This paper aims to help fill a gap in the literature on Li-ion battery electrode materials due to the absence of measured elastic constants needed for diffusion induced stress models. By examining results from new first principles density functional theory (DFT) calculations of LiCoO2, LiMn2O4, (and their delithiated hosts, CoO2 and MnO2), LixAl alloys, and data from the extant literature on LiFePO4 (and FePO4), LiTi2O4 (and Li2Ti2O4), LixSi, LixSn and lihtium graphite-interaction-compounds, a compelling picture. emerges on the dependency of the elastic properties on Li concentration. Specifically, three distinct categories of behavior are found: (a) the averaged Young's moduli change very minimally upon lithiation of the spinet and olivine structures; (b) lithiation induced stiffening is observed only when new and stronger bonds between the Li ions and the host materials are formed in layered compounds; and (c) for-alloy-forming-electrode materials, such as Si, beta-Sn and Al, the averaged Young's moduli of lithiated compounds follow the linear rule of mixtures. The tendency of ductile or brittle behavior electrode materials is investigated with the Pugh criterion, and a ductile to brittle transition was found to occur during lithiation of Al and beta-Sn, but not in Si. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.