International Journal of Inorganic Materials 3 (2001) 191–200

Nanostructured materials for energy storage

*L.F. Nazar , G. Goward, F. Leroux, M. Duncan, H. Huang, T. Kerr, J. GaubicherDepartment of Chemistry, University of Waterloo, Waterloo, Ontario, Canada

Received 15 March 2001; accepted 20 March 2001

Abstract

Traditional electrode materials for lithium-ion storage cells are based on materials which have both mixed electron and ion transport1(for Li ). They are typically crystalline layered structures such as metal oxides that have high redox potentials, and act as positive

electrodes; and graphitic carbons capable of reversible uptake of Li at low potentials which act as negative electrodes. Recently, however,nanostructured solid state materials, which are comprised of two or more compositional or structural phases, have been considered. Thisnew area has been particularly exploited in the area of negative electrode design, where the intimate mix of components at the nanoscalepermits and enhances Li reversibility. It also include cathode materials where materials that function on the basis of intergrowth structures(internal composites) have been found to be beneficial; and insulating materials where the limitations to electron transport must beovercome by judicious design of nanostructured composites. The research trends and future prospects are discussed. 2001 Publishedby Elsevier Science Ltd.

Keywords: Nanocomposite; Li-ion battery; Metal oxide anodes; Vanadium phosphates

1. Introduction During ‘discharge’, the positive electrode becomes reduced1(and Li is inserted), and the negative electrode is

1The increasing role of technology in our lives necessita- oxidized (Li is extracted). The converse occurs duringtes the development of advanced energy storage tech- ‘charge’. The electrochemical binding energy differencenologies, ranging from portable electronic devices to for lithium between the two host lattices drives the electronhybrid electric vehicles. Considered the most advanced of transport through the external circuit and hence does thethe available rechargeable technologies, lithium ion bat- useful ‘work’.teries rely on solid state intercalation compounds that are Many developments in solid state chemistry in the lastable to reversibly intercalate lithium within both electrode decade have resulted in some promising new discoveriesstructures. Both cell voltage and capacity are governed by for both electrodes. Increasingly it is apparent that somethe materials chemistry. The capability to improve that very interesting characteristics either result from develop-chemistry resides in a better understanding of the processes ment of nanostructured domains within materials duringthat occur in the bulk in addition to the development of the initial cycles of the cell, or the deliberate constructionsome new materials based on nanostructured concepts. of such nanostructures. The partial disorder inherent to

Traditional electrode materials are based on crystalline these materials can have a substantial impact, affecting thelayered structures such as metal oxides that have high kinetics and/or thermodynamics of the materials in fun-redox potentials; and graphitic carbons capable of revers- damental and beneficial ways. Various synthesis-by-designible uptake of Li at low potentials. The former are referred approaches, or in situ creation methods can be used toto as cathodes (positive electrodes); the latter as anodes tailor such materials for both negative and positive elec-(negative electrodes). The reactions in the cell involve trodes.electron transfer between the two host materials such that This concept got its footing with the attempted develop-simultaneous redox reactions take place at both electrodes ment of new high-capacity anodes, an area that presents a

1concurrent with the insertion /deinsertion of the Li . major challenge at present, and where the ‘nano’ approachis ideally suited. The attempted development of materialsover the past few years has made it clear that one*Corresponding author.

E-mail address: lfnazar@uwaterloo.ca (J. Gaubicher). component alone will not suffice. Many of the most

1466-6049/01/$ – see front matter 2001 Published by Elsevier Science Ltd.PI I : S1466-6049( 01 )00026-5

