Li 4 Ti 5 O 12 /Co 3 O 4 Composite for Improved Performance in Lithium-Ion Batteries

To take advantage of the best properties of both LTO and Co 3 O 4 , Li 4 Ti 5 O 12 /Co 3 O 4 composites were synthesized by the solution combustion and solution precipitation methods. The diffraction patterns of Li 4 Ti 5 O 12 (LTO), Co 3 O 4 , and LTO/Co 3 O 4 composites were well-indexed as a cubic spinel structure with the Fd-3m space group. As shown in the FE-SEM images, the Co 3 O 4 particles are attached on the surface of LTO, especially in the pore, and this structure is expected to prevent volume expansion and particle aggregation of Co 3 O 4 particles. As expected, the LTO/Co 3 O 4 composite had higher capacity than that of LTO, with a decreased slope and an extended plateau region over the voltage range of 0.01–3.0 V. LTO/Co 3 O 4 -20 delivered a reversible discharge capacity of 300 mAh g − 1 at a constant current density of 160 mA g − 1 . Also, it exhibited only a slight capacity drop with increasing current density. Galvanostatic intermittent titration technique and X-ray absorption near-edge structure spectroscopy were also conducted to investigate the characteristics of LTO/Co 3 O 4 -20. © The

Spinel Li 4 Ti 5 O 12 (LTO) has an excellent reversibility of Li-ion intercalation/de-intercalation and exhibits zero-strain volume change during cycling with excellent safety performance. Moreover, LTO has a high voltage plateau at 1.55 V vs. Li/Li + , which can avoid the formation of metallic lithium. Therefore, LTO has been investigated to use as the anode material of lithium ion batteries for energy storage, electric vehicles, and hybrid electric vehicles. [1][2][3] However, unfortunately LTO has a low electronic conductivity with a low theoretical capacity of 175 mAh g −1 . These drawbacks restrict its applications in high-power storage devices. 4,5 Many efforts have been investigated for improving its low capacity, including composites with metal oxides such as TiO 2 , 6 SiO 2 , 7 SnO 2 , 8 and ZnO. 9 These metal oxides have a high theoretical capacity, however, they have a high irreversible capacity caused by volume expansion and particle aggregation, resulting in poor cycling performance. 10,11 Among them, Co 3 O 4 has the same spinel crystal structure as LTO and it can accommodate up to 8 Li + ions per formula unit according to the redox reaction Co 3 O 4 + 8Li → 4Li 2 O + 3Co 0 . The theoretical capacity of Co 3 O 4 can be calculated as 890 mAh g −1 from this proposed reaction. 12,13 In this work, we attempted to overcome the low theoretical capacity of LTO and the poor cycling performance of Co 3 O 4 by synthesizing the LTO/Co 3 O 4 composites. Firstly, pristine LTO was synthesized by a two-step solution-combustion method and then LTO/Co 3 O 4 composites were synthesized by a solution precipitation method.

Experimental
Materials synthesis.-LTO was synthesized using a solutioncombustion reaction. 14 Titanium (IV) isopropoxide [Ti{OCH(CH 3 ) 2 } 4 ] was slowly dropped into distilled water under 10 • C. After white precipitates (TiO(OH) 2 ) were observed, nitric acid was added and the mixture was constantly stirred until the appearance turned to transparent. Then, lithium nitrate was dissolved in distilled water and glycine was added as a fuel. After stirring at 80 • C for 5 h to form a viscous gel, the precursor was dropped into an alumina crucible preheated to 500 • C. This crucible was placed in a muffle furnace and heated at 750 • C for 12 h. LTO/Co 3 O 4 composites were synthesized by using precipitation reaction of Co 2+ ion. The as-prepared LTO was uniformly dispersed in distilled water under stirring for 30 min, after that a certain quantity of Co(NO 3 ) 6 · H 2 O was dropped into the solution with mass ratios of 20 and 40, respectively. Then, NH 3 · H 2 O was slowly added into the above suspension and then stirred for several hours. The precursor powders were filtered and washed three times with distilled water and dried under vacuum z E-mail: ryuks@ulsan.ac.kr at 80 • C for 12 h. The dried powders were calcined at 450 • C for 1 h to obtain LTO/Co 3 O 4 composites. In addition, Co 3 O 4 was synthesized using the same process without the addition of LTO.
The morphology of the samples was observed by field emission scanning electron microscope (FE-SEM) using a JSM-6500F (JEOL, Japan) with an accelerating voltage of 10 kV. The Co 3 O 4 amounts in LTO/Co 3 O 4 composites were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a 720-ES (Varian, USA).
In situ X-ray absorption spectroscopy (XAS) measurements were carried out on the BL10C beamline at the Pohang Light Source. To control the X-ray photon energy, a Si (111) double crystal monochromator was used.
Electrochemical measurement.-The working electrodes were made by mixing active materials, conducting agent (carbon black), and binder (PVDF) with a weight ratio of 80:10:10. The CR 2032 coin-type cells were assembled in an argon-filled glove-box using lithium metal foil as the counter electrode. 1.0 M LiPF 6 /EC+DMC (1:2 in volume) electrolyte and Celgard polypropylene separator were used. Cyclic voltammetry was performed using a WBCS3000 battery tester (WonATech, Korea) with a scan rate of 0.1 mV s −1 in the voltage range of 0.01-3.0 V. Galvanostatic charge-discharge measurements were conducted with current densities ranging from 16 mA g −1 to 1600 mA g -1 at room temperature. The galvanostatic intermittent titration technique (GITT) was conducted using a SP-300 (Biologic, France) with a current density of 10 mA g −1 in the voltage range of 0.01-3.0 V.  The average crystalline size of Co 3 O 4 can be calculated by the Debye-Scherrer formula, as follows:

