New Trends in Intercalation Compounds for Energy Storage
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New Trends in Intercalation Compounds for Energy Storage edited by
C.Julien Universite Pierre el Marie Curie, Laboratoire des Milieux Desordonnes el Heterogenes, Paris, France
J.P. Pereira-Ramos CNAS/LESCQ, Thiais, France
aod
.....
" Springer-Science+Business Media, B.V.
Proceedings of the NATO Advanced Study Institute on New Trends in Intercalation Compounds for Energy Storage Sozopol, Bulgaria 22 September-2 October 2001
A C.I. P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-1-4020-0595-4 ISBN 978-94-010-0389-6 (eBook) DOI 10.1007/978-94-010-0389-6
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TABLE OF CONTENTS
Part 1. Lectures
Intercalation compounds for energy storage C. Julien, J.P. Pereira-Ramos and A. Momchilov .................................................. 1
Lithium intercalation compounds - The reliability of the rigid-band model C. Julien ................................................................................................................. 9
Overview of carbon anodes for lithium-ion batteries K. Zaghib and K. Kinoshita .................................................................................... 27
Electronic structure of various forms of solid state carbons - Graphite intercalation compounds
J. Conard ................................................................................................................ 39
From intercalation compounds to inserted clusters Li in carbon superanodes for secondary batteries
J. Conard ................................................................................................................ 63
Lithium NMR in lithium-carbon solid state compounds J. Conard and P. Lauginie .. ....... ................... ......... .................. ...................... ........ 77
Critical review of HlCarbon literature and ab-initio research for a chemical site between two coroners
F. Marinelli, R.J.-M. Pellenq and J. Conard .......................................................... 95
Carbon-based negative electrodes of lithium-ion batteries obtained from residua of the petroleum industry
R. Alcantara, R1 Fernandez-Madrigal, P. Lavela, 1L. Tirado, 1M Jimenez- Mateos, C. Gomez de Salazar, R. Stoyanova and E. Zhecheva .............................. 101
Hydrogen in metals J. Huot .................................................................................................................... 109
vi
Effects of composition in LalNi-based intermetallic compounds used as negative electrodes in Ni-MH batteries
R. Baddour-Hadjean, J.P. Pereira-Ramos, M. Latroche and A. Percheron-Guegan ............................................................................................. 145
Lithium insertion compounds for energy storage A. Manthiram ........................................................................................................... 157
Chemical and structural stabilities of layered oxide cathodes A. Manthiram .......................................................................................................... 177
In situ preparation of composite electrodes : antimony alloys and compounds R. Alcantara, F.J. Fernandez-Madrigal, P. Lavela, C. Perez-Vicente and J.L Tirado ....................................................................................................... 193
On the use of in-situ generated tin-based composite materials in lithium-ion cells R. Alcantara, F.J. Fernandez-Madrigal, P. Lavela, C. perez-Vicente and J.L Tirado ....................................................................................................... 201
Physical chemistry of lithium intercalation compounds c. Julien ................................................................................................................. 209
Lattice dynamics of manganese oxides and their intercalated compounds c. Julien and M. Massot ......................................................................................... 235
Physical chemistry and electrochemistry of intercalation in disordered compounds C. Julien and B. Yebka ........................................................................................... 253
Modified host lattices for Li intercalation with improved electrochemical properties J.P. Pereira-Ramos, S. Bach, S. Franger, P. Soudan and N. Baffier ..................... 269
Surface science investigations of intercalation reactions with layered metal dichalcogenides
W. Jaegermann and D. Tonti .................................................................................. 289
Conductive polymers and hybrid materials as insertion electrodes for energy storage applications
P. Gomez-Romero .................................................................................................. 355
An electrochemical point of view on the intercalation compounds A. Momchilov ......................................................................................................... 377
Manganese dioxides promising cathode materials for lithium batteries B. Banov ...... .......... .... .... .............. .... ... ......... .... ... ......... ......... .... ..... ...... ..... ...... ....... 393
vii
Part 2. Seminars
Impedance of diffusion of inserted ions. Simple and advanced models J. Bisquert ............................................................................................................... 405
Dielectric relaxation spectroscopy for probing ion/network interactions in solids F. Henn, S. Devautour and J. e. Giuntini ............................................................... 413
Cations mobility and water adsorption in zeolites G. Maurin, S. Devautour, P. Senet, J.e. Giuntini and F. Henn ............................ 421
Strategies to improve the cycling performance of lithium storage alloys M Wachtler, M. Winter and J.O. Besenhard ......................................................... 429
Nanoscaled containers for hydrogen J.D. Dragieva, Ch.D. Deleva, MA. Mladenov and P.P. Zlatilova ......................... 433
Nanocrystalline materials for lithium batteries e. W Kwon, S.J. Hwang, A. Poquet, N. Treuil, G. Campet, J. Portier and J.H. Choy ......................................................................................................... 439
Study of fluorinated graphite intercalation compounds J.P. Asanov, P.P. Semyannikov and V.M Paasonen .............................................. 447
Insertion of rare-earth metals into AgI-based compounds - First evidence of disordering and strong modification of 13- and y-AgI crystal structures
A.L. Despotuli ... ....................................................................................................... 455
Electronic structure of oxygen in delitiated LiTM02 studied by electron energy-loss spectrometry
J. Graetz, R. Yazami, e.e. Ahn, P. Rez and B. Fultz .............................................. 469
Short-range ColMn ordering and electrochemical intercalation of Li into Li[Mn2_yCOy]04 spinels. O<y:s;; 1
E. Zhecheva, R. Stoyanova, P. Lavela and l.-L. Tirado ........................................ .475
Limitation of cathode electrolyte reaction in lithium ion batteries H. Omanda, T. Brousse and D.M Schleich ............................................................. 483
The nature of the phase transition in LixMn204 J. Marzec, M. Marzec and J. Molenda ................................................................... 489
viii
Morphology control of electrode materials for Li batteries through sol-gel technique S. V. Pouchko, AK. Ivanov-Schitz, T.L. Kulova, A.M. Skundin and E.P. Turevskaya ...................................................................................................... 493
Cryochemical processing of cathode materials for lithium-ion batteries O.A. Brylev, O.A. Shlyakhtin, A V Egorov, T.L. Kulova, AM. Skundin and S. V. Pouchko .......................................................................................................... 497
Amorphous and active carbon: the quantum chemistry view on the structure V.D. Khavryutchenko, A V. Khavryutchenko and V V Strelko .............................. 501
Mechanochemical synthesis of intercalation lithium transition-metal oxide compounds: some aspects of mechanism
N. V. Kosova ............................................................................................................ 507
Electronic state of ions in mechanochemically prepared intercalation lithium-transition metal oxide compounds
N V. Kosova, E. T. Devyatkina, VF. Anufrienko, V. V. Kaichev, V.!. Buhktiyarov, S. V. Vosel, NT. Vasenin and T.V Larina ............................................................... 515
Part 3. Posters
A. Castro-Couceiro, S. Castro-Garcia, M.A. Sefiaris-Rodriguez, C. Rey-Cabezudo and C. Julien .............................................................................. 523
Structural and electrochemical properties of V 205 thin films obtained by atomic layer chemical vapor deposition (ALCVD)
A. Mantoux, J.e. Badot, N Baffier, J. Farcy, J.P. Pereira-Ramos, D. Lincot and H. Groult ............ _ ............................................................................................ 531
Influence of thermal treatment and atmospheres on the electrochemistry of V 205 as lithium insertion cathode
A.K. Cuentas-Gallegos and P Gomez-Romero ...................................................... 535
Dielectric dispersion and kinetics properties of Bi2Se3 intercalated by molecular iodine 1.1. Grygorchak and N.K. Tovstyuk ......................................................................... 539
High-frequency capacitor nanostructure formation by intercalation l.l. Grygorchak, B.a. Seredyuk, K.D. Tovstyuk and B.P. Bakhmatyuk .................. 543
Method of synthesis of electrode materials with controlled particle size for lithium batteries
S. Uzunova, B. Banov and A. Momchilov ............................................................... 545
IX
Synthesis of cobalt substituted Li nickelates from a high dispersity mixed oxide precursor R. Moshtev, P. Zlatilova, I. Bakalova and S. Vassilev ........................................... 551
Application of the current interruption method for the measurement of the ohmic resistivity of LiNi1_yCoy0 2 cathodes as a function of the state of discharge
R. Moshtev, P. Zlatilova, I. Bakalova and S. Vassilev ........................................... 557
Pulsed microplasma cluster source technique for synthesis of nanostructured carbon films P. Milani, P. Piseri, E. Barborini and I.N. Kholmanov .......................................... 561
Oxygen intercalation in strontium ferrite: evolution of thermodynamics and electron transport properties
M. V. Patrakeev, J.A. Shilova, E.B. Mitberg, A.A. Lakhtin, I.A. Leonidov and V.L. Kozhevnikov ............................................................................................. 565
Insertion of aluminium into a boron icosahedral hollows as the first step of nanofilaments
