Xiao
Liu
ab,
Shuai
Zhang
a,
Peng
Zhang
*c,
Zongmin
Zheng
d,
Feng
Bai
a and
Qi
Li
*a
aKey Lab for Special Functional Materials of Ministry of Education, National & Local Joint Engineering Research Center for High-efficiency Display and Lighting Technology, Collaborative Innovation Center of Nano Functional Materials and Applications, School of Materials Science and Engineering, Henan University, Kaifeng 475004, P. R. China. E-mail: nanokingdom@163.com
bCollege of Chemistry and Chemical Engineering, Henan Key Laboratory of Function-Oriented Porous Materials, Luoyang Normal University, Luoyang, 471934, P. R. China
cDepartment of Chemistry, College of Sciences, Northeastern University, Shenyang, 110819, Liaoning, P. R. China
dNational Engineering Research Center for Intelligent Electrical Vehicle Power System, College of Mechanical and Electrical Engineering, Qingdao University, Qingdao, Shandong 266071, P. R. China
First published on 30th November 2022
Tin dioxide (SnO2) with a high theoretical capacity of 1494 mA h g−1 has great potential to break through the capacity limitation of the conventional graphite anode (372 mA h g−1) in lithium-ion batteries. However, its practical application still faces several obstacles such as high volumetric expansion and poor electrical conductivity. To solve these problems, innovative design and synthesis of SnO2-based nanocomposite structures are necessary. Herein, we demonstrate an integrated design of a hierarchical pitaya-like P-SnO2/C@NC core–shell nanostructure which includes the core of SnO2 nanoparticles (∼4–12 nm) uniformly embedded in the porous carbon sphere and the shell of a continuous nitrogen-doped carbon (NC) layer. Specifically, during repetitive lithiation and delithiation processes, the ultrasmall SnO2 nanoparticles reduce the internal stress greatly, the porous carbon matrix provides buffer space for a large volume change, and the N-doped carbon shell further guarantees the whole structure unit sufficient electrical conductivity and structural stability. Consequently, the resultant battery exhibits a reversible capacity of 936.8 mA h g−1 after 100 cycles at 100 mA g−1 and even an average capacity of 460.0 mA h g−1 at a high current density of 3.2 A g−1. The excellent electrochemical performance of pitaya-like SnO2/C@NC proves the efficacy of this structure design and thus provides significant reference for the construction of other electrode materials in rechargeable alkali metal ion batteries.
SnO2 + 4Li+ + 4e− Sn + 2Li2O | (1) |
Sn + 4.4Li+ + 4.4e− Li4.4Sn | (2) |
According to the reaction pathways, nanostructural design of SnO2, such as nanowires, nanotubes and hollow nanospheres, is believed to be a promising method to provide better reversibility because of its controllable internal strain caused by lithiation/delithiation.32–35 Meanwhile, nanostructured SnO2 can effectively shorten the Li-ion diffusion pathway and consequently improve the rate capability to some extent.36,37 However, the intrinsic low electronic conductivity of SnO2 still constrains its rate capability; likewise pulverization caused by the large volume expansion limits the cycling stability.38 To solve these two problems, loading SnO2 nanostructures on various types of conductive carbon substrates is put forward to improve the conductivity and structural stability of SnO2.39–43 Simultaneously, the porous design of the carbon substrate is developed to further accommodate the large volume change and promote the rapid transmission of Li+.44 Nevertheless, SnO2 nanoparticles that directly grow on the surface layer of the carbon materials would still migrate and aggregate during long cycles.31,32 Therefore, for driving SnO2 to substantially overcome its defects and further increase the stability, realization of an advanced and integrated structural innovation is urgently needed.