Results and Discussion
where λ is the wavelength of the X-ray radiation taken 0.15406 nm for Cu Kα, θ is the Bragg angle, D the crystallite size, k the Scherer constant as 0.89, and β the full width at half maximum (FWHM) of the diffraction peak (311) measured at 2θ in radians. 15 The calculated average crystalline size of Co 3 O 4 is 44.6 nm. Rietveld refinement results using Fullprof program are shown in Fig 2b and   In the first cycle of Co 3 O 4 , an irreversible cathodic peak is observed at around 0.7 V, which is attributed to the electrochemical reduction reaction of Co 3 O 4 with Li + and the formation of solid electrolyte interphase (SEI) layer by the electrolyte decomposition. The anodic peak at around 2.1 V is attributed to the oxidation reaction of Co 3 O 4 . 16 In the second cycle, the main cathodic peak shifts to 1.1 V and the peak current rapidly decreases compared  with the first cycle due to the irreversible electrode reaction. 17 In contrast, the cathodic peak negatively shifts and the anodic peak positively shifts due to the hysteresis. 18 LTO/Co 3 O 4 -20 exhibits two pairs of redox peaks corresponding to the LTO and Co 3 O 4 peaks and the intensity of the LTO peak is greater than that of Co 3 O 4 . In the second cycle, the cathodic peak for Co 3 O 4 is increased to 1.31 V, and it leads to decrease the gap between cathodic and anodic peak, indicating that the polarization is reduced. The peak intensity of LTO and Co 3 O 4 is changed, but integral areas are remained during cycling. However, LTO/Co 3 O 4 -40 has a higher Co 3 O 4 peak and a lower LTO peak, and the two peaks overlap at around 1.0 V.
The discharge-charge curves of LTO, Co 3 O 4 , LTO/Co 3 O 4 -20, and LTO/Co 3 O 4 -40 at a current density of 10 mA g −1 are shown in Fig. 5. The voltage range is expanded from 1.0-2.6 V to 0.01-3.0 V to observe the influence of Co 3 O 4 . LTO delivers the initial and second discharge capacity of 281.9 and 243.9 mAh g −1 , respectively. The irreversible capacity is attributed by the formation of an SEI layer. The voltage profile of LTO has three plateaus at around 1.5, 0.7, and 0.5 V. These plateaus correspond to the intercalation of 3 moles of lithium ions per mole of LTO. Especially, the voltage plateau at 1.5 V is related to the insertion of lithium ion into the octahedral (16c) site according to the general mechanism. At the same time, 3 mol of lithium ions located at the 8a site is transported to the octahedral (16c) site. The SEI layer is formed at around 0.7 V. It has been reported that the extra capacity can be delivered by intercalating another 2 mol of lithium ions into the vacant tetrahedral (8a) sites below 0.6 V. This mechanism is formulated as follows. 19 Li 3(8a) LiTi 5 4+ (16d) O 12(32e) + 5e − + 5Li + → Li 2(8a) Li 6(16c) LiTi 5 3+ (16d) O 12(32e) [2] Also, they suggested that the extra capacity caused by the steep voltage region below 0.6 V may negatively affect its practical utilization. LTO/Co 3 O 4 -20 and LTO/Co 3 O 4 -40 deliver initial discharge  capacities of 392.1 and 579.5 mAh g −1 , respectively. These results approximately correspond to the calculated theoretical capacity. The charge-discharge curves of composite materials show the combined properties of LTO and Co 3 O 4 . In addition, the voltage profile of LTO/Co 3 O 4 -20 is closer to that of LTO, whereas, the profile of LTO/Co 3 O 4 -40 is more similar with that of Co 3 O 4 . Therefore, the extended plateau will be utilized by synthesizing a complex with The quasi-open circuit voltage (QOCV) curves of the samples obtained by GITT measurement are shown in Fig. 6. The electrochemical cells were charged and discharged at a constant current density of 10 mA g −1 for 1 h with a relaxation time of 2 h. As shown, the profiles can be divided into two regions based on the characteristics of the curves. The up and down-side of the colored blocks indicate the charge and discharge processes, respectively. Relatively small polarization is observed in Region I, indicating that Region I is related to the LTO. In contrast, Region II with large polarization is related to the Co 3 O 4 . This suggestion is associated with the volume expansion of Co 3 O 4 particles during cycling and they can lead to weakness of electrical contact between Co 3 O 4 and the conducting agent particles. 20 We can suggest that, therefore, the charge-discharge profile of LTO/Co 3 O 4 -20 is more affected by LTO than Co 3 O 4 . In contrast, the characteristic of LTO/Co 3 O 4 -40 is more related to Co 3 O 4 .
