Facile controlled synthesis of hierarchically structured mesoporous Li₄Ti₅O₁₂/C/rGO composites as high-performance anode of lithium-ion batteries

Abstract

In this paper, a facile strategy is proposed to controllably synthesize mesoporous Li₄Ti₅O₁₂/C nanocomposite embedded in graphene matrix as lithium-ion battery anode via the co-assembly of Li₄Ti₅O₁₂ (LTO) precursor, GO, and phenolic resin. The obtained composites, which consists of a LTO core, a phenolic-resin-based carbon shell, and a porous frame constructed by rGO, can be denoted as LTO/C/rGO and presents a hierarchical structure. Owing to the advantages of the hierarchical structure, including a high surface area and a high electric conductivity, the mesoporous LTO/C/rGO composite exhibits a greatly improved rate capability as the anode material in contrast to the conventional LTO electrode.

Key words： Li₄Ti₅O₁₂; phenolic-resin-based carbon; mesoporous composite; graphene

1 Introduction

As one of the most advanced rechargeable batteries, lithium-ion batteries (LIBs) have attracted tremendous attention in the past decade due to its great application potential in different fields such as electric vehicles, portable electronics, and even large-scale energy storage [1]. Although being well developed for portable electronics and other small-scale devices, there is still a need to make great efforts to further improve the energy density and stability of LIBs for their applications in electrical vehicles. The commercially available LIBs suffer from several technical issues, including unsatisfied cycling stability, capacity and safety, which require screening the alternative electrode materials [2–5]. Considerable research efforts have been made on the anode material, among which spinel Li₄Ti₅O₁₂ (LTO) is treated as the promising one, as it exhibits a flat voltage plateau around 1.5 V (versus Li/Li⁺), which is able to inhibit the formation of SEI and in turn suppress lithium dendrite deposition on the anode surface during the charge/discharge cycling. In addition, the zero-strain characteristics of LTO ensures the excellent reversibility, the outstanding structural stability as well as the fast lithium-ion mobility, which exhibits a great potential for the realization of high-rate LIBs [6–9].

One of the critical issues associated with LTO is the low electronic conductivity (<10⁻¹³ S/cm), for which severe polarization and poor rate-performance would arise when it works as the electrode material. Meanwhile, the conventional micrometer-size LTO particles always deliver a low specific capacity due to the sluggish lithium migration within its bulk structure [10]. Therefore, prior to putting the LTO material into practical applications, the electrochemical performance needs to be improved. For instance, it is reported by Liu et al. that LTO with a particle size of 100 nm is prepared by the two-phase interfacial reaction (cyclohexane/water) sol-gel method. After 50 cycles, the nano-LTO still delivers a discharge capacity of 150 mAh/g and 126.6 mAh/g at 10 C and 20 C [11]. Zhu et al. designed a kind of 3D hollow LTO microspheres by hydrothermal reaction of lithium peroxotitanate complex solution and a calcination treatment. The material delivers a rate capability of 139, 125, and 108 mAh/g at 10, 20, and 30 C. After 1000 cycles at 30 C, it still has a 94% capacity retention [12]. A kind of Li₄Ti₅O₁₂ with a porous morphological structure is prepared by utilizing the combustion method. The capacity of particles obtained at 700°C reaches 125 mAh/g at 10 C [13]. More comparisons are listed in supplementary information.

In Refs. [14,15], carbon-coated composite materials with a mesoporous structure have been designed and synthesized, in which the unique structure with large surface areas and large pore volumes has proven to improve the rate capability of LIBs. The important effect of the carbon coating has also been verified in Refs. [4–6]. Herein, a kind of mesoporous LTO/C composite embedded in graphene matrix (denoted as LTO/C/rGO) as the LIB anode have been proposed and designed, which consists of an LTO core, a phenolic-resin-based carbon shell, and a porous frame constructed by rGO, to facilitate the transport of both lithium ions and electrons. First, the carbon coating is formed by resol, which limits the size of the primary nanoparticles. Then, the GO is assembled by the hydrogen bond effect. Finally, the hierarchical structure is formed by the calcination process. The hierarchical structure offers unique advantages, including the increased electrode-electrolyte interfacial area and the reduced mechanical lattice strain upon lithium insertion/desertion benefited from the mesoporous carbon coated LTO, as well as the formation of a highly conductive network derived from the rGO matrix. The experimental results clearly demonstrate that the mesoporous LTO/C/rGO composite exhibits a much better rate-performance than the conventional LTOs does.

2 Experimental section

2.1 Materials

Phenol, formalin solution (37%, mass fraction), sodium hydroxide, hydrochloric acid, and ethanol were prepared by Shanghai Chemical Industrial Co., Ltd. (Shanghai, China). Tetrabutyl titanate (TBT), CH₃COOLi·2H₂O (99.95%), and F127 (Pluronic block copolymer EO₁₀₆-PO₇₀-EO₁₀₆) were obtained from Sigma-Aldrich Corp.

