Nano/micrometer porous conductive network structure Li₄Ti₅O₁₂@C/CNT microspheres with enhanced sodium-storage capability as an anode material

Abstract

Li₄Ti₅O₁₂@C/CNT microspheres, wherein CNTs were firmly anchored to Li₄Ti₅O₁₂@C nanoparticles, were prepared via a facile spray drying method and subsequently annealed in an argon atmosphere, exhibiting long cycling stability (charge/discharge capacities of 85.45/86.18 mA h g⁻¹ after 500 cycles at 500 mA g⁻¹) and excellent rate capability (charge capacity of 61.16 mA h g⁻¹ after 10 cycles at 1000 mA g⁻¹). The special spherical structure design is not only beneficial to improving the structural stability and reaction kinetics of the electrode materials during the long-term extraction–insertion of sodium ions but also supplies numerous interfacial sites to store more sodium ions.

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1. Introduction

Li₄Ti₅O₁₂, with a spinel structure, is a promising anode material of lithium-ion batteries because of its excellent cycling stability, stable discharge voltage platform, and high lithium-ion diffusion coefficient.^1,2 However, its further development is severely restricted by its low theoretical specific capacity and poor electronic conductivity.^3–5 In addition, flatulence occurs due to the side reactions between electrolyte and Li₄Ti₅O₁₂, leading to rapid capacity fading and serious security threats.^6,7 Research shows that the capacity can be improved by reducing the particle size,⁸ and a high rate capability can be realized by coating with a carbon layer⁹ or compositing with CNTs^10,11 with good conductivity. Carbon layers coated on the surface of Li₄Ti₅O₁₂ particles can prevent direct contact between the electrolyte and Li₄Ti₅O₁₂, preventing the appearance of flatulence. Although the reversible capacity and conductivity of Li₄Ti₅O₁₂ have been greatly improved, the healthy development of lithium-ion batteries is strictly limited by the lack of lithium resources.

Sodium-ion batteries have been regarded as an ideal alternative to lithium-ion batteries due to the abundance of sodium resources and their low cost, and have been widely studied in recent years.^12–18 Similar to lithium-ion batteries, CNTs and carbon layers can be used to enhance the sodium-storage capability of Li₄Ti₅O₁₂. A free-standing LTO-C/RGO electrode was prepared by modified vacuum filtration, which exhibited superior rate capability (98.7 mA h g⁻¹ at 5C) and long cycling performance (114 mA h g⁻¹ at 2C after 600 cycles).¹⁹ Wang et al. reported free-standing CNT/Li₄Ti₅O₁₂/C composite nanofibers, achieving a high reversible capacity (119 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹) and excellent rate capability (77 mA h g⁻¹ at 50 0 mA h g⁻¹).²⁰ Although the rate capability and cycling stability have been greatly improved via various effective methods, the yield of the product is too low to satisfy the practical application of sodium-ion batteries. Thus, it is particularly important to find a method for large-scale production. Spray drying is a method for large-scale production that is widely used in the field of lithium-ion batteries.^21–28 However, its application in the field of sodium-ion batteries is rarely reported. A Li₄Ti₅O₁₂@C/rGO electrode was fabricated via the spray drying method, which shows superior sodium storage performance. A capacity retention of 95% is maintained after 1000 cycles at 5C at room temperature. Even when the ambient temperature increases to 60 °C, the capacity retention is still as high as 95% after 500 cycles at 10C.²⁹

Hence, nano/micrometer porous structure Li₄Ti₅O₁₂@C/CNT microspheres were prepared via a facile spray drying method and subsequently annealed in an argon atmosphere in this work. As a comparison, Li₄Ti₅O₁₂ hollow nanospheres were synthesized by the solvothermal method to identify the superiority of the Li₄Ti₅O₁₂@C/CNT microsphere structure. The results of electrochemical performance indicate that the special nano/micrometer porous structure of the Li₄Ti₅O₁₂@C/CNT microspheres is crucial to improving their sodium-storage capability. Compared with the Li₄Ti₅O₁₂ hollow nanospheres, the Li₄Ti₅O₁₂@C/CNT microspheres show superior rate capability and longer cycling life. Thus, the Li₄Ti₅O₁₂@C/CNT microspheres are a promising anode material for sodium-ion batteries.

