Sn, which offers large theoretical capacity (994 mAh/g), is a potentially useful anode material in lithium-ion batteries. However, Sn exhibits irreversible capacity loss through mechanical degradation. This occurs due to the large volume changes (~ 300%) associated with the formation of different phases during the electrochemical lithiation. The kinetics of phase transformations play an important role in controlling the performance of Sn anodes. To investigate the phase kinetics, potentiostatic lithiations at selective potentials were conducted to induce desired phase transformations in electroplated Sn thin films. The Multi-Beam Optical Stress Sensor technique was used to measure the in situ stress evolution. Kinetic models were applied to simulate the phase boundary propagation, and to extract kinetic parameters, such as Li diffusivities and reaction rate coefficients of phase transformations. The kinetic analysis was also implemented in studying the connection between Li diffusion and stress evolution observed in the experiments. Furthermore, the mechanical property of the Sn anodes changes with Li concentration. The elastic modulus of Sn anodes at different states of charge was determined from the measured elastic response of the lithiated Sn film during a small-scale delithiation. X-ray diffraction and focused-ion beam milling were performed to characterize the phase composition in the samples after experiments. This system serves as a canonical setup to understand the diffusion with moving phase boundary and mechanical property measurement of thin films. The results from this work are useful for future research on reliability, mechanical failure, and cell design of Sn anode electrodes.
"Phase Kinetics and Mechanical Characteristics of Sn Anodes in Li-ion Batteries"
Materials Science Engineering Theses and Dissertations.
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