(Invited) Grain Texture and Transport in Sintered Lithium Cobaltite
Authors: Tanner Cameron W 
Published: February 2024

Reversible exchange of lithium from lithium cobaltite (LCO) with the layered rock-salt structure was first reported by Goodenough in 1980 [1]. It has since become a critical component in the cathode of lithium-ion batteries that power portable electronics like cell phones, laptop computers, and cordless power tools. The rate of lithium diffusivity in LCO particles ranges from 10-7 to 10-11 cm²/s depending upon the state of charge and the method of measurement [2]. Lithium diffusivity in LCO because of its layered structure is also dependent upon crystallographic direction. In thin-film, micro-batteries (TFMB’s), the crystallographic orientation of LCO is controlled through choice of conditions for deposition and crystallization [3]. The diffusivity of lithium is significantly greater within rather than across the basal plane [4]. Orientations of (104) and (110) are preferred for rate performance.

Continuous, roll-to-roll sintering of LCO ribbon with thicknesses down to 10 µm was recently demonstrated [5]. The LCO ribbon because it is sintered and self-supporting can in principle serve as a mechanical support to reduce the proportions of inactive battery components to dramatically increase energy density. A possible way of creating a solid-state battery with LCO ribbon is to capitalize on the LiPON solid electrolyte from TFMB’s. Both composite and dense cathodes can be envisioned. Energy densities greater than 1500 Wh/L are possible. The practicality and design of a cathode-supported battery depends upon lithium diffusion in the LCO and charge transfer resistance at the interface with the separator. Ideally, transport is sufficiently facile that acceptable rate performance in a battery is realized with cathode thickness of greater than 20 µm.

In this work, we report on characterization of lithium diffusivity and ohmic losses in sintered LCO with controlled grain textures. LCO cathodes with thicknesses of 15-30 µm with random and (003) grain textures were prepared by tape casting and rapid sintering. The (hk0) texture was made by taking advantage of the (003) textured green tape; approximately 200 layers were stacked and cold-welded to form a block with a thickness of ~8 mm. The sintered block was diced to expose the edge with the favorable (hk0) orientation. Electron backscatter diffraction maps and pole figures that illustrate the grain structure and texture are shown in Fig. 1. The texture is so strong that material is orthotropic in character.

Lithium diffusivity and ohmic cell resistances were determined as a function of state of charge by galvanostatic intermittent titration [7]. An average diffusivity to use as a single figure of merit was also determined by fitting of capacities measured as a function of rate of discharge from a cut-off potential of 4.3 V. Rates of lithium diffusion in sintered LCO cathodes with the (hk0) texture is 0.09 µm²/s and approximately twelve-fold greater than for the (003) texture, 0.0076 µm²/s. Ohmic loss was also found to depend upon grain texture. It was approximately 60-100 Ωcm² for (hk0) texture and >500 Ωcm² for (003). A reasonable explanation for the elevated resistance is that charge transfer occurs more easily with the (hk0) texture.

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Figure 1