In this work, we have designed and synthesized a core–shell structured SnO2-based composite where ultrasmall SnO2 nanoparticles are uniformly embedded in porous carbon nanospheres and there is a continuous N-doped carbon coating layer on the surface (denoted as P-SnO2/C@NC). The pitaya-like composites not only improve the electrical conductivity of SnO2, but also alleviate its expansion and aggregation during the lithiation/delithiation process. More importantly, the N-doped carbon shell can act as an armor to ensure the structure stability of the SnO2/carbon composite during cycling. As expected, the obtained P-SnO2/C@NC gives a high reversible capacity of 936.8 mA h g−1 at 100 mA g−1 after 100 cycles and superior rate performance. The excellent performance can be attributed to the unique pitaya-like structure of P-SnO2/C@NC and thus provides significant reference for the synthesis of other electrode materials for rechargeable alkali metal ion batteries.
To gain insight into the novel structure of P-SnO2/C@NC, two typical SnO2-based materials including SnO2 nanoparticles embedded in carbon nanospheres (denoted as SnO2/C) and yolk–shell SnO2 nanospheres were obtained from P-SnO2/C@NC for comparison (Fig. 2a). After insufficient calcination in air to remove the N-doped carbon layer, SnO2/C nanospheres were obtained. According to the TEM image, SnO2/C exhibited a similar morphology to P-SnO2/C@NC, but the surface was coarse and SnO2 nanoparticles were exposed on the surface. When the carbon matrix is completely removed, yolk–shell SnO2 nanospheres are obtained. Similar yolk–shell metallic oxide nanospheres have been synthesized using a developed sequential template method by Wang's and Lou's groups46,47 In addition, in order to confirm the role of the barrier matrix, SnO2 nanoparticles loaded on graphene nanosheets (denoted as SnO2/GSs) were also synthesized by a previously reported method.48 From the SEM and TEM images (Fig. S7a and S7b†), it can be seen that a large amount of ultrasmall SnO2 nanoparticles anchored on GSs. All the diffraction peaks of SnO2/C, yolk–shell SnO2 and SnO2/GSs can be indexed to rutile SnO2 (Fig. 2b). It is worth noting that the diffraction peaks of SnO2/GSs are relatively broad, indicating the smaller crystal size of SnO2 nanoparticles, which is consistent with the SEM and TEM results. The weight ratios of SnO2 in P-SnO2/C@NC, SnO2/C, yolk–shell SnO2 and SnO2/GSs determined by thermogravimetric analysis (TGA) are 58.1%, 68.4%, 97.5% and 37.9%, respectively (Fig. 2c). The decrease of N content (Fig. 2d) and increase of Sn content (Fig. S8†) also proved that the N-doped carbon coating layer and carbon matrix were gradually removed. In the Raman spectra, the D band and G band corresponding to disordered and graphitic carbon centered at 1355 and 1575 cm−1, respectively (Fig. 2e). The ID/IG ratio can be used to estimate the average size of the graphitic domains, and the value for P-SnO2/C@NC and SnO2/C is about 0.85 and 0.88, respectively, indicating a high graphitization degree after calcination. Nitrogen adsorption–desorption isotherms of P-SnO2/C@NC, SnO2/C, yolk–shell SnO2 and SnO2/GSs show a typical type-IV curve with a hysteresis loop (Fig. 2f). The corresponding pore size distributions of these samples based on the Barrett–Joyner–Halenda (BJH) model demonstrate the existence of abundant mesoporous (Fig. 2g). Such a porous structure gives rise to a high Brunauer–Emmett–Teller (BET) specific surface area of P-SnO2/C@NC, SnO2/C, yolk–shell SnO2 and SnO2/GSs of 248, 230, 44 and 165 m2 g−1, respectively. The numerous mesopores in all the samples provide the function of accommodating the large volume change and also accelerating the lithium insertion/extraction during charge and discharge processes. Through the design and comparison of these pertinent samples, we could further learn the critical role of the NC coating layer as well as the importance of structure optimization.
