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Mesoporous Spinel Li4Ti5O12 Nanoparticles for High Rate Lithium-ion Battery Anodes
Electrochimica Acta 133 (2014) 578–582
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Mesoporous Spinel Li4 Ti5 O12 Nanoparticles for High Rate Lithium-ion Battery Anodes Weijian Liu a , Dan Shao a,b , Guoen Luo a , Qiongzhi Gao a , Guangjie Yan a , Jiarong He a , Dongyang Chen c , Xiaoyuan Yu a,∗ , Yueping Fang a,∗∗ a
The institute of Biomaterial, College of Science, South China Agricultural University, Guangzhou, Guangdong 510642, China CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China c Department of Chemistry, Northwestern University, 2145 Sheridan Road, Illinois 60208-3113, USA b
a r t i c l e
i n f o
Article history: Received 1 March 2014 Received in revised form 10 April 2014 Accepted 13 April 2014 Available online 21 April 2014 Keywords: Lithium titanate Mesoporous structure Nanoparticle Anode material Lithium-ion battery
a b s t r a c t Mesoporous spinel lithium titanate (Li4 Ti5 O12 ) nanoparticles with the diameter of 40 ± 5 nm and the pore-size distribution of 6 - 8 nm were prepared by a simple hydrothermal method. As an anode material for lithium-ion batteries, these spinel Li4 Ti5 O12 mesoporous nanoparticles exhibited desirable lithium storage properties with an initial discharge capacity of 176 mAh g−1 at 1 C rate and a capacity of approximately 145 mAh g−1 after 200 cycles at a high rate of 20 C. These excellent electrochemical properties at high charge/discharge rates are due to the mesoporous nano-scale structures with small size particles, uniform mesopores and larger electrode/electrolyte contact area, which shortens the diffusion path for both electrons and Li+ ions, and offers more active sites for Li+ insertion-extraction process. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) are among the most advanced electrochemical energy storage systems for a wide range of green applications, such as hybrid electric vehicles and plug-in hybrid electric vehicles, owing to its advantages in long-term cost saving, gravimetric and volumetric energy density, and environment friendliness [1,2]. In general, the kinetic problems in the used electrode materials limit the high power performance of LIBs. Under high power densities, the electrode materials in LIBs must possess rapid ionic and electronic diffusion [3,4]. The currently commercialized graphite anode cannot satisfactorily meet the rate performance and safety requirements of the high power LIBs, especially in applications like electric vehicles and hybrid electric vehicles. Spinel structured Li4 Ti5 O12 (LTO) is attracting wide interest as an anode material for high rate lithium ion batteries owing to the below two advantages [5,6]. First, this material has a stable charge/discharge plateau voltage at approximately 1.55 V (vs. Li+ /Li), which can avoid the reduction of organic electrolyte and
∗ Corresponding author. Tel.: +86 20 85280323; fax: +86 20 85285026. ∗∗ Corresponding author. Tel.: +86 20 85285565; fax: +86 20 85285565. E-mail addresses: [email protected] (X. Yu), [email protected] (Y. Fang). http://dx.doi.org/10.1016/j.electacta.2014.04.072 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
deposition of metallic lithium, thus making batteries more safe. Additionally, the zero-strain LTO displays small volume change in the charge/discharge process, which makes a structural stability, benefiting for a good reversibility and long cycling life. However, its low electronic conductivity results in poor rate performance, and therefore many approaches have been developed to overcome its poor electronic conductivity. It is well known that the morphologies and structures of the LTO are critical factors in lithium intercalating/deintercalating activity and cycling stability [7,8]. Nanostructural LTO is potential to exhibit improved rate performance because of the shorter transport path lengths of lithium ions and electrons [9]. Mesoporous electrode materials have also been regarded as great potential applications in LIBs [9–13]. Very recently, mesoporous spinel LTO microspheres [14] and mesoporous LTO-C microspheres [15], which all presented an excellent high-rate performance as an anode material for LIBs, were synthesized through a hydrothermal process and a solvothermal process, respectively. However, the fabrication of mesoporous LTO nanoparticles (M-LTO NP) with large specific surface area and pore volume, which can provide the efficient transport of lithium ions and electrons in the high-rate lithium ion batteries, is still a challenge. For these reasons, renewed efforts have been made to attain nanosized mesoporous spinel LTO with super electrochemical performance. In this work, we report a facile and scalable two steps approach towards the M-LTO NPs. The M-LTO NPs were synthesized via a
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simple hydrothermal process in LiOH ethanol/water solution from commercial titanium dioxide nanoparticles and subsequent thermal treatment. Three features have been achieved herein: (i) small particle size (about 40 ± 5 nm), (ii) uniform mesoporous configuration (pore diameter: 6 - 8 nm and pore volume: 0.34 cm3 g−1 ) and (iii) enhanced conductivity. These features provide a large number of open channels as passages of the electrolyte and improve the diffusion rate of lithium ions during the cycling processes. Therefore, the as-prepared M-LTO NPs with excellent cycle durability and high rate capacity is a huge potential for the high power storage lithium anode materials. 