Cathodes for Lithium Ion Batteries : The Benefits of Using Nanostructured Materials

As celas de íon lítio disponíveis comercialmente, as quais são as mais avançadas entre as baterias recarregáveis disponíveis até agora, empregam óxidos de metais de transição microcristalinos como catodos, os quais funcionam como matrizes de inserção de lítio. Em busca por uma melhor performance eletroquímica, o uso de nanomateriais no lugar dos materiais convencionais tem emergido como excelente alternativa. Nesta revisão nós apresentaremos uma breve introdução sobre as motivações de usar materiais nanoestruturados como catodos em baterias de íon-lítio. Para ilustrar tais vantagens apresentamos exemplos de pesquisas relacionadas com a preparação e dados eletroquímicos dos mais usados catodos em nanoescala, tais como LiCoO 2 , LiMn 2 O 4 , LiMnO 2 , LiV 2 O 5 e LiFePO 4 .

][5] The reason for this relevance is that compared to traditional rechargeable batteries such as, lead acid and Ni-Cd, the lithium-ion battery shows several advantages, such as lighter in weight, smaller in dimension and higher energy density. 1 Moreover, although capacity values may be similar to other rechargeable systems, voltages are approximately three times higher, affording higher energy. 1 In general, the commercial lithium-ion batteries use graphite-lithium composite, Li x C 6 , as anode, lithium cobalt oxide, LiCoO 2 , as cathode and a lithium-ion conducting electrolyte.When the cell is charged, lithium is extracted from the cathode and inserted at the anode.On discharge, the lithium ions are released by the anode and taken up again by the cathode (Figure 1).
Owing to the importance of lithium ion batteries, these cells are still object of intense research to enhance their properties and characteristics.9][20][21][22] However, most of these efforts are concentrated in new cathode materials, since the most used cathode material (LiCoO 2 ) is expensive and is somewhat toxic.
The active cathode material of a secondary lithium ion battery is a host compound, where lithium ions can be inserted and extracted reversibly during the cycling process.The main requirements for cathode materials are: (i) the transition metal ion in the insertion compound cathode should have a large work function to maximize the cell voltage, (ii) the insertion compound should allow an insertion/extraction of a large amount of lithium to maximize the cell capacity, (iii) the lithium insertion/extraction process should be reversible with no or minimal changes in the host structure over the entire range of lithium insertion/ extraction, (iv) chemical stability for both redox forms of cathode couple, (v) the insertion compound should support good electronic and Li + conductivities to minimize cell polarizations and, (vi) the voltage profile should be relatively continuous, without large voltage steps that can complicate power managements in devices.Finally, from a commercial point of view, the insertion compound should be inexpensive, environmentally friendly and lightweight to minimize the battery weight.
In the last years, the use of nanomaterials for Liion cathodes instead conventional materials has become very attractive to improve the performance of lithium rechargeable batteries.4][25][26][27][28] In one paper published in 2003, Bueno and Leite 29 discuss the effects of the nanocrystalline state on the performance of Li-ion batteries in a conceptual point of view, where two types of size effects are discussed: (i) trivial size effects which rely on solely on the increased surface to volume ratio and (ii) true size effects, which also involve changes of local materials properties.Based on this paper and other reviews, motivations for using nanomaterials will be discussed.
Nanoparticles have critical nucleation radii (CNRs) that are larger than the particle diameter.Since the structural transitions to thermodynamically undesired structures can only occur if the particle radius is larger than the critical nucleation radius for that phase, 30 it is possible to eliminate such transitions by using nanoparticles with CNRs larger than the particle radius.Thus, small particles would accommodate more easily the structural changes occurring during the cycling process where lithium are inserted and extracted.
In nanoscopic particles the charge accommodation occurs largely at or very near the surface and the smaller the particles are, the larger the portion of these constituent atoms is at the surface.For example, for a 1 nm MnO 2 particle, the fraction of Mn(IV) ions at the surface will be 0.5, while for a 10 nm particle it will be 0.05.Further, for a 10 nm particle the fraction of Mn(IV) ions within next-nearest neighbor distance of the surface is 0.75.This fact will reduce the need for diffusion of Li + in the solid phase, greatly increasing the charge and discharge rate of the cathode.This will also reduce the volumetric changes and lattice stresses caused by repeated Li + insertion and expulsion. 31,32he Li + diffusion distances will be significantly reduced and Li + diffusion within the particle to compensate reduction of non-surface cation sites.For example, for a 10 nm particle and a bulk diffusion coefficient of Li + in typical insertion materials of approximately 10 -10 cm 2 s -1 , 33 the time required for Li + to diffuse a distance equal to the particle radius is 2.5 × 10 -3 s.Thus, Li + diffusion within the particle to compensate reduction of non-surface Mn sites is not a significant kinetic barrier during cycling.At high discharge rate, high Li + ion insertion flux density and slow Li + transport result in concentration polarization of lithium ion within the electrode material.This causes a drop in cell voltage, which results in termination of the discharge before the maximum capacity of the electrode materials is used.Decreasing the average diffusion distance, while keeping the mass constant, increases the surface are of the electrode and lowers current density.This results in the delay of concentration polarization to higher current values and, consequently, increased electrode rate capability.
