(Invited) Design of the Separators for Li-Ion and Lithium Metal Batteries

Abstract

Functionalization of separators is one path to increase the performance of Li-ion (LIB) and lithium metal batteries (LMB). This is a research area within LIBs not very extensively explored. A separator should prevent direct contact between the positive and negative electrodes and provide good ionic transport pathways. Commercial separators are usually made of polyolefin with low thermal stability and low electrolyte wettability. In this study, cellulose of a green algae has been used to make new separator materials via a simple paper making method. The resulting separator material has several advantages compared to commercial separator materials in ionic conductivity, thermal stability, electrolyte wettability and type of pore distribution. With this material as the basis two types of advanced separator functionalities was designed and will be described in the presentation. 1) Redox-active separators based on the use of a redox-active conducting polymer, polypyrrole (PPy) and a natural polymer, polydopamine (PDA) where the PPy-based redox-active separator is designed to contribute capacity to the cathode of a LIB, while the PDA-based redox-active separator is proposed to be used on the anode side. 2) It is known that a homogeneous current distribution is beneficial for the battery performance. Therefore, separators with homogenous pore distributions have been manufactured to study the influence of the pore distribution on the Li deposition/stripping behavior and composite cathode utilization in LMBs. One example is a sandwich-structured separator composed of two cellulose nanofiber (CNF) surface layers and an intermediate glass microfiber (GMF) and then a CNF composite layer on the other side. The CNF surface layers, of the sandwich-separator, have homogeneous distribution of nano-sized pores, which favours a homogeneous current distribution at both electrodes. The intermediate GMF/CNF layer, however, has excellent ionic transport due to a macro porous structure of the material. This separator exhibits a good electrolyte wettability and better thermal stability compared to a classical Celgard separator. The reason for this is the use of the hydrophilic and thermally stable CNFs and GMFs. The combination of nano-sized and micro-sized fibers used in this separator yields a higher ionic conductivity than that for the commercial separator (1.14 vs. 0.49 mS cm−1). The performance is studied for both Li anodes and LiFePO4 composite cathodes and the results show that use of separators with high porosities and homogeneous surface pore distributions can improve the performances (e.g. capacities and stabilities) of LMBs considerably. The presentation will highlight the importance of proper separator/electrode interactions. The present approach constitutes a practical engineering strategy for the production of separators with nano/micro fibers and a promising route for the development of LMBs with improved safety and enhanced electrochemical performances.