Enhanced lithium storage property of Na-doped Li2Na2Ti6O14 anode materials for secondary lithium-ion batteries

Mengmeng Lao, Peng Li, Xiaoting Lin, Lianyi Shao, Miao Shui*, Nengbing Long, Dongjie Wang and Jie Shu*
Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province, People's Republic of China. E-mail: sergio_shu@hotmail.com; shujie@nbu.edu.cn; shuimiao@nbu.edu.cn; Fax: +86-574-87609987; Tel: +86-574-87600787

Received 13th March 2015 , Accepted 5th May 2015

First published on 5th May 2015


Abstract

In this paper, a series of Na-doped Li2Na2Ti6O14 samples are synthesized by a simple solid-state reaction method through Li-site substitution with Na. Morphology observation shows that all five materials are well crystallized with a particle size in the range of 150–300 nm. Electrochemical analysis shows that Li1.95Na2.05Ti6O14 exhibits lower charge–discharge polarization (0.05 V) than that (0.11 V) of other Li2−xNa2+xTi6O14 samples (x = 0.00, 0.10, 0.15, 0.20). As a result, Li1.95Na2.05Ti6O14 has the highest initial charge capacity of 243.6 mA h g−1, and maintains a reversible capacity of 210.7 mA h g−1 after 79 cycles. For comparison, Li2−xNa2+xTi6O14 (x = 0.00, 0.10, 0.15 and 0.20) samples only hold a reversible capacity of 159.1, 203.5, 190.1 and 156.7 mA h g−1, respectively. Moreover, Li1.95Na2.05Ti6O14 also delivers the best rate performance compared with the other four samples, with a charge capacity of 221.1 mA h g−1 at 200 mA g−1, 211.9 mA h g−1 at 300 mA g−1, and 198.7 mA h g−1 at 400 mA g−1. Besides, the reversible in situ structural evolution proves that Li1.95Na2.05Ti6O14 is a stable host for lithium storage. All the improved electrochemical properties of Na-doped Li2Na2Ti6O14 should be attributed to the Na-doping with low content, which reduces the charge–discharge polarization and improves the ionic conductivity.


1 Introduction

Because of their high energy density, high output working potential and long cycling life, rechargeable lithium ion batteries have been widely used in portable equipment, such as electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles.1,2 But the intrinsically poor safety characteristics hinder the large scale deployment of lithium ion batteries. One safety issue is that dendritic lithium may grow on the surface of the anode at high rates due to the low Li insertion potential of conventional graphite materials approaching 0.0 V versus Li/Li+.3

To replace the carbonous materials, various titanium based oxides have been proposed as high performance anode materials for lithium ion batteries.4–9 Compared to widely reported Li4Ti5O12, Li2MTi6O14 (M = Sr, Ba, Pb) are a novel kind of hosts for lithium storage.10–14 The stable framework, lower resistivity and lower working potential (1.4 V versus 1.55 V of Li4Ti5O12) attract the materials scientists all over the world to make this compound become a promising lithium storage anode material. For instance, the solid state formation mechanism of Li2SrTi6O14 anode is investigated using starting materials of SrCO3, Li2CO3, and anatase TiO2 by ex situ X-ray diffraction (XRD).10 It is found that the optimal calcination parameter to form pure Li2SrTi6O14 is 950 °C for 8 h. To improve the packing density, mesoporous TiO2 brookite is used as a template and reactant in a sol–gel synthesis.11 As a result, Li2SrTi6O14 anode shows impressive performance with a reversible capacity of 120 mA h g−1 at C/14 rate and 92.0 mA h g−1 at 4 C rate.

