Investigation of the degradation of SnO2 electrodes for use in Li-ion cells

SnO2 nanoparticle layers were synthesized as electrodes, assembled in lithium half-cells and tested by combining cyclic voltammetry and long-term cycling behavior, supplemented by XPS- and SEM-investigations. The cycling behavior of SnO2 shows characteristic features: large difference between discharging and charging capacity for the first cycles, pronounced negative slope, corresponding to huge degradation of specific capacity and a shallower slope after achieving coulombic efficiency around 100%. The areas under the charge and discharge curves of the voltammograms are assumed to be a measure for the amount of oxidation and reduction, respectively. The integrated reduction and oxidation areas are not equal after the first cycle (58:42). The whole reaction of the cycle can be described as partially reversible. Consequently, residuals remain, not contributing to capacity in the next cycle. However, after achieving coulombic efficiency around 100%, the area ratio becomes 50:50. The goal of XPS investigation is to show the evolution of the phases while crossing specific potentials. It was shown that the SEI is appearing for the first time by crossing the potential of 1 V. This SEI is dominated by lithium carbonate but also consists of a complex mixture of different lithium compounds and other decomposition products of the electrolyte.

► Combination of cyclic voltammetry and long term battery cycling on the same half-cell.
► For the first time, areas under the curves were evaluated and set into relation.
► Amount of Li-ions involved in the process, due to reduction and oxidation.
► Evolution of the SEI during cycling, were carried out with ex-situ XPS measurements.