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Interpenetrated Networks between Graphitic Carbon Infilling and Ultrafine TiO2 Nanocrystals with Patterned Macroporous Structure for High-Performance Lithium Ion Batteries Wenji Zheng, Zhijun Yan, Yan Dai, Naixu Du, Xiaobin Jiang, Hailing Dai, Xiangcun Li,* and Gaohong He State Key Laboratory of Fine Chemicals, Chemical Engineering Department, Dalian University of Technology, Linggong Road No. 2, Dalian 116024, China S Supporting Information *
ABSTRACT: Interpenetrated networks between graphitic carbon infilling and ultrafine TiO2 nanocrystals with patterned macropores (100−200 nm) were successfully synthesized. Polypyrrole layer was conformably coated on the primary TiO2 nanoparticles (∼8 nm) by a photosensitive reaction and was then transformed into carbon infilling in the interparticle mesopores of the TiO2 nanoparticles. Compared to the carbon/graphene supported TiO2 nanoparticles or carbon coated TiO2 nanostructures, the carbon infilling would provide a conductive medium and buffer layer for volume expansion of the encapsulated TiO2 nanoparticles, thus enhancing conductivity and cycle stability of the C−TiO2 anode materials for lithium ion batteries (LIBs). In addition, the macropores with diameters of 100−200 nm in the C−TiO2 anode and the mesopores in carbon infilling could improve electrolyte transportation in the electrodes and shorten the lithium ion diffusion length. The C−TiO2 electrode can provide a large capacity of 192.8 mA h g−1 after 100 cycles at 200 mA g−1, which is higher than those of the pure macroporous TiO2 electrode (144.8 mA h g−1), C−TiO2 composite electrode without macroporous structure (128 mA h g−1), and most of the TiO2 based electrodes in the literature. Importantly, the C−TiO2 electrode exhibits a high rate performance and still delivers a high capacity of ∼140 mA h g−1 after 1000 cycles at 1000 mA g−1 (∼5.88 C), suggesting good lithium storage properties of the macroporous C−TiO2 composites with high capacity, cycle stability, and rate capability. This work would be instructive for designing hierarchical porous TiO2 based anodes for high-performance LIBs. KEYWORDS: TiO2, macropore, mesopore, Li ion battery, carbon
1. INTRODUCTION
theless, the electrochemical properties such as long-term cycle stability at high current density and the rate capability of these electrodes gradually degrade due to disintegration of the TiO2 nanoparticles from the carbon or graphene supports and their large volume change during the Li ion insertion−extraction process.9−11 On the other hand, the inferior electrochemical performance could be attributed to the pulverization and rearrangement of the TiO2 active materials because the carbon or graphene coatings cannot conformably encapsulate the primary tiny TiO2 nanoparticles.12−14 In addition, graphitiza-
Designing various nanostructured electrodes has been attracting wide interests to improve the power/energy densities and cycle stability of rechargeable lithium ion batteries (LIBs).1,2 Recently, TiO2 is becoming a particularly attractive electrode material due to its electrochemical stability, highly reversible lithiation process, and abundance in nature.3,4 However, the TiO2 anode electrodes generally exhibit poor long-term cycle stability and rate property due to their inferior electronic conductivity and slow lithium ion diffusion process in LIBs.5,6 To further improve the electrochemical performance of the TiO2 electrodes, various carbon or graphene based TiO2 composite materials have been fabricated to boost the electronic conductivity of the electrode materials.7,8 Never© 2017 American Chemical Society
Received: February 16, 2017 Accepted: June 1, 2017 Published: June 1, 2017 20491
DOI: 10.1021/acsami.7b02345 ACS Appl. Mater. Interfaces 2017, 9, 20491−20500
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Figure 1. Preparation of polypyrrole−TiO2 hybrids and their conversion into C−TiO2 composite (a); SEM and cut-TEM images of the macroporous TiO2 (b−d) and the C−TiO2 composite (e−g).
