On Rate-Limiting Mechanisms in Nickel Manganese Cobalt and Lithium Iron Phosphate Cathodes: The Interplay of Low and High Current Constraints

Abstract

Lithium-ion batteries lie at the heart of the energy transition away from fossil fuels. They allow us to store intermittent renewable energy and dispatch it on demand in the form of electricity. Mobile applications of lithium-ion batteries, such as in electric vehicles, require high energy densities as the battery needs to propel its own mass, in addition to that of the rest of the car. Much work has been devoted towards this end, with current commercial state-of-the-art lithium-ion batteries reaching energy densities of 200-300 Wh/kg. However, The United States Department of Energy has set the ambitious goal of developing a cell with energy density above 500 Wh/kg that can cycle for more than 1000 cycles and costs less than $60/kWh . Increasing the energy density of a cell often involve trade-offs with respect to cycling stability, discharge rate, and cost. Understanding these trade-offs is crucial. In this dissertation, we develop a physics-based framework for characterizing and understanding discharge curves, rate performance data, and their relationship to the physical rate-limiting mechanisms evolving within the cell. With these models in mind, we assemble the largest, highest resolution, and highest quality rate performance data set of its kind available in literature; consisting of 58 LiNi_(0.6)Mn_(0.2)Co_(0.2)O_2 and 86 LiFePO_4 lithium metal anode cells, across five loadings and three porosities, discharged through a densely staggered series of galvanostatic discharge currents. We identify four different rate-limiting mechanisms that play a role in the capacity underutilization observed in LiNi_(0.6)Mn_(0.2)Co_(0.2)O_2 and LiFePO_4 cathodes. At low currents, LiNi_(0.6)Mn_(0.2)Co_(0.2)O_2 cathodes are limited by the ionic solid-diffusion in the active particles, while LiFePO_4 cathodes are limited by the phase transformation rate of the active particles. We find that the sharp capacity drop at high currents is generally caused by the growing Ohmic and charge-transfer potentials, and not by the ionic liquid-diffusion limits often ascribed to it. The ionic liquid-diffusion limits emerge in the thickest and most dense of cathodes, and requires a very low threshold voltage to be visible beyond the Ohmic/charge- transfer limit. The experimental data points to an optimal dense cathode thickness of 140-160 um and our modeling suggests that the optimal thickness for cells with porous anodes would be much smaller. Overall, this work provides a broad and nuanced framework for understanding the design considerations in Lithium-ion cells.