Electrochemical Stress Measurements of Potential Induced Structural Changes in Sn Anodes for Li Ion Batteries

We report on electrochemical surface stress measurements of thin film Sn electrodes for Li-ion battery anodes at potentials <2.0 V vs. Li/Li⁺. In most anodes, mechanical changes are dominated and controlled by Li-host interactions. Graphite, Si, and a Au model system exhibit compressive stress resulting from Li insertion. Calculations support these experimental results. Anodes experience tensile stress resulting from Li removal. This tensile stress may create cracks, and cause capacity loss.

In contrast, Sn surfaces exhibit major compressive and tensile surface stress changes even before Li insertion. These features originate in conversion reactions between SnOx on the electrode surface and Li forming Sn and LiOx.  During the cathodic scan, an intense compressive feature at ca. 0.7 V vs. Li/Li⁺ is observed. A major tensile release at ca. 0.6 V vs. Li/Li⁺ follows this compressive feature. Changes in the amount of oxygen in the surface film affect the electrochemical stress measurement dramatically, while having a more modest impact on the voltammetry.  The SnOx/Li conversion reaction impacts the ability of Sn and its alloys to serve as an anode material for a Li ion battery.

We also report the results of matrix assisted laser desorption (MALDI) time of flight (TOF) mass spectrometry (MS) analysis of Sn electrodes. In a mixture of ethylene carbonate and dimethyl carbonate, long chain oligomers are observed following the first cycle. These oligomers are decomposed in the subsequent cycles showing that Sn surfaces form an unstable SEI. This decomposition results in oligomerized species, which are different from those formed at the end of the first cycle. We will discuss potential and solvent dependent oligomerization mechanisms and their effect on mechanical properties of Sn electrodes.