tion of the amorphous carbon coatings is also necessary to facilitate the diffusion of lithium ions and electrons in electrode materials.15,16 Formation of appropriate porous structure in electrode materials is another effective way to enhance electrochemical performance of LIBs.2,5,17−19 For the micropores (50 nm) and mesoporous (2−50 nm) structures is necessary to offer a high surface area and large amount of electroactive sites in electrode materials, as well as to facilitate pore accessibility for ions21,22 and accommodate more electrolytes for an electrochemical reaction.3,23 Thus, preparation of hierarchical porous structure with mesopores and macropores is a promising strategy to improve rate performance of the electrodes. Herein, we report a novel C−TiO2 electrode material with ordered macropores in the composites and mesopores in the carbon infilling. The walls are composed of small TiO2
nanoparticles (∼8 nm), and the interparticle mesopores are well-filled with graphitic carbon matrix, suggesting that each primary tiny TiO2 nanoparticle in the wall is conformably coated by graphitic carbon layers. When being applied as anode materials for LIBs, C−TiO2 composites show a higher lithium ion storage performance compared with the reported carbon− TiO2 based electrode materials. The unique interpenetrating network between carbon infilling and the primary TiO2 particles is crucial to improve electron transportation in the composites and accommodate volume change of the TiO2 nanoparticles during the Li insertion and extraction process. Also, the conformal carbon coating on the TiO2 nanoparticles would prevent them from agglomeration and pulverization upon cycling.4,7 In addition, the macropores (100−200 nm) in the C−TiO2 anode and mesopores in the carbon infilling facilitates transportation of electrolytes and shorten lithium ion diffusion lengths. The LIBs with the C−TiO2 electrode as an anode electrode delivers a high capacity of ∼200 mA h g−1 at 200 mA g−1 with a Coulombic efficiency of nearly 100%. More 20492
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Figure 2. (a) HAADF image of the macroporous C−TiO2 from the side view; (b−d) EDS mapping of Ti, O, and C elements, respectively, implying uniform distribution of carbon in the C−TiO2 composites; (e) HAADF of the macroporous C−TiO2 from the top view, (f−h) EDS mapping of Ti, O, and C elements; (i) SAED and HRTEM of the fringe TiO2 nanoparticles with ultrathin carbon coating; (j) HRTEM image of C−TiO2 composites.
graphitic carbon coating on the primary tiny TiO2 nanoparticles to improve its conductive property and cycle stability for LIBs anode materials. Inspired by this idea, pyrrole monomers were first introduced into the interior of the TiO2 framework and were subsequently locally polymerized in the interparticle mesopores with the primary tiny TiO2 nanoparticles as photosensitizer under UV light irradiation, resulting in formation of porous polypyrrole−TiO2 composites.26,27 Macroporous C−TiO2 hybrids were obtained after carbonization treatment of the polypyrrole−TiO2 composites. In this work, with the polypyrrole as a carbon source, it is expected that the photosensitive reaction can favor formation of a well-adhered interface between the polymer and the primary TiO 2 nanoparticles, where the polypyrrole polymer can conformably coat the primary TiO2 nanoparticles, subsequently forming a stronger combination between the carbon layer and TiO2 in the
importantly, the electrode possesses high rate capabilities and long-term cycle stability, still showing a capacity of ∼140 mA h g−1 after 1000 cycles at a high current density of 1000 mA g−1 (∼5.88 C).
2. RESULTS AND DISCUSSION TiO2 materials with macroporous structure would facilitate diffusion and transportation of reactant molecules and ions within the frameworks.24,25 This structure is attractive for highperformance LIBs anode materials because the ordered macropores can accommodate more electrolytes for an electrochemical reaction and improve Li ion transportation. Figure 1a shows the formation process of the C−TiO2 composites with the macroporous TiO2 material as a template. It is supposed that the interparticle mesopores in the wall can be filled by graphitic carbon matrix, thus forming conformal 20493
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Figure 3. (a) Wide-angle XRD patterns, (b) Raman spectrum, (c) XPS spectrum, and (d) N2 adsorption/desorption isotherms of the C−TiO2 composite.
