Nano Metal Oxides for Li-Ion Batteries

Juchen Guo and Chunsheng Wang

University of Maryland

Nanoscale and nanostructured metal oxides have drawn tremendous interests from the researchers working in the field of energy storage and energy conversion technologies in recent years. In this chapter, the state of the art of nano-metal oxide materials in Li- ion batteries will be discussed in attempt of a comprehensive over- view.

Lithium-ion battery has been a very importance category of rechargeable batteries since its first commercialization by Sony in 1991. It has been widely used in portable consumer electronics such as laptop computers, digital camera, small power tools, etc. Howev- er, its potential is not limited to such small devices due to several unique merits: Li-ion battery has the highest energy density among all types of rechargeable batteries that are currently on the market. It also has relatively low self discharge rate. Because of these virtues, interests in Li-ion batteries keep growing for defense, aerospace, 2 smart grid system and automotive applications. To satisfy the de- mands of these emerging applications, the next generation of Li-ion batteries must achieve a holistic and striking advancement from the current technology, specifically in four criteria - energy density, dis- charging and charging rate (power density), safety feature, and cycle stability. The energy density, power density and cycle stability of Li- ion batteries are mainly determined by electrode materials and structures. Enhancement of the safety feature ultimately depends on development of non-flammable electrolyte and solid electrolyte to re- place the current liquid electrolyte consisting of highly flammable organic solvents. The use of nano metal oxides (nanoscale or nanostructured) as anode materials, cathode materials, and electrolyte ad- ditives has greatly enhanced the performance of Li-ion batteries due to their unique chemical and structural properties.

14.1 Classification of Electrode Materials for Li-Ion Batteries

Fig. 1 Schematic of Li-ion battery with graphite anode and LixMO2 cathode in the state of discharge

Before going into further discussion, it is necessary to briefly introduce how Li-ion battery works. For instance, the most common cathode material is lithium cobalt oxide (LiCoO2), and anode material is graphite as shown in Figure 1. Typical electrolyte consists of lithium salts like lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4) dissolved in organic solvent such as mix- 3 ture of propylene carbonate and diethyl carbonate. During the battery charging process, Li atoms in LiCoO2 become ions, migrating to graphite anode across the electrolyte and inserting into the gaps between the graphene layers. The reverse process takes place in the battery discharge: Li atoms stored in the layered graphite become ions, migrating to the LiCoO2 cathode and inserting into layers of octahedral lattices formed by cobalt and oxygen atoms. This type of lithiation/delithiation mechanism is referred as intercalation reaction in which Li is stored in layer-structured materials such as graphite and lithium cobalt oxide. Nowadays, the intercalation mechanism has been generalized to refer to all topotactic reactions of Li-ions inserting into the interior of the lattice of the host materials of which the structures are not limited to be layered. Another lithiation/delithiation mechanism is based on reversible redox reactions between metal oxide and Li (Li+), and it is referred as conversion reaction. According to conversion mechanism, lithiation takes place through reduction of metal oxide by Li to produce metal and lithium oxide, and delithiation takes place through oxidation of the formed metal by lithium oxide. Beside these two mechanisms, in a few binary inter-metallic AB compounds, Li will reversibly displace

A to form LixB, and the formed LixB has a strong structural relationship with parent AB compound. This mechanism is referred as displacement reaction. All current metal oxide electrode materials for Li-ion batteries can be sorted into these three categories, except tin dioxide (SnO2) based materials of which the lithiation/delithiation process combines conversion and alloying reactions. Therefore, for 4 the purpose of articulation, all the metal oxide electrode materials will be discussed with respect to their different lithiation/delithiation mechanisms.

14.2 Advantage & Disadvantage of Nano-Electrode Materials

The advantages of nano electrode materials come from their nanometer characteristic length and their tremendously large surface area. Generally speaking, the smaller size can shorten the Li- ion/electron transport pathways, and enhance the phase transformation. The large surface area can also speed up the charge transfer reaction kinetics due to the increased contact area with electrolyte. En- hanced Li insertion/extraction kinetics can lead to higher rate performance, even novel lithiation/delithiation mechanism. Higher surface area also can enhance the capacity through the surface Li storage mechanism [1]. Moreover, nano electrode material can better accommodate the mechanical strain induced by the concomitant volume change in the lithiation/delithiation process, thus improving the cycle stability. Unfortunately, the disadvantages of nano electrode materials are also from their nano characteristic scale and large surface area. The nanoscale materials will lower the packing density of the electrode thus resulting in low overall energy density of the batteries. Also, the large surface area will promote larger amount of side reactions at the electrolyte/electrode interfaces. Therefore, the development of nano electrode materials should focus on the direction to the 5 optimized properties balancing the advantages and the disadvantages.

14.3 Nano Metal Oxide Anode Materials

14.3.1 Intercalation Metal Oxides Materials with layered structure (graphite for anode and Li-

CoO2 for cathode) are the natural choice as the Li host material [2], but not the only ones. Materials with tunneled structures (such as spinel) can also be used as Li storage host with intercalation mechanism. Among them, Li4Ti5O12 and TiO2 are the two most intensively studied metal oxide anode materials.

