directly output a broad range of currents and voltages, from several volts to tens of volts. We hope that the REWOD-based energy harvesting can provide a novel technology platform for a broad range of new electronic products and enable reduction of cost, pollution, and other problems associated with the wide-spread battery use.
12:15 PM - F1.5
Stretchable Electrochemical Capacitors: Power for Stretchable Electronics.
Pritesh Hiralal 1 2 , Darryl Cotton 2 , Yinglin Liu 2 , Piers Andrew 2 , Gehan Amaratunga 1 Show Abstract
1 Engineering, University of Cambridge, Cambridge United Kingdom, 2 Nokia Research Center, Nokia, Cambridge United Kingdom
Stretchable electronics allows a number of uses and tolerances which are not possible with rigid or even flexible electronics (flexible electronics or printed circuit boards). Whereas application of flexible electronics is limited to flat substrates, stretchable electronics can cover moving parts, such as joints in robotic elements, and also curved substrates or unusual materials such as silk, paper, leather etc. However, under stretch conditions, materials face large strains and changes in shape. Components need to be fabricated which can tolerate and function under these conditions. Substrate and interconnects should be made stretchable rather than flexible or rigid. Extensive efforts to advance stretchable electronics, including the integration of active components like diodes, transistors and integrated circuits, as well as sensors and actuators have been made. Here we demonstrate a power source, electrochemical capacitors, constructed onto copper substrates patterned on an elastomer, which withstand stretch ratios up to 100% before failure and deliver capacitances of 150mF/cm2. The fabrication process allows for mass production with roll-to-roll techniques based on printing and laminating. With stretchable interconnects, the electrochemical capacitors allow the construction of self-powered stretchable electronic devices.
12:30 PM - **F1.6
Low Voltage and Flexible Electronics Based on Graphene.
Manish Chhowalla 1 Show Abstract
1 , Rutgers University, Highland Park, New Jersey, United States
Graphene and its derivatives possess unique properties that make them attractive for both nano- and macro-electronics. Gaphene holds tremendous promise for large area electronics where the research is motivated by enabling low cost and flexible devices. Flexible displays, radio-frequency identification tags and large-surface sensor networks are examples of some macro-electronic devices that would benefit from the use of high mobility graphene as the channel material. Although the use of lightweight substrates and flexible active materials are useful towards making large area electronics technology portable, it is also necessary to develop devices that provide minimal power dissipation that can be powered by small batteries or by near-field radio-frequency coupling. In this presentation, I will present results of our work demonstrating how graphene can be an extremely useful for low power, flexible electronics. Specifically, I will present our work on developing a simplified photolithographic method, which effectively allows building complex high performance device architectures by means of fast and cost effective sheet-to-sheet solution processing. I will describe the versatility of symmetric reduced graphene source - drain (S-D) electrodes in organic electronics applications. In addition, I will describe discrete OFETs and graphene FETs based on a novel device architecture comprising a self-assembled monolayer (SAM) nanodielectrics. The latter is widely recognized as an elegant approach to reducing the operating voltage and power consumption in electronic devices.
F2: New Energy Systems II
Monday PM, November 28, 2011
Back Bay C (Sheraton)
2:30 PM - **F2.1
GaN and SiC Materials Technology for Chipscale Mobile Power.
