Modelling and Validation of High-Energy Density 30µm Thin-Film Solid-State LiCoO2 Cell: 1D Cahn-Hilliard Phase Separation Model

Abstract

LiCoO2 (LCO) dominates the industry of high energy density Li-batteries cathodes, with almost 20% of market share. Since its introduction in 1980, scientific community produced a rich literature; in particular, several studies suggest maximizing the loaded weight of active material as an interesting route for energy density increase: over the whole weight of the cell, the impact of electrolyte, current collectors, and anode decreases, determining an increase in volumetric and gravimetric energy density. Notwithstanding, unclear correlation between electrode thickness and delivered capacity, altogether with an extremely short cycle-life and poor rate-capability, reported for cathode thicker than 10µm [1], are now halting the development of cells with thick electrodes for both solid-state deposited (PVD, pulse laser, electroplating) and tape-casted cathode.We focused our work on unveiling the causal link between thickness and available capacity in LCO cathode, providing a simple formula to relate the latter with cathode dimension, discharge current, and temperature. We propose solid-state thin-film configuration in a lithium-free battery as a test vehicle to study pure LCO cathode. This configuration introduces no additional dynamic limitation like grain-to-grain diffusion.We exploited both model and experimental results to delve into the understanding of interfacial lithium accumulation during discharge and we linked the related capacity loss to phase separation occurring around 75% of intercalated lithium [2-3]. In order to interpret both dynamic and static results we have implemented a 1D model based on Cahn-Hilliard equation [4] and solved it using COMSOL. We will provide results to foster a division of LCO cathode into two domain: from 50% to 75% of lithium intercalation, a high-rate zone, which capacity can be delivered at high C-rate without any limitation for any considered LCO thickness; from 75% to 100%, a low-rate zone, which can provide capacity only at very low C-rate.We validated the model realizing 5, 10, 20 and 30µm thick LCO cathode in a thin-film all-solid-state configuration, with LIPON as solid electrolyte, tested for discharge current density from 0.2 to 2 mAcm-2. We also present the 30µm thick LCO cathode with the highest capacity reported for thin-film configuration so far (>1mAhcm-2) and provide strong evidences of linear correlation between delivered capacity and cathode thickness. The results obtained for all-solid-state thin-film cells, can be extended to all LCO cathode, whatever the deposition technique used, as long as the particle size is the same order of the present work and the main limiting factor from the dynamic point of view is the solid diffusion in the cathode.