Reduced Graphene Oxide Composites

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Preparation of Lithium Titanate/Reduced Graphene Oxide Composites with 3D “Fishnet-Like” Conductive Structure via a Gas-Foaming Method for High-Rate Lithium-Ion Batteries Tao Meng, Fenyun Yi, Honghong Cheng, Junnan Hao, Dong Shu, Shixu Zhao, Chun He, Xiaona Song, and Fan Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15525 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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ACS Applied Materials & Interfaces

Preparation of Lithium Titanate/Reduced Graphene Oxide Composites with 3D “Fishnet-Like” Conductive Structure via a Gas-Foaming Method for High-Rate Lithium-Ion Batteries Tao Meng a, Fenyun Yi a,d, Honghong Cheng a, Junnan Hao a, Dong Shu a,b,d*, Shixu Zhao a, Chun He c, Xiaona Song a, Fan Zhang a a

School of Chemistry and Environment, South China Normal University, Guangzhou

510006, P. R. China b

Engineering Research Center of Materials and Technology for Electrochemical

Energy Storage (Ministry of Education) , Guangzhou 510006, P. R. China c

School of Environmental Science and Engineering, Sun Yat-sen University,

Guangzhou 510275, P. R. China d

Base of Production, Education & Research on Energy Storage and Power Battery of

Guangdong Higher Education Institutes, Guangzhou 510006, P. R. China *Corresponding author E-mail: [email protected] (Dong Shu).

Keywords: lithium titanate; graphene; fishnet-like; ammonium chloride; gasfoaming method Abstract Using

ammonium

chloride

(NH4Cl)

as

the

pore-forming

agent,

three-dimensional (3D) “fishnet-like” lithium titanate/reduced graphene oxide (LTO/G) composites with hierarchical porous structure are prepared via a gas-foaming method. SEM and TEM images show that in the composite prepared with the NH4Cl 1

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concentration of 1 mg mL−1 (1-LTO/G), LTO particles with sizes of 50~100 nm disperse homogeneously on the 3D “fishnet-like” graphene. The nitrogen-sorption analyses reveal the existence of micro/mesopores, which is attributed to the introduction of NH4Cl into the gap between the graphene sheets that further decomposes into gases and produces hierarchical pores during the thermal treatment process. The loose and porous structure of 1-LTO/G composites enables the better penetration of electrolytes, providing more rapid diffusion channels for lithium ion. As a result, the 1-LTO/G electrode delivers an ultrahigh specific capacity of 176.6 mAh g−1 at a rate of 1 C. Even at 3 C and 10 C, the specific capacity can reach 167.5 and 142.9 mAh g−1, respectively. Moreover, the 1-LTO/G electrode shows excellent cycle performance with 95.4% capacity retention at 10 C after 100 cycles. The results demonstrate that the LTO/G composite with these properties is one of the most promising anode materials for lithium-ion batteries. 1. Introduction With the depletion of oil resource and the deterioration of the environmental pollution, development of renewable energy is imminent.1,2 In numerous emerging energy technologies, the lithium-ion battery (LIB) is a more reliable one with high energy density, superior cycle stability and benign towards the environment.3-5 The spinel lithium titanate (LTO) is regarded as a promising anode material for high-performance LIBs.6-9 It shows zero crystal strain due to its less than 0.2% of volume change during the insertion/extraction of Li+, leading to the superior cycle stability.10-12 In addition, it exhibits a high operating voltage of nearly 1.55 V (vs. 2


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Li/Li+), avoiding the formation of a solid-electrolyte interface (SEI) layer on the electrode surface.13,14 Moreover, under a high operating voltage, several issues such as the lithium dendrite deposition on the anode surface can be overcome, enabling its high safety. However, pristine LTO suffers from a relatively low theoretical specific capacity (175 mAh g−1), a poor Li+ conductivity (< 10-8 cm2 S−1),8 and a low electronic conductivity (< 10−13 S cm−1).15 These drawbacks lead to the poor rate performance, limiting its large-scale application in energy storages. Up to now, tremendous efforts have been made to solving the above-mentioned problems, including reducing the particle size of LTO,16,17 doping ions,18,19 and coating active materials on the surface of LTO.20,21 Among these strategies, the preparation of composite materials of LTO combined with carbons is a very effective way to enhance the electronic conductivity of LTO, thereby improving the rate performance. Graphene, an atomically thin two-dimensional carbonaceous material, has a high surface area (~2600 m2 g−1), an admirable electronic conductivity, excellent thermal properties, and a high mechanical strength.22-25 Consequently, it has been regarded as an ideal carbon source to enhance the electrochemical properties of LTO.26,27 In order to fully use the large surface area of graphene and expand the application of graphene in different fields, 3D graphene with the crumpled or foam structure has received widespread attention. For example, Niu et al. synthesized 3D porous graphene hybrid architectures decorated with a variety of materials by a hydrothermal method, enhancing their specific applications in both photocatalysis and energy storage.28 To date, many synthesis approaches of LTO/graphene (LTO/G) 3



