Study of Bi-layer electrodes for lithium-ion batteries through simulation and experiment

Abstract

The need to develop secondary lithium-ion batteries (LIBs) with high-energy and high-power density is imperative for the advancement of portable devices, electric vehicles (EV), and integrated renewable energy systems in order to meet future energy demands while reducing fossil fuel dependency and mitigating global environmental issues. LIB designed for high energy density suffer from low power density. Therefore, increasing energy density without compromising power density represents a great challenge for battery researchers and manufacturers. One of the promising avenues for improving energy and power density is electrode engineering through grading a thick (≥200 µm) electrode by spatially varying microstructures (i.e., porosities and/or particle sizes). To date, most of the reports on graded electrodes have focused on the modelling effort to predict cell performance. While previous modelling studies have predicted both considerable and marginal improvement in cell performance, very few experimental studies have been conducted to validate the performance of such electrode designs. In addition, the simulation results discussed in previous reports on next-generation graded electrodes do not recognize the effect of ball milling conditions on microstructural, transport and kinetic parameters.

At first, this thesis focusses on investigating the effect of material processing conditions on NMC particle morphology and the resulting microstructural (i.e., porosity, tortuosity), transport (solid phase diffusivity) and kinetic (reaction rate constant) properties of synthesized single-layer cathodes. The cathode microstructures composed of small particles are found to be less porous and more tortuous than the cathode microstructures composed of big particles. An increase in solid-phase diffusivity and reaction rate constants due to decrease in crystallinity and increase in interfacial surface area respectively are observed for cathodes with small particles. Next, these experimentally obtained parameters are used to develop a coupled electrochemical-thermal model and simulate the performance of a 400 µm thick bi- layer cathode with two different particle sizes and porosities in each layer. The simulation results predict that higher electrode utilization and discharge capacity at higher C-rates (1-4C) can be achieved by positioning large particles with higher porosity in a layer next to the separator and small particles with lower porosity in a layer next to the current-collector. The bi-layer cathodes also show promising performance for both energy and power applications. Motivated by the simulation results, a 200 µm thick bi-layer cathode was synthesized in order to compare cell performance against monolayer cathodes. The bi-layer cathode exhibited high discharge capacity with increasing C-rates (1-2C) compared to monolayer cathodes during initial cycling performance. However, long term cycling (100 cycles) analysis reveals that bi-layer cathode retains no advantage over a conventional electrode (i.e., a monolayer cathode composed of big particles) in a half-cell configuration. Finally, the benefits of grading such microstructures are contrasted against the constraints associated with their synthesis through traditional slurry-casting methods.