TEM images from the side and top views in Figure 2a,e further confirm well preservation of the open-ended macropores with an aligned arrangement. The EDS image of the pores proves the existence of carbon, titanium, and oxygen elements, and the carbon element was homogeneously distributed in the hierarchical TiO2 framework (Figure 2d,h). The high-resolution TEM image in Figure 2i shows that the fringe primary nanoparticles are also conformably coated by ultrathin graphitic carbon layers (∼5 nm, as denoted by arrows). The interplanar distance of the primary TiO2 nanoparticles is about ∼0.351 nm, agreeing with the (101) plane of anatase TiO2.34−36 The selected area electron diffraction pattern (SAED; inset in Figure 2i) proves the polycrystalline property of the TiO2 crystal. The high-resolution TEM image in Figure 2j clearly demonstrates the presence of many TiO2 crystallites with a diameter of 5−10 nm (red cycles), and the void space between the TiO2 nanoparticles are well-filled by the carbon matrix with some mesopores (red arrows). The mesopores in the carbon infillings may be due to the contract effect of the polypyrrole matrix during the pyrolysis process at high temperature. Thus, both the macro- and mesopore structures in the C−TiO2 electrode are well-preserved compared with the pure TiO2 template. X-ray diffraction patterns (XRD) in Figure 3a further prove that the TiO2 and C−TiO2 composites possess a wellcrystallized anatase phase (JCPDS No. 21-1272). The crystalline sizes were estimated to be about 7.9 and 8.6 nm for the TiO2 and C−TiO2 crystals, respectively, by applying the Scherrer equation to the (101) diffraction peak, agreeing well with the HRTEM results in Figure 1d,g. In addition, the weak diffraction peak at about 24° is attributed to the (002) peak of graphitic carbon.4 In this work, it is supposed that the ordered macropores of 100−200 nm could facilitate entrance of pyrrole monomers into the interior of the TiO2 and the interparticle mesopores and were then locally polymerized on the surface of
C−TiO2 hybrids than other C−TiO2 composites prepared by solution or sol−gel methods.4,16 The SEM and cut-TEM images in Figure 1b,c show that TiO2 materials with ordered macropores and interparticle mesopores in the wall were prepared by a bisurfactant soft template approach in this work. Further, the high-resolution TEM image in Figure 1d proves that the wall is composed of primary tiny TiO2 nanoparticles with a diameter of about 8 nm, and mesopores of 5−10 nm among the primary TiO 2 nanoparticles are observed. The presence of the interparticle mesopores would increase the accessible surface area and molecules/ion transportation within the TiO2 material. In contrast, the macroporous structure in the C−TiO2 hybrids is well-preserved though the wall becomes rougher in Figure 1e, which can be ascribed to the carbon layer on the TiO2 nanoparticles.26,28 From the TEM images in Figure 1f,g and Supporting Information Figure S1, the ambiguous boundary of the TiO2 nanoparticles can be ascribed to the conformable carbon coatings which result from the locally polymerized polypyrrole layers.29−31 Compared with the carbon or graphene supported TiO 2 nanoparticles or carbon coated TiO 2 nanostructures, the interpenetrating networks between the carbon infilling and the primary TiO2 particles is crucial to improve the interfacial stability and shorten the lithium ion diffusion length. The carbon matrix would provide an ideal conductive medium and buffer layer for volume expansion of the encapsulated TiO2 nanoparticles, thus enhancing the electronic conductivity and long-term cycle stability of C− TiO2 anode materials for LIBs.16,32,33 In addition, the ordered macropores are conducive to accommodation and transportation of electrolytes for an electrochemical reaction. High-angle annular dark field (HAADF) TEM and energy dispersed spectrum (EDS) technology were used to investigate the microstructure of the C−TiO2 composites. The HAADF20494
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Figure 4. (a) CV curves of TiO2 and C−TiO2 electrodes at a scan rate of 0.1 mV/s; (b) electrochemical impedance spectra (EIS) of TiO2 and C− TiO2 electrode materials.
