A homogenization study of the effects of cycling on the electronic conductivity of commercial lithium-ion battery cathodes

State-of-the-art image acquisition, image analysis, and modern homogenization theory are used to study the effects of cycling on commercial lithium-ion battery cathodes’ ability to conduct electronic current. This framework allows for a rigorous computation of an effective, or macroscale, electronic conductivity given an arbitrarily complicated three-dimensional microstructure comprised of three different material phases, i.e., active material, binder (polymer mixed with conductive carbon black), and electrolyte. The approach explicitly takes into account the geometry and is thus a vast improvement over the commonly used Bruggeman approximation. We apply our framework to two different types of lithium-ion battery cathodes before and after cycling. This leads us to predict an appreciable decrease in the effective electronic conductivity as a direct result of cycling. In addition, we present an ad-hoc “neighbor counting” methodology which meaningfully quantifies the effect of binder detaching from the surface of the active material due to the internal mechanical stresses experienced under operating conditions, thereby supporting the results of the homogenization calculations

3D framework structure of a new lithium thiophosphate, LiTi2(PS4)(3), as lithium insertion hosts

The M2(PS4)3 (M = transition metal) structure has been investigated as a new open 3D-framework structure for electrode materials for lithium rechargeable batteries. One of these compounds, LiTi2(PS4)3, was synthesized from solid state reaction. From the single crystal X-ray diffraction analysis, it was observed that the LiTi2(PS4)3 crystallizes in the hexagonal space group P6cc (No. 184) with a = 19.8978 (4) A, c = 11.5198(3) A, and Z = 8. The resemblance of the powder X-ray diffraction pattern and the lattice parameters obtained from single crystal X-ray diffraction measurement indicates that the crystal structure is isomorphous with NaTi2(PS4K This structure is built of TiS6 octahedra linked by edges to PS4 tetrahedra and vice versa to build a 3D framework. Most attractively, very wide tunnels are obtained along the c-axis where lithium ions are mobile. Electrochemical insertion of lithium into LiTi2(PS4)3 was carried out. The initial structure of LiTi2(PS4)3 was changed with initial insertion of lithium, but ???7 Li per formula unit could be inserted in the modified structure without decomposition of the starting material. However, such a large capacity was not retained on cycling.close121