2. Experimental section

2.1 Chemicals

Tetrabutyl titanate (C₁₆H₃₆O₄Ti), lithium acetate (CH₃COOLi), titanium dioxide (TiO₂), absolute ethanol, and sucrose (C₁₂H₂₂O₁₁) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Carbon nanotubes (CNTs, 95%) were purchased from Chengdu Organic Chemicals Co., Ltd. All the chemical reagents were used without further purification. Deionized water from a Milli-Q system (Millipore, Bedford, MA) was used in all the experiments.

2.2 Fabrication of the Li₄Ti₅O₁₂@C/CNT microspheres

The synthetic process of the Li₄Ti₅O₁₂@C/CNT microspheres is shown in Fig. 1. Firstly, CNTs and deionized water were mixed and stirred for 2 h to obtain a well dispersed CNT dispersion, and sucrose was dissolved in deionized water to prepare a sucrose solution. CH₃COOLi, TiO₂, sucrose solution, and the CNT dispersion were mixed in a suspension. Then, the suspension was sprayed onto a Li₄Ti₅O₁₂@C/CNT microsphere precursor using a spray dryer. Finally, the Li₄Ti₅O₁₂@C/CNT microsphere precursor was annealed under an argon atmosphere at 800 °C for 10 h to synthesize the Li₄Ti₅O₁₂@C/CNT microspheres.

Fig. 1 The synthetic process of the Li₄Ti₅O₁₂@C/CNT microspheres.

2.3 Fabrication of the Li₄Ti₅O₁₂ hollow nanospheres

The synthetic process of the Li₄Ti₅O₁₂ hollow nanospheres is shown in Fig. S1,† which is mainly composed of SiO₂ nanospheres, the SiO₂@Li₄Ti₅O₁₂ nanosphere precursor, and Li₄Ti₅O₁₂ hollow nanospheres.

2.3.1 Fabrication of the SiO₂ nanospheres. Firstly, 32 mL absolute ethanol and 1.5 mL deionized water were mixed and stirred for 30 min to obtain a well dispersed ethanol solution. Then, 35 mL concentrated ammonia was mixed with ethanol solution and stirred for 1 h to prepare a mixed solution. Finally, 3 mL tetraethyl orthosilicate was added to the mixed solution and stirred for 5 h. SiO₂ nanospheres were synthesized via centrifugation and dried at 80 °C for 10 h.

2.3.2 Fabrication of the SiO₂@Li₄Ti₅O₁₂ nanosphere precursor. Firstly, 0.6 g SiO₂ nanospheres were dispersed in 100 mL absolute ethanol and stirred for 2 h to prepare a homogeneous suspension. Secondly, a 20 mL mixed solution containing 0.132 g lithium acetate and 0.850 g tetrabutyl titanate was poured into the suspension and stirred for 2 h at room temperature to obtain a mixed solution. Thirdly, the mixed solution was poured into a hydrothermal autoclave and maintained at 200 °C for 36 h. Finally, the SiO₂@Li₄Ti₅O₁₂ nanosphere precursor powder was fabricated via centrifugation and subsequently dried at 80 °C for 12 h.

2.3.3 Fabrication of the Li₄Ti₅O₁₂ hollow nanospheres. Firstly, the synthesized SiO₂@Li₄Ti₅O₁₂ nanosphere precursor powder was dispersed in a 100 mL 0.5 M KOH solution and stirred for 12 h in a water bath at 60 °C. Then, the product was washed by ultrasonication 3 times, and subsequently dried in a vacuum at 80 °C for 10 h. Finally, the dried powder was annealed under an air atmosphere at 800 °C for 12 h to prepare the Li₄Ti₅O₁₂ hollow nanospheres.

2.4 Material characterization

The crystal structures were examined by X-ray diffraction (XRD, Bruker D8 Advance). The microstructures of the materials were analyzed by scanning electron microscopy (S-3400N) and transmission electron microscopy (JEM2100). The Raman spectra were recorded on a Raman spectrometer (LabRAM HR). The weight loss of the materials was detected by thermogravimetric analysis (Diamond TG/DTA). The N₂ adsorption–desorption isotherms were plotted to analyze the specific surface area (BELSORP-miniII).

