- Email: [email protected]
Enhanced lithium-storage performance of Li4Ti5O12 coated with boron-doped carbon layer for rechargeable Li-ion batteries
Solid State Ionics 324 (2018) 191–195
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Enhanced lithium-storage performance of Li4Ti5O12 coated with borondoped carbon layer for rechargeable Li-ion batteries
T
⁎
Shu Tian , Xiaojun Wang, Jie Yang Department of Physics, Lvliang University, Lvliang, Shanxi 033001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li4Ti5O12 Anode B-doped carbon Rate property Li-ion batteries
In this study, the B-doped carbon coating has been applied to enhance the lithium-storage performance of Li4Ti5O12 anode material for Li-ion batteries. The designed B-doped carbon improved Li4Ti5O12 composite (abbreviated as Li4Ti5O12@C-B) is successfully synthesized using a sol-gel approach followed by a microwave heating route. The XRD and HRTEM results demonstrate that the well-crystallized Li4Ti5O12 particles are uniformly coated with the B-doped carbon layer with a thickness of about several nanometers. The existence of boron doping in the carbon layer is also confirmed by XPS and EDX mapping. Compared with the undoped Li4Ti5O12@C, the Li4Ti5O12@C-B anode shows superior electrochemical performances such as higher reversible capacity and better cycling stability. The enhanced lithium-storage property can be attributed to the doped boron element which could bring many amazing improvements on the carbon coating. Therefore, this novel strategy of B-doped carbon coating can be widely used to improve the electronic conductivity of other electrode materials for electrochemical energy storage.
1. Introduction Nowadays, rechargeable Li-ion batteries with high-energy density and long cycle-life are extending to automotive and stationary energy storage applications such as hybrid electric vehicles (HEVs), electric vehicles (EVs), portable electronic devices and renewable energy integration [1, 2]. As one of the promising candidates, spinel Li4Ti5O12 anode is regarded as the most attractive candidate for substitution of the commercial graphite in energy storage due to its good safety, low cost and excellent cycling stability [3, 4]. Li4Ti5O12 material can accommodate up to 3 Li+ per formula unit, resulting in an end-phase Li7Ti5O12 with a theoretical capacity of 175 mAh g−1 [5]. Furthermore, it shows a flat discharge profile at about 1.55 V (vs. Li+/Li), which makes it safe by avoiding the formation of solid electrolyte interphase film [6, 7]. Unfortunately, the commercialization of Li4Ti5O12 anode is restricted by two problems, including the bad electronic conductivity (ca. 10−13 S cm−1) and sluggish Li+-ion diffusion (ca. −13 −9 2 −1 10 –10 cm s ) [8]. In this context, various approaches have been devoted in the past decades to solve these challenges. One general method is to prepare the nanostructured Li4Ti5O12 particles to reduce the Li+-ion diffusion pathway [9, 10]. Additionally, heteroatom doping such as Co2+ [11], V5+ [12] and Mg2+ [13], has an enhancing effect on the intrinsic
⁎
electronic conductivity for the Li4Ti5O12 material. The most effective approach is to construct the conductive networks by carbon materials [4, 7, 8, 14–16] amount the Li4Ti5O12 particles. Especially, the carbon coating can greatly inhibit the growth of Li4Ti5O12 particles during the annealing process, prevent the direct contact of Li4Ti5O12 with the electrolyte solution and improve the apparent electronic conductivity of Li4Ti5O12. Recently, it is reported that doping boron into the carbon layer can further enhance the electronic conductivity of the bulk [17]. However, to the best of our knowledge, there are rare reports to explain in-depth how B-doping can further enhance the electrochemical property of carbon-coated Li4Ti5O12 composite so far. Herein, the B-doped carbon is adopted to improve the lithium-storage performance of Li4Ti5O12 anode for rechargeable Li-ion batteries. The designed Li4Ti5O12@C-B composite has been synthesized through a sol-gel method followed by a microwave heating route. Compared with the undoped Li4Ti5O12@C electrode, the Li4Ti5O12@C-B shows superior electrochemical performances such as higher reversible capacity and better cycling stability. This can be attributed to the doped boron element which could bring many amazing improvements on the carbon coating. As a result, the B-doped carbon coating is an effective approach to improve the apparent electronic conductivity of Li4Ti5O12 for electrochemical energy storage.
Corresponding author. E-mail address: [email protected] (S. Tian).
https://doi.org/10.1016/j.ssi.2018.07.009 Received 5 June 2018; Received in revised form 8 July 2018; Accepted 8 July 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
Fig. 1. (a) XRD patterns and (b) Raman spectra of Li4Ti5O12@C and Li4Ti5O12@C-B; XPS spectra core level of (c) C1s and (d) B1s for the Li4Ti5O12@C-B composite.
Fig. 2. SEM images of (a) Li4Ti5O12@C and (b) Li4Ti5O12@C-B; (c) The EDX elemental mapping images of Li4Ti5O12@C-B sample.
192
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
Fig. 3. TEM and HRTEM images of (a,b) Li4Ti5O12@C and (c,d) Li4Ti5O12@C-B particles.
2. Experimental
content of carbon.
