Synthesizing nano-sized (∼20 nm) Li₄Ti₅O₁₂ at low temperature for a high-rate performance lithium ion battery anode

Abstract

In this paper, we developed a novel strategy to synthesize nano-sized Li₄Ti₅O₁₂ (LTO) by hydrothermal method and calcination. X-ray diffraction and high resolution transmission electron microscopy were performed to characterize the structures and morphologies of these samples. Highly crystalline and pure-phase Li₄Ti₅O₁₂ synthesized at low calcination temperature of 500 °C has been reported for the first time. This nanocrystalline LTO was tested as the anode material for lithium ion batteries, and exhibited excellent reversible capacities of 166, 162, 155, 142 and 123 mAh g⁻¹ at current densities of 1 C, 2 C, 5 C, 10 C and 20 C, respectively. It also demonstrated good capacity retention and high coulombic efficiency values at all current rates. This excellent electrochemical performance makes our LTO a promising anode material for high energy/power density lithium ion batteries.

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Recently, rechargeable lithium ion batteries (LIBs) have attracted more and more attention as some of the most promising candidates for use in electric vehicles (EVs), stationary power storage devices and portable power sources for micro-electric devices.^1–5 However, the cycle life and safety are the key obstacles hindering its wider application in these fields.⁶ Spinel Li₄Ti₅O₁₂ (LTO) is one of the most attractive negative electrode materials that may solve these problems.^7,8 LTO displays excellent stability, owing to its volume expansion/contraction (<1%) with lithium ion intercalation and deintercalation, compared to approximately 9% volume change if carbon materials are used as anodes in commercial LIBs.² Furthermore, LTO exhibits high plateau voltage in the Li insertion/deinsertion potential at approximately 1.55 V (versus Li⁺/Li).⁹ As a consequence, it does not form a solid electrolyte interface with high resistance.¹⁰ However, Li₄Ti₅O₁₂ has some disadvantages, such as its very low theoretical capacity of 175 mAh g⁻¹ and pretty low electronic conductivity (ca. 10⁻¹³ S cm⁻¹), thus it is not satisfactory for such applications.^2,11,12

Several studies have been done in order to solve these problems, such as reducing the particle size and coating conductive materials on the Li₄Ti₅O₁₂ surface, or doping with some metal ions.⁹ Coating conductive materials on the surface to enhance the electrical conductivity of the electrode is one of the common methods. Carbon is the most inexpensive and widely used material for modifying Li₄Ti₅O₁₂, because it can increase the electrical conductivity.^13–15 Doping LTO particles with different metal ions, such as Ag⁺, Ru²⁺, Cr⁴⁺ and V⁵⁺, to increase their intrinsic conductivity has been widely studied so as to obtain a satisfactory power density.^16–22 Decreasing the particle size was one of the most useful ways, as it could shorten the diffusion path of electrons and lithium ions while enlarging the contact area between the electrode and electrolyte.²³ For instance, Liu et al. used F127 to prepare Li₄Ti₅O₁₂ with an average size of 20 nm.²³ Xia et al. prepared a series of Li₄Ti₅O₁₂ particles with average grain sizes of around 50 nm.^9,24 Sun et al. reported a micro-sized Li₄Ti₅O₁₂ material composed of nanoscale (∼100 nm) primary particles.²⁵ In these reports, relatively high calcination temperatures (>700 °C) and complex steps were required to obtain highly crystalline Li₄Ti₅O₁₂, which inevitably induces the growth of Li₄Ti₅O₁₂ particles and makes its commercial production difficult. So far, it remains a great challenge to develop a facile approach to synthesize nano-sized and highly crystalline Li₄Ti₅O₁₂ at low temperatures.

Here, we report a novel strategy to synthesize highly crystalline Li₄Ti₅O₁₂ (∼20 nm). To the best of our knowledge, this is the first report on the preparation of highly crystalline Li₄Ti₅O₁₂ with a small particle size of ∼20 nm using a low synthesis temperature (500 °C). The as-derived nanocrystalline Li₄Ti₅O₁₂ sample was tested as the anode material for lithium ion batteries, exhibiting a superior reversible capacity of 123 mAh g⁻¹ at 20 C and good cycling performance even at high current densities.

Experimental

Preparation of nano-sized LTO

All the reagents were purchased from China National Medicines Corp., Ltd without any purification.

The LTO nanoparticles were fabricated by a hydrothermal method. Typically, ammonium hydroxide (12.5% v/v, AR) was dripped into 200 ml TiOSO₄ (1 M, AR) solution with the aid of ultrasonication, until the pH of the solution was about 6. Subsequently, the TiO₂ was collected by centrifuging and washing with deionized water more than 3 times.

5.0 g TiO₂ (solid content 20%) and 0.5 g LiOH·H₂O (AR) were dispersed in 30 ml deionized water and stirred for more than 30 min. Then the suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The precipitate was separated by vacuum-filtering and washed with deionized water several times. In order to remove the excess water of the precursor, it was kept in a vacuum desiccator at −40 °C for 24 h. Finally, the LTO precursor was calcined at different temperatures in air to obtain the nano-sized Li₄Ti₅O₁₂ materials.