composites, such as sol-gel methods, solid state routes, and hydrothermal methods, have been intensively investigated. For instance, Liu et al. prepared LTO/G composites (G = 0.5 wt.%) via a sol-gel method, which could deliver a specific capacity of 170.7 mAh g−1 at 1 C.29 Oh et al. reported a composite of graphene-wrapped LTO particles (G = 5 wt.%) prepared by a solid state route that could exhibit a discharge capacity of 122 mAh g−1 at a high rate of 30 C.30 Chen et al. employed a small quantity of graphene sheets (G = 1.2 wt.%) to modify LTO nano-particles by a hydrothermal method. The obtained LTO/G composites displayed a capacity of 187 mAh g−1 at 1 C.31 However, the above-mentioned LTO/G composites are almost stacked tightly so that the electrolyte could hardly fully infiltrate, hindering the redox reaction of lithium ions. Besides, it is challenging to make nano-sized LTO particles uniformly attach on the graphene sheets to form homogeneous LTO/G composites. Actually, the existence of hierarchical pores can help loose the material to form a 3D conductive network structure. Additionally, the electrolyte can penetrate the electrode material more deeply, providing more rapid diffusion channels for lithium ion and further improving the electrochemical performance. In this work, using NH4Cl as the pore former, we designed a 3D “fishnet-like” LTO/G composite with a hierarchical porous structure via a gas-foaming method. A small amount of graphene employed in composites can not only increase the electronic conductivity and the specific surface area, but also limit the secondary growth of LTO nano-sized particles during the calcination process. In particular, the 4


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unique loose 3D “fishnet-like” structure can avoid the restacking of graphene so that electrolyte can fully infiltrate electrode materials, providing more rapid diffusion channels for lithium ions and significantly improving the rate performance. In the fabrication process of LTO/G composites, it is found that the concentration of NH4Cl solution has a great influence on electrochemical properties. The preparation of LTO/G through gas-foaming method, to our best knowledge, has not been reported yet. In addition, the influence of different foaming agent concentrations on electrochemical properties of composite materials was discussed. Notably, the as-prepared sample (1-LTO/G) with the NH4Cl concentration of 1 mg mL−1 exhibited the best electrochemical performance, which delivered an ultrahigh discharge capacity of 176.6 mAh g−1 at 1 C, a superior cyclic stability with a capacity retention of 95.4%, and approximately 100% Coulombic efficiency after 100 cycles at 10 C. 2. Experimental 2.1 Material fabrication The fabrication process of LTO/G composites is shown in Figure 1. First, 0.87 g LiOH·H2O was dissolved in 50 mL ethanol to form solution A. 8.5 mL tetrabutyl titanate (TBT) was dispersed in 30 mL ethanol to form solution B. Then, solution A was added dropwise into solution B with vigorously stirring for 3 h until the yellow transparent solution was gradually transformed into a white suspension C. Graphite oxide (GO) was obtained from nature flake graphite powders via a modified Hummers method.32 60 mg GO was dispersed in 60 mL deionized water with ultrasonic for 5 h to form a colloidal suspension. Subsequently, the GO suspension 5



was centrifuged and the upper liquid was added dropwise to suspension C. After 1 h of the ultrasonic treatment, the mixture was transferred into a polytetrafluoroethylene (PTFE)-lined autoclave and then placed into a microwave reactor at 180 °C for 30 min. After cooling down to room temperature, 50 mL NH4Cl solutions with different concentrations (0, 1, and 3 mg mL−1) were added dropwise into the above mixture with further ultrasonic for 5 h. Then, the mixture was dried in an oven at 80 °C for 24 h. The composites were calcined at 800 °C for 12 h under a nitrogen atmosphere to obtain LTO/G composites. The as-prepared samples with the NH4Cl concentration of 0, 1, and 3 mg mL−1 were denoted as 0-LTO/G, 1-LTO/G, and 3-LTO/G, respectively. For comparison, the pristine LTO material was calcined at 800 °C for 12 h in the air without the addition of GO and NH4Cl.

Figure 1. Scheme illustration of the preparation of LTO/G materials. 2.2 Material characterization

6


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The crystalline phases were examined via powder X-ray diffraction (XRD) on a D/MAX 2200 VPC X-ray generator with the 2θ ranged from 5o to 80o. Raman spectroscopy was conducted with a Renishaw InVia Raman spectrometer using the laser excitation at 633 nm. The morphology and microstructure of samples were investigated by a scanning electron microscope (SEM, ZEISS Ultra-55), a transmission electron microscope (TEM, JEM-2100HR), and a high-resolution transmission electron microscope (HRTEM). The specific surface area and pore size distribution were analyzed through nitrogen adsorption-desorption techniques (ASAP-2020). The content of graphene in the sample was determined by thermogravimetric analysis (TGA, STA409PC) under air atmosphere from 30 °C to 700 °C at a heating rate of 10 °C min−1. 2.3 Electrochemical measurements The electrochemical performance tests were carried out using CR2032-type coin cells. The anode electrode was constructed by mixing the active material, polyvinylidene fluoride (PVDF), and acetylene black (Super P) at a weight ratio of 8: 1: 1 in the N-methyl-pyrrolidone (NMP) solvent to obtain a uniform slurry. Subsequently, the slurry was homogeneously pasted onto a pure Cu foil and dried in vacuum oven at 60 °C for 24 h. After that, the Cu foil with active materials was cut into a disc with the diameter of 14 mm. A lithium metal foil was used as the counter electrode and the separator is Celgard 2500 membrane. The electrolyte was composed of 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC = 1: 1, in volume). The cells were assembled in a glove box filled with 7



