Thermodynamic aspects of the reaction of lithium with SnP2O7 based positive electrodes

The reaction of lithium with tin pyrophosphate, 4Li(+) + SnP2O7 -> Li4P2O7+ Sn, which yields a nanocomposite formed by tin and lithium pyrophosphate, has been probed electrochemically by step potential electrochemical spectroscopy. The thermodynamic characteristics (i. e., Delta G, Delta H and Delta S) of the formation of nanosized tin particles have been determined from data obtained under equilibrium conditions at different temperatures. A first order change in free energy clearly reflects the transformation of alpha tin to beta tin at 286 +/- 1 K. This shows that for the electrochemically produced nanocrystalline tin phase, the transition is at the same temperature as that expected for bulk materials. Interestingly this transition is not significantly limited by kinetics in the way that bulk tin metal is infamous for in "tin plague" and so we have been able to derive values for these important thermodynamic parameters. Taking into account the absence of long range order in the nanocomposites, the observations indicate that performance of batteries based upon tin oxides as the anode precursor may be affected by small changes of temperature around the transition point. The thermodynamics for this displacive reaction are found to be of a similar order to those obtained for a typical intercalation reaction. However, it seems that the entropy contribution to the free energy dominates for the displacive reaction, which is likely due to the formation of nanosized tin particles. (c) 2007 The Electrochemical Society.