The concept of using the B2O4 framework of an AB2O4 (A = Li) spinel material as host structure for Li-ions storage was original- ly proposed by Thackeray et al. in 1980s [3]. Lithium titanium oxide, Li4Ti5O12, a ceramic material having a defect tunneled

([Li1/3Ti5/3]O4) structure, was initially proposed as an anode material by Colbow et al. in 1989 [4] and tested by Ferg et al. [5] and Ohzuku et al. [6] in early 1990s. Li4Ti5O12 can be lithiated over the composition range of Li4+xTi5O12 (0 < x < 3) at about 1.55 V potential versus Li/Li+, and its theoretical lithiation capacity is 175 mAh g-1. Despite its moderate lithiation capacity, the particular advantage of Li4Ti5O12 comparing to other spinel anodes was that it is a “zero-strain” intercalation material. The defect Li1/3Ti5/3O4 spinel framework exhibits minimal volume change during Li-ions insertion and extraction so that the crystal structure is better retained, thus resulting in better cycle life. Another character of Li4Ti5O12 is its lithiation voltage of 1.55 6

V versus Li/Li+, which is considered to have two-faced effects. On one hand, the 1.55 V lithiation voltage is higher than the decomposition voltage of the organic solvents in the electrolyte. Therefore using Li4Ti5O12 as the anode material can eliminate the formation of Solid Electrolyte Interface (SEI) film, which a considerable cost efficiency factor. Also, the higher lithiation voltage significantly reduces the possibility of the lithium metal plating at the anode, so that the safety can be enhanced. On the other hand, using Li4Ti5O12 anode sacrifices the full cell working voltage because of its higher lithiation voltage compared to graphite (0.1 to 0.2 V versus Li/Li+).

Since pure Li4Ti5O12 is an electric insulator, the advantage of nanoscale Li4Ti5O12 is the extraordinary enhancement of Li insertion/extraction kinetics. The mean Li-ion diffusion time in an ideal anode particle can be approximately expressed using the following equation, if assuming Fickian diffusion.

L is the diffusion distance and D is the Li-ion diffusivity in the material. Based on this equation, the advantage of nanoscale electrode material is obvious: the resultant short diffusion distance can reduce the diffusion time significantly. For instance, if the particle size is reduced to 100 nanometers from 1 micrometer, Li-ion diffusion time can be decreased 100 times. Another advantage of nanoscale electrode materials for charge transfer kinetics enhancement is their large surface area which results in large contact surface between electrode and electrolyte. As an electrically insulating material, the 7

electronic conductivity of Li4Ti5O12 increases during the lithiation reaction from the outer surface directing inward, which is not critically problematic for lithiation, since the Li+/e- transport takes place at the outer layer anyway. During delithiation, as Li being extracted, the conductivity starts to decrease from the outer layer of the Li4Ti5O12 particle. Therefore, the delithiation process has worse kinetics than lithiation. Fast separation of Li+ and e- is critical to achieve fast charge/discharge rate, which can be achieve by reduce the Li+/e- transport pathway by using nanoscale Li4Ti5O12 materials.

Because of these advantages, nanoscale Li4Ti5O12 anode materials have been extensively studied. Among them, Kim and Cho

[7] reported synthesis and electrochemical performance of Li4Ti5O12 nanorods. The diameter of the reported Li4Ti5O12 nanorods is about 100 nm diameter as shown in Figure 2a. The notable merit of this material is its very promising discharge rate capacity. As shown in Figure 2b, the reversible first discharge capacity was 165 mAh g-1 under cycling rate of 0.l C (16 mA g-1), and no capacity fading was observed up to 30 cycles between 1 and 2.5 V. At rates of 0.5 C, the first capacity at 0.5 C was identical to that at 0.1 C. Very small capacity decreases with increasing current were observed at 5 and 10 C (1600 mA g-1) rates, the capacity retention was 95 and 93%, showing 157 and 155 mAh g-1, respectively. As a comparison, the electrochemical performance of Li4Ti5O12 particles with 700 nm diameter is distinctly worse. Though this cannot be used as the direct evidence of the superiority of nanorods over nanoparticles due to their differ- 8 ent characteristic size, it clear demonstrate the significant advantage of nanoscale materials by enhancing charge transfer kinetics.

Fig. 2 TEM image of the Li4Ti5O12 nanorods, (b) rate capacity test of the Li4Ti5O12 nanorods at different C rates [7]

In light of the great promise of Li4Ti5O12 as Li intercalation anode materials, researcher naturally began to investigate titanium dioxides as candidates of anode materials because of their higher theoretical lithiation capacity. The Li intercalation reaction to TiO2 can be generally expressed as the following reaction:

+ - xLi + TiO2 + xe ↔ LixTiO2 Full lithiation should lead to the formation of lithium titanium oxide

-1 in formula of LiTiO2 (x = 1) with 335 mAh g theoretical capacity. This reaction takes place in the voltage range from 1.5 to 1.8 V.

Therefore, like Li4Ti5O12, using TiO2 as the anode materials can avoid anode passivation and also enhance the safety feather. The investigation on TiO2 was actually not a recent idea: it started in 1980s and continued in 1990s [8-11]. However, the sluggish performance of the earlier TiO2 materials had merely attracted lukewarm atten- tion. The TiO2 research really took off in virtue of the development of nanotechnology. To date, there have been four types of titanium dioxides being reported to have lithiation capacity, and they are rutile, brookite, anatase and bronze (TiO2(B)). These polymorphs are the only known naturally occurring TiO2 forms to date, even bronze is rare. Rutile is the most common and the most thermodynamically 9

stable form of TiO2. Anatase and brookite can be converted to rutile upon heating in a temperature range of 700-1000 °C. The basic physical and structural properties of these TiO2 polymorphs are list- ed in Table 14.1 [12].

