Nanoporous-Carbon As an Intercalation Host Material to Enable Multivalent Ion Electrochemical Energy Storage

We study the use of nanoporous-carbon (NPC) as an electrically conductive host material for Mg²⁺ intercalation.  Negligible progress for multivalent energy storage via intercalation into existing carbonaceous materials has been made due to insufficient interstitial/interplanar spacing within the crystallographic structures to host the larger sizes of partially-solvated multivalent ions.  However, NPC has high surface area with open and accessible porosity, controllable via mass density, which can minimize the diffusional resistance.  Here, we grow NPC directly on stainless-steel disks, which are used as anodes to fabricate coin cells with solid Mg cathodes and a Grinard-based electrolye.  NPC mass density is controlled during growth, ranging from 0.06 – 1.0 g/cm³, and correlates directly with electrochemical characterizations.  In particular, cyclic voltammetry (CV) scans for NPC with density ~ 0.5 g/cm³ are consistent with the reversible intercalation of Mg ions.  Higher density NPC yields capacitive behavior, most likely resulting from the smaller interplanar spacings between graphene sheet fragments and tighter domain boundaries; lower density NPC results in asymmetrical CV scans, consistent with possible structural degradation resulting from mass transport through such a soft carbon material.

Briefly, NPC grows via pulsed-laser deposition with controllable mass densities ranging from ~ 0.05 to 2.0 g/cm³.  The internal structure of NPC self-assembles during growth and consists of nano-fragments of aligned graphene sheet stacks with interplanar spacings expanded by as much as 55% compared to crystalline graphite.  Indeed, typical crystalline domain sizes are only 1 – 2 nm in length, as determined by high-resolution transmission electron microscopy; hence, NPC consists of randomly-oriented graphene nano-crystallites with expanded interplanar spacings, and with a plethora of grain boundaries to enable rapid diffusion of species into an entire volume.  Ionic intercalation into graphite is well known.  Reducing the mass density of NPC increases the interplanar spacings.  Therefore, the advantages of using NPC to study Mg²⁺intercalation include the tunability of the interplanar spacings and the ability to deposit it onto any substrate material without using binder materials, both increasing the fraction of available carbon surface atoms and simplifying the interpretation of measurements.  This suggests the potential to successfully develop NPC as an intercalation host for multivalent energy storage.

This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.