Letter www.acsami.org
Enhanced Reaction Kinetics and Structure Integrity of Ni/SnO2 Nanocluster toward High-Performance Lithium Storage Yinzhu Jiang,*,† Yong Li,† Peng Zhou,† Shenglan Yu,† Wenping Sun,*,‡ and Shixue Dou‡ †
State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Key Laboratory of Novel Materials for Information Technology of Zhejiang Province and School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China ‡ Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia S Supporting Information *
ABSTRACT: SnO2 is regarded as one of the most promising anodes via conversion-alloying mechanism for advanced lithium ion batteries. However, the sluggish conversion reaction severely degrades the reversible capacity, Coulombic efficiency and rate capability. In this paper, through constructing porous Ni/SnO2 composite electrode composed of homogeneously distributed SnO2 and Ni nanoparticles, the reaction kinetics of SnO2 is greatly enhanced, leading to full conversion reaction, superior cycling stability and improved rate capability. The uniformly distributed Ni nanoparticles provide a fast charge transport pathway for electrochemical reactions, and restrict the direct contact and aggregation of SnO2 nanoparticles during cycling. In the meantime, the void space among the nanoclusters increases the contact area between the electrolyte and active materials, and accommodates the huge volume change during cycling as well. The Ni/SnO2 composite electrode possesses a high reversible capacity of 820.5 mAh g−1 at 1 A g−1 up to 100 cycles. More impressively, large capacity of 841.9, 806.6, and 770.7 mAh g−1 can still be maintained at high current densities of 2, 5, and 10 A g−1 respectively. The results demonstrate that Ni/SnO2 is a high-performance anode for advanced lithium-ion batteries with high specific capacity, excellent rate capability, and cycling stability. KEYWORDS: lithium-ion batteries, anode, SnO2, Ni, reaction kinetics
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because of intrinsically high surface free energy and spontaneous atom migration,9 which eventually causes the electrode pulverization and performance decay.10 On the other hand, incorporation of carbon encapsulation layers can improve the electrical conductivity and decrease the electrode polarization.11 However, the conversion reaction kinetics between Sn and SnO2 is not improved significantly because of the sluggish nature of the conversion reaction, especially at high current densities. Zhou et al. demonstrated unprecedentedly high capacity in SnO2 NC@N-RGO hybrid nanomaterial.1 However, Sn nanoparticles resided in the cycled electrode, indicating fully reversible conversion reaction was not maintained. Additionally, although Zhao et al. reported that complete conversion reaction of SnO2 was achieved and maintained up to 100 cycles (at a low current density of 0.15 A g−1) for SWNTs@SnO2@PPy composite, the specific capacity dropped significantly upon increasing current density.12 Recently, conductive metal materials (such as Au, Ag, and Ni) have been demonstrated to enhance the electrochemical reaction kinetics and maintain the structure integrity of the electrode materials.13−15 Moreover, nanosized transition metals can act as the catalyst to promote the decomposition of Li2O, which is extremely beneficial for the conversion reaction
nO2 has attracted considerable attention as a promising anode for lithium-ion batteries (LIBs) because of its numerous appealing features, including high theoretical capacity, abundance, low cost, and environmental benignity.1,2 Typically, the theoretical capacity of SnO2 is 782 mAh g−1 based on alloying mechanism (xLi+ + xe− + Sn ↔ LixSn (0 ≤ x ≤ 4.4)),3,4 and the value is doubled to 1482 mAh g−1 in case that the conversion reaction in SnO2 is fully reversible (SnO2 + 4Li+ + 4e−↔ Sn + 2Li2O).1,5,6 However, such conversion reaction is proved to be sluggish and might become irreversible during cycling, which limits the rate capability and induces capacity decline upon cycling.3 Moreover, the extra volume change during the conversion reaction between Sn and SnO2 aggravate the structure degeneration and further deteriorate the cycling performance.7 Therefore, the full governing on conversion reaction of SnO2-based electrode is of high urgency and significance for achieving large reversible capacity with high Columbic efficiency, high rate capability, and long cycle life. There have been various attempts ranging from fabricating SnO2 nanomaterials to coating or encapsulating SnO2 with conductive carbon to achieve reliable performance. Engineering SnO2 nanostructures is beneficial to facilitate the electrochemical reaction kinetics, especially for conversion reaction. Meantime, nanostructured materials can also effectively accommodate partial strain associated with volume change, leading to enhanced cycling stability.8 Unfortunately, these nanostructured materials tend to aggregate during cycling © XXXX American Chemical Society
Received: September 10, 2015 Accepted: November 18, 2015
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DOI: 10.1021/acsami.5b08303 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces kinetics.16 Our recent work demonstrated significantly enhanced rate capability after incorporating Ag nanoparticles into porous Fe2O3 film anode.13 The homogeneously dispersed Ag nanoparticles established a well-organized conductive network and greatly accelerated the conversion reaction even at high rates.13 As inspired by the previous reports, incorporating metallic nanostructures into SnO2 anodes might induce a considerable performance enhancement in reversible capacity, cycling stability, as well as rate capability. In this paper, Ni/SnO2 nanoclusters were prepared as anode for LIBs by pulsed spray evaporation chemical vapor deposition (PSE-CVD) (Figure 1), and more experimental details can be
features are beneficial to fast and durable lithiation/delithiation reactions. The Ni/SnO2 nanoclusters were demonstrated to exhibit enhanced reaction kinetics and outstanding lithium storage capability. The as-deposited Ni/SnO2 composite film was first characterized by XRD to identify its phase structure (Figure 2a). Only a broad peak can be discerned, indicating that SnO2 in the composite is not well crystallized and has very small grain size. It is hard to distinguish the existence of metallic Ni since the peaks of Ni and the substrate overlap.18 Ni/SnO2 film was then deposited on copper foil for further structure characterization. As shown in the inset of Figure 2a, a weak peak centered at around 44.6° can be attributed to the (111) planes of cubic Ni (JCPDS card No. 04−0850).18 For bare SnO2 film, besides the strong peaks of the steel substrate, three diffraction peaks can be assigned to (110), (101), and (211) planes of tetragonal SnO2 (JCPDS card no. 41−1445).19 XPS was performed to investigate the electronic structure and bonding nature of Ni/SnO2 film. As demonstrated in Figure 2b, the binding energies of 486.60 (Sn 3d5/2) and 495.00 eV (Sn 3d3/2) have a spin−orbit splitting of 8.4 eV, suggesting Sn4+ bounds oxygen in the SnO2 matrix.20 The Ni 2p3/2 (Figure 2c) peak with a binding energy of 852.48 eV verifies the metallic nature of the embedded Ni nanoparticles, which is consistent with the reported values.21,22 For the oxygen core level (O 1s), the peaks at lower (530.50 eV) and higher (532.00 eV) energies can be attributed to the O2− bound to Sn4+ and the chemisorbed oxygen, respectively (Figure 2d).13 The XRD and XPS results confirm that composite film is composed of metallic Ni and SnO2. The surface morphologies of both Ni/SnO2 composite and bare SnO2 films were characterized by SEM. The SnO2 film displays a smooth dense surface (Figure S1a), which consists of densely packed superfine nanoparticles (
