Phase transition behavior of a processed thermal battery.

In order to fully understand the transition mechanism of a processed thermal battery, Micro Xray Diffraction (i-XRD) and Micro X-ray Fluorescence (i-XRF) were employed to characterize various quench states. The transition of interest in this paper is the formation of Li7Si3 from Li13Si4 (initial anode material). The unit cell conditions during Li13Si4 transition to Li7Si3 show a contraction of 0.04 and 0.02 A (a and b-axis respectively), an expansion in the c-axis of 0.02 A, and an overall reduction in the cell volume from 541.13 A to 539.21 A for the Li13Si4 orthorhombic (Pbam) component. The contraction in the a-b plane results from the loss of lithium atoms, whereas the expansion in the c-axis direction is due to reorganization of the lithium and silicon within the unit cell. Transition processing also requires that an excess of 32⁄3 moles lithium (within anode region) react with sulfur (within cathode region) to form Li2S. Micro-XRF confirms a definite migration of sulfur through the separator region during the transition. These results both explain peak shifts (in anode regions) and the formation of Li2S during transition states.