Numerous materials have been studied for use as lithium ion battery cathodes, including metal oxides, metal sulfides, conducting polymers and poly(sulfides).Among them, transition metal oxides are the most successful materials because of their chemical and structural stability, high lithium ion capacity and favorable electrical properties, such as high conductivity of lithium ions and electrons.
6][37][38][39] In addition, recently the olivine LiFePO 4 was proposed as possible cathode as lithium ion batteries. 40n layered LiMO 2 (M=Co, Mn), the lithium and the metallic (M 3+ ) ions occupy alternate (111) planes of the cubic rock salt structure.The lithium ion intercalates into or deintercalates reversibly from the MO 2 layers.Spinel LiMn 2 O 4 has a cubic spinel structure (Fd 3 m), where Li + ions occupie the 8a tetrahedral sites with manganese occupying the octahedral (16d) site and the other octahedral site (16c) is vacant.When Li + diffuses within the structure, first it moves from the 8a site to the neighbor empty octahedral 16c site, then to the next 8a site in such a way that the Li + ion takes the diffusion path (8a-16c-8a).The olivine structure of LiFePO 4 has a hexagonallyclose-packed oxygen array with corner-shared FeO 6 octahedral and PO 4 tetrahedral.The lithium ions are octahedrally coordinated to oxygen, forming edge-sharing chains of LiO 6 octahedral (Figure 2). 41In a lithium cell, electrochemical extraction of lithium from LiFePO 4 is accompanied by a direct transition to FePO 4 , in which the Fe 2+ ions are oxidized to Fe 3+ , leaving the olivine FePO 4 framework intact.The diffusion pathway for the lithium ions in LiFePO 4 was only recently discussed in one paper where one dimensional diffusion mechanism is proposed, distinctly from the two-dimension diffusion plane observed in layered transition metals oxides and the threedimensional diffusion channels in spinel LiMn 2 O 4 . 42n general, bulk transition metal oxides are prepared by solid-state reactions, which involve repeated heat process at high temperatures.Such processes generally afford the thermodynamically more stable phases and in general, microcrystalline materials are obtained.Because of these undesirable aspects, several methodologies, such as coprecipitation, sol-gel process with/without template, synthesis by precursor, ion-exchange reaction and hydrothermal, which use lower temperature processing and mild conditions have been tested in order to obtain particles with better control of morphology and smaller size.This review will focus on the preparation and electrochemical performance of nanostructured transition metal oxides commonly used in lithium ions batteries.

Lithium Iron Phosphate
Lithium iron phosphate, LiFePO 4 , is one of the most recent materials reported as cathode for lithium ion battery. 40Perhaps, a reason that has contributed to this delay is that LiFePO 4 has structure significantly different from layered and spinel lithium metal oxides.Fe-based cathode materials are environmentally compatible, cheap, simple in processing and thermal stability comparable to those of LiCoO 2 , LiNiO 2 and LiMn 2 O 4 . 43,44iFePO 4 can act as cathode material due to its high discharge potential around 3.4 V versus Li/Li + and moderate theoretical capacity (170 mA h g -1 ). 40In spite of these advantages, the olivine LiFePO 4 presents low conductivity (σ~10 -9 S cm -1 ) and thereby its electrochemical performance is limited, resulting in poor rate capability.