2. Experimental 2.1. Synthesis of mesoporous Li4 Ti5 O12 nanoparticles (M-LTO NPs) 0.5 g of commercial TiO2 nanoparticles with the diameter of about 20 nm was mixed with 35 mL, 5 M LiOH ethanol-water (1: 4, v/v) solution under ultrasonic treatment for 1 h, in order to make the TiO2 nanoparticles disperse in the LiOH solution evenly. After ultrasonic treatment, the suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave and then placed in an oven at 180 ◦ C for 12 h. The white powder obtained was washed repeatedly with distilled water and ethanol for several times to remove the traces of hydroxides on the particle, dried in a vacuum oven at 70 ◦ C for 24 h, and subsequently calcined at 700 ◦ C for 1 h under air atmosphere to yield the Li4 Ti5 O12 sample, M-LTO NPs. 2.2. Characterization and electrochemical properties The structures of samples were determined by X-ray diffraction (XRD, D/Max-Ш X-ray diffractometer with Cu Ka radiation). The morphology of the resultant samples was studied with transmission electron microscopy (TEM, JEOL JEM-2100HR). The specific surface area of the prepared sample was evaluated by N2 gas adsorption and desorption measurements (Gemini-2390, Micromeritics) with Brunauer–Emmett–Teller (BET) method. The electrochemical performances were tested by CR2025 coin cells. The working electrodes were fabricated by mixing asprepared M-LTO NPs (80 wt%), carbon black (electronic conductive additive, 10 wt%) and polyvinylidene fluoride (binder, 10 wt%) in N-methylpyrrolidinone. This slurry was coated on copper foil and dried at 80 ◦ C overnight under vacuum. The mass of active electrode material was estimated to be about 20.0 mg/cm2 . Coin cell assembly was carried out in a glove box filled with ultra-pure argon, using lithium metal as the counter anode, 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (EC:DMC) (1:1, v/v) as electrolyte and micro-porous polypropylene film (Celgard 2400) as a separator. The specific capacity was calculated based on the active mass of M-LTO NPs in the electrode, while there may be minor contribution from carbon black. Galvanostatic charge/discharge tests were performed at room temperature on a LAND CT-2001A cell program-control test system at various rates with cut-off voltages of 0.5 and 3.0 V vs. Li/Li+ . Electrochemical impedance spectroscopic (EIS) measurements were carried out on an IM6e electrochemical workstation over the frequency range between 1 MHz and 10 mHz and applying an AC signal of 5 mV. The cyclic voltam-mograms (CV) experiments were conducted using Potentiostat/Gallanostat Model (Perkin-Elmer 273A, EG&E). 3. Results and discussion The crystal structure of the as-prepared sample obtained by a simple hydrothermal process in LiOH ethanol/water solution from
Fig. 1. XRD pattern of the M-LTO NPs.
commercial titanium dioxide nanoparticles and subsequent thermal treatment has been determined by X-ray diffraction (XRD) (see Fig. 1). The sharp and clear diffraction peaks clearly indicate a good crystallization of the sample. All of the identified peaks can be perfectly indexed to the face-centered cubic spinel Li4 Ti5 O12 (JCPDS 49-0207). The XRD result suggests that the nice spinel Li4 Ti5 O12 structure has been obtained via a simple hydrothermal process and subsequently thermal treatment. Fig. 2a shows a typical transmission electron microscope (TEM) image of the as-prepared LTO sample. It indicates the presence of well-shaped, relatively homogeneous nanoparticles with an average diameter of 40 ± 5 nm. They are much smaller than the LTO particles prepared by traditional methods reported before [16,17]. To show further insight into the detailed nanostructure of the as-prepared LTO sample, highmagnification TEM images of the obtained LTO nanoparticles are presented in Fig. 2b, c and d. Obviously, they reveal a porous nature of the LTO particles. Particularly, the higher magnified images of the LTO particles (Fig. 2c and 2d) clearly show nice mesoporours structures (about 5 nm in pore size). Nitrogen-sorption studies carried out on the obtained LTO sample show a type-IV adsorption–desorption isotherm with an H2 -type hysteresis, confirming that the sample is mesoporous (Fig. 3). The Brunnauer–Emmett–Teller (BET) surface area is 36.76 m2 g−1 . The total pore volume amounts 0.34 cm3 g−1 . The Barrett–Joyner–Halenda (BJH) pore-size distribution curve (see the inset in Fig. 3) indicates that the mesoporous LTO materials exhibit a good pore-size distribution, and its average pore diameters is about 6 - 8 nm, being consistent with the TEM observation. Control experiments have shown that the M-LTO NPs could be formed only with the right reagents combination and reaction conditions. First, the presence of ethanol was necessary in the reaction to produce the M-LTO NPs; only the large LTO particles were obtained in the absence of ethanol. Second, the LiOH concentrations and the ratio of ethanol and water significantly influenced morphologies of the final the LTO products. The electrochemical properties of the M-LTO NPs were evaluated (Fig. 4-5). The initial charge/discharge profiles for the M-LTO NPs anode material between 0.5-3.0 V at increasing rates from 1 to 20 C are shown in Fig. 4. The M-LTO NPs sample delivers specific capacities of 176 mAh g−1 at 1 C rate, very close to the theoretical capacity (175 mAh g−1 ) of Li4 Ti5 O12 . It is well known that it is hard to obtain the theoretical capacity in a bulk or micrometer sized particle. The high capacities indicate that the mesoporous structure of M-LTO NPs was well developed, and the Li+ ion insertion/extraction took place at entire part of the mesopores in M-LTO NPs due to facile lithium ion diffusion through the mesoporous Li4 Ti5 O12 nanoparticles and such resulted higher utilization rate of material. The initial charge/discharge curve of the M-LTO at 1 C shows very
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Fig. 2. (a) a TEM image of the M-LTO NPs. The TEM images of the M-LTO NPs: (b) low magnification, (c, d) high magnification.