192 L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200

promising materials in fact form nanostructures in situ as SnO, SnO and Sn BPO established that the initial Li2 2 6

during the Li discharge process, and this offers the uptake results in an irreversible reaction to form Li O and2

possibility of targeted design of composites. More recently, metallic Sn. The subsequent Li uptake that results in thethe concept has been extended to new cathode composites. reversible capacity is based on alloying of the Li with theThese include materials where extensive doping of the Sn centres as outlined above, to ultimately form Li Sn4.4

structure is required to achieve desired properties; to nanoparticles embedded in the Li O matrix [9,10]. A2

materials that function on the basis of intergrowth struc- similar mechanism was thought to be responsible for Litures (internal composites); and insulating materials where uptake in the tin-oxide composite glasses. The detailedthe limitations to electron transport must be overcome by studies which followed, based on a variety of spectro-judicious design of nanostructures. What follows is a brief scopic and diffraction methods, revealed that the ‘Li–Sn’overview of some developments based on our work and alloy nanoparticles are highly disordered at the limit ofthat of others that highlights key points in anode and deep discharge [11–16]. The proximity of oxygen in thecathode design based on this theme. matrix (either on the bulk of the particles or within them)

may give rise to this disorder — and in any case, certainlyaffects the thermodynamics of the system.

2. Nanocomposites as negative electrodes The reactions occurring within the tin composite glassestake place in a size-limiting regime, where the thermo-

Negative electrode materials which have been previous- dynamics are strongly affected by the high surface energyly studied primarily include main-group metals (Al, Si, Ge, of the particles [12]. The reaction between Li and Sn,Sn, Pb, Sb and Bi) which react with Li at low potential to which forms crystalline Li Sn under high-temperature4.4

form Li alloys with a stoichiometry as high as 4.4 Li /M conditions can be much more complex on a nanoscopic(e.g. Li Sn ). Unfortunately, formation of the alloy is level. First, the kinetics of the system limit the formation22 5

associated with a very large volume expansion. This of ordered ‘bulk’ phases at ambient temperature, even inultimately pulverizes the electrode material and results in a tin oxide itself, but especially in the dilute tin-basedloss of electrical contact between the material grains on glasses. Secondly, the tin clusters formed on Li uptake arecycling. The resulting rapid capacity fade prevents their in close proximity with both lithium and oxygen as a resultpractical use as electrodes. To overcome this problem, of these particles having a very high surface area: volumerecent efforts have turned to the encapsulation of the main ratio. We have suggested that partial oxygen incorporation

1group metal center in an matrix which is conducive to Li within the bulk (as an intersitial element) may be respon-transport (and ideally, to electron transport, in addition to sible for some of the disorder observed in the X-raypossessing mechanical flexibility). diffraction patterns of the fully discharged material, as well

7Multiphase electrode materials composed of intermetal- as unusually low frequency Li NMR shifts indicative oflic compounds, MM9, in which only one component forms Li in the proximity of both reduced Sn and oxygen [12].an alloy with lithium, exhibit better cycling performance Oxygen is present as an interstitial component in mainthan a single phase host. This has been attributed to the group compounds such as Zr Sn O and La Sn O and is5 3 5 3

formation of an ‘inactive’ M9 matrix that surrounds the implicated in a range of intermetallics [17]. The possibleactive alloying M metal which helps maintain the integrity interstitial, and surface oxygen can facilitate the rever-of the microstructure of the composite electrode during the sibility of Li uptake by the Sn. The surface energy of these(de-)alloying process. Materials of this type include com- particles allows the ‘back-reaction’ of lithium with oxygenposites derived from mixtures of SnFe /SnFe C [1–3], or to take place at a lower potential than predicted from2 3

intermetallics such as InSb and Cu Sn [4–7]. In the simple considerations that exclude surface energy contribu-6 5

Sn/Fe case, for example, reaction of Li induces a de- tions.composition into (Fe1Sn1SnFe C) followed by Li alloy- This is corroborated by Mossbauer studies [13–15], and3

ing with the Sn centres. In general, however, these EXAFS studies [11,12], in addition to recent X-rayapproaches have failed to yield stable systems in which scattering studies employing the pair-distribution functionreversible Li insertion processes occur highly reversibly. method, which all give evidence of oxygen incorporation /An alternative approach is to use metal oxides, in which disorder at low potential. The availability of matrix boundoxygen can play an important role in the process. oxygen to participate (surface or interstitial) will be

determined by the surface energy of the electrochemically2.1. Cyclability and reversibility of the metal–oxygen active particles, hence the more nanosized are the aggre-coordination environment in main group metal oxides gates, the more reactive they will be. A model which

summarizes this is shown in Fig. 1.Recently, a new anode material comprised of an amor-

phous, multi-element composite oxide glass 2.2. Transition metal oxidesII(Sn Al B P O ), containing ‘active’ Sn centers inspiredx y z p n

much interest in the area of main-group metal oxide The same beneficial mix of metal and metal-oxide onmaterials [8]. The first studies on related compounds such the nanoscale appears to be responsible for promising

