The Newly Observed Reaction Mechanism of Mesoporous SnO2 Electrode 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. SnO₂ 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 SnO₂ to Sn and formation of a non-decomposable Li₂O matrix. Although there have been reports in the electrochemical behavior and performance improvement on SnO₂as 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 SnO₂ electrode material because of its higher abnormal capacity. Mesoporous SnO₂ 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 SnO₂ 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 SnO₂ 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 mesoporous SnO₂ which clearly show that SnO₂ structure changed into amorphous phase after point 4. Upon discharging below 0.2V amorphous SnO₂ phase converts into metallic phases which are indicated by broad peaks around 22 and 38 degree. To further study the mechanism we carried out ex-situ XAS measurements on mesoporous SnO₂ anode samples (Fig. 1 (b)). The peaks at 1.59Å and 2.60Å in the Fourier transform spectrum of extended X-ray absorption fine structure (EXAFS) represent Sn-O bond and Li-Sn bonds, respectively. [3] Irreversible capacity during first cycle of SnO₂ is related to conversion reaction and Li_4.4Sn is formed as a product of alloying reaction. Sn-O peak intensity was decreased gradually by progressing discharge and Li- Sn peak was revealed by alloying reaction after point 3. Moreover, through the EXAFS data of the first charge, we can verify the existence of Sn-O bond of conversion reaction at the last region and conversion reaction is associated with capacity of mesoporous SnO₂after initial discharge. These results were related to high abnormal capacity of mesoporous SnO₂ and more detailed discussion will be presented at the time of meeting.

1. I. A. Courtney and J. R. Dahn, J. Electrochem. Soc., 144, 2045 (1997)

2. I. A. Courtney and J.R. Dahn, J. Electrochem. Soc.,144, 2943 (1997)

3. A. N. Mansour, S. Mukerjee, X. Q. Yang and J. McBreen, J. Electrochem. Soc., 147, 869 (2000)