2.5 Sodium-ion battery electrochemical measurements

Firstly, the prepared powder, Super P, and sodium carboxymethyl cellulose (weight ratio: 7 : 1 : 2) were mixed to prepare a slurry. Then, the slurry was uniformly coated on copper foil and dried under vacuum at 80 °C for 12 h. Thirdly, coin cells (CR2016) were assembled in a glovebox filled with argon. Sodium metal foil and glass fiber were used as the counter electrode and separator, respectively. NaClO₄ was mixed with ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 v/v) to prepare the electrolyte. The charge/discharge, rate capability, and cycling performance were tested using a Land Battery instrument. Cyclic voltammetry and electrochemical impedance spectroscopy were performed on a electrochemical workstation (CHI604D).

3. Results and discussion

The phases of the Li₄Ti₅O₁₂ nanospheres and Li₄Ti₅O₁₂@C/CNT microspheres are analyzed by X-ray diffraction to estimate the purity of the phase (Fig. 2). The diffraction peaks of the two materials are essentially similar, exhibiting that the crystal structure of Li₄Ti₅O₁₂ has not been changed by the coated carbon layers and composite CNTs. The pure Li₄Ti₅O₁₂ phases are detected and other phases are not found, showing that the Li₄Ti₅O₁₂ nanospheres and Li₄Ti₅O₁₂@C/CNT microspheres are synthesized successfully.

Fig. 2 XRD patterns of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres.

Fig. 3 shows the microstructures of the Li₄Ti₅O₁₂ hollow nanospheres and Li₄Ti₅O₁₂@C/CNT microspheres. The SiO₂ nanospheres (Fig. S2a†) with a diameter of about 200 nm show good dispersion, which is conducive to the coating of the Li₄Ti₅O₁₂ shell. Li₄Ti₅O₁₂ was successfully coated on the surface of the SiO₂ nanospheres, and SiO₂@Li₄Ti₅O₁₂ core–shell nanospheres were successfully prepared (Fig. S2b†). The size of the Li₄Ti₅O₁₂ hollow nanospheres ranges from 50 nm to 300 nm (Fig. 3a), which is beneficial to the size effect. The microspheres with a diameter of 1–10 μm exhibit good dispersion. The outer surface structure of the microspheres consists of Li₄Ti₅O₁₂ nanoparticles and CNTs with a diameter of 50 nm (Fig. S3a†), and a network structure was formed by interweaving between the Li₄Ti₅O₁₂ nanoparticles and CNTs (Fig. 3b and S3b†). In addition, numerous nanometer and micrometer-scale pores were produced due to the interweaving action.

Fig. 3 SEM images of (a) the Li₄Ti₅O₁₂ hollow nanospheres and (b) Li₄Ti₅O₁₂@C/CNT microspheres; inset represents the outer surface structure of a microsphere.

TEM and HRTEM images of the Li₄Ti₅O₁₂ hollow nanospheres and Li₄Ti₅O₁₂@C/CNT microspheres are shown in Fig. 4. The shell thickness of the Li₄Ti₅O₁₂ hollow nanospheres is about 10 nm (Fig. 4a), which is too thin to resist the stress from the long charge and discharge processes. The lattice spacing of 0.486 nm corresponds to the (111) crystal facet of the Li₄Ti₅O₁₂@C/CNT microspheres. Meanwhile, the carbon layers coated on the surface of the Li₄Ti₅O₁₂ nanoparticles measure about 1 nm. The coated carbon layers can not only improve the electroconductibility but also prevent direct contract between the electrolyte and Li₄Ti₅O₁₂ to reduce side effects.

Fig. 4 TEM images of the (a) Li₄Ti₅O₁₂ hollow nanospheres and (b) Li₄Ti₅O₁₂@C/CNT microspheres. (c) HRTEM image of the Li₄Ti₅O₁₂@C/CNT microspheres.

