Article pubs.acs.org/JPCC
Ni/C Hierarchical Nanostructures with Ni Nanoparticles Highly Dispersed in N‑Containing Carbon Nanosheets: Origin of Li Storage Capacity Liwei Su, Zhen Zhou,* and Panwen Shen Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, People’s Republic of China S Supporting Information *
ABSTRACT: Ni/C hierarchical composites, which consist of Ni nanoparticles highly dispersed in N-containing carbon nanosheets, were prepared via a facile, economical, and green route, and the electrochemical Li storage performance was investigated. On the basis of the available lithium storage mechanisms, Ni nanoparticles are inert to react with Li+ and contribute nothing to electrochemical Li storage. However, the composites exhibited an unexpected reversible capacity of 1051 mAh g−1 after 30 cycles and 635 mAh g−1 after 100 cycles at the current density of 200 mA g−1. Such high reversible capacity cannot be simply ascribed to the Li insertion/extraction in carbon nanosheets. Instead, we proposed a possible origin of the reversible capacity, the electrochemical catalysis of Ni nanoparticles on the reversible formation/decomposition of some components in solid electrolyte interface films. These findings can further understand the role of transition-metal nanoparticles in lithium storage and open new doors for exploiting advanced materials for Li ion batteries and other energy-storage devices.
1. INTRODUCTION Lithium ion batteries (LIBs) are considered to be indispensable energy storage devices in terms of high energy/power density, environmental friendliness, and considerable safety with continuous advancement.1−3 Currently, great efforts have focused on exploring advanced anode materials to substitute for the commercialized graphite due to its low capacity (372 mAh g−1) and unsatisfactory high-rate performances, far from the dramatically increasing demand of mobile power sources.4 Diverse promising substitutes such as TiO2,5−7 Si,8−10 and transition-metal oxides (MOs)11,12 have been explored, which correspond to three lithium storage mechanisms: insertion/ extraction (type I), alloying/dealloying (type II), and conversion reactions (type III), respectively. The conversion mechanism can be presented as, taking NiO as an example, NiO + 2Li+ + 2e− ⇔ Ni + Li2O. During discharging, NiO is reduced to Ni nanoparticles, accompanied by the lithium storage capacity of 718 mAh g−1. Nickel nanoparticles and composites have received tremendous attention over decades owing to their potential applications in various fields including magnetism, optics, electronics, catalysis, and supercapacitors.13−15 However, they are rarely adopted as main active materials for lithium storage. On the basis of the type III mechanism mentioned above, Ni nanoparticles cannot directly react with Li+ anymore, and the formation of Ni nanoparticles demonstrates the end of the electrochemical procedure for lithium storage. Note that extra © 2012 American Chemical Society
capacity phenomena of NiO electrodes were widely disclosed in previous reports, especially NiO nanostructures combined with high-conductivity carbon materials.16−18 As a typical example, 3D hierarchical NiO−graphene composites exhibited a reversible capacity of 1056 mAh g−1 after 50 cycles, 50% higher than the theoretical value of NiO. The newly generated Ni nanoparticles during the discharge process should play a critical role in activating or promoting new electrochemical behaviors for lithium storage. To clarify the detailed function of Ni nanoparticles for lithium storage, we prepared Ni/C nanocomposites herein through a simple, economical, and green sol−gel route without surfactants or toxic substances. Compared with the previous preparation methods of Ni/C composites,13,19−21 this sol−gel route needs no expensive apparatus, reagents, or complicated processes and hence is suitable for large-scale production. More importantly, the as-obtained Ni/C hierarchical composites consist of uniform Ni/C nanosheets in which Ni nanoparticles are highly dispersed into N-containing carbon nanosheets. The small particle size and good spatial distribution in thin carbon nanosheets are beneficial for the direct contact of active materials with electrolytes and resultant excellent cycling performances for lithium storage.22 When tested as anodes in Received: October 11, 2012 Revised: October 24, 2012 Published: October 25, 2012 23974
dx.doi.org/10.1021/jp310054b | J. Phys. Chem. C 2012, 116, 23974−23980
The Journal of Physical Chemistry C
Article
LIBs, the as-prepared Ni/C nanocomposites displayed a reversible capacity much higher than the theoretical values of graphite and NiO. Possible mechanisms were discussed for the formation of this unique hierarchical structure and the existence of extra capacity beyond the three traditional mechanisms for lithium storage. This work might enlighten us on further understanding the role of transition-metal nanoparticles in lithium storage.