Isostructural to Li2SrTi6O14, Li2Na2Ti6O14 reveals lower lithiation potential of 1.25 V with open channels in the structure enabling the reversible insertion of lithium ions.15–19 Although Li2Na2Ti6O14 can be obtained at 600 °C by sol–gel method, it shows a low reversible capacity of about 75 mA h g−1.15 To find appropriate preparation parameters, a comparison between sol–gel route and solid state method is undertaken for Li2Na2Ti6O14 at different sintering temperatures.16 It can be found that Li2Na2Ti6O14 prepared by sol–gel method at 700 °C reveals the highest initial charge specific capacity of 106.6 mA h g−1 and the best rate properties than any other samples. To further improve the electrochemical properties, the coating by conductive additives, such as copper/carbon, carbon nanotube, graphene, carbon black, is made on Li2Na2Ti6O14.17,18 As a result, modified Li2Na2Ti6O14 exhibits lower electrochemical polarization, quicker kinetic behavior and improved lithium storage capability compared to bare Li2Na2Ti6O14. However, few investigations have been reported to enhance the electrochemical performance of Li2Na2Ti6O14 by doping.20

In this paper, five Na substituted Li2−xNa2+xTi6O14 (x = 0, 0.05, 0.10, 0.15, 0.20) samples are synthesized by a simple solid-state method. The structure, morphology and electrochemical properties of as-prepared sample are described and compared by using various analytical methods. An investigation is carried out to give new insights of Na doping into the evolutions of structural property, ion diffusion, rate performance, charge–discharge behavior of Li2Na2Ti6O14.

2 Experimental

2.1 Sample preparation

In the sample preparation procedure, all the chemical reagents are of analytical grade. Via a simple solid-state method, Li2−xNa2+xTi6O14 (x = 0, 0.05, 0.10, 0.15, 0.20) samples are prepared using stoichiometric amount of CH3COOLi·2H2O (99.0%), CH3COONa·3H2O (99%) and anatase TiO2 (5–10 nm, 99.8%). The starting materials are mixed by planetary ball milling for 15 hours in ethanol to obtain homogeneous slurry. The resulting slurry is dried at 80 °C for 24 hours, and then calcined at 800 °C for 10 hours in air atmosphere to obtain the final Li2−xNa2+xTi6O14 samples.

2.2 Material characterization

The phase identifications are performed on a Bruker D8 Focus X-ray diffractometer with nickel-filtered Cu Kα radiation (λ = 1.5418 Å), operating at 40 kV and 40 mA. In situ X-ray diffraction (XRD) patterns are collected by the same instrument. The samples are scanned between 5 to 80° (2θ degree) using a scan speed of 0.2° min−1. The surface morphologies of Li2−xNa2+xTi6O14 samples are observed by a L30 S-FEG field emission scanning electron microscope (SEM), conducting at 10 kV.

2.3 Electrode preparation and battery assembly

The working electrodes are prepared by dispersing a mixture of as-prepared active material, carbon black and polyvinylidene fluoride with a weight ratio of 8[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 in N-methyl-2-pyrrolidinone to form homogeneous slurry. After mixing, the slurry is pasted on copper foil evenly and then the film is dried at 120 °C in a vacuum oven for 12 hours, followed by cutting into sheets with a diameter of 15 mm. The two-electrode coin-type cells are assembled in an Ar-filled glove box by using Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.2) film as working electrode, lithium metal foil as counter electrode, Whatman glass fiber as separator and 1 mol L−1 LiPF6 in a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) mixture of ethylene carbonate and dimethyl carbonate as electrolyte.

2.4 Electrochemical evaluation

Galvanostatic charge–discharge tests are conducted on Land CT2001A multiple battery test system at a current density of 100 mA g−1 between 0.0 and 3.0 V. Cyclic voltammograms (CVs) are performed via a computer-controlled CHI 660D electrochemical workstation at a scan rate of 0.1 mV s−1 between 0.0 and 3.0 V. Electrochemical impedance spectroscopy (EIS) analysis is carried out by CHI 660D electrochemical workstation with an oscillating potential of 5 mV in the frequency ranging from 105 to 10−2 Hz. The cells for EIS observation are three-electrode system with metal lithium foils as reference and counter electrodes. All the tests are carried out at room temperature.