L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200 193

Fig. 1. Scheme showing the uptake of Li by SnO.

properties exhibited by a growing number of transition repeated discharge to very low potential. Thus, if themetal oxides. Low voltage Li uptake with very high voltage window is restricted above this level, the Moreversible capacity has been reported for LiMVO (M5 essentially shuttles between oxygen poor / lithium rich4

Zn, Cd, Ni) [18,19], Li MO and Li M V O (M5 (discharge) and oxygen rich / lithium poor (charge) amor-x z x y 12y z

transition metal) [20] and in amorphous or semicrystalline phous phases with improved cyclability being the result.RVO (R5In, Fe) [21], MV O (M5Fe, Mn, Co) [22], and Complementary findings in Fe and Co vanadate systems on4 2 6

A MoO [23,24]. These materials are fascinating, since the Li uptake have also been revealed by in situ X-ray,x 3

¨Li capacity can be accounted for by complete reduction of XANES, Mossbauer and microscopy studies studiesthe metal centers in most cases (in fact, over reduction is [27,28] We believe that the recently reported behavior ofusually observed), yet no Li–M alloys are known when M iron borates such as Fe BO and Fe BO as low-potential3 6 3 3

is a transition metal. Li uptake materials is also based on similar principles [29].When these materials were first reported, the mechanism These findings led to the investigation of simple nano-

responsible for the Li uptake and reversibility was not sized metal oxides MO based on the rock-salt structureunderstood; today a complete understanding remains elu- (M5Co, Fe, Ni). Here, high-resolution TEM studies of thesive although much progress has been made. The initial material at deep discharge demonstrated the presence of

˚work on Li uptake in partially reduced molybdenum oxides metal nanoparticles about 20 A in diameter. Again, thesuch as Li MoO showed by means of a combination of highly divided, high-surface energy nature of the0.25 37Li NMR, XRD and XAS probes, that on Li reaction at nanoparticles facilitates the back-reaction with oxygenlow potential a complex amorphous nanocomposite is from the lithium oxide matrix to reform the metal oxide onformed. It consists of a disordered, highly oxygen deficient charge, this time in a semicrystalline form [30]. Thelithium/molybdenum-oxide (a lithium molybdenum sub- reaction, particularly reversible for Co, but less so for Feoxide) in intimate association with lithium oxide [23,24]. and Ni, can be simplified as; [30]The latter accounts for the irreversible capacity observed

1 2Li 1 e 1 MO ↔ Li O 1 M2by electrochemical studies. Subsequent EXAFS andXANES studies revealed that although metallic Mo isformed on deep discharge, a molybdenum oxide is regener- 2.3. Mixed Fe–Sn oxidesated on oxidation [25,26]. The depth of discharge isimportant in controlling the metal–oxygen environment. The Sn oxide, transition metal oxide, and Sn–Fe inter-At potentials deep enough to result in complete reduction metallic studies cited above opened avenues to investigateof the metal, the molybdenum retains most of its oxygen oxide materials containing both transition metal and main-coordination environment (Fig. 2). The latter is only lost at group metal active centers. An interesting class of materi-

194 L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200

Fig. 3. Structure of Li Ca Sn Fe O (from Ref. [31]).0.6 0.4 0.6 1.4 4

als based on the inherently open architecture of CaFe O2 4

are suitable candidates. This structure (Fig. 3) displays ahigh degree of compositional variety: a solid solutionexists on Sn substitution for Fe with an accompanyingremoval of Ca for Li, the end member having thestoichiometry Li Ca Sn Fe O , and relatively high0.6 0.4 0.6 1.4 4

ionic conductivity. These materials can be reversiblydischarged to low potential, although as in the case ofother oxides, irreversibly capacity that presumably arisesfrom the partial formation of Li O is observed. The2

