Preparation and electrochemical properties of Ca-doped Li4Ti5O12 as anode materials in lithium-ion battery

Accepted Manuscript PII: DOI: Reference:

S0013-4686(13)00402-7 http://dx.doi.org/doi:10.1016/j.electacta.2013.03.006 EA 20113

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

27-1-2013 26-2-2013 2-3-2013

Please cite this article as: Q. Zhang, C. Zhang, B. Li, S. Kang, X. Li, Y. Wang, Preparation and Electrochemical properties of Ca-Doped Li4 Ti5 O12 as Anode materials in Lithium-Ion Battery, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.03.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and Electrochemical properties of Ca-Doped Li4Ti5O12 as Anode materials in Lithium-Ion Battery

Department of Environmental Science and engineering, Fudan University, shanghai 200433, PR China

School of Environment and Architecture, University of Shanghai for Science and

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Qianyu Zhanga, Chengli Zhanga, Bo Lia, Shifei Kanga, Xi Lia,*, Yangang Wangb,*

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Technology, Shanghai 200093, PR China

* Corresponding author. Tel/Fax: +86 21 65642789.

E-mail addresses:

[email protected] (X. Li), [email protected] (Y.G. Wang).

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Highlights Ca-doped Li4Ti5O12 samples were prepared by a simple solid-state method. The high-rate performance of Li4-xCaxTi5O12 (0≤ x ≤ 0.2) anode was first reported. Li3.9Ca0.1 Ti5O12 shows the best high-rate performance.

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Abstract

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Ca-doped lithium titanates with the formula of Li4-xCaxTi5O12 (x=0, 0.05, 0.1, 0.15, 0.2) were synthesized as anode materials by a simple solid-state reaction in an air

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atmosphere. The phase structure, morphologies and electrochemical properties of the prepared powders were systematically characterized by X-ray diffraction (XRD),

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scanning electron microscopy (SEM) and cyclic voltammetry (CV), respectively. XRD revealed that the Ca-doping caused no change on the phase structure and highly

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crystalline Li4-xCaxTi5O12 (0≤ x ≤ 0.2) powders without any impurity were obtained. SEM images showed that all samples had similar particulate morphologies and the

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particle size distribution was in the range of 1-2 μm. It was observed that Ca-doped

lithium titanates employed as the anode materials of lithium-ion batteries delivered excellent electrochemical performances, and sample Li3.9Ca0.1Ti5O12 exhibited a

higher specific capacity, better cycling performance and rate capability than other samples. The Li3.9Ca0.1Ti5O12 material showed discharge capacities of 162.4 mAh·g-1, 148.8 mAh·g-1 and 138.7 mAh·g-1 after 100cycles at 1 C, 5 C and 10 C charge-discharge rates, respectively. Electrochemical impedance spectroscopy (EIS) revealed that the Li3.9Ca0.1Ti5O12 electrode exhibited the highest electronic conductivity and fastest lithium-ion diffusivity, which indicated that this novel

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Li3.9Ca0.1Ti5O12 material was promising as a high-rate anode material for the lithium-ion batteries. Keywords: Lithium titanate; Anode material; Doping; Lithium-ion battery

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1. Introduction

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With growing concerns over the global warming effect coming from the production

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of carbon dioxide and the energy crisis of fossil fuels, developing the next-generation electric vehicles (EVs), hybrid electric vehicles (HEVs) and plug-in hybrid electric

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vehicles (PHEVs) to save oil and to decrease exhaust emissions are very significant [1]. As energy storage devices with high power and high energy density, lithium-ion

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batteries (LIBs) have been widely utilized as power sources for EVs, HEVs and PHEVs [2]. However, the conventional carbonaceous materials used as anodes in

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commercial LIBs have safety issues owing to a low Li-intercalation potential

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approaching almost 0 V (vs. Li/Li+) [3]. Recently, the spinel Li4Ti5O12 (LTO) has been

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viewed as a promising alternative to carbonaceous material due to its high insertion potential at around 1.55 V (vs. Li/Li+), which can avoid the dendritic lithium growth on the anode surface at high charging rate [4]. Besides, LTO has excellent Li-ion insertion and extraction reversibility and structure stability with almost negligible volume change during charge-discharge process [5]. Thus, LTO is indeed a potential candidate material for high-rate LIBs. Despite the above mentioned advantages, the LTO shows poor electronic conductivity due to the empty Ti 3d state with a band energy of about 2eV [6], which seriously hinders its high rate performance [7]. To enhance the electronic conductivity

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of LTO, several methods have been proposed, including: (ⅰ) coating with conductive materials, such as amorphous carbon, carbon nanotube or a metallic conducting layer [8-15]; (ⅱ) reducing particle size [16-21]; (ⅲ) doping with metal ions (such as Na+

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[22], Zn2+ [23], Mg2+ [24], AI3+ [25], Co3+ [26], Ni2+ [27], Mn4+ [27], La3+ [28], Zr4+

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[29, 30], Ru4+ [31, 32], V5+ [33], Nb5+ [34, 35], Ta5+ [36]) or non-metal ions (such as

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F- [37], Br- [38]) in Li, Ti or O sites.