argon gas. After aging for 12 h, the electrochemical performance of cells was tested. The galvanostatic discharge-charge tests were performed at cut-off voltages ranged from 1.0 to 3.0 V by a Land CT2001 battery tester. Cyclic voltammogram (CV) was recorded from 1.0 to 3.0 V at different scan rates on an electrochemical workstation (CHI660E). Electrochemical impedance spectroscopy (EIS) data were collected from the same workstation by applying a sine wave with an amplitude of 5 mV over the frequency ranged from 10-2 to 105 Hz. 3. Results and Discussion The XRD spectra of as-prepared GO, pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G are shown in Figure 2a. The diffraction peak at 11° (2θ) is the characteristic peak of GO.33,34 The peaks of pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G samples are quite consistent without any impurity-related peaks, indicating that the addition of GO has no effect on the crystal structure of LTO. The strong and sharp peaks reveal that LTO in the composites is highly crystalline, which are corresponding to (111), (311), (400), (331), (333), (400), and (531) planes of a face-centered cubic spinel structure with Fd-3m space group, respectively.35 No obvious diffraction peaks of carbon are detected in 0-LTO/G, 1-LTO/G and 3-LTO/G samples, which is possibly due to the low content of graphene in the composites. To prove this, TGA analyses of 0-LTO/G, 1-LTO/G, and 3-LTO/G samples in the air atmosphere were carried out to determine the graphene content, as shown in Figure 2b. In all case, graphene oxide can be completely decomposed at 700 °C. The curves show a similar shape, but the contents of graphene are slightly different, which are ~1.7, 1.8, and 2.0 wt. %. in 8


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0-LTO/G, 1-LTO/G, and 3-LTO/G, respectively.

Figure 2. (a) XRD patterns of the GO, pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G samples; (b) TG curves of the 0-LTO/G, 1-LTO/G and 3-LTO/G samples. The morphologies of the pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G samples were investigated by a field-emission SEM. As shown in Figures 3a and 3b, the original size of the pristine LTO particles is around 200 nm. Unfortunately, the pristine LTO particles undergo sintering and secondary growth after calcination treatments. From Figures 3c, 3e, and 3g, the composites have a loose structure and the LTO grains are homogeneously distributed on the surfaces of graphene. Graphene can be observed in the gaps among LTO nano-sized particles as well as on the surface of LTO particles. It acts as a barrier to prevent LTO nano-sized particles from generating serious agglomeration during the thermal treatment process. Meanwhile, the gaps among LTO nano-sized particles can form a small quantity of meso/macropores. Figure 3d further confirms that LTO grains anchor on the surface of graphene, and graphene aggregates slightly. Figure 3f shows that the LTO particles are about 100 nm in diameter and are wrapped by 3D “fishnet-like” graphene. Moreover, the particle size is much smaller than that of the pristine LTO particle. It is worth noting that the 9



decomposition and evaporation of NH4Cl result in the abundant micro/mesopores in “fishnet-like” graphene (the pores are marked in Figures 3f, 3h and S1.), which can increase the contact areas between electrolyte and active material and provide plenty of diffusion paths for lithium ions, improving the electrochemical performance of LTO/G composites. However, when the concentration of NH4Cl solution further increases, the “fishnet-like” graphene in 3-LTO/G trends to restack and aggregate seriously (Figure 3h) due to the high concentration of NH4+ and Cl-.

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Figure 3. SEM images of the (a and b) pristine LTO; (c and d) 0-LTO/G; (e and f) 1-LTO/G and (g and h) 3-LTO/G samples. To further explore the microstructure of the as-prepared samples, TEM observation was conducted. Figures 4a and 4b show the pristine LTO particles agglomerate into a secondary particle with the size of ~500 nm. From Figure 4c, the 11



LTO particle size in the 0-LTO/G composite is ~300 nm, indicating that graphene can inhibit the secondary growth of LTO particles. As shown in Figure 4e, in the 1-LTO/G composite, LTO particles with sizes of 50~100 nm anchored uniformly on the surface of graphene, which is identical with the SEM results. Hence, the addition of NH4Cl is responsible for the significant difference between 0-LTO/G and 1-LTO/G composites. Compared to 1-LTO/G, due to the absence of gas expansion from the decomposition of NH4Cl under high temperatures, the graphene in 0-LTO/G aggregates slightly so that its barrier effect during the thermal treatment is not obvious, resulting in the agglomeration of LTO particles. It can be found from Figures 4e and 4g that the size of LTO particle in 3-LTO/G is similar to that in 1-LTO/G, but the graphene in 3-LTO/G (Figure 4g and Figure S2b) suffers from more serious aggregation than that in 1-LTO/G. The reason is that the high concentrations of NH4+ and Cl- lead to the aggregation of graphene, as discussed in the SEM results. The HRTEM images of pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G composites are given in Figures S2a, 4d, 4f, and 4h, respectively. We can observe an interplanar distance of 0.48 nm for the lattice fringes in three figures, which are assigned to the (111) plane of LTO. In Figure 4d, an amorphous strip with an identical thickness of ~3 nm is formed, and a large amorphous region appears (marked as graphene), indicating that LTO particles adhesion on the surface of graphene sheets. The same phenomenon can be clearly seen in Figure 4f, which further confirms that LTO grains anchored on the surface of graphene. The boundary of LTO grain and graphene sheet is also obvious in Figure 4h for 3-LTO/G. The insets in Figures 4d, 4f, and 4h reveal that 0-LTO/G, 1-LTO/G, and 12


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3-LTO/G are polycrystalline. These rings could be assigned to the (111), (311), (400), (333), and (440) planes of LTO, agree well with XRD results.