crystals formation A.!. Kharlamov, Ch. Trapalis, N. V. Kirillova, S. V. Loytchenko, V. V. Fomenko and A.A. Kharlamova ............................................................................................. 573
Structure, microstructure and magneto-transport properties of Prl_xBxC003_11(B2+=Ba2+, Ca2+) perovskite materials
B. Rivas-Murias, M. Simchez-Andujar, J. Rivas, A. Fondado, J. Mira and M.A. Seiiaris-Rodriguez .................................................................................. 577
Electronic structure of LiMn204 Q.-H. Wu, A. ThifJen and W Jaegermann .............................................................. 585
Electrochemical behaviour of manganese spinel obtained by a controlled particle size method at low temperature
B. Banov, S. Uzunova, A. Momchilov and l. Uzunov ............................................. 591
Li-Si system studies as possible anode for Li-ion batteries I. Samaras, L. Tsiakiris, S. Kokkou, O. Valassiades and Th. Karakostas .............. 597
Synthesis. structural and thermodynamical characterization of Mm(NiCo )5_xAlx alloys S. Bliznakov, E. Lefterova, M. Mitov, L. Bozukov, A. Popov and I. Dragieva ....... 601
Structural and electrochemical studies of Li-Co-Cr-O oxides prepared by wet chemistry N. Amdouni, C. Julien and H. Zarrouk ................................................................... 607
Influence on physico-chemical properties of LiFexMn2_x04 upon iron doping K. Swierczek.1. Marzec, M. Marzec and J. Molenda ............................................. 613
Phase transition disappearance in Li1+liMn2_804 depended on lithium excess M. Molenda and R. Dziembaj ................................................................................. 615
x
Processes of deintercalation of lithium fluoride out of exhausted cathode materials of lithium batteries
A.A. Evtukh, v.N. Plakotnik and I. V. Goncharova ............................................... 619
Electrochemical properties of nanoparticies produced by borohydride reduction M. Mitov, S. Bliznakov, A. Popov, l. Dragieva and l. Markova ............................. 623
Problems of the intercalated layer structures C.D. Tovstyuk and C.C. Tovstyuk ........................................................................... 629
Cryochemically processed Li2Cu02 for lithium-ion batteries A. V. Egorov, O.A. Brylev, a.A. Shlyakhtin, T.L. Kulova, A.M. Skundin and Yu. D. Tretyakov ..................................................................................................... 633
Investigation of charge carrying in Li-intercalated ordinary and oxidized graphite-like materials
V.S. Kuts and V. V. Strelko ...................................................................................... 635
Thermodynamics and kinetics of lattice gases: statistical mechanics perspective V.S. Vikhrenko, G. S. Bokun and Y.G. Groda ......................................................... 641
Electrical and electrochemical properties of LiNi1•yCOy0 2 prepared by sol-gel method L. EI-Farh, S. Ziolkiewicz, M. Benkaddour and C. Julien ..................................... 643
Author index ............................................................................................................... 645
Subject index .............................................................................................................. 649
PREFACE
This volume is based on the lectures at the NATO Advanced Study Institute, entitled 'New Trends in Intercalation Compounds for Energy StorageN
, held at Sozopol, Bulgaria, from September 22 till October 2, 2001. It attracted almost 82 participants from 18 different countries. A total of 38 lectures has been provided during this AS!.
The meeting combined different types of scientists from advanced experts to aspiring young researchers. It aimed at stimulating future developments by providing­ across borders-cross-fertilisation and exchange between previously unconnected groups. This is reflected in the contents of the volume which covers the lectures given. The book also contains in a second and third parts seminar and poster presentations mostly from younger participants with valuable complementation and specifications to the lectures. The subject of intercalation compounds is a major development in high technology which bears considerable industrial potential. It is important to give the opportunity to young scientists and engineers to be rapidly updated by the best experts in the field. Meeting of western world specialists with scientists from the newly freed eastern countries is considered as an eminent priority because it concerns the basic training of future engineers and the modern industrial development of these countries. The selection of the participants was carefully planned with these perspectives in mind.
With its topics the Advanced Study Institute constitutes an attempt to bring together in an organised manner the areas work on new technologies for uniting materials scientists with chemists, electrochemists and physicists with the hope of increasing communication and understanding of various aspects of sophisticated materials. Recent advances in electrochemistry and materials science have opened the way for the evolution of entirely new types of systems for energy storage, the rechargeable lithium-ion batteries, the electrochroms, the hydrogen containers, etc. with greatly improved electrical performance and other desirable characteristics.
The book encompasses all the branches linked in the progress from fundamentals to applications: from description and modelling of different materials to technological use, from general diagnostics to methods related to technological control and operation of intercalation compounds. Designing devices with higher specific energy and power will require a deeper understanding of materials properties and performance. In this way the status of materials and advanced efforts based the development of new substances for energy storage are covered.
The main topics developed of this book are as follows. Two brief reviews presented here introduce the field of intercalation compounds
with special emphasis to energy storage. Engineering design and optimisation of materials are summarised.
The following six lectures concern the principles and technological developments of carbon-based materials; carbon anodes have been treated in all forms: amorphous carbons, graphite and GICs, hard carbons, fibres, nanotubes, fullerenes, etc. Parameters determining the potentials and capacities of electrochemical cells, thtO fundamental aspects of intercalation reactions linked to battery operation are reported. Materials
xi
xii
obtained from residua of the petroleum industry find also some special emphasis. By a number of lectures great weight is also given to recent work concerning the synthesis and structural characteristics of new compounds. Application of alternative anodes. in particular oxides and antimony alloys for lithium batteries, are extensively discussed.
The excellent examples of intercalation compounds for energy storage are. of course. metal hydrides. Hydrogen insertion is experimentally evidenced in metals. alloys and nanocomposites. The discussion on the structural modifications occurring in various insertion compounds is presented in a systematic way, where the interplay between theory and empirical data are emphasised. One lecture on new compositions aOO structures of Ni-based intermetallic compounds covers the fundamental background on which Ni-MH batteries optimisation is envisaged. Several lectures on the present status and progress in the field of conductive polymers and hybrid materials as insertion electrodes for energy storage applications are combined in this volume to an extensive review.
In addition to structural and physico-chemical properties of intercalation materials. in which ions and electrons are exchanged along the charge transfer cycles, synthesis processes are widely evoked. Here. we have complementary sets of lectures which covered all aspects of material growth. Low-temperature route. mechanical milling. etc. are evoked. One of these sets dealt with transition-metal oxides prepared via wet chemistry method, namely sol-gel process. Specific modem developments aOO openings to applications for modified electrodes. i.e.. doped and substituted materials. are discussed with a luxury of precision and details. The discussion around these sets of lectures gave the prospective for todais batteries and projected tomorrow's power sources.
A set of surface science investigations such as XPS, LEEDS. EELS. etc .• which are probes for electronic structure, are also treated. It is high vacuum physics aOO technology that allows in-situ intercalation of alkali metals in layered frameworks. An extensive discussion on the present status of research in advanced diagnostic techniques for investigation of the charge transfer was presented at the Institute.
It was also important to consider the practical ways in which the intercalation compounds are applied in devices such as lithium-ion batteries. The present status of the technology. difficulties encountered. and advances to be expected are widely examined. These aspects were covered in a set of lectures addressing the fundamental reactivity arxl safety of advanced lithium batteries. The effects of growth characteristics and preparation techniques on the performance of materials. were examined in detail.
By the structure of the program and the quality of the lectures this Institute constitutes an excellent basis for further development of scientists and engineers in this rapidly growing new field of intercalation compounds applications.
December 2001
ACKNOWLEDGEMENTS
It is a pleasure to acknowledge with gratitude the award from NATO Assistant Secretary General for Scientific and Environmental Affairs which was possible the Advanced Study Institute on "New Trends in Intercalation Compounds for Energy Storage" within a partnership programme on high technology. We are grateful to the NATO Science Committee and the Scientific and Environmental Affairs Division for their interest and helpful attitude to arrange the meeting.
The help given by NATO to participants from Greece and by NSF to students from USA is also appreciated for the preparation and planning of the Institute.
Our thanks for financial and organisational support are also directed to the Institut des Hautes Etudes pour Ie Developpement de la Culture, de la Science et de la Technologie en Bulgarie, Paris, and its director Professor Minko Balkanski (Universite Pierre et Marie Curie, Paris).
We are indebted to our colleagues, lecturers and members of the ASI Scientific Committee, for their valuable advise. We want to address our special thank to our colleagues coming from North America, Prof. Jacques Huot and Dr. Karim Zaghib who, despite a very difficult situation following the appalling events of September 11 th, reached to Sozopol on time.