flat plateau at 1.55 V, which demonstrates the characteristic of a Li4 Ti5 O12 /Li7 Ti5 O12 two-phase reaction. The cell discharge voltage exhibits a tendency to decrease with the increasing discharge rate which could be attributed to the electrode polarization. The as-prepared M-LTO NPs show outstanding rate performance. Fig. 5 shows the influence of discharge rate on the capacity retention, inset is the cycle performance of M-LTO NPs at 20 C for 200 cycles. The specific discharge capacity of about 171, 167, 162 and 151 mAh g−1 was obtained at a rate of 1, 5, 10 and 20 C, respectively. Although the capacity decreases regularly with increasing in discharge rate,
the M-LTO NPs still remains a high capacity with excellent capacity retention. The capacity remains a discharge capacity of 145 mAh g−1 after 200 cycles even at the high rate of 20 C as shown in inset graph of Fig. 5, the total capacity fade is about 4.0% during 200 cycles. Furthermore, it is worth noting that the capacity of M-LTO NPs can be completely recovered to 169 mAh g−1 when the discharge rate returns to 1 C rate after continuous 20 C rate cycles, indicating that the M-LTO NPs material has good electrochemical reversibility and structural stability even after high rate charge and discharge cycles. Especially, the M-LTO NPs exhibited
Fig. 3. N2 adsorption-desorption BET isotherm of the M-LTO NPs; the inset shows the pore size distribution of the M-LTO NPs.
Fig. 4. The initial charge/discharge curves for the M-LTO NPs between 0.5 and 3.0 V at various rates.
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Fig. 7. Electrochemical impedance spectra of the M-LTO NPs and conventional LTO sample. (inset graph: the relationships between −1/2 and Z’) . Fig. 5. Rate performance of the M-LTO NPs at various rates (inset graph: cycle performance of M-LTO NPs at 20 C for 200 cycles).
a remarkable cycling and rate capability behaviour, which is even superior to the performance reported for LTO nanotubes [5], LTO nanowires [18], mesoporous spinel LTO microspheres [14], porous LTO microspheres [19] and mesoporous LTO-C microspheres [15]. According to the result, the excellent electrochemical properties of the M-LTO NPs anode material at high charge/discharge rates are due to the nanosized mesoporous structure. The benefit of nanosized mesoporous structure can be rationalized by two courses: (1) mesoporous nanomaterials provide a favorable framework that ensures short diffusion path for Li-ions to migrate from the core of the particles to their surface through the porous structure; (2) the large surface area of the nanosized mesoporous structure ensures good contact between electrode and electrolyte, offering more active sites for the electrochemical reactions. To gain further insight to the effect of mesoporous particles on the electrochemical properties of M-LTO NPs electrode, cyclic voltammetry (CV) and electrochemical impedance spectroscopy measurements (EIS) were performed to evaluate the Li+ insertion/extraction kinetic process. Fig. 6 shows the typical cyclic voltammetry (CV) curves of the M-LTO NPs between 0.5 and 2.5 V at different scan rates. All curves show a pair of reversible redox peaks, which attributed to the solid state redox of Ti3+ /4+ . It is well-known that the relation between peak currents and scan rates indicates the different electrochemical reaction characteristics, including diffusion-controlled or surface-confined charge-transfer processes [20,21]. As shown by inset graph in Fig. 6, a linear relationship between the oxidation/reduction peak current and square root of the scan rates demonstrate a diffusion-controlled reaction for the M-LTO NPs. Fig. 7 shows Nyquist plots obtained from the as-prepared MLTO NPs electrode and the LTO conventionally prepared by high temperature method. As can be seen, the semicircle in the middle frequency range shows the charge transfer resistance (Rct ).
