Diffusion Control of Organic Cathode Materials in Lithium Metal Battery Using a Graphite Modified Separator

Abstract

Energy storage technology is a critical research area for the success of portable electronic devices and electrical transportation. Such applications need affordable, durable, safe and environmentally friendly battery materials with high energy density. Organic cathode materials are currently promising candidates because they fulfill most of these requirements for an active battery material1. However, they usually suffer from a major limitation, namely their solubility in organic electrolytes2. Even a very low solubility translates into a decreased capacity upon cycling due to the loss of active material. Among organic cathode materials, conjugated carbonyl compounds have been intensively scrutinized because of a combination of desirable properties, such as low cost, good theoretical capacity, reversible oxidative behavior, high discharge potential and commercial availability. For example, 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) is an inexpensive red pigment that is widely investigated as an active material for energy devices. However, Li-PTCDA batteries suffer from poor and irreversible cycling stability due to the dissolution in the electrolyte. Even more problematic, the dissolved PTCDA migrates through the porous separator and deposits on the anode surface causing irreversible damage3. In order to solve this problem, chemical modifications such as polymerisation, functionalization and immobilization on carbon materials have improved cycling stability. However, these modified cathode materials, which are often prepared by complex processes, contain considerable amounts of inactive mass that cause a decreases of the energy density. We developed a Janus membrane, which consists of two layers – a commercial polypropylene separator (CelgardTM) and a 300-600 nm layer of exfoliated graphite that was applied by a simple and environmentally friendly process. The submicron graphite layer is only permeable to Li+ and it drastically improves the battery performance, as measured by capacity retention and high coulombic efficiency, even at 2C rates. Post-mortem analysis of the battery indicates that the new membrane protects the anode against corrosion, and cathode dissolution is reduced. This graphite-based membrane is expected to greatly expedite the deployment of lithium batteries using organic moities as active materials.