Nanoscale thermal transport and elastic properties of lithiated amorphous Si thin films

Silicon is the heart of modern electronics and due to it is high theoretical storage capacity it also attracted much attention as a possible anode material for the next generation Li-ion batteries. Heat conduction is one of the major properties in the development of Li-ion batteries for energy conversion systems and it is of paramount importance to have comprehensive understanding of heat dissipation on the scale of electrochemical storage device. In this work we report ex-situ study of nanoscale thermal transport and elastic properties of lithiated amorphous Si (a-Si) anode films using picosecond time-domain thermoreflectance (TDTR). Radio frequency (rf) magnetron sputtering was used to deposit a ~330 nm thick a-Si films on glass substrate. Near-surface nanoscale thermal transport measurements show 40% increase in thermal conductivity of a-Si upon electrochemical lithiation reaching up to 2.2 W m^-1K-¹. This sizeable increase might be due to Li⁺ ion-mediated heat conduction during lithiation process. The standard deviation of measured thermal conductivity was slightly higher likely due to inhomogeneous lateral and cross-plane Li⁺ ions distribution in the sub-surface film region. Nanosecond laser pulsed induced surface acoustic waves (SAWs) measurements showed the decrease in Young’s modulus after lithiation on nanometre scale, which is attributed to volumetric expansion of Si upon Li⁺ ions insertion.