In order to enhance and optimize the electronic conductivity of LiFePO 4 , several groups are dedicating their researches to these materials.Up to now, there is no way to change intrinsically the electrical conductivity of LiFePO 4 .Thus two alternative methods have been reported.One is the reduction of the grain size of the sample and consequently the diminution of the diffusion length, both for electrons and ions 45 and the other is the manufacture of nanocomposites of LiFePO 4 with a conductive phase, such as carbon. 46sually, bulk LiFePO 4 is prepared by solid-state methods, where a stoichiometric mixture of iron compound, ammonium dihydrogen phosphate, NH 4 .H 2 PO 4 , and lithium hydroxide is heated and calcined several times. 47Depending on the synthetic conditions, the capacity of the LiFePO 4 can reach high values, such as 115 mA h g -1 . 48ecently, nanocrystalline LiFePO 4 powders were prepared by two different methods.One has involved the heating of amorphous nano-sized LiFePO 4, prepared by chemical lithiation of amorphous FePO 4 , at 550 o C, for different periods 49 and the other was a liquid phase methodology. 50The nanoparticles (100-150 nm) prepared by the first method have shown excellent capacity of 160 mA h g -1 with marked capacity loss.The same material prepared by the second methodology (5-50 nm) has exhibited structural stability with no capacity loss during  41 cycling process, in spite of its low specific capacity (90 mA h g -1 ).
In the search for better performance (higher capacities and cyclability), the preparation of composites of LiFePO 4 and carbon in nanoscopic scale to improve their electronic conductivity has emerged as excellent alternative.
These nano-sized LiFePO 4 /C composites were prepared by different methods, such as, modified solid state reaction, 46 liquid-based method using sugar as carbon source, 51 emulsion drying process 52 and citric acid based sol-gel route. 53In general, these nanocomposites have shown excellent performance, both in terms of specific capacity and capacity retention.Large agglomerates with hard definition of these nanoparticles LiFePO 4 and carbon were observed (Figure 3).The samples synthesized by solid-state reaction have shown larger particle size and poor particle size distribution due to the successive calcinations.
Both the small particle size and the enhanced electronic conductivity were responsible to the higher capacity retention upon cycling and the superior rate capability.Moreover, the carbon addition should not affect the structure of the nanocomposite but probably improves its kinetics in terms of capacity and cycling life due to the intra and inter conductivity of the particles.The addition of carbon can also act as nucleation sites for the growth of LiFePO 4 particles, resulting smaller particles. 51,52,54

Vanadium Oxides
Vanadium forms several binary oxides.[57][58] Among these vanadium derivatives, the layered vanadium pentoxide, LiV 2 O 5 has been the most studied in spite of its low discharge voltage, low electric conductivity and slow diffusion kinetics of lithium ion. 59,60 3][84] Some authors have used hexadecylamine 82,83 as templating molecule while others microporous polycarbonate filtration membranes. 84The electrochemical data obtained of the material prepared in presence of the primary amine have shown marked fading under cycling, probably due to the presence of residual organic material.This problem is avoided when synthetic membranes are used as template and initial capacity higher than 200 mA h g -1 and excellent reversibility for at least 100 cycles were showed.XRD data have indicated that the tubular structure is preserved, even after prolonged cycling.
In addition to the well-known synthesis of vanadium pentoxide nanotubes starting from vanadium (V) alkoxides, two alternative routes by using two novel non-alkoxide reagents, such as VOCl 3 and commercial V 2 O 5 as vanadium source and primary amines as templates or intercalates were proposed. 85The advantages of these methods are low cost and easy handling of commercial V 2 O 5 .The morphology and the composition of these nanotubes are quite similar to those obtained by vanadium (V) alkoxides.The average lengths were smaller than other nanotubes synthesized using conventional starting material.
V 2 O 5 nanoparticles synthesized by combustion flamechemical vapor condensation process have presented 30% higher retention capacity than conventional materials and up to 50% in case of lower discharge voltage.Initial specific capacities of the nanocrystalline and commercial V 2 O 5 material of 390 mA h g -1 and 350 mA h g -1 have reached 310 mA h g -1 and 190 mA h g -1 , respectively after eight cycles, showing that the capacity loss is much lower for nano-V 2 O 5 than for commercial V 2 O 5 . 50n order to enhance the intrinsic electric conductivity of vanadium pentoxide, nanocomposites of V 2 O 5 and conducting polymers or carbon have been target of study, mainly those prepared as nanostructured materials.
