The lithium intercalation/de-intercalation process in all TiO2 polymorphs depends
typically on their structure, particle size, morphology, and surface area (194,196,197). Therefore, use of nanostructured TiO2 is one approach to increasing
the rate performance and achieving better energy storage capacity and longer cycling life than that of bulk materials (198–200). Nanostructured TiO2 materials
provide a large surface area which results in high electrode-electrolyte contact area and hence distributes the current density and increases the charge/discharge rate. Additionally, the nanomaterial provides short transport path lengths for electrons
and Li+ which permit the use of materials with low ionic conductivity in the battery and minimizes strain during the insertion/extraction of Li+ (174,201,202).
Nanostructured forms of TiO2 in 1D and various morphologies including nanowires
(203), nanotubes (204,205), nanorods (169,206), hollow nanospheres (207,208), and nanoribbons (205) , composites with carbon (209,210), tin (167,211), silicon (92), and also thin films (212,213) have been reported to show a higher capacity and much improved capacity retention and rate capability in comparison to bulk materials.
The first synthesized TiO2 nanotubes from amorphous TiO2 were reported in 1998.
Since then, a lot of effort has been spent in the development of anode materials based on nanostructured TiO2 in various particle morphologies.
Regarding the rutile form of TiO2, the low lithium diffusion in the ab- plane is a key
factor in improving the charge/ion transport properties and improving electrochemical properties. It is necessary to address this issue by fabricating nanostructured rutile with a small diameter in the ab- plane (191). Nanosized rutile (133,191,214,215) has been reported to show much improved electrochemical properties by decreasing the crystallite size from the micro to the nanoscale. Up to 0.85 moles of Li+ can be inserted into nanosized rutile at room temperature in comparison with only 0.1–0.25 mol of Li into microsized rutile (214).
The difference in the behaviour of Li+ insertion between micro and nanosized rutile was shown by Hu et al. in 2006 (191). Nanosized rutile with a needle-like shape has a diameter of ~5 nm corresponding to the ab- plane. This nanotructured material exhibits a reversible insertion of ~0.5 mol Li+ per mol TiO2 at room
temperature. This insertion/extraction of ~0.5 mol Li+ using nanosized rutile was confirmed by Reddy et al. (133). Hu et al. also suggested that up to 0.15 Li is stored at the surface (191). In contrast, Wagemaker et al. demonstrated that a fraction of ∼0.15 actually enters the rutile host structure, accompanied by a slight change in the lattice parameters (216).
Tarascon et al. demonstrated that the rutile nanoparticles (10 nm x 200 nm in size) can accommodate 0.85 moles of Li+ during the first reduction and 0.5 mole of Li is reversibly cycled with a capacity of 150 mA h/g after 60 cycles. The charge- discharge profile in Figure 1-6 (a), suggests that the Li insertion occurs through two solid solution domains followed by irreversible phase transformation of electro-
active LiTiO2 (rock-salt type) (214). These results are supported by in situ TEM
investigation of the structural transformation of rutile nanowire (NW) in which a single-crystal rutile TiO2 nanowire underwent electrochemical lithiation (217). The
transformation of the NW rutile to a monoclinic non-reversible intermediate structure was observed in LixTiO2 upon lithium insertion while upon full lithiation (
0.85) the transformation resulted in a rock-salt structure (Fm-3m), Figure 1-6 (b).
Figure 1-6: (a) Galvanostatic cycling curves of different rutile TiO2 samples between 3 V and 1 V with rate 20 C, Adapted from Electrochemistry Communications, 9 /2, Baudrin E, Cassaignon S, Koelsch M, Jolivet JP, Dupont L, Tarascon JM., Structural evolution during the reaction of Li with nano-sized rutile type TiO2 at room temperature, 337–42, Copyright (2007), with permission from Elsevier (214), (b) Schematics of structural transition of rutile LixTiO2 from primitive tetragonal to a fully lithiated phase, adapted from (217) with permission of The Royal Society of Chemistry.