3 Results and discussion

Fig. 1 presents the XRD patterns of as-prepared Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) obtained at a temperature of 800 °C. All these patterns are collected on Bruker D8 Focus X-ray diffraction instrument at a room temperature. It can be observed from the XRD curves that the diffraction peak positions and the relative intensities of the prepared samples are well matched with the standard powder diffraction file of JCPDS card no. 52-0690, and it is clear that the diffraction patterns of all samples are similar with each other. Here, a twin peak appears at 65 °C in Fig. 1 corresponding to (824) and (040) peaks. It also shows from the graph that no impurities are observed in Li2Na2Ti6O14 and Na doped Li2Na2Ti6O14 samples. All the results obtained from the above means that the low content Na doping (0 ≤ x ≤ 0.2) does not change the main crystal structure of Li2Na2Ti6O14. For a clear observation, the peak position variation of (024) plane is enlarged and shown in Fig. 1b. The compounds with Na doping content of x = 0.05 and 0.10 exhibit no variation of the diffraction peak, revealing that low Na doping does not affect the lattice parameter of Li2Na2Ti6O14. However, the Bragg positions of the Na-doping Li2−xNa2+xTi6O14 samples with x = 0.15, 0.20 slightly shift to lower diffraction angles based on the enlarged (024) peak in Fig. 1b, indicating the lattice parameter of Li2−xNa2+xTi6O14 gradually increases after Na doping. The increased lattice parameter should be ascribed to the larger Na+ (0.97 Å) replacement of the smaller Li+ (0.68 Å) in the sites.
image file: c5ra04427h-f1.tif
Fig. 1 XRD patterns of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples. (a) Full patterns, (b) local patterns.

Fig. 2 provides the typical SEM photographs of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples. It is apparent that all the five prepared samples are similar with each other, having a relative uniform morphology with narrow size distribution between 150 and 300 nm. It also can be found that the size of Li2Na2Ti6O14 is relatively smaller than another four samples, which may attribute to the bigger Na+ replacement of the smaller Li+ in the structure.


image file: c5ra04427h-f2.tif
Fig. 2 SEM images of Li2−xNa2+xTi6O14 samples. (a and b) x = 0.00, (c and d) x = 0.05, (e and f) x = 0.10, (g and h) x = 0.15, (i and j) x = 0.20.

The CV curves of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) are presented in Fig. 3. All the tests are conducted at room temperature under a scan rate of 0.1 mV s−1 between 0.0 and 3.0 V. As is shown in Fig. 3a–e, a pair of characteristic redox peaks can be observed at around 1.17 and 1.19 V for five samples, regarded as the signature of two lithium-ion per formula insertion into and extraction from the Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) framework,14,16 which is in consistence with the charge–discharge potential plateaus in Fig. 4. According to the formula of Li2−xNa2+xTi6O14, a total of six Ti4+ can be reduced to Ti3+ after a theoretical evaluation. Thus, the broad peak around 0.0–0.1 V is associated with another four lithium-ion formula storage in the compound. And all the CV peaks are narrow and sharp, corresponding to the rapid kinetic process for lithium ions transportation in the lattices. Besides, pristine Li2Na2Ti6O14 and Li1.95Na2.05Ti6O14 exhibit much obvious reduction peaks at 1.178 V and 1.205, respectively, and a sharper oxidation peaks at 1.362 V than other Li2−xNa2+xTi6O14 (0.10 ≤ x ≤ 0.20) samples. The peak current reduces relatively rapid along with the increase of Na doping concentration from 0.05 to 0.20, revealing a drop in the electrode kinetics. Moreover, the initial three CV curves of Li2Na2Ti6O14 and Li1.95Na2.05Ti6O14 coincide with each other and show higher peak current than the other three ones, indicates that Li2Na2Ti6O14 and Li1.95Na2.05Ti6O14 have higher electrochemical reaction activity and reversibility. The difference between anodic and cathodic peaks can be mainly attributed to the slow lithium ion diffusivity in solid-state body of bulk Li2−xNa2+xTi6O14 (0.10 ≤ x ≤ 0.20).13 The potential difference (Δφp) of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) electrodes between oxidation and reduction peaks is listed in Table 1. It is obvious that the potential separation (Δφp value) of pristine Li2Na2Ti6O14 (about 1.82 mV) is much larger than those of Na-doped samples. It suggests that Na doping is beneficial to the reversible intercalation and deintercalation of Li+ in the structure, and then enhances the reversibility of Li2Na2Ti6O14. Furthermore, Li1.95Na2.05Ti6O14 expresses the smallest potential separation among all the materials, indicating that Li1.95Na2.05Ti6O14 may own the best electrochemical performance for its fast electron transfer kinetics and outstanding cycling reversibility.


