The low-temperature (400 °C) coating of few-layer graphene on porous Li₄Ti₅O₁₂viaC₂₈H₁₆Br₂ pyrolysis for lithium-ion batteries

Abstract

Porous Li₄Ti₅O₁₂ coated with few-layer graphene was prepared via the low-temperature pyrolysis of C₂₈H₁₆Br₂ at 400 °C. The coating layer was very thin and uniform. The coated sample shows superior Li storage performance compared with the as-prepared sample. Capacities of 131 and 104 mA h g⁻¹ can be reached at current rates of 5 and 10 C, respectively. Moreover, cyclic performance is significantly improved after coating. The capacity decreases from 144.6 to 124.4 mA h g⁻¹ after 2400 cycles at a current rate of 2 C in a half cellversusLi/Li⁺, with high capacity retention of 86%.

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Lithium-ion batteries (LIBs) have been considered one of the most promising power sources for various types of electric vehicles and large-scale energy storage because of their high energy density, high power density, and environmental friendly features.^1–3 Thus, the search for electrode materials with high power density, long cycle life, and good safety features has become a key issue in the past several years. Spinel Li₄Ti₅O₁₂ has attracted increasing attention as an anode material for LIBs because of the minimal change in its lattice parameter and relatively high voltage during Li⁺ insertion/extraction; these attributes lead to potentially good cyclability and high safety.^4–12 However, pure Li₄Ti₅O₁₂ has limited applications in electric vehicles and large-scale storage because of its low electronic conductivity and moderate Li⁺ diffusion coefficient.^13–15

Strategies such as decreasing particle size and carbon coating have been proposed to solve the problems of pure Li₄Ti₅O₁₂. Carbon coating can enhance the surface electrical conductivity of electrode materials, and decreasing particle size can reduce the Li⁺ diffusion length; both are effective ways of improving rate performance.^16–19 Typically, carbon coating of electrode materials is conducted at over 700 °C.^8,11,20–23 At such a high temperature, some electrode materials, including lithium transition metal oxides, may be reduced or may become unstable during the carbon coating process.²⁴ Thus, few reports on the carbon coating of lithium transition metal oxides have been published. Furthermore, high temperature pyrolysis means more energy consumption, which is contrary to our aim of energy saving. Our group has recently reported porous Li₄Ti₅O₁₂ coated with N-doped carbon derived from ionic liquid at 600 °C, which significantly improved the high rate and cyclic performance of the sample.²⁵ However, this working temperature is still too high for the coating of some lithium transition metal oxides. The low-temperature coating of a highly conductive surface layer has been an essential technology for optimizing poorly conductive and less thermally stable anode and cathode materials for LIBs.

Graphene, a two-dimensional one-atom thick sheet of carbon, has attracted considerable attention in the field of nanoscience because of its high surface area of over 2600 m² g⁻¹, excellent thermal and mechanical properties, and superior electrical conductivity.^26–30 Recently, many graphene based composites have been widely investigated as electrode materials for LIBs and supercapacitors.^31–33 However, most of them are mechanical mixtures of graphene and the active materials or are prepared in a complicated way.

10,10′-Dibromo-9,9′-bianthryl (C₂₈H₁₆Br₂) has a highly symmetrical structure and high carbon content (up to 65.66%). Its molecular structure is shown in Fig. 1. Recently, Cai et al.³⁴ reported the use of C₂₈H₁₆Br₂ as a precursor for the preparation of graphene on the (111) surface of Au and Ag at 400 °C, in which Au and Ag were used as substrates. These may also act as catalysts. As shown in Fig. 1, the precursor undergoes dehalogenation to form single covalent C–C bonds between each monomer to create polymer chains at around 200 °C. Then, the polymer chains are subject to cyclodehydrogenation to form graphene nanoribbons at around 400 °C. Here, we extended this approach to coating the electrode materials with few-layer graphene at a rather low temperature of 400 °C. The rough surfaces of the electrode materials (e.g., Li₄Ti₅O₁₂) were successfully coated with few-layer graphene using C₂₈H₁₆Br₂ as a precursor.

Fig. 1 Scheme for the pyrolysis process of the precursor.

The porous Li₄Ti₅O₁₂ microspheres were prepared by spray drying.³⁵ The primary particle size was about 50 nm, and these nanoparticles aggregated to form porous microspheres (the morphologies of the as-prepared Li₄Ti₅O₁₂ are shown in Fig. S1†). Then, the porous Li₄Ti₅O₁₂ powder was mixed with appropriate proportions of the C₂₈H₁₆Br₂ solution. After the evaporation of the organic solvent, the mixture of C₂₈H₁₆Br₂ and Li₄Ti₅O₁₂ was heat-treated at 400 °C to obtain the resultant composite. The experimental details of the synthetic approach and its characterizations are described in the experimental section in the ESI†.

