Nanoporous Carbon As a Three-Dimensional Graphene Anode and Si Scaffold for Li-Ion Batteries
Authors: Harrison Katharine Lee , Wolak Matthaeus A. , Siegal Michael P. , Sava Gallis Dorina F. 
Published: February 2020

Graphite is the state-of-the-art anode for Li-ion batteries, but graphite has limited lithium storage capacity (372 mAh/g) due to the small inter-planar spacing (3.35 Å) between graphene sheets. Graphene, in the form of reduced graphene oxide (RGO), has recently been demonstrated as an alternative anode. The theoretical capacity of graphene is double (744 mAh/g) that of graphite; Li+ can adsorb on both sides of the graphene sheets because the sheets pack at a lower density than graphite due to disorder and the presence of functional groups formed during synthesis. Typically, RGO anodes are fabricated by preparing a slurry of RGO, conductive carbon, and binder. This process does not allow control of the interplanar spacing between the graphene sheets and leads to a porous, disorganized electrode. RGO electrodes typically exhibit capacity fade and copious solid electrolyte interphase (SEI) formation.

We study graphene as an anode material for Li-ion batteries via deposition of nanoporous carbon (NPC) by pulsed laser deposition (PLD). This binderless deposition provides a ‘clean’ three-dimensional graphene material, enabling systematic and controlled lithiation studies. NPC grows at room temperature, is stable up to 600 °C, and assembles as nanometric, randomly-oriented stacks of several aligned graphene sheets. Most importantly, the interplanar spacing can be controlled during deposition such that the mass density can be varied from 2.25 g/cm3 (graphite-like) to 0.1 g/cm3 (55% expanded interplanar spacing relative to graphite). The ability to control interplanar spacing allows systematic study to understand how interplanar spacing affects lithiation of graphene.

NPC demonstrates higher specific capacity than the theoretical values for graphene, likely arising from the plethora of grain boundaries and large, controlled interplanar spacing between sheets. We find that lower-density NPC typically leads to higher capacity, perhaps arising from the larger spacing between sheets for lithiation. The Coulombic efficiency (CE) of the first cycle also correlates with mass-density, such that the higher capacity, lower mass-density samples also exhibit lower CE. This indicates SEI forms more readily during the first cycle in lower mass-density samples. The CE rises after the first two cycles to settle in the range of 98 to >99% and is not very dependent on NPC density, indicating that SEI formation is minimal after the first few cycles.

In addition to enabling systematic, controlled lithiation studies in a three-dimensional form of graphene, NPC is also useful as a scaffold to support higher capacity materials. We demonstrate this concept using Si with a theoretical capacity (~4200 mAh/g) much larger than carbons. Si suffers from >300% volume expansion upon alloying with Li, causing severe capacity fade and excessive SEI formation as the material pulverizes with electrochemical cycling. Many strategies have improved capacity fade. However, reversible capacity after many cycles remains far lower than theoretical. RGO and Si nanocomposites have been prepared to cushion volume expansion, but pulverization of the composite can still occur because the Si particles are large relative to the spacing between RGO sheets. Our current work focuses on depositing Si atoms and/or small clusters between NPC sheets (Si-NPC composites). We take advantage of the expanded NPC interplanar spacing to provide a scaffold where Si atoms can reside and are free to expand with lithiation without pulverizing the NPC host.

Si-NPC composites are prepared by co-depositing Si during the NPC PLD process. We have successfully deposited at least 7 atomic % Si in NPC. The formation of composites between Si and NPC is confirmed by mass-density measurements, which are below the mass-densities of other potential products such as Si, graphite, and silicon carbide (all greater than 2.2 g/cm3). Initial data shows improved capacity with addition of Si. The CE in the first cycle is comparable to NPC without Si and CE rises to >99% in some samples. The high CE values indicate little additional SEI formation with Si addition and suggest that these composites do not pulverize with cycling, which would expose new surface area for SEI to form. We expect that the Si-NPC architectures can be tuned for high capacity, CE, and cycle life.

We thank Lyle Brunke for assistance growing NPC films, Graham Yelton, Kyle Fenton, and Kyle Klavetter for discussions, and Carlos Gutierrez for programmatic guidance. This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.