image file: c5ra04427h-f3.tif
Fig. 3 CVs of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples. (a) Li2Na2Ti6O14, (b) Li1.95Na2.05Ti6O14, (c) Li1.9Na2.1Ti6O14, (d) Li1.85Na2.15Ti6O14, (e) Li1.8Na2.2Ti6O14.

image file: c5ra04427h-f4.tif
Fig. 4 The 1st (a), 10th (b), 20th, (c) 50th (d) and 79th (e) charge–discharge curves of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples at a current density of 100 mA g−1.
Table 1 Potentials of the redox peaks in the CVs for Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples
Samples φpa (V) φpc (V) Δφpa (mV)
a Δφp = φpaφpc.
Li2Na2Ti6O14 1.360 1.178 182
Li1.95Na2.05Ti6O14 1.362 1.205 157
Li1.9Na2.1Ti6O14 1.355 1.190 165
Li1.85Na2.15Ti6O14 1.362 1.198 164
Li1.8Na2.2Ti6O14 1.354 1.186 168


Fig. 4 depicts the charge and discharge curves upon the 1st, 10th, 20th and 50th cycles of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples at the potential range of 0.0–3.0 V. As is shown in Fig. 4a, all above-mentioned compounds exhibit one charge plateau and one discharge plateau, similar to the previous report of Li2Na2Ti6O14.14–17 For the initial cycle of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20), the potential drops rapidly from open-circuit potential to 1.35 V, and then the discharge potential maintains for a long flat platform at a average potential of 1.24 V. Finally, a long slope appears between 0.0 and 1.2 V. During the charging process, the charge plateaus of Li2−xNa2+xTi6O14 (x = 0.00, 0.10, 0.15 and 0.20) are at about 1.34 V. In contrast, the plateau of Li1.95Na2.05Ti6O14 keeps nearly at 1.30 V, and then rapidly increases to the cut-off potential. It reveals that Li1.95Na2.05Ti6O14 has lower charge–discharge polarization (0.05 V) than that (0.11 V) of Li2−xNa2+xTi6O14 (x = 0.00, 0.10, 0.15, 0.20), which is also in correspondence with the results of CV curves. Besides, it is evident that the charge plateau of Li1.95Na2.05Ti6O14 are much longer than those of Li2−xNa2+xTi6O14 (x = 0.00, 0.10, 0.15, 0.20) in all of four graphs. After 50 cycles, Li1.95Na2.05Ti6O14 still exhibits the discharge and charge plateaus at around 1.16 and 1.33 V, respectively. Its charge–discharge polarization remains much lower than those of Li2−xNa2+xTi6O14 (x = 0.00, 0.10, 0.15, 0.20), which further confirm the superior cycling performance of Li1.95Na2.05Ti6O14.

