October 4-9, 2015
Recent advances in nanoindentation of lithiated silicon are described. Silicon, due to its high theoretical specific capacity for electrochemical lithium incorporation, has emerged as one of the most appealing materials to replace conventional graphitic anodes in lithium ion batteries. But inserting lithium into silicon causes a large volume expansion (~300%) that can promote fracture during lithiation/delithiation cycling, thereby reducing the practical capacity of silicon lithium ion battery anodes. Understanding these mechanical behavior problems is essential for their management and control. Current FEM modeling is based on only estimates of the elastic and properties of lithiated Si. Better knowledge of these mechanical properties is required.
We have developed a method for conducting nanoindentation experiments on lithiated silicon under the protection of a mineral oil and have made accurate measurements of hardness and modulus of amorphous silicon as a function of lithium content. We have also observed and characterized viscoplastic creep in lithiated silicon, using both time-dependent displacement measurements and time-dependent stiffness measurements using the CSM feature of the nanoindenter and the analysis method introduced by Maier, Merle, Göken and Durst. We find that viscoplastic flow can be described as a stress-driven, thermally activated process with an activation volume comparable to the scale of a molecular unit of Li15Si4. These findings point to the importance of incorporating viscoplasticity into lithiation/delithiation models. These more subtle mechanical behavior effects may need to be included in future modeling. It is hoped that these studies will be useful in the design of silicon electrodes for advanced battery systems.
William Nix and Lucas Berla, "Mechanical properties of lithiated silicon: A candidate electrode for lithium ion batteries" in "Nanomechanical Testing in Materials Research and Development V", Dr. Marc Legros, CEMES-CNRS, France Eds, ECI Symposium Series, (2015). http://dc.engconfintl.org/nanomechtest_v/41