There are two different nanocomposites of vanadium pentoxide described in the literature.One is composed of nanofibers of vanadium pentoxide and a conducting polymer, polyaniline 78 while the other uses single-wall  51 carbon nanotubes (SWNTs) to electrically "wire" the poorly conducting V 2 O 5 nanoparticles. 86It is notable that the V 2 O 5 /SWNT composite has shown much better performance than that obtained with poly(aniline), both in terms of specific capacity as in capacity retention.From the TEM images (Figure 4), it is possible to note that the V 2 O 5 /SWNT nanocomposite possesses high pore volume that ensures electrolyte access throughout the electrode, while in the V 2 O 5 /PANI composite a more compact structure is observed.In addition, we believe that the appreciable mechanical property of the SWNT and the electric conductivity between the vanadium pentoxide nanofibers are much more effective than the connectivity promoted by polyaniline.Probably, the addition of carbon nanotubes provides electronic conduction without blocking electrolyte access to the V 2 O 5 nanofibers.
This argument is corroborated by the comparison between nanocomposites of V 2 O 5 with carbon black and single-wall carbon nanotubes.It believes that the traditional composite with carbon black particles forms aggregates, which may occlude the vanadium oxide surface (Figure 5), like the nanocomposites prepared with the conducting polymer, PANI.In the case of V 2 O 5 /SWNT nanocomposites in spite of the intimate contact between the two components, the access to the electrolyte is facilitated, improving the electrochemical performance of the resulting cell.This structural difference of nanocomposites with SWNT and other compounds is due to the similar morphology and dimensional scale of the SWNT and the prepared vanadium oxide. 87other vanadium oxide containing V 5+ ions that has received much attention is LiV 3 O 8 .This compound is formed by distorted [VO 6 ] octahedral sites connected via shared edges and vertices to form [V 3 O 8 ] -strands that are stacked one upon another to form quasi-layers, where lithium ions are situated.During lithiation V 3 O 8 framework remains intact and there is no change in the unit cell volume.These features are attractive to possible use as cathode in lithium ion batteries. 88,891][92][93] In general, large particles are obtained due to the use of high temperature.
It is well known that the synthetic procedures as well as the post treatments have significant influences on the  electrochemical properties of LiV 3 O 8 .Several reports have mentioned the importance of both crystallinity and particle size on the electrochemical properties of this material. 94iV 3 O 8 nanorods were prepared by a mixture of LiOH, V 2 O 5 and NH 4 OH after hydrothermal reaction, followed by evaporation and posterior calcinations at different temperatures. 95The sample heated at 300 o C has afforded the best electrochemical performance, showing high discharge capacity of 300 mA h g -1 in the range of 1.8 -4.0 V and 92% of capacity retention after 30 cycles (Figure 6).Notably, the value of 300 mA h g -1 is considerably higher than the capacities of 220-270 mA h g -1 obtained for LiV 3 O 8 prepared by other methods. 58,90,96,97TEM micrographies and XRD data have showed that heat treatment performed at high temperatures cause changes in the LiV 3 O 8 crystallinity and morphology and larger and more crystalline particles are obtained at 600 o C. Both the small size and the disordered structure are responsible for this appreciable electrochemical response.
VO 2 (B) has been emerged as another good option as cathode material for lithium ion batteries since 1996. 98ts structure is built up by distorted octahedral VO 6 that shares both corners and edges, where lithium ions can occupy both octahedral and tetrahedral sites.Commonly, VO 2 (B) is obtained from the reduction of potassium vanadate (K 3 VO 4 ) using potassium borohydride in aqueous media.When this compound is heated above 350 o C, it begins to transform irreversibly to the more stable rutile VO 2 (R) as revealed by XRD data, which is not an attractive cathode. 99Because of this, the methodology to synthesize VO 2 (B) is the key point and the search for new methods is still object of interest.
Following this conventional methodology, VO 2 (B) metastable was prepared using two different reducting agents, such as KBH 4 and sodium dithionite. 56,100The observed capacities were higher than that of bulk VO 2 (B). 101In addition, comparing the electrochemical performance of lithium cells using these two samples of VO 2 (B), it is evident that the powder prepared with sodium dithionite in LiOH medium has exhibited slight higher capacity than the sample prepared with KBH 4 .
Using a surfactant-assisted hydrothermal method at 180 o C, vanadium dioxide nanorods were also prepared. 102hen they are used as cathode, the resulting cell has shown initial discharge specific capacity of 306 mA h g -1 at 3.6-1.5V, which value is significantly superior to those obtained with nanoparticles prepared by Kannan et al. 56,100 This nanorod can maintain 80% of its initial capacity.