Figure 4. TEM images of the (a and b) pristine LTO; (c and d) 0-LTO/G; (e and f) 1-LTO/G and (g and h) 3-LTO/G samples. The Raman spectra of 0-LTO/G, 1-LTO/G, and 3-LTO/G are given in Figure 5a. 13



All products show two main peaks at ~1350 and ~1560 cm−1, which can be assigned to the D- and G-band of graphene, respectively.36,37 The D-band stands for the disorder induced in sp2-bond carbon. The ID/IG ratios of 0-LTO/G, 1-LTO/G, and 3-LTO/G composites are 1.04, 1.18, and 1.10, respectively, indicating that numerous defects appear in the graphene layer. The high ID/IG value of 1-LTO/G implies the existence of more edges, defects, and disordered carbons, which are beneficial for its electrochemical performance.38 The 2D band exists in the Raman spectra of three samples, indicating that the graphene is multilayered.39 To investigate the pore size distribution and specific surface area of the samples, the N2 adsorption-desorption measurement was conducted. As presented in Figure 5b, four N2 adsorption-desorption isotherms can be classified as type IV. The Brunanuer-Emmett-Teller (BET) surface areas of the pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G composites are calculated to be 4.7, 10.0, 23.8, and 16.7 m2 g−1, respectively. The specific surface area of 1-LTO/G is five times of that of the pristine LTO. There are no obvious hysteresis loops in the adsorption-desorption isotherms of pristine LTO and 0-LTO/G, implying the existence of few mesopores. On the contrary, the adsorption-desorption isotherms of both 1-LTO/G and 3-LTO/G samples show a hysteresis loop in the range of ca. 0.4-1.0 P/P0, indicating the existence of abundant meso/macropores. The corresponding pore size distribution curves (Figure 5b inset) present two mean pore size distribution peaks centered at 1.76 and 10.9 nm for 1-LTO/G and 1.93 and 11.2 nm for 3-LTO/G, which means the existence of micro/mesopores. The pore size distribution of the two samples shows a 14


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high similarity, indicating the NH4Cl concentration had little effect on the pore size distribution of the LTO/G material in the range of 1 to 3 mg mL-1. A large proportion of micro/mesopores is caused by the decomposition of NH4Cl, and a fraction of meso/macropores is attributed to the gaps among LTO nano-sized particles due to the barrier of graphene, highly consistent with the SEM results.

Figure 5. (a) Raman spectra of the 0-LTO/G, 1-LTO/G and 3-LTO/G samples; (b) Nitrogen adsorption/desorption isotherms at 77 K of the pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G samples (inset is the pore size distribution of the 1-LTO/G and 3-LTO/G samples). Figure 6a exhibits the CV curves of the pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G composite electrodes at a scanning rate of 0.1 mV s−1. A pair of obvious redox peaks appears at around 1.55 V in the CV curves of four samples, which can be attributed to the intercalation/deintercalation of Li+ in the spinel LTO (Li4Ti5O12 + 3Li+ + 3e- ↔ Li7Ti5O12).40 The redox peaks of LTO/G composite electrodes are sharper and stronger than those of the pristine LTO electrode, particularly the 1-LTO/G composite electrode. The well-resolved and sharp redox peaks indicate the rapid kinetics and admirable efficiency of the insertion/extraction process of lithium 15



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ions in the spinel lattice. It could be inferred that graphene acts as a barrier to prevent LTO particles from aggregating and secondary growing during the thermal treatment process. The nano-sized LTO/G particles possess a larger specific surface area which could increase the contact areas between the electrolyte and active material. The 3D “fishnet-like” structure of graphene can improve the electric conductivity and the number of micro/mesopores, providing more migration channels for lithium ions. The different performance of 0-LTO/G, 1-LTO/G, and 3-LTO/G composite electrodes indicates that the concentration of NH4Cl solution has a great effect on the electrochemical properties. An appropriate concentration of NH4Cl solution can not only create micro/mesopores but also reduce the aggregation of graphene oxide. In order to further explore the kinetic mechanism of Li+ insertion/extraction in the 1-LTO/G composite electrode, the CV tests at scan rates range of 0.1-1.5 mV s−1 were carried out. From Figure 6b, we can observe a pair of symmetrical redox peaks. When the scan rate increases, the redox peaks shift slightly but maintain the symmetrical shape, revealing the low polarization of the 1-LTO/G composite electrode. The following equation for the diffusion-limited process: 41

Ip = (2.69 × 105) n3/2AD1/2Cʋ1/2

(1)

Where A is the surface area of the electrode (1.54 cm2), C is the concentration of reactant, D is the lithium ion diffusion coefficient, and n represents the number of electron transfers. The peak current (Ip) is proportional to the square root of the scan rate (ʋ1/2). As presented in Figure 6b inset, there is a good linear relationship between the square root of the scan rate and the peak current, indicating that the reaction is 16


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controlled by the diffusion of lithium ions42-44. The value of D of 1-LTO/G is calculated to be 5.76 × 10−10 cm2 s−1, which is three times that of LTO (1.6 × 10−10 cm2 s−1). The great improvement of diffusion dynamics can result in better electrochemical performance. To sum up, the rapid redox reaction and the low polarization degree of 1-LTO/G composite electrode are benefited from the synergistic effect of the hierarchical porous 3D “fishnet-like” conductive network structure and the nano-sized LTO particles.

