High Initial Reversible Capacity and Long Life of Ternary SnO2-Co-carbon Nanocomposite Anodes for Lithium-Ion Batteries
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SnO2-Co-carbon nanocomposites were in-situ prepared from Co-based metal–organic frameworks and showed a remarkably high initial Coulombic efficiency (82.2%) and a capacity of ~ 800 mAh g−1 at a high current density of 5 A g−1.
Facile approach for designing highly reversible and stable electrodes for next-generation high-performance lithium-ion batteries.
KeywordsUltrafine SnO2 nanostructures ZIF-67 frameworks Enhanced initial Coulombic efficiency Reversible conversion reaction
Over the past few years, the applications of lithium-ion batteries (LIBs) have extended from consumer electronics to power batteries. This impressive progress achieved in this field suggests that LIBs will continue to be a dominant power source for electric vehicles in the next decade . In the pursuit of further improvement of LIBs, various efforts have been made to rationalize their design and to develop advanced electrode materials with high specific capacity, prolonged life span, and good rate capability [2, 3]. Graphite is the most widely used LIB anode. However, it exhibits a limited specific capacity of 372 mAh g−1. Therefore, in 2011, Sony Corporation produced novel LIBs (Nexelion). Specifically, their anodes consisted of Sn-Co–C composites. Various tin-based anodes have also been fabricated in order to develop high-performance LIBs [2, 3, 4]. Among these anodes, tin oxide (SnO2) anodes have been extensively studied because SnO2 can store Li+ via a two-step reaction and shows a high theoretical specific capacity of 1494 mAh g−1. In the Li+ storage of SnO2, the first step is the conversion reaction (SnO2 + 4Li → Sn + 2Li2O), which generates a capacity of 731 mAh g−1. In the subsequent lithiation/delithiation process, an alloying reaction (Sn + 4.4Li → Li4.4Sn) occurs, delivering a capacity of 763 mAh g−1 [6, 7, 8]. However, there are two major challenges in developing SnO2 anodes with a high specific capacity: (i) capacity loss induced by the huge volume variations (greater than 300%) generated during cycling, (ii) the irreversibility of the conversion reaction, which reduces the initial Coulombic efficiency (ICE) of the anode [5, 6, 7].
Various attempts have been made to overcome these limitations. For example, various carbon-based SnO2 composites have been developed to accommodate the volume expansions caused by cycling in order to achieve cycling stability [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. However, this carbon composite approach cannot improve the ICE of SnO2 anodes. Hu et al. reported that SnO2 electrodes with Sn grains < 11 nm in diameter show a completely reversible conversion reaction (Sn + Li2O → SnO2) . Transition metals (M = Cu, Fe, Mn, Co, etc.) or metal oxides are used to stabilize nanostructured SnO2 and can also improve the reversibility of the conversion reaction between Li2O and SnO2 [4, 20, 21, 22, 23, 24, 25]. Inactive metals can buffer the expansion of Sn particles (to coarsen them) and migrate to the Sn/Li2O surface so that Sn can remain active with Li2O. Thus, the introduction of transition metals can improve the ICE of SnO2 electrodes. To sum up, there are mainly three mainstream solutions for developing high-performance SnO2-based anodes: (1) designing a unique carbon-based structure including the surface coating to suppress the full volume expansions [26, 27] while providing an expansion space [8, 28, 29, 30, 31], (2) preparing ultrafine SnO2 nanoparticles to aggrandize the grain boundaries and alleviate the mechanical strain and improve the reversibility of the conversion reaction , and (3) introducing transition metals or forming intermetallic alloys to make the conversion reaction reversible and mitigate the expansion of Sn simultaneously [4, 20, 21, 22, 23].
Recently, metal–organic frameworks (MOFs) with inorganic (Co and Zn) and organic molecules have been used as a novel 3D porous carbon source for developing adjustable templates to anchor guest transition metals [32, 33, 34]. In this study, we fabricated a novel ternary SnO2-Co-C composite by mixing ultrafine SnO2 nanoparticles with a Co-based MOF (ZIF-67) (denoted as N-u-SCC) to develop LIB anodes with high ICE and long-term cycle stability. ZIF-67 serves as a sacrificial template for the formation of Co additives and 3D porous carbon frameworks. This well-designed structure showed the advantages of the unique 3D carbon-based nanostructure with in situ formed Co additives and suppressed the volume expansions and Sn coarsening of the lithiated SnO2. This improved the cycling performance of the anodes and rendered the conversion reaction highly reversible. The 3D porous carbon framework served as an excellent carrier for SnO2 (to be anchored) and improved the conductivity of the entire composite while providing enough space for volume variations during the lithiation/delithiation process. The in situ formed Co additives not only prevented the covering of SnO2 by Li2O and alleviated the volume expansions, but also served as good electron conductors. As a result, the N-u-SCC-2 electrode showed a high ICE of 82.2% (average level). In addition, the electrodes showed extraordinary specific capacity (~ 975 mAh g−1 after 100 cycles at 0.2 A g−1), high capacity retention (78.6% after 100 cycles at 0.2 A g−1), excellent rate capability (a reversible capacity of ~ 800 mAh g−1 under the current density of 5 A g−1), and prolonged life span.
