Understanding the Limitations in Battery Components for Improving Energy Density under Extreme Fast Charging (XFC) Conditions

Presently, there are ~2 million electric vehicles (EVs) in the US, which account for only less than 2% market share in automobile sector. The poor adoption of EVs is attributed to three key issues, range anxiety, charging time, and higher cost of EVs. Most EVs take ~30 minutes to charge using DC Fast charger, with less than 200 miles. To change consumer perception towards EVs, it’s important to reduce charging time to that of fueling a conventional IC engine vehicle with a range of more than 200 miles. To reach the U.S Department of Energy goal of extreme fast charging(XFC) along with high energy density >200 Wh kg^-1, a combined improvement in electrode architecture, electrolyte, and separator membrane would be crucial.

This dissertation focuses on improving the energy density of lithium-ion batteries by understanding the bottlenecks in separator membrane, cathode design, and anode architecture under XFC conditions. We develop novel hybrid anode architecture via freeze tape casting with directionally aligned solid particles that improve the rate performance of graphite by ~20% at 5C rate. The enhanced rate performance is attributed to low tortuosity and shorter diffusion pathways of the freeze cast electrodes. Next, we investigate various cathode (LiMn_0.6Ni_0.2Co_0.2O₂) design parameters (electrode porosity and mass loading) for developing high energy density electrodes for XFC application. Increasing mass loading from 11.5 mg cm^-2 to 25 mg cm^-2 reduces the rate performance due to mass transport limitation and underutilization of thick electrodes. While, reducing the electrode porosity helps in improving rate performance and gravimetric energy density of the cell. In regard to separator membrane, this work demonstrates that Celgard 2500 has excellent electrolyte wettability, 2.23 Ω cm^-2 less in resistance, attributed to the high porosity and low tortuosity of Celgard 2500 and improved rate performance at 3C. To address this issue of thermal shrinkage and self-discharge, separators are coated with ceramics that lead to high electrolyte wettability, excellent thermal stability (0.6% shrinkage vs 5.8% for uncoated membrane) at 130°C and lower self-discharge after 350 hours. These results indicate that separator membrane, cathode design, and anode architecture play a significant role in enabling XFC.