Figure 6. (a) CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 1.0-3.0 V of the pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G samples; (b) CV curves of the 1-LTO/G sample at different scan rates between 1.0-3.0 V (the inserted image is the peak currents against square roots of scan rate). The charge and discharge curves of the materials at different rates from 0.1 to 20 C are shown in Figures 7a-7d. Compared to the pristine LTO, the LTO/G composites have higher specific capacities at all tested rates. Particularly, the 1-LTO/G composite electrode delivers the highest discharge capacities of 206, 182.8, 176.6, 167.5, 161, 142.9, and 121.6 mAh g−1 at 0.1, 0.5, 1, 3, 5, 10, and 20 C, respectively, while the pristine LTO electrode has the lowest discharge capacities of 176.9, 134.5, 108.2, 71.2, 17



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57.9, 45.2, and 37.9 mAh g−1, respectively. As a comparison, a relatively high concentration of NH4Cl solution was used for preparing 3-LTO/G. The graphene has occurred slightly aggregation before the thermal treatment because of the high concentration of NH4+ and Cl-, even though the foaming effect could relief the aggregation and produce pores. Finally, from the obtained SEM and TEM images, it is found that the agglomeration of graphene in 3-LTO/G is more serious than that in 1-LTO/G. The graphene in 1-LTO/G is more loosen and porous than that in 3-LTO/G, which consequently presents a larger specific surface area and a better penetration of electrolytes. The size of LTO particles in 1-LTO/G is smaller than that in 3-LTO/G, thus

shortening

the

transfer

distance

and

enabling

more

quickly

intercalation/deintercalation of lithium ions. These above-mentioned factors contribute to the best electrochemical performance of the 1-LTO/G electrode. It is worth noting that the 1-LTO/G electrode has an ultrahigh discharge capacity of 176.6 mAh g−1 at 1 C, which is slightly higher than the theoretical specific capacity. Figure 7f displays the relationship between the current density and the voltage difference between the charge/discharge platform (∆E),45 from which the 1-LTO/G electrode demonstrates the lowest polarization. This can be ascribed to the following three factors: Firstly, the 3D “fishnet-like” structure of graphene improves the electric conductivity. Secondly, the micro/mesopores of composites provide more migration channels for lithium ions. Thirdly, the nano-sized LTO particles shorten the transfer distance of lithium ions.

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Figure 7. Charge/discharge curves of the (a) pristine LTO, (b) 0-LTO/G, (c) 1-LTO/G and (d) 3-LTO/G at various current densities from 0.1 to 20 C rate; (e) Rate capability and (f) the relationship between the current density and the voltage difference between the charge/discharge platform (∆E) of the pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G. Figure 7e and Figure 8a show the rate capability results of the samples, from which the 1-LTO/G electrode has the best rate performance. When the discharge rate returns to 0.5 C after 40 cycles, the capacity of four samples can be almost fully recovered due to the “zero-strain” structure of LTO. Table S1 compares the rate performances of 1-LTO/G with other LTO-based materials reported previously. Obviously, the as-prepared 1-LTO/G composite has better rate performance than those of the state-of-the art LTO-based materials. The lower polarization and excellent rate performance of the 1-LTO/G electrode can be attributed to the relatively large specific surface area, small particle size, and massive hierarchical porous structure. 19



Particularly, the 3D “fishnet-like” conductive network of graphene provides abundant migration paths for electrons and the hierarchical porous structure provides more channels for lithium ions. The cycle performance of the as-prepared samples at 1 C is shown in Figure 8b. Due to the “zero-strain” structure of LTO, pristine LTO, 0-LTO/G, 1-LTO/G, and 3-LTO/G electrodes show the superior capacity retention of 95%, 96.9%, 95.3%, and 95% after 100 cycles, respectively. Figure S3 is the charge and discharge profiles of the 1st, 25th, 50th, 75th, and 100th cycle of 1-LTO/G. The charge and discharge platform has little changes and the specific capacity decreases slowly, indicating the excellent stability of the 1-LTO/G electrode during cycling. As shown in Figure 8c, the 1-LTO/G electrode also demonstrates outstanding cyclic stability at high rates of 5 C and 10 C. After 100 cycles, it has the capacity retention of 93.5% and 95.4% at rates of 5 C and 10 C, respectively, with a Coulombic efficiency of approximately 100% throughout the whole cycling. The excellent Coulombic efficiency throughout the whole cycling at high rates show superior electrochemical Li+ intercalation/deintercalation reversibility into/out of the electrode. The high reversibility and good stability of the electrode may benefit from the zero-strain structure and 3D “fishnet-like” conductive structure of 1-LTO/G composites, which can create fast Li+ diffusion and high electrical conductivity. The EIS tests were performed to further investigate the electrochemical property of the samples. As shown in Figure 8d, each impedance spectrum consists of a semicircle and a straight line in the high-frequency region and low-frequency region, respectively. The diameter of the semicircle in the high-frequency region represents 20


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the resistance (Rct) related to the charge transfer across the electrode interface, while the straight line in the low-frequency region stands for the diffusion of lithium ion into the bulk phase of the material.46,47 The inserted image presents the equivalent circuit. According to the Nyquist plots, the Rct value of the 1-LTO/G is 32 Ω, while the value of the pristine LTO is 106 Ω. Notably, the 1-LTO/G electrode has the lowest charge transfer impedance among the electrodes, indicating the faster transfer rates of lithium ions and electrons. The reason for 1-LTO/G with the lowest Rct may be as follows: i) the existence of graphene acts as a barrier to prevent LTO particles from aggregation and enhances the electrical conductivity; ii) the introduction of an appropriate amount of NH4Cl not only creates micro/mesopores but also reduces the aggregation of graphene and this factor contribute to the charge transfer impedance of 1-LTO/G electrode is lower than that of 3-LTO/G electrode; iii) the hierarchical porous 3D “fishnet-like” conductive structure provides more rapid diffusion channels for lithium ion and electronic.