2 Experimental Section
All the chemicals used in this work were analytically pure and were commercially available. Commercial SnO2 and Co(NO3)2·6H2O, 2-methylimidazole were purchased from Shanghai Macklin Biochemical Co. Ltd. Na2SnO3 and urea were purchased from Xilong Scientific.
2.1 Synthesis of Ultrafine SnO2
Ultrafine SnO2 was prepared by modifying the method reported by Lou et al. . In a typical reaction, 2.5 mmol of NaSnO3·4H2O and 16 mmol of urea were added into a solution of 145 mL of H2O and 15 mL of ethanol and the resulting mixture was stirred for 1 h. The reaction mixture was then transferred to a Teflon-lined stainless-steel autoclave, which was heated in an oven at 190 °C for 15 h. The reaction mixture was centrifuged to obtain precipitates, which were dried at 80 °C overnight and annealed at 550 °C for 4 h.
2.2 Synthesis of ZIF-67 Frameworks
Part of basic facts of the N-u-SCC composites
Mole dosage of Co(NO3)2·6H2O (mmol)
Mole dosage of 2-Melm (mmol)
Carbon content (%)
2.3 Synthesis of N-c-SCC and N-u-SCC Composites
The N-doped commercial SnO2-Co-C (denoted as N-c-SCC) and N-u-SCC composites were prepared using a method similar to that used for the preparation of the ZIF-67 frameworks. The only difference was that 0.2 g of SnO2 (commercial or ultrafine) was added into solution A followed by sonication for 0.5 h. After the sonication, solution A was stirred for another 10 min for better dispersion. Then, solution B was quickly added to solution A and the resulting mixture was stirred vigorously for another 3 min. The reaction mixture was then static aged for 22 h. The precipitates separated from the solution by centrifugation were freeze-dried and then annealed at 550 °C for 2 h.
2.4 Material Characterization
The morphology of the as-prepared samples was examined by a SUPRA 55 field-emission scanning electron microscope (FESEM). Transmission electron microscopy (TEM) examinations were carried out on a JEOL JEM 2100F at 200 kV. The elemental mapping and energy-dispersive X-ray spectroscopy (EDS) measurements of the samples were taken on an energy-dispersive X-ray spectrometer equipped with the JEOL 2100F microscope. The powder X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Ultima IV with Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) analysis was carried out using an Axis Ultra DLD spectrometer.
2.5 Electrochemical Measurements
The electrochemical performance of the as-prepared composites was evaluated using R2032 coin-type half cells assembled in a glove box filled with argon. The oxygen and moisture contents of the glove box were < 0.5 ppm. The electrodes were prepared by confecting a slurry containing the active materials, carbon black, and carboxymethyl cellulose with a ratio of 7:2:1 in a solution of water and ethanol. This slurry solution was stirred for 8 h and was then casted onto a Cu foil and dried at 80 °C for 12 h under vacuum. The active material loading on each electrode was 0.9–1.1 mg. A solution of 1 M LiPF6 in ethylene carbonate and diethyl carbonate (at a volume ratio of 1:2) containing 10 wt% fluoroethylene carbonate (FEC) was used as the electrolyte. The electrochemical measurements of the electrodes were taken using a Neware battery tester over the potential range of 0.01–3.0 V. The specific capacity of the composites was calculated using their whole masses. Cyclic voltammetry (CV) measurements were taken on an electrochemical workstation (CHI 660C) over the voltage range of 0.01–3 V at a scan rate of 0.1 mV s−1.
3 Results and Discussion
The valence states of Sn and Co in the N-u-SCC-2 composite were analyzed by XPS. As can be observed from Fig. 5c, Sn showed a main valence of Sn4+. This is consistent with the observation that SnO2 was the main phase of this composite. In addition, some Sn present on the surface showed a valence of zero. This can be attributed to the reduction of metallic Sn by carbon and Co during the annealing process (as observed from the XRD patterns). Oxygen vacancies were generated at this point. The Co 2p spectra (Fig. 5d) of the near-surface region of the composite showed three types of valence (Co, CoO, and Co2O3). The increasing valence could be assigned to the oxidation of metallic Co. The small amount of oxidation in the near surface of the composite could not be detected by XRD. Figure 5e, f shows the N 1s and C 1s spectra of the N-u-SCC-2 composite and confirms the presence of N and C in it. Moreover, the high-resolution N 1s XPS spectra showed that the composite consisted of pyridinic N (398.7 eV) and pyrrolic N (400.5 eV) (Fig. S5). The pyridinic N and pyrrolic N contents of this composite were 80.3% and 19.7%, respectively (Fig. S5). These results indicate the N-doped C composite SnO2 was the main active phase of the N-u-SCC-2 composite.