2.2 Synthesis of Mesoporous LTO/C/rGO Composite

Figure 1 exhibits the schematic diagram of the synthesis process. In short, the carbon coating is formed by adding resol first, which limits the size of the primary nanoparticles. Then, the GO is assembled by hydrogen bond effect. Finally, the hierarchical carbon-coating structure is formed.

In detail, resol precursors (20% (mass fraction) in ethanol) could be obtained by a simple base-catalyzed process according to Refs. [15,16]. Then, the mesoporous LTO/C composite embedded in graphene matrix (LTO/C/rGO) was obtained by the co-assembly process. In brief, the molar ratio of TBT and CH₃COOLi·2H₂O in the ethanol solution was about 0.85: 1. When the tetrabutyl titanate (TBT) is added into the ethanol solution, the hydrolysis reaction happens and TiO₂ is formed. Then, the LTO is synthesized by

{\mathrm{5TiO}}_{\mathrm{2}}+\mathrm{4LiOH}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to {\mathrm{Li}}_{\mathrm{4}}{\mathrm{Ti}}_{\mathrm{5}}{\mathrm{O}}_{\mathrm{12}}+{\mathrm{6H}}_{\mathrm{2}}\mathrm{O}.

Meanwhile, 0.5 g F127 was added into the above mixture solution under strong stirring. Then, 5.28 g of TBT and 5.0 g of resol precursor solution with a certain amount of graphene oxide (GO) were added dropwise (10% of the theoretical mass of LTO) and kept stirring for 5 h. After that, the mixture was transferred to a hood at 50°C overnight, and the obtained composite was subjected to heat treatment at 120°C for 24 h. Finally, the above powder materials were calcined at 700°C in Ar atmosphere with a heating rate of 5°C/min. Using the Ar/H₂ mixture as calcination atmosphere may lead to the formation of oxygen defect [17]. Actually, the carbon will also perform reducibility in high temperature. The effect of oxygen defect on LTO was not considered.

Fig.1 Schematic diagram of the synthesis process of LTO/C/rGO.

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2.3 Characterization

The morphologies and structures of the LTO composite were measured by using scanning electron microscopy (SEM, NERCN-TC-006) and transmission electron microscopy (TEM, NERCN-TC-010-1). The X-ray diffraction patterns of the above composite were recorded by the D/max-2600PC (XRD, NERCN-TC-007). Thermogravimetric analysis (TGA, NERCN-TC-013) was performed at a heating rate of 5°C/min in air atmosphere.

2.4 Electrochemical testing

The electrochemical performance was tested via assembling CR2025 coin cells, which used Li metal as the counter and reference electrode. The cyclic voltammetry (CV) results were recorded on CHI760 electrochemical workstation with a scan rates of 0.1 mV/s. The galvanostatic charge/discharge performances were measured between 1.0 and 3.0 V (versus. Li⁺/Li) through LAND battery stations.

2.5 Results and discussion

Figure 2(a) shows the XRD pattern of carbon-coated LTO composite (LTO/C). The characteristic peaks at 18°, 35°, 43° and 60° represent a typical LTO structure and the other two weak peaks near 25° may be ascribed to the formation of TiO₂, which has been also reported in Refs. [18,19].

Fig.2 Characterization data of LTO/C/rGO

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Therefore, the sign of TiO₂ can be observed in the XRD data. The residual TiO₂ is unavoidable in this process. According to the weak peak in the XRD data, the content of LTO is much more than that of TiO₂. Thus, the effect of TiO₂ will not be discussed. According to the XPS data in Fig. 2(b), the characteristic peaks of Ti at 459.6 eV and 465.2 eV describe the Ti 2p_3/2 and Ti 2p_1/2 of Ti⁴⁺. The peak at 531 eV and 532.9 eV of O 1s could be ascribed to Ti-O and C-O [20]. The XRD and XPS data suggest that the LTO is obtained successfully by the proposed method. According to the TGA results in Fig. 3, there is a main step observed after 500°C which suggests that the pure LTO composite is left. There are three stages before 500°C (0°C–100°C, 100°C–300°C and 300°C–500°C). The mass loss (about 4%) in 0°C–100°C and 100°C–300°C may come from the evaporation of the adsorbed water and crystal water. When the temperature reaches 300°C–500°C, the amorphous carbon starts to break down. rGo is added during the synthesis of LTO/C/rGO composite, for which the mass loss is more than that of the LTO/C composite. The carbon content within LTO/C and LTO/C/rGO is about 10% and 12% (mass fraction) respectively (the part of water is removed). Considering the fact that the only difference between the preparation processes of LTO/C/rGO and LTO/C is the addition of graphene, the graphene content within LTO/C/rGO is 2% (mass fraction) only.