the primary TiO2 nanoparticles under UV light irradiation.27,37 In this process, the pyrrole molecules were oxidized by valence band holes (h+) from the TiO2 photosensitizer, resulting in formation of an amount of pyrrole radical cation intermediates and subsequent polymerization.38,39 Compared with the previous carbon−TiO2 electrode materials, the photocatalyzed polymerization here is expected to form a conformal polymer coating on the surface of the primary TiO2 nanoparticles with a covalently bonded interaction between the organic and inorganic phases.31,40 The polymer coating was then carbonized into graphitic carbon layers on the TiO2 particle surface after heat treatment of the polypyrrole−TiO2 composites.4,41 Simultaneously, the phase transformation of the TiO2 nanoparticles from anatase to rutile and their growth/aggregation can be suppressed by the polymer coatings under the hightemperature pyrolysis process.42−44 Raman spectra of the C−TiO2 composites were also studied in Figure 3b. The peaks at 150, 199, 395, 510, and 642 cm−1 can be ascribed to the Eg(1), Eg(2), B1g, A1g, and Eg(3) T−O vibration modes of anatase TiO2, respectively.29,31 The peaks at 1355 and 1585 cm−1 correspond to the D and G bands of carbon.41 Moreover, the high intensity of the D band indicates a graphitic property of the carbon matrix.45 Figure 3c shows the X-ray photoelectron spectroscopy (XPS) of the C−TiO2 sample; two major peaks at 458.4 and 464.1 eV are ascribed to the typical Ti 4+ 2p3/2 and Ti4+ 2p1/2, respectively.1 For the C element (Figure S2a), the main peak at the binding energy of 285.0 eV is assigned to C−C and defect-containing sp2hybridized graphitized carbon (CC). The wide peak at 288.6 eV can be assigned to the C−O or OC−O band, suggesting the possible formation of strong Ti−O−C interaction bonds between the TiO2 nanoparticles and the carbon layer.3,41 For the XPS spectrum of N1S (Figure S2b), the major binding energy at 399.6 eV can be due to the N−H band, while the peak at 401.0 eV is due to the positively charged -N+ atom. The results further confirm the transformation of polypyrrole into the carbon layers.40,46−50 In Figure 3d, the nitrogen sorption isotherms of the macroporous TiO2 and C−TiO2 are typical type-II curves, with combination forms of a distinct H2-type hysteresis loop in the P/P0 range of 0.2−0.8, and a well-defined H3 hysteresis loop in the P/P0 range of 0.8−1.0. The specific surface area of the C−TiO2 was calculated to be 31.3 m2 g−1, which was a little lower than that of the TiO2 (41.5 m2 g−1); the decrease is probably due to the occupation of some interparticle mesopores by the carbon matrix. In addition, the two samples exhibit a narrow pore size distribution at about 4 nm and large
meso- and macropores of 10−60 nm which facilitate electrolyte accommodation. Figure 4a shows the cyclic voltammetry (CV) of the TiO2 and C−TiO2 electrode materials at a scan rate of 0.1 mV s−1 within a voltage range of 1−3 V (vs Li+/Li). A pair of anodic and cathodic peaks is observed in both electrodes at about 2.16 and 1.67 V, which correspond to lithium insertion and extraction reactions in anatase lattice.2,35 The higher current density of C−TiO2 electrode than that of the TiO2 is attributed to its higher level charge separation and electrochemical activity. In addition, the lower polarization in the initial CV cycle (Figure S3a,b) also proves the enhanced electronic conductivity of the C−TiO2 electrode resulting from the graphitic carbon coating.4 From the Nyquist plots in Figure 4b, the semicircle and sloping straight line can be associated with charge transfer process and solid-state diffusion of lithium, respectively. In the high-frequency