The Raman test results of the Li₄Ti₅O₁₂@C/CNT microspheres are shown in Fig. S4.† The presence of carbon is identified by the D and G peaks of the Li₄Ti₅O₁₂@C/CNT microspheres. An I_D/I_G ratio greater than 1 indicates that the coated carbon layers are highly graphitized. The disordered D-band peak located at 1340 cm⁻¹ and graphitic G-band peak located at 1591 cm⁻¹ represent the crystal defects of carbon atoms and sp³ hybridization, respectively. Abundant defects are conducive to the diffusion of sodium ions. In addition, the defects accelerate the transmission of electrons to improve conductibility. Fig. S5† presents the thermogravimetric curve of the Li₄Ti₅O₁₂@C/CNT microspheres. Three stages of weight loss were found. The 0.27 wt%, 7.38 wt% and 3.74 wt% weight losses are attributed to the loss of adsorbed water, coated carbon layers, and CNTs, respectively, indicating that the carbon layers were successfully coated on the surface of the Li₄Ti₅O₁₂ nanoparticles and the CNTs were successfully composited with the Li₄Ti₅O₁₂ nanoparticles, which is consistent with the results in Fig. 3b and 4b. The N₂ adsorption/desorption isotherms of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres were recorded to test the specific surface area. The specific surface area of the Li₄Ti₅O₁₂ hollow nanospheres is slightly lower than that of the Li₄Ti₅O₁₂@C/CNT microspheres (Fig. S6†), which is the result of the special porous spherical structure. The abundant nanometer and micrometer-scale pores, distributed on the surface of the Li₄Ti₅O₁₂@C/CNT microspheres, are conducive to increasing the contact area between the electrode and electrolyte and increasing the specific surface area.

Fig. 5a exhibits the cycling stability curves of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres at a current density of 100 mA g⁻¹ for 150 cycles. The obvious capacity attenuation phenomenon appeared in the first two cycles, which is the result of the decomposition of the electrolyte. The charge/discharge capacities of the Li₄Ti₅O₁₂@C/CNT microspheres are both significantly higher than those of the Li₄Ti₅O₁₂ hollow nanospheres at different cycles. Moreover, the Li₄Ti₅O₁₂@C/CNT microspheres show superior cycling stability. The charge/discharge capacities of the Li₄Ti₅O₁₂@C/CNT microspheres decreased from 172.1/174.9 mA h g⁻¹ to 122.8/124.1 mA h g⁻¹ after 150 cycles. By contrast, the Li₄Ti₅O₁₂ hollow nanospheres exhibit poorer cycling stability; the charge/discharge capacities decay from 171.4/174.3 mA h g⁻¹ to 82.2/84.6 mA h g⁻¹ after 150 cycles. The longer cycling stability of the Li₄Ti₅O₁₂@C/CNT microspheres is attributed to their special structural design. Moreover, the abundant nanometer and micrometer-scale pores distributed on the surface of the microspheres can effectively relieve the huge volume change from the repeated extraction–insertion of sodium ions. In addition, the CNTs play a key role in enhancing the structural stability of the microspheres. The Li₄Ti₅O₁₂ nanoparticles are firmly anchored by the CNTs, forming microspheres with an extremely stable structure. The cycling stability test results of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres at a current density of 500 mA g⁻¹ for 500 cycles are shown in Fig. S7† to further confirm the superior cycling stability of the Li₄Ti₅O₁₂@C/CNT microspheres. The charge/discharge capacities (85.45/86.18 mA h g⁻¹) of the Li₄Ti₅O₁₂@C/CNT microspheres are higher than those of the Li₄Ti₅O₁₂ hollow nanospheres (13.52/14.07 mA h g⁻¹) after 500 cycles, and the capacity attenuation of the Li₄Ti₅O₁₂@C/CNT microspheres is much lower than that of the Li₄Ti₅O₁₂ hollow nanospheres, indicating the superior cycling stability of the Li₄Ti₅O₁₂@C/CNT microspheres. The rate performances of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres were tested to identify the superior conductibility of the Li₄Ti₅O₁₂@C/CNT microspheres (Fig. 5b). The capacity decay rate of the Li₄Ti₅O₁₂ hollow nanospheres is significantly faster than that of the Li₄Ti₅O₁₂@C/CNT microspheres with the increase in current density. A charge capacity of 61.16 mA h g⁻¹ is maintained at 1000 mA g⁻¹, and a 138.81 mA h g⁻¹ charge capacity is retained when the current density recovers to 50 mA g⁻¹ (Li₄Ti₅O₁₂@C/CNT microspheres). As a comparison, the charge capacity of the Li₄Ti₅O₁₂ hollow nanospheres decays from 132.94 mA h g⁻¹ (50 mA g⁻¹) to 18.32 mA h g⁻¹ (1000 mA g⁻¹), and the charge capacity recovers to 119.38 mA h g⁻¹ when the current density decreases to 50 mA g⁻¹. The excellent rate capability of the Li₄Ti₅O₁₂@C/CNT microspheres can be attributed to the coated carbon layers and CNTs. The carbon layers coated on the surface of the Li₄Ti₅O₁₂ nanoparticles are beneficial to improving the electronic conductivity, promoting the transfer of electrons, and enhancing the rate capability of Li₄Ti₅O₁₂. A spherical conductive network structure was formed due to the synergetic effect of the CNTs and carbon layers, which provides more transmission paths for electrons.