Lithium Cobalt Oxide
The most used transition metal oxide as cathode in commercial lithium-ion cells is layered LiCoO 2 .Lithium cobalt oxide was suggested for the first time as cathode in 1980 34 and ever since it has been widely investigated due to its favorable electrochemical properties, such as structural stability under cycling and its easy preparation with high quality.However, this material is very expensive and its theoretical capacity is relatively low (~ 130 mA h g -1 ), since the cobalt system goes through several phases as the lithium content varies from 1 to 0 and in practice it cycles just 0.5/Co. 103n general, LiCoO 2 is prepared by solid state reaction at high temperature.This methodology is a simple and an inexpensive method, the precursor being simply ground and mixed together before being calcined at high temperature.Solid state reactions to afford LiCoO 2 may start from many different precursors, such as, Li 2 O and CoO, Li 2 CO 3 and Co 3 O 4 , Li 2 CO 3 and Co, LiOH and Co 3 O 4 and others, but independent of Co precursor, LiCoO 2 formation reaction goes through the reaction of Li precursor and Co 3 O 4 . 35Owing to insufficient mixing, low reactivity of the starting materials and calcinations at 850-900 o C for several hours, LiCoO 2 produced by solid state reactions usually has non-homogeneity, irregular morphology and a broad particle size distribution, which characteristics influence significantly on the electrochemical properties of the product.][114] Although these methods require a much lower calcination time and shorter calcination temperature, the synthesis of  95 particles with size under 100 nm is very hard due to their tendency to agglomerate; therefore, the search for new synthetic methodologies or even the modification of the old ones is still necessary.
Recently, the use of triblock copolymer surfactant poly(ethylene oxide)-block-poly(propylene oxide)-blockpoly(ethylene oxide) (P123) as soft template agent to synthesize nanosized LiCoO 2 by modified sol-gel method was proposed. 115The size of the sample was around 50-100 nm, depending on the calcinations temperature (Figure 7).Samples prepared at 850 o C have presented the highest initial discharge capacity (150 mA h g -1 ) with good cycling stability (Figure 8).In spite of the larger size, the samples calcinated at higher temperatures have showed higher charge/discharge capacities, due to their better crystalline structure, which is verified by XRD data.
Another method used to prepare LiCoO 2 nanoparticles was the co-precipitation in ethanol with mechanical stirring and posterior calcinations at different temperatures. 116This method has produced nanoparticles with shape of thin polygons and size in the range of 20-100 nm, the particle size increasing with the temperature.The electrochemical performance of a cell based on these nanoparticles was excellent even at high discharge rate.To illustrate, specific capacities around 100 mA h g -1 for a 50 C and 130 mA h g -1 for 10 C rates were obtained.The capacity retention was moderate, showing 75% of retention in the 16 th cycle.The problem herein is that the nanometer sized lithium cobalt oxide is very easy to agglomerate and thereby it is hard to disperse and mix it with carbon black and binder to produce the cathode.Thus, the contact resistance of a cathode using this nano-size LiCoO 2 is much higher than that of commercial one, which explains the pronounced capacity fading.
Besides the low capacity discharge, LiCoO 2 goes to thermal decomposition.This instability is due to the reactive tetravalent Co in the delithiated state.Above 150 o C, such material collapses, which reaction is exothermic and the energy stored in the battery is released as heat.During collapse, oxygen is released, which can combust the organic electrolyte and evolve more heat. 117,118 I][121][122][123] In the search for new coatings, aluminum phosphates (AlPO 4 ) have been pointed out as a good alternative coating due to more uniform coating layer when compared to other coatings (Al 2 O 3 and ZrO 2 ).Moreover, the thermal stability of the AlPO 4 coated LiCoO 2 was comparable to that of LiMn 2 O 4 , which is the most stable compound thermally.The reason to explain why AlPO 4 nanoparticlecoated LiCoO 2 has superior performance to bare LiCoO 2 or other metal-oxide coated cathodes, may be attributed to the strong P=O bond, which is very resistant to chemical attack.The direct coating of AlPO 4 nanoparticles on  LiCoO 2 nanoparticles has shown a drastic improvement in both the safety and the electrochemical properties of LiCoO 2 cathodes. 124Up to now, the AlPO 4 -nanoparticle coated LiCoO 2 has shown approximately 99% retention of capacity (150 mA h g -1 ) at 1 C, even after 20 cycles.The increase in the thickness of the AlPO 4 coating has led to improvements of the electrochemical performance and the thermal stability. 125,126Figure 9 shows the retention capacity and some Co dissolution from bare and coated cathodes after 50 cycles.Cobalt dissolution from coated cathodes was significantly reduced when compared with the bare sample.Based on these results, the retention capacity appears to correlate with the Co dissolution suppressed by the AlPO 4 nanoparticles.