The N-u-SCC-2 electrode showed a reversible discharge capacity of ~ 975 mAh g−1 after 100 cycles (Fig. 6d) at a current density of 0.2 A g−1, which corresponds to a capacity retention ratio of 78.6% (when compared with the 2nd cycle). On the other hand, the N-u-SCC-1 and pure SnO2 electrodes exhibited much smaller reversible discharge capacities of 504 and 226.0 mAh g−1, respectively, after 100 cycles. The first discharge capacities of the N-u-SCC-2, N-u-SCC-1, and pure SnO2 electrodes were 1365.2, 990.5, and 1736.7 mAh g−1, respectively. This high capacity retention of the N-u-SCC-2 electrode can be attributed to its high carbon content, which provided more space for volume variations along with a larger Sn/Co contact area to inhibit the volume expansion, thus increasing the cycling life of the electrode. It should be noted that the N-u-SCC-2 electrode showed an average ICE of 82.2% because of the formation of SEI layers. This improved the reversibility of the reactions between SnO2 and Li. On the other hand, the N-u-SCC-1 and pure SnO2 electrodes showed an ICE of 68.1% and 59.7%, respectively (Fig. S6). The N-u-SCC-2 electrode exhibited smaller internal resistance than the other two electrodes, as revealed by the electrochemical impedance spectroscopy (EIS) measurements shown in Fig. S7a. The EIS measurements of the N-u-SCC-2 electrode after 100 cycles were also taken. An equivalent circuit consisting of the resistances of the electrolyte (Re), charge transfer (Rct), and constant phase elements and Warburg impedance was proposed to fit the impedance data. Low Rct values were obtained after 100 cycles (179–32 Ω), suggesting the enhanced charge transfer kinetics (Table S4). In addition, the N-u-SCC-2 electrode showed excellent rate capability. Figure S8a shows the discharge/charge curves of the electrode at different current densities. All the curves showed similar trend with the same discharge/charge platform. Furthermore, a reversible capacity of ~ 800 mAh g−1 was obtained when the current density was increased to 5 A g−1. At the current densities of 0.2, 0.5, 1, 2, and 5 A g−1, discharge capacities of ~ 1500, 1240, 1090, 965, and 800 mAh g−1, respectively, were obtained. As the current returned to 0.2 A g−1, the capacity became stable, as shown in Fig. S8b. This extraordinary rate capability of N-u-SCC-2 was not due to its unique nanostructure consisting only of Co and carbon framework, but can be attributed to the oxygen vacancies, which improved the conductivity and transportation of Li+ [28, 31, 35, 36, 37, 38, 39, 40]. The N-u-SCC-2 electrode exhibited a cycling life of up to 450 cycles and maintained a reversible capacity of ~ 760 mAh g−1 at the current density of 0.5 A g−1, as shown in Fig. 6e. After 150 cycles, the diffusion kinetics of lithium ions improved after the initial activation. The optimization of the SEI layer at the initial stage leads to capacity fading because of its breakdown and reconstruction. The cycling performance of an electrode improves after the formation of a stable SEI layer. The long-term cycling performance of the N-u-SCC-2 electrodes at relatively high current densities of 1 A g−1 for 200 cycles and 2 A g−1 for 300 cycles was also evaluated, as shown in Fig. S9. As can be observed from the figure, the electrode exhibited a desirable long-term cycling performance.
In summary, a novel ternary SnO2-Co-C nanocomposite (N-u-SCC) was successfully prepared via a simple and low-cost synthesis method. In this design, the in situ formation of Co additives from the Co-based ZIF-67 framework rendered the SnO2 conversion reaction highly reversible and the N-doped carbon frameworks efficiently mitigated the structural degradation of SnO2 while facilitating electronic transport and ionic diffusion. Accordingly, the optimized N-u-SCC electrodes exhibited excellent electrochemical performance with high ICE (average 82.2%), outstanding rate performance (800 mAh g−1 at 5 A g−1), and long-term cycling performance (~ 760 mAh g−1 after 400 cycles at a current density of 0.5 A g−1). These findings will be helpful for developing highly reversible and stable electrodes for next-generation high-performance LIBs.
This work is financially supported by the National Key R&D Program of China (No. 2016YFA0202602) and the National Natural Science Foundation of China (Grant Nos. 21503178 and 21703185). We are grateful to Hongfei Zheng of Xiamen University for his technical support in transmission electron microscopy. This work is also supported by XMU Undergraduate Innovation and Entrepreneurship Training Programs (Grants No. 2017X0695 for Huijiao Yang and Xiaocong Tang).
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