An In Situ Study of the Electrochemical Reaction of Li with Nanostructured Sn30Co30C40

The reaction of lithium with nanostructured Sn 30 CO 30 C 40 alloy prepared by mechanical attrition was studied using in situ X-ray diffraction (XRD) and in situ 119 Sn Mossbauer effect spectroscopy. During the first discharge (insertion of Li into the alloy) of Li/Sn 30 Co 30 C 40 cells, similarities between the structures of fully lithiated Sn 30 Co 30 C 40 and lithiated Sn were observed by XRD. Both XRD patterns showed evidence for the Sn tetrahedra, with neighboring Li atoms, as found in Li 22 Sn 5 . This implies that fully lithiated Sn 30 Co 30 C 40 is composed of nanoscale regions of Li 22 Sn 5 , nanoscale Co, and lithiated disordered carbon. In situ 119 Sn Mossbauer spectroscopy results were consistent with this interpretation and showed the presence of unlithiated Sn atoms at the bottom of the discharge. This observation agrees well with the fact that attrited Sn 30 Co 30 C 40 samples only reach about two-thirds of the theoretical capacity. During the first recharge, in situ results from both techniques show that Co is rebonded to the Sn atoms in the CoSn grains as Li is removed. Overall, the insertion of lithium in nanostructured Sn 30 Co 30 C 40 is believed to proceed via the following reaction where only the fully charged (left) and fully discharged (right) states are indicated: [132(1 - z) + 40y]Li + Sn 30 Co 30 C 40 ↔ (1 - z)(6Li 22 Sn 5 + 30Co) + zCoSn + 40Li y C, where y ≈ 0.5.

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