Fig. 6. The cyclic voltammograms of the M-LTO NPs at different scan rate (inset graph: the relationships between peak current and square root of the scan rate).
The M-LTO NPs electrode exhibits much lower Rct than that of the conventionally prepared. This lower Rct result suggests that the MLTO NPs electrode possess better kinetic behavior. In additionally, Nyquist plots obtained from the M-LTO NPs electrode before cycle and after 5th , 50th cycle tests are shown in Fig. 8. As can be seen, Rct of the 5th and 50th cycles do not change much indicates that electrochemical reversibility was established after activation of the initial cycles. This result suggests that the kinetic behavior and SEI of the electrode are stable, which implies excellent chemical stablility and excellent strain accommodation of M-LTO NPs electrode and a relatively superior cyclability of the cell. To further investigate the electrode kinetics, the chemical diffusion coefficients of lithium ions (DLi + ) in M-LTO NPs electrode and the LTO conventionally prepared are estimated by the EIS method [16,17] (see Fig. 7). As shown in the inset left figure in Fig. 7, the graph of Z against ω−1/2 is a straight line with the slope of . Base on the slope of Z against ω−1/2 , which is governed by Eq. (1) [16], the diffusion coefficint values (DLi + ) of the Li+ in the samples can be calculated by Eq. (2) [17]. Z = Rs + Rct + Rf + ω−1/2
(1)
D = 0.5R2 T 2 /A2 n4 F 4 C 2 2
(2)
Where the meanings of is the Warburg factor; n is the number of electrons per molecule during oxidization; ω the angular frequency in the low frequency region, Rct the charge-transfer resistance; D the diffusion coefficient; R the gas constant; T the absolute temperature; F Faraday’s constant; A the area of the electrode surface; and, C the molar concentration of Li+ . The calculated DLi + values are 3.0 ×10−14 cm2 s−1 for the M-LTO NPs electrode and 1.2 × 10−14 cm2 s−1 for the LTO conventionally prepared, respectively. It is noticeable that the diffusion coefficients of the M-LTO NPs can be enhanced due to the higher mobility for Li+ diffusion of the M-LTO NPs, indicating the nano mesoporous particles of small size effect, effectively reduces the embedded distance of Li+ , and the mesoporous structure can provide convenient
Fig. 8. Electrochemical impedance spectra of the M-LTO NPs after different cycles.
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diffusion channels for lithium ion, which directly led to improved high-rate capabilities of the anode material. As previous reported [15,18], the mesoporous nanostructures M-LTO NPs with high surface area induce the intimate contact between the electrolyte and active electrode materials to maximize the contact area, which provide high surface area, more reaction sites and lower activation energy for lithium ion insertion/extraction. Especially, the large surface area introduced by the mesoporous nanostructure building blocks of each M-LTO NP favor the creation of an easy and shorter diffusion pathway for ionic and electronic diffusion, resulting in extremely good power performance. In addition, the good crystallinity of the M-LTO NPs possesses the stability of the spinel crystal structure and exhibits a zero-volume expansion over the whole stoichiometric range. From the previous experiment results, it demonstrated that our mesoporous nanostructure M-LTO NPs material architecture could be tolerant to the high charge/discharge current, which is a desirable characteristic required for high power application. 4. Conclusions In summary, the mesoporous spinel Lithium titanate (Li4 Ti5 O12 ) nanoparticles were prepared by a facile hydrothermal method, which converted titanium oxide nanoparticles to the spinel M-LTO NPs in a mixed solvent of ethanol/water and followed by heat treatment. The as-prepared M-LTO NPs exhibited a capacity of 176 mAh g−1 at 1 C rate and a capacity of 151 mAh g−1 at a very high rate of 20 C with excellent capacity retention performance. The M-LTO NPs possessed faster charge-transfer kineties and higher ion-diffusion coefficients than the conventional LTO, as evidenced by the EIS results. We speculated that the main reason for this enhanced rate performance was arisen from the mesoporous nanostructures, small particle size (about 40 ± 5 nm), uniform mesoporous topology (pore diameter: 6 - 8 nm and pore volume: 0.34 cm3 g−1 ) and enhanced conductivity of the M-LTO NPs. These M-LTO NPs were therefore promising anode materials for high power lithium ion batteries applications. Acknowledgement This research was supported by NSF of China (20963002, 21173088 and 51003034), Science and Technology Planning Project of Guangdong Province (2009B010900025 and 2010B080701072), Guangdong Natural Science Foundation (9151064201000039), the key Academic Program of the 3rd phase “211 Project” of South China Agricultural University, the Key Laboratory of Renewable Energy and Gas Hydrate Program of the Chinese Academy of Sciences (No. 2010002).