To further conduct investigation of the electrochemical properties for Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20), the cycling performance and corresponding cycling Coulombic efficiencies upon repeated cycles are also shown in Fig. 5. It can be clearly observed that Li1.95Na2.05Ti6O14 express the highest charge capacity of 243.6 mA h g−1 for the first cycle, while Li2Na2Ti6O14, Li1.9Na2.1Ti6O14, Li1.85Na2.15Ti6O14 and Li1.8Na2.2Ti6O14 only deliver the initial charge capacity of 240.1, 229.0, 214.0 and 225.2 mA h g−1, respectively. Besides, Li1.95Na2.05Ti6O14 still remains the charge capacity of 210.7 mA h g−1 after 79 repeated cycles, which proves that appropriate amount Na doping is beneficial to improve the cycling stability of Li2Na2Ti6O14. On the other hand, Li2Na2Ti6O14, Li1.9Na2.1Ti6O14, Li1.85Na2.15Ti6O14 and Li1.8Na2.2Ti6O14 only hold the reversible capacity of 159.1, 203.5, 190.1 and 156.7 mA h g−1, respectively, implying high Na doping content goes against the cycling performance. Therefore, it is known that the superior cycling performance of Li1.95Na2.05Ti6O14 is attributed to the low content Na doping in the structure, which may improve the lithium ion diffusivity and structural stability of Li2Na2Ti6O14. Furthermore, the corresponding cycling Coulombic efficiencies upon repeated cycles are shown in Fig. 5b. Among the five compounds, Li1.95Na2.05Ti6O14 still presents the highest average cycling efficiency of 99.0%, while the others hold the cycling Coulombic efficiency of nearly 94.5–98.0%, which further indicates the outstanding cycling performance of Li1.95Na2.05Ti6O14.


image file: c5ra04427h-f5.tif
Fig. 5 (a) Cycle performance and (b) corresponding Coulombic efficiency of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples.

Fig. 6a displays the Nyquist plots before cycles for the as-prepared Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20). The enlarged Nyquist curves of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) in the high frequency region are also shown in Fig. 6a as an insert graph. All the curves have one semi-circle in the high-frequency region and a straight line in low-frequency region, which are related with the charge transfer process and lithium ion diffusion behavior. The EIS patterns can be used to calculate the lithium diffusion coefficient (DLi), which is an important factor to evaluate the electrochemical kinetics of compounds. Here, the DLi of lithium ion can be calculated by the following equation:4,21

 
image file: c5ra04427h-t1.tif(1)
where R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature for battery testing (298 K), A is the surface area of electrode (1.766 cm2), n is the number of electrons transfer during the electrochemical reaction, F is the Faraday constant (94[thin space (1/6-em)]850 C mol−1), CLi is the concentration of lithium ion in the compound, and the Warburg factor σ can be calculated from the plots in the low-frequency region, and the relationship of σ with Zre is as follows:
 
image file: c5ra04427h-t2.tif(2)


image file: c5ra04427h-f6.tif
Fig. 6 (a) EIS patterns before cycles and (b) corresponding Zre vs. ω−0.5 curves at the low frequency range for Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples.

The Zreσ plots are shown in Fig. 6b, and a linear characteristic could be seen from the curves. Based on the eqn (1) and (2), the calculated diffusion coefficients are listed in Table 2. Observed from Table 2, it is clear that Li1.95Na2.05Ti6O14 possesses the highest lithium ion diffusion coefficient of 1.11 × 10−14 cm2 s−1, while the pristine Li2Na2Ti6O14 only reveals a low lithium ion diffusion coefficient of 1.57 × 10−15 cm2 s−1. With the increase of Na doping content, the lithium ion diffusion coefficient finally reduces to 4.37 × 10−15 cm2 s−1 with x = 2.0 in Li2−xNa2+xTi6O14. It suggests that appropriate amount Na doping is beneficial to improve the lithium ion diffusivity and kinetic behavior of Li2Na2Ti6O14. Therefore, it is expected that Li1.95Na2.05Ti6O14 may have an outstanding rate property.

