Insights into Electrochemical Behavior, Phase Evolution and Stability of Sn upon K-alloying/de-alloying via In Situ Studies

We report here results based on electrochemical K-alloying/de-alloying and associated in-situ studies with Sn in K 'half cells'. The as-recorded reversible K-capacity of similar to 245 mAh/g agree with the 'final' phase (K4Sn4) observed in in-situ synchrotron XRD scans at the end of one K-alloying half cycle. The electrochemical cycling and in-situ XRD results indicate that K4Sn4 forms via one-step phase transformation with no prior Sn-K solid solution formation and reverts back to beta-Sn after de-potassiation half cycle (leading to similar to 85 +/- 6% first cycle coulombic efficiency). Interestingly, no notable evidence for occurrence of irreversible surface reaction could be found during the 1st discharge. However, strong evidences for the same were recorded (at similar to 1.3-1.4 V, against K/K+) after completion of one full K-alloying/de-alloying cycle. In-situ monitoring of stress developments in the Sn film electrodes during galvanostatic cycling indicated the occurrence of mechanical instability not only during K-alloying/de-alloying induced phase transformations, but also upon occurrence of the surface reactions (as supported by SEM observations), which also lead to development of compressive stresses by itself. Accordingly, galvanostatic cycling within restricted cell voltage window of 1.2-0.01 V, as against 2-0.01 V, suppressed the irreversible surface phenomena and improved the cyclic stability. (C) 2017 The Electrochemical Society. All rights reserved.