Michael Dudley 1 , Krishna Shenai 2 Show Abstract
1 Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States, 2 Electrical Engineering and Computer Science Department, University of Toledo, Toledo, Ohio, United States
Both Silicon Carbide (SiC) and Gallium Nitride (GaN) have been touted to be the industry workhorses for 21st century energy-conversion power electronics, especially important for realizing the “smart grid” of tomorrow that must efficiently and reliably integrate distributed renewable energy sources in a cost-effective manner as well as for the demands of mobile electronic systems. However, their potential at present is significantly hampered because of large densities of material defects (100's to 1,000's per cm2 in SiC and 1,000,000’s per cm2 in GaN) which result in high manufacturing cost, limited voltage and current ratings, significant device de-rating, excessive power loss, and poor reliability. Experimental results accumulated over the past two decades by researchers around the world clearly suggest that, for the case of SiC, non-micropipe defects present in the bulk and epitaxial material cause severe degradation in power electronics device performance and reliability. The same crystal defects also limit the voltage and current ratings of devices, and severely hinder the development of cost-effective, efficient and reliable power electronics systems. Under high electric field and charge injection conditions, these crystal defect sites lead to enhanced generation of local microplasma and cause degradation in the forward current conduction and reverse voltage-blocking characteristics of high-voltage SiC Schottky barrier and PiN junction power diodes. More than a decade ago, a severe degradation in the diode breakdown voltage with increased diode area was demonstrated; this phenomenon was linked to the increased defect density in the SiC material . Early deployment of high-voltage SiC Schottky diodes in computer/telecom power supplies resulted in repeated field-returns. This type of device failure was attributed to dv/dt-related premature breakdown caused by excessive charge generation in the space charge region of a reverse-biased high-voltage SiC Schottky barrier diode with a high density of dislocations. The problem becomes particularly acute with increased dv/dt stress and at elevated temperatures. Subsequently, the basal plane dislocations (whose density has been tied to screw dislocations) were shown to cause forward voltage increase in PiN diodes when stressed at constant current that eventually led to thermal runaway and device destruction. This phenomenon has been attributed to defect movement and generation activated by the energy released from minority carrier recombination in a wide bandgap semiconductor operating under high-level charge injection condition. Similar susceptibility to material defects is expected in GaN power devices. In this paper we assess the influence of defects on the performance of power devices fabricated from both SiC and GaN and discuss potential paths forward.
3:00 PM - F2.2
Thin Film Coil Design Consideration for Wireless Power Transfer Flat Panel Display.
Jun Yu 1 , Sungsik Lee 1 , Arman Ahnood 1 , Arokia Nathan 1 Show Abstract
1 London Centre for Nanotechnology, University College London, London United Kingdom
Wireless power transfer has been an interesting and challenging topic since early 20th century when engineer Nikola Tesla first proposed this idea. Based on this, some recent research has been carried out on strongly coupled magnetic resonance theory. However, the receiver coils reported were relatively large-sized and non-thin-film based, and as such they are not applicable to displays or small mobile devices. In this work, we present a wireless power receiver system based on thin-film technology, and it could be integrated with thin-film display panel to produce a low cost system and a high throughput. In addition, the thin-film power receiver could be scaled down using metallisation during the process of making display, and suggests a possibility to be used for smaller mobile devices. A thin film based wireless power transfer circuit via strong magnetic resonance coupling is introduced. In particular, wireless power transfer through thin film technology is examined by transmission between an AC power transmitter and two receiver coils. Receiver thin film coils are the vital parts of the transmission, and planar spiral coils are used because of the ease of fabrication and reduction of metal layers. The optimal coil parameters are chosen according to the thin film fabrication limitations in order to obtain the maximum transfer efficiency. We fabricated the first receiver coil on the glass substrate and then deposited dielectric for isolation. The second receiver coil is placed on top of the dielectric layer. For fixed system parameters, the variation of transfer efficiency depending upon transfer distance is analysed. A comparison is made between theoretical and experimental results which show a good agreement. It is suggested that the thin film based wireless power transfer technology can be an integral part of the display system. Design Improvements as well as direction for further investigation will also be discussed.
3:15 PM - **F2.3
Power Passives for Chipscale Power Management.
Brian Morgan 1 , Sarah Bedair 1 , Christopher Meyer 1 2 , Lin Xue 2 , Christopher Dougherty 2 , Rizwan Bashirullah 2 , David Arnold 2 Show Abstract
1 , Army Research Lab, Adelphi, Maryland, United States, 2 , Univ. of Florida, Gainesville, Florida, United States
We present research focused on developing mm-scale power converters for power management of emerging microsystem applications, such as sensor nodes or micro-autonomous robotics. High frequency (~100MHz) CMOS boost converter architectures have been developed to drive the needs for inductance, capacitance and efficiency of miniature power passives, targeting the <1W power level. Therefore, this talk will highlight progress in high voltage CMOS converter topologies, high performance air-core MEMS inductors, and novel work on nanoparticle-based passives (capacitors & inductors) using a fluidic self assembly delivery system.