The cycling performances up to 50 cycles at a current density of 160 mA g −1 are shown in the Fig. 7. The discharge capacity of LTO is constantly maintained over 200 mAh g −1 with no significant de-crease. The initial discharge capacity of Co 3 O 4 is 1000 mAh g −1 with rapid decrease during cycling. This capacity fade is mainly attributed to the volume expansion and particle aggregation. LTO/Co 3 O 4 -20 delivers an initial discharge capacity of 319.8 mAh g −1 with excellent cycling performance. This result may be caused by the synergistic effect of LTO and Co 3 O 4 . Meanwhile, LTO/Co 3 O 4 -40 shows an initial discharge capacity of 580.4 mAh g −1 , which is higher than that of LTO/Co 3 O 4 -20. However, it has a large irreversible capacity and poor cycling performance, indicating that LTO could not act as a buffer due to the excess Co 3 O 4 particles on the LTO surface.
The dissembled electrodes after 50 cycles are presented in Fig. 8 to check the state of electrodes. LTO electrode maintained its origin state during cycling, however, Co 3 O 4 electrode had many severe cracks resulting from the large volume expansion. On the other hand, LTO/Co 3 O 4 -20 electrode shows the relatively clear state, indicating that the LTO matrix can prevent the volume expansion of Co 3 O 4 during cycling. Fig. 9 indicates the rate capabilities at various current densities ranging from 160 mA g −1 to 1600 mA g −1 . LTO delivers a discharge capacity of 230.9 mAh g −1 and 200.4 mAh g −1 at a current density of 160 mA g −1 and 1600 mA g −1 , respectively. Co 3 O 4 delivers an initial discharge capacity of 1000 mAh g −1 at a current density of 160 mA g −1 . However, the capacity dramatically decreases to 50 mAh g −1 at a current density of 1600 mA g −1 . LTO/Co 3 O 4 -20 delivers a discharge capacity of 307.3 mAh g −1 , which is slightly higher than that of LTO, at a current density of 160 mA g −1 and maintains the capacity as the current density increases. In contrast, the discharge capacity of LTO/Co 3 O 4 -40 was significantly decreased with increasing current densities.   Fig. 9b. The K-edge XANES spectra are formed by transitions of the 1s electron at the inner shell to unoccupied states with appropriate symmetry. 21 The weak triplet pre-edge (peak A) at around 4971 eV corresponds to the transition of 1s→3d unoccupied states, which is the electric dipole-forbidden and quadruple-allowed transition due to the hybridization of 3d and 4p orbitals. 22,23 The shoulder peaks (peak B) at higher energy represent a shakedown process transition of 1s→4p caused by ligand-to-metal charge transfer (LMCT). The strongest absorption main edge (peak C) is the pure   dipole-allowed 1s→4p transition without the shakedown process. 21 These pre-edge peaks decrease due to the decrease in distortion of the octahedral during the discharge process. The main Ti K-edge at approximately 4990 eV gradually shifts to the lower energy region during the lithium ion insertion process, indicating that the oxidation state of titanium is reduced. These changes, especially, are most visible at points 3-7 and 16-21. On the other hand, the peaks have no visible change at the points 8-11 and 22-24. These results correspond to Fig. 6, indicating that Ti contributes only to the region of LTO. Therefore, the extended plateau of capacity is introduced by the addition of Co 3 O 4 .

Conclusions
We attempted to overcome the low theoretical capacity of LTO and the poor cycling performance of metal oxides by synthesizing LTO/Co 3 O 4 composites. LTO particles can trap Co 3 O 4 particles and they can prevent the volume expansion and particle aggregation of Co 3 O 4 . As expected, LTO/Co 3 O 4 composites have higher capacity than that of LTO. QOCV curves show that LTO/Co 3 O 4 -20 is more dominated by LTO, whereas LTO/Co 3 O 4 -40 has similar characteristics with Co 3 O 4 . The XANES results show that the Ti K-edge changes at certain region, which is specified as Region I in GITT results.