Of great value for the present success of this Institute and the future development of the Centre for Scientific Culture in Bulgaria is the personal involvement of Dichko Fotev, administrator of the Sozopol Pochivna Basa of Bulbank.
We also wish to thank Miss Delphine Julien for giving so much of her energy and helping the organisation of the Institute.
Finally, sincere thanks must go to the authors of the papers. Without their timely submission of their manuscripts of high quality, publication of these proceedings would not have been possible.
xiii
C. JULIEN i , J.P. PEREIRA-RAMOS2 and A. MOMCHILOV3
J LMDH, UMR 7603, Universite Pierre et Marie Curie 4 place Jussieu, case 86, 75252 Paris cedex 05, France 2 LESCO-CNRS, 2 rue Henri-Dunant, 94232 Thiais, France 3CLEPS, Bulgarian Academy of Sciences Acad. G. Bonchev Bl. 10, Sofia 1113, Bulgaria
As society becomes increasingly more dependent on electricity, the development of systems capable of storing directly or indirectly this secondary energy form will be a crucial issue for the 21st century. Batteries, which are devices converting the energy released by spontaneous chemical reactions to electricity work, have some extraordinary properties in these regards. They store and release electrical energy; they are portable and can be used flexibly with a short lead time in manufacture. In this brief introduction, we show the important functions of intercalation compounds used in advanced systems for energy storage.
1. Energy Storage Ability
Energy storage (ES) can be obtained through various ways depending on the stored energy:
- mechanical, like water stored behind a dam (potential energy) or like high speed heavy wheel used to start marine engines (kinetic energy),
- electrical in capacities (voltage is equivalent to potential energy) or in superconductive coils with high values current (equivalent to kinetic energy),
- chemical by storing separately two chemical elements like Li and F, or molecules like H2S04 and H20 able to react when in contact, producing a high value bonding energy. Also, ES in explosives where the energy is obtained by chemical reorganisation pertains to this class. Both use some kind of stored potential energy, differences of the chemical potential.
- electrochemical storage is a variant of the chemical one where the stored energy depends on the difference of bonding energy between two different compounds of the same element, or ion, one used as anode, the other one as a cathode. Classical example is the lead battery where the oxidation degree of lead changes from one electrode to the other one. Now a new very important component appears in the form of the electrolyte able to transport the ion, i.e. in that case sol. Two configurations exist for this ES: either the chemical components are compressed together, with a mechanical separator, between the electrodes able to give only one discharge and then we speak of primary battery. or the system is reversible (secondary) and able to be recharged electrically, as we do pump water at the top of a dam. Another application is to use the high surface area
C. Julien et al. (eds.), New Trends in Intercalation Compounds for Energy Storage, 1-8. © 2002 Kluwer Academic Publishers.
2
of nano-structures with possibly intercalation compounds to produce thanks to thin layers of charged species high value capacities. Values such as Fig are now easily produced with the constraints due to double-layer formation and potential stability (the earth capacity in space is -1 F !).
The purpose of these lectures is to study in details the electronic structure and the new bonding modes present in intercalation compounds of various morphologies, and with intercalated species, to satisfy the storage properties.
2. The Sustained Energy
It is well kwon that the present production on use of energy displays serious problems to the global environment, particularly in relation to greenhouse gas emission such as carbon dioxide which provokes climate modification [1]. The challenge in moving towards global energy sustainability can be assessed by the trends in the use of fuels for primary energy supplies. Table 1 gives statistics reported by the International Energy Agency (lEA). The total primary energy supply in the world, in megatons oil equivalent (Mtoe), was 5096 Mtoe in 1998 [2]. The lEA's forecast of the world demand is 13,700 Mtoe in 2020.
TABLE 1. Total primary energy supply by fuel (in % terms) for the world and forecast (lEA data [I]).
Energy supply 1973 1998 2010 2020
Oil 44.9 53.2 38.8 3~.3
Coal, biomass and waste 36.1 24.4 28.4 28.7
Gas 16.3 18.8 23.6 25.2
Nuclear 0.9 1.3 5.8 4.4
Hydroelectric power 1.8 2.1 2.6 2.6
Geothermal, wind, solar and heat 0.1 0.2 0.7 0.8
The goal of global energy sustainability implies the replacement of all fossil fuels (oil, coal, natural gas) by renewable energy sources (geothermal, biomass, hydrogen, batteries, etc.). The large explosion of systems able to store energy may be considered to be due to influences related to economy and connected to basic problems in industrialised countries from the economical, environmental, technological and political points of view. Many technologies were recently developed for producing, storing and saving electricity; they include lithium and lithium-ion batteries, fuel cells, electrochromics, supercapacitors, etc. To reach the goal of a high specific energy and energy density, two fundamental requirements must be met by electrode materials: a high specific charge, in mAhlg and a large difference between standard redox potential of the respective electrode reaction leading to a high cell voltage. These preconditions are usually achieved by reactions of insertion electrode materials [3-4].
As electrical energy storage is required to power microelectronics, i.e. cell phones and pagers, stand-by power systems, it is obvious to consider the recent tendency of the technology. Figure 1 shows the evolution of voltage at which the semiconductor devices operate in a cellular phone. The voltage decrease is related to the thickness of integrated circuits. This picture demonstrates that the powering voltage could reach the range 2-3 V for integrated circuit with a thickness of -0.2 mm. Of course, this issue led to the design of batteries for telecommunication devices.
8.0 .............. ..,....., ....... ,....,...,....,.....-r"T"T""'r'..,....,,....,...r"T""'I"'"T......,r-T""1
Year 96 98 00
Figure 1. The evolution of the voltage at which the semiconductor devices operate in a cellular phone.
3. Intercalation Compounds
3
In intercalation systems such as Li in graphite, hydrogen in metals, and alkali metals in transition-metal dichalcogenides [5], the guest atom occupies certain host sites preferentially at low temperatures while others remain empty. Because of this specificity of the interactions between the guest atoms and the intercalation lattice sites, a wide variety of intercalated structures can form, including ordered vacancy compounds. This topotactic intercalation mechanism is the basic concept for the material's application as an electrode in rechargeable batteries, electrochromic displays, smart windows, etc. The topotactic insertion-extraction reactions occur by diffusion in one-dimensional (ID) channels, by 2D diffusion or by 3D diffusion. Typical examples are given in Table 2.
Low dimensional materials are particularly susceptible to intercalation reactions due to the presence of the weak van der Waals forces between either strongly bonded chains or layers, but three dimensional materials can also be host to intercalation provided that interstitial sites exist and that these are accessible to the incoming guest. This usually implies the presence of channels which will aid diffusion. Further, some amorphous substances can be used and this area is receiving increasing attention. An intercalation reaction is topotactic in nature; the reaction does not involve diffusive rearrangement of the host atoms. Moreover, the guest species in such reaction may be neutral, an electron donor, or an electron acceptor. For instance, water is neutral guest as interlayer species in clays; oxygen is an electron acceptor into the intergrowth structure of Ba2 YCU306, which converts to superconductor; lithium is an electron donor in secondary-batteries electrodes such as LixTiS2, LixV20 s, or Li\.xMn204' In addition to the intrinsic properties of the intercalation compounds itself, the geometric design of an insertion-compound electrode is critically important [6]. The principle of the geometric design is similar for every application. An important strategy in the design and optimisation of an electrode is the use of smaller intercalation-compound particles. The smaller the particle size, the shorter is the distance a guest species must diffuse in the solid electrode and the smaller the change of the volume to surface area of the individual particles during a discharge/Charge cycle (intercalationldeintercalation process). Nevertheless, a compromise has to be found to balance the particle size and the grain boundaries.
4
Dimension
Na,W03. Li,KFeSz graphite. coke. hard carbon (LiC6) Li. TiSz. Li.MoSz intergrowth Baz YCu306+d Lij.xCoOz. Lij .• NiOz Hz.MgzNi. LaNisH6 LixVzOs. Lij.,V30S. H,MnOz. LixMnOz Lij.,Mnz04. Li,Fez(S04)3. Li4l3Tis1304
4. Systems and Cell Development
The reason of the widespread application of intercalation compounds is their electrochemical insertion ability (electroinsertion) which is intrinsically simple and reversible. The term electroinsertion refers to solid-state redox reaction involving electrochemical charge transfer coupled with introduction of mobile guest ions into the lattice of the solid host.
4.1. LITHIUM-ION BATTERIES
Intercalation compounds appear mainly as a help for electrochemical storage. The idea of using materials that undergo insertion reactions as the electrochemically active components of batteries began to be explored and accepted in the early 1970s [7]. Two approaches consist in designing rechargeable lithium batteries with the use of insertion compounds. The first system utilises an insertion material as a lithium-ion accepting cathode material and a lithium metal as the negative electrode, the so-called lithium­ metal battery, i.e. LilffiS2 and LilN60 13 cells. The second system consists in using two open-structured materials as electrodes (Table 3), in which the lithium ions can be shuttled from one intercalation compound acting as lithium-ion source to another lithium­ ion accepting material. This type of battery is commonly known as a lithium-ion (Li-ion) battery, i.e. LixCJILiCo02 and LixCJILiMn204 cells.