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Mesoporous Spinel Li4 Ti5 O12 Nanoparticles for High Rate Lithium-ion Battery Anodes Weijian Liu a , Dan Shao a,b , Guoen Luo a , Qiongzhi Gao a , Guangjie Yan a , Jiarong He a , Dongyang Chen c , Xiaoyuan Yu a,∗ , Yueping Fang a,∗∗ a
The institute of Biomaterial, College of Science, South China Agricultural University, Guangzhou, Guangdong 510642, China CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China c Department of Chemistry, Northwestern University, 2145 Sheridan Road, Illinois 60208-3113, USA b
a r t i c l e
i n f o
Article history: Received 1 March 2014 Received in revised form 10 April 2014 Accepted 13 April 2014 Available online 21 April 2014 Keywords: Lithium titanate Mesoporous structure Nanoparticle Anode material Lithium-ion battery
a b s t r a c t Mesoporous spinel lithium titanate (Li4 Ti5 O12 ) nanoparticles with the diameter of 40 ± 5 nm and the pore-size distribution of 6 - 8 nm were prepared by a simple hydrothermal method. As an anode material for lithium-ion batteries, these spinel Li4 Ti5 O12 mesoporous nanoparticles exhibited desirable lithium storage properties with an initial discharge capacity of 176 mAh g−1 at 1 C rate and a capacity of approximately 145 mAh g−1 after 200 cycles at a high rate of 20 C. These excellent electrochemical properties at high charge/discharge rates are due to the mesoporous nano-scale structures with small size particles, uniform mesopores and larger electrode/electrolyte contact area, which shortens the diffusion path for both electrons and Li+ ions, and offers more active sites for Li+ insertion-extraction process. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) are among the most advanced electrochemical energy storage systems for a wide range of green applications, such as hybrid electric vehicles and plug-in hybrid electric vehicles, owing to its advantages in long-term cost saving, gravimetric and volumetric energy density, and environment friendliness [1,2]. In general, the kinetic problems in the used electrode materials limit the high power performance of LIBs. Under high power densities, the electrode materials in LIBs must possess rapid ionic and electronic diffusion [3,4]. The currently commercialized graphite anode cannot satisfactorily meet the rate performance and safety requirements of the high power LIBs, especially in applications like electric vehicles and hybrid electric vehicles. Spinel structured Li4 Ti5 O12 (LTO) is attracting wide interest as an anode material for high rate lithium ion batteries owing to the below two advantages [5,6]. First, this material has a stable charge/discharge plateau voltage at approximately 1.55 V (vs. Li+ /Li), which can avoid the reduction of organic electrolyte and
∗ Corresponding author. Tel.: +86 20 85280323; fax: +86 20 85285026. ∗∗ Corresponding author. Tel.: +86 20 85285565; fax: +86 20 85285565. E-mail addresses: [email protected] (X. Yu), [email protected] (Y. Fang). http://dx.doi.org/10.1016/j.electacta.2014.04.072 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
deposition of metallic lithium, thus making batteries more safe. Additionally, the zero-strain LTO displays small volume change in the charge/discharge process, which makes a structural stability, benefiting for a good reversibility and long cycling life. However, its low electronic conductivity results in poor rate performance, and therefore many approaches have been developed to overcome its poor electronic conductivity. It is well known that the morphologies and structures of the LTO are critical factors in lithium intercalating/deintercalating activity and cycling stability [7,8]. Nanostructural LTO is potential to exhibit improved rate performance because of the shorter transport path lengths of lithium ions and electrons [9]. Mesoporous electrode materials have also been regarded as great potential applications in LIBs [9–13]. Very recently, mesoporous spinel LTO microspheres [14] and mesoporous LTO-C microspheres [15], which all presented an excellent high-rate performance as an anode material for LIBs, were synthesized through a hydrothermal process and a solvothermal process, respectively. However, the fabrication of mesoporous LTO nanoparticles (M-LTO NP) with large specific surface area and pore volume, which can provide the efficient transport of lithium ions and electrons in the high-rate lithium ion batteries, is still a challenge. For these reasons, renewed efforts have been made to attain nanosized mesoporous spinel LTO with super electrochemical performance. In this work, we report a facile and scalable two steps approach towards the M-LTO NPs. The M-LTO NPs were synthesized via a
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simple hydrothermal process in LiOH ethanol/water solution from commercial titanium dioxide nanoparticles and subsequent thermal treatment. Three features have been achieved herein: (i) small particle size (about 40 ± 5 nm), (ii) uniform mesoporous configuration (pore diameter: 6 - 8 nm and pore volume: 0.34 cm3 g−1 ) and (iii) enhanced conductivity. These features provide a large number of open channels as passages of the electrolyte and improve the diffusion rate of lithium ions during the cycling processes. Therefore, the as-prepared M-LTO NPs with excellent cycle durability and high rate capacity is a huge potential for the high power storage lithium anode materials. 