Electrochemical, spectroscopic, and thermal investigations of LiSn2(PO4)3 and Sn3(PO4)2 anodes during the first discharge

The first discharge of crystalline LiSn2(PO4)(3) and Sn-3(PO4)(2) is investigated with infrared spectroscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and impedance spectroscopy. Prior to discharging, both compounds have a rich vibrational structure in the mid- and far-IR; however, discharging to 1.00 V completely destroys this structure. Specifically, the PO43- intramolecular stretching and bending vibrations collapse into broad bands while the lattice modes vanish for both compounds. The resulting mid- and far-IR spectra are consistent with the formation of highly disordered Li3PO4. Carbonate bands appear in the mid-IR spectra when either compound is discharged to 1.00 V. These bands are assigned to the products of an electrolyte decomposition reaction catalyzed by the metallic tin that forms in these electrodes. The infrared spectra suggest that Li+ ions do not strongly interact with the Li3PO4 or Li2CO3 during the Li-Sn alloying process. Residual amounts of unalloyed metallic tin are detected with XRD and DSC. The Sn-3(PO4)(2) electrodes result in considerably larger amounts of residual tin than LiSn2(PO4)(3). These differences may be explained by considering the relative concentration of tin produced when each compound is discharged. Slow Li+ ion diffusion probably causes a small amount of tin in the center of the larger aggregates to remain unalloyed. (c) 2005 The Electrochemical Society. All rights reserved.