region, the C−TiO2 electrode shows a lower internal resistance (22.6 Ω) than the TiO2 electrode (29.1 Ω) with a decrease of ∼28.8%, suggesting faster charge transfer in the C−TiO2 electrode. In the moderate- and low-frequency regions, the small semicircle and large straight line slope of C−TiO2 electrode imply a lower charge transfer resistance and faster lithium ion diffusion in the solid state of the electrode, respectively.2,4,41 To demonstrate the advantages of the macroporous C−TiO2 composites, their lithium storage properties have been evaluated as anode materials for LIBs. Figure 5a shows the first, second, third, and 100th discharge/charge curves of the C−TiO2 electrode at a current density of 200 mA g−1 within a voltage window of 1−3 V vs Li+/Li. The obvious plateaus at about 1.7 and 2.0 V can be ascribed to lithium insertion and extraction process for anatase TiO2.2,34 For the C−TiO2 electrode, the initial discharge and charge capacities are 252.1 and 219 mA h g−1, respectively, with a high Coulombic efficiency of 86.8%. Moreover, the identical discharge/charge curves between the second and 100th cycles shows the electrode possessing a stable electrochemical property and still delivers a high capacity of 192.8 mA h g−1 after 100 cycles. For the TiO2 electrode, however, the initial discharge/charge capacities are only 124.3 and 118.8 mA h g−1, respectively (Figure 5b). The results increase to 154.9 and 150.5 mA h g−1 in the second cycle, and the discharge capacity is 144.8 mA h g−1 after 100 cycles (Figure 5c), which is smaller than that of the C−TiO2 electrode (192.8 mA h g−1). From Figure 5d and Figure S3c, the C−TiO2 electrode also exhibits a high rate performance at the current rate range of 0.1−10 A g−1. The average discharge capacities of 215.8, 191.1, 20495
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Figure 5. (a and b) Charge/discharge curves of the macroporous C−TiO2 and TiO2 electrodes at 200 mA g−1; (c) cycle performance of C−TiO2 and TiO2 electrodes at a constant current density of 200 mA g−1; (d) rate capacity properties of the two electrodes within the current range of 0.1− 10 A g−1; (e) long-term cycle stability of the C−TiO2 electrode at 1 A g−1 for 1000 cycles; (f) rate capability of the C−TiO2 electrodes with and without macroporous structure; (g) cycle performance of the two C−TiO2 electrodes at a current density of 200 mA g−1.
166.9, 141.2, 123.6, 90.6, and 70.0 mA h g−1 were obtained at current rates of 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g−1, respectively, and a high capacity of 208.8 mA h g−1 is obtained when the current density is returned to 0.1 A g−1. Furthermore, the contribution of carbon to the capacity of the composites can be neglected (Figure S3e). In contrast, the capacities of the corresponding TiO2 are only 169.5, 159.2, 140.6, 119.4, 93.2, 55.5, and 33.3 mA h g−1 at the same current rates from 0.1 to 10 A g−1. Importantly, the C−TiO2 electrode exhibits a long-
term cycle stability by delivering a high capacity of ∼140 mA h g−1 even after 1000 cycles at 1000 mA g−1 (∼5.88 C, 1 C = 170 mA g−1; Figure 5e), with 91.2% retention of the highest discharge capacity (152.3 mA h g−1 at the 12th cycle, Figure S3d). In terms of the specific capacity, rate capability, and long cycle stability, the macroporous C−TiO2 composites have a superior lithium storage property compared with the reported TiO2 based electrode materials (Table S1). 20496
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Figure 6. (a and b) SEM and HRTEM of C−TiO2 electrode after 1000 cycles; (c) HAADF-TEM of the C−TiO2 composites after 1000 cycles at 5.88 C; element mapping of (d) Ti, (e) O, and (f) C, respectively.