The powder X-ray diffraction (XRD) patterns of the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂ samples coated with few-layer graphene (Li₄Ti₅O₁₂/graphene) are shown in Fig. S2†. All the peaks of both samples can be indexed in accordance with those of pure Li₄Ti₅O₁₂ (JCPDS card no. 880673). No difference in the patterns between the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples is observed, indicating that the crystalline structure of Li₄Ti₅O₁₂ is maintained after coating via the low-temperature approach at 400 °C.

High resolution transmission electron microscopy (HRTEM) and Raman measurements were carried out to investigate the distribution and composition of the coating layer on the Li₄Ti₅O₁₂ surface. The HRTEM images of the Li₄Ti₅O₁₂/graphene sample are shown in Fig. 2, which clearly reflects that the resultant material is polycrystalline. The crystalline planes of Li₄Ti₅O₁₂ (111) and (400) with crystal spacings of 0.482 and 0.209 nm can be clearly observed. This result further confirms that the crystalline structure is maintained after coating. Interestingly, an ultrathin coating layer is observed on the surface of the Li₄Ti₅O₁₂ sample. The thickness of the coating layer is about 1 nm and the space between the two graphene coating layers is 0.35 nm, which is approximately equal to the plane spacing of graphite. Raman spectroscopy is a powerful tool for detecting carbon and investigating its degree of crystallization. Fig. 3a shows the Raman spectra of the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples. For the Li₄Ti₅O₁₂/graphene sample, some new bands appear while some weaken compared with the as-prepared Li₄Ti₅O₁₂ sample. The new bands located at 1352 and 1614 cm⁻¹ correspond to the D and G bands, respectively, which are typical bands of graphitic materials. The G band is a typical zone center vibration mode of graphite crystallites, corresponding to the ordered sp² bonded carbon, whereas the D band is an edge vibration mode or disorder layer.^36–38 The sharp bands and narrow full width at half maximum of the G and D bands indicate a high degree of graphitization in the carbon coating layer, which is impossible to achieve with other carbon precursors (e.g., sugar) pyrolyzed at such a low-temperature. The inset in Fig. 3a shows the Raman spectrum of the Li₄Ti₅O₁₂/graphene sample at the range of 2000–3000 cm⁻¹. One peak located at 2703 cm⁻¹ corresponds to the 2D band, which is a characteristic band of graphene. On the basis of the Raman spectroscopy and HRTEM results, we can conclude that the coating layer is few-layer graphene. Other new bands, such as those located at 1269 and 1225 cm⁻¹, are in good agreement with those reported by Cai et al.³⁴ The Raman bands of Li₄Ti₅O₁₂ become so weak in the Li₄Ti₅O₁₂/graphene sample because the conductive graphene coating layer reduces the penetration depth of the incident laser. Thermogravimetric/differential thermal analysis (TG/DTA) was performed to estimate the carbon content of the Li₄Ti₅O₁₂/graphene sample. Fig. 3b shows the TG/DTA curves of the Li₄Ti₅O₁₂/graphene sample under an oxygen atmosphere. The exothermic peak located at 460 °C corresponds to the oxidation of carbon. The TG curve shows that the carbon content is 5.91 wt%, which approximates to the theoretical value.

Fig. 2 HRTEM images of the Li₄Ti₅O₁₂/graphene sample.

Fig. 3 (a) Raman spectra of the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples, the inset is the Raman spectrum of the Li₄Ti₅O₁₂/graphene sample at range of 2000–3000 cm⁻¹; (b) TG/DTA curves of the Li₄Ti₅O₁₂/graphene sample.