Table 2 The Li+ diffusion coefficients calculated from EIS patterns for Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples
Samples x = 0.00 x = 0.05 x = 0.10 x = 0.15 x = 0.20
DLi (cm2 s−1) 1.57 × 10−15 1.11 × 10−14 1.13 × 10−14 2.81 × 10−15 4.37 × 10−15
σ (Ω s−0.5) 615.5 226.5 229.2 460.1 368.6


Fig. 7 delivers the initial charge–discharge curves of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples at 200, 300 and 400 mA g−1. It can be found that Li2Na2Ti6O14, Li1.95Na2.05Ti6O14 and Li1.9Na2.1Ti6O14 show a flat plateau at around 1.34 V for the charge plot and a flat plateau at about 1.20 V for the discharge plot. In contrast, Li1.85Na2.15Ti6O14 and Li1.8Na2.2Ti6O14 display the discharge and charge plateaus at 0.98 and 1.48 V, respectively. The results indicate that Li2Na2Ti6O14, Li1.95Na2.05Ti6O14 and Li1.9Na2.1Ti6O14 maintain lower electrochemical polarization than Li1.85Na2.15Ti6O14 and Li1.8Na2.2Ti6O14 during high rate discharge–charge cycles, even at a current density of 400 mA g−1. It also suggests that Li2Na2Ti6O14, Li1.95Na2.05Ti6O14 and Li1.9Na2.1Ti6O14 may have better kinetic properties at high rates.


image file: c5ra04427h-f7.tif
Fig. 7 The comparative charge–discharge curves of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples at different current densities. (a) 200 mA g−1, (b) 300 mA g−1, and (c) 400 mA g−1.

The rate performances of five Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples are presented in Fig. 8. Extended galvanostatic cycles of the Li2−xNa2+xTi6O14 electrodes are performed at increasing current density of 150, 200, 250, 300, 350 and 400 mA g−1. Viewed from Fig. 8, it is obvious that the pristine Li2Na2Ti6O14 only maintains the lithium storage capacity of 206.5 mA h g−1 at 200 mA g−1, 194.6 mA h g−1 at 300 mA g−1, and 186.6 mA h g−1 at 400 mA g−1. With the increase of Na doping content, the rate performance of Li2−xNa2+xTi6O14 shows an improvement at low Na content and then presents a decrease at high Na content. Compared with other four samples, Li1.95Na2.05Ti6O14 apparently delivers a better performance with the charge capacity of 221.1 mA h g−1 at 200 mA g−1, 211.9 mA h g−1 at 300 mA g−1, and 198.7 mA h g−1 at 400 mA g−1. This result is roughly consistent with the cycling performance as shown in Fig. 5a. From what have discussed above, it can be concluded that the incorporation of low sodium content x = 0.05 provokes positive effect on ionic conductivity and the electrochemical properties for Li2Na2Ti6O14.


image file: c5ra04427h-f8.tif
Fig. 8 Rate performance of Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples.

To observe the structural evolution of Na-doped Li2Na2Ti6O14, in situ XRD technique is used to investigate the reversibility of Li1.95Na2.05Ti6O14. The description of in situ cell and its preparation process can be found in our previous paper.22 Fig. 9–11 shows that the characteristic diffraction peaks of Li1.95Na2.05Ti6O14 gradually shift to lower Bragg positions during the lithiation process. With a reverse delithiation reaction, all the featured peaks can go back the original positions. It suggests that the structural change of Li1.95Na2.05Ti6O14 is highly reversible during the charge–discharge process. As a result, the stable host structure of Li1.95Na2.05Ti6O14 can ensure the long-term repeated electrochemical cycles.


image file: c5ra04427h-f9.tif
Fig. 9 Overall in situ XRD patterns of Li1.95Na2.05Ti6O14 during the charge–discharge process.

image file: c5ra04427h-f10.tif
Fig. 10 Selected in situ XRD patterns of Li1.95Na2.05Ti6O14 with the same background.

image file: c5ra04427h-f11.tif
Fig. 11 The evolution of relative intensity versus Bragg position of Li1.95Na2.05Ti6O14 during the charge–discharge process.

