Titanium oxide (TiO₂) has received much attention as the most promising alternative to the conventional graphite anode of Li-ion batteries for hybrid electric vehicle (HEV) and electric vehicle (EV) applications, primarily due to its advantageous material properties, such as enhanced safety, good capacity retention during cycling and non-toxicity [1-2]. Although TiO₂ anode exhibits higher operating voltage (~1.7 V vs. Li/Li⁺) than carbon-based anode (~0.1 V vs. Li/Li⁺) and it reduces the overall cell voltage accordingly, the high operating voltage of TiO₂ anode can prevent the formation of a SEI (solid electrolyte interface) layer under 1 V, which can lead to irreversible capacity loss [3-5]. The performance of TiO₂ anode for Li-ion batteries depends strongly on the crystalline phase, the morphology, and the porosity of the structure. Recently, nanostructured TiO₂ materials, such as nanoparticles, nanorods, nanowires, and nanotubes have been studied to improve the performance of TiO₂ anode, due to their high rate capability, resulting from the large surface area that provides innumerable reaction sites and short diffusion length for Li⁺ [6].

Various synthesis methods have been developed for the preparation of TiO₂ nanomaterials, such as hydrolysis, sol-gel, hydrothermal reaction, templating, and anodization [4,5,7-9]. The hydrolysis reaction is a very simple method for the synthesis of TiO₂ nanoparticles which are more appropriate to use as anode materials of Li-ion batteries, primarily due to their easy mass production and application to slurry process. However, most TiO₂ nanoparticles synthesized by the hydrolysis reaction exhibit a wide particle size distribution which is often inhomogeneous with respect to the location inside the electrode and leads to nonuniform electrochemical performance [7]. Herein, we introduce a new synthesis method to prepare the TiO₂ nanoparticles of a narrow particle size distribution as well as a homogeneous dispersion throughout the electrode, which can be achieved by electron beam (E-beam) irradiation during the synthesis process [10-12]. The E-beam irradiation has been used to induce graft polymerization or cross-linking of various separators for Li-ion-batteries to enhance electrochemical performances because the high density of the E-beam can create a large amount of radicals over entire polymer samples [13-15].

In this work, we investigated the effects of E-beam irradiation on the synthesis of TiO₂ nanoparticles and their electrochemical performance as alternative anode materials for Li-ion batteries, which were then compared with that of TiO₂ nanoparticles synthesized by normal hydrolysis reaction.

Fig. 1 shows the SEM images of the hydrolysis synthesized TiO₂ nanoparticles and the E-beam irradiated TiO₂ nanoparticles. All samples were composed of nano-sized primary particles and secondary particles of micro-size, which were formed by aggregation of the primary particles. The hydrolysis TiO₂ sample showed a wide size distribution. The primary particles of hydrolysis TiO₂ presented a fine size of 100~500 nm, but they were easily agglomerated by the attractive force between individual particles due to their fine size. The agglomerated primary particles were further aggregated to the secondary particles with 5~10 µm size. On the other hand, the primary and secondary particles of E-beam TiO₂ showed not only narrower particle size distribution, but also smaller particle size than that of hydrolysis TiO₂. Although the primary particles of E-beam TiO₂ presented finer particle size of 100~200 nm, their agglomeration was suppressed by the E-beam irradiation, resulting in smaller secondary particles of 0.5~1 µm Resultingly, the BET specific surface area of E-beam TiO₂ (25.6 m² g⁻¹) was larger than that of hydrolysis TiO₂ (10.6 m² g⁻¹).

In Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the agglomeration and stability of particle dispersions are determined by the sum of the attractive and repulsive forces between individual parti-cles. And the attractive force is due to the van der Waals force [16]. To overcome the van der Waals force and suppress the agglomeration between individual particles, the electrostatic repulsive forces caused by high surface charge, thicker double layer, and steric forces have to be applied to individual particles [16]. Typically, when the agglomerated nanoparticles are added to a liquid, they can be separated by overcoming the weaker attractive force in several ways, whereas the aggregated nanoparticles cannot be separated. It is believed that the weaker attractive force between the agglomerated primary particles of TiO₂ nanoparticles was easily broken by the E-beam irradiation. The E-beam irradiation through H₂O contributes to the ionization/excitation of H₂O molecules forming a number of transient species and stable products [17,18]. Among all species generated during H₂O radiolysis, hydroxyl radicals (^•OH) are the most reactive with the TiO₂ surface, as a very strong oxidizing agent [19]. It is reported that the ^•OH induced by H₂O radiolysis contribute to the formation of hydroxyl (OH^-) groups, which increase the charge and wettability of the TiO₂ surface [17,19,20]. Therefore, it is evident that the E-beam irradiation on TiO₂ nanoparticles with H₂O suppressed the agglomeration between individual TiO₂ nanoparticles, leading to smaller primary and secondary particles with narrower size distribution.

Fig. 2 exhibits the particle size distribution of the hydrolysis synthesized TiO₂ nanoparticles and the E-beam irradiated TiO₂ nanoparticles. The E-beam TiO₂ presented not only narrower particle size distribution, but also smaller particle size than that of the hydrolysis TiO₂. Although the average E-beam TiO₂ particle size (noted as D50) was 202.73, smaller than that of the hydrolysis TiO₂ (489.84 nm), the smallest particle size (noted as D10) of E-beam TiO₂ (178.07 nm) was slightly bigger than that of hydrolysis TiO₂ (100.17 nm). It is reported that the E-beam irradiation increased the diffusivity of atoms by several orders of magnitude compared to that in the ther-mal treatment step during synthesis. The precipitation of nanoparticles occurs not only at distinctly lower temperatures, but also at considerably higher rates in the irradiation process [10,12]. This demonstrates that the E-beam irradiation can lead to a fast grain growth of the primary TiO₂ nanoparticles, resulting in a bigger smallest particle size. However, subsequent grain growth of the primary TiO₂ nanoparticles was suppressed during the E-beam irradiation, because the diffusion of atoms for the growth of TiO₂ nanoparticles was inhibited by the separation of individual TiO₂ nanoparticles. Thus its average particles size is smaller that that of the hydrolysis sample.