A good reversibility is frequently obtained and the voltage, up to 4 volts, depends on the chosen intercalation compounds, mainly the oxide in that case. But a good knowledge of the sites where the active species is stored, is useful because we need to keep almost constant the potential energy of the free element (e.g. the potential energy of Li between graphene sheets is practically the same as that of free metal). Basically, the charge-discharge reactions of lithium batteries involve the generation of lithium ions and their migration across the electrolyte and insertion into the crystal lattice of the host electrode materials (Fig. 2). For instance, lithium reacts with LiCo02 according to the reaction
(1)
in which 0 represents the vacant octahedral sites. We need also to keep attention to the mobility of the working species, related to the diffusion through the potential barriers around the sites (e.g. frozen Li in Li-batteries stops their efficiency at -30°C), and to the parasitic reactions with the electrolyte. the mechanical barrier, etc.
5
Any liquid or solid lithium-ion conductor may be used as a suitable electrolyte. Solutions of lithium salts in aprotic organic solvent or solvent mixtures are examples of liquid electrolytes, while lithium ion conducting polymer membranes are examples of solid electrolytes.
Positive electrode Electrolyte Negative electrode
Figure 2. The principle of lithium-ion battery.
Figure 3 shows the cell voltage vs. capacity for various intercalation compounds used in lithium batteries. In the past few years, Li-ion batteries have been introduced into the consumer market, particularly the cellular phone and camcorder segments [8]. Li-ion batteries excel through their high cell voltage, low weight and volume for given stored energy, favourable power output and long cycle life. These outstanding features have led to considering Li-ion batteries for electric vehicle (EV) applications [9].
5.0
LiCoQ,
> a-MnO z. MO~ •• V02 • '-' LiV3 0s 11.} TiS z 00 ~
f- • .CrO z ...... 2.0 M'i '0 > MoQ, • • TiOz
1.0 f- .WQ.
I I I
100 200 300 400 500 Capacity (mAh/g)
Figure 3. Cell voltage versus capacity for various intercalation compounds used in lithium batteries.
6
TABLE 3. Electrochemical equivalents of negative and positive electrodes used in lithium-ion batteries.
Materials Molar weight Density Reversible Specific capacity
(glcm3) range (Ox) (mAhlg) Negative electrodes U.C6(C) 72.06 2.0 0.5 186
U.~(g) 72.06 2.25 1.0 372
Wch 215.8 12.11 1.0 124
MoCh 127.9 6.47 0.5 105
U413 Tis1304 153.1 1.0 ISS
Positive electrodes UMn204 180.8 S.16 1.0 148
UCOO2 97.9 4.28 0.5 137
UNiCh 97.6 4.78 0.7 192
4.2. Ni-MH BATTERIES AND HYDROGEN STORAGE
Intermetallic compounds can store reversibly large quantities of hydrogen through solid gas reactions to form intermetallic hydrides, which has prompted their application in the field of the electrochemical and chemical storage.
The battery application concerns the use of intermetallic hydrides as negative· electrodes in the nickel-metal hydride battery to replace the toxic heavy metal Cd electrode in Ni-cadmium batteries. Major activities in the direction of developing commercial Ni-MH rechargeable battery for portable devices concern the study of ABn-
type intermetallic compounds (A: rare earth; B: transition metal; 2~n~5). More particularly, two classes of binary or pseudo-binary intermetallic compounds are currently being developed: AB2-type alloys (Laves type) are largely investigated due to their inherent high hydrogen absorption capacity, but they suffer from passivation, slow activation and corrosion. On the other hand multi component ABs-type alloys (Haucke phase CaCus) are now widely used in commercial batteries, despite some improvements are still needed from kinetics and cycle life behavior. Electrodes made from ABs compounds and AB2 Laves phases type alloys allow to reach electrode capacities between 200 and 350 mAhlg and energy densities which are 30% higher and more than those of conventional Ni-Cd cells.
In addition to their application as rechargeable battery electrodes, metallic hydrides may be used for chemical storage as they can store hydrogen at low pressure while having volumetric densities comparable to liquid hydrogen. In addition to their utilisation as rechargeable-battery electrodes, insertion compounds may be used for chemical storage. Metal hydrides are considered a promising means of hydrogen storage mainly because they store hydrogen at low pressure while having volumetric densities comparable to liquid hydrogen. These systems are inherently safe because the release of hydrogen in metal hydrides being an endothermic process [10]. Furthermore, the concept of nano-crystallinity is a new issue for hydrogen storage. Table 4 lists the density and hydrogen storage capacity in various systems.
For instance, hydrogen may be stored more densely and conveniently in a hydride such as H2xMg2Ni than as a liquid. It can be retrieved from the hydride by modest heating in the thermally controlled reversible reaction
(2)
7
TABLE 4. The density and hydrogen storage capacity in various systems.
System Density (mol H:z/dm3) Storage capacity H2 wt. %
MgH2 55 7.7
LaNisH6 52 1.4
gas (275 K, 1 bar) 0.045 100 Iiquid(20K) 35 100
Hydrogen storage in porous carbon structures near the normal conditions of temperature and pressure has triggered a great deal of research in order to provide storage facility for transportation systems [11]. The case of hydrogen insertion in carbon is specific, quite different from light alkaline metals. It has not been proposed for energy-chemical storage, but the hydrogen storage itself in dense forms is problematic and if carbon succeeds in this task, it should be considered as a chemical storage.
4.3. ELECTROCHROMICS
Reduction or oxidation of a transparent, i.e. white-powder, host changes its colour, which makes possible an electrochromic displays or a smart window. For example, white W03
film associated with an acidic electrolyte can be used in an electrochromic display since the reversible reaction
(3)
products a dark blue tungsten bronze, Hx W03• Paired electrochromic reactions have been used to construct a smart window [12].
4.4. ULTRACAPACITORS
For applications in which significant energy is needed in pulse form, traditional capacitors as used in electronic circuits cannot store enough energy in the volume and weight available. For these applications, the development of high energy density capacitors has been undertaken by various groups around the world [13]. The simplest capacitors store energy by charge separation in a thin layer of dielectric material that is supported by metal plates that act as the terminals for the device. The energy stored in a capacitor is given by 1I2CY2, where C is its capacitance and Y is the voltage between the plates. In an ultracapacitor, the electrodes are fabricated from high surface area, porous material having pores of diameter in the nanometer range. There are carbon black, aerogel carbon, anhydrous RU02 or doped conducting polymers providing capacitance in the range 100-1300 F/cm3• Projections of future developments using carbon indicate that energy densities of 10 mWh/g or higher are likely with power densities of 1-2 Wig.
s. Concluding Remarks
Investigations on intercalation compounds constitute a multidisciplinary research of modem material science and technology. There is also considerable global activity in wider applications on insertion reactions that constitute a breakthrough with these multi­ component materials in the development of storage energy. The fascination with electrical energy storage is driven by the potential superior performance of materials, by environmental necessity, and by the fundamental challenges these technologies present.
8
The most advanced technology using intercalation compounds concerns the search of high performing systems for energy storage such as metal-hydride batteries, lithium­ ion batteries, electrochromics, etc. All these systems need optimised intercalation compounds. Recently, new applications for nano-structured intercalation compounds have been proposed in thin-films micro-batteries for powering sensors, biomedical devices, credit cards, etc.
The aim of this NATO-ASI is to stimulate an intense discussion on the fundamental and technological properties and prospects of a large variety of intercalation materials such as graphite, carbonaceous compounds, oxides, chalcogenides, clays, metal hydrides and other related materials.
References
1. NOAA (2001) Washington, USA (http://w.w.w.al.noaa.gov).ThegreenhousegasesincludeC02.CA4. 03, N20 and CFC. In 2000, the atmospheric concenttation of CD2 was 368 ppm against 280 ppm in 1750. Each year, 5 metric tons of carbon dioxide are added to the atmosphere for each person in the USA.
2. International Energy Agency (2000) Key World Energy statisticsjrom the lEA, 2000 Edition, Paris, France. 3. Winter, M., Besenhard, J.O., Spahr, M.E., and Novak, P. (198) Adv. Mater, 10,725. 4. Julien, C. and Nazri, G.A. (2001) in U.S. Nalwa (ed.) Handbook of Advanced Electronic and Photonic
Materials, Academic Press, San Diego, vol. 10, 99. 5. Whittingham, M.S. (1978) Prog. Solid State Chem. 12,41. 6. Julien, C. and Nazri, G.A. (1994) Solid State Batteries: Materials Design and Optimization, Kluwer,
Boston. 7. Armand, M. (1980) in D.W. Murphy, 1. Broadhead and B.C.U. Steele (eds.), Materialsfor Advanced
Batteries, Plenum Press, New York, p. 145. 8. Nagaura, T. (1991) Prog. Batteries Solar Cells 10, 218. 9. The following considerations are based on the performance of the General Motors Impact EV with a curb
weight of 1,350 kg. The car would achieve a range of -150 miles on the highway and -120 miles in city driving using a 24-kWh Li battery weighting -160 kg.