2. Experimental 2.1. Synthesis of mesoporous Li4 Ti5 O12 nanoparticles (M-LTO NPs) 0.5 g of commercial TiO2 nanoparticles with the diameter of about 20 nm was mixed with 35 mL, 5 M LiOH ethanol-water (1: 4, v/v) solution under ultrasonic treatment for 1 h, in order to make the TiO2 nanoparticles disperse in the LiOH solution evenly. After ultrasonic treatment, the suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave and then placed in an oven at 180 ◦ C for 12 h. The white powder obtained was washed repeatedly with distilled water and ethanol for several times to remove the traces of hydroxides on the particle, dried in a vacuum oven at 70 ◦ C for 24 h, and subsequently calcined at 700 ◦ C for 1 h under air atmosphere to yield the Li4 Ti5 O12 sample, M-LTO NPs. 2.2. Characterization and electrochemical properties The structures of samples were determined by X-ray diffraction (XRD, D/Max-Ш X-ray diffractometer with Cu Ka radiation). The morphology of the resultant samples was studied with transmission electron microscopy (TEM, JEOL JEM-2100HR). The specific surface area of the prepared sample was evaluated by N2 gas adsorption and desorption measurements (Gemini-2390, Micromeritics) with Brunauer–Emmett–Teller (BET) method. The electrochemical performances were tested by CR2025 coin cells. The working electrodes were fabricated by mixing asprepared M-LTO NPs (80 wt%), carbon black (electronic conductive additive, 10 wt%) and polyvinylidene fluoride (binder, 10 wt%) in N-methylpyrrolidinone. This slurry was coated on copper foil and dried at 80 ◦ C overnight under vacuum. The mass of active electrode material was estimated to be about 20.0 mg/cm2 . Coin cell assembly was carried out in a glove box filled with ultra-pure argon, using lithium metal as the counter anode, 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (EC:DMC) (1:1, v/v) as electrolyte and micro-porous polypropylene film (Celgard 2400) as a separator. The specific capacity was calculated based on the active mass of M-LTO NPs in the electrode, while there may be minor contribution from carbon black. Galvanostatic charge/discharge tests were performed at room temperature on a LAND CT-2001A cell program-control test system at various rates with cut-off voltages of 0.5 and 3.0 V vs. Li/Li+ . Electrochemical impedance spectroscopic (EIS) measurements were carried out on an IM6e electrochemical workstation over the frequency range between 1 MHz and 10 mHz and applying an AC signal of 5 mV. The cyclic voltam-mograms (CV) experiments were conducted using Potentiostat/Gallanostat Model (Perkin-Elmer 273A, EG&E). 3. Results and discussion The crystal structure of the as-prepared sample obtained by a simple hydrothermal process in LiOH ethanol/water solution from
Fig. 1. XRD pattern of the M-LTO NPs.
commercial titanium dioxide nanoparticles and subsequent thermal treatment has been determined by X-ray diffraction (XRD) (see Fig. 1). The sharp and clear diffraction peaks clearly indicate a good crystallization of the sample. All of the identified peaks can be perfectly indexed to the face-centered cubic spinel Li4 Ti5 O12 (JCPDS 49-0207). The XRD result suggests that the nice spinel Li4 Ti5 O12 structure has been obtained via a simple hydrothermal process and subsequently thermal treatment. Fig. 2a shows a typical transmission electron microscope (TEM) image of the as-prepared LTO sample. It indicates the presence of well-shaped, relatively homogeneous nanoparticles with an average diameter of 40 ± 5 nm. They are much smaller than the LTO particles prepared by traditional methods reported before [16,17]. To show further insight into the detailed nanostructure of the as-prepared LTO sample, highmagnification TEM images of the obtained LTO nanoparticles are presented in Fig. 2b, c and d. Obviously, they reveal a porous nature of the LTO particles. Particularly, the higher magnified images of the LTO particles (Fig. 2c and 2d) clearly show nice mesoporours structures (about 5 nm in pore size). Nitrogen-sorption studies carried out on the obtained LTO sample show a type-IV adsorption–desorption isotherm with an H2 -type hysteresis, confirming that the sample is mesoporous (Fig. 3). The Brunnauer–Emmett–Teller (BET) surface area is 36.76 m2 g−1 . The total pore volume amounts 0.34 cm3 g−1 . The Barrett–Joyner–Halenda (BJH) pore-size distribution curve (see the inset in Fig. 3) indicates that the mesoporous LTO materials exhibit a good pore-size distribution, and its average pore diameters is about 6 - 8 nm, being consistent with the TEM observation. Control experiments have shown that the M-LTO NPs could be formed only with the right reagents combination and reaction conditions. First, the presence of ethanol was necessary in the reaction to produce the M-LTO NPs; only the large LTO particles were obtained in the absence of ethanol. Second, the LiOH concentrations and the ratio of ethanol and water significantly influenced morphologies of the final the LTO products. The electrochemical properties of the M-LTO NPs were evaluated (Fig. 4-5). The initial charge/discharge profiles for the M-LTO NPs anode material between 0.5-3.0 V at increasing rates from 1 to 20 C are shown in Fig. 4. The M-LTO NPs sample delivers specific capacities of 176 mAh g−1 at 1 C rate, very close to the theoretical capacity (175 mAh g−1 ) of Li4 Ti5 O12 . It is well known that it is hard to obtain the theoretical capacity in a bulk or micrometer sized particle. The high capacities indicate that the mesoporous structure of M-LTO NPs was well developed, and the Li+ ion insertion/extraction took place at entire part of the mesopores in M-LTO NPs due to facile lithium ion diffusion through the mesoporous Li4 Ti5 O12 nanoparticles and such resulted higher utilization rate of material. The initial charge/discharge curve of the M-LTO at 1 C shows very
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Fig. 2. (a) a TEM image of the M-LTO NPs. The TEM images of the M-LTO NPs: (b) low magnification, (c, d) high magnification.