2.1. Preparation of samples
2.3. Electrochemical measurements
The B-doped carbon-decorated Li4Ti5O12 sample was synthesized using a sol-gel method combined with the microwave heating route. In a typical process, the C16H36O4Ti was firstly dispersed in the ethanol solution with sonication for 30 min. Secondly, the mixture of CH3COOLi, glucose (carbon source) and boric acid (boron source) dissolved in the distilled water and ethanol was drop-wise added into the above suspension under magnetic stirring. Afterwards, the solution was heated at 80 °C under continuous stirring to evaporate the residual distilled water and ethanol until the gel was formed. After drying at 110 °C for 10 h, the obtained precursor was irradiated at 360 °C for 3 min and irradiated at 750 °C for 5 min by using a microwave tube furnace under an argon atmosphere to get the Li4Ti5O12@C-B product. For comparison, the undoped Li4Ti5O12@C sample was also synthesized using the same method without adding the boron source.
The lithium-storage performances of the two anode materials were investigated using the CR2032 cells with lithium metal as the counter electrode and Celgard 2400 as the separator. The 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v) was used as the electrolyte solution. The working electrodes were prepared by casting 85 wt% active material, 10 wt% conductive carbon black and 5 wt% polyvinylidene fluoride (PVDF) in the N-methyl-2pyrrolidone (NMP) solution. Then, the obtained slurry was casted on the clean Cu foil and dried at 100 °C overnight. The average mass loading for the working electrode was about 2.5 mg cm−2. The charge/ discharge tests were carried out between 1.0 and 2.5 V (vs. Li+/Li) at room temperature using a Land-CT2001A tester. The charge and discharge capacities were calculated based on the Li4Ti5O12 material. Electrochemical impedance spectroscopy (EIS) measurement was performed on a CHI 660E electrochemical workstation in the frequency range of 100 kHz–0.01 Hz with the perturbation of 5 mV.
2.2. Structure and morphology characterizations
3. Results and discussion
The crystal structures of the as-synthesized samples were performed through the X-ray diffraction (XRD, Bruker D8/Germany) with Cu Kα radiation (λ = 0.15418 nm). Raman spectrum was carried out by the laser Raman spectrometer with the 514 nm Ar+-ion laser. The morphology was studied through a scanning electron microscopy (SEM, Philips XL 300) equipped with an energy dispersive X-ray spectroscopy (EDX). The microstructures of the obtained composites were also analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL 2100F). XPS of Li4Ti5O12@C-B was conducted with an Al Kɑ source to analyze the chemical valence state of elements. The elemental analysis for the samples was performed to calculate the
Fig. 1a shows the XRD patterns of the as-synthesized Li4Ti5O12@C and Li4Ti5O12@C-B composites. It can be noted that all the diffraction peaks of both samples are indexed well as the spinel-type structure of Li4Ti5O12 (JCPDS No. 49-0207) with the space group of Fd3m. The results are in good agreement with the previously reported literatures [7, 8, 15]. Besides, no other impurities can be observed in the XRD profiles. Meanwhile, no peaks related to the carbon layer are detected for both samples, suggesting that the carbon decomposed from glucose is amorphous. In all, these results reveal that the sol-gel method 193
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
Fig. 4. (a–c) Rate performances of Li4Ti5O12@C and Li4Ti5O12@C-B at different rates between 1.0 and 2.5 V; (d) Nyquist plots of Li4Ti5O12@C and Li4Ti5O12@C-B electrodes.
carbon layer. The TEM and HRTEM images for the Li4Ti5O12@C and Li4Ti5O12@C-B particles are shown in Fig. 3. The Li4Ti5O12@C sample shows spherical shape with a diameter of about 80 nm (Fig. 3a) and the surface of particle is coated with a thin carbon layer (Fig. 3b). As illustrated in Fig. 3c,d, it can be noted that the particle size and the coated carbon layer of Li4Ti5O12@C-B sample are not changed after boron doping. The B-doped carbon layer can significantly improve the electronic conductivity of electrode materials [17, 22]. According to the elemental analysis, the residual carbon contents for Li4Ti5O12@C and Li4Ti5O12@C-B samples are about 4.27 wt% and 4.13 wt%, respectively. Fig. 4a shows the initial charge/discharge curves of Li4Ti5O12@C and Li4Ti5O12@C-B anodes at a low rate of 0.2 C in the potential range of 1.0–2.5 V. It can be found that both electrodes display very flat charge and discharge plateaus at about 1.55 V, revealing the characteristic of two-phase reaction based on the redox couple of Ti4+/Ti3+ [23, 24] via the following reaction: Li4Ti5O12 + 3 Li+ +3 e− ↔ Li7Ti5O12. Differently, the Li4Ti5O12@C-B shows a higher discharge capacity of 169.8 mA h g−1 than the undoped Li4Ti5O12@C anode (166.7 mA h g−1). The high-rate capabilities of the electrodes at various current rates are illustrated in Fig. 4b. Compared with the undoped Li4Ti5O12@C, the Li4Ti5O12@C-B shows higher discharge capacities of 167.2, 162.1, 153.4 and 136.7 mA h g−1 at high rates of 1, 2, 5 and 10 C, respectively. The improved rate performance of Li4Ti5O12@C-B can be assigned to the boron doping, which can bring many amazing improvements on the carbon layer. The boron doping can change the electronic structure and enhance the electrical properties of carbon layer, and break the electroneutrality of carbon material and create many active sites [25, 26]. The introduction of boron also increases the number of hole-type charge carriers, which can greatly enhance the