F3: Mobile Systems and Solid State Ionics
Monday PM, November 28, 2011
Back Bay C (Sheraton)
4:15 PM - **F3.1
Towards Self-Sustaining Energy Usage for Mobile Devices.
Arman Ahnood 1 , Jacqueline Edge 2 , Arokia Nathan 1 , Shahin Ashtiani 5 , Arun Madan 4 , Pritesh Hiralal 3 , Gehan Amaratunga 3 Show Abstract
1 London Center for Nanotechnology, University College London, London United Kingdom, 2 Dept of Physics and Astronomy, University College London, London United Kingdom, 5 School of Electrical & Computer Engineering, University of Tehran, Tehran Iran (the Islamic Republic of), 4 , MVSystems Inc., Golden, Colorado, United States, 3 Electrical and Computer Engineering, University of Cambridge, Cambridge United Kingdom
Long battery life is an important requirement for mobile devices and has often been satisfied by either improving energy efficiency of the device or increasing its battery’s energy density. An alternative to these approaches is for handheld devices to recycle some of their own energy consumed or harvest part of their energy from ambient sources. Displays are ideal for implentation of this approach due to their relatively high power consumption, large external surface area and the use of transparent substrates (glass or plastic). This talk discusses the design and implementation of a mobile energy system in mobile systems. As an example we conceder devices with flat panel displays including AMOLD and LCD. It starts by considering the key requirements for successful integration of photovoltaic energy system. The gain from energy recycling and harvesting from a display module are analysed and discussed. In addition to electrical energy generation, mobile energy systems store the generated energy in a suitable energy storage device and therefore require charging circuits. Various options for charging circuits and energy storage devices are reviewed, and a suitable circuit based on thin film transistors (TFTs) is discussed. Based on this an energy harvesting system, using amorphous silicon (a-Si:H) photovoltaic array, a-Si:H TFT charging circuit and thin film supercapacitor is proposed and fabricated. The general system performance is analysed and potential improvement methods are identified. The application of thin film technology, and low fabrication temperature opens the possibility of seamlessly integrating the energy system with display panels on rigid or flexible substrates.
4:45 PM - **F3.2
Polymeric Membranes for Lithium Batteries.
Stefan Manuel 1 , N. Angulakshmi 1 Show Abstract
1 Electrochemical Power Systems Division, Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi India
Energy consumption relying on fossil fuels is forecasted to cause a severe problem in the world economics and ecology mainly because of depleting resources and increasing environmental concerns. Developing alternative energy storage or conversion devices with high power and energy densities is under serious consideration as a viable alternative. Lithium-ion batteries, fuel cells, and supercapacitors (SCs) are considered strongly as major contenders for power source applications. The state-of -art employ different cell components and chemistries. The commercialized lithium –ion batteries use a graphite carbon anode, a lithium intercalated transition metal oxide cathode separated by a liquid electrolyte comprised of lithium salts dissolved in organic solvents. On the other hand, lithium polymer battery has several advantages over its liquid counter part which include no- leakage of electrolyte, high energy density and flexible geometry. The development of polymeric membranes for lithium batteries has gone through three stages namely (i) dry solid (ii) gel polymer and (iii) nanocomposite polymer electrolytes. The present talk focuses the development of novel nanocomposite polymer electrolytes (NCPE) with different lithium salts and inert fillers and the cycling profile of Li/NCPE/LiFePO4 cells at 70°C. The significance of ionic liquid based gel polymer electrolytes will be discussed. The polymeric membranes prepared by electrospinning for applications in lithium batteries will also be presented. References1.M. Armand, J.-M. Tarascon, Nature 451 (2008) 652-657.2.A. Manuel Stephan, T.P.Kumar, N. Angulakshmi, J. Phys.Chem. B. 113 (2009) 1962-1972.3.A. Manuel Stephan Eur.Polym.J, 42 (2006) 21-42.
5:15 PM - F3.3
Chemical, Thermal and Interfacial Characterization of B2O3 Doped Glass Ceramic Electrolytes for Rechargeable Lithium Batteries.