Fig. 5 (a) Cycling curves of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres at a current density of 100 mA g⁻¹, (b) rate performance of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres, (c) charge and discharge profiles of the Li₄Ti₅O₁₂@C/CNT microspheres, and (d) Nyquist plots of Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow spheres.

The charge/discharge curves of the Li₄Ti₅O₁₂@C/CNT microspheres are shown in Fig. 5c. The stable voltage platforms located at 1.07 V and 0.78 V correspond to the charge voltage platform and discharge voltage platform, respectively, implying that the structure of the Li₄Ti₅O₁₂@C/CNT microspheres is extremely stable. On the contrary, the Li₄Ti₅O₁₂ hollow nanospheres display a higher charge voltage platform (1.21 V) and lower discharge voltage platform (0.65 V) due to polarization (Fig. S8†). The polarization of the Li₄Ti₅O₁₂@C/CNT microspheres is effectively alleviated by the coated carbon layers and composited CNTs, which greatly improve the electroconductivity and promote the kinetic properties.

The first three cycles of cyclic voltammetry were performed to analyze the electrochemical reaction process of the Li₄Ti₅O₁₂@C/CNT microspheres during the process of Na-ion insertion–extraction (Fig. S9†). The reduction/oxidation peaks located at 0.72/1.12 V are attributed to the sodium-ion insertion–extraction of the Li₄Ti₅O₁₂ nanoparticles in the first cycle. Compared with the first cycle, the reduction peaks of the last two cycles have been changed significantly, which is due to the irreversible sodium insertion process. The cyclic voltammetry curves of the first three cycles almost overlap, indicating the excellent cycling stability of the Li₄Ti₅O₁₂@C/CNT microspheres.

Electrochemical impedance spectroscopy of the Li₄Ti₅O₁₂@C/CNT microspheres and Li₄Ti₅O₁₂ hollow nanospheres was performed to identify the superiority of the spherical conductive network structure (Fig. 5d). The charge transfer resistance of the Li₄Ti₅O₁₂@C/CNT microspheres is lower than that of the Li₄Ti₅O₁₂ hollow nanospheres, which is attributed to the superior conductivity of the Li₄Ti₅O₁₂@C/CNT microspheres. Compared with the Li₄Ti₅O₁₂ hollow nanospheres, the Li₄Ti₅O₁₂@C/CNT microspheres are composed of carbon layers and CNTs, and a conductive network structure is formed due to the synergy of the carbon layers and CNTs. The spherical structure of the Li₄Ti₅O₁₂@C/CNT microspheres is essentially maintained at 500 mA g⁻¹ for 500 cycles (Fig. S10†), which further indicates the stable spherical structure of the Li₄Ti₅O₁₂@C/CNT microspheres.

4. Conclusions

Herein, Li₄Ti₅O₁₂@C/CNT microspheres with numerous nanometer and micrometer-scale pores were synthesized, which show excellent sodium-storage capability. The charge/discharge capacities of 85.45/86.18 mA h g⁻¹ are maintained after 500 cycles at 500 mA g⁻¹, exhibiting long cycling stability. The CNTs tightly anchored to the Li₄Ti₅O₁₂@C nanoparticles, and the various nanometer/micrometer-scale pores distributed on the surface of the microspheres play a vital role in improving the cycling stability. The CNTs ensure the stability of the spherical structure during the long-term extraction–insertion of sodium ions. The abundant pores are able to effectively relieve the volume expansion from the charge/discharge processes. In addition, a charge capacity of 61.16 mA h g⁻¹ is kept at a high current density of 1000 mA g⁻¹, showing high rate capability, which is the result of the coated carbon layers and CNTs. The carbon layers coated on the Li₄Ti₅O₁₂ nanoparticles greatly improve the electroconductivity of the Li₄Ti₅O₁₂ nanoparticles. Meanwhile, a 3D continuous conductive network is formed via the tandem action of the CNTs. There are correlations between the Li₄Ti₅O₁₂@C nanoparticles. The prepared Li₄Ti₅O₁₂@C/CNT microspheres indicate that the design of the nanometer/micrometer porous conductive network structure is an effective method to enhance the sodium-storage capability, which provides a new idea for the fabrication of anode materials for high-performance sodium-ion batteries.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51725101, 11727807, 51672050, 61790581), the Ministry of Science and Technology of China (973 Project No. 2018YFA0209102) and the Science and Technology research project of Jiangxi Provincial Department of Education (GJJ200338).