combined with the microwave heating route is an effective and rapid approach to synthesize the Li4Ti5O12-based electrodes for Li-ion batteries. To prove the existence of carbon layer in the composites, Raman spectra of Li4Ti5O12@C and Li4Ti5O12@C-B were performed as illustrated in Fig. 1b. For both samples, two broad peaks centered at 1347 and 1604 cm−1 are observed, which can be assigned to the D-band and G-band of carbon material, respectively. The D-band is attributed to the disordered graphitic structure, whereas the G-band is due to the existence of graphite carbon [18]. Generally, the intensity ratio of D-band and G-band (ID/IG) can be used to estimate the degree of disorder and defects of carbon material [19, 20]. The Li4Ti5O12@C-B sample has a higher value of ID/IG (1.06) than that of Li4Ti5O12@C (0.94), which means more defects are existed in the B-doped carbon layer [17]. The XPS spectra core level of C1s and B1s of Li4Ti5O12@C-B composite are shown in Fig. 1c,d. As described in Fig. 1c, the C1s peak is deconvoluted into two peaks located at 284.1 and 286.9 eV, representing the C]C and CeO bonds respectively [21]. It is also observed from Fig. 1d that a broad peak at 190.7 eV can be observed, which is assigned to the B1s spectrum [22]. The SEM images of the obtained Li4Ti5O12@C and Li4Ti5O12@C-B particles are shown in Fig. 2a,b. Obviously, it can be seen that both Li4Ti5O12@C and Li4Ti5O12@C-B samples have the small particle size distribution in the range of 70–100 nm. This small particle size is benefit to shorten the diffusion distance of Li+-ion during charge/discharge procedure [9, 10]. The EDX elemental mapping images of Li4Ti5O12@C-B sample are illustrated in Fig. 2c, which indicate that the Ti, O, C and B elements are homogeneously distributed in the composite and the boron heteroatom is successfully doped into the structure of 194
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
conductivity of carbon materials. Moreover, the ultrahigh-rate performance of Li4Ti5O12@C-B at 20 C is also shown in Fig. 4c. It delivers a reversible discharge capacity of 114.1 mAh g−1 for the first cycle and an excellent capacity retention ratio of 96.7% after 100 cycles. To further analysis the impact of B-doped carbon coating on the electrochemical kinetics of Li4Ti5O12, the EIS measurements of Li4Ti5O12@C and Li4Ti5O12@C-B were performed as shown in Fig. 4d. It is clearly that the sharps of Nyquist plots for both electrodes are similar. The inset in Fig. 4d gives an equivalent circuit to simulate the electrochemical impedance spectroscopy data. Herein, the Rs represents the resistance of the electrolyte, electrode as well as separator, corresponding to the intercept at high-frequency; CPE is the double layer capacitance; Rct and Zw represent the charge transfer resistance and Warburg impedance, respectively [27]. The impedance spectra of the two anodes are composed of a depressed semicircle and a straight line, which can be assigned to the charge transfer resistance at the electrode/ electrolyte interface and the solid-state diffusion of ions in the electrode material [27, 28]. Clearly, the Li4Ti5O12@C-B exhibits a lower Rct than the undoped Li4Ti5O12@C electrode, as evidenced by the reduced diameter of the semicircle in the high-frequency region in the EIS curves. The results reveal that the Li4Ti5O12@C-B has low ion transport resistance and high electronic conductivity due to the doped boron element.
electron microscopy, Adv. Mater. 24 (2012) 3233–3238. [6] Y.Q. Wang, L. Gu, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery, J. Am. Chem. Soc. 134 (2012) 7874–7879. [7] H. Liu, G. Wen, P. Gao, Enhanced rate performance of nanosized Li4Ti5O12/graphene composites as anode material by a solid state-assembly method, Electrochim. Acta 171 (2015) 114–120. [8] G.N. Zhu, H.J. Liu, J.H. Zhuang, C.X. Wang, Y.G. Wang, Y.Y. Xia, Carbon-coated nano-sized Li4Ti5O12 nanoporous micro-sphere as anode material for high-rate lithium-ion batteries, Energy Environ. Sci. 4 (2011) 4016–4022. [9] Y. Sun, H. Dong, Y. Xu, Y. Zhang, C. Zhao, D. Wang, Z. Liu, D. Liu, Incorporating cyclized-polyacrylonitrile with Li4Ti5O12 nanosheet for high performance lithium ion battery anode material, Electrochim. Acta 246 (2017) 106–114. [10] Y. Gu, Y. Zhu, Z. Tang, Y. Zhang, Y. Yang, L. Wang, Design and synthesis of dualphase Li4Ti5O12-TiO2 nanoparticles as anode material for lithium ion batteries, Mater. Lett. 131 (2014) 118–121. [11] H.X. Liu, C. Su, X. Li, Y. Guo, Fabrication of N-doped carbon-coated Li4Ti5-xCoxO12 anode for lithium-ion batteries, Solid State Ionics 320 (2018) 113–117. [12] S. Saxena, A. Sil, Role of calcination atmosphere in vanadium doped Li4Ti5O12 for lithium ion battery anode material, Mater. Res. Bull. 96 (2017) 449–457. [13] W. Wang, B. Jiang, W. Xiong, Z. Wang, S.Q. Jiao, A nanoparticle Mg-doped Li4Ti5O12 for high rate lithium-ion batteries, Electrochim. Acta 114 (2013) 198–204. [14] B. Ren, W. Li, A.J. Wei, X. Bai, L. Zhang, Z.F. Liu, Boron and nitrogen co-doped CNT/Li4Ti5O12 composite for the improved high-rate electrochemical performance of lithium-ion batteries, J. Alloys Compd. 740 (2018) 784–789. [15] H.D. Gu, F. Chen, C.B. Liu, J.C. Qian, M. Ni, T.T. Liu, Scalable fabrication of coreshell structured Li4Ti5O12/PPy particles embedded in N-doped graphene networks as advanced anode for lithium-ion batteries, J. Power Sources 369 (2017) 42–49. [16] L. Zheng, X. Wang, Y. Xia, S. Xia, E. Metwalli, B. Qiu, Q. Ji, S. Yin, S. Xie, K. Fang, S. Liang, M. Wang, X. Zuo, Y. Xiao, Z. Liu, J. Zhu, P. Muller-Buschbaum, Y. Cheng, Scalable in situ synthesis of Li4Ti5O12/carbon nanohybrid with supersmall Li4Ti5O12 nanoparticles homogeneously embedded in carbon matrix, ACS Appl. Mater. Interfaces 10 (2018) 2591–2602. [17] C. Wang, Z.Y. Guo, W. Shen, Q.J. Xu, H.M. Liu, Y.G. Wang, B-doped carbon coating improves the electrochemical performance of electrode materials for Li-ion batteries, Adv. Funct. Mater. 