Nutan Gupta 1 , Rachid Yazami 1 , Madhavi Srinivasan 1 Show Abstract
1 School of Material Science and Engineering, Nanyang Technological University, Nanyang Avenue, Jurong West-05, Singapore, Singapore, Singapore
Abstract The state-of-art Li-metal batteries using organic liquid electrolytes are plagued with issues associated with their limiting performance, thermal instability, flammability and corrosiveness with lithium metal. The phosphate-based glass ceramic electrolytes provide protective layers in the solid state that conduct lithium ions at ambient temperature but are electronically insulating has been proposed as a possible solution in future rechargeable lithium batteries. This work focuses on the production of glass and glass-ceramics of the LiO-Al2O3-SiO2-TiO2-P2O5 (LASTP) system obtained by melting and quenching followed by crystallization of a precursor glass, which shows a tendency for homogeneous nucleation. The electrochemical stability of the prepared glass-ceramic phases against lithium was studied in Li-Li symmetric cells with well-sintered LASTP pellets with B2O3 (1-10 mol.%) as electrolyte. The symmetric cell was heated at 60°C for 4 hours to improve the interfacial contact between lithium and solid electrolyte and after heating, the contact resistance was optimized for the symmetric cell study. Additional confirmative insight on the electrochemical stability of the phases was obtained from Li/liquid electrolyte/LASTP/Li cells. These results are necessary for understanding of origins of interfacial resistance in Lithium batteries and suggest pathways for material optimization.Keywords: Glass ceramic; rechargeable; lithium batteries; solid electrolytes; symmetric cell. *Email: Madhavi@ntu.edu.sg
F4: Poster Session: Energy Conversion and Storage for Mobile Energy Systems I
Tuesday AM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - F4.1
Finite Element Simulations on Scaling Effects of Thermoelectric Generators.
Nicholas Williams 1 , Ali Gokirmak 1 , Helena Silva 1 Show Abstract
1 Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut, United States
In order for extensive adoption of thermoelectric generators (TEGs) into power generation, considerable improvements must be realized in the efficiency of thermal-electric direct energy conversion. Small-scale structures have been proposed for potential improvements in efficiency. In this study, two dimensional finite element simulations using Synposys Sentaurus TCAD software are employed for silicon based TEGs to examine the effects of scaling from mm to sub-μm range on device performance in a wide temperature range.
For these simulations, the geometry is composed of a single TEG that is sandwiched between two, 170 μm thick thermal contacts emulating the sheet metal encasing. Simulated TEG structures consist of a p-type and a n-type crystalline silicon columns which are electrically connected by a top copper contact where each leg has a separate bottom metal contact. The TEG is electrically isolated from the 170 μm thermal contacts by a 20 nm thick layer of SiO2. The n and p-legs are completely encapsulated by SiO2. Temperature dependent material parameters for electrical resistivity, thermal conductivity, Seebeck coefficient, and heat capacity are included in the model.
A heat source is applied to the top surface of the top metal contact while the bottom of the bottom contact is held at 300 K. Short circuit current, open circuit voltage, resistance, and power density are extracted for temperature gradients from 50 to 1350 K. Simulation results show that the optimum column height scales down for higher temperature gradients.
9:00 PM - F4.10
Optimization of the Cathode Material Li2MP2O7.
Hui Zhou 1 , Shailesh Upreti 1 , Natasha Chernova 1 , M. Stanley Whittingham 1 , Archit Lal 2 Show Abstract
1 Chemistry and Materials, State University of New York at Binghamton, Binghamton, New York, United States, 2 , Primet Precision Materials Inc., Ithaca, New York, United States
We have recently reported a novel solid solution of Li2FeyMn1-yP2O7 (0 ≤ y ≤ 1) synthesized through a “wet” method. The initial electrochemical tests indicate that lithium can be reversibly inserted and extracted, and the compounds can work as cathode materials for lithium-ion batteries, especially the Li2FeP2O7. However, poor electronic conductivity and large particle size resulted in incomplete electrochemical performance of our initial samples. Here we report the results of the optimization of these series by nanosizing, carbon-coating and substitution. For the carbon-coated nano Li2FeP2O7 the initial charge capacity increases from 90 mAh/g to 155 mAh/g, and an improvement in the rate capability as well as cyclability is observed. Reversible capacity exceeding the theoretical for one lithium, 110 mAh/g, may point towards the oxidation of Fe beyond 3+. Changes in the structure and oxidation state of transition metals during lithium insertion/extraction are being studied by ex-situ and in-situ x-ray diffraction (XRD) and absorption (XAS). Ex-situ XRD with the GITT technique and in-situ XRD data are consistent with each other and reveal that the basic structural framework remains intact as we charge and discharge the material electrochemically. The intensity variations observed in x-ray reflections are currently understood in terms of iron migration between different sites of the crystal lattice during Li extraction/reinsertion. Specially designed cathode is being studied under synchrotron XRD to investigate this electrochemically driven topotactic, yet reversible, reaction. Detailed structural refinements, local structures from XAS and magnetic susceptibility data at different states of charge and discharge will be discussed. This work is supported by the US Department of Energy, Office of FreedomCAR and Fuel Partnership through the BATT program at Lawrence Berkeley National Laboratory.