Metallic ion doping has been considered to be an effective way to improve

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high-rate discharge capacity and cycling stability through the increases in electronic conductivity and lithium diffusion coefficient in LTO [39, 40]. The increase in

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electronic conductivity can be obtained by doping a cation with higher oxidation state into the tetrahededral 8a Li+ site or the octahedral 16d Ti4+ site [36]. For example, the

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doping of Zn2+ into the tetrahededral Li+ 8a site or V5+ into the octahedral 16d Ti4+ site can effectively increase the electronic conductivity of LTO [23,33]. However, to

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the best of our knowledge, no study about Ca-doped LTO as an anode material has been reported. In this work, Ca-doped LTO in the form of Li4-xCaxTi5O12 (0< x ≤ 0.2) were prepared by a solid-state method to improve the rate capability of the LTO electrodes. This method is highly attractive for synthesis of high-performance LTO electrodes owing to its simplicity, economy and efficiency. The effects of Ca2+ doping

on the structure, morphology and electrochemical characteristics of LTO were also investigated. 2. Experimental 2.1. Material preparation and characterization

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Ca doped and undoped LTO (Li4-xCaxTi5O12, x= 0, 0.05, 0.1, 0.15 and 0.2) were synthesized by a solid-state reaction. Stoichiometric Li2CO3, TiO2 and CaO used as raw materials were mixed by ball milling for 8 h in acetone slurry, followed by drying

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at 120
for 10 h. Finally, the products were calcined at 850
for 12 h in an air

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atmosphere to obtain the samples. 5% excessive Li was added to compensate for the

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Li volatilization during the high temperature heating process. The crystal structure of the as-prepared materials was identified by XRD using Cu-Kα radiation (10º≤ 2θ ≤

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80º). SEM was used to investigate the particle size and morphologies of the as-prepared powders.

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2.2. Battery preparation

The electrochemical characteristics were evaluated by means of two-electrode

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CR2032 coin cells. The sample slurry was prepared by mixing active material powders with conductive carbon (acetylene black) and binder (PVDF) at a weight

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ratio of 82:10:8 in N-methyl-2-pyrrolidine (NMP). Subsequently, the slurry was coated on a copper foil using the doctor blade technique and dried at 120
for 10 h to evaporate the NMP solvent. The electrode foil was punched to 12 mm diameter discs, which were used to assemble the coin cells in an Ar glove box where both moisture and oxygen content were less than 1ppm. Li foil was used as the counter and reference electrode in the cell. Celgard 2400 was the separator. The electrolyte solution was 1 M LiPF6 dissolved in a 1:1:1 mixture by volume of ethlylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC). All of the coin cells were cycled between 1 V and 2.5 V (vs. Li/Li+).

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2.3. Electrochemical measurement Electrochemical tests were carried out by using the above coin-type half cells. Galvanostatic charge-discharge measurements were performed on Land CT2001A

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(Wuhan, China) tester at 1 C, 5 C and 10 C, respectively. For the rate performance

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measurement, the current was varied from 0.5 C to 20 C. Cyclic voltammetry (CV)

was measured on an electrochemical workstation (CHI 660E) between 1.0 and 2.5 V

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(vs. Li/Li+) at a scanning rate of 0.5 mV·s-1. Electrochemical impedance spectra (EIS)

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measurements were also measured at the electrochemical workstation with a ±5 mV AC signal and a frequency range from 10 mHz to 1 MHz. All experiments were

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carried out at room temperature (25
). 3. Results and discussion

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Fig. 1(a) shows the XRD patterns of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) powders. The diffraction peaks of all investigated samples, which were in accordance with the

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standard diffraction peaks of LTO with PDF number of 71-0426, can be indexed for a cubic spinel structure with the space group of Fd3m. No impurity peaks were detected

even when x=0.2, indicating that the doped Ca2+ ions have successfully entered the

lattice structure of LTO, which is in good accordance with the EDX result shown in Fig. 3. The sharp diffraction peaks were observed for all samples, suggesting the formation of good crystallinity. The peak position variation of (111) plane is enlarged and shown in Fig. 1(b). The (111) peak shifts to low angle with Ca doping, denoting that the doped LTO samples have larger lattice constant than pristine LTO. The lattice parameters of the samples obtained according to the Rietveld method are shown in

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Table 1. In order to observe the effect of the concentration of Ca substitution in LTO on the lattice constant, the lattice constant as a function of Ca substitution corresponding to the

Table 1 is shown in Fig.2. It can be clearly seen that the lattice parameter increases

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almost linearly with the increased amount of Ca-doping, which may be ascribed to the

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larger Ca2+ (0.1 nm) that occupied the site of the smaller Li+ (0.076 nm) and the

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transition of a certain amount of Ti3+ ion (0.067 nm) to Ti4+ ion (0.0605 nm) as charge compensation. As a result, the crystal lattice expended to a certain extent and the

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expansive channels reduced the block for the diffusion of Li ions, and then increased the ionic conductivity of LTO. Therefore, it can be expected that the doped samples

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would exhibit higher specific capacity compared to the pure LTO sample. Fig. 4 shows SEM images of the as-prepared LTO powders with and without Ca

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doping. All samples are well crystallized and possess similar morphology with homogeneous distribution of micro-sized (1-2 μm) particles.

