Grain boundary metal–insulator transitions in polycrystalline LiCoO2

A thermodynamically consistent variational framework is developed to show that the metal–insulator transition in polycrystalline LiCoO2 is driven by grain boundary lithium segregation, the interfacial misorientation, and the size of the abutting grains. In general, high-angle grain boundaries and grains possessing high curvature favor phase transformation to Li-rich phases. In particular, the insulating O3(I) phase wets high-angle grain boundaries in a manner analogous to grain boundary premelting, at least Δc=0.15 before the O3(II)-O3(I) equilibrium phase boundary, c=0.75. A critical misorientation as a function of the macroscopic lithium content, Δθc(c) exists above which the grain boundaries undergo a metal-insulating transition. The critical misorientation decreases with composition until at c=0.81, the percolation threshold is reached, forming an insulating grain boundary network that dramatically suppresses the electrical conductivity of polycrystalline LCO. The O3(I) phase extends into the O3(II) grain interiors through a diffusion-limited process, while the O3(II) phase extends into the H1-3 grain interiors through a phase transformation limited process. Results suggest that the fabrication of textured LCO microstructures will delay the metal–insulator transition. Specifically, textured microstructures exhibiting grain boundary misorientations Δθ<6∘ will suppress the formation of the insulating O3(I) phase up to the O3(II)-O3(I) phase boundary, enabling the use of an additional 64.6 Wh/kg in specific energy density and 16.4 Ah/kg of charge between c=0.75 and c=0.81.