Table 14.1 Data for TiO2 polymorphs for anode materials [12]

The main problem of TiO2 polymorphs as anode materials is their poor Li+ and electron conductivity so that the Li lithiation/delithiation reaction kinetics was largely hindered. Recent studies suggested that the TiO2 lithiation/delithiation reaction kinetics, ultimately the rate performance, are closely related to its crystal structural properties such as site occupation, local coordination and energetic [13-15]. For instance, the thermodynamically stable posi- tions for Li insertion in rutile are the octahedral sites in the ab planes. It has been proved that the Li+ diffusion in rutile is anisotrop- ic: the theoretical diffusion coefficient of Li-ion along ab plane is 8 orders of magnitude lower than that along the c axis (10-14 cm2 s-1 and 10-6 cm2 s-1, respectively) [16-22]. The repulsive interaction between the Li-ions diffusing along the c axis may slow down the diffusion, and the Li-ion pairs in the ab plane may also block the Li-ion diffusion along the c axis [18, 22]. Therefore, the low Li insertion capacity of rutile is mainly restrained by poor Li-ion transport kinetics. Recent studies suggested that the lithiation capacity of rutile could be increased by reducing the structural size. Jiang et al. reported full lithiation in the first cycle using nano-sized needlelike rutile 10 particles (15 nm) for the first time [23]. Also, 0.7 Li per unit of rutile can be extracted in the first delithiation. Reducing the rutile particle size has three-fold advantages. Firstly, it decreases the Li+ and electron diffusion pathway. Secondly, the mechanical strain during Li insertion is reduced. Finally, because of the enormously enhanced surface area, Li surface storage capacity is increased [24]. Other re- markable rutile works include the rutile nanorods (10 nm × 40 nm) reported by Hu et al. [24] and nanowires (10 nm × 200 nm) reported by Baudrin et al. [25] The later rutile nanowires demonstrated much superior capacity than bulk rutile and even nano-size rutile particles as shown in Figure 3.

-1 Fig. 3 (a) Galvanostatic cycling curves of rutile TiO2 samples using a 30 mAh g current between 3 V and 1 V at 20 °C; (b) the capacity retention for these samples

Although the brookite has been less reported for its Li storage capacity compared to other TiO2 polymorphs, the reported performance of brookite indicated strong dependence on the particle size. The 10 nm sized brookite particles delivered reversible capacity of 170 mAh g-1 for more than 40 cycles, reported by Reddy et al [26, 27]. For anatase, the Li+ diffusion in it is more facile comparing to rutile because of its looser lattice structure. Upon the Li uptake into anatase, its original lattice structure of tetragonal body-centered

I41/amd space group changes to orthorhombic pmn21 space group when 0.5 Li per unit of anatase is inserted [28]. Also, the lithiation 11 results in 4 % volume expansion thus causing rapid capacity fade for bulk anatase material [29]. Reducing the anatase particle size again can shorten the Li-ion diffusion pathway and increase the Li surface storage capacity due to the large surface area [30, 31]. For example, Gao et al. reported first discharge and charge capacities of 340 and 200mAh g-1, respectively, for the anatase nanotubes with 10-15 nm diameters and 200-400 nm lengths [32].

Fig. 4(a) TEM image of TiO2(B) wires [33]; (b) TEM image of TiO2(B) tubes

[34]; (c) variation of potential with Li content for TiO2(B) nanowires and TiO2(B) nanotubes cycled under identical conditions [34]

TiO2(B) (bronze) is rare in nature so that all reported TiO2(B) anode material to date was synthesized. The advantage of bronze comparing to other TiO2 polymorphs is its more open lattice structure which facilitates the Li insertion. Armstrong et al. synthesized

TiO2(B) nanowires (20-40 nm diameter and 2-10 µm length) [33] and nanotubes (10-20 nm outer diameter, 5-8 nm inner diameter and ~ 1 µm length) [34] as shown in Figure 16.4a and 4b, respectively.

The TiO2(B) nanowires demonstrated superior lithiation capacity

-1 (Li0.91TiO2, 305 mAh g specific capacity) to the bulk material (Li0.71-

-1 TiO2, 240 mAh g specific capacity). During the lithiation process, there is no detectable volume change taking place due to the more open lattice structure. Comparing to nanoscale TiO2(B) particles with similar diameter, even both showed similar first cycle lithiation capacity, the capacity retention of the nanowires was far more better. 12

The TiO2(B) nanotubes showed marginally higher lithiation capacity

-1 (Li0.98TiO2, 325 mAh g specific capacity) than the nanowires. How- ever, the TiO2(B) nanowires demonstrated better kinetics in spite of lager diameter. As shown in Figure 16.4c, the plateaus of the charge/discharge curves of TiO2(B) nanowires are flatter and closer to each other, which indicates small overpotential. The irreversible capacity of the nanowires is also smaller.

TiO2 polymorphs have demonstrated very attractive electrochemical properties as anode materials, such as higher lithiation voltage avoiding electrode passivation and enhancing safety feature.