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Figure 8. (a) Rate capability of the pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G; (b) Cycle performance of the pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G at 1C; (c) Cycling performance and Coulombic efficiency versus cycle number at 5 C and 10 C of the 1-LTO/G; (d) EIS of the pristine LTO, 0-LTO/G, 1-LTO/G and 3-LTO/G (the inserted image is equivalent circuit). 4. Conclusions In summary, using NH4Cl as a pore-forming agent, the hierarchical porous LTO/G composites were prepared via a gas-foaming method. In the 1-LTO/G composite, graphene has a unique 3D “fishnet-like” conductive structure, which not only avoids the sintering and secondary growth of LTO particles but also improves the electric conductivity. In addition, the micro/mesopores of composites provide more 22


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rapid diffusion channels for lithium ions. Moreover, the nano-sized LTO particles shorten the transfer distance of lithium ions. For the above reasons, the 1-LTO/G composite electrode achieved a remarkable discharge capacity of 176.6 mAh g−1 at 1 C and an excellent rate capacity of 142.9 mAh g−1 at a high rate of 10 C. Furthermore, outstanding cycle performance with 95.4% capacity retention at 10 C after 100 cycles and a low electrode polarization were obtained. As a result, the 1-LTO/G composite material might be a promising anode material for high-performance LIBs in commercial applications. Acknowledgements The authors wish to acknowledge the following financial supporters of this work: the National Natural Science Foundation of China (Grant No. 21673086, 21273085 and 51578556), the Scientific and Technological Plan of Guangdong Province (lithium ion capacitor and No. 2014A020216009), the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030313376 and 2015A030308005). Associated content Supporting Information Available: SEM image of the1-LTO/G；HRTEM image of the pristine LTO; TEM image of the 3-LTO/G; The voltage profiles for 1-LTO/G at a rate of 1C; Comparison of rate performance of 1-LTO/G with other LTO-based materials reported before. References (1) Shi, Y.; Wang, J.-Z.; Chou, S.-L.; Wexler, D.; Li, H.-J.; Ozawa, K.; Liu, H.-K.;

23



Wu, Y.-P. Hollow Structured Li3VO4 Wrapped with Graphene Nanosheets In Situ Prepared by a One-Pot Template-Free Method as an Anode for Lithium-Ion Batteries.

Nano Lett. 2013, 13, 4715-4720. (2) Zheng, Y.; Zhou, T.; Zhao, X.; Pang, W. K.; Gao, H.; Li, S.; Zhou, Z.; Liu, H.; Guo, Z. Atomic Interface Engineering and Electric-Field Effect in Ultrathin Bi2MoO6 Nanosheets for Superior Lithium Ion Storage. Adv. Mater. 2017, 29, 1700396. (3) Wang, Y.; Zhao, J.; Qu, J.; Wei, F.; Song, W.; Guo, Y.-G.; Xu, B. Investigation into the Surface Chemistry of Li4Ti5O12 Nanoparticles for Lithium Ion Batteries, ACS

Appl. Mater. Interfaces 2016, 8, 26008-26012. (4) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: a Review. Energy Environ. Sci. 2011, 4, 3243-3262. (5) Liu, Y.; Zhou, T.; Zheng, Y.; He, Z.; Xiao, C.; Pang, W. K.; Tong, W.; Zou, Y.; Pan, B.; Guo, Z.; Xie, Y. Local Electric Field Facilitates High-Performance Li-Ion Batteries. ACS Nano 2017, 11, 8519-8526. (6) Ni, H.; Song, W.-L.; Fan, L.-Z. A Strategy for Scalable Synthesis of Li4Ti5O12/Reduced Graphene Oxide toward High Rate Lithium-Ion Batteries.

Electrochem. Commun. 2014, 40, 1-4. (7) Jung, H.-G.; Myung, S.-T.; Yoon, C. S.; Son, S.-B.; Oh, K. H.; Amine, K.; Scrosati, B.; Sun, Y.-K. Microscale Spherical Carbon-Coated Li4Ti5O12 as Ultra High Power Anode Material for Lithium Batteries. Energy Environ. Sci. 2011, 4, 1345-1351. 24


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Long, D. H.; Jeong, M.-G.; Lee, Y.-S.; Choi, W.; Lee, J. K.; Oh, I.-H.; Jung, H.-G. Coating Lithium Titanate with Nitrogen-Doped Carbon by Simple Refluxing for High-power Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 10250-10257. (9) Zhu, G.-N.; Wang, Y.-G.; Xia, Y.-Y. Ti-Based Compounds as Anode Materials for Li-Ion Batteries. Energy Environ. Sci. 2012, 5, 6652-6667. (10) Yi, T.-F.; Yang, S.-Y.; Xie, Y. Recent Advances of Li4Ti5O12 as a Promising Next Generation Anode Material for High Power Lithium-Ion Batteries. J. Mater. Chem. A 2015, 3, 5750-5777. (11) Haridas, A. K.; Sharma, C. S.; Rao, T. N. Donut-Shaped Li4Ti5O12 Structures as a High Performance Anode Material for Lithium Ion Batteries. Small 2015, 11, 290-294. (12) Wu, H.-C.; Lee, E.; Wu, N.-L.; Jow, T. R. Effects of Current Collectors on Power Performance of Li4Ti5O12 Anode for Li-Ion Battery. J. Power Sources 2012,