References

1. X. L. Jia, Y. F. Kan and X. Zhu, et al., Building flexible Li₄Ti₅O₁₂/CNT lithium-ion battery anodes with superior rate performance and ultralong cycling stability, Nano energy, 2014, 10, 344–352.
2. J. H. Park, S. W. Kang and T. S. Kwon, et al., Spray-drying assisted synthesis of a Li₄Ti₅O₁₂/C composite for high rate performance lithium ion batteries, Ceram. Int., 2018, 44(3), 2683–2690.
3. F. Zhang, F. Y. Yi and T. Meng, et al., In Situ Supramolecular Self-Assembly Assisted Synthesis of Li₄Ti₅O₁₂-Carbon-Reduced Graphene Oxide Microspheres for Lithium-Ion Batteries, ACS Sustainable Chem. Eng., 2019, 7(1), 916–924.
4. W. B. Zhu, Z. Y. Zhuang and Y. M. Yang, et al., Synthesis and electrochemical performance of hole-rich Li₄Ti₅O₁₂ anode material for lithium-ion secondary batteries, J. Phys. Chem. Solids, 2016, 93, 52–58.
5. H. G. Jung, J. Kim and B. Scrosati, et al., Micron-sized, carbon-coated Li₄Ti₅O₁₂ as high power anode material for advanced lithium batteries, J. Power Sources, 2011, 196(18), 7763–7766.
6. I. Belharouak, G. M. Koening and T. Tan, et al., Performance Degradation and Gassing of Li₄Ti₅O₁₂/LiMn₂O₄ Lithium-Ion Cells, J. Electrochem. Soc., 2012, 159, A1165–A1170.
7. J. Li, S. Huang and S. F. Li, et al., Synthesis and electrochemical performance of Li₄Ti₅O₁₂/Ag composite prepared by electroless plating, Ceram. Int., 2017, 43(2), 1650–1656.
8. Z. X. Sun, L. J. Sun and S. W. Koh, et al., Photovoltaic-powered supercapacitors for driving overall water splitting: A dual-modulated 3D architecture, Carbon Energy, 2022, 1–12.
9. G. N. Zhu, H. J. Liu and J. H. Zhuang, et al., Carbon-coated nano-sized Li₄Ti₅O₁₂ nanoporous micro-sphere as anode material for high-rate lithium-ion batteries, Energy Environ. Sci., 2011, 4(10), 4016–4022.
10. B. Q. Hu, X. S. Zhou and J. Xu, et al., Excellent Rate and Low Temperature Performance of Lithium-Ion Batteries based on Binder-Free Li₄Ti₅O₁₂Electrode, Chemelectrochem, 2020, 7, 3,  DOI:10.1002/celc.201901914.
11. L. Deng, W. H. Yang and S. X. Zhou, et al., Effect of carbon nanotubes addition on electrochemical performance and thermal stability of Li₄Ti₅O₁₂ anode in commercial LiMn₂O₄/Li₄Ti₅O₁₂ full-cell, Chin. Chem. Lett., 2015, 26(12), 1529–1534.
12. G. Hasegawa, K. Kanamori and T. Kiyomura, et al., Hierarchically Porous Li₄Ti₅O₁₂ Anode Materials for Li- and Na-Ion Batteries: Effects of Nanoarchitectural Design and Temperature Dependence of the Rate Capability, Adv. Energy Mater., 2015, 5(1), 1400730.
13. S. Ghosh, S. Mitra and P. Barpanda, et al., Sonochemical Synthesis of Nanostructured Spinel Li₄Ti₅O₁₂ Negative Insertion Material for Li-ion and Na-ion Batteries, Electrochim. Acta, 2021, 392, 139026.
14. S. H. Gong, J. H. Lee and D. W. Chun, et al., Effects of Cr doping on structure and electrochemical properties of Li₄Ti₅O₁₂ nanostructure for sodium-ion battery anode, J. Energy Chem., 2021, 59, 465–472.
15. M. Kitta, T. Kojima and R. Kataoka, et al., Realizing the Single-Phase Spinel-Type Sodium Titanium Oxide with the Li₄Ti₅O₁₂-like Structure for Building Stable Sodium-Ion Batteries, ACS Appl. Mater. Interfaces, 2020, 12(8), 9322–9331.