Besides the aluminum phosphates coatings, MgO nanoparticles were also tested as protective layer in LiCoO 2 .Zhao et al. 127 have reported the deposition of nanocrystalline MgO on commercial LiCoO 2 via sol-gel method.During heat treatment and charge-discharge process, Mg 2+ ions have diffused into the LiCoO 2 layers, acting as a pillar between CoO 2 layers and stabilizing the initial structure.The electrochemical performance of this modified LiCoO 2 has shown initial discharge capacity of more than 130 mA h g -1 maintained even after 40 cycles, while only 13 mA h g -1 for pristine LiCoO 2 was registered (Figure 10).These electrochemical results are a quite similar to those obtained for AlPO 4 coated LiCoO 2 electrodes; however the thermal stabilities of the latter samples were much more pronounced.The addition of a large amount of MgO (above 1% mol) is not favorable, since this compound is not electroactive.

Manganese Oxides
The use of manganese oxides in rechargeable lithium cells has been stimulated due to their low cost and small environmental impact.In this review, we will be centered on the spinel phase (LiMn 2 O 4 ) and on layered manganese oxide (LiMnO 2 ).
The discharge curve for Li x Mn 2 O 4 has two concentration portion (Figure 11), occurring near 4 V (region I and II) and 3 V (region III), which correspond to the addition of one more lithium, resulting in Li 2 Mn 2 O 4 . 128,129In the composition range, 0<x<1, at 4 V, the structure of Li x Mn 2 O 4 remains cubic.At 3 V, in the composition range 1.0<x<2.0 the cubic spinel phase transforms into an ordered tetragonal rock-salt phase, NaCl-type structure.
The spinel phase only cycles well for 0.5 Li/Mn, since discharge process for this material is normally limited to the upper plateau around 4V.The principal reason for the degradation at potentials below 3.5 V is presumably the transition from cubic phase to tetragonal phase.][132][133] Traditionally, spinels are produced by annealing lithium compounds (LiOH, Li 2 CO 3 , Li 2 NO 3 , LiI) with manganese oxides, acetates or hydroxides.This annealing process, done in air at high temperature, affects both morphology and structural characteristics of the target product.5][136][137] Thus, a sustained effort of many researchers has gone into the development of new synthetic methods to yield nanocrystalline LiMn 2 O 4 particles, such as acetate based chemical solution route, 138 reduction of manganese dioxide by glucose, 139 modified citrate route, 140 ultrasonic spray pyrolysis, 141 sol-gel using citric acid, 142 nitrate based chemical solution route, 143 mechanochemical synthesis, 144 sol-gel method, 27 self-combustion reaction, 145 glycine nitrate combustion process 146 and solid state reaction followed by ball milling. 147epending on the synthetic procedure, nanosized LiMn 2 O 4 with different physical and chemical properties, such as crystallinity, amount of combined water, specific surface area, porosity and conductivity were obtained.These characteristics affect the electrochemical properties of a resulting cell using these materials as cathodes, but it is certain that their small size is crucial to improve the electrochemical performance compared with batteries using conventional microcrystalline particles.
In general, the procedures listed above have produced nanoparticles with grain size ranging from 10 to 150 nm.In some cases, it is clearly seen that large particles consisted by several agglomerate nanocrystallines were obtained. 138,147 Een in these cases, in spite of the large size, due to the presence of nano-size sub-grains inside the relatively large particle, the performance electrochemical is better than the results obtained for bulk LiMn 2 O 4 .After calcinations at different temperatures, it was observed that at high temperatures, larger particles and more crystallinity were obtained (Figure 12).To illustrate, the XRD patterns of the spinel LiMn 2 O 4 prepared by self-combustion reaction (SCR) calcinated at different temperature are shown in Figure 13. 145urther heating at higher temperature leads to gradual sharping of the diffraction peaks, which is an indicative of improved crystallinity of the samples.This fact was also confirmed by detailed analysis of the peak broading combined with SEM. 145The discharge capacity values have varied from 100 to 150 mA h g -1 at 4 V in most of the cells using these nano-sized LiMn 2 O 4 .However, the capacity retentions have varied accentually depending on the sample used.For example, the batteries using nanoparticles produced by acetate based chemical solution route 138 have shown severe capacities fading.For the other cases, the capacity retention was much high, ranging from 70 to 90% after several cycles.Such difference among nano-sized samples might be related to the uniformity of particles shape and size.In other words, a cathode consisting of homogeneous particle is expected to have a regular network which can maintain a uniform intercalation reversibility of each particle through repeated cycles.On the contrary, heterogeneous and partially agglomerated particles may have different reactivity, which disparity among particles with various sizes is considered to increase as the cycling proceeds, resulting in a decrease in the reversibility of the electrode.The nanoparticles produced by solid state reaction followed by ball milling process have demonstrated almost no capacity fading, showing that this process significantly changes particle characteristics not only on the formation of nanometer-scale grains in an agglomerate particle, but also on the generation of lattice strain and partial oxidation of manganese ions.Finally, reactions that involve vigorous gas evolution, such as the glycine nitrate process (GNP), often produce highly open nanostructured powders.However, it is worth to note that the detailed nanostructured of the powder depends on the combustion parameters.