flat plateau at 1.55 V, which demonstrates the characteristic of a Li4 Ti5 O12 /Li7 Ti5 O12 two-phase reaction. The cell discharge voltage exhibits a tendency to decrease with the increasing discharge rate which could be attributed to the electrode polarization. The as-prepared M-LTO NPs show outstanding rate performance. Fig. 5 shows the influence of discharge rate on the capacity retention, inset is the cycle performance of M-LTO NPs at 20 C for 200 cycles. The specific discharge capacity of about 171, 167, 162 and 151 mAh g−1 was obtained at a rate of 1, 5, 10 and 20 C, respectively. Although the capacity decreases regularly with increasing in discharge rate,
the M-LTO NPs still remains a high capacity with excellent capacity retention. The capacity remains a discharge capacity of 145 mAh g−1 after 200 cycles even at the high rate of 20 C as shown in inset graph of Fig. 5, the total capacity fade is about 4.0% during 200 cycles. Furthermore, it is worth noting that the capacity of M-LTO NPs can be completely recovered to 169 mAh g−1 when the discharge rate returns to 1 C rate after continuous 20 C rate cycles, indicating that the M-LTO NPs material has good electrochemical reversibility and structural stability even after high rate charge and discharge cycles. Especially, the M-LTO NPs exhibited
Fig. 3. N2 adsorption-desorption BET isotherm of the M-LTO NPs; the inset shows the pore size distribution of the M-LTO NPs.
Fig. 4. The initial charge/discharge curves for the M-LTO NPs between 0.5 and 3.0 V at various rates.
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Fig. 7. Electrochemical impedance spectra of the M-LTO NPs and conventional LTO sample. (inset graph: the relationships between −1/2 and Z’) . Fig. 5. Rate performance of the M-LTO NPs at various rates (inset graph: cycle performance of M-LTO NPs at 20 C for 200 cycles).
a remarkable cycling and rate capability behaviour, which is even superior to the performance reported for LTO nanotubes [5], LTO nanowires [18], mesoporous spinel LTO microspheres [14], porous LTO microspheres [19] and mesoporous LTO-C microspheres [15]. According to the result, the excellent electrochemical properties of the M-LTO NPs anode material at high charge/discharge rates are due to the nanosized mesoporous structure. The benefit of nanosized mesoporous structure can be rationalized by two courses: (1) mesoporous nanomaterials provide a favorable framework that ensures short diffusion path for Li-ions to migrate from the core of the particles to their surface through the porous structure; (2) the large surface area of the nanosized mesoporous structure ensures good contact between electrode and electrolyte, offering more active sites for the electrochemical reactions. To gain further insight to the effect of mesoporous particles on the electrochemical properties of M-LTO NPs electrode, cyclic voltammetry (CV) and electrochemical impedance spectroscopy measurements (EIS) were performed to evaluate the Li+ insertion/extraction kinetic process. Fig. 6 shows the typical cyclic voltammetry (CV) curves of the M-LTO NPs between 0.5 and 2.5 V at different scan rates. All curves show a pair of reversible redox peaks, which attributed to the solid state redox of Ti3+ /4+ . It is well-known that the relation between peak currents and scan rates indicates the different electrochemical reaction characteristics, including diffusion-controlled or surface-confined charge-transfer processes [20,21]. As shown by inset graph in Fig. 6, a linear relationship between the oxidation/reduction peak current and square root of the scan rates demonstrate a diffusion-controlled reaction for the M-LTO NPs. Fig. 7 shows Nyquist plots obtained from the as-prepared MLTO NPs electrode and the LTO conventionally prepared by high temperature method. As can be seen, the semicircle in the middle frequency range shows the charge transfer resistance (Rct ).