16. G. B. Xu, L. W. Yang and X. L. Wei, et al., MoS₂-Quantum-Dot-Interspersed Li₄Ti₅O₁₂ Nanosheets with Enhanced Performance for Li- and Na-Ion Batteries, Adv. Funct. Mater., 2016, 26(19), 3349–3358.
17. C. J. Chen, H. H. Xu and T. F. Zhou, et al., Integrated Intercalation-Based and Interfacial Sodium Storage in Graphene-Wrapped Porous Li₄Ti₅O₁₂ Nanofibers Composite Aerogel, Adv. Energy Mater., 2016, 6(13), 1600322.
18. M. Kitta, R. Kataoka and S. Tanaka, et al., Spinel-Type Sodium Titanium Oxide: A Promising Sodium-Insertion Material of Sodium-Ion Batteries, ACS Appl. Mater. Interfaces, 2019, 2(6), 4345–4353.
19. G. B. Xu, Y. Tian and X. L. Wei, et al., Free-standing electrodes Composed of carbon-coated Li₄Ti₅O₁₂ nanosheets and reduced graphene oxide for advanced sodium ion batteries, J. Power Sources, 2017, 337, 180–188.
20. J. Q. Wang, W. H. Li and Z. Z. Yang, et al., Free-standing and binder-free sodium-ion electrodes based on carbon-nanotube Li₄Ti₅O₁₂ nanoparticles embedded in carbon nanofibers, RSC Adv., 2014, 4(48), 25220–25226.
21. T. Lan, Q. Qiao and F. X. Hu, et al., Electrochemical Oscillation during the Galvanostatic Charging of Li₄Ti₅O₁₂ in Li-Ion Batteries, J. Phys. Chem. C, 2021, 125(27), 14549–14558.
22. J. Xu, D. M. Zhang and Z. P. Zhang, et al., A high performance all-vanadate-based Li-ion full cell, J, Mater. Chem. A, 2021, 9(6), 10345–10353.
23. C. W. Chang-Jian, B. C. Ho and C. K. Chung, et al., Doping and surface modification enhance the applicability of Li₄Ti₅O₁₂ microspheres as high-rate anode materials for lithium ion batteries, Ceram. Int., 2018, 44(18), 23063–23072.
24. W. C. Chien, Z. H. Wu and Y. C. Hsieh, et al., Electrochemical performance of Li₄Ti₅O₁₂ anode materials synthesized using a spray-drying method, Ceram. Int., 2020, 46(17), 26923–26935.
25. T. Lan, Q. Qiao and F. X. Hu, et al., Electrochemical Oscillation during the Galvanostatic Charging of Li₄Ti₅O₁₂ in Li-Ion Batteries, J. Phys. Chem. C, 2021, 125(27), 14549–14558.
26. J. H. Park, S. W. Kang and T. S. Kwon, et al., Spray-drying assisted synthesis of a Li₄Ti₅O₁₂/C composite for high rate performance lithium ion batteries, Ceram. Int., 2018, 44(3), 2683–2690.
27. E. C. Cho, C. W. Chang-Jian and J. A. Chou, et al., MWCNT-embedded Li₄Ti₅O₁₂ microspheres interfacially modified with polyaniline as ternary composites for high-performance lithium ion battery anodes, Ceram. Int., 2020, 46(5), 6801–6810.
28. D. B. Ruan, M. S. Kim and B. Yang, et al., 700 F hybrid capacitors cells composed of activated carbon and Li₄Ti₅O₁₂ microspheres with ultra-long cycle life, J. Power Sources, 2017, 366, 200–206.
29. H. K. Roh, G. W. Lee and S. Haghighat, Polyol-mediated carbon-coated Li₄Ti₅O₁₂ nanoparticle/graphene composites with long-term cycling stability for lithium and sodium ion storages, Chem. Eng. J., 2020, 385, 123984.

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Footnote

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra04977e

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