The Newly Observed Reaction Mechanism of Mesoporous SnO 2 E lectrode Investigated by Synchrotron X-ray Techniques

Graphitic carbon is widely used as anode material due to its low cost, good cycle life, and very stable capacity in most commercial lithium-ion batteries (LIBs). However, capacity of carbon anode (372mAh/g and 830mAh/mL) is limited by the reversible electrochemical intercalation of lithium ions in its structure. So, the search of new anode material has been ongoing to achieve the higher capacity. SnO 2 has been widely studied in the last decade as one of the potential candidates for anode materials due to its higher specific lithium storage capacity (783mAh/g). [1, 2] But, its poor capacity retention over long-term charge-discharge cycling has prevented its use as commercial anode material in LIBs. This problem has been associated with its alloying reaction which results in large volume changes of electrode material during electrochemical cycling. Also, an irreversible conversion reaction occurs prior to the alloying reaction, which results in the reduction of SnO 2 to Sn and formation of a non-decomposable Li 2 O matrix. Although there have been reports in the electrochemical behavior and performance improvement on SnO 2 as anode materials for Li-ion batteries, it is still difficult to prove the reaction mechanism and abnormal capacity clearly. For accurate explanation of reaction mechanism and abnormal capacity, it is important to analyze each region systematically. We studied mesoporous SnO 2 electrode material because of its higher abnormal capacity. Mesoporous SnO 2 was synthesized by sol-gel method by using the KIT-6 template. SEM & EDS were used to confirm the successful synthesis of this electrode material. Additionally, we performed diverse electrochemical tests such as EIS, GITT and cyclic voltammetry. The first discharge capacity of mesoporous SnO 2 was 2009.60mAh·g -1 and the charge capacity was 1048.43 mAh·g -1 . Compared with the theoretical specific capacity, the extra discharge capacity was associated with the formation of a solid electrolyte interphase (SEI) layer generated by an irreversible insertion/extraction of Li-ions into host structures or Li alloying reactions and by possible interfacial Li storage. In this work, we have tried to explain the electrochemical reaction mechanism of meso-porous SnO 2 by using ex situ X-ray Diffraction (XRD) and X-ray Absorption Spectroscopy (XAS) during cycles. Before the experiment, we were subdividing points in discharge/charge curves. Fig. 1 (a) shows ex-situ XRD patterns during first discharge of