9:00 PM - F4.12
New Li2MnSiO4/C Nanocomposite Cathode Materials.
Shu Zhang 1 , Xiangwu Zhang 1 Show Abstract
1 Fiber and Polymer Science, College of Textiles, North Carolina State University, Raleigh, North Carolina, United States
The ever-growing demand for compact, high-energy density electrochemical energy storage has led to the development of lithium-ion batteries. In order to meet high performance target, a new electrode materials- Li2MnSiO4 for lithium-ion battery has been studied. Theoretically, the exchange of two electros per reaction is available for this material with a theoretical capacity of 333 mAh/g, which is higher than the capacities of most current cathode materials. However, due to the intrinsic low electronic conductivity of Li2MnSiO4, less than one electron transfer has been practically realized. To overcome the major drawback of Li2MnSiO4, a novel approach to fabricate Li2MnSiO4/C nanocomposite was developed to improve the electronic conductivity by electrospinning and heat treatment. A precursor solution containing PAN as the carbon source, and lithium acetate, manganese acetate and tetraethyl orthosilicate as the Li2MnSiO4 precursor was first electrospun into nanofibers. Then, the fibrous mat was heat treated in a furnace at 700°C. Both Li2MnSiO4 and carbon nanofibers were formed simultaneously in this step. Cells using this material as the cathode showed a reversible discharge capacity of around 220 mAh/g, indicating more than one electron transfer reaction. However, more study is required to improve the cycling performance of this material.
9:00 PM - F4.13
LiCoO2 Core-Li3VO4 Shell Structure with Enhanced Cyclability as Cathode Materials in Li-Ion Batteries.
Xiong Pu 1 , Liang Yin 1 , Choongho Yu 1 Show Abstract
1 Department of Mechanical Engineering, Texas A & M University, College Station, Texas, United States
Core-shell structures are effective in improving the electrochemical properties of active electrode materials in Li-ion batteries. The surface modification may overcome some disadvantages of core materials, such as poor electrical conductivity (e.g. LiFePO4 with carbon shell), poor cyclability (e.g. Si with carbon shell) and structure instability (e.g. LiCoO2 with Al2O3 shell). LiCoO2 possesses a theoretical capacity as high as 275 mAh/g, but a half of its theoretical capacity is utilized because of the structural instability and dissolution of Co4+ that often happen when it is overcharged at a voltage higher than 4.2 V. Appropriate surface modifications with core-shell structures, however, may solve this problem and improve both capacity and cyclability of LiCoO2. Here, a novel LiCoO2-Li3VO4 core-shell nanostructure is presented. LiCoO2 nanoparticles have been synthesized by a hydrothermal method. Crystalline lithium vanadate (Li3VO4) coatings with different weight percentages on the LiCoO2 nanoparticles have been prepared by annealing them with NH4VO3. When charging/discharging between 2.4 and 4.4 V, both capacity and capacity retention after 50 cycles of the coated samples was improved as compared to those of pristine LiCoO2 nanoparticles. This is because that coating layer of crystalline Li3VO4 not only suppresses the reaction between LiCoO2 with an electrolyte but also contributes to energy storage in the charging/discharging window. This work suggests a novel promising approach compared to other core-shell structures with inactive materials (e.g., Al2O3) for the shell.