24 (2014) 5511–5521. [18] A. Sadezky, H. Muckenhuber, R. Niessner, U. Poschl, Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information, Carbon 43 (2005) 1731–1742. [19] Z.S. Wu, W. Ren, L. Gao, J. Zhao, Z. Chen, B. Liu, D. Tang, B. Yu, C. Jiang, H.M. Cheng, Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation, ACS Nano 3 (2009) 411–417. [20] Z.T. Liu, X.S. Qin, H. Xu, G.H. Chen, One-pot synthesis of carbon-coated nanosized LiTi2(PO4)3 as anode materials for aqueous lithium ion batteries, J. Power Sources 293 (2015) 562–569. [21] L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu, J. Ma, Y. Ma, Z. Hu, Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction, Angew. Chem. Int. Ed. 50 (2011) 7132–7135. [22] X.L. Su, T. Huang, Y.G. Wang, A.S. Yu, Synthesis and electrochemical performance of nano-sized Li4Ti5O12 coated with boron-doped carbon, Electrochim. Acta 196 (2016) 300–308. [23] Y.H. Shang, Outstanding lithium-storage performance of carbon-free Li4Ti5O12 anode material for rechargeable lithium-ion batteries, Solid State Ionics 322 (2018) 39–43. [24] A.J. Wei, W. Li, L.H. Zhang, B. Ren, X. Bai, Z.F. Liu, Enhanced electrochemical performance of a LTO/N-doped graphene composite as an anode material for Li-ion batteries, Solid State Ionics 311 (2017) 98–104. [25] J. Han, L. Zhang, S. Lee, J. Oh, K. Lee, J. Potts, J. Ji, X. Zhao, R. Ruoff, S. Park, Generation of B-doped graphene nanoplatelets using a solution process and their supercapacitor applications, ACS Nano 7 (2013) 19–26. [26] Y. Zhao, L. Yang, S. Chen, X. Wang, Y. Ma, Q. Wu, Y. Jiang, W. Qian, Z. Hu, Can boron and nitrogen co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 135 (2013) 1201–1204. [27] C. Wang, Z.Y. Guo, W. Shen, A.L. Zhang, Q.J. Xu, H.M. Liu, Y.G. Wang, Application of sulfur-doped carbon coating on the surface of Li3V2(PO4)3 composites to facilitate Li-ion storage as cathode materials, J. Mater. Chem. A 3 (2015) 6064–6072. [28] Y.K. Zhou, J. Wang, Y.Y. Hu, R. O'Hayre, Z.P. Shao, A porous LiFePO4 and carbon nanotube composite, Chem. Commun. 46 (2010) 7151–7153.
4. Conclusions In summary, the boron-doped carbon-decorated Li4Ti5O12 composite has been designed and synthesized by using a sol-gel method followed by the microwave heating route. Based on the analysis of XRD, Raman spectrum, HRTEM, XPS and EDX mapping, it can be noted that the well-crystallized Li4Ti5O12 nanocrystals are uniformly coated with carbon layer and the boron element has been successfully doped into the structure of carbon material. Besides, the electrochemical measurements reveal that the as-prepared Li4Ti5O12@C-B shows good lithium-storage performance when used as anode material for Li-ion batteries. It delivers a reversible discharge capacity of 114.1 mAh g−1 at a high rate of 20 C and an excellent capacity retention ratio of 96.7% after 100 cycles. Meanwhile, it also shows a smaller charge transfer resistance than the undoped Li4Ti5O12@C electrode, which can be attributed to the free carriers donated by boron. Thus, we can speculate that the Li4Ti5O12@C-B composite can be used as the next-generation anode material for Li-ion batteries. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Research on advanced materials for Li-ion batteries, Adv. Mater. 21 (2009) 4593–4607. [3] H. Ge, L. Chen, W. Yuan, Y. Zhang, Q.Z. Fan, H. Osgood, D. Matera, X.M. Song, G. Wu, Unique mesoporous spinel Li4Ti5O12 nanosheets as anode materials for lithium-ion batteries, J. Power Sources 297 (2015) 436–441. [4] Z.H. Yu, L. Wang, L.H. Jiang, Design and synthesis of N-doped graphene sheets loaded with Li4Ti5O12 nanocrystals as advanced anode material for Li-ion batteries, Ceram. Int. 42 (2016) 16031–16039. [5] X. Lu, L. Zhao, X. He, R. Xiao, L. Gu, Y.S. Hu, H. Li, Z. Wang, X. Duan, L. Chen, J. Maier, Y. Ikuhara, Lithium storage in Li4Ti5O12 spinel: the full static picture from
195
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Enhanced lithium-storage performance of Li4Ti5O12 coated with borondoped carbon layer for rechargeable Li-ion batteries
T
⁎
Shu Tian , Xiaojun Wang, Jie Yang Department of Physics, Lvliang University, Lvliang, Shanxi 033001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Li4Ti5O12 Anode B-doped carbon Rate property Li-ion batteries
In this study, the B-doped carbon coating has been applied to enhance the lithium-storage performance of Li4Ti5O12 anode material for Li-ion batteries. The designed B-doped carbon improved Li4Ti5O12 composite (abbreviated as Li4Ti5O12@C-B) is successfully synthesized using a sol-gel approach followed by a microwave heating route. The XRD and HRTEM results demonstrate that the well-crystallized Li4Ti5O12 particles are uniformly coated with the B-doped carbon layer with a thickness of about several nanometers. The existence of boron doping in the carbon layer is also confirmed by XPS and EDX mapping. Compared with the undoped Li4Ti5O12@C, the Li4Ti5O12@C-B anode shows superior electrochemical performances such as higher reversible capacity and better cycling stability. The enhanced lithium-storage property can be attributed to the doped boron element which could bring many amazing improvements on the carbon coating. Therefore, this novel strategy of B-doped carbon coating can be widely used to improve the electronic conductivity of other electrode materials for electrochemical energy storage.
