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Multi-Yolk-Shell SnO2/Co3Sn2@C Nanocubes with High Initial Coulombic Efficiency and Oxygen Reutilization for Lithium Storage Liwei Su, Yawei Xu, Jian Xie, Lianbang Wang, and Yuanhao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10450 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016
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ACS Applied Materials & Interfaces
Multi-Yolk-Shell SnO2/Co3Sn2@C Nanocubes with High Initial Coulombic Efficiency and Oxygen Reutilization for Lithium Storage
Liwei Sua,* Yawei Xua, Jian Xiea, Lianbang Wanga, Yuanhao Wangb,*
a
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China b
Faculty of Science and Technology, Technological and Higher Education Institute of
Hong Kong, Hong Kong *Corresponding author:
[email protected] (L. Su),
[email protected] (Y. Wang)
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ABSTRACT The challenging problems of SnO2 anode material for lithium ion batteries are the poor electronic conductivity and the low oxygen reutilization due to the irreversibility of Li2O generated in the initial discharge leading to a theoretical initial Coulombic efficiency (ICE) of only 52.4%. Different from these strategies, this work proposes a novel strategy to level up the oxygen reutilization in SnO2 by introducing Co3Sn2 nanoalloys which can release Co atoms to reversibly react with Li2O instead. According to this protocol, multi-yolk-shell SnO2/Co3Sn2@C nanocubes are designed and successfully prepared using hollow CoSn(OH)6 nanocubes as precursors followed a hydrothermal carbon coating and calcination treatment. The unique multi-yolk-shell nanostructure offers adequate breathing space for the volumetric deformation during long-term cycling. Moreover, the removal of Li2O allows a high electronic conductivity and resultant rate performance. As a result, the efficient reutilization of oxygen enables a high ICE of 71.7% and a reversible capacity of 1003 mAh g-1 after 200 cycles at 100 mA g-1. Cyclic voltammetry (CV), cycling performance at different voltage windows, and X-ray photoelectron spectroscopy (XPS) confirm the proposed mechanism. This strategy employing oxygen-poor metals or alloys provides a novel approach to enhance the oxygen reutilization in SnO2 for higher reversibility. KEYWORDS: Coulombic efficiency; Lithium ion batteries; Multi-yolk-shell structures; Oxygen reutilization; Reversible conversion 2
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ACS Applied Materials & Interfaces
1. INTRODUCTION With the ever-increasing demand of high performance energy storage devices, lithium ion batteries (LIBs) have been widely applying in portable electronics and electric vehicles mainly due to ultrahigh energy/power density,1,2 while the limited capacity of graphite-based materials dominating the anode market seriously restricts the energy output density.3 As a potential alternative, SnO2 has drawn researchers’ great attention in terms of promising theoretical capacity and appropriate operation voltages.4,5 The lithium storage reactions are generally presented as Eqns. (1) and (2). SnO2 + 4Li+ + 4e- → Sn + 2Li2O
Eqn. (1)
Sn + 4.4Li+ + 4.4e- ↔ Li4.4Sn
Eqn. (2)
The first step is generally believed to be irreversible and the newly-derived Sn is the actual active component for lithium storage, giving a theoretical capacity of 781 mAh g-1 (2.1 times that of graphite) and huge volumetric deformation of ~300%.6
The large volume swings during cycling can result in severe electrode
pulverization and successive formation of solid-electrolyte interphase (SEI) films, and are generally accommodated via preparing SnO2 nanomaterials (such as nanospheres,7 nanotubes,8 nanorods,9 nanowires,10 hollow,11 and yolk-shell nanostructures12) and/or incorporating with conductivity materials (involving pyrolysis carbon,13 carbon nanotubes,14 and graphene nanosheets15). Among them, hollow and yolk-shell structure facilitate the connection of electrolyte and active 3
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materials, promote Li+ ion diffusion, and offer more voids internally for volume variation.16-20 Another paramount disadvantage is the low initial Coulombic efficiency (ICE). According to Eqns. (1) and (2), O2- ions in the first step (Eqn. 1) are reversibly fixed in Li2O, indicating that a maximum of 4.4Li+ ions participate in the reversible conversion (Eqn. 2) rather than total 8.4Li+ ions involved. Thus, the theoretical value of ICE is only 52.4%, which hinders its performance in essence.21 If the oxygen can be totally reutilized, the theoretical capacity of SnO2 is as high as 1494 mAh g-1 (~4 times that of graphite).22 It is significant but remains a challenge to accomplish 100% reutilization of the oxygen in SnO2 until now.23,24 Tremendous efforts have been focusing on how to improve the reversibility of SnO2. It is well known that pristine SnO2 material is a typical semiconductor with a band gap of 3.6 eV. Guo et al. believed that the low electronic conductivity restricted the reversibility to a great extent.25 Therefore, they prepared carbon coated SnO2 hollow microspheres and found the existence of SnO in the charged electrode using high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Furthermore, quite a few works show that the reversibility of SnO2 can be realized by reducing the particle size especially to quantum dot (QD,