Characterization

The thermogravimetry and differential scanning calorimetry curves of the Li₄Ti₅O₁₂ precursor were recorded on a 2960 SDT from room temperature to 800 °C with a heating rate of 5 °C min⁻¹ under air flow. The crystal structures of the powders were studied using an X-ray diffraction (XRD) system (18KW D/MAX2200V PC Rigaku) with CuKa radiation from 10 to 70°. High resolution transmission electron microscopy (HRTEM, JEOL-2010F) was used to characterize the morphologies of the powders.

The electrochemical performances were measured with coin cells, in which lithium metal foil was used as the counter electrode. The electrolyte employed was 1 M solution of LiPF₆ in ethylene carbonate and dimethyl carbonate (EC + DMC) (1 : 1 in volume). The active material powder (80 wt%), acetylene black (Super P, supplied by Timcal Inc. 10 wt%) and polyvinylidene fluoride (PVDF) binder (10 wt%) were homogeneously mixed in N-methyl pyrrolidinone (NMP) solvent with magnetic stirring. After stirring for 24 h, the slurry was coated uniformly onto copper foil. Finally, the electrode was dried under vacuum at 120 °C for 8 h. Cell assembly was carried out in an argon-filled glove box ([O₂] < 1 ppm, [H₂O] < 1 ppm). The coin cells were cycled with different current densities between cut-off voltages from 2.5 to 1.0 V on a CT2001A cell test instrument (LAND Electronic Co. Ltd) at 20 °C. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range from 100 kHz to 10 mHz with an electrochemical workstation (CHI660E).

Results and discussion

Fig. 1 shows the TG-DSC curves of the precursor powders with a heating rate of 5 °C min⁻¹ from 25 °C to 800 °C in an air atmosphere. The first step of weight loss, observed between room temperature and about 200 °C, was mainly due to the vaporization of absorbed water. It corresponds to an endothermic peak around 100 °C on the DSC curve. The weight loss in the second step (mainly at 500 °C) was attributed to the decomposition of Ti–OH and Li–OH bonds. When the temperature was above 400 °C, the thermogravimetry (TG) curve showed a nearly constant weight, indicating that the decomposition was complete. Therefore, it was necessary to sinter the precursor mixture above 400 °C to get the well crystallized LTO phase.

Fig. 1 The TG-DSC curves of the precursor.

Fig. 2 showed the XRD patterns of the Li₄Ti₅O₁₂ (LTO) precursor calcined at different temperatures. From 400 °C to 600 °C, the diffraction peaks became more sharply and well defined, indicating that the crystal intensity improved with the increasing temperature. For the sample calcined at 500 °C, the diffraction peaks at 2θ = 18.4, 35.6, 37.1, 43.3, 47.4, 57.2, 62.8 and 66.1° could be indexed to the cubic spinel LTO (JCPDS card no. 49-0207) with an Fd3̄m space group. In addition, a tiny reflection at 2θ = 45.2° was detected when the LTO precursor calcined at 600 °C. It suggested the existence of trace impurities and proved that the temperature was too high. Therefore, pure-phase and highly crystalline LTO could be obtained at the relatively low temperature of 500 °C. By using Scherrer’s formula based on the (111) peak, the grain size of Li₄Ti₅O₁₂ (500 °C) was estimated to be 17.5 nm, which was much smaller than those reported in other publications.^26–28

Fig. 2 The XRD patterns of LTO calcined at different temperatures.

The nano-sized LTO featured a uniform particle size distribution as revealed by the typical TEM images, which are showed in Fig. 3. Fig. 3d indicates that the particle size of the prepared LTO-500 was 18 nm, which was consistent with the XRD results. The dimensions of LTO-400 were similar to those of LTO-500, as shown in Fig. 3b. However, its lattice fringes were not clear, owing to its low degree of crystallinity. In contrast, the size of LTO-600 became almost 50 nm when the temperature increased to 600 °C, as shown in Fig. 3e. The HRTEM image of the LTO-600 surface is showed in Fig. 3f.

Fig. 3 TEM and HRTEM images of LTO-400 (a and b), LTO-500 (c and d) and LTO-600 (e and f).

It was found that the surface of the LTO-600 was melted and the lattice fringes were disordered, which meant that the calcination temperature was too high. This bad surface may had negative effect on Li ion insertion/deinsertion and its contact with the electrolyte. Fig. 3d shows that the entire grain particle of Li₄Ti₅O₁₂ was highly crystalline and further revealed that the crystalline region with clear lattice fringes had an interplanar spacing of 0.48 nm, consistent with the (111) atomic planes of the spinel structure. These results proved that the diameter of the LTO was affected by the temperature. The higher temperature was not only increasing the diameter of the grains, but also liquating its surface. That bad surface structure may lead to a bad electrochemical performance.