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In order to demonstrate the effect of Ca-doping on improving the rate capability of

the electrodes, the cyclic performances of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples at different current rates are shown in Fig. 5. For each stage the charge-discharge processes of the samples are taken for 10 cycles. It can be clearly seen that the undoped LTO sample exhibits a high discharge capacity and good cycling stability at low charge-discharge rate. At 0.5 C, the initial discharge capacity of LTO is 175.8 mAh·g-1, while the doped samples display slight lower discharge capacity and their initial discharge capacities (when x= 0.05, 0.10, 0.15, 0.20) are 172.5 mAh·g-1, 174.7

mAh·g-1, 171.4 mAh·g-1 and 169.2 mAh·g-1, respectively. This may be attributed to

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the Ca2+ occupied the 8a Li+ site that would reduce the amount of Li+ in the LTO. Since almost all the electrochemical energy comes from Li+ insertion and extraction during charge-discharge process, the decrease of the amount of active lithium ions

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caused by Ca doping can reduce the initial discharge capacity. However, better

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electrochemical performance for Ca-doped LTO materials was found with the increase

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of charge-discharge rate. As the discharge current rate increases, the capacity of LTO sample quickly decreases, while the doped samples manifest less capacity degradation

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than the LTO sample. Among the doped samples, the Li3.9Ca0.1Ti5O12 shows excellent high-rate capability. The specific capacity of the Li3.9Ca0.1Ti5O12 sample exceeds the

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pristine LTO from 1 C rate, though it has a lower specific capacity than the undoped one at 0.5 C. Even at 20 C, its discharge capacity is 123.4 mAh·g-1, which is about

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70.6% of the initial discharge capacity at 0.5 C, whereas LTO shows much lower

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capacity, only 22.4 mAh·g-1 at 20 C. The rate capacity of LTO is mainly limited by its

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poor electronic conductivity. The substitution of Ca2+ for Li+ can improve the

conductivity. According to the charge neutrality condition, Ca2+ occupied a Li+ site

forces a Ti4+ transfer to Ti3+. This increases the concentration of the electron, which corresponds to the location of Ti3+ at the Ti4+ site [36]. Despite the doping of Ca

cation into the tetrahedral 8a site would lower the Li+ conductivity, this effect is

weaker than the increase in electronic conductivity and enhanced Li+ diffusion caused by Ca2+ doping. Hence, an improvement in rate capability is achieved. Moreover, as the current rate returned to 0.5 C again, a stable capacity of 166 mAh·g-1 can be obtained without any decay in the following 10 cycles, demonstrating that the

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Li3.9Ca0.1Ti5O12 electrode has an excellent reversibility and stability. All the results above indicate that Li3.9Ca0.1Ti5O12 is a promising anode material with high rate capability, good cycling stability and reversibility.

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The cycling performance of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples at the rates of

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1 C, 5 C and 10 C are shown in Fig. 6. At 1 C, Li3.9Ca0.1Ti5O12 reaches a reversible capacity of 162.1 mAh·g-1 after 100 cycles, which keeps 93.3% of its initial discharge

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capacity (173.8 mAh·g-1) and is higher than that of the LTO (157.8 mAh·g-1). As

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shown in Fig. 6(a), all samples display good cycle performance, which is due to the near zero volume variation of LTO-based spinel structure during the charge-discharge

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process, and the Ca doping do not change its structure characteristics. At 5 C, the 100th discharge capacities of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) are 92.9 mAh·g-1, 132.5

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mAh·g-1, 148.8 mAh·g-1, 131.2 mAh·g-1, and 120.1 mAh·g-1 for different amount of dopant (x=0, 0.05, 0.10, 0.15 and 0.20), respectively. It is clearly observed from Fig.

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6(b) that doped LTO electrodes exhibit higher reversible capacities than that of pristine LTO. At 10 C (Fig. 6(c)), the discharge capacity of Li3.9Ca0.1Ti5O12 remains 138.7 mAh·g-1 even after 100 cycles, while the corresponding value of the LTO is decreased to 50.1 mAh·g-1. The improvement in high rate capacity of Li3.9Ca0.1Ti5O12

may attribute to the enlarged lattice volume as the result of Ca2+ doping, which is

beneficial for the insertion and extraction of Li+. According to the above results, it can be concluded that Li3.9Ca0.1Ti5O12 has the highest discharge capacity and excellent cyclic stability. Cyclic voltammograms (CVs) of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) electrodes obtained at