10. Uuot, 1. (2001) in this Book. II. Marelli, F., Pellenq, RJ.M, and Conard, J. (2001) in this Book. 12. Campet, G., Portier, J., Wen, SJ., Morel, B., Bourrel, M., and Chabagno, J.M. (1992) Active Passive Elec.
Compo 14, 225. 13. Burke, A. (2000) J. Power Sources 91,37.
LITmUM INTERCALATION COMPOUNDS. THE RELIABILITY OF THE RIGID-BAND MODEL
C.JULIEN Laboratoire des Milieux Desordonnes et Heterogenes, UMR 7603 Universite Pierre et Marie Curie 4 place Jussieu, case 86, 75252 PARIS cedex 05, France
Numerous layered structured compounds are interesting materials in which lithium intercalation occurs primarily without destruction of the host lattice. In many cases a rigid band model is a useful first approximation for describing the changes in electronic properties of the host material with intercalation [W.Y. Liang, Mater. Sci. Eng. B 3 (1989) 139]. This review paper presents results obtained on transition-metal chalcogeoide compounds and effects of lithium intercalation on transition-metal oxides as well. We observed, nevertheless, that the rigid-band model is not applicable to all of the layered intercalation materials. One may argue that the applicability of the rigid-band model may be taken as a test for the properties most desirable in a good intercalation material. This needs yet to be more extensively documented for their promising applications as insertion electrode in solid state batteries.
1. Introduction
Layered compounds, in particular the transition-metal dichalcogenies (TMDs) arxl transition-metal oxides (TMOs), can be intercalated with a wide range of both organic and inorganic materials which may have a profound influence on the physical properties of the host compound. The intercalation reaction in these compounds is driven by charge transfer from the intercalant to the host layered compound conduction band and thus electron-donating species can take place in such a reaction. The reversible ion-electron transfer reaction is classically represented by the scheme
(1)
in the usual case where (H) is the host material, A an alkali metal and x the molar intercalation fraction. The electronic transport plays an important role in such reaction toward the formation of intercalation compounds. It also governs the phase transitions as the parameter expansion of the host structure has an electronic component. Consequently, it is possible to consider three classes of intercalation reaction that correspond to the different steps in the delocalisation of the transferred electrons. The level of acceptance can be either a discrete atomic state, or a molecular level of a discrete polyatomic entity existing in the structure, or part of a conduction band.
The rigid-band model (RBM) is a useful approximation for describing the changes in electronic properties of the host material with intercalation. Sellmyer [1] distinguishes two versions of the rigid-band model for dilute solid solutions which might be called the
9
C. Julien et al. (eds.), New Trends in Intercalation Compounds for Energy Storage, 9-26. © 2002 Kluwer Academic Publishers.
10
electron-gas RBM and the screened-impurity RBM. In the former, due mainly to Jones [2], the valence electrons are regarded essentially as in plane wave states and the only effect of alloying with an element having a valence difference tlZ, is to change the free electron density to a new value, that obtained simply by scaling the valences of the solvent and solute according to their atomic fractions in the alloy. In the screened­ impurity RBM of Friedel [3], it is recognised that the electron gas cannot support an electric field at long distances from the charge impurity. It can be shown with a dielectric-screening or Thomas-Fermi level that the conduction electron will redistribute themselves to screen out the Coulomb field. In this case, any charged impurity added to a solid will polarise the solid, and attract to itself.
EA
c e I. ca e U.
Figure I. The fundamental question concerning the evolution of the Fermi level.
The concept of rigid-bands implies chemical stability of the system. From the energetic point of view, this means the total energy of the substance is little affected by the addition of intercalant electrons. The consequence is that the structure too is stable, and the only energy band involved in intercalation is the narrow "d" conduction band in TMD materials for instance. These are precisely the properties most desirable in a good cathode material which provide features such as stable voltage against ageing and mechanical durability [4]. It is most important, therefore, to investigate theoretically and experimentally how well the approximation can apply in a system employed for the lithium battery cathode. In this paper the validity of the rigid-band model is demonstrated by optical experiments and band structure examination.
2. Lesson I. Lithium Intercalation in TiS2
Electronic band structure ofTMD materials have been generally calculated using simplest molecular-orbital arguments [5]. The schematic band structure of the MX2 compounds with octahedral and trigonal prismatic co-ordination are shown in Fig. 2. In the simple picture, during intercalation, the donating-electrons will occupied one of the empty d­ bands. The simplest approximation to the band structure of an intercalation compound is just that of the parent host compound with the Fermi level moved up to accommodate the extra electrons.
In LixTiS2 the magnitude of the Hall coefficient decreases with increasing lithium content, confirming the occurrence of electron transfer from the intercalate to the host [6- 8]. The electron concentration is TiS2 before intercalation is 3. Ix 1020 cm·3 indicating that
11
this material is of stoichiometry Til.0044S2 [8]. Upon lithium intercalation we observe a large decrease of the resistivity as well as of the Hall coefficient. The carrier concentration in electrointercalated samples increases to 5x1021 and 9.6x1021 cm-3 for x=0.25 and x=O.5, respectively. The Hall coefficient RH of all the samples is nearly temperature independent, as would be expected for a normal metal.
octahedral trigonal prismatic (a) (b) (a) (b)
Density of states N(E)
Figure 2_ Schematic band structures for all the TMDs compounds with octahedral and trigonal prismatic co­ ordination. Sketches show the electronic structures before (a) and after (b) lithium intercalation with the respective position of the Fermi level.
Klipstein et al. [6] explain this behaviour by a model involving the interplay between inter-pocket and intra-pocket scattering of electrons by longitudinal acoustic phonons, whereby the increase in Fermi surface dimensions reduces the restriction on the wave-vector of phonons that may take part in the scattering process, implying that, as the carrier concentration increases, a should tend towards unity and, simultaneously, the temperature T min' below which the In(p-po) versus In(T) curve starts deviating from linearity, should increase. This model, which was originally based on studies in pristine TiS2 with a varying degree of stoichiometry, was later verified to remain valid for higher carrier concentrations, such as in TiS2 intercalated with lithium via the BuLi technique [7] or intercalated with hydrazine [9].
Electron transfer is also apparent in the optical properties of this system. Fig. 3 shows the absorption spectra, in the energy range 0.5-6.0 eV of pure and Li-intercalated TiS2 [10]. In the spectrum of the intercalation complex it is clearly seen that it shows free carrier absorption below 1 eV for LiTiS2• We also observe interband transitions which are the first direct allowed transitions from the p valence to the d conduction band at the L point of the Brillouin zone. Moving into the spectrum of Lix TiS2, the onset of inter-band transitions is seen to have shifted to higher energies and the oscillator strength under the absorption band is roughly halved. Beal and Nulsen [10] argue that this is exactly that one would expect if the dz band is now half-full following saturation of the intercalation complexes.
12
Energy (eV)
Figure 3. Room-temperature absorption spectra of TiS2 and LiTiS2 (after [10]). The schematic band structure shows that the rigid-band model may be used for the intercalation complexes.
Another optical experiment is the infrared reflectivity carried out on Li-electro­ intercalated TiS2 [8]. Fig. 4 shows the temperature dependence of the reflectivity spectra of pure TiS2 and Li intercalated TiS2 single crystals. We observe a large shift in the plasma edge for Li1.0TiS2 with respect to pure TiS2. According to the single carrier Drude model, the analysis of the dielectric function gives the values as follows. In TiS2• the plasma edge lies around 1200 cm-1, whereas in LiTiS2 the plasma edge occurs at about 4000 cm-1 giving plasma frequencies of 1360 and 4100 cm-1, respectively, if we take into account that the high-frequency dielectric constant remains similar to that of the pristine material and if we consider the electron effective mass as obtained by Isomaki et al. [11]. At the L-point of the Brillouin zone, Isomaki et al. estimate m.=O.4mo along the a-axis. This assumption implies that the optical effective mass mopt has a value higher than 1.3ffio. In the present studies, the Drude analysis gives a carrier concentration of 1.7x1022 cm-3 for complete intercalation of TiS2 at x=l. This is in excellent agreement with the theoretically expected value of 1.75x1022 cm-3 and very close to the value of 2.2x1022 cm-3 determined from Hall measurements.
