Direct Tracking of SnO2-Li+ Reactions in Confined Carbon Nanospace
Authors: Notohara Hiroo , Urita Koki , Moriguchi Isamu 
Published: December 2024

SnO2 has a high theoretical capacity of 1492 mAh/g-SnO2 by (1) a conversion reaction and (2) an alloying-dealloying reaction, described as follows: SnO2 + 4 Li+ + 4 e- ⇌ Sn + 2 Li2O (1) Sn + 4.4 Li+ + 4.4 e- ⇌ Li4.4Sn (2)

However, the fact of the matter is that the SnO2 electrode hardly exhibits such high capacity due to poor reversibility of lithiation and delithiation reactions. The irreversibility is largely a result of an aggregation of Sn metal particles in Li2O matrix through the lithiation and delithiation reactions.[1] The coarsening of Sn in Li2O matrix reduces the interface area for the reverse conversion reaction and leads large void in the Li2O matrix at the delithiation process.[2] The prevention of Sn coarsening is critical for improving the reversibility of the conversion reaction. The nanosizing of SnO2 and the confinement of reaction spaces were especially effective to improve the reversibility of the conversion reaction.[3] However, the effects on the phase change of SnO2 in sub-nanometer space have not been clarified. It is important to understand the mechanisms that provide high reversibility to maximize the performance of rechargeable batteries. In this study, we present the lithiation and delithiation reactivity and structural changes of SnO2 confined in sub-nanometer space using the inside space of single-walled carbon nanotubes (SWCNT). The SnO2-loaded SWCNT composite (SnO2@CNTs) was obtained by rinsing with water after introducing SnCl2 vapor into the SWCNT.[4] Charge-discharge characteristics of SnO2@CNTs were evaluated in the coin-type cells using Li metal counter and 1 M LiFP6 in EC-DMC electrolyte. The structural changes in the SnO2@CNTs during charge-discharge were evaluated using ex situ and in situ scanning transmission electron microscope (STEM) observations. In situ STEM observations were performed using a probe-type in situ TEM holder; a metal Li foil with an oxidized surface was used as the counter electrode, and SnO2@CNTs were used as the working electrode. The counter and working electrodes were contacted in the TEM column, and in situ TEM observations were conducted during the charging and discharging processes of the SnO2@CNTs. Structural characterization showed that SnO2 nanocrystals were loaded within the inner space of the SWCNTs. A small amount of SnO2 particles was loaded onto the SWCNT bundle surfaces. The ex situ STEM observations clearly show that reversible lithiation and delithiation of the SnO2 electrode were achieved in the carbon nanospaces. In contrast, the SnO2 particles on the outer surface of the SWCNTs changed to large Sn particles after the initial delithiation process, indicating that the reverse conversion reaction of SnO2 particles on the surface of SWCNTs is irreversible. To investigate the dynamics of the reversible SnO2 and Li-ion reactions in the SWCNTs, in situ STEM observations were performed. SnO2 particles in the carbon nanospace expanded into the carbon pores during the lithiation process. The LixSn and Li2O phases produced in the conversion reaction had domains of approximately 1 nm. Aggregation of Sn and LixSn particles did not occur as in the bulk material, and the aggregation of Sn particles was less likely to occur in the carbon nanospace. In the delithiation process, the LixSn and Li2O phases were converted back to SnO2 particles. The results of our study showed the morphological reversibility of SnO2 electrode was achieved by the nano-confinement against SnO2 in sub-nm space and leaded the reversibility of lithiation and delithiation reactions.[5]

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