1. Introduction Nowadays, rechargeable Li-ion batteries with high-energy density and long cycle-life are extending to automotive and stationary energy storage applications such as hybrid electric vehicles (HEVs), electric vehicles (EVs), portable electronic devices and renewable energy integration [1, 2]. As one of the promising candidates, spinel Li4Ti5O12 anode is regarded as the most attractive candidate for substitution of the commercial graphite in energy storage due to its good safety, low cost and excellent cycling stability [3, 4]. Li4Ti5O12 material can accommodate up to 3 Li+ per formula unit, resulting in an end-phase Li7Ti5O12 with a theoretical capacity of 175 mAh g−1 [5]. Furthermore, it shows a flat discharge profile at about 1.55 V (vs. Li+/Li), which makes it safe by avoiding the formation of solid electrolyte interphase film [6, 7]. Unfortunately, the commercialization of Li4Ti5O12 anode is restricted by two problems, including the bad electronic conductivity (ca. 10−13 S cm−1) and sluggish Li+-ion diffusion (ca. −13 −9 2 −1 10 –10 cm s ) [8]. In this context, various approaches have been devoted in the past decades to solve these challenges. One general method is to prepare the nanostructured Li4Ti5O12 particles to reduce the Li+-ion diffusion pathway [9, 10]. Additionally, heteroatom doping such as Co2+ [11], V5+ [12] and Mg2+ [13], has an enhancing effect on the intrinsic
⁎
electronic conductivity for the Li4Ti5O12 material. The most effective approach is to construct the conductive networks by carbon materials [4, 7, 8, 14–16] amount the Li4Ti5O12 particles. Especially, the carbon coating can greatly inhibit the growth of Li4Ti5O12 particles during the annealing process, prevent the direct contact of Li4Ti5O12 with the electrolyte solution and improve the apparent electronic conductivity of Li4Ti5O12. Recently, it is reported that doping boron into the carbon layer can further enhance the electronic conductivity of the bulk [17]. However, to the best of our knowledge, there are rare reports to explain in-depth how B-doping can further enhance the electrochemical property of carbon-coated Li4Ti5O12 composite so far. Herein, the B-doped carbon is adopted to improve the lithium-storage performance of Li4Ti5O12 anode for rechargeable Li-ion batteries. The designed Li4Ti5O12@C-B composite has been synthesized through a sol-gel method followed by a microwave heating route. Compared with the undoped Li4Ti5O12@C electrode, the Li4Ti5O12@C-B shows superior electrochemical performances such as higher reversible capacity and better cycling stability. This can be attributed to the doped boron element which could bring many amazing improvements on the carbon coating. As a result, the B-doped carbon coating is an effective approach to improve the apparent electronic conductivity of Li4Ti5O12 for electrochemical energy storage.
Corresponding author. E-mail address: [email protected] (S. Tian).
https://doi.org/10.1016/j.ssi.2018.07.009 Received 5 June 2018; Received in revised form 8 July 2018; Accepted 8 July 2018 0167-2738/ © 2018 Elsevier B.V. All rights reserved.
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
Fig. 1. (a) XRD patterns and (b) Raman spectra of Li4Ti5O12@C and Li4Ti5O12@C-B; XPS spectra core level of (c) C1s and (d) B1s for the Li4Ti5O12@C-B composite.
Fig. 2. SEM images of (a) Li4Ti5O12@C and (b) Li4Ti5O12@C-B; (c) The EDX elemental mapping images of Li4Ti5O12@C-B sample.
192
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
Fig. 3. TEM and HRTEM images of (a,b) Li4Ti5O12@C and (c,d) Li4Ti5O12@C-B particles.
2. Experimental
content of carbon.
2.1. Preparation of samples
2.3. Electrochemical measurements
The B-doped carbon-decorated Li4Ti5O12 sample was synthesized using a sol-gel method combined with the microwave heating route. In a typical process, the C16H36O4Ti was firstly dispersed in the ethanol solution with sonication for 30 min. Secondly, the mixture of CH3COOLi, glucose (carbon source) and boric acid (boron source) dissolved in the distilled water and ethanol was drop-wise added into the above suspension under magnetic stirring. Afterwards, the solution was heated at 80 °C under continuous stirring to evaporate the residual distilled water and ethanol until the gel was formed. After drying at 110 °C for 10 h, the obtained precursor was irradiated at 360 °C for 3 min and irradiated at 750 °C for 5 min by using a microwave tube furnace under an argon atmosphere to get the Li4Ti5O12@C-B product. For comparison, the undoped Li4Ti5O12@C sample was also synthesized using the same method without adding the boron source.