density of 200 mA g−1, which are lower than those of the C− TiO2 electrode. The results further confirm that the dual porous structure is crucial to improve the electrochemical performance of the LIBs anode materials.20,22
The outstanding electrochemical performance of the porous C−TiO2 composite as anode material for LIBs can be ascribed to its unique structures. First, the primary tiny TiO 2 nanocrystals could enhance the interfacial contact area with the electrolyte and promote reversible lithium insertion/ extraction process.2,5 Second, the conformal graphitic carbon layers could improve electronic conductivity of the electrode, buffer the expansion/shrinkage of TiO2 nanoparticles, and prevent them from agglomeration and pulverization upon cycling, thus achieving high rate performance and long-term cycle stability at high current density. Third, the dual meso- and macroporous structures can effectively improve the electrolyte accommodation, transportation, and accessibility with active sites for an electrochemical reaction.3 Moreover, the macroporous structure and carbon coating of the C−TiO2 electrode after 1000 cycles is still be distinguished (Figure 6a), confirming again the excellent structural robustness of the C−TiO2 electrode materials. The HRTEM in Figure 6b shows that the primary TiO2 nanoparticles are still conformably coated by the graphitic carbon layer, and the results can be further confirmed by the uniform distribution of Ti and C elements in the materials after 1000 cycles at 5.88 C (Figure 6c−f). The carbon infilling in the interparticle mesopores could buffer volume change of TiO2 crystals and improve the cycle stability of the active materials. To further investigate contributions of the patterned macroporous structure to the high specific capacity and rate capability of the C−TiO2 electrode, C−TiO2 composites without ordered macropores were synthesized and their electrochemical performance was studied (Figure S4). It can be clearly observed that the rate capability of the C−TiO2 electrode without the patterned macropores are inferior to that of the patterned macroporous C−TiO2 electrode within the same current range of 0.1−10 A g−1 (Figure 5f,g). The former delivers a specific capacity of 137.5 mA h g−1 for the initial cycle and maintains at 128 mA h g−1 after 100 cycles at a current
3. CONCLUSIONS C−TiO2 electrode material with ordered macropores and interparticle mesopores has been successfully fabricated by local polymerization of pyrrole monomers on the surface of the TiO2 nanoparticles and their subsequent carbonization. When being applied as an anode material, the C−TiO2 electrode delivers a high initial capacity of 252.1 mA h g−1 at 200 mA g−1, and is still maintained at 192.8 mA h g−1 after 100 cycles, which is higher than those of the pure macroporous TiO2 electrode (144.8 mA h g−1 after 100 cycles) and the C−TiO2 (128 mA h g−1 after 100 cycles) composite electrode without the macroporous structure. Importantly, the C−TiO2 electrode exhibits high rate capability and discharge capacity of about 140 mA h g−1 even after 1000 cycles at 1 A g−1 (5.88 C), demonstrating the superior lithium storage property of the macroporous C−TiO2 composites in terms of specific capacity, rate capability, and long cycle life. This work would be instructive for designing hierarchical porous TiO2 based anodes for high-performance lithium ion batteries. 4. EXPERIMENTS AND METHODS Preparation of the Macroporous TiO2 and C−TiO2 Samples. The macroporous TiO2 was synthesized by using a bisurfactant soft template. In a typical process, 1.8 g of dioctyl sulfosuccinate sodium salt (Sigma-Aldrich) and 1.5 g of L-R-phosphatidylcholine (Soy-95%, Avanti Polar Lipid) were dissolved into 5 mL of isooctane under ultrasonication treatment. Then, a gel-like soft template was obtained with gradual addition of 100 mL of deionized water to the above surfactant mixture solution. Subsequently, 5 mL of titanium isopropoxide (TIP; Sigma-Aldrich) was added to the gel phase under vortex shaking, and the resulting mixture was immediately sheared by using a viscometer spindle at a rate of 24.0 s−1 for about 1 20497