Intrigued by the advanced structural features of P-SnO2/C@NC, we have evaluated the electrochemical performance of the above samples. EIS results show that the charge transfer resistance of P-SnO2/C@NC is slightly higher than that of SnO2/GSs, but obviously lower than that of SnO2/C and yolk–shell SnO2 (Fig. 3a), indicating the superior conductivity of P-SnO2/C@NC. The decreased conductivity of SnO2/C and yolk–shell SnO2 can be attributed to the removal of the conductive N-doped carbon layer and the carbon matrix during the post-calcination process (Fig. 2a). Fig. 3b shows the CV curves of P-SnO2/C@NC in the initial three cycles, and a pronounced reduction peak at 0.72 V in the first cycle can be assigned to the reduction of SnO2 to Sn and the formation of a solid-electrolyte-interphase layer (SEI), and another reduction peak at 0.05 V can be attributed to the alloying process of Li and Sn.44 During the anodic scan, a strong peak at 0.59 V and a broad peak at 1.23 V correspond to the delithiation of LixSn to Sn and the oxidation of metallic Sn, respectively.30 In the subsequent cycles, the CV curves almost overlap, suggesting the excellent electrochemical reversibility of the P-SnO2/C@NC electrode. However for SnO2/C, yolk–shell SnO2 and SnO2/GSs, the peak current decreases continuously with the CV cycling (Fig. S9a–c†). In addition, when comparing the third cycle of P-SnO2/C@NC, SnO2/C, yolk–shell SnO2 and SnO2/GSs (Fig. S9d†), P-SnO2/C@NC shows the smallest peak separation (70 mV) for the reversible peaks located near 1.20 V, demonstrating the less overpotential for the phase transformation between Sn and SnO2 for the P-SnO2/C@NC electrode than the others. Galvanostatic charge/discharge tests were further performed to evaluate the electrochemical performance of the above samples in the potential window of 3.0–0.005 V at a constant current density of 0.1 A g−1 (Fig. 3c and S10†). After 100 cycles, P-SnO2/C@NC still delivers a discharge capacity of about 936.8 mA h g−1 with a high coulombic efficiency close to 99.0%. The excellent capacity retention should benefit from improved electron transfer with an N-doped carbon coating layer and the ultrasmall SnO2 nanoparticle embedded in a porous carbon matrix. In contrast, the capacity of SnO2/C, yolk–shell SnO2 and SnO2/GSs significantly decreased down to 250.5, 107.7 and 342.3 mA h g−1, respectively. The first coulombic efficiency (CE) for P-SnO2/C@NC, SnO2/C, yolk–shell SnO2 and SnO2/GSs was 76.3%, 59.2%, 58.6% and 57.9%, respectively. The loss of capacity is mainly caused by the irreversible formation of SEI films. From the second cycle, the CE of P-SnO2/C@NC increases quickly to nearly 100%, and poor conductivity and no carbon matrix are responsible for the low capacity retention for SnO2/C and yolk–shell SnO2. Though SnO2/GSs has good conductivity, the exposure of SnO2 nanoparticles on the surface of GSs inevitably leads to the aggregation during cycling, thus giving low capacity retention. Fig. 3d compares the rate performance of P-SnO2/C@NC, SnO2/C, yolk–shell SnO2 and SnO2/GSs at current densities of 0.1, 0.2, 0.4, 0.8, 1.6 and 3.2 A g−1. The P-SnO2/C@NC electrode also exhibits optimal performance at high current densities, and the average value is 851.0, 791.0, 720.2, 647.9, 562.3 and 455.5 mA h g−1, respectively. More importantly, the discharge capacity can be recovered to 805.2 mA h g−1 when the current density returns to 0.1 A g−1. In contrast, SnO2/C, yolk–shell SnO2 and SnO2/GSs give a much lower capacity of 225.8, 83.4 and 40 mA h g−1 at 3.2 A g−1, respectively. In addition, the cycling performance of the P-SnO2/C@NC electrode at a high current density of 0.4 A g−1 can maintain 690 mA h g−1 even after 100 cycles (Fig. S11†). These results demonstrate the excellent energy density and high-rate capability of the P-SnO2/C@NC composite.