The nanoscale TiO2 further improved the charge transfer reaction kinetics by shortening the Li+ and electron transport pathway. Howev- er, there are still a few intrinsic disadvantages of TiO2 that may need further investigation. As it has relatively higher lithiation potential

+ versus Li/Li redox couple, the full cells with TiO2 anode and typical cathode are subject to lower cell voltage. However, exceptions may be possible if a high potential cathode material can be found. One example is TiO2(B) nanotubes/Li[Ni0.5Mn1.5]O4 cell reported by Armstrong et al [35], which could achieve a 3 V overall cell potential. The other serious problem of nanoscale TiO2 materials is the continuously irreversible capacity on every cycle, except one report by Armstrong et al [33]. The irrecoverable capacity is mainly attributed to the electric insulating nature of the TiO2. As previously mentioned in this chapter, the electric insulation can hurt Li extraction more than insertion. Because the outer layer of the particle become electric insulating, it will be more difficult to extract all inserted Li. 13

The current development indicates that even the dimension of TiO2 has been reduced to scale of tens of nanometers, the transport pathway was still not efficient enough for fast electron and Li+ separation to completely deplete the inserted Li. This problem can definitely jeopardize the real application of TiO2 as Li-ion battery anode. One solution is to incorporate electric conductive nano-sized composite into TiO2 to form nanostructured material. One great example is that

Guo et al. recently reported a mesoporous RuO2-anatase TiO2 composite [36]. The RuO2 nanoparticles formed an electric conductive network in the mesoporous TiO2 structure so that the electrochemical performance was enhanced. Liu and coworkers reported a hybrid nanostructure of rutile TiO2 and graphene [37]. Both works demonstrated reduced irreversible capacity and largely improved fast charge/discharge performance. Other reported intercalation metal oxides include oxides of vanadium, niobium from Group 5B and molybdenum, tungsten from Group 6B in the Periodic Table of the Elements. The concept of using these oxides as Li-ion battery electrode taking the advantage of their layered lattice structures was proposed by Whittingham et al. in the 1970s [38]. Binary vanadium oxides with octahedral or distorted octahedral coordination are known for all oxidation states between V5+ and V2+. The typical high lithiation-delithiation voltages of most of the vanadium oxides makes them suitable as cathode materials.

One exception is VO2(B), a vanadium dioxide formula having mon- oclinic metastable shear structure. Its advantage as Li storage material is its structural stability arising from an increased edge sharing 14 and the consequent resistance to lattice shearing during cycling [39].

VO2(B) received particular interest as an anode material for aqueous electrolyte Li-ion batteries because of its proper electrode potential

+ of 2.5V versus Li/Li [40]. Flower-like VO2(B) nanoparticles have been synthesized by Zhang et al [41], and tested as anode material coupled with LiMn2O4 cathode. The flowerlike VO2(B) demonstrated superior electrochemical properties to VO2(B) nanobelt and carambola-like nanoparticles [41]. However, the Li-ion batteries with aqueous phase electrolyte based on VO2(B) suffers very low energy density which is about 75 mAh g-1 at the first cycle. It can be attributed to the cell voltage restrict to avoid H2O decomposition.

Beside VO2(B), some vanadium oxide based compounds such as

LiVO2 [42], LiV3O8 [43], MnV2O6 [44], RVO4 (R = In, Cr, Fe) [45] and LiMVO4 (M = Cd, Co, Zn, Ni, Cu and Mg) [46] were also studied as anode materials. The niobium oxides have similar properties as vanadium oxides. Ternary niobium oxide such as Ag nanoparticle-doped LiNbO3 [47] was investigated as the anode materials, as well as KNb5O13 and K6Nb10.8O30 [48]. LiNbO3 demonstrated low lithiation voltage (< 0.5 V) and high first cycle capacity. However, the first delithiation capacity is only 12% to 13% of the first lithiation one. These niobium oxides demonstrated capacities between 100 and 200 mAh g-1 within the voltage range of 1.0 to 1.5 V versus Li/Li+. Though there has not been nanoscale or nanostructured niobium oxides reported to date, it can be speculated that these types of material can possess enhanced electrochemical performance. 15

The molybdenum dioxide (MoO2) and tungsten dioxide

(WO2) as Li-ion battery anode material were systematically investigated by Auborn and Barberio in 1987 for the first time [49]. MoO2 nano-particles were synthesized by Yang et al. from reduction of

Molybdenum trioxide (MoO3) [50]. This rutile-like MoO2 material demonstrated a reversible capacity of 318 mAh g-1 for 20 cycles at 5 mA cm-2 current density. Also, 85% of the reversible capacity was within the range of 1.0 to 2.0 V. The same research group also synthesized MoO2 tremella-like nanoparticles consisting of nanosheets, which showed reversible capacity about 600 mAh g-1 at 0.5 mA cm-2 current density [51]. Superior electrochemical performance was achieved by Dillon and coworkers based on the nanoscale MoOx consisting of Mo, MoO2 and MoO3 phases [52]. Reversible capacity

-1 of 600 mAh g over 50 cycles at C/5 charge/discharge rate. MoO3 and tungsten trioxide (WO3) are also promising Li intercalation materials for their layered structure. The lithiation/delithiation potential of WO3 is around 4 V so that it is a cathode material [53]. On the other hand, MoO3 has proper Li intercalation potential as anode ma-

-1 terial and relatively high theoretical capacity of 1117 mAh g . MoO3 has an orthorhombic crystal structure composed of distorted MoO6 octahedral layers. The reported electrochemical performance of the nanoscale MoO3 materials varies depending on different morphologies. Lithiated MoO3 nanobelt (200 nm wide and 2-6 µm long) was reported by Mai et al [54]. Compared with non-lithiated MoO3 nanobelt, the lithiated one showed lower lithiation capacity, but better cycle stability and less lithiation-delithiation hysteresis. Howev- 16 er, the reported capacity (~ 250 mAh g-1) was significantly lowered than the theoretical capacity and the plateaus of the lithiation-delithiation voltage plot are above 2.25 V which was not ideal for anode due to the resultant low full cell potential. Recently, similar MoO3 nanobelt with a uniform carbon coating was reported by Hassan et al [55]. The reported result was very attractive: the carbon-coated

-1 MoO3 nanobelt could retain capacity of 1064 mAh g after 50 cycles with 0.1C between 3.0 and 0.05 V, and no trend of fade was observed. The first cycle lithiation capacity (~1300 mAh g-1) exceeded the theoretical capacity which could be attributed to the formation of the solid electrolyte interphase (SEI) film or the surface Li storage.

