Damian Burch

Particle Size Dependence of the Ionic Diffusivity

Diffusion constants are typically considered to be independent of particle size with the benefit of nanosizing materials arising solely from shortened transport paths. We show that for materials with one-dimensional atomic migration channels, the diffusion constant depends on particle size with diffusion in bulk being much slower than in nanoparticles. This model accounts for conflicting data on LiFePO(4), an important material for rechargeable lithium batteries, specifically explaining why it functions exclusively on the nanoscale.

Size-Dependent Spinodal and Miscibility Gaps for Intercalation in Nanoparticles

Using a recently proposed mathematical model for intercalation dynamics in phase-separating materials ( Singh , G. K. , Ceder , G. and Bazant , M. Z. Electrochimica Acta 2008 , 53 , 7599. ), we show that the spinodal and miscibility gaps generally shrink as the host particle size decreases to the nanoscale. Our work is motivated by recent experiments on the high-rate Li-ion battery material LiFePO(4); this serves as the basis for our examples, but our analysis and conclusions apply to any intercalation material. We describe two general mechanisms for the suppression of phase separation in nanoparticles, (i) a classical bulk effect, predicted by the Cahn-Hilliard equation in which the diffuse phase boundary becomes confined by the particle geometry; and (ii) a novel surface effect, predicted by chemical-potential-dependent reaction kinetics, in which insertion/extraction reactions stabilize composition gradients near surfaces in equilibrium with the local environment. Composition-dependent surface energy and (especially) elastic strain can contribute to these effects but are not required to predict decreased spinodal and miscibility gaps at the nanoscale.

Phase-Transformation Wave Dynamics in LiFePO4

A general continuum model has recently been proposed for the dynamics of ion intercalation in a single crystal of rechargeable-battery electrode materials [1]. When applied to strongly phase-separating, highly anisotropic materials such as LiFePO4, phase-transformation waves are predicted between the lithiated and unlithiated portions of a crystal. In this paper, we extend the analysis of the wave dynamics, and we describe a new mechanism for current capacity fade through the interactions of these waves with defects in the material.