Fig. 3 presents the XRD patterns of the hydrolysis synthesized TiO₂ nanoparticles and the E-beam irradiated TiO₂ nanoparticles. The crystallographic structures of both samples had been confirmed to be the tetragonal anatase phase (JCPDS card no. 21-1272, S.G.: I41/amd). Although the peak position of both samples corresponded well, the peak width of E-beam TiO₂ was slightly wider than that of hydrolysis TiO₂ for all peaks. This demonstrates that the E-beam TiO₂ had smaller the crystallite size than the hydrolysis TiO₂. The crys-tallite size, which is different than the particle size could be estimated by a full width at half maximum (FWHM) of XRD peaks using the Scherrer equation [21]. The crystallite size of the E-beam TiO₂ and hydrolysis TiO₂ was 23.4 and 29.1 nm for (101) peak, and 19.1 and 24.0 nm for (200) peak, respectively. It is expected that the E-beam TiO₂ with smaller crystallite size would be favorable to the diffusion of Li⁺ through TiO₂ nanoparticles, resulting in a better electrochemical performance.

Fig. 4 shows the charge-discharge curves for the first two cycles of the hydrolysis synthesized TiO₂ nanoparticles and the E-beam irradiated TiO₂ nanoparticles. All samples presented almost the same voltage profile during the charge-discharge. At the first cycle, the discharge capacity and coulombic efficiency of hydrolysis TiO₂ and E-beam TiO₂ were 197.67 mAh g⁻¹ and 85.75%, and 198.36 mAh g⁻¹ and 86.14%, respectively. And at the 2^nd cycle, the E-beam TiO₂ also exhibited slightly bigger discharge capacity (194.74 mAh g⁻¹) and higher coulombic efficiency (97.65%) than those of hydrolysis TiO₂ (193.60 mAh g⁻¹ and 97.52%). The better electrochemical performance of E-beam TiO₂ was demonstrated well in the cycling performance and rate capability tests, as presented in Fig. 5 and 6. In spite of the similarity in the crystallinity and initial discharge capacity at 0.2 C (the first cycle) of both samples, there are notable differences in the initial discharge capacity at 0.5 C (the 3^rd cycle) and capacity retention capability. After 50 cycles, the E-beam TiO₂ exhibited higher discharge capacity of 148.50 mAh g⁻¹, corresponding to 77.97% of its initial discharge capacity at 0.5 C (the 3^rd cycle), whereas the hydrolysis TiO₂ showed lower discharge capacity of 123.55 mAh g⁻¹, corresponding to 67.35% of its initial discharge capacity at 0.5 C (the 3^rd cycle). This indicates that the insertion/extraction of Li⁺ into/from the E-beam TiO₂ was preferable to that into/from the hydrolysis TiO₂.

The rate capability, in common with the cycling performance, of E-beam TiO₂ was superior to that of hydrolysis TiO₂. There was no significant difference between both samples at relatively low current density (< 0.2 C). However, an abrupt decrease in the discharge capacity of hydrolysis TiO₂ was observed with an increase in the current density (> 0.5 C), which indicates that the insertion/extraction of Li⁺ into/from the hydrolysis TiO₂ was kinetically limited at a rela-tively high current density. At the high current density of 10 C, the discharge capacity (100.99 mAh g⁻¹ = 50.74%) of E-beam TiO₂ was much higher than that of hydrolysis TiO₂ (61.92 mAh g⁻¹ = 31.60%). The better electrochemical performance of E-beam TiO₂ than that of hydrolysis TiO₂ was attributed to smaller particle size and narrower particle size distribution, resulting in the large surface area that provided innu-merable reaction sites and short diffusion length for Li⁺ through TiO₂ nanoparticles. The homogeneous arrangement of E-beam TiO₂ nanoparticles also contributed to improvement of the electrical contact between individual TiO₂ nanoparticles and acetylene black in the elec-trode, which led to better electrochemical performance of E-beam TiO₂ than that of hydrolysis TiO₂.

The effects of E-beam irradiation on the synthesis of TiO₂ nanoparticles and their the electrochemical performance as alternative anode materials for Li-ion batteries are investigated and compared with that of TiO₂ nanoparticles synthesized by normal hydrolysis reaction. All samples were composed of nano-sized primary particles and micro-sized secondary particles which were formed by aggregation of the primary particles. The E-beam irradiation on TiO₂ nanoparticles suppressed the agglomeration between individual TiO₂ nanoparticles, resulting in a smaller particle and crystallite size with a narrower size distribution. These features of E-beam irradiated TiO₂ nanoparticles contributed to not only a larger surface area that provided innumerable reaction sites and short diffusion length for Li⁺ through TiO₂ nanoparticles, but also improvement of the electrical contact between individual TiO₂ nanoparticles and acetylene black in the electrode. Therefore, the TiO₂ nanoparticles induced by E-beam irradiation present better cycling performance and rate capability than the TiO₂ nanoparticles synthesized by normal hydrolysis reaction.