197, 301-304. (13) Kim, J.; Kim, S.-W.; Gwon, H.; Yoon, W.-S.; Kang, K. Comparative Study of Li(Li1/3Ti5/3)O4 and Li(Ni1/2−xLi2x/3Tix/3)Ti3/2O4 (x = 1/3) Anodes for Li Rechargeable Batteries. Electrochim. Acta 2009, 54, 5914-5918. (14) Sun, X.; Radovanovic, P. V.; Cui, B., Advances in Spinel Li4Ti5O12 Anode Materials for Lithium-Ion Batteries. New J. Chem. 2015, 39, 38-63. (15) Tang, Y.; Liu, L.; Zhao, H.; Jia, D.; Liu, W. Porous CNT@ Li4Ti5O12 Coaxial Nanocables as Ultra High Power and Long Life Anode Materials for Lithium Ion 25



Batteries. J. Mater. Chem. A 2016, 4, 2089-2095. (16) Sha, Y.; Zhao, B.; Ran, R.; Cai, R.; Shao, Z. Synthesis of Well-Crystallized Li4Ti5O12 Nanoplates for Lithium-Ion Batteries with Outstanding Rate Capability and Cycling Stability. J. Mater. Chem. A 2013, 1, 13233-13243. (17) Chou, S.-L.; Wang, J.-Z.; Liu, H.-K.; Dou, S.-X. Rapid Synthesis of Li4Ti5O12 Microspheres as Anode Materials and Its Binder Effect for Lithium-Ion Battery. J.

Phys. Chem. B 2011, 115, 16220-16227. (18) Jiang, C.; Ichihara, M.; Honma, I.; Zhou, H. Effect of Particle Dispersion on High Rate Performance of Nano-Sized Li4Ti5O12 Anode. Electrochim. Acta 2007, 52, 6470-6475. (19) Zhang, Q.; Lu, H.; Zhong, H.; Yan, X.; Ouyang, C.; Zhang, L. W6+ & BrCodoped Li4Ti5O12 Anode with Super Rate Performance for Li-Ion Batteries. J. Mater.

Chem. A 2015, 3, 13706-13716. (20) Zhao, E.; Qin, C.; Jung, H.-R.; Berdichevsky, G.; Nese, A.; Marder, S.; Yushin, G. Lithium Titanate Confined in Carbon Nanopores for Asymmetric Supercapacitors.

ACS nano 2016, 10, 3977-3984. (21) Wang, P.; Zhang, G.; Cheng, J.; You, Y.; Li, Y.-K.; Ding, C.; Gu, J.-J.; Zheng, X.-S.; Zhang, C.-F.; Cao, F.-F. Facile Synthesis of Carbon-Coated Spinel Li4Ti5O12/Rutile-TiO2 Composites as an Improved Anode Material in Full Lithium-Ion Batteries with LiFePO4@N-Doped Carbon Cathode. ACS Appl. Mater.

Interfaces 2017, 9, 6138-6143. (22) Hao, J.; Liao, Y.; Zhong, Y.; Shu, D.; He, C.; Guo, S.; Huang, Y.; Zhong, J.; Hu, 26


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L. Three-Dimensional Graphene Layers Prepared by a Gas-Foaming Method for Supercapacitor Applications. Carbon 2015, 94, 879-887. (23) Li, X.; Zhang, C.; Xin, S.; Yang, Z.; Li, Y.; Zhang, D.; Yao, P. Facile Synthesis of

MoS2/Reduced

Graphene

Oxide@Polyaniline

for

High-Performance

Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 21373-21380. (24) Zhou, T.; Zheng, Y.; Gao, H.; Min, S.; Li, S.; Liu, H. K.; Guo, Z. Surface Engineering and Design Strategy for Surface-Amorphized TiO2@Graphene Hybrids for High Power Li-Ion Battery Electrodes. Adv. Sci. 2015, 2, 1500027. (25) Zhang, C.; Peng, X.; Guo, Z.; Cai, C.; Chen, Z.; Wexler, D.; Li, S.; Liu, H. Carbon-Coated SnO2/Graphene Nanosheets as Highly Reversible Anode Materials for Lithium Ion Batteries. Carbon 2012, 50, 1897-1903. (26) Sun, Y.; Wu, Q.; Shi, G. Graphene Based New Energy Materials. Energy

Environ. Sci. 2011, 4, 1113-1132. (27) Kim, H.-K.; Jegal, J.-P.; Kim, J.-Y.; Yoon, S.-B.; Roh, K. C.; Kim, K.-B. In Situ Fabrication of Lithium Titanium Oxide by Microwave-Assisted Alkalization for High-Rate Lithium-Ion Batteries. J. Mater. Chem. A 2013, 1, 14849-14852. (28) Niu, Z.; Liu, L.; Zhang, L.; Shao, Q.; Zhou, W.; Chen, X.; Xie, S. A Universal Strategy to Prepare Functional Porous Graphene Hybrid Architectures. Adv. Mater. 2014, 26, 3681-3687. (29) Liu, H.-p.; Wen, G.-w.; Bi, S.-f.; Wang, C.-y.; Hao, J.-m.; Gao, P. High Rate Cycling Performance of Nanosized Li4Ti5O12/Graphene Composites for Lithium Ion Batteries. Electrochim. Acta 2016, 192, 38-44. 27