here has been significant work over the last few years with the objective of optimizing the performance of the spinel LiMn 2 O 4 by the substitution of different cations.4][155][156] The presence of Ni stabilizes the octahedral spinel sites and although the intercalation of lithium decreases the average valence of manganese to values below 4.5, the distortion of the tetrahedral phase is not observed.In addition, when the charge voltage increases above 4.3 V to 4.9 V, a new voltage plateau around 4.7 V appears, which suggest its use as cathode material for a 5V-lithium ion battery. 156,157  general, LiNi 0.5 Mn 1.5 O 2 is produced by solid state reactions, where microparticles are obtained.In the last years, LiNi 0.5 Mn 1.5 O 2 nanoparticles were prepared by two different methods.Lee et al. 157 have prepared pure nanosized LiNi 0.5 Mn 1.5 O 4 powder by composite carbonate process, which is a combination of sol-gel and solid-state methods.Later, LiNi 0.5 Mn 1.5 O 4 spinel was synthesized by single step sucrose aided self-combustion method, where mesopores were observed among the particles. 158In both cases, the samples are clusters (1-4 μm) composed of nano-sized small particles.When used as cathode in lithium ion batteries, discharge capacities around 140 mA h g -1 and excellent cyclability after several cycles were obtained in both cases.These nanosized materials have shown excellent electrochemical performance, even at high temperature, both in terms of cyclability and specific capacity.
When cobalt is added into spinel LiMn 2 O 4 to afford LiMn 1.8 Co 0.2 O 4 , this doping process also increases the stability of the spinel structure and consequently improves the cyclability.][161] In general, this compound has been prepared by solidstate reactions and precipitation techniques.However, these synthetic methodologies have led to powders with large size and less controlled morphology. 151,162 Mn 1.8 Co 0.2 O 4 nanoparticles (20-34 nm) were prepared via reverse micelle process, 163 followed by calcination.The smallest particles were obtained using water to oil ratio (W/O) around 1/5 and cathodes using these nanoparticles have shown excellent cyclability and good capacity (160 mA h g -1 ).When larger W/O volume ratio was used and hence larger particles were obtained, the discharge capacity decreased markedly, besides the excellent cyclability.
In order to combine the superior electrochemical performance of layered structures with the advantages of a manganese-based chemistry, LiMnO 2 has appeared as a good substitute for spinel LiMn 2 O 4 due to its higher theoretical capacity of 285 mA h g -1 . 164In this compound, Li and Mn ions occupy octahedral sites arranged in an alternating zig-zag configuration of edge-sharing [LiO 6 ] and [MnO 6 ] octahedral.However, when LiMnO 2 is used as cathode in a Li-ion cell, it tends to transform toward the spinel structure only by a minor rearrangement of the cations and like spinel LiMn 2 O 4 shows severe capacity fading with cycling.
As a way to overcome some of these difficulties encountered with crystalline LiMnO 2 , amorphous or nanocrystalline structures have attracted increasing attention.][167][168][169][170] Wu et al. 171 have proposed a new method to prepare nanosized orthorhombic LiMnO 2 (35 nm) consisted by two-step liquid phase thermal process at low temperature.In the first step, the precursor Mn 3 O 4 was prepared via a solvothermal route, where potassium permanganate powder was reduced with a primary alcohol, such as ethanol and methanol.Followed by hydrothermal process in lithium hydroxide aqueous solution at 160-180 o C, the precursor Mn 3 O 4 was transformed to the nanocrystalline o-LiMnO 2 .The initial charge-discharge curves are distinct from spinel phases, but the o-LiMnO 2 phase is transformed to spinel-like LiMn 2 O 4 gradually.The cell using this material as cathode has shown initial reversible specific capacity of 225 mA h g -1 with moderate capacity fading (20%) after 30 cycles (Figure 14).