4 Conclusions

Via Li-site substitution with Na, five Li2−xNa2+xTi6O14 (0 ≤ x ≤ 0.20) samples are prepared by a simple solid-state method. XRD results reveal that all the as-prepared samples are single-phase without any impurity. Morphological characterization shows that all the five samples are well-crystallized products with a homogeneous particle size between 150 and 300 nm. Electrochemical analysis shows that Li1.95Na2.05Ti6O14 exhibits the highest reversible capacity and the best rate property among all the Li2Na2Ti6O14-type anodes. Compared with other four samples, Li1.95Na2.05Ti6O14 can deliver a better rate performance with the charge capacity of 221.1 mA h g−1 at 200 mA g−1, 211.9 mA h g−1 at 300 mA g−1, and 198.7 mA h g−1 at 400 mA g−1. EIS results prove that the improved lithium storage performance of Na-doped Li2Na2Ti6O14 is related to the improved ionic conductivity and the decreased redox polarization via Li-site substitution with Na. Besides, the reversible structural change of Li1.95Na2.05Ti6O14 as observed by in situ XRD proves its stable host structure for repeated lithium storage.

Acknowledgements

This work is sponsored by Ningbo Key Innovation Team (2014B81005) and Ningbo Natural Science Foundation (2014A610042). The work is also supported by opening project of Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CAS (PCOM201408) and K. C. Wong Magna Fund in Ningbo University.