31 41capacity is enhanced on substitution of Fe for Sn inthe framework and is highest for the compositionLi Ca Sn Fe O . On introducing Sn into the struc-0.6 0.4 0.6 1.4 4

ture the reversible capacity is also substantially increasedcompared with the parent material. Although there is alarge irreversible component to the redox process duringfirst discharge–charge, the materials can sustain a stablereversible capacity of .600 mAh/g, about twice that ofgraphitic carbon within the voltage window of 3.0–0.005V (Fig. 4). The profile of the differential capacity plotssuggest there is no phase separation to Li /Sn alloy phaseson reduction, but rather a lithium-rich, oxygen deficientSn/Fe /oxide matrix is formed. These results are alsosupported by XAS measurements which suggest that Sn–Fe alloy nanophases are formed at intermediate potential,in close proximity with oxygen [31–33].

Finally, it is worth noting that all of these materialsdisplay a relatively high average charge potential, andlarge hysteresis consistent with the necessity of oxygen

Fig. 2. (a) Electrochemical profile of A MoO (A5Li, Na) on dischargex 3 transport within the bulk during oxidation. Critical to theof Li, charge and second discharge, showing the points at which samples

functioning of these materials, therefore, is the intimatewere extracted for XAS analysis; (b) pseudo-radial distribution functionmixture at the nanoscale regime, of metal sub-oxide (orplots (RDF) for electrode materials taken at the points indicated in (a).

Distances are not corrected for phase shift. pure metal, depending on the degree of Li uptake) and

L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200 195

3.2. Conductor /insulator nanostructured materials

An even clearer illustration of the nanostructured ap-proach in cathodes is provided by a class of materials thatdespite their poor conductivity, can function as practicalelectrodes through the formation of ‘bad conductors /goodconductor’ composites. On the poor conductor side arematerials such as metal oxide /polyanion frameworks basedon the olivine, NASICON and VOPO frameworks. These4

structures consist of MO octahedra bridged by XO (X56 4

P, Si, B, S) tetrahedra. Some members of this family haveexploited in the past for their fast-ion conduction prop-erties but more recently they have engendered muchinterest as electrode materials for Li-ion batteries. Owing

1to their structural properties, the Li can easily be in-serted /deinserted from the host; while incorporation of a

Fig. 4. Reversible capacity corresponding to Li insertion intransition metal, M conveys redox properties on theLi Ca Sn Fe O in the voltage window 3.0–0.005 V at a current0.5 0.5 0.5 1.5 4systems. A range of different materials where M isdensity of 50 mA/g (from Ref. [31]).primarily Fe, Ti or V, organized in different structuralmotifs with the XO tetrahedra have been examined to4

oxygen from the lithium oxide matrix that facilitates the date, and shown to be promising intercalation hosts for1oxygen recovery process. reversible Li -based redox chemistry [37–51]. (As first

shown by Goodenough) incorporation of the XO moiety4

raises the cell voltage by comparison with the corre-sponding oxide, allowing the voltage to be tailored by

3. Nanostructured materials as positive electrodes structural changes and chemical substitution [39,41–45].41 31Thus, the V /V couple in NASICON Li V (PO ) is36x 2 4 3

51 413.1. Nanodomains in spinel /layered material composites at about the same voltage as the V /V couple in thevanadyl phosphates, Li VOPO [46], but much higher16x 4

The best known cathode materials are the two-dimen- than the same couple in V O . Vanadyl phosphates are2 5

sional layered oxides LiCoO and LiNiO and the three particularly appealing due to their high theoretical capacity2 251 41dimensional spinel LiMn O where about 0.5 Li per and useful potential of the V /V redox couple (|3.85–2 4

transition metal atom can be reversibly extracted. The first 4.0 V).is used in commercial cells, despite its high cost that All of these materials are, however, electronic insulatorsprohibit the production of large cells; the other two have and hence prohibit electron transport. The formation oflimitations due to safety problems (LiNiO ) and stability nanostructured composite is a viable approach to overcom-2