The lithium-storage performances of the two anode materials were investigated using the CR2032 cells with lithium metal as the counter electrode and Celgard 2400 as the separator. The 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v) was used as the electrolyte solution. The working electrodes were prepared by casting 85 wt% active material, 10 wt% conductive carbon black and 5 wt% polyvinylidene fluoride (PVDF) in the N-methyl-2pyrrolidone (NMP) solution. Then, the obtained slurry was casted on the clean Cu foil and dried at 100 °C overnight. The average mass loading for the working electrode was about 2.5 mg cm−2. The charge/ discharge tests were carried out between 1.0 and 2.5 V (vs. Li+/Li) at room temperature using a Land-CT2001A tester. The charge and discharge capacities were calculated based on the Li4Ti5O12 material. Electrochemical impedance spectroscopy (EIS) measurement was performed on a CHI 660E electrochemical workstation in the frequency range of 100 kHz–0.01 Hz with the perturbation of 5 mV.
2.2. Structure and morphology characterizations
3. Results and discussion
The crystal structures of the as-synthesized samples were performed through the X-ray diffraction (XRD, Bruker D8/Germany) with Cu Kα radiation (λ = 0.15418 nm). Raman spectrum was carried out by the laser Raman spectrometer with the 514 nm Ar+-ion laser. The morphology was studied through a scanning electron microscopy (SEM, Philips XL 300) equipped with an energy dispersive X-ray spectroscopy (EDX). The microstructures of the obtained composites were also analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL 2100F). XPS of Li4Ti5O12@C-B was conducted with an Al Kɑ source to analyze the chemical valence state of elements. The elemental analysis for the samples was performed to calculate the
Fig. 1a shows the XRD patterns of the as-synthesized Li4Ti5O12@C and Li4Ti5O12@C-B composites. It can be noted that all the diffraction peaks of both samples are indexed well as the spinel-type structure of Li4Ti5O12 (JCPDS No. 49-0207) with the space group of Fd3m. The results are in good agreement with the previously reported literatures [7, 8, 15]. Besides, no other impurities can be observed in the XRD profiles. Meanwhile, no peaks related to the carbon layer are detected for both samples, suggesting that the carbon decomposed from glucose is amorphous. In all, these results reveal that the sol-gel method 193
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
Fig. 4. (a–c) Rate performances of Li4Ti5O12@C and Li4Ti5O12@C-B at different rates between 1.0 and 2.5 V; (d) Nyquist plots of Li4Ti5O12@C and Li4Ti5O12@C-B electrodes.
carbon layer. The TEM and HRTEM images for the Li4Ti5O12@C and Li4Ti5O12@C-B particles are shown in Fig. 3. The Li4Ti5O12@C sample shows spherical shape with a diameter of about 80 nm (Fig. 3a) and the surface of particle is coated with a thin carbon layer (Fig. 3b). As illustrated in Fig. 3c,d, it can be noted that the particle size and the coated carbon layer of Li4Ti5O12@C-B sample are not changed after boron doping. The B-doped carbon layer can significantly improve the electronic conductivity of electrode materials [17, 22]. According to the elemental analysis, the residual carbon contents for Li4Ti5O12@C and Li4Ti5O12@C-B samples are about 4.27 wt% and 4.13 wt%, respectively. Fig. 4a shows the initial charge/discharge curves of Li4Ti5O12@C and Li4Ti5O12@C-B anodes at a low rate of 0.2 C in the potential range of 1.0–2.5 V. It can be found that both electrodes display very flat charge and discharge plateaus at about 1.55 V, revealing the characteristic of two-phase reaction based on the redox couple of Ti4+/Ti3+ [23, 24] via the following reaction: Li4Ti5O12 + 3 Li+ +3 e− ↔ Li7Ti5O12. Differently, the Li4Ti5O12@C-B shows a higher discharge capacity of 169.8 mA h g−1 than the undoped Li4Ti5O12@C anode (166.7 mA h g−1). The high-rate capabilities of the electrodes at various current rates are illustrated in Fig. 4b. Compared with the undoped Li4Ti5O12@C, the Li4Ti5O12@C-B shows higher discharge capacities of 167.2, 162.1, 153.4 and 136.7 mA h g−1 at high rates of 1, 2, 5 and 10 C, respectively. The improved rate performance of Li4Ti5O12@C-B can be assigned to the boron doping, which can bring many amazing improvements on the carbon layer. The boron doping can change the electronic structure and enhance the electrical properties of carbon layer, and break the electroneutrality of carbon material and create many active sites [25, 26]. The introduction of boron also increases the number of hole-type charge carriers, which can greatly enhance the
combined with the microwave heating route is an effective and rapid approach to synthesize the Li4Ti5O12-based electrodes for Li-ion batteries. To prove the existence of carbon layer in the composites, Raman spectra of Li4Ti5O12@C and Li4Ti5O12@C-B were performed as illustrated in Fig. 1b. For both samples, two broad peaks centered at 1347 and 1604 cm−1 are observed, which can be assigned to the D-band and G-band of carbon material, respectively. The D-band is attributed to the disordered graphitic structure, whereas the G-band is due to the existence of graphite carbon [18]. Generally, the intensity ratio of D-band and G-band (ID/IG) can be used to estimate the degree of disorder and defects of carbon material [19, 20]. The Li4Ti5O12@C-B sample has a higher value of ID/IG (1.06) than that of Li4Ti5O12@C (0.94), which means more defects are existed in the B-doped carbon layer [17]. The XPS spectra core level of C1s and B1s of Li4Ti5O12@C-B composite are shown in Fig. 1c,d. As described in Fig. 1c, the C1s peak is deconvoluted into two peaks located at 284.1 and 286.9 eV, representing the C]C and CeO bonds respectively [21]. It is also observed from Fig. 1d that a broad peak at 190.7 eV can be observed, which is assigned to the B1s spectrum [22]. The SEM images of the obtained Li4Ti5O12@C and Li4Ti5O12@C-B particles are shown in Fig. 2a,b. Obviously, it can be seen that both Li4Ti5O12@C and Li4Ti5O12@C-B samples have the small particle size distribution in the range of 70–100 nm. This small particle size is benefit to shorten the diffusion distance of Li+-ion during charge/discharge procedure [9, 10]. The EDX elemental mapping images of Li4Ti5O12@C-B sample are illustrated in Fig. 2c, which indicate that the Ti, O, C and B elements are homogeneously distributed in the composite and the boron heteroatom is successfully doped into the structure of 194