Fig.3 TGA curves of LTO/C and LTO/C/rGO under air atmosphere at a heating rate of 5°C/min.

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Figure 4 demonstrates the SEM images of the LTO/C and LTO/C/rGO composites. As can be observed in Figs. 4(a) and 4(b), the prepared LTO/C composite has a spherical structure, and the corresponding particle size is about 500 nm. After the co-assembly with graphene, a hierarchical structure, consisting of a LTO core, a carbon shell, and a graphene-based porous framework, could be observed in Figs. 4(c) and 4(d). It can be noticed that the LTO/C particles are closely wrapped by the rGO nanosheets. Moreover, the nanaoparticle is only 100–200 nm. The promoted structure not only improves the specific surface area and electric conductivity of LTO, but also protects the structure integrity of the composite electrode during the charge/discharge cycling [21,22].

Fig.4 Typical SEM images of LTO/C and LTO/C/rGO.

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All kinds of the LTO composites are further observed by TEM and HRTEM as manifested in Fig. 5. As can be noticed in Figs. 5(a) and 5(b), the dark and light areas come from the secondary particle of LTO/C and the amorphous carbon layers. The LTO/C composite displays a sphere shape and such large particles are packed by numerous nanoparticles, which is beneficial to form the mesoporous structure and significantly increase the surface area. In detail, the primary nanoparticles shown in Fig. 5(b) display a high LTO crystallinity with the size between 5 and 10 nm, which are coated by the amorphous carbon layers through the carbonization process of phenolic resin [23,24]. As shown in Figs. 5(c) and 5(d), a graphene area among the LTO/C composite is observed, which is consistent with the SEM characterization. As discussed above, the mesoporous LTO/C/rGO shows the potential as LIBs anode to achieve a high rate capability and a good cycling stability due to the greatly increased conductivity and electrode-electrolyte interfacial area [25,26].

Fig.5 Typical TEM and HRTEM images of LTO/C and LTO/C/rGO.

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To explore the electrochemical performances of LTO/C/rGO, CR2025 coin cells are assembled by using as-prepared LTO/C/rGO as the anode and Li metal as the reference electrode. Figure 6 exhibits the CV curves of the first 5 cycles at a scan rate of 0.1 mV/s. Two strong redox peaks near 1.72 V and 1.45 V are observed, which comes from the typical extraction and insertion of lithium ions within the LTO crystal. This phenomenon demonstrates the good electrode kinetics of the LTO/C/rGO electrode [27,28].

Fig.6 CV curves of the initial 5 cycles at a scan rate of 0.1 mV/s.

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Furthermore, the capacity performance of the LTO/C/rGO composite as an anode material is evaluated by the galvanostatic charge/discharge process at different current densities. Figure 7 presents the typical charge/discharge profiles. The specific capacity at 0.1, 1, 2, 5, and 10 C are 175, 162, 153, 147, and 140 mAh/g, respectively. More importantly, at the high rates of 20 and 30 C, the reversible capacities still deliver high values of 127 and 119 mAh/g, which demonstrates the superior rate-performance of LTO/C/rGO.

Fig.7 Typical charge/discharge profiles of LTO/C/rGO composite at different rates.

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To verify the effect of the hierarchical structure in rate capacities, different LTO samples (including bulk LTO, LTO/C, and LTO/C/rGO) are further tested at 1, 2, 5, 10, 20, and 30 C. As displayed in Fig. 8, it is remarkable that the LTO/C/rGO composite always delivers the highest capacities at each current density.

Fig.8 Rate capacities of different electrodes when cycled at the various rates from 1 to 30 C.

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In addition, the cycling performance of the obtained LTO/C/rGO composite anode is evaluated at a high rate of 10 C. As shown in Fig. 9, there is almost no capacity decline even after 100 cycles (the Coulomb efficiency reaches the theoretical maximum), demonstrating the excellent stability of the LTO/C/rGO composite. The improved electrochemical performance could be attributed to the advantages benefited from the hierarchical structure, i.e., the higher electric conductivity from the carbon-coating and graphene matrix, and the larger electrode/electrolyte interface area from the mesoporous structure.

Fig.9 Cycling performance of LTO/C/rGO composite at 10 C.

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3 Conclusions

In this paper, a facile route is proposed for synthesizing mesoporous Li₄Ti₅O₁₂/C composite embedded in graphene matrix via the co-assembly of LTO precursor, GO, and phenolic resin. The obtained LTO/C/rGO composite presents a hierarchical structure, which makes the increased electric conductivity and electrode/electrolyte interface area possible. Due to the unique structure, the LTO/C/rGO composite exhibits a superior rate-performance, which is even able to deliver an ultrahigh capacity of 119 mAh/g at 30 C. The experimental results clearly demonstrate that the LTO/C/rGO composite material is highly suitable for the anode of high-rate LIBs.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0101312) and the National Natural Science Foundation of China (Grant No. 21975157).

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