Fast charging of batteries is the key to increase the adoption of electric vehicles. All the components of a battery: anode, cathode, electrolyte and separator, contribute to the limiting factors of fast charging. Separators are an important part of Li-ion batteries that allows lithium ions to shuttle between the electrodes and isolates the electronic flow. While majority of the efforts are focused on novel electrode architecture and electrolyte formulation for high rate capabilities, studies on separators are mostly pertaining to their mechanical and physical properties, and little attention has been paid to their effect on rate performance. To achieve the DOE goal of fast charging time of less than 10 minutes and maintaining a cell energy density >180 Wh/kg, it requires improvement in every cell component including separator.

In this study, we report a comprehensive analysis of the physical properties, electrochemical performance, and high rate capabilities of the widely used battery separator Celgard 2325 and Celgard 2500. It is shown in figure 1-a that Celgard 2500 performs better than Celgard 2325 at higher c-rates (1-5C) during both constant charge and constant discharge protocol. This can be attributed to the fast ion diffusion through the Celgard 2500 due to its more porous structure and better electrolyte uptake capability. Figure 1-b shows that Celgard 2500 has lower contact angle compared to 2325 in water and electrolyte, indicating it has higher surface tension, which leads to better electrolyte wettability. It is demonstrated that increasing porosity in separator can improve rate performance and facilitate fast charging.