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a scan rate of 0.5 mV·s-1 between 1 V and 2.5 V are shown in Fig. 7. It is demonstrated that there are a pair of sharp and reversible redox peaks for each sample, indicating the good electrode kinetic of all anodes. It can be seen from Fig. 7 that the

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CVs of all investigated electrodes are similar, suggesting that Ca-doping do not

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change the electrochemical reaction process of LTO. The oxidation peaks located at

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about 1.70 V (vs. Li/Li+) and the reduction peaks occurred at around 1.50 V (vs. Li/Li+) are attributed to the processes of Li deintercalation and intercalation, respectively. It is

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typical behavior for two-phase reaction based on the redox couple of Ti3+/Ti4+ during Li+ insertion and extraction processes. The corresponding electrochemical reaction is

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[24]: Li4Ti5O12 + 3Li+ + 3e-

Li7Ti5O12

E= 1.5 V

(1)

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The voltage differences between anodic and cathodic peaks reflect the polarization degree of the electrode. The voltage differences of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2)

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electrodes between anodic and cathodic peaks are shown in Table 2. It can be observed that the Ca-doped LTO electrodes have lower potential differences than the pristine LTO, which indicate that Ca-doping is favorable for reducing the electrode polarization. Besides, the Li3.9Ca0.1Ti5O12 shows the least potential difference among

all the doped samples, suggesting that too high amount of doping is adverse. Thus, the optimal ratio of Ca-doping is 0.1, which exhibits the lowest electrode polarization. Fig.8 shows the initial discharge-charge curves of the LTO and Li3.9Ca0.1Ti5O12 electrodes at various current rates from 0.5 C to 20 C. It can be seen that the discharge plateau decreases with increasing discharge current, suggesting increased polarization

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for both the electrodes. The discharge plateau of Li3.9Ca0.1Ti5O12 decreases more slowly than LTO. With the increase of discharge current rate from 0.5 C to 10 C, the plateau voltage decreases by 0.18 V and 0.04 V for LTO and Li3.9Ca0.1Ti5O12,

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respectively. However, with the future increase of discharge rate to 20 C, the plateau

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voltage of LTO decreases drastically and even no obvious discharge plateau can be

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found, while it still reaches 1.46 V for the Li3.9Ca0.1Ti5O12 electrode with an obvious discharge plateau. Moreover, it is evident that the LTO exhibits higher potential

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difference, and the reversible capacity of 169.7 mAh·g-1 at 0.5 C drops to 26.2 mAh·g-1 at 20 C. In contrast, the Li3.9Ca0.1Ti5O12 electrode exhibits an excellent rate

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capacity due to the relatively smaller potential difference during high current rate shown in Fig. 8(b). The discharge capacities of Li3.9Ca0.1Ti5O12 are 169.2 mAh·g-1 at

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0.5 C and 122.4 mAh·g-1 at 20 C. These results clearly indicate that Ca-doping can effectively improve the rate performance of LTO.

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Fig. 9 shows the electrochemical impedance spectra (EIS) curves of the

Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) electrodes at the voltage of 1.55 V after the first cycle. EIS are fitted by using an equivalent circuit. All the EIS curves are composed of a depressed semicircle at the high to intermediate frequency range, and there is a straight line at lowest frequency region. The high frequency semicircle is related to the charge transfer resistance at the active material interface, while the sloping line at the low frequency end indicates the Warburg impedance caused by a semi-infinite diffusion of Li+ ion in the electrode. In the equivalent circuit, Rs is the ohmic

resistance of electrolyte; Rct is the charge transfer resistance; CPE is placed to

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represent the double layer capacitance and passivation film capacitance; Zw represents the Warburg impedance [24, 36]. As seen from Fig.9, the charge transfer resistance (Rct) of Ca-doped electrodes is much lower than that of the pure LTO, indicating that

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Ca doping is favorable to improve the electronic conductivity. Moreover, the

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increased slope in the low frequency end for the Ca-doped samples demonstrates that

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Ca-doping can improve Li+ migration in LTO. Some of the parameters fitted by ZView software are listed in Table 3. It can be seen that the exchange current density

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(ίº = RT/nFRct) of the Li3.9Ca0.1Ti5O12 sample is the highest among all samples. Thus, the charge-transfer reaction of the Li3.9Ca0.1Ti5O12 electrode took place more than

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other electrodes. These demonstrate that the Li3.9Ca0.1Ti5O12 sample has the highest electronic conductivity and exhibit the best electrochemical performance. Therefore,

performance.

