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
First published on 5th May 2015
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.
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.
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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.
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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.
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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. |
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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. |
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.
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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
![]() | (1) |
![]() | (2) |
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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 Zre–σ 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.
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.
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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.
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.
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Fig. 11 The evolution of relative intensity versus Bragg position of Li1.95Na2.05Ti6O14 during the charge–discharge process. |
This journal is © The Royal Society of Chemistry 2015 |