(Li Mn O ). Cycling over the full Li range in the spinel ing the low electronic conductivity that impedes their11x 2 4

(0,x,1) involves a two phase reaction arising from the practical exploitation. Control of particle size is critical (toJahn–Teller distortion which occurs when more than 50% reduce the path length for transport). Furthermore, forma-

31of the octahedral sites are occupied by Mn . This causes tion of an intimate interface of the conductive additivestructural distortion on cycling, and loss of capacity. A with that of the active material is necessary to improvenew layered material, monoclinic LiMnO having the same transport properties. Mechanical grinding can be used to2

structure as LiCoO appeared to be a promising candidate overcome some limitations, but the most successful ap-2

for solutions to these limitations. It was found that on proaches involve the design of graphitic carbon/phosphatecycling LiMnO however, conversion from the layered nanostructured composites based on a range of different2

morphology to the spinel occurred, causing rapid capacity techniques including grafting, and coating that will befading [34]. Further work on doped variants, including illustrated in the following examples.

9Li(Mn M )O suggest that doping with M95Ni or Co12y y 2

limits the extent of conversion to spinel during cycling,and vastly improves the stability [35,36]. The conversion 3.3. Grafting: V O2 5

that does occur results in nanodomains of lithiated spinelwithin the layered phase. It is this domain structure which To address the problem of shortening the pathlength,has been attributed to the improved cycling stability. On and improving both the ionic and electronic conductivityintercalation presumably the stress imposed by the Jahn– of nanostructured materials, our initial efforts centered onTeller induced phase transition is dissipated by these V O , a widely studied cathode material that has kinetic2 5

domain walls, resulting in less capacity fading. limitations both for ionic and electronic transport [53,54].

196 L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200

This was seen as a starting point for the investigations. carbon particles can be achieved by grafting polyelec-1Various improvements on V O have been attempted in trolytes on their surface (Fig. 6) . The image of V O –C-2 5 2 5

recent years, including aerogels [55–58], V O prepared by PEG shows an grouping of well separated particles on the2 5

electrodeposition in the presence of surfactants, and substrate with a mean diameter of about 85 nm, whichlayered polymer /V O nanocomposites [59–61], that show consists of the particle size of the carbon core (| 44 nm)2 5

1enhancement of Li diffusion [62,63]. None of the above with a polyelectrolyte /metal oxide coating of about 40 nm.approaches takes advantage of an engineered composite The electrochemical response of the hybrid nanostruc-design, however. Our nanostructured approach relies on tured material over a wide range of current densities —the grafting of polymer electrolyte chains and metal oxide measured by standard electrochemical methods and afterlayers onto the surface of electronically conductive thorough drying of the material — is greatly enhanced‘graphitic’ carbon. Although V O serves to illustrate the compared to a standard V O sol–gel based composite2 5 2 5

concept, the method is also applicable to other materials. electrode. It exhibits an enhanced electrochemical responseFurthermore, as the polymer-functionalized carbon is at high rates, and improved electrochemical cycling stabili-readily dispersed (‘soluble’) in polar solvents, the conduc- ty owing to its unique microstructure. The combination oftive particles can also be used to impregnate the interstices possible variation in the components, along with goodof insulating or poorly conductive high surface-area ma- specific capacity at extremely high discharge /charge ratesterials with accessible micropores. (as much as 2000 mA/g for V O , makes this concept very2 5

We prepared the nanocomposites by chemically wiring attractive for a variety of materials (Fig. 7). The averageboth polyelectrolytes and the active metal oxide (V O ) redox potential of the system, of about 3 V is useful for2 5

onto the surface of semigraphitic carbon black nanoparti- lithium–polymer cell applications, where metallic lithiumcles that serve as an electronically conductive core. Our acts as the anode. We are now adapting the process toprocedure is illustrated in Fig. 5. We chose acetylene black create VOPO –carbon composites, by direct conversion of4

as the core carbon support, because of its well known high the V O to VOPO in situ on the carbon surface2 5 4

electronic conductivity and small particle size (averagediameter¯44 nm). The electrolyte, polyethylene glycol