In order to have an in-depth understanding of the lithium storage behavior in P-SnO2/C@NC, SnO2/C and yolk–shell SnO2 electrodes, a series of CV tests with scan rates between 0.1 and 10 mV s−1 were carried out (Fig. 4). According to a power law relationship i = aνb (logi = loga + blogν, where i is the peak current, ν is the scan rate, a and b are adjustable parameters), b values obtained from the slope of the plot of logi vs. logν can be used to describe the reaction kinetics (Fig. 4d). There are two well-defined conditions: b = 0.5 and b = 1.0. For b = 0.5, the limiting case would be a diffusion-controlled process, while b = 1.0 is representative of a surface controlled process, which also means that the reaction rate is rapid.28 At a low scan rate of 0.1–1.0 mV s−1, all b values are calculated in the range of 0.8–1.0 for P-SnO2/C@NC (Table S1†), indicating a major surface controlled process, while for SnO2/C and yolk–shell SnO2 (Fig. S12 and Table S1†), b values for the conversion reaction drop to 0.7, implying a combination of solid state diffusion and surface limited reactions. At higher scan rates between 1.0 and 10 mV s−1, for P-SnO2/C@NC, all b values are still greater than 0.5, while for SnO2/C and yolk–shell SnO2, some b values are below 0.5, which means that SnO2/C and yolk–shell SnO2 are mainly controlled by the diffusion process at a higher scan rate.
To understand the difference in the electrochemical performance, the morphology and structure changes of the corresponding electrodes after charge–discharge cycles were investigated. SEM images reveal that P-SnO2/C@NC (Fig. 5a) and SnO2/C (Fig. 5c) still maintain the spherical shape after 100 cycles, while the framework of yolk–shell SnO2 collapsed, which accounts for its low electrochemical performance (Fig. S13a†). TEM images were obtained to further investigate the inner structure. It can be seen that the SnO2 nanoparticles become smaller in P-SnO2/C@NC (Fig. 5b), while the SnO2 nanoparticles agglomerate into larger ones in SnO2/C (Fig. 5d), and no complete microsphere is observed in yolk–shell SnO2 (Fig. S13b†). These results unambiguously demonstrate the key role of the N-doped carbon coating layer in preventing the agglomeration of SnO2 nanoparticles and maintaining the stability of the structure unit. The disappearance of the characteristic XRD peaks of rutile SnO2 in both P-SnO2/C@NC and SnO2/C suggests the amorphous nature of SnO2 after cycling (Fig. 5e). The XPS results demonstrate that Sn 3d peaks slight shift to higher energies after charging (Fig. 5f), and the variation could be attributed to a more negative ion combining with Sn4+.49 The possible reaction mechanism can be conjectured as follows: due to the poor electrical conductivity of SnO2/C and yolk–shell SnO2, the reduction of SnO2 nanoparticles starts at the contact point between the current collector and the SnO2 nanoparticles; particles away from the current collector must wait for the reaction front to arrive from adjacent particles, thus the particle size will grow gradually. While for P-SnO2/C@NC, the reduction front radiates inward from the entire surface due to the enhancement of surface conductivity by the N-doped carbon coating layer. After the completion of the reduction reaction, each nanosphere transformed to Li2O and fine LixSn precipitates decorated the carbon matrix. These fine precipitates are too small to crack, and thus they can be reversibly oxidized to SnO2 and finally result in a stable capacity reversibility in this system.
Fig. 5 (a, c) SEM and (b, d) TEM images of P-SnO2/C@NC (a, b) and SnO2/C (c, d) electrodes after cycling. (e) XRD pattern and (f) XPS spectra of P-SnO2/C@NC and SnO2/C after cycling. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr05702f |
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