Solid State Ionics 324 (2018) 191–195
S. Tian et al.
conductivity of carbon materials. Moreover, the ultrahigh-rate performance of Li4Ti5O12@C-B at 20 C is also shown in Fig. 4c. It delivers a reversible discharge capacity of 114.1 mAh g−1 for the first cycle and an excellent capacity retention ratio of 96.7% after 100 cycles. To further analysis the impact of B-doped carbon coating on the electrochemical kinetics of Li4Ti5O12, the EIS measurements of Li4Ti5O12@C and Li4Ti5O12@C-B were performed as shown in Fig. 4d. It is clearly that the sharps of Nyquist plots for both electrodes are similar. The inset in Fig. 4d gives an equivalent circuit to simulate the electrochemical impedance spectroscopy data. Herein, the Rs represents the resistance of the electrolyte, electrode as well as separator, corresponding to the intercept at high-frequency; CPE is the double layer capacitance; Rct and Zw represent the charge transfer resistance and Warburg impedance, respectively [27]. The impedance spectra of the two anodes are composed of a depressed semicircle and a straight line, which can be assigned to the charge transfer resistance at the electrode/ electrolyte interface and the solid-state diffusion of ions in the electrode material [27, 28]. Clearly, the Li4Ti5O12@C-B exhibits a lower Rct than the undoped Li4Ti5O12@C electrode, as evidenced by the reduced diameter of the semicircle in the high-frequency region in the EIS curves. The results reveal that the Li4Ti5O12@C-B has low ion transport resistance and high electronic conductivity due to the doped boron element.
electron microscopy, Adv. Mater. 24 (2012) 3233–3238. [6] Y.Q. Wang, L. Gu, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery, J. Am. Chem. Soc. 134 (2012) 7874–7879. [7] H. Liu, G. Wen, P. Gao, Enhanced rate performance of nanosized Li4Ti5O12/graphene composites as anode material by a solid state-assembly method, Electrochim. Acta 171 (2015) 114–120. [8] G.N. Zhu, H.J. Liu, J.H. Zhuang, C.X. Wang, Y.G. Wang, Y.Y. Xia, Carbon-coated nano-sized Li4Ti5O12 nanoporous micro-sphere as anode material for high-rate lithium-ion batteries, Energy Environ. Sci. 4 (2011) 4016–4022. [9] Y. Sun, H. Dong, Y. Xu, Y. Zhang, C. Zhao, D. Wang, Z. Liu, D. Liu, Incorporating cyclized-polyacrylonitrile with Li4Ti5O12 nanosheet for high performance lithium ion battery anode material, Electrochim. Acta 246 (2017) 106–114. [10] Y. Gu, Y. Zhu, Z. Tang, Y. Zhang, Y. Yang, L. Wang, Design and synthesis of dualphase Li4Ti5O12-TiO2 nanoparticles as anode material for lithium ion batteries, Mater. Lett. 131 (2014) 118–121. [11] H.X. Liu, C. Su, X. Li, Y. Guo, Fabrication of N-doped carbon-coated Li4Ti5-xCoxO12 anode for lithium-ion batteries, Solid State Ionics 320 (2018) 113–117. [12] S. Saxena, A. Sil, Role of calcination atmosphere in vanadium doped Li4Ti5O12 for lithium ion battery anode material, Mater. Res. Bull. 96 (2017) 449–457. [13] W. Wang, B. Jiang, W. Xiong, Z. Wang, S.Q. Jiao, A nanoparticle Mg-doped Li4Ti5O12 for high rate lithium-ion batteries, Electrochim. Acta 114 (2013) 198–204. [14] B. Ren, W. Li, A.J. Wei, X. Bai, L. Zhang, Z.F. Liu, Boron and nitrogen co-doped CNT/Li4Ti5O12 composite for the improved high-rate electrochemical performance of lithium-ion batteries, J. Alloys Compd. 740 (2018) 784–789. [15] H.D. Gu, F. Chen, C.B. Liu, J.C. Qian, M. Ni, T.T. Liu, Scalable fabrication of coreshell structured Li4Ti5O12/PPy particles embedded in N-doped graphene networks as advanced anode for lithium-ion batteries, J. Power Sources 369 (2017) 42–49. [16] L. Zheng, X. Wang, Y. Xia, S. Xia, E. Metwalli, B. Qiu, Q. Ji, S. Yin, S. Xie, K. Fang, S. Liang, M. Wang, X. Zuo, Y. Xiao, Z. Liu, J. Zhu, P. Muller-Buschbaum, Y. Cheng, Scalable in situ synthesis of Li4Ti5O12/carbon nanohybrid with supersmall Li4Ti5O12 nanoparticles homogeneously embedded in carbon matrix, ACS Appl. Mater. Interfaces 10 (2018) 2591–2602. [17] C. Wang, Z.Y. Guo, W. Shen, Q.J. Xu, H.M. Liu, Y.G. Wang, B-doped carbon coating improves the electrochemical performance of electrode materials for Li-ion batteries, Adv. Funct. Mater. 