Fig. 6. The cyclic voltammograms of the M-LTO NPs at different scan rate (inset graph: the relationships between peak current and square root of the scan rate).
The M-LTO NPs electrode exhibits much lower Rct than that of the conventionally prepared. This lower Rct result suggests that the MLTO NPs electrode possess better kinetic behavior. In additionally, Nyquist plots obtained from the M-LTO NPs electrode before cycle and after 5th , 50th cycle tests are shown in Fig. 8. As can be seen, Rct of the 5th and 50th cycles do not change much indicates that electrochemical reversibility was established after activation of the initial cycles. This result suggests that the kinetic behavior and SEI of the electrode are stable, which implies excellent chemical stablility and excellent strain accommodation of M-LTO NPs electrode and a relatively superior cyclability of the cell. To further investigate the electrode kinetics, the chemical diffusion coefficients of lithium ions (DLi + ) in M-LTO NPs electrode and the LTO conventionally prepared are estimated by the EIS method [16,17] (see Fig. 7). As shown in the inset left figure in Fig. 7, the graph of Z against ω−1/2 is a straight line with the slope of . Base on the slope of Z against ω−1/2 , which is governed by Eq. (1) [16], the diffusion coefficint values (DLi + ) of the Li+ in the samples can be calculated by Eq. (2) [17]. Z = Rs + Rct + Rf + ω−1/2
(1)
D = 0.5R2 T 2 /A2 n4 F 4 C 2 2
(2)
Where the meanings of is the Warburg factor; n is the number of electrons per molecule during oxidization; ω the angular frequency in the low frequency region, Rct the charge-transfer resistance; D the diffusion coefficient; R the gas constant; T the absolute temperature; F Faraday’s constant; A the area of the electrode surface; and, C the molar concentration of Li+ . The calculated DLi + values are 3.0 ×10−14 cm2 s−1 for the M-LTO NPs electrode and 1.2 × 10−14 cm2 s−1 for the LTO conventionally prepared, respectively. It is noticeable that the diffusion coefficients of the M-LTO NPs can be enhanced due to the higher mobility for Li+ diffusion of the M-LTO NPs, indicating the nano mesoporous particles of small size effect, effectively reduces the embedded distance of Li+ , and the mesoporous structure can provide convenient
Fig. 8. Electrochemical impedance spectra of the M-LTO NPs after different cycles.
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diffusion channels for lithium ion, which directly led to improved high-rate capabilities of the anode material. As previous reported [15,18], the mesoporous nanostructures M-LTO NPs with high surface area induce the intimate contact between the electrolyte and active electrode materials to maximize the contact area, which provide high surface area, more reaction sites and lower activation energy for lithium ion insertion/extraction. Especially, the large surface area introduced by the mesoporous nanostructure building blocks of each M-LTO NP favor the creation of an easy and shorter diffusion pathway for ionic and electronic diffusion, resulting in extremely good power performance. In addition, the good crystallinity of the M-LTO NPs possesses the stability of the spinel crystal structure and exhibits a zero-volume expansion over the whole stoichiometric range. From the previous experiment results, it demonstrated that our mesoporous nanostructure M-LTO NPs material architecture could be tolerant to the high charge/discharge current, which is a desirable characteristic required for high power application. 4. Conclusions In summary, the mesoporous spinel Lithium titanate (Li4 Ti5 O12 ) nanoparticles were prepared by a facile hydrothermal method, which converted titanium oxide nanoparticles to the spinel M-LTO NPs in a mixed solvent of ethanol/water and followed by heat treatment. The as-prepared M-LTO NPs exhibited a capacity of 176 mAh g−1 at 1 C rate and a capacity of 151 mAh g−1 at a very high rate of 20 C with excellent capacity retention performance. The M-LTO NPs possessed faster charge-transfer kineties and higher ion-diffusion coefficients than the conventional LTO, as evidenced by the EIS results. We speculated that the main reason for this enhanced rate performance was arisen from the mesoporous nanostructures, small particle size (about 40 ± 5 nm), uniform mesoporous topology (pore diameter: 6 - 8 nm and pore volume: 0.34 cm3 g−1 ) and enhanced conductivity of the M-LTO NPs. These M-LTO NPs were therefore promising anode materials for high power lithium ion batteries applications. Acknowledgement This research was supported by NSF of China (20963002, 21173088 and 51003034), Science and Technology Planning Project of Guangdong Province (2009B010900025 and 2010B080701072), Guangdong Natural Science Foundation (9151064201000039), the key Academic Program of the 3rd phase “211 Project” of South China Agricultural University, the Key Laboratory of Renewable Energy and Gas Hydrate Program of the Chinese Academy of Sciences (No. 2010002).
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