(30) Shi, Y.; Wen, L.; Li, F.; Cheng, H.-M. Nanosized Li4Ti5O12/Graphene Hybrid Materials with Low Polarization for High Rate Lithium Ion Batteries. J. Power

Sources 2011, 196, 8610-8617. (31) Chen, C.; Huang, Y.; Zhang, H.; Wang, X.; Li, G.; Wang, Y.; Jiao, L.; Yuan, H. Small Amount of Reduce Graphene Oxide Modified Li4Ti5O12 Nanoparticles for Ultrafast High-Power Lithium Ion Battery. J. Power Sources 2015, 278, 693-702. (32) Zheng, Y.; Zhou, T.; Zhang, C.; Mao, J.; Liu, H.; Guo, Z., Boosted Charge Transfer in SnS/SnO2 Heterostructures: Toward High Rate Capability for Sodium-Ion Batteries. Angew. Chem. Int. Ed. 2016, 55, 3408-3413. (33) Chen, S.; Chen, P.; Wu, M.; Pan, D.; Wang, Y. Graphene Supported Sn–Sb@Carbon Core-Shell Particles as a Superior Anode for Lithium Ion Batteries.

Electrochem. Commun. 2010, 12, 1302-1306. (34) Chen, S. Q.; Wang, Y. Microwave-Assisted Synthesis of a Co3O4-Graphene Sheet-on-Sheet Nanocomposite as a Superior Anode Material for Li-Ion Batteries. J.

Mater. Chem. 2010, 20, 9735-9739. (35) Xiong, L.-z.; He, Z.-q.; Yin, Z.-l.; Chen, Q.-y. Preparation and Characterization of SnO2-Li4Ti5O12 Composite by Sol-Gel Technique. T. Nonferr. Metal. Soc. 2010, 20, s267-s270. (36) Peng, L.; Zhang, H.; Fang, L.; Zhang, Y.; Wang, Y. Novel Peapoded Li4Ti5O12 Nanoparticles for High-Rate and Ultralong-Life Rechargeable Lithium Ion Batteries at Room and Lower Temperatures. Nanoscale 2016, 8, 2030-2040. (37) Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, 28


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electron-Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47-57. (38) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (39) Thakur, S.; Karak, N. Green Reduction of Graphene Oxide by Aqueous Phytoextracts. Carbon 2012, 50, 5331-5339. (40) Verde, M. G.; Baggetto, L.; Balke, N.; Veith, G. M.; Seo, J. K.; Wang, Z.; Meng, Y. S., Elucidating the Phase Transformation of Li4Ti5O12 Lithiation at the Nanoscale.

ACS nano 2016, 10, 4312-4321. (41) Cai, Y.; Huang, Y.; Jia, W.; Wang, X.; Guo, Y.; Jia, D.; Sun, Z.; Pang, W.; Guo, Z. Super High-Rate, Long Cycle Life of Europium-Modified, Carbon-Coated, Hierarchical Mesoporous Lithium-Titanate Anode Materials for Lithium Ion Batteries.

J.Mater. Chem. A 2016, 4, 9949-9957. (42) Wang, X.; Wang, L.; Wang, J.; Chen, T. Study on the Electrochemical Behavior of Poly(ferrocenylsilane) Films. J. Phys. Chem. B 2004, 108, 5627-5633. (43) Lai, C.; Dou, Y. Y.; Li, X.; Gao, X. P. Improvement of the High Rate Capability of Hierarchical Structured Li4Ti5O12 Induced by the Pseudocapacitive Effect. J. Power

Sources 2010, 195, 3676-3679. (44) Zhang, J.; Cai, Y.; Wu, J.; Yao, J. Graphene Oxide-Confined Synthesis of Li4Ti5O12 Microspheres as High-Performance Anodes for Lithium Ion Batteries.

Electrochim. Acta 2015, 165, 422-429. 29



(45) Xu, N.; Sun, X.; Zhang, X.; Wang, K.; Ma, Y. A Two-Step Method for Preparing Li4Ti5O12-Graphene as an Anode Material for Lithium-Ion Hybrid Capacitors. RSC Adv. 2015, 5, 94361-94368. (46) Liu, W.; Zhou, T.; Zheng, Y.; Liu, J.; Feng, C.; Shen, Y.; Huang, Y.; Guo, Z. Hierarchical Structural Evolution of Zn2GeO4 in Binary Solvent and Its Effect on Li-ion Storage Performance. ACS Appl. Mater. Interfaces 2017, 9, 9778-9784. (47) Yu, R.; Zhang, C.; Meng, Q.; Chen, Z.; Liu, H.; Guo, Z. Facile Synthesis of Hierarchical Networks Composed of Highly Interconnected V2O5 Nanosheets Assembled on Carbon Nanotubes and Their Superior Lithium Storage Properties. ACS Appl. Mater. Interfaces 2013, 5, 12394-12399.

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Reduced Graphene Oxide Composites

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