References

  1. J. M. Tarascon and M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature, 2001, 414, 359–367 CrossRef CAS PubMed.
  2. M. Armand and J. M. Tarascon, Building better batteries, Nature, 2008, 451, 652–657 CrossRef CAS PubMed.
  3. S. S. Zhang, The effect of the charging protocol on the cycle life of a Li-ion battery, J. Power Sources, 2006, 161, 1385–1391 CrossRef CAS PubMed.
  4. T. Yi, H. Liu, Y. Zhu, L. Jiang, Y. Xie and R. Zhu, Improving the high rate performance of Li4Ti5O12 through divalent zinc substitution, J. Power Sources, 2012, 215, 258–265 CrossRef CAS PubMed.
  5. T. Takashima, T. Tojo, R. Inada and Y. Sakurai, Characterization of mixed titanium–niobium oxide Ti2Nb10O29 annealed in vacuum as anode material for lithium-ion battery, J. Power Sources, 2015, 276, 113–119 CrossRef CAS PubMed.
  6. J. Akimoto, K. Kataoka, N. Kojima, S. Hayashi, Y. Gotoh, T. Sotokawa and Y. Kumashiro, A novel soft-chemical synthetic route using Na2Ti6O13 as a starting compound and electrochemical properties of H2Ti12O25, J. Power Sources, 2013, 244, 679–683 CrossRef CAS PubMed.
  7. W. Cho, M. S. Park, J. H. Kim and Y. J. Kim, Interfacial reaction between electrode and electrolyte for a ramsdellite type Li2+xTi3O7 anode material during lithium insertion, Electrochim. Acta, 2012, 63, 263–268 CrossRef CAS PubMed.
  8. J. C. Pérez-Flores, A. Kuhn and F. García-Alvarado, Synthesis, structure and electrochemical Li insertion behaviour of Li2Ti6O13 with the Na2Ti6O13 tunnel-structure, J. Power Sources, 2011, 196, 1378–1385 CrossRef PubMed.
  9. J. Liu, Y. Li, X. Wang, Y. Gao, N. Wu and B. Wu, Synthesis process investigation and electrochemical performance characterization of SrLi2Ti6O14 by ex situ XRD, J. Alloys Compd., 2013, 581, 236–240 CrossRef CAS PubMed.
  10. D. Dambournet, I. Belharouak, J. Ma and K. Amine, Template-assisted synthesis of high packing density SrLi2Ti6O14 for use as anode in 2.7 V lithium-ion battery, J. Power Sources, 2011, 196, 2871–2874 CrossRef CAS PubMed.
  11. J. Liu, X. Sun, Y. Li, X. Wang, Y. Gao, K. Wu, N. Wu and B. Wu, Electrochemical performance of LiCoO2/SrLi2Ti6O14 batteries for high-power applications, J. Power Sources, 2014, 245, 371–376 CrossRef CAS PubMed.
  12. I. Koseva, J. P. Chaminade, P. Gravereau, S. Pechev, P. Peshev and J. Etourneau, A new family of isostructural titanates, MLi2Ti6O14 (M = Sr, Ba, Pb), J. Alloys Compd., 2005, 389, 47–54 CrossRef CAS PubMed.
  13. X. T. Lin, P. Li, L. Y. Shao, M. Shui, D. J. Wang, N. B. Long, Y. L. Ren and J. Shu, Lithium barium titanate: a stable lithium storage material for lithium-ion batteries, J. Power Sources, 2015, 278, 546–554 CrossRef CAS PubMed.
  14. D. Dambournet, I. Belharouak and K. Amine, MLi2Ti6O14 (M = Sr, Ba, 2Na) lithium insertion titanate materials: a comparative study, Inorg. Chem., 2010, 49, 2822–2826 CrossRef CAS PubMed.
  15. S. Y. Yin, L. Song, X. Y. Wang, Y. H. Huang, K. L. Zhang and Y. X. Zhang, Reversible lithium storage in Na2Li2Ti6O14 as anode for lithium ion batteries, Electrochem. Commun., 2009, 11, 1251–1254 CrossRef CAS PubMed.
  16. K. Wu, D. Wang, X. Lin, L. Shao, M. Shui, X. Jiang, N. Long, Y. Ren and J. Shu, Comparative study of Na2Li2Ti6O14 prepared by different methods as advanced anode material for lithium-ion batteries, J. Electroanal. Chem., 2014, 717–718, 10–16 CrossRef CAS PubMed.
  17. K. Wu, J. Shu, X. Lin, L. Shao, M. Lao, M. Shui, P. Li, N. Long and D. Wang, Enhanced electrochemical performance of sodium lithium titanate by coating various carbons, J. Power Sources, 2014, 272, 283–290 CrossRef CAS PubMed.
  18. K. Wu, X. Lin, L. Shao, M. Shui, N. Long, Y. Ren and J. Shu, Copper/carbon coated lithium sodium titanate as advanced anode material for lithium-ion batteries, J. Power Sources, 2014, 259, 177–182 CrossRef CAS PubMed.
  19. K. Wu, J. Shu, X. Lin, L. Shao, P. Li, M. Shui, M. Lao, N. Long and D. Wang, Phase composition and electrochemical performance of sodium lithium titanates as anode materials for lithium rechargeable batteries, J. Power Sources, 2015, 275, 419–428 CrossRef CAS PubMed.
  20. M. Lao, X. Lin, P. Li, L. Shao, K. Wu, M. Shui, N. Long, Y. Ren and J. Shu, Preparation and electrochemical characterization of Li2+xNa2−xTi6O14 (0≤ x ≤0.2) as anode materials for lithium-ion batteries, Ceram. Int., 2015, 41, 2900–2907 CrossRef CAS PubMed.
  21. T. Yi, S. Yang, X. Li, J. Yao, Y. Zhu and R. Zhu, Sub-micrometric Li4−xNaxTi5O12 spinel as anode material exhibiting high rate capability, J. Power Sources, 2014, 246, 505–511 CrossRef CAS PubMed.
  22. J. Shu, M. Shui, D. Xu, Y. L. Ren, D. J. Wang, Q. C. Wang, R. Ma, W. D. Zheng, S. Gao, L. Hou, J. J. Xu, J. Cui, Z. H. Zhu and M. Li, Large-scale synthesis of Li1.15V3O8 nanobelts and their lithium storage behavior studied by in situ X-ray diffraction, J. Mater. Chem., 2012, 22, 3035–3043 RSC.

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