Also, the volume change induced MoO3 pulverization was observed from the SEM image. MoO3 nanoparticles (~ 20 nm) were synthesized by Lee and coworkers [56]. These MoO3 nanoparticles demonstrated superior cycle stability and great capacity: Above 600 mAh g-1 capacity could be delivered after 150 cycles between 3 V and 0.005V with C/2 charge/discharge rate. The performance was compared to micron-sized MoO3 particles of which the performance is far inferior. Therefore, the excellent performance of MoO3 nanoparticles can be attributed to their nano size. As summery for the intercalation metal oxide anode materials, they all possess layered or tunneled structures. Typical lithiation reaction always occurs with concomitant lattice volume expansion which has negative influence on the cycle stability. The lithiation capacity of intercalation metal oxides is limited by the availability of the lattice sites and suppressed by the redox competition with other 17 phases. Furthermore, most of the intercalation metal oxides have low Li+ diffusivity and electric conductivity. Therefore, the slow ion/electron transport kinetics is really the bottleneck to achieve full capacity and fast charge/discharge rate. The advantages of nanoscale materials mainly are the shortened Li+ and electron transport pathway and the enhanced surface area. As the result, the kinetics can be greatly enhanced. Higher grain interface area in nanoscale materials can also enhance the storage of the Li. The problem of the current intercalation metal oxide anode materials is the irreversible capacity (low coulombic efficiency), which can still remain even the material size is reduced to ten of nanometers. This observation may indicate that reducing size is not the panacea, the conductivity must also be enhanced. The current solution for this problem is conductive layer coating and conductive composite doping.

14.3.2 Conversion Metal Oxide Materials

In 2000, Poizot et al. reported a new lithiation-delithiation mechanism of Li storage in transition metal oxides which can be used as anode materials [57]. This new mechanism is different from the aforementioned Li insertion/extraction, and can generally be expressed as

2xLi + MOx ↔ M + xLi2O (M = Fe, Ni, Co, Cu, Mn, Cr, Ru, Zn) 18

During the lithiation (metal oxide reduction) reaction, the metal oxide is reduced by lithium at a certain potential. In the delithiation

(metal oxide oxidation) reaction, metal is oxidized by Li2O to their original valence state. This mechanism is referred as conversion reaction. The lithiation through metal oxide reduction has actually been well recognized and investigated for a long time. For instance,

Li and manganese dioxide (MnO2) is one anode/cathode pair for pri- mary Li battery. However, this reaction was considered as irreversible until the investigation of Poizot et al. The detailed study shows that the lithiation resulted in 2-5 nm metal grains embedded in an amorphous Li2O matrix [57]. This nanostructure greatly reduces the Li+ and electron transport pathway, and facilitates moving

+ Li through the Li2O phase and electrons through the metal phase, thus making reversible lithiation/delithiation possible. Therefore, this conversion mechanism is indeed one excellent example of the applications enabled by the nanotechnology. However, the delithiation capacity is not completely matched with the lithiation capacity. Most of the results of conversion metal oxide anode materials showed considerable amount of irreversible capacity in the first cycle. The only exception is ruthenium dioxide reported by Balaya et al. [58], because RuO2 has unusually high electric conductivity. However, RuO2 is not a practical anode material because of its rarity. The cause of the irreversibly capacity is the insufficient Li+ and electron conductivity for complete delithiation. Another common problem for metal oxide anode materials caused by the same reason is the large hysteresis of charge/discharge volt- 19 age profile. Ideal electrode should have voltage hysteresis as small as possible to have high energy density efficiency. The work of Li et al. clearly demonstrated that reducing the particle size of the metal oxide could effectively lower the potential hysteresis [59]. Since the original study by Poizot et al, numerous investiga- tions on nanostructured metal oxides with conversion mechanism have been carried out. To date, the most commonly investigated metals include iron, cobalt, nickel, copper, manganese, and chromi- um. All valence states of these metal oxides can theoretically be used as anode materials. However, the conversion of high valence metal oxides is complex comparing to the low valence ones with rock-salt like structure. Larcher and coworkers demonstrated the Li- inserted intermediates in forms of LixCo3O4 and LixFe2O3 for conversion of Co3O4 and α-Fe2O3, respectively [60, 61]. Both Co3O4 and α-

Fe2O3 possess tunneled spinel structures, thus Li initially inserting into these materials following the intercalation mechanism. This process had been long recognized. However, Larcher et al. found out that further lithiation could result in deep structural modification to the rock-salt structure. Finally the metallic phase was formed as nano-grains dispersed in Li2O matrix. Similar process could be ap- plied to Fe3O4. The formation of the Li-intercalated intermediate phase is closely related to the material size [61]. For instance, 1 Li could be taken into LixFe2O3 when nano-sized α-Fe2O3 was used, whereas x is only about 0.05 for micron-sized α-Fe2O3. For anode application, the metal oxides should have high capacities which make higher valence metal oxides be the better choice such as Cr2O3, 20

