Ab initio thermodynamics of phase-separating and cation-disordered cathodes for Li-ion batteries

Abstract

In order to accelerate the electrification of the automotive fleet, the energy density and power density limitations of commercial Li-ion battery cathodes (layered LiMO2) must be overcome. In this thesis, we use ab initio methods to gain critical insights on two important classes of alternative Li-ion battery cathodes, namely high-capacity Li-excess cation-disordered rocksalts and high-rate LiFePO4 In the first part of this thesis (Chapters 3 and 4), we provide the first voltage-based design rules for high-capacity cation-disordered rocksalts. We demonstrate that, depending on the transition metal species, cation disorder can increase or decrease the average voltage of lithium transition metal oxides, and hence increase or decrease the total energy density of these compounds In particular, the disordered Ni3+/4+ voltage is found to be high (~4.4V), value at which it is likely to be preceded by oxygen activity. We further investigate the effect of cation-disorder on the voltage slope of lithium transition metal oxides, which controls the total capacity accessible below the stability limit of the electrolyte. We demonstrate that cation-disorder increases the voltage slope by increasing the Li site energy distribution and by enabling Li occupation of high-voltage tetrahedral sites. We further demonstrate that the voltage slope increase upon disorder is smaller for high-voltage transition metals, and that short-range ordering and Liexcess contribute in reducing the inaccessible capacity at high voltage upon disorder. In the second part of this thesis (Chapter 5), we resolve the apparent paradox between the high Li diffusivity in phase-separating LiFePO 4 and the persistence of thermodynamically unstable solid-solution states during (dis)charge at low to moderate C-rates. We demonstrate that, even under rate conditions such that relaxation to a two-phase state is kinetically possible, the thermodynamically favorable state in a single particle is not a sharp interface but rather a diffuse interface with an intermediate solid-solution region that occupies a significant fraction of the particle volume. Our results not only explain the persistence of solid-solution regions at low to moderate C-rates in nano-LiFePO4, but also explain the observations of stable intermediate solid-solution states at an ac interface in particles quenched from a solid solution.