3.4. Binding: VOPO composites4(PEG, M 5 4600) was grafted onto its surface, after firstw

partially oxidizing the carbon to create functional surfaceAs the introduction of the PO moiety into vanadium4groups [64–66]. The polyelectrolyte-functionalized carbon

oxides raises the redox potential by about 1 V, vanadylor C-PEG, was then coated with an aqueous dispersion of

phosphates, VOPO can act as a cathode materials based4V O lamellae (‘sol’) [67,68], and dried at 1408C to yield V IV2 5 on the reversible V /V couple centered at about 4 VV O –C-PEG). AFM images of the C-PEG and V O –C-2 5 2 5

1 2PEG clearly show that almost complete dispersion of the VOPO 1 Li 1 e ↔ LiVOPO4 4

A previous vanadyl phosphate that has been recentlyinvestigated, b-VOPO , exhibited extremely poor rate4

performance, in addition to limited reversible Li intercala-tion capacity. At practical rates, very little Li could bereversibly intercalated. We sought to improve both elec-tronic and ionic conduction factors by reducing the particlesize and generating nanostructured carbon composites.Particles of the epsilon phase, e-VOPO phase were grown4

by hydrothermal crystallization of VPO4, followed byoxidation under mild conditions. Chemical lithiation of thismaterial to form a-LiVOPO , followed by mechanical4

grinding with carbon and addition of the ‘soluble’ carbonparticles to coat the phosphate particle surface bothresulted in marked improvements. Over 70% theoreticalcapacity (|100 mAh/g), and stable cycling behavior at 4V at good rates was achieved (Fig. 8) [52]. Notably,despite the structural similarity of the a-LiVOPO and4

1AFM was performed using a Digital Instruments NanoScopeE scanningprobe microscope, where samples of C-PEG and V O /(C-PEG) for AFM2 5

were prepared by dispersing the material in methanol using ultrasound forFig. 5. Scheme illustrating the process for coating V O xerogel on 10 min, and then depositing the suspension onto mica and drying it at2 5

polyethylene glycol-grafted carbon black. 608C for 2 h.

L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200 197

Fig. 6. AFM image of V O –C-PEG nanoparticles on a mica substrate.2 5

e-VOPO frameworks which should facilitate Li insertion / reversible Li extraction, even after extensive milling with4

extraction (Fig. 9), a-LiVOPO prepared in larger particle carbon. Therefore, both the reduced particle size accessed4

form by solid state routes shows virtually no capacity for by the preparation method, and the formation of the carbonparticle composite appear to be responsible for the im-proved kinetics.

Fig. 7. Capacity as a function of current density for the V O xerogel,2 5

V O –C, and V O –C-PEG as labelled, in the voltage window 3.8–1.8 V.2 5 2 5

In all cases, working electrodes were prepared from the nanocomposite,acetylene black, and polyvinylidene fluoride (PVDF) with weight ratio of85, 10 and 5%, respectively, with a loading of the active material of 3–4 Fig. 8. Comparison of the capacity on cycling of an a-LiVOPO –20%4

mg. After evaporation of the solvent, all the electrodes were heated at carbon-composite electrode at C/10 (16 mA/g) rate in the voltage range1508C for at least 2 h prior to cell assembly. Swagelock-type cells were 3.0–4.5 V, for a-LiVOPO made by the hydrothermal route / lithiation /4

assembled in an argon filled glove box using 1.0 M LiPF /EC-DMC as carbon binding, versus that made by conventional high-temperature6

the electrolyte. methods without the nanostructured approach.

198 L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200

Fig. 9. Comparison of the structures of a-LiVOPO (electrode in the discharged state) and the structure of e-VOPO (electrode in the charged state).4 4

3.5. Coating: LiFePO 4. Conclusions4

Goodenough et al. first reported that the olivine, A wide range of nanostructured materials, preparedLiFePO could act as a cathode material based on the either by deliberate design or by fortuitous reactions are4

II IIIreversible Fe /Fe couple centered at 3.4 V often advantageous for enhancing electronic, ionic, oroxygen transport in electrode materials for electronic