Later, nanosized orthorhombic LiMnO 2 with high purity and good crystallinity was synthesized by a simple process, involving controlled oxidation of Mn(OH) 2 followed by in situ exchange of lithium ions under mild conditions.The nanosized o-LiMnO 2 has exhibited excellent initial discharge capacity of 220 mA h g -1 with good cycling performance (89% retention after 30 cycles). 172o far, there are a limited number of methods to prepare nanocrystalline LiMnO 2, thereby this field research is still open to improvements.
It is interesting to state that the transformation from orthorhombic to spinel-like structure becomes easier when the degree of stacking faults increases.Thus, o-LiMnO 2 with a low number of stacking faults could display better cyclability than a high stacking faulted phase when cycled between 2.0 V and 4.3 V.
Therefore, nanomaterials with disordered lamellar structures are expected to play a role in inhibiting the rapid phase transformation observed in spinel LiMn 2 O 4 , because the irregular layer arrangement may provide a high-energy barrier against the phase conversion.][175][176][177][178][179] The resulting elementary layers, so-called "nanosheets" can act as inorganic building blocks to build up novel nanostructured systems.The restacking of these nanosheets via flocculation produces a disordered layered structure with irregular orientation (Figure 15).In 2003, colloidal MnO 2 nanosheets have been successfully obtained by delaminating an acid exchanged form of KMnO 2 with tetrabutylammonium (TBA) ions, followed by restacking via flocculation with lithium ions. 180These nanosheets have shown high two-dimensionality associated with a thickness comparable to molecules, showing a number of interesting aspects with respect to highly two-dimensional anisotropy and ultra thin thickness in nanometer scale.
The resulting disordered sample underwent electrochemical Li + insertion/extraction with smooth cycling curves, showing specific capacity of 190 mA h g -1 in the first cycle, followed by 220 mA h g -1 in the next cycle and continuous decrease, as shown in Figure 16.After 20 cycles, only a very small plateau was showed at 4.0 V, but no apparent one at 3.0 V, which is characteristic of spinel phase, indicating the preservation of the initial phase.This smooth charge-discharge slope without well defined plateaus was apparently different of layered LiMnO 2 , which generally undergoes phase transformation into spinel in the first cycles accompanied by evolution of a clear voltage profile feature of spinel structure with 3 V and 4 V. Probably, this capacity fading can be attributed to the formation of some inactive electrochemical phase or the dissolution of the material.

Concluding Remarks
The use of nanomaterials for energy storage devices, such as batteries, is a rapidly growing field with tremendous potential.However, such research is only in its infancy and is likely to undergo dramatic evolutionary and revolutionary changes as insight into nanoarchitectures becomes clearer through scientific investigation in this research area.
In this review we present the motivations to use nanostructured transition metal oxides as cathodes in lithium ion batteries in place of the conventional microcrystalline ones.It is believed that such nanomaterials mitigate the problem of slow diffusion because the distance that lithium ion must diffuse in the solid state is limited to the radius of the nanoparticles.Moreover, nano-size particles have large surface area to volume ratio; therefore charge accommodations of these nanoscopic particles will take place largely at the surface, avoiding stress-induced structural changes (e.g.Jahn-Teller distortions in Li x Mn 2 O 4 ).

Figure 1 .
Figure 1.Schematic diagram of a lithium-ion battery.

Figure 2 .
Figure 2. Layered, spinel and olivine structures of positive electrode materials for lithium batteries.Reprinted by permission from Macmillan Publishers Ltd., copyright (2002).41

Figure 7 .
Figure 7. SEM images of LiCoO 2 calcined at various temperatures (a) 450 o C and (b) 850 o C for 12 h.Reprinted with permission from © 2005, Elsevier.115

Figure 9 .
Figure 9. Plot of the retention capacity and Co dissolution in the bare and AlPO 4-coated LiCoO 2 cathodes.Reprinted with permission from © 2003, Elsevier.125

Figure 11 .
Figure 11.Open circuit curve of Li x Mn 2 O 4 at 30 o C. Reprinted with permission from © 2001, Elsevier. 128

Figure 12 .
Figure 12.TEM photographs of LiMn 2 O 4 spinels prepared by sol-gel annealed at different temperatures (a) 350 o C (b) 450 o C (c) 550 o C and (d) 650 o C. Reproduced with permission from © 2004, The Electrochemical Society, Inc.27