We assume in the spirit of the rigid-band model [4] that intercalation does not change appreciably the conduction band effective mass, nor the high-frequency dielectric constant of the host material. The charge transfer dn from the alkali-metal atoms to the d-conduction band of the host compound can be directly calculated from the difference between 00/ before and after intercalation. Here dn is expressed in terms of the number of electrons transferred per Ti atoms. Using this method we have dn=0.9±0.1 electrons. The uncertainty of 0.1 electrons is thought to be a reasonable estimate in view of the
13
assumptions made. It is interesting to note the large increase of the plasma damping factor from 310 to 2160 cm-l in Til.oosS2 and Li1.0Ti1.00sS2' respectively_ This increase is observed in the energy-loss function by the broadening of the plasmon peak The damping factor can be expressed as follows
r = lit = q/m*~H' (2)
where ~H is the Hall mobility of free carriers_ The observed increase of r suggests a decrease of the Hall mobility or a modification of the effective mass tin the intercalated sample_ In Li!.OTi1.00SS2 the electron mobility measured by Hall effect has a value of 1.9 cm2V-ls-l at room temperature [8]_ This value can be related with those given in the literature of 13_5 and 035 cm2y-ls-l for TiS2 and LiTiS2, respectively [71-
100 100
.... • u u
50 50
.. -. 0 0 500 1000 1500 2000 2500 1000 2000 3000 4000 5000
Wavenumber (em-I) Wavenumber (em-I)
(a) (b) Figure 4. IR reflectivity spectra of (a) TiS2 and (b) LiTiS2 as a function of temperature.
IR reflectivity spectra of the Li1.oTiLoosS2 sample shows surprising departure from ordinary Drude behaviour, and there is not a strong change in the IR spectra as a function of the sample temperature in comparison with that in pure material [12]. The dip in the reflectivity is close to the plasma frequency rop=4180 cm:l extracted from analysis of the data using a Drude-like model with a frequency dependent relaxation time, as
lIt(x,T,ro) = xto + a[(pT)2 +ro2]. (3)
A good fit to the optical data is achieved with the scattering rate given by Eqn_ (3) which reduces to the ideal electron-electron scattering behaviour in the isotropic three-
14
dimensional effective mass model [13]. For Li intercalated TiS2 sample, the temperature and frequency components of Eqn. (2) are strongly screened by the first term (x'to) as shown in Fig. 5. This may be due to the complete filling of the d-band associated with a very low Hall mobility. In this case, it is difficult to evaluate the optical mobility because the quantity o.>'t» 1 is no more valid. Considering that Hall measurements on the LiJ.oTiJ.oosS2 sample give NH=1.8x1022 cm-3 and that the Fermi energy obtained by optical reflectivity measurements is Ep=4180 cm-1=0.52 eV, we may estimate the electron effective mass m*=0.49 mo. This value is very close of that in pure material reported by Isomaki et al. [11]. In conclusion, it can be seen from the electrical and optical properties of the Li- intercalated TiS2 presented above that, s-bands aside, they can all be explained in terms of the rigid-band model. It is worth mentioning here that optical absorption results by Scholz and Frindt [14] on Ag-intercalated TiS2 also agree with this model.
L­o ~ 900 'l--
g' ·50 E to
o 100 200 300 Temperature (K)
Figure 5. Temperature dependence of the damping factor (inverse relaxation time) for (a) TiS2 and (b) LiTiS2•
3. Lesson II. Lithium Intercalation in TaSz
Among the group-V TMDs, TaS2 has perhaps been the subject of greatest interest, because of the fascination range of structural and electronic properties that this material exhibits. Due to valence electron occupying their di band, this metallic compound can exist in either 1 T-, 2H-, or 4H-structure [15]. As a consequence of the switching from octahedral (Oh) to trigonal prismatic (TP) co-ordination, the shift of the dz2 band to lower energies occurs gradually. The absorption spectrum of pure 2H-TaS2 shows a Drude edge below 1 e V associated with the free-carrier absorption in this material owing to the half­ filled dz2 band. After intercalation, the Drude edge disappears and the first dz2-1d transition shifts toward lower energy. These changes are attributed to the gradual filling of the dz2 band by electron transferred from lithium.
The absorption spectrum of IT-TaS2 is shown in Fig. 6. Above 1.5 eV, the absorption bands are associated with the dZ\---7dz22 and dz\---7d transitions, whereas the band above -3.5 eV owing to p---7di2 transition. Charge transfer from Li to the host
15
lattice increases the population of the dz2 band and raise the Fermi level Ep to a new energy, E' p. The displacement of the strong absorption edge around -3 eV indicates a considerable lowering of the dz2 band with respect to its position in the pristine material. The lowering of the dz2 band is attributed to the filling with electrons donated by Li, as well as the modification of the crystal structure, e.g. an increase of the cIa ratio after intercalation [16]. These results provide further support to the argument that, upon lithium intercalation the rigid-band model is not entirely applicable in 1 T -TaS2.
c ..J6 I ..... c z4 e Q
~2 < ,----- ........ _----,' -- O~1~--2~--3~--4~--~5--~6
Energy (eV)
Figure 6. Absorption spectra of pure IT-TaS2 and IT-LixTaS2• The schematic band structure shows the lowering of the dz band upon Li intercalation (after [15]).
4. Lesson III. Lithium Intercalation in 2H-MoS2
Among the group-VI TMD, MoS2 is one of the materials where intercalation reactions induce a transition of the host related to local ligand field modification. In that particular case, molybdenum presents a trigonal prismatic sulphur co-ordination which changes to the octahedral one (TP~Oh transition) [17-19]. The structure modification is accompanied by an increase of the M-X band ionicity in agreement with the respective stability of the new atomic arrangement, the Coulomb repulsion between partially charged ligands favouring the octahedral form. Also, comparison of the d-band density cf states for 2H-MoS2 and hypothetical I T-LiMoS2 show that the occupied bands which contain six states are lower in the case of the Oh phase corresponding to the glide
16
process between Mo and S atoms. This is a fine example of destabilisation through lithium reduction.
The transformation from (TP) to (Oh) co-ordination is attributed to a process which is driven by a lowering of the electronic energy for the octahedral structure when electrons are donated from Li to the MoS2 layer upon intercalation [4]. The octahedral transformation starting at x=<O.l completes around x=1.0 and is preserved on subsequent cycling. Cell discharge-charge occurs in the range 2.2-1.3 V with a mid-discharge voltage of -1.7 V. Electron diffraction studies on the Li-MoS2 system have shown that this transformation is accompanied by a 2aox2ao superlattice formation [20].
:t:' S ...l ,,; > (: Gl
!. r-: .' . , t:rN: : ~ .
0.0 0.5 1.0 1.5 2.0 2.5 x(Li) in Lix MoS2
Figure 7. (a) Discharge curve of LilILixMoS1 and (b) related incremental capacity, -rJxjaV.
Fig. 7a-b shows the cell voltage vs. composition of Li/ILixMoS2 and the related variation in the inverse derivative voltage, -ax/iN, at ambient temperature [19]. The natural sample has a 2H-MoS2 structure and upon Li intercalation behaves as a two-phase system. The first phase is the initial material and the second one is the 1 T -structure of Lil.()MoS2 which appears at x=1 [17]. The incremental capacity (Fig. 7b) that the cathode material exhibits a complex intermediate behaviour; at least four states can be observed up to Li\.oMoS2' but the analysis of such a feature is very difficult because the validity of the Fick law requires that the host material remains single phased. However, it is interesting to note that, in this case, the cathode is highly strained and each of the above states can be described approximately with an interaction energy, UO,!' of the intercalant. At room temperature the difference in standard potential UO,! of the successive states is very small.
17
The structure observed at x::0.25 in the incremental capacity (Fig. 7b) may be related with the superlattice formation identified in the Raman scattering measurements of LiO.3MoS2 by Sekine et al. [21-22] and in the electronic microscopy diffraction mode by Chrissafis et al. [20] corresponding to a 2aox2a., superlattice which is interpreted as a pseudo-staging on the basal hexagonal lattice (Fig. 8). However, we do expect that LixMoS2 is accompanied by the raising of the Fermi level due to the charge transfer from Li intercalation. This is depicted in Fig. 9 where the temperature dependence of the electrical conductivity of lithium intercalated MoS2. Undoped MoS2 is an-type semiconductor with o-exp(-E/kBT), where E.=0.05 eV, and Lio.3MoS2 is a highly degenerate semiconductor with 0-rIA. Here, we must be aware of the limitations of the rigid band model. It is probably non appropriate for LixMoS2 because the fully occupied di states in the pristine material imply new electrons to enter the next higher "d" band with a change of the cIa ratio associated with the destabilisation of the host lattice.
Figure 8. Li intercalation of MoS2 in n-butyllithium. (a) After to min. defects are created near the edges and in the steps of the specimen denoted by arrows. (b) After 2 h intercalation. superlattice spots appear (denoted by the letter s). which are indexed as (11201120). Notice also the splitting of the main spots (denoted by the letter M). (c) A microcrograph taken from the same area reveals that the specimen is heavily defected owing to intercalation. (d) Distribution of lithium vs. Distance from the edges of the specimen as revealed by SSNTD images.
