Lithium-ion batteries (LIBs) have been recognized as major part in the development of electrical devices such as portable device, energy storage system (ESS) and electric vehicle (EV) due to high energy storage-demand. Graphite material is the most widely used as anode part in commercial LIBs due to its low cost and good cycle life. However, due to limited capacity (~372mAh/g) of graphite, alternative anode materials based on conversion or alloying reaction have been extensively studied, which shows exceptional high capacity. Among these anode materials, SnO₂ has been attracted as one of the promising anode materials, which stores high electric energy by conversion and alloying reaction during first discharge process: (1) SnO₂ + 4Li⁺+ 4e^- → Sn + 2Li₂O (Conversion reaction; 711mAh/g), (2) Sn + xLi⁺+ xe^- → Li_xSn (0<<x<<4.4) (Alloying reaction; 993mAh/g of Sn or 783mAh/g of SnO₂). In this lithium storage mechanism, only alloying reaction has been contributed to reversible capacity after the first irreversible discharge process.[1] Currently, there are several reports that some nano-structured and composite SnO₂ materials deliver exceptional high reversible capacity beyond their theoretical capacity based on alloying reaction without clear explanation of additional capacity.[2,3] In our previous work, for elucidating the additional capacity and the detailed lithium storage mechanism of nano-structured SnO₂, we performed systematic analysis about the mesoporous SnO₂ anode material using synchrotron radiation based analyses such as XRD and XAS, and reported novel electrochemical reaction mechanism. Ordered mesoporous SnO₂ prepared by hard templating showed a reversible capacity of about 1100 mAh/g, which is higher than the predicted value based on the alloying reaction mechanism of Sn, formed from conversion reaction of SnO₂ with lithium. Synchrotron XAS analysis combined with XRD revealed that some portion of the Li₂O phase decomposes to form the SnO_x phase with Sn upon delithiation (additional conversion reaction), parallel with dealloying of Li_xSn (alloying reaction), which leads to unexpected high capacity of an ordered mesoporous SnO₂ material.[4] Namely, high performance of SnO₂ anode material is highly related to the additional conversion reaction during electrochemical cycling and stable structure for accommodating volume expansion.

In this study, to understand the main reason for the difference in electrochemical performance between bulk and mesoporous SnO₂, we performed XAS experiment about bulk SnO₂ anode material in same procedure with our previous report, and carried out quantitative analysis using EXAFS spectra of bulk and mesoporous sample as well. Besides, for maximizing the performance of SnO₂ anode material for LIBs, we prepared SnO₂/rGO composite, synthesized by simple refluxing method using graphene oxide, SnCl₄ and ascorbic acid. Composition of metal oxide nanoparticles and rGO systems is the efficient methods to improve electrochemical performance in terms of rate performance and cyclability. In metal oxide/rGO composite anode materials, rGO provide faster lithium ion diffusion pathway through rGO sheets and metal-O-C linkage at the interface, and ensure stable structure during volume expansion aroused from electrochemical cycling by strong interaction between rGO sheets and metal oxide nanoparticles.[5]

The discharge/charge capacity of bulk, mesoporous, and SnO₂/rGO composite were 1644, 1073mAh/g; 2059, 1148mAh/g; and 1752, 1185mAh/g, respectively. Except the irreversible capacity during the initial discharge, reversible capacity is similar to each other. However, compared to bulk and mesoporous SnO₂, SnO₂/rGO composite material shows prominent cyclability, and its capacity retention after 100^th cycles is over 100%. This phenomenon may be related to the stable rGO source and exfoliation process during electrochemical cycling, respectively.[6] Using synchrotron radiation based techniques, we found that high reversible capacity of SnO₂/rGO composite comes from alloying reaction and partial conversion reaction as same as bulk and mesoporous SnO₂. Additionally, through further XAS experiment using cycled samples, we confirmed key factors in the high performance of SnO₂/rGO composite anode material. More detailed discussion will be presented at the time of meeting.

Reference

[1] I. A. Courtney et al., Journal of The Electrochemical Society 144 (1997) 2045-2052

[2] Z. Ying et al., Appl. Phys. Lett. 87 (2005) 113108

[3] J. Lin et al., ACS Nano 7, (2013) 6001–6006

[4] H. Kim et al., Chemistry of materials 26 (2014) 6361-6370

[5] X.-Y. Shan et al., J. Mater. Chem. A 2 (2014) 17808–17814

[6] J. Wang et al., J. Am. Chem. Soc. 133 (2011) 8888–8891