Mn2O3, and Fe2O3. They also should have low potentials, in terms of which the merit is in order of Cr2O3 > Mn2O3 > Fe2O3 based on theoretical calculation [62]. The spherical Cr2O3 nanoparticle reported by Hu et al. exhibited a lithiation voltage lower than 0.5 V versus Li/Li+ and an initial lithiation capacity of 1200-1400 mAh g-1which is higher than its theoretical capacity of 1058 mAh g-1 [63]. The excess capacity can be attributed to the surface Li storage mechanism. The average delithiation voltage of Cr2O3 is about 1.2 V which is also much lower than that of most of the other metal oxides. Therefore,

Cr2O3 seems like a better suitable conversion metal oxide anode than the others.

Fig. 5(a) SEM image of the α-Fe2O3 nanotube arrays; (b) TEM image and SAED pattern of α-Fe2O3 nanotubes; (c) cycle performance at C/5 rate for α-Fe2O3 nanotube arrays and carbon coated α-Fe2O3 nanotube arrays

Beside nanospheres, a wide variety of nanoscale metal oxides have been synthesized and tested as anode materials in the last decade. The geometries include nanotube or nanowire arrays [64- 74], nanoflakes [75], nanospindles [76], flower-like [77, 78], hollow sea-urchin-like nanoparticles [79], and mesoporous structures [80, 81]. Among them, Chen and coworkers reported the synthesis and electrochemical performance of α-Fe2O3 (hematite) nanotubes as anode material [64]. The reported α-Fe2O3 nanotubes demonstrated very high initial lithiation capacity. The α-Fe2O3 nanoflakes reported by Reddy et al. demonstrated stable charge-discharge capacity above 21

600 mAh g-1 up to 80 cycles despite of the 70% irreversible capacity at the first cycle [75]. Recently, Liu et al. reported carbon-coated α-

Fe2O3 nanotube arrays with impressive capacity retention as shown in Figure 16.5 [74]. Lou et al. reported a mesoporous nanoneedle structure of Co3O4 as shown in Figure 16.6 [81]. The mesoporous structure could arguably enhance the charge transfer kinetics and cycle stability, thus resulting in promising electrochemical performance. Beside the aforementioned usual metal oxides, carbon coated ZnO nanorod arrays were also investigated by Liu et al. as anode material [82].

Fig. 6 (a) SEM image of the mesoporous Co3O4 nanoneedles; (b) cycle perfor-

-1 mance at current density of 150 mAh g for mesoporous Co3O4 nanoneedles prepared from different temperatures

Despite some attractive characters of the metal oxide anode materials, there are still some intrinsic disadvantages of these materials. Firstly, most of them have relatively high lithiation potential (above 1 V versus Li/Li+) which will cause low overall cell voltage. Secondly, most of the reported delithiation capacities of these metal oxides can only be achieved in a wide potential window, typically from a near-zero lower limit to upper limit of 3 V. Obviously not entire such capacities can be accounted for real-life battery applications. Beside these intrinsic problems, there are also some formida- ble technical difficulties including large irreversible capacity (low coulombic efficiency) and large charge/discharge hysteresis (typical- 22 ly about 1 V) both of which are due to the poor electronic properties. A number of techniques have been used to enhance the kinetics through improving the electronic properties, including carbon coating and metal doping [77, 83]. However, substantial improvement has not been achieved. Therefore, metal oxide anode materials still remain as a concept rather than realistic choice for Li-ion batteries.

14.3.3 Displacement Metal Oxide Materials

The concept of displacement mechanism is to displace one metal A from a binary inter-metallic AB by lithium reduction. The AB proves a host framework for the inserting and extracting metal A and Li, respectively. Therefore, the intense volume change by direct alloying can be limited. To date, there has been only one reported metal oxide, namely Cu2.33V4O11, obeying the displacement mechanism [84]. In this material, Cu inserted in [V4O11]n layered structures. 5.6 Li per Cu2.33V4O11 can be reversibly inserted and extracted into the layered structure via displacement reaction. The total capac-

-1 ity of Cu2.33V4O11 is about 270 mAh g , However, the lithiation potential is pretty high at 2.5 V which makes it more suitable as a cathode material.

14.3.4 Tin Dioxides Based Anode Materials SnO2 is another well-investigated anode material. Unlike other metal oxide anode materials, the lithiation/delithiation process of

SnO2 is a combination of conversion and allaying mechanisms 23 which can be described as follows: In the first lithiation reaction,

SnO2 is irreversibly reduced to metallic Sn by Li:

+ - 4Li + 4e + SnO2 → 2Li2O + Sn Further lithiation obeys the reversible allaying reaction:

+ - xLi + xe + Sn ↔ LixSn

Therefore, SnO2 conversion only takes place as the initial part of the first lithiation. The following cycles follow the allaying/dealloying reaction between Sn and Li. Sn has very high lithiation capacity: In theory as many as 4.4 Li can be inserted in 1 Sn. Such high capacity will induce severe volume change which can result in particle pulverization, thus causing rapid capacity fade. The advantage of using