24 (2014) 5511–5521. [18] A. Sadezky, H. Muckenhuber, R. Niessner, U. Poschl, Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information, Carbon 43 (2005) 1731–1742. [19] Z.S. Wu, W. Ren, L. Gao, J. Zhao, Z. Chen, B. Liu, D. Tang, B. Yu, C. Jiang, H.M. Cheng, Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation, ACS Nano 3 (2009) 411–417. [20] Z.T. Liu, X.S. Qin, H. Xu, G.H. Chen, One-pot synthesis of carbon-coated nanosized LiTi2(PO4)3 as anode materials for aqueous lithium ion batteries, J. Power Sources 293 (2015) 562–569. [21] L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, Q. Wu, J. Ma, Y. Ma, Z. Hu, Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction, Angew. Chem. Int. Ed. 50 (2011) 7132–7135. [22] X.L. Su, T. Huang, Y.G. Wang, A.S. Yu, Synthesis and electrochemical performance of nano-sized Li4Ti5O12 coated with boron-doped carbon, Electrochim. Acta 196 (2016) 300–308. [23] Y.H. Shang, Outstanding lithium-storage performance of carbon-free Li4Ti5O12 anode material for rechargeable lithium-ion batteries, Solid State Ionics 322 (2018) 39–43. [24] A.J. Wei, W. Li, L.H. Zhang, B. Ren, X. Bai, Z.F. Liu, Enhanced electrochemical performance of a LTO/N-doped graphene composite as an anode material for Li-ion batteries, Solid State Ionics 311 (2017) 98–104. [25] J. Han, L. Zhang, S. Lee, J. Oh, K. Lee, J. Potts, J. Ji, X. Zhao, R. Ruoff, S. Park, Generation of B-doped graphene nanoplatelets using a solution process and their supercapacitor applications, ACS Nano 7 (2013) 19–26. [26] Y. Zhao, L. Yang, S. Chen, X. Wang, Y. Ma, Q. Wu, Y. Jiang, W. Qian, Z. Hu, Can boron and nitrogen co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 135 (2013) 1201–1204. [27] C. Wang, Z.Y. Guo, W. Shen, A.L. Zhang, Q.J. Xu, H.M. Liu, Y.G. Wang, Application of sulfur-doped carbon coating on the surface of Li3V2(PO4)3 composites to facilitate Li-ion storage as cathode materials, J. Mater. Chem. A 3 (2015) 6064–6072. [28] Y.K. Zhou, J. Wang, Y.Y. Hu, R. O'Hayre, Z.P. Shao, A porous LiFePO4 and carbon nanotube composite, Chem. Commun. 46 (2010) 7151–7153.
4. Conclusions In summary, the boron-doped carbon-decorated Li4Ti5O12 composite has been designed and synthesized by using a sol-gel method followed by the microwave heating route. Based on the analysis of XRD, Raman spectrum, HRTEM, XPS and EDX mapping, it can be noted that the well-crystallized Li4Ti5O12 nanocrystals are uniformly coated with carbon layer and the boron element has been successfully doped into the structure of carbon material. Besides, the electrochemical measurements reveal that the as-prepared Li4Ti5O12@C-B shows good lithium-storage performance when used as anode material for Li-ion batteries. It delivers a reversible discharge capacity of 114.1 mAh g−1 at a high rate of 20 C and an excellent capacity retention ratio of 96.7% after 100 cycles. Meanwhile, it also shows a smaller charge transfer resistance than the undoped Li4Ti5O12@C electrode, which can be attributed to the free carriers donated by boron. Thus, we can speculate that the Li4Ti5O12@C-B composite can be used as the next-generation anode material for Li-ion batteries. References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Research on advanced materials for Li-ion batteries, Adv. Mater. 21 (2009) 4593–4607. [3] H. Ge, L. Chen, W. Yuan, Y. Zhang, Q.Z. Fan, H. Osgood, D. Matera, X.M. Song, G. Wu, Unique mesoporous spinel Li4Ti5O12 nanosheets as anode materials for lithium-ion batteries, J. Power Sources 297 (2015) 436–441. [4] Z.H. Yu, L. Wang, L.H. Jiang, Design and synthesis of N-doped graphene sheets loaded with Li4Ti5O12 nanocrystals as advanced anode material for Li-ion batteries, Ceram. Int. 42 (2016) 16031–16039. [5] X. Lu, L. Zhao, X. He, R. Xiao, L. Gu, Y.S. Hu, H. Li, Z. Wang, X. Duan, L. Chen, J. Maier, Y. Ikuhara, Lithium storage in Li4Ti5O12 spinel: the full static picture from
195