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4. Conclusions

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the Ca doping of x=0.10 is the appropriate amount to get excellent high rate

The spinel Li4-xCaxTi5O12 (x=0, 0.05, 0.10, 0.15, 0.2) were synthesized by a simple

solid-state method in an air atmosphere. XRD results show that Ca2+ has entered the

lattice of LTO and did not change the electrochemical reaction process as proved by CV and charge-discharge tests. Even though the material has a particle size of 1-2 μm,

Ca-doped LTO shows very high rate capability and excellent stability and reversibility. From overall performance point of view, the Li3.9Ca0.1Ti5O12 sample exhibits the best rate capability. After 100 cycles, its discharge capacity of 138.7 mAh·g-1 with a 90.8% retention at 10 C is much higher than that of LTO, only 50.1 mAh·g-1 with a retention

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of 15.4% at the same current rate. It can be attributed to its higher electronic conductivity and lithium-ion diffusivity compared to the LTO, suggesting that Ca-doping is beneficial to the improvement in high-rate capability of the LTO. All the

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evidences demonstrate that the Li3.9Ca0.1Ti5O12 electrode is a promising anode

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material for lithium-ion batteries.

Acknowledgements

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This work was carried out with financial supports from National Natural Science Foundation of China (Grant No. 61171008), National Natural Science Foundation of

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China (Grant No.21103024), Shanghai Pujiang Rencai Project (No. 09PJ1401400), China Postdoctoral Science Foundation Funded Project (Grant No. 20100480534),

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and China Postdoctoral Science Foundation Special Funded Project (Grant No. 21103024) . This research was also supported by Dalian Mingjia Jinshu Products

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Limited Company，Shanghai Jubo Energy Technology Limited Company and Suzhou Baotan New Energy Limited Company on field and fund.We would like to thank Shaolong Li and Wei Yuan for experimental technique support.

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References

[1] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Ti-based compounds as anode materials for Li-ion

an

batteries, Energy Environ. Sci. 5 (2012) 6652.

[2] N. Ohta, K. Takada, L.Q. Zhang, R.Z. Ma, M. Osada, T. Sasaki, Enhancement of

M

the High-Rate Capability of Solid-State Lithium Batteries by Nanoscale Interfacial Modification, Adv. Mater. 18 (2006) 2226.

te

d

[3] K. S. Park, A. Benayad, D. J. Kang, S. G. Doo, Nitridation-driven conductive Li4Ti5O12 for lithium ion batteries, J. Am. Chem. Soc. 130 (2008) 14930.

Ac ce p

[4] S.S. Zhang, The effect of the charging protocol on the cycle life of a Li-ion battery, J. Power Sources 161 (2006) 1385.

[5] K.C. Hsiao, S.C. Liao, J.M. Chen, Microstructure effect on the electrochemical property of Li4Ti5O12 as an anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7242.

[6] T. Brousse, P. Fragaud, R. Marchand, D.M. Schleich, O. Bohnke, K. West, All oxide solid-state lithium-ion cells, J. Power Sources, 68 (1997) 412. [7] L.F. Shen, C.Z. Yuan, H.J. Luo, X.G. Zhang, K. Xu, F. Zhang, In situ growth of Li4Ti5O12 on multi-walled carbon nanotubes: novel coaxial nanocables for high

Page 14 of 34

rate lithium ion batteries, J. Mater. Chem. 21 (2011) 761. [8] S. Huang, Z. Wen, J. Zhang, X. Yang, Improving the electrochemical performance of Li4Ti5O12/Ag composite by an electroless deposition method, Electrochim.

ip t

Acta 52 (2007) 3704.

cr

[9] G.J. Wang, J. Gao, L.J. Fu, N.H. Zhao, Y.P. Wu, T. Takamura, Preparation and

us

characteristic of carbon-coated Li4Ti5O12 anode material, J. Power Sources 174 (2007) 1109.

an

[10] J.J. Huang, Z.Y. Jiang, The preparation and characterization of Li4Ti5O12/carbon nano-tubes for lithium ion battery, Electrochim. Acta 53 (2008) 7756.

M

[11] Y.G. Wang, H.M. Liu, K.X. Wang, H. Eiji, Y.R. Wang, Synthesis and

d

electrochemical performance of nano-sized Li4Ti5O12 with double surface

te

modification of Ti(III) and carbon, H.S. Zhou, J. Mater. Chem. 19 (2009) 6789. [12] H.Y. Yu, X.F. Zhang, A.F. Jalbout, X.D. Yan, X.M. Pan, H.M. Xie, R.S. Wang,

Ac ce p

High-rate characteristics of novel anode Li4Ti5O12/polyacene materials for Li-ion secondary batteries, Electrochim. Acta 53 (2008) 4200.

[13] T. Yuan, X. Yu, R. Cai, Y.K. Zhou, Z.P. Shao, Synthesis of pristine and carbon-coated Li4Ti5O12 and their low-temperature electrochemical performance,

J. Power Sources 195 (2010) 4997.

[14] B. Zhang, Z.D. Huang, S.W. Oh, J.K. Kim, Improved rate capability of carbon coated Li3.9Sn0.1Ti5O12 porous electrodes for Li-ion batteries, J. Power Sources 196 (2011) 10692. [15] H.F. Ni, L.Z. Fan, Nano-Li4Ti5O12 anchored on carbon nanotubes by liquid phase

Page 15 of 34

deposition as anode material for high rate lithium-ion batteries, J. Power Sources 214 (2012) 195. [16] A. Guerfi, S. Sévigny, M. Lagace, P. Hovington, K. Kinashita, K. Zaghib,

ip t

Nano-particle Li4Ti5O12 spinel as electrode for electrochemical generators, J.

cr

Power Sources 119 (2003) 88.

us

[17] K. Nakahara, R. Nakajima, T. Matsushima, H. Majima, Preparation of particulate Li4Ti5O12 having excellent characteristics as an electrode active material for

an

power storage cells, J. Power Sources 117 (2003) 131.