SnO2 as anode material instead of Sn is that the initial conversion reaction will produce nanoscale Sn grains dispersed in the Li2O matrix. The Li2O matrix is inert in the alloying/dealloying reactions, and can function as the cushion structure to accommodate the Sn volume change. In addition to the small grain size, Sn is an electronic conductor which can facilitate high capacity and fast charge/discharge rate. Therefore, using SnO2 as anode materials can significantly enhance the cycle stability of the anode. However, it is achieved on the sacrifice of large irreversible capacity in the first cycle. The reported nanoscale SnO2 anode materials include nanospheres [85, 86], nanowires [87, 88], flower-like nanoparticles [89], and porous cage-like nanospheres [90]. Figure 16.7 shows the morphology and performance of the porous cage-like SnO2 anode materials reported by Yu and coworkers. 24

Fig. 7 (a) SEM images of thin film of SnO2 based porous spheres; (b) cyclability of SnO2 based composite film of porous spheres

14.4 Nano Metal Oxide Cathode Materials

Lithium transition metal oxides and lithium transition metal phosphates represent the most successful cathode electrode materials for Li-ion batteries. When using lithium metal anode, some metal oxides such as MnO2, V2O5 can be used as cathodes. The power and cycling life of low-potential cathodes have been greatly improved by simple shrinking particle size from micro- to nano-scale due to short diffusion length, high electrode/electrolyte interface, and fast phase transformation. However, high contact area between electrode and electrolyte in nano-scale high-potential cathodes also promote decomposition of the electrolyte and formation of a solid electrolyte interface layer on the surface of the particles, resulting in fading of cycle life especially at high temperature [91, 92]. For Li-Mn-O cathode, the use of small particles also increases undesirable dissolution of Mn. Surface coating with a nanolayer of inert oxides (SnO2,

Al2O3, MgO, ZrO2) can alleviate Mn dissolution and SEI formation but also decreases the reaction rate, scarifying the benefit of nano-s- cale particle.

14.4.1 Nanoscale Cathode Materials 25

A great improvement in rate performance and cycling stability of V2O5 nanotubes [93] and nanowires [94] and LiCoO2 nanowires have been reported [95]. High-quality single crystalline cubic spinel LiMn2O4 nanowires were synthesized by Hosono et al.

[96] using Na0.44MnO2 nanowires as a self-template. These single crystalline spinel LiMn2O4 nanowires show high thermal stability and excellent performance at high rate charge-discharge with excellent cycle stability. Spinel LiMn2O4 nanorods have an average diameter of 130 nm and length of 1.2 μm were also synthesized by Cui’s group [97] using a simple solid-state reaction. The LiMn2O4 nanorod cathodes have a high charge storage capacity at high power rates compared with commercially available powders.

Nano LiFePO4 cathodes have been extensively investigated in term of rate performance and cycling stability. LiFePO4 has a gravimetric capacity of 170 mAh g-1, low cost, high thermal and chemical stability, less reactive with electrolyte due to low potential, and very flat discharge potential, which make this cathode as promising cathode for hybrid electric vehicle batteries. The lithiation and delithiation of bulk LiFePO4 involves a phase transformation between LixFePO4 (x = 0.032) and Li1-yFePO4 (y = 0.038) at 3.48V [98, 99] and Li ion transport mainly along the (010) direction. However, the miscibility gap and equilibrium phase transformation potential are highly size-dependent. This miscibility gap is reduced to values of y = 0.12 and x = 0.06 when particle size decreases to 40 nm, as shown in Figure 8 [100]. Values of y 26

= 0.17 and x = 0.12 are obtained for 34 nm particles [101]. The dra- matic shrinking of the miscibility gap at nano-scale particle size is clear seen in Figures 16.8 and 16.9. Stoichiometric 30-40 nm particles of LiFePO4 exhibit two-phase behavior over 70% of composition range, while highly defective materials with the same particle size demonstrates solid-solution behavior over the entire composition range [102]. The size dependence of miscibility gap has been explained by the increasing contribution of elastic energy induced by the formation of coherent two-phase interphase in small particle. The coexistence of two crystallographic phases within one particle leads to a phase boundary energy penalty, due to the difference in lattice parameters of the phases. This strain-induced energy can destabilize a two-phase coexistence in small particles, decreasing the energy gain from phase transformation and narrowing the miscibility gap.

Fig. 8 OCV curves measured for LixFePO4 at room temperature with various mean particles sizes of 200, 80, and 40 nm. Reproduced from reference [100] with permission

Fig. 9 Temperature-dependent of LiFePO4 with different particle sizes. Repro- duced from reference [102] with permission

Fig. 10 Equilibrium potentials of LiFePO4 with different particle sizes measured by GITT [105] 27

The particle size of LiFePO4 not only changes the miscibility gap, but also affects the equilibrium potential and potential hysteresis [104, 105]. The particle size changes the equilibrium phase dia- gram of LiFePO4, as demonstrated in Figure 16.9 [103]. Our results demonstrates that equilibrium discharge potential of 40nm LiFePO4 is 8 mV higher than bulk LiFePO4, as evidenced in Figure 16.10. The narrowed miscibility gap and increased equilibrium discharge potential of nano-LiFePO4 greatly reduces the accommodation energy of phase transformation (Figure 16.11) and increases the interface mobility of phase transformation (Figure 16.12). Therefore, use of nano-scale LiFePO4 not only reduces the Li-ion diffusion path, but also enhances the phase transformation kinetics, resulting in a great increase in rate performance of LiFePO4.