2. EXPERIMENTAL SECTION Preparation of Ni Nanoparticles Highly-Dispersed in Carbon Nanosheets. All reagents are of analytic grade and were used without further purification. In a typical synthesis, 10.00 g urea, 1.00 g citric acid, and 0.2 g NiCl2•6H2O were dissolved in the solution of 150 mL of ethanol and 50 mL of distilled water with continuous agitation at 75 °C for 3 h until the formation of a gel; then, the gel was transferred to an oven to keep 100 °C overnight. The as-prepared precursor was heated at two-step high temperatures of 350 °C for 4 h and 650 °C for 10 h in an Ar atmosphere. The obtained black products (symbolized as S1) were stored for further characterization. To clarify the effect of Ni nanoparticles on the lithium storage capability of Ni/C nanosheets, we added NiCl2•6H2O with different contents of 0.4, 0.6, 0.8, 1.2, and 2.4 g to the same solution, and the as-prepared products were symbolized as S2 to S6, respectively. As a reference, carbon nanosheets were also obtained under the same condition, except the addition of NiCl2•6H2O. Material Characterization. The samples were characterized by X-ray diffraction (XRD) (Rigaku D/Max III diffractometer with Cu Kα radiation, λ = 1.5418 Å), scanning electron microscopy (SEM, FEI Nanosem 430 field-emission gun scanning electron microscope), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) as well as energy-dispersive X-ray spectroscopy (EDS) (FEI Tecnai G2F-20 field-emission gun transmission electron microscope), thermogravimetric/differential thermal analysis (TG-DTA, Rigaku PTC-10A TG-DTA analyzer), elementary analyzer (vario EL CUBE, elementar), and X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical). Electrochemical Measurements. In test cells, lithium metal was used as the counter and reference electrode. The working electrodes were composed of active materials, acetylene black (AB), and polytetrafluoroethylene (PTFE) at the weight ratio of 15:3:2. The average weight of the working electrodes was approximately 2 mg. The electrolyte was 1 M LiPF6 dissolved in a 1:1:1 mixture of ethylene carbonate (EC), ethylene methyl carbonate (EMC), and dimethyl carbonate (DMC). The cells were assembled in a glovebox filled with high-purity argon (O2 and H2O < 1 ppm). Discharge/charge measurements were performed between the potential range of 0.01 and 3.00 V (vs Li/Li+) under a LAND-CT2001A instrument at room temperature. The specific capacity was calculated on the mass of the whole composites (Ni + C). Cyclic voltammetry (CV) was performed at a scanning rate of 0.1 mV s−1 between 0.01 and 3.00 V (vs Li/Li+) at room temperature.
Figure 1. XRD patterns of Ni/C nanosheets obtained with different NiCl2•6H2O contents.
to S1 to S6, respectively. When the NiCl2•6H2O contents were 0.2 and 0.4 g, there is only a broad peak at ∼26° corresponding to carbon. When the content of NiCl2•6H2O increases to 0.6 g (S3), the characteristic peaks located at 44.3°, 51.6°, and 76.2° start to appear, well-assigned to (111), (200), and (220) planes of cubic Ni with the space group of Fm3m (JCPDS 1-1258), respectively, and become stronger with increasing NiCl2•6H2O contents due to larger particles or better crystallization. No other peaks are observed in all XRD patterns, indicating that no other phase exists in the obtained Ni/C composites. The morphology and structure of the as-prepared Ni/C composites (S3 as a typical example) are presented in Figure 2.
3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the obtained samples with different NiCl2•6H2O contents from 0.2 to 2.4 g corresponding
Figure 2. SEM images of Ni/C nanosheets (S3) with different magnifications (A,B). TEM (C,D), HRTEM (E), and SAED (F) images of S3. 23975
dx.doi.org/10.1021/jp310054b | J. Phys. Chem. C 2012, 116, 23974−23980
The Journal of Physical Chemistry C
Article
Figure 3. TEM images of highly dispersed Ni/C nanosheets with different Ni contents. (A−F correspond to S1−S6, respectively.)
Figure 4. Cycling performances of Ni/C nanosheets (S1−S6) with different Ni contents and N-containing carbon nanosheets at 200 mA g−1 (A) and discharge/charge profiles (B) and cycling performances of S3 for the initial 100 cycles at 200 mA g−1 (C) and at different current densities (D) for the initial 30 cycles.
The sample is dominated by very large plate-like micrometerscale agglomerations (Figure 2A), which have hierarchical nanostructures and consist of hundreds of nanosheets with the thickness of