[18] Y. Lan, X.P. Gao, H.Y. Zhu, Z.F. Zheng, T.Y. Yan, F. Wu, S.P. Ringer, D.Y. Song,

M

Titanate Nanotubes and Nanorods Prepared from Rutile Powder, Adv. Funct.

d

Mater. 15 (2005) 1310.

te

[19] W. Wang, J.G. Tu, S.B. Wang, J.G. Hou, H.M. Zhu, S.Q. Jiao, Ru-doped Li4Ti5O12 anode materials for high rate lithium-ion batteries, Electrochim. Acta

Ac ce p

68 (2012) 254.

[20] Y. S. Lin, M.C. Tsai, J.G. Duh, Self-assembled synthesis of nanoflower-like Li4Ti5O12 for ultrahigh rate lithium-ion batteries, J. Power Sources 214 (2012)

314.

[21] K.M. Kim, K.Y. Kang, S. Kim, Y.G. Lee, Electrochemical properties of TiO2 nanotube-Li4Ti5O12 composite anodes for lithium-ion batteries, Curr. Appl. Phsi. 12 (2012) 1199.

Page 16 of 34

[22] D. Pal, J.L. Pandey, Effect of paramagnetic manganese ions doping on frequency and high temperature dependence dielectric response of layered Na1.9Li0.1Ti3O7

ip t

ceramics, Bull. Mater. Sci. 33 (2010) 691.

[23] T.F. Yi, H.P. Liu, Y.R. Zhu, L.J. Jiang, Y. Xie, R.S. Zhu, Improving the high rate

cr

performance of Li4Ti5O12 through divalent zinc substitution, J. Power Sources

us

215 (2012) 258.

[24] A.Y. Shenouda, K.R. Murali, Electrochemical properties of doped lithium titanate

an

compounds and their performance in lithium rechargeable batteries, J. Power

M

Sources 176 (2008) 332.

[25] R. Cai, S.M. Jiang, X. Yu, B.T. Zhao, H.T. Wang, Z.P. Shao, A novel method to

d

enhance rate performance of an AI-doped Li4Ti5O12 electrode by post-synthesis

Ac ce p

8013.

te

treatment in liquid formaldehyde at room temperature, J. Mater. Chem. 22 (2012)

[26] Y.J. Hao, Q.Y. Lai, J.Z. Lu, X.Y. Ji, Effects of dopant on the electrochemical properties of Li4Ti5O12 anode materials, Ionics 13 (2007) 369.

[27] W.M. Long, X.Y. Wang, S.Y. Yang, H.B. Shu, Q. Wu, Y.S. Bai, L. Bai, Electrochemical properties of Li4Ti5-2xNixMnxO12 compounds synthesized by sol-gel process, Mater. Chem. Phys. 131 (2011) 431. [28] T.F. Yi, Y. Xie, Q.J. Wu, H.P. Liu, L.J. Jiang, M.F. Ye, R.S. Zhu, High rate cycling performance of lanthanum-modified Li4Ti5O12 anode materials for lithium-ion batteries, J. Power Sources 214 (2012) 220.

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[29] X. Li, M.Z. Qu, Z.L. Yu, Structural and electrochemical performance of Li4Ti5-xZrxO12 as anode material for lithium-ion batteries, J. Alloys Compd. 487 (2009) L12.

ip t

[30] F. Gu, G. Chen, Z.H. Wang, Synthesis and electrochemical performances of

cr

Li4Ti4.95Zr0.05O12/C as anode material for lithium-ion batteries, J. Solid State

us

Electrochem. 16 (2012) 375.

[31] C.Y. Lin, Y.R. Jhan, J.G. Duh, Improved capability and rate capability of

an

Ru-doped and carbon-coated Li4Ti5O12 anode material, J. Alloys Compd. 509 (2011) 6965.

M

[32] Y.R. Jhan, J.G. Duh, Electrochemical performance and low discharge cut-off voltage behavior of ruthenium doped Li4Ti5O12 with improved energy density,

te

d

Electrochim. Acta 63 (2012) 9.

[33] Z.J. Yu, X.F. Zhang, G.L. Yang, J. Liu, J.W. Wang, R.S. Wang, J.P. Zhang, High

Ac ce p

rate capability and long-term cyclability of Li4Ti4.9V0.1O12 as anode material in

lithium ion battery, Electrochim. Acta 56 (2011) 8611.

[34] B.B. Tian, H.F. Xiang, L. Zhang, Z. Li, H.H. Wang, Niobium doped lithium titanate as a high rate anode material for Li-ion batteries, Electrochim. Acta 55 (2010) 5453.

