Architecture Transformations of Ultrahigh Areal Capacity Air Cathodes for Lithium‐Oxygen Batteries

Abstract

Lithium‐oxygen (Li−O2) batteries have the highest known theoretical energy density but are still not practical for use in most applications. A key aspect that has been largely overlooked in the field is how to improve the reversible areal capacity of the air cathode to a competitive level (>5–10 mAh cm−2). For carbon‐based air cathodes, increase of areal capacity requires a corresponding increase of carbon mass loading. As a result, the electrode thickness also increases which impedes the mass transport of Li ions and O2 needed for battery reactions. It is therefore imperative to investigate not only the air cathode composition, but also its dimensional architecture, for the effective function of high areal capacity Li−O2 batteries. Herein, we present the use of several representative additives of carbon‐based nanomaterials, including carbon black, two different diameters of multi‐walled carbon nanotubes, and graphene materials from two sources, into a high mass loading (10 mg cm−2), ultrathick (∼100 μm) air cathode architectural platform matrix based on dry‐compressed holey graphene. The performances of these all‐carbon composite air cathodes differ from that of the neat holey graphene, but not by a simple addition or subtraction effect. In addition, the additive effects to full discharge and curtailed cycling experiments are significantly different. The presented results strongly suggest that the architectural volume expandability is critical for the full discharge properties. However, the cycling performance depends more upon the resilience of the air cathode architecture (“breathability”), which does not necessarily correlate with its expandability.

Li-Ion Permeability of Holey Graphene in Solid State Batteries: A Particle Dynamics Study

“…Significant progress has been made recently by taking advantage of the unique dry compressibility of hG 10 for high mass loading, high areal capacity electrodes in a variety of energy-storage applications, including supercapacitors, 11 Li-ion batteries, 12,13 Li−S/selenium (Se) batteries, 14,15 and Li-oxygen (O 2 ) batteries. 16,17 These electrodes for liquid electrolyte batteries were all prepared under solvent-free and binder-free conditions in straightforward dry-pressing processes enabled by hG. The role for hG is not only as a conductive additive but also as a drypressable matrix and a scaffold/binder.…”

“…Holey graphene (hG), also known as graphene nanomesh, is a structural derivative of graphene, formed by producing nanometer-sized holes (i.e., nanopores) using a variety of techniques − such as electron or ion-beam bombardment, nanolithography, templated growth, liquid-phase oxidation, chemical activation, gaseous-phase etching, or guided etching with catalytic or reactive nanoparticles. Significant progress has been made recently by taking advantage of the unique dry compressibility of hG for high mass loading, high areal capacity electrodes in a variety of energy-storage applications, including supercapacitors, Li-ion batteries, , Li–S/selenium (Se) batteries, , and Li-oxygen (O 2 ) batteries. , These electrodes for liquid electrolyte batteries were all prepared under solvent-free and binder-free conditions in straightforward dry-pressing processes enabled by hG. The role for hG is not only as a conductive additive but also as a dry-pressable matrix and a scaffold/binder.…”

Scalable Dry-Pressed Electrodes Based on Holey Graphene

“…In addition to the enhanced through-plane ion transport as an advanced replacement for graphene and other conductive carbon, hG can now conveniently serve as a unique conductive scaffold. The use of hG enables electrode fabrication through dry pressing, which is a facile, solvent-free, and room-temperature method that is universally applicable to various energy storage systems including, but not limited to, supercapacitors, Li ion batteries, , Li–S/Se batteries, − and Li–O 2 /air batteries. − As a conductive scaffold, hG not only can be pressed by itself into neat carbon electrodes but also can serve as a compressible matrix for hosting active battery materials or catalysts that otherwise cannot be dry pressed into robust architectures themselves. For the latter, because of the highly exfoliated nature of hG powder before compression (apparent density <10 mg cm –3 ), the active battery materials or the functional electrocatalysts may consist of a high fraction (e.g., 90 wt %) of the total weight, but the composite power mixture retains a large volume and thus remains compressible.…”

“…(b) Cartoon illustrations (not drawn to scale) showing architectural transformations of (top) an hG/graphene composite air cathode under full discharge and (bottom) an hG/MWNT (MWNT stands for multiwalled carbon nanotube) composite air cathode under curtailed cycling conditions. Reproduced with permission from ref . Copyright 2021 Wiley-VCH.…”

“…In an effort to reveal the mechanism of such ultrahigh areal capacity and whether further improvement can be made, all-carbon composite air cathodes were fabricated using hG as the dry-press host matrix but with other carbon materials as fillers (with a weight ratio of 5:1) . Interestingly, almost all of the carbon material fillers, including carbon black, carbon nanotubes, and chemically reduced graphene (c-G), reduced the full discharge capacity, with some by as much as 70%.…”