To evaluate the cycling performance of the prepared samples, the charge/discharge capacities of LTO-500 and LTO-600 were measured at different current rates (1 C, 2 C, 5 C, 10 C and 20 C). For each stage, the batteries were cycled 50 times. The capacities of LTO-500 were 166, 162, 155, 142 and 123 mAh g⁻¹ at 1 C, 2 C, 5 C, 10 C and 20 C, respectively, and the capacities of LTO-600 were 163, 160, 148, 116 and 74 mAh g⁻¹ at 1 C, 2 C, 5 C, 10 C and 20 C, respectively. In addition, the capacities of LTO-400 were 164, 150, 137, 112 and 56 mAh g⁻¹ at 1 C, 2 C, 5 C, 10 C and 20 C, respectively. The low capacity and bad rate performance of LTO-400 were due to its low degree of crystallinity.

The difference in capacity between LTO-500 and both LTO-400 and LTO-600 became more pronounced with the increased current rate. As shown in Fig. 4, a very stable cycling ability was observed for LTO-500 at each current rate. The capacity loss was less than 0.1% per cycle at all the measured current rates, indicating the high stability of the nano-sized LTO in repeated cycles. In addition, the coulombic efficiency of the nano-sized LTO approached 100% for each cycle. Although the capacities of LTO-600 at high current rates were lower than those of LTO-500, the capacities remained stable at all current rates owing to its high degree of crystallinity. The bad rate performance of LTO-600 may due to its larger particle size and melted surface, which also proved that the little particle size and good surface could contribute to the electrochemical performance of LTO. However, the electrochemical performance of LTO-400 became worse with the increasing current rate. The reason that the capacity of LTO-400 so poor was its lower degree of crystallinity. In a word, the performance of LTO-500 was much higher than that of the other LTO electrodes.^2,29–31

Fig. 4 The cycle performance of LTO-400, LTO-500 and LTO-600.

The discharge curves of the LTO electrode cycled using different current densities with voltage limits of 1–2.5 V were showed in Fig. 5. Flat discharge plateaus at about 1.55 V are observed, suggesting that the discharge plateau of LTO-500 was better than those of LTO-600 and LTO-400, owing to its small size and better degree of crystallinity.

Fig. 5 The discharge curves of the LTO-400, LTO-500 and LTO-600 electrodes.

We then collected electrochemical impedance spectroscopy (EIS) measurements to understand the intrinsic origins of the improved high-rate performance of the LTO samples. Fig. 6 shows the electrochemical impedance spectra of the LTO-500 and LTO-600 measured at a stable voltage of 1.55 V. The data were analyzed by using the equivalent circuit model. In this model, R_s represents the ohmic resistance including total resistance of the electrolyte, separator, and electrical contacts. R_ct is the charge-transfer resistance and CPE is the constant phase-angle element involving double layer capacitance. Z_W (Warburg impedance) reflects the solid-state diffusion of Li⁺ in the bulk of the active material.

Fig. 6 Electrochemical impedance spectra of the pure LTO-500 and LTO-600 nanocomposite electrodes at a voltage of 1.55 V.

It appeared that the R_ct of LTO-500 (119 Ω) was much smaller than that of LTO-600 (258 Ω). We further calculated the charge-transfer kinetic parameter i₀ (exchange current density) using the following equation (i₀ = RT/nFR_ct) where R_ct is the gas constant, T is the absolute temperature, n is the number of electrons involved in the charge-transfer reaction, and F is the Faraday constant. The derived i₀ value of LTO-500 (0.2 mA cm²) was higher than that of LTO-600 (0.09 mA cm²). The accelerated charge-transfer kinetics of the LTO-500 sample could be ascribed to the smaller particle size and larger electrode/electrolyte contact area.

Meanwhile, the nanoscale grain size of LTO-500 dramatically shortened the diffusion length of the lithium ions, thus significantly improving the lithium storage kinetics in the bulk of the active material. According to the above analysis, the accelerated charge-transfer kinetics and lithium-storage kinetics contribute collectively to the promotion of the rate performance for LTO-500. Obviously, the samples calcined at 600 °C suffer from capacity fading. The alternating-current impedance was also increased. This phenomenon was caused by the existence of impurities in the surface with high calcination temperatures. This is because the increased calcination temperature could induce grain growth, thus adversely affecting the charge-transfer and lithium storage kinetics.

Conclusions

To summarize, we designed a novel approach to synthesize nano-sized Li₄Ti₅O₁₂ at low temperature. Pure-phase and highly crystalline Li₄Ti₅O₁₂ with a small particle size of ∼20 nm was obtained at the relatively low calcination temperature of 500 °C. Owing to the small diameter of the LTO-500 nanoparticles, the LTO anode exhibited a high reversible capacity of 123 mAh g⁻¹ at 20 C and excellent cycling performance even at high current densities. The intrinsic advantages of Li₄Ti₅O₁₂ combined with its high-rate performance makes our nano-sized LTO a promising anode material for the development of high energy density lithium batteries for the plug-in hybrid electric vehicle and electric vehicle markets.

Acknowledgements

The authors thank Mr Pengfei Hu and Mr Jianchao Peng for their help in HRTEM characterization and Professor Dayang Wang for discussion. This work was financially supported by Shanghai Municipal Science and Technology Commission (11160704000, 12nm0500100 and 13DZ2292100). The research was also supported by Baoshan District Science and Technology Commission of Shanghai (no. bkw2013142).

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