[35] B.B. Tian, H.F. Xiang, L. Zhang, H.H. Wang, Effect of Nb-doping on electrochemical stability of Li4Ti5O12 discharged to 0 V, J. Solid State Electrochem. 16 (2012) 205. [36] J. Wolfenstine, J.L. Allen, Electrical conductivity and charge compensation in Ta

Page 18 of 34

doped Li4Ti5O12, J. Power Sources 180 (2008) 582. [37] S. Huang, Z. Wen, Z. Gu, X. Zhu, Preparation and cycling performance of

ip t

Al3+and F− co-substituted compounds Li4AlxTi5−xFyO12−y, Electrochim. Acta 50 (2005) 4057.

cr

[38] G.D. Du, N. Sharma, V.K. Peterson, J.A. Kimpton, D.Z. Jia, Z.P. Guo, Br-Doped

us

Li4Ti5O12 and Composite TiO2 Anodes for Li-ion Batteries: Synchrotron X-Ray and in situ Neutron Diffraction Studies, Adv. Funct. Mater. 21 (2011) 3990.

an

[39] S.H. Huang, Z.Y. Wen, X.L. Yang, Z.H. Gu, X.H. Xu, Li4Ti5O12/Ag composite as electrode materials for lithium-ion battery, J. Power Sources 148 (2005) 72.

M

[40] R. Cai, T. Yuan, R. Ran, X.Q. Liu, Z.P. Shao, Preparation and re-examination of

Ac ce p

te

(2011) 68.

d

Li4Ti4.85Al0.15O12 as anode material of lithium-ion battery, Int. J. Energy Res. 35

Page 19 of 34

ip t cr us

Figure and Table Captions

Fig. 1. (a) XRD patterns and (b) enlarged (111) peaks of Li4-xCaxTi5O12 (x= 0, 0.05,

an

0.1, 0.15 and 0.2) samples.

Fig.2. The effect of the concentration of Ca substitution in LTO on the lattice constant. EDX spectrum of the Li3.8Ca0.2Ti5O12 powder.

M

Fig.3.

Fig. 4. SEM images of Li4-xCaxTi5O12 samples: (a) x=0; (b) x=0.05; (c) x=0.10; (d)

te

d

x=0.15; (e) x=0.20.

Fig. 5. The rate capability measurements of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples

Ac ce p

at various cycling rates.

Fig. 6. The cycling performance of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples at (a) 1

C, (b) 5 C and (c) 10 C. Fig.7.

Cyclic voltammograms of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples. Scan rate: 0.5

mV·s-1. Fig.8.

Initial discharge-charge curves of (a) Li4Ti5O12 and (b) Li3.9Ca0.1Ti5O12 at

different rates. Fig. 9. Electrochemical impedance spectra of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) electrodes. Inset is the equivalent circuit.

Page 20 of 34

Table 1 Lattice parameters of synthesized Li4-xCaxTi5O12 (x= 0, 0.05, 0.1, 0.15, 0.2) samples. Table 2 Potential differences between anodic and cathodic peaks for the synthesized

ip t

Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2)

Ac ce p

te

d

M

an

us

cr

Table 3 Impedance parameters of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) electrodes

Page 21 of 34

Table 1 Lattice parameters of synthesized Li4-xCaxTi5O12 (x= 0, 0.05, 0.1, 0.15, 0.2)

0.8359

x=0.05

0.8371

x=0.10

0.8379

x=0.15

0.8385

x=0.20

0.8390

cr

x=0.00

us

α(nm)

Ac ce p

te

d

M

an

Sample

ip t

samples.

Page 22 of 34

Table(s)

ip t

Table 2 Potential differences between anodic and cathodic peaks for the synthesized

φa (V)[a]

φc (V)[b]

1.750

1.465

x=0.05

1.717

1.488

x=0.10

1.707

1.506

x=0.15

1.734

1.509

x=0.20

1.751

1.519

[a] Anodic peak.

229 201 225 232

M

[b] Cathodic peak.

285

an

x=0

ΔE / mV[c]

us

Sample

cr

Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples after first cycle.

Ac ce p

te

d

[c] ΔE = φa - φc.

Page 23 of 34

Table(s)

Samples

Rs (Ω)

Rct (Ω)

CPE-T (μF)

x = 0.00

3.817

53.84

1.763

x = 0.05

2.731

37.90

4.546

x = 0.10

1.537

16.87

7.238

x = 0.15

2.522

38.11

4.412

x = 0.20

3.312

60.45

3.119

ip t

Table 3 Impedance parameters of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) electrodes j (mA·cm-2) 0.3922

1.4286

0.8058 0.4036

Ac ce p

te

d

M

an

us

cr

0.8121

Page 24 of 34

10 C

40

120 100 80

x=0 x=0.05 x=0.10 x=0.15 x=0.20

60 40 20

an

20

140

us

Discharge capacity / mAh g-1

160

x=0 x=0.05 x=0.1 x=0.15 x=0.2

60

0

cr

180

80

-Z / 

ip t

Graphical abstract

0

0

20

40

60

80

0

100

Z /

20

40

60

80

100

Ac ce p

te

d

M

Cycle number

Page 25 of 34

ip t

(444)