Following the above structural transformation, phonon spectroscopy offers an excellent way of quantifying the degree of anisotropy not only by distinguishing inter-
18
and intra-layer normal modes but also determining the shear moduli in different directions. The Raman spectrum of 2H-MoS2 at room temperature is shown in Fig. 10. It exhibits four Raman-active bands that are the intra-layer A!g mode at 407 cm·! involving motion along the c-axis, the intra-layer ~g mode at 382 cm·! involving motion in the based plane, the E!g mode at 286 cm·! and the rigid-layer (RL) mode at 32 cm-! of ~g symmetry. This last mode is of interlayer type involving rigid motion of neighbouring sandwiches in opposite phase.
1000
Temperature (K)
Figure 9. Temperature dependence of the electrical conductivity of lithium intercalated MoSz.
5 D
100 200 300 400 Raman shift (cm-I)
Figure 10. Raman spectra of 2H-MoSz natural crystal as a function of Li content lithium.
The Raman spectra of LixMoS2 with x=O.1 and x=O.3 (Fig. 10) display the structural changes from the B-phase (2H structure) to a a.-phase (IT structure) upon Li
19
intercalation [21-22]. This transformation from trigonal prismatic to octahedral c0-
ordination has been attributed to a process which is driven by a lowering of the electronic energy for the octahedral structure when electrons are donated from Li to the MoS2 layer on intercalation [7.1]. The octahedral transformation in LixMoS2 starts at x=O.l arxl completes around x=l. For a degree of intercalation x:::::O.l, the Raman intensity is considerably reduced (by a factor 5) and we observe two new bands: a broad peak located at 153 cm-' (A-line) and a weak peak situated at 205 em-' (B-line) and the intensity of the RL mode is reduced. The two pristine intra-layer modes can still be observed, with little shift in frequency, but both are split to give weak additional side bands towards lower energies (C- and D-line). These band are attributed to the Davydov pairs of the optical phonon branches. For x=O.3, the spectrum of LixMoSz is modified compared with the former ones. The RL mode is not longer recorded. All other lines are still observed. We remark the small shift in frequency of the lattice modes of MoS2 [22].
A simple model has been used to calculate the frequencies of the new modes appearing after Li intercalation. The intercalation mode is given by
(4)
where m, and m2 are the masses of the MoS2 molecule and of the Li atom, respectively, and k is the force constant between the Sand Li atoms. We estimate k=8.23x103 dynlcm, which is much smaller than the intra-layer force constants.
For a degree of intercalation x=0.3, we assume that LixMoS2 is a two-phase system. The following changes on the lattice dynamics can be expected. The RL mode disappears because the elementary cell of the IT-structure contains only one molecular unit (3 atoms per sandwich). The symmetry changes from D6h to D3d and the new symmetry allows only the two Raman active AlB and Eg modes which are representative of the intra­ layer atomic motions. The weak spacing expansh.>n observed upon Li intercalation arxl the difference of the molybdenum co-ordination do not modify significantly the frequency of these modes. A simplest calculation gives a change of about 6% in frequency. Thus, we can trust the validity of the lattice dynamics model using a 2H-structure [22].
5. Lesson IV. Lithium Intercalation in Mo03
The molybdenum oxides display a varieties of structural types involving linked Mo06
octahedra whose arrangements are favourable for intercalation process. These materials offer high voltages and composition intervals accessible for lithium intercalation are wide. The interest of a-Mo03 arises from its layered structure presenting open channels for fast Li-ions diffusion, a higher electrochemical activity vs. LilLi+ than that of chalcogenides and the highest chemical stability among the oxide lattices [23]. Fig. 11 shows the first discharge charge curves of a LiJ/Mo03 cell using a well-crystallised film. This film grown by sputtering technique on nickel substrate in oxygen partial pressure of 100 mTorr displays the electrochemical features of the a-Mo03 phase [24].
Fig. 12 shows the temperature dependence of electrical conductivity of LixMo03 (O.O=:;x::S;O.3) intercalated by electrochemical titration. One observes a clear departure from semiconducting behaviour of pure Mo03 material to degenerate semiconductor of Li­ intercalated Mo03 even at low Li content The metallic features are also observed in the temperature dependence of the Hall coefficient. The observation of plasma absorption in this material implies carrier concentration of at least 10'8 electrons/cm3 indicating a weak variation of the free-electron effective mass and of the high-frequency permittivity. In
20
Mo03, the bonding framework is composed of five O(p ) and three MO(~g) orbitals which interact to form nand n* bands [25]. As the antibon~ing n* states hold the extra electrons supplied by the inserted lithium, Lio.3Mo03 may be expected to exhibit two­ dimensional electronic conductivity. The narrowing of the conduction band is expected to lead to an increase in the effective electron mass which can affect the position of the Drude edge in LixMo03 phases. The conductivity of Mo03 is believed to exist because of the electron hopping between MoM and Mos+ sites. The nature of the conductivity variation observed in Fig. 11 corresponds to a steadily decrease with the addition of intercalants. Intercalation of lithium ions in the Mo03 structure is believed to lower the valency state of molybdenum ions by transfer of electrons from lithium to molybdenum. The relative concentration of MoM and Mos+ ions results in the lowering of the conductivity towards a insulator behaviour. Further experiments are needed to elucidate the mechanism of the charge transfer occurring in transition-metal oxide compounds but the rigid-band model seems adequate in LixMo03 which found technological application in electrochromic rear mirror in automotive industry.
.-., 3.5 +
~ 3.0 o-l ...; > 2.5 :> '-" cu bIl :I 2.0 '0 > ~ 1.5 U
1.0 0 0.3
1.5
Figure 11. First discharge charge curves of a LilIMo03 cell using a well-crystallised film grown by sputtering technique Ni substrate (after [24)).
10-1 I
LixMoOs '0 '. c:: 0 10-4 r '. u '. ", .. . . , . x=O.OO r . .. . ' . ..
10-5 • I I I I I
2 4 6 8 10 12 14 IOOOff (K-I)
Figure 12. Arrhenius plot of the electrical conductivity of a.-Mo03 and LixMo03•
21
Infrared absorption studies of LixMo03 compounds revealed a transition from metallic to small-polaron features [26]. After intercalation the lattice vibration spectrum is completely screened by the free electrons in the host material. The Drude edge contribution, i.e. plasmon feature, is responsible of the metallic absorption due to high electron density in Lio.3Mo03 as shown in Fig. 13. The free-carrier absorption coefficient can be expressed by
a = wp2 't' [n c (l+Clh2)], (5)
where wp is the plasma frequency, 't' is the relaxation time of the free carrier, n is the refractive index and c the light velocity. Using Eqn. (5), the fit of experimental data gives a carrier concentration of 5xlO16 cm-3 in Lio.3Mo03. This value is in good agreement with the Hall measurements. The temperature dependence of the absorption coefficient shows a small increase of a with temperature, which can be attributed to the fact that Li­ intercalated Mo03 is a degenerate semiconductor for x=O.3.
1.6
1.4
-e 0 0.6 ell
Wavenumber (em-I)
Figure 13. IR absorption spectra of a-Mo03 and Lio,Mo03.
Nadkarni and Simmons [27] studied the electrical properties of Mo03 and reported that there is a donor band between the conduction and the valence bands due to oxygen vacancies. Mo03 has the outer electron configuration 4s5 5s1. If Mo03 is considered to be ionic, i.e. composed only of Mo(VI) and 0 2- ions, the valence band would be composed of oxygen 2p states and the conduction band of empty 4 d and 5s states [28]. Upon Li intercalation the electrical conductivity of LixMo03 increases by two orders of magnitude and the temperature dependence of 0' shows important changes in the conduction mechanism. The semiconducting character of Mo03 gradually disappears and, for a degree of intercalation of x = 0.3, the material exhibits a metallic behaviour.
6. Lesson V. Lithium Intercalation in V60 13
V 6013 is a black material which derives from the Re03 structure and is intermediate in composition between V02 and V20 5• In this family, V30 7 and V40 9 appear to have an intermediate structure between that of V60 13 and V20 5• The monoclinic structure of V60 13
22
contains edge-shared distorted Y06 octahedra forming single and double zig-zag chains linked together by further edge sharing comer-shared. The resulting sheets (single am double) are interconnected by comer sharing, thus giving a tridimensional framework [29]. This structure contains tri-capped cavities joined through shared square faces. The three open faces of the cavity should permit lithium-ion diffusion along (010) with the possibility of exchange between pairs of adjacent channels. Stoichiometric Y 6013 can be written as (y4i-Mys+MQ2-)13 as far as the valency state of the vanadium ions are concerned.