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ACS Applied Materials & Interfaces h. To remove the soft template, the sheared sample was dried at 60 °C and calcined at 400−550 °C with ramping the temperature 50 °C every 30 min, and with a continuous calcination at 550 °C for 3 h to get macroporous TiO2 electrode materials. In addition, TiO 2 nanoparticles without ordered porous structure were synthesized by adding the precursor TIP to deionized water under magnetical stirring, followed by calcination at 500 °C for 3 h. For the preparation of the macroporous polypyrrole−TiO2 hybrids, a mixture solution was first obtained by dispersing 0.4 g of TiO2 into 100 mL of 0.2 M pyrrole aqueous solution under ultrasonication. To improve entrance of the pyrrole monomers into the macroporous TiO2, the above suspension was then magnetically stirred in the dark for 1 h. The photopolymerization reaction was initiated with the suspension solution toward UV light irradiation (mercury vapor lamp with a center wavelength of 254 nm, 15 W). The reaction was continued for 60 h with an interval pyrrole addition every 24 h (0.1 M, according to the solution). The darkish precipitates were collected, dried, and calcined at 700 °Cin N2 for 3 h (1 °C/min), to obtain the macroporous C−TiO2 electrode materials. The content of the carbon in the C−TiO2 composites was determined to be about 6.5% by an element analyzer. For the preparation of the C−TiO2 composites without macroporous structure, the process was the same as that of the macroporous C−TiO2 electrode material except that the TiO2 nanoparticles without ordered porous structure were used. Characterization. The porous structures of the TiO2 and C−TiO2 electrode materials were observed by a Hitachi-4800 field-emission scanning microscope (FESEM) and JEOL-2010F transmission electron microscope (TEM), respectively. The compositions of the C−TiO2 sample were studied by X-ray photoelectron spectroscopy (ESCALAB 250Xi). The content of carbon in the composites was determined by an inductively coupled plasma emission (ICP; Optima 2000DV). X-ray diffraction (XRD) patterns were obtained by a D/ MAX-2400 diffractometer (Cu Kα radiation, 0.154 nm). The nitrogen adsorption−desorption isotherms were measured at 77.35 K using a Micromeritics AUTOSORB-1-MP. Electrochemical Performance. The electrochemical performances of the TiO2 and C−TiO2 electrode materials were tested by using CR2025 coin-type cells, consisting of the cathode and a lithium metal anode, separated by a porous polyethylene film (Celgard 2325). The cathode contains 80% of active material, 10% of super P as conductive additive, and 10% of poly(vinyllidene fluoride) (PVDF5130) as a binder. Then the electrodes were dried at 100 °C in a vacuum oven overnight, and the loading mass of the active materials (including TiO2 and carbon) was ∼1 mg/cm2. Then the CR2025-type coin cells were assembled in a glovebox for electrochemical characterization. A nonaqueous solution (1:1 for ethylene carbonate and dimethyl carbonate) with 1 M LiPF6 was used as the electrolyte. The cells were galvanostatically charged and discharged at a current density of 200 mA g−1 within the voltage range of 1−3 V (the capacity of the C−TiO2 composites was calculated based on the total mass of TiO2 and carbon). For the rate capability test, the charge/discharge currents gradually increased from 0.1 to 0.2, 0.5, 1, 2, 5, and 10 A g−1 and then restored to 0.1 A g−1. The long-term cycle stability of the cells was tested by charge/discharge at 1 A g−1 within 1−3 V for 1000 cycles.
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macroporous C−TiO2 electrode with previous various TiO2 based materials (Table S1) (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaobin Jiang: 0000-0003-0262-4354 Xiangcun Li: 0000-0003-1647-676X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Natural Science Foundation of China (Grants 21476044 and 21676043), Fundamental Research Funds for the Central Universities (Grant DUT15QY08), and the financial support from Changjiang Scholars Program (Grant T2012049) are greatly appreciated.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02345. Morphologies of the macroporous TiO2 and C−TiO2 composite electrode materials (Figure S1), XPS spectra of the C−TiO2 material (Figure S2), electrochemical performance of the C−TiO2 electrode (Figure S3), SEM image of the C−TiO2 electrode material without patterned macroporous structure (Figure S4), and comparison of electrochemical performance of the 20498
DOI: 10.1021/acsami.7b02345 ACS Appl. Mater. Interfaces 2017, 9, 20491−20500
Research Article
ACS Applied Materials & Interfaces
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