In contrast, Wagemaker et al. have argued that the fully lithiated form, LixTiO2
(x=0.85), has a layered monoclinic structure similar to a hexagonal structure. This view is supported by structural investigation using neutron diffraction in which a needle-shaped nanocrystalline rutile (11 nm x 11 nm x 43 nm) was chemically lithiated, then the exact lithium position, maximum chemical intercalation fraction and phase transitions were determined (216). The results reveal that the Li+ intercalation in the nanosized rutile occurs with two phase transitions, see Figure 1-7; with compositions up to x < 0.5, the phase transition is from the tetragonal rutile structure to the monoclinic P2/m space group. This intermediate phase is very similar to the tetragonal rutile structure, and is therefore referred to as P2/m(RUT). With a maximum composition of x = 0.85, and may be with the maximum theoretical composition x= 1.0, the phase transformation is from the monoclinic P2/m(RUT) structure to a layered monoclinic structure (P2/m space group). This layered structure is closely related to the hexagonal structure, hence it is referred to as P2/m(HEX) (216).
Figure 1-7: Structural evolution of nanoneedle rutile upon Li+ intercalation, where x indicates the composition (LixTiO2). Adapted with permission from Chemistry of Materials, 20 /9, Borghols WJH, Wagemaker M, Lafont U, Kelder EM, Mulder FM, Impact of nanosizing on lithiated rutile TiO2, 2949–55. Copyright (2008) American Chemical Society (216).
The dependence of Li insertion reactions on rutile particle size has been investigated by Zhou’s team (171,215). Their results revealed progressively increasing Li capacity, and Li-ion solubility with decreasing particle size. By decreasing the particle size to 15 nm, a capacity of 378 mAh/ g was delivered at
first discharge and subsequent stable capacity of 200 mAh/ g corresponding to 0.6 Li per one molecule of rutile was observed over 20 cycles. However, with a particle size of 300 nm, a capacity of 110 mAh/ g was delivered at initial cycle then dropped to 50 mAh/ g at the 20th cycle. These results suggest that the improvement in capacity and Li-ion intercalation are related to the nanosize characteristics. This finding was also confirmed by another group (202).
Wohlfahrt-Mehrens et al. reported on the electrochemical insertion of Li+ in nanosized rutile prepared by sol–gel chemistry (154). Also, they demonstrated that extending the potential window of the rutile, which is typically 1-3 V, to potential windows of 0.5-3 V and 0.1-3 V results in an excellent reversible capacity of 315 mAh/ g and a high rate capacity of 196 mAh/ g at 5C. The cycling stability over 1000 cycles is excellent due to the good capacity retention (218).
Recently, Hassoun et al. (219) repeated the construction of a full-cell using nano rutile TiO2 (10 nm) as anode and the LiFePO4 as cathode. It delivered a reversible
capacity of 150 mAh/ g with a potential window of 0.8- 3.8 V at room temperature with cyclability for 20 cycles.
Recently, the fabrication of rutile electrodes in the form of dandelion-like superstructures has been reported. Dandelion-like structures consist of numerous inter-aggregated single-crystalline nanorods of rutile; hence, this structure provides a larger specific surface area and the single-crystalline nanorod provides a stable structure and short path lengths for Li+ diffusion and electronic transport, which improve the rate and cycle performances of the battery. Particularly, intensive research has been conducted to synthesise dandelion-like rutile superstructures. Dandelion-like rutile TiO2 microspheres were synthesized for the first time by a
hydrothermal method (220). Kim et al. reported the synthesis of dandelion-like rutile TiO2 nanostructures through hydrolysis and demonstrated the best
electrochemical performance (128 mAh/ g after 50 cycles) (221). Recently, Sun et al. synthesized novel dandelion-like rutile superstructures via a hydrolysis route (222) in which nanosized rutile rods (6 nm) were grown along the c-axis, [001] direction, which facilitates the transport of lithium ions and electrons. The results from Sun et al. show that the synthesized rutile TiO2 microspheres have a high
reversible capacity of 242 mAh/g (0.72 Li/Ti) and an excellent rate capability of 116mAh/ g at 20 C. Both groups reported that the dandelion-like rutile
superstructures exhibit good rate and cycle performances as anode materials of lithium ion batteries.
In general, the use of nanostructured materials as electrodes is more advantageous than bulk materials and nanostructuring is a well-recognized strategy for improving the capacity and cycling behaviour of TiO2. However, the
polarization increases by decreasing the crystallite size to the nanoscale. The large surface area of nanocrystalline materials increases the electrolyte-electrode contact area and leads to more side reactions with the electrolyte, resulting in a greater thickness of the solid electrolyte interface (SEI) layer than in the bulk TiO2
and therefore hindering the transport of electrons and Li ions in and out of the rutile structure.