The rate capability and long cyclic performance of the Li₄Ti₅O₁₂/graphene sample were investigated and compared with those of the as-prepared Li₄Ti₅O₁₂ sample. As shown in Fig. 4a, the Li₄Ti₅O₁₂/graphene sample exhibits a slightly lower capacity at a low current rate than the Li₄Ti₅O₁₂ sample. However, the capacity of the as-prepared Li₄Ti₅O₁₂ sample rapidly decreases with increasing current rate. Although the capacity of the Li₄Ti₅O₁₂/graphene sample is 153 mA h g⁻¹ at a current rate of 0.5 C (0.5 C means 3 mol Li insertion into Li₄Ti₅O₁₂ in 2 h), this capacity can be maintained at 134 and 104 mA h g⁻¹ at current rates of 5 and 10 C, respectively. In contrast, the capacities of the Li₄Ti₅O₁₂ sample are only 61 and 30 mA h g⁻¹ at current rates of 5 and 10 C, respectively. The Li₄Ti₅O₁₂/graphene sample with a 3.51 wt.% carbon content (Fig. S3†) also shows improved rate performance, whose capacities at 5 and 10 C are 134.6 and 77.6 mA h g⁻¹, respectively (Fig. S4†). The long cyclic performance is shown in Fig. 4b. After 2400 cycles, a 124.4 mA h g⁻¹ capacity can be maintained for the Li₄Ti₅O₁₂/graphene sample at a current rate of 2 C, with the capacity retention of 86% and coulombic efficiency of 100% in all cycles except the first cycle. On the other hand, the capacity of the as-prepared Li₄Ti₅O₁₂ sample decreases to 100 mA h g⁻¹ at the same current rate after 120 cycles. Electrochemical impedance spectroscopy measurements were carried out to compare the charge transfer resistances of the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples. Fig. 5 shows the Nyquist plots of both electrodes at a fully charged state of 2.2 V and their fitting results, determined using the equivalent circuit. The experimental values (circles) fit well with the calculated values (lines), and the Rct values (charge transfer resistance) of the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples extracted from the fitting results are 1048 and 172 Ω, respectively. Therefore, the improved rate performance of Li₄Ti₅O₁₂/graphene can be attributed to the decreased resistance after few-layer graphene coating on the surface of Li₄Ti₅O₁₂. The graphene coating layer enhances the surface/interface electrical conductivity of the Li₄Ti₅O₁₂ particles and the electrical contact among particles. After this, the layer facilitates the Li insertion/extraction process, as demonstrated by the decreased charge transfer resistance of the coated sample. In addition, the uniformly coated graphene layers prevent direct contact between the Li₄Ti₅O₁₂ particles and the electrolyte, which may also solve the well-known problem of gas-release in Li₄Ti₅O₁₂ based batteries. Both factors are favorable to achieving excellent long cyclic performance.

Fig. 4 (a) Discharge and charge capacities of the as-prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples at different current rates (△ Li₄Ti₅O₁₂ discharge, ▲ Li₄Ti₅O₁₂ charge, □ Li₄Ti₅O₁₂/graphene discharge and ■ Li₄Ti₅O₁₂/graphene charge); (b) cyclic performance of Li₄Ti₅O₁₂/graphene, the inset is the cyclic performance of the as-prepared Li₄Ti₅O₁₂ sample.

Fig. 5 Nyquist plots for the Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples, as well as the fitting results (line) using an equivalent circuit shown in the lower inset. The higher inset is the enlarged plot.

Furthermore, few-layer graphene coated on Li₂MnO₃ was also prepared using the same approach (see the HRTEM image in Fig. S5†). Preliminary results show that the crystalline structure and chemical state of Mn remain unchanged after few-layer graphene coating. The investigation of the Li storage behavior of the coated sample is still ongoing and will be discussed in a forthcoming paper. The extension of the proposed approach to future work indicates that it is a versatile technique.

In conclusion, we have demonstrated that the graphene resulting from the C₂₈H₁₆Br₂ precursor can also be formed on the rough surfaces of lithium transition metal oxides at a temperature as low as 400 °C (e.g., Li₄Ti₅O₁₂), not only on the (111) surface of Au or Ag.³⁴ The rate performance and long cyclic performance of the Li₄Ti₅O₁₂ sample coated with few-layer graphene are quite promising. The process of this coating approach is relatively simple, and, more importantly, the pyrolysis temperature of the used precursor can be as low as 400 °C. In addition, this new graphene coating approach could easily be extended to other active electrode materials for electrochemical devices.

Acknowledgements

We thank the invaluable discussions with Prof. Michel Armand. This work was supported by funding from “863” Project (2009AA033101), “973” Projects (2010CB833102), NSFC (50972164), Science and Technology Planning Project of Guangdong Province (2010A090602001), CAS project (KJCX2-YW-W26), the 100 Talent Project of the Chinese Academy of Sciences and Scientific Research Key Project Fund of Ministry of Education (109111).

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Footnote

† Electronic supplementary information (ESI) available: SEM images for the as-prepared Li₄Ti₅O₁₂ sample; XRD patterns for the as prepared Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/graphene samples; TG curve and rate performance for the Li₄Ti₅O₁₂/graphene sample with a carbon content of 3.51 wt%; HRTEM image for the Li₂MnO₃/graphene. See DOI: 10.1039/c2ra01263d

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