Fig. 11 Discharge accommodation energies of LiFePO4 with different particle size [105]

Fig. 12 Interface mobilities of (a) bulk and (b) nano LiFePO4 obtained from phase transformation GITT [105]

14.4.2 Nanostructured Cathode Materials

Although use of nano-scale cathode materials can enhances the power of cathode, the tap density and energy density drop drasti- cally as the particle size decreases [106]. So the use of nano-scale 28 materials might lead to high power but could result in very low volumetric energy storage. To avoid low volumetric energy density and high reactive surface, but retain the advantage of the nano-scale, at- tention has turned to nanostructured cathode materials. Nanostruc- tured cathode for Li-ion battery has been reviewed by Wang and Cao [107]. The benefit of use nanostructured cathode materials was evidenced from LiMnO2 cathode. Li can be lithiated/delithiated in

LixMnO2 spinel over the range of 0 < x < 2. Cycling is usually confined to the range 0 < x < 1 to avoid the transformation of cubic

LiMn2O4 to tetragonal Li2Mn2O4, which leads to a marked loss of capacity. However, the lattice stress caused by Jahn-Teller distortion can be accommodated more easily in the case of nano-domain structure. The entire nano-domains can spontaneously change between cubic and tetragonal structures during lithiation/delithiation. There- fore, the capacity retention is greatly improved compared with the normal bulk LiMnO2 [108].

14.5 Nano Metal Oxides in Electrolyte

The conventional electrolyte for Li-ion batteries is Li salt solution based on electrochemical stable organic solvents. The advantage of the conventional electrolyte is the high Li-ion conductivity, but it is undermined by the flammability of the organic solvent that is a serious safety concern. There are several strategies attempting to 29 solve this problem, including using aqueous electrolytes, solid ceramic electrolytes, ionic liquid electrolytes and solid polymeric electrolytes [109]. To the best knowledge of the authors, the first three methodologies do not involve application of nanostructured metal oxides. Therefore, only the application of nanostructured metal oxides in polymeric electrolytes will be discussed in this section. In addition to the potential safety enhancement, solid polymeric electrolytes can also provide the simplicity of manufacture and a wide variety of battery geometries. Strictly speaking, the solid polymeric electrolytes only refer to those solvent free membranes based on the mixture of Li salts and polymers. Among them, the most attractive membranes are the ones based on poly(ethylene oxide) (PEO) and a variety of Li salts such as LiPF6 or LiCF3SO3 [110]. The conduct of Li-ions in these membranes is through the complexation between the ether groups in PEO and the Li-ions. However, the ionic conductivity is poor at room temperature so that the real-life application of this type of electrolyte has not been achieved. A very effective method to enhance the conductivity of the PEO electrolyte is addition of nano-sized metal oxide particles as filler, such as TiO2, Al2O3, and SiO2 [111] as shown in Figure 16.13. A long recognized effect of fillers, traditionally referred as plasticiz- er, is to lower the degree of crystallinity of the polymer. PEO is a polymer that tends to crystallize, so that the crystalline domains will block the Li-ions transport in the amorphous regions. The metal oxide nanoparticle fillers can inhibit the PEO chain to crystallize at lower temperature. Furthermore, the ionic conductivity enhancement 30 upon addition of metal oxide nanoparticles was also explained by the space charge theory [112]. Accordingly, the ionic could be promoted by the Lewis acid-base interactions between the surface state of the metal oxide nanoparticles with both the polymer chains and the anion of the Li salt. This hypothesis could be proved by the close relationship between the degree of conductivity and the filler surface modification. It was demonstrated that the sulfate-promoted superacid zirconia (S-ZrO2) fillers could greatly improve both the ionic conductivity and the Li-ion transference number due to its high acidic surface state [113].

Fig. 13 Arrhenius plots of the conductivity of filler-free PEO-LiClO4 and of nanocomposite PEO-LiClO4.10 wt% TiO2 or 10 wt% Al2O3 was used based on total weight (PEO:LiClO4 = 8:1 in all cases) [111]

14.6 Conclusion and Outlook

The state of the art of application of nanoscale and nanostructured metal oxides in development of Li-ion batteries was sum- marized and discussed in this chapter. A large number of metal oxides can be used as anode and cathode materials in Li-ion battery according to their specific structural and chemical properties. The advantages of metal oxides with nano-dimension are undoubted: nano- sized metal oxides can enhance the lithiation/delithiation kinetics owing to their smaller size and larger surface area. Therefore, the 31 rate capacity can be significantly enhanced. Even new lithiation/delithiation mechanism, conversion reaction, can be enabled. However, the smaller size and large surface area also bring some downside effects including low pack density (low energy density) and large surface side reaction. Future investigation should em- phasize to compress these shortcomings without sacrifice the merit of the nanostructured metal oxides. One possible means to achieve this goal is to introduce mesoporous (pore size between 2 to 50 nm) structure into bulk metal oxides to form hierarchical nanostructures. For instance, the electric conductive additive, or dopant should be uniformly dispersed in the body of metal oxide, and form interconnected networks. The aforementioned RuO2 incorporated anatase TiO2 mesoporous composite [36] could serve a good example of this type of structure. The overall bulk size could lead to higher packing density, and the uniformly mixed nanoscale metal oxide domains and nanoscale conductive network can provide superior charge transfer kinetics. In addition, mesoporous structure can facilitate the contact with electrolyte. Therefore, this true three-dimensional structure could exhibit better performance than the current majority zero-dimensional (particle) and one-dimensional (tube or wire) structures. 32

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