A Direct Comparison of a Power and an Energy-Optimized Li-Ion Pouch Cell: Characterization, Ageing and Modelling

Abstract

Driven by the incredible transformation of the energy sector over the past 30 years lithium-ion batteries (LIBs) have seen continuous development and incredible improvements in terms of energy density, lifetime and power performance. Depending on their applications, Li-ion cells are normally optimized for either supplying as much power as possible or storing as much energy as possible, or a compromise between the two. A power optimized cell has typically a lower energy density and thinner electrode layers, compared to an energy optimized cell with higher energy density and thicker electrodes. One pathway to increase the performance of the modern LIBs is to move from conventional to thick electrode designs: this will result in an increased ratio of active components (anode/cathode) to inactive components (current collector, separator, casings) delivering improved energy densities. However, increasing the electrode thickness leads to several new challenges, such as e.g., reduced mechanical strength, higher tortuosity, and poorer rate performance, significantly affecting cell performance and ageing characteristics.In this study we aim to develop a fundamental understanding of electrode performance and limitations through the evaluation of both, high-energy and high-power electrodes from state-of-the-art commercial LIBs. For this purpose, we selected two renowned large-scale commercial Li-ion pouch cells with similar geometry, chemistry, and capacity: one optimized for power and the other for energy.Both cell types were tested following a test matrix including different temperatures, currents, and State-of-Charge limits as well as with and without mechanical compression of the cell during cycling. A large dataset with diagnostic data was recorded during ageing. This included, e.g., dQ/dV (Incremental capacity analysis, ICA), DC resistances and entropy changes. An especially strong influence of the current on the cycle life was observed for the energy-optimized cells. Cells without mechanical pressure had considerably shorter cycle life compared to cells cycled with mechanical pressure. Detailed monitoring of the pressure evolution across the pouch surface revealed significant pressure increases and thus electrode expansion throughout the cycling.Cell-tear-down analysis was done to reveal differences in battery design, especially electrode structure and thicknesses. The extracted materials were structurally characterized by, e.g., SEM, XRD, and x-ray computed tomography, and assessed electrochemically in coin full- and half-cells. The obtained data was then used to fully parameterize a P2D electrochemical model.The combination of detailed experiments coupled to P2D modelling provided valuable insights into influence of electrode thickness and morphology on battery performance and ageing, as well as degradation mechanisms specific to electrodes with increased thicknesses.