The structure of Y 6013 is interesting from an electrochemical viewpoint due to the theoretical maximum limit of lithium uptake giving a energy density of 890 mWh/g. The stoichiometric Y 6013 structure is believed to accommodate 8 Li per formula unit as determined by the available electronic sites rather than the structural cavities [30]. The limit corresponds to a situation when all the vanadium ions are present in the trivalent y3+ state. As a function of the stoichiometry, the maximum uptake goes to 1.35 Li for Y02.18 oxide. Reversible chemical and electrochemical insertion of lithium into Y60 13
was first demonstrated by Murphy et al. [31-32], and its potential as an active cathode material in practical batteries has since been more fully investigated [30]. The discharge curve exhibits three distinct plateaus, reflecting the sequential ftlling of unequivalent sites in the host structure_
,......., -S ~ en '-' t:l
2.0
0.0
-2.0
-4.0
-6.0
-8.0
lOOOff (1(-1)
Figure 14. Arrhenius plot of the electrical conductivity of V 6013 and Li. V 6013 (OS;xS;6).
Fig. 14 shows the Arrhenius plot of the electrical conductivity of Y60 13 and Li,V6013 (0~x~6). The pure material has a conductivity of lxlO-2 S/cm at room temperature and exhibits a semiconductor behaviour. The electronic conduction in Y60 13
is due to the electron hopping between y4i- and Vs+ states. The nature of the conductivity observed in Fig. 14 corresponds to a steadily decrease with addition of Li-ions in the Li.V60 13 framework. Intercalation of Li-ions is believed to lower the valence state of vanadium ions by transfer of electrons. The relative concentration of reduced cations results 10 the lowering of the conductivity towards a poor electronic semiconductor.
23
Electronic conductivity of pressed V 6013 powders indicates a sharp fall in two steps with increasing Li content [33-34]. For lithiated V60 13, we observe a continuous decrease of the electrical conductivity. Lowest conductivity of 5xlO-4 S cm-l has been measured in Li6V60 13. This is also accompanied by an increase in activation energy, a general phenomenon observed in any oxide with small-polaron conduction.
Infrared studies of Li. V 6013 compounds revealed the transition from metal-like to small-polaron features (Fig. 15). In pure V 6013' one has a Drude edge around 200 cm- l
which is the contribution of the free charge-carriers. For lithiated V 6013' we observe a continuous decrease of the electrical conductivity which is also recorded in the far-infrared spectrum by the disappearance of the Drude absorption [26].
1.0
~ 0.2
Wavenumber (em-!)
7. Lesson VI. Lithium Intercalation in LiCo02
Lithiated transition metal oxides with a layered, a-NaFeOrtype, structure such as LiMe02 (Me=Ni, Co) have been a great interest as positive electrode materials tor rechargeable lithium batteries. LiCoOz has been proposed as cathode for lithium battery by Mizushima et al. in 1980 [35] and currently it is being used in commercial rechargeable Li-ion batteries by Sony [36]. Fig. 16 shows the potential curve of LiCo02
during the first charge in the potential range 2.5-4.2 V vs. LilLi+. The charged capacity was 155 mAhig for the cathode and matched well with published data [37].
Fig. 17 displays the FTIR absorption spectra of LixCo02 cathode-active materials as a function of the lithium content. As predicted from the factor group analysis, one observes four distinct bands in the FTIR absorption spectrum of pristine LiCo02• They are located at 269, 420, 539, and 602 cm- l . The spectrum of LiCo02 matches well with those reported previously [38-40]. A closer examination of the shape of the high­ vavenumber band at 602 cm- l indicates that a shoulder exists at 646 cm- I . We remark the shape of the IR band at 269 cm- l which corresponds to an oscillator with a great strength. The infrared-active bands shown in Fig. 17 are generally broader that those observed in Raman spectroscopy [41]. The broadening of IR bands is attributed to the average oxidation state of Co, which are oxidised into the COIV state during charge of the cell, and to the random distribution of Li+ ions in the interlayer space.
24
~ ;:: 0 LiCo02 > 3.0 --CI)
2.5 0.4 0.5 0.6 0.7 0.8 0.9 1.0
x(Li) in LixCo~ Figure 16. The potential curve of LiCo02 during the first charge in the potential range 2.5-4.2 V vs. LilLi' (0.1 mAlcm2 current density).
tI) ..... ..... s:: ::s Q) (.)
100 200 300 400 500 600 700 Wavenumber (em-I)
Figure 17. FfIR absorption spectra of LiCo02 as a function of the Li concentration.
To get a better understanding of the vibrational spectra of the layered LiCo02 with R-3m space group, we consider a structure which consists of compressed Co06 and
25
elongated Li06 octahedra that yields distinct vibrations in two different frequency regions, i.e. at 400-650 cm-! there are bands due to Co06 vibrations, while the Li06 vibrations are within the region 200-400 cm-!. Infrared bands located in the high-frequency region, i.e. at 602 and 539 cm-!, are attributed to Co-O stretching and O-Co-O bending motion, respectively. The low-frequency band situated at 269 cm-! involves the motion of Li atoms against their oxygen neighbours in an octahedral environment. This peak is related to an asymmetric stretching vibration of Li06 units [40].
There are obvious modifications of the FfIR spectrum of LiCoOz upon lithium 00- intercalation (Fig. 17) as follows. (a) We observed a decrease of the oscillator strength and a broadening of all the IR-bands which can be associated to a disorder induces by the departure of Li-ions located between two CoOz blocks. The broadening of the low­ frequency band can be also attributed to the random distribution of the Li-ions remaining in the host matrix. (b) No frequency change is observed for the high-wavenumber bands which are assigned to the Co06 vibrations. Thus, as expected, we can conclude that the CoOz layers are not affected significantly by the lithium de-intercalation process. (c) A significant shift of the low-frequency band is recorded. This band shifts toward the low­ energy side from 269 to 258 cm-! in Lio.sCoOz. The frequency shift corresponds to the increase in the interlayer spacing due to an increase of the repulsive interactions between two adjacent negatively charged CoOz layers upon de-lithiation. Thus, the interlayer force constant is reduced by about 8%.(d) The increase of the far-infrared absorption of Lio.4CoOz sample is attributed to the Drude edge due to the change in the electrical conductivity of the material. This suggests the existence of collective delocalised electrons. These results agree well with the data of electrical measurements which show that LiCoOz has a semiconductor-like conductivity while Li,CoOz exhibits almost a temperature-independent conductivity [42].
LiCoOz is a p-type semiconductor (band gap Eg=2.7 eV) [43] while Li,CoOz for x$0.75 has a metal-like behaviour. Li,CoOz is predicted to have partially filled valence bands for x lower than 1.0 [44]. For every Li removed from LiCoOz lattice, an electron hole is created within the valence band. For x<0.75, we expect that there are sufficient holes to allow for a sign:ficant degree of screening, and in this regime, the hole states in the valence bands are likely to be delocalised such that Li,CoOz exhibits metallic electronic properties. This behaviour is clearly observed in the FTIR absorption spectra where absorption by holes are observed in the low-wavenumber region. As pointed out by Van der Ven et al. [44] the occurrence of delocaIised holes contribute to the free energy of the electrode, influencing both the energetic and entropic terms. This could be at the origin of the two-phase region observed in the potebtial curves of the Li/lLiCoOz cells.
From the infrared data, it is also interesting to remark that LiCoOz is less sensitive to lithium content due to the higher bond covalency in the CoOz slabs than LiNiOz does. Consequently, the strong bond covalency in LiCoOz, with reduced Co-O bond distance, results in stabilisation of CoIII in low-spin ground state, and reduces the electronic conductivity of the compound. By de-intercalating lithium into materials, the repulsion of the negatively charged CoOzlayers increases and the Co4+/C03+ redox couple offers the possibility of electronic transfer. This cation oxidation results in an increase of the conductivity due to the decrease of the covalent character of the CoOz slabs. XRD and FfIR data seem to be in good accordance with such a structural model.
Acknowledgments
The author wishes to thank Dr. Bouziane Yebka for his assistance in experimental works
26
on lithium intercalation in oxides. Dr. Michel Massot is gratefully acknowledged for his contribution in Raman scattering measurements.
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OVERVIEW OF CARBON ANODES FOR LITHIUM-ION BATTERIES
Karim ZAGHIBa and Kim KINOSHIT Ab a Institut de Recherche d' Hydro-Quebec, 1800 boul, Lionel-Boulet Varennes, Quebec, Canada, J3X 1 S1 bEnvironmental Energy Technologies Division Lawrence Berkeley National Laboratory 1 Cyclotron Road, Berkeley, CA 94720 USA
Commercial lithium-ion batteries utilize metal oxide (lithiated Co oxide) pOSItive electrodes, non-aqueous solvent containing LiPF6 as the electrolyte and carbon negative electrodes. There has been extensive research to identify the optimum carbon to meet requirements such as high capacity, low irreversible capacity loss, long cycle life, low co