1 2 applications as energy storage materials. In contrast toLiFePO ↔ FePO 1 Li 1 e4 4

attempting to tailor a single material, nanocomposites canExtraction of lithium from the structure, however, was provide a optimum way of crafting desired characteristicslimited to 0.6Li due to the difficulty in accessing the redox for a target application. Namely, the nanostructured ap-sites. A novel, and successful solution to the problem proach allows one to tailor the material to suit the need, ordeveloped by Armand et al. is to form a thin graphitic to compensate for the limitations inherent within onelayer on the surface of LiFePO particles by means of a material alone. For negative electrode materials, the les-4

carbon-based coating formed either in situ during nuclea- sons learned from a careful and detailed attention to thetion of the phosphate particles, or postprecipitation. This mechanism of Li uptake by metal oxides will hopefullymethod, along with control of particle size, results in lead to even more promising materials in the future.utilization of over 95% of theoretical capacity with reason- Ideally, these materials will be prepared by design. Issuesable rate capabilities at 3.4 V [69–71]. Other methods of still concern the the improvement of oxygen mobility informing LiFePO /carbon composites with higher amounts these materials, and the role that order and microstructure4

of carbon have yielded similar results [72]. This class of in the transition metal oxide materials at various stages ofmaterial, along with other phosphates, are attractive for discharge and charge play in the reversible Li insertion /practical applications owing to the useful voltage range; uptake process. For positive electrode materials, two areasthe ability to structurally and chemically adjust the po- of nanostructured materials based on intergrowth structurestential described above, and the low cost of the materials in the nano-regime may lead the way to new and betterthemselves. cathodes operating at 3.5–4.0 V with good rate capa-

L.F. Nazar et al. / International Journal of Inorganic Materials 3 (2001) 191 –200 199

[22] Leroux F, Piffard Y, Ouvrard G, Mansot J-L, Guyomard DM. Chembilities. Novel approaches are needed here to blend theMater 1999;11:2948.structures at the nanometer level.

[23] Leroux F, Goward GR, Power WP, Nazar LF. Electrochem Solid-State Lett 1998;1:201–5.

[24] Leroux F, Nazar LF. Solid State Ionics 2000;133:37–50.Acknowledgements [25] Leroux F, Nazar LF. Mat. Res. Soc. Symp. Proc. New Materials for

Batteries and Fuel Cells. II 1999;575:173.[26] Leroux F, Ouvrard G, Nazar LF. Adv Mater, submitted forThis work was supported by the National Sciences and

publication.Engineering Council of Canada through the Research and [27] Denis S, Baudrin E, Orsini F, Ouvrard G, Touboul M, TarasconStrategic grant programs. I would like to thank my PDF, J-M. J Power Sources 1999;81:79.Dr. Fabrice Leroux (now at the CNRS Clermont-Ferrand, [28] Denis S, Baudrin E, Orsini F, Tarascon J-M. Chem Mater

2000;12:3733.´France) and Professor Guy Ouvrard (Universite de Nantes[29] Rowsell J, Gaubicher J, Nazar LF. J. Power Sources, in press.and the IMN, France) for their work on the XAS studies of[30] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nature

the metal oxide materials; and my PDF Dr. Joel Gaubicher 2000;496:407.´(now at the Universite de Paris, VI, France) for his [31] Duncan M, Nazar LF. Mat Res Soc Symp Proc Solid State Ionics V

contribution to the latter stage of the vanadium phosphate 1999;548:71.[32] Duncan M, Ouvrard G, Nazar LF, Gaubicher J. Abstract No. 235,work. I most gratefully acknowledge my colleagues and

The Electrochemical Society Meeting Abstracts, Hawaii, 1999.students at the University of Waterloo, in particular[33] Duncan M, Ouvrard G, Nazar LF, Gaubicher J, Rowsell J. J

Professor W.P. Power, Gillian Goward, Tracy Kerr, Huan Electrochem Soc, submitted for publication.Huang and Morven Duncan for their contributions which [34] Shao-Horn Y, Hackney SA, Armstrong AR, Bruce PG, Gitzendannerare cited in the references. R, Johnson CS, Thackeray MM. J Electrochem Soc 1999;146:2404.

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