Mechanics and transport modeling of particulate and fibrous network: Toward design of improved nickel metal hydride and lithium-ion battery technologies

The main goals of this work were to gain insight into the transport and mechanical properties of the particulate and fibrous network materials widely used as electrodes in NiMH and Li-Ion batteries. During electrochemical cycling, the networks experience multiaxial loading, resulting in degradation and morphology changes of the electrode materials. A stochastic finite element model was used to investigate the effects of morphologic parameters on material properties.; This work comprised two studies, of mechanics and transport, respectively, in porous materials. In a mechanics study, beam elements were used to model segments of the networks; several assumptions were investigated to model bond properties and local failures. In the transport study, another stochastic finite element model was developed, wherein particles were modeled as generalized ellipses. A four-point-probe experimental technique was used to measure electrode conductivity, for validation of the models.; In the mechanics study, network simulations and two-beam models showed that use of the Euler beam assumption was adequate for the nickel networks of interest, since the shortest segments in the networks apparently served only as rigid connections; most of the network deformation occurred with bending of the longer aspect ratio segments. A torsion spring bond assumption was found suitable for modeling imperfect bonding and curved fibers in the networks. The scale effect was found to be important; simulations less than one staple length in size resulted in unrealistically stiff networks. In the transport study, simulation results showed moderate increases in fiber or whisker aspect ratio significantly improved conductivity, offering immediately practical advice for manufacturers.; Overall, the capability of determining the effect of particle shape on material properties makes the stochastic model superior to continuum approaches for porous media. Further work will include extension to 3D materials, and coupling of models with electrochemical performance.