(533) (622)

(440)

(511)

x=0.20

(531)

(400) (331)

(311)

(111)

(a)

Intensity / a.u.

x=0.15

cr

x=0.10

x=0.00 10

20

30

40

50

60

70

80

an

2/ degree

us

x=0.05

M

(b)

x=0.20

Ac ce p

te

d

Intensity / a.u.

x=0.15

17.0

17.5

x=0.10 x=0.05 x=0.00

18.0

18.5

19.0

19.5

20.0

2 / degree

Fig. 1. (a) XRD patterns and (b) enlarged (111) peaks of Li4-xCaxTi5O12 (x= 0, 0.05,

0.1, 0.15 and 0.2) samples.

Page 26 of 34

Figure(s)

ip t

0.840

cr

0.838

us

0.837

0.836

0.835 0.00

0.05

0.10

an

Lattice parameters / nm

0.839

0.15

0.20

M

Li4-xCaxTi5O12 sample

Ac ce p

te

d

Fig.2. The effect of the concentration of Ca substitution in LTO on the lattice constant.

Page 27 of 34

M

an

us

cr

ip t

Figure(s)

d

EDX spectrum of the Li3.8Ca0.2Ti5O12 powder.

Ac ce p

te

Fig.3.

Page 28 of 34

Ac ce p

te

d

M

an

us

cr

ip t

Figure(s)

Fig. 4. SEM images of Li4-xCaxTi5O12 samples: (a) x=0; (b) x=0.05; (c) x=0.10; (d) x=0.15; (e) x=0.20.

Page 29 of 34

Figure(s)

0.5C

0.5C

1C

3C

160

5C

10C

140

cr

20C

120 100

60 40 20

us

x=0 x=0.05 x=0.10 x=0.15 x=0.20

80

0 0

10

20

30

an

Discharge capacity / mAh g-1

180

ip t

200

40

50

60

70

M

Cycle number

Fig. 5. The rate capability measurements of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples

Ac ce p

te

d

at various cycling rates.

Page 30 of 34

Figure(s)

(a) 180

ip t

160

cr

140

x=0 x=0.05 x=0.10 x=0.15 x=0.20

120

100 0

20

40

60

80

(b) 180

5C

160

M

140 120 100 80

d

x=0 x=0.05 x=0.10 x=0.15 x=0.20

60 40

te

Discharge capacity / mAh g-1

100

an

Cycle number

us

Discharge capacity / mAh g-1

1C

20 0

Ac ce p

0

20

40

60

80

100

80

100

Cycle number

(c)

180

10 C

Discharge capacity / mAh g-1

160 140 120 100 80 60 40 20

x=0 x=0.05 x=0.10 x=0.15 x=0.20

0 0

20

40

60

Cycle number

Fig. 6. The cycling performance of the Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples at (a) 1 C, (b) 5 C and (c) 10 C.

Page 31 of 34

Figure(s)

ip t

1.0

cr

0.0

-1.5

x=0 x=0.05 x=0.10 x=0.15 x=0.20

-2.0 2.5

2.0

an

-1.0

us

-0.5

M

Current / mA

0.5

1.5

1.0

+

Fig.7.

Cyclic voltammograms of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) samples. Scan rate: 0.5

Ac ce p

mV·s-1.

te

d

Potential vs. (Li / Li ) / V

Page 32 of 34

Figure(s)

(a) Li4Ti5O12 2.6

5C

10C

3C

1C

ip t

20C

2.2 2.0

cr

1.8

0.5C

1.6 1.4 1.2 1.0

5C

10C

20C

3C

0.8 0

20

40

60

80

100

120

(b) Li3.9Ca0.1Ti5O12

2.2

180

1C

d

2.0 1.8

0.5C

te

1.6 1.4

1C

1.2

Ac ce p

1.0

20C 10C5C

0.8

0

Fig.8.

160

M

2.4

+

140

20C10C 5C 3C

2.6

Potential vs. (Li / Li ) / V

1C

an

Specific Capacity / mAh g-1

us

+

Potential vs. (Li / Li ) / V

2.4

20

40

60

80

100

120

140

3C 160

180

Specific Capacity / mAh g-1

Initial discharge-charge curves of (a) Li4Ti5O12 and (b) Li3.9Ca0.1Ti5O12 at

different rates.

Page 33 of 34

Figure(s)

us

40

cr

x=0 x=0.05 x=0.1 x=0.15 x=0.2

60

-Z / 

ip t

80

0 0

20

40

an

20

60

80

100

M

Z /

Fig. 9. Electrochemical impedance spectra of Li4-xCaxTi5O12 (0 ≤ x ≤ 0.2) electrodes.

Ac ce p

te

d

Inset is the equivalent circuit.

Page 34 of 34