Lithium-ion batteries (LIBs) find applications ranging from portable electronics to electric vehicles, due to their higher energy densities (~ 250 Wh.hr kg^-1) than other rechargeable batteries. Since the invention of LIBs, commercialized anodes have mostly consisted of carbon-based materials such as natural graphite, mesophase carbon microbeads (MCMB) massive artificial graphite (MAG), and hard carbons. However, the employment of LIBs in electric vehicles is increasing demand for higher energy density LIBs. Graphite-based anodes are limited to a charge storage capacity of 372 mAh g^-1 due to the tight interplanar spacing (3.35 Å) between the graphene sheets. Several alternative anode materials have been proposed to replace graphite. In particular, Si and Sn alloys are extensively studied, but enormous volume change while cycling is hindering their commercialization. Lithium titianate (Li₄Ti₅O₁₂) anodes are successfully commercialized, but low lithium storage capacity (175 mAh g^-1) and high voltage (~1.5 V) prevents their use for high energy density applications. Carbon allotropes with modified structures and crystallinity can achieve higher capacity relative to graphite, so they are potential candidates to enable higher energy density LIBs. Carbon materials vary drastically in terms of interplanar spacing, termination groups on the edges of graphite/graphene fragments, defects, and porosity within the carbon structures. These variations significantly affect the Coulombic efficiency, lithium storage capacity, irreversible capacity loss during the first cycle, and the voltage range for lithiation.

We will present an investigation of nanoporous carbon (NPC) prepared via pulsed laser deposition as a model system to understand lithiation mechanisms in carbon anodes. NPC is grown directly on stainless steel current collectors at room temperature, resulting in randomly-oriented stacks consisting of a few layers of aligned nm-sized graphene sheets. NPC mass density can be controlled from 2.0 g/cm³ to below 0.1 g/cm³ by varying the carbon deposition energetics, enabling the tuning of interplanar spacing and porosity within NPC. Low-density NPC exhibits as much as a 55% larger interplanar spacing compared to graphite-like high-density NPC and larger open pore spaces between stacks of sheets. The ability to control interplanar spacings and porosity in otherwise identical carbon samples allows for a systematic study to understand how these characteristics affect lithiation and de-lithiation processes in graphene-like carbon materials.

We will discuss how the mass density of this 3D graphene-like NPC nanomaterial affects lithiation mechanisms and will demonstrate that NPC anode storage capacity systematically increases with decreasing mass density. High capacity is enabled by a plethora of grain boundaries, large and controlled interplanar spacings between graphene sheets, and nanopores between the randomly-aligned sheet fragments, all potentially contributing to extra lithium storage sites. The primary mechanisms for lithium storage in nanoporous carbon varies with mass density. For example, the capacity increases with cycling for the lowest mass density materials, potentially indicating that lithium plates within the nanopores as electrolyte penetrates deeper into the structure with each cycle. NPC mass density also correlates with Coulombic efficiency, such that lower density NPC demonstrates lower coulombic efficiency. This may be related to solid electrolyte interphase formation in the lower mass density samples since the electrolyte is more likely to penetrate into the structure and enable higher carbon-electrolyte interface surface areas. However, initial data suggests that lower Coulombic efficiency with lower mass density is likely related to the trapping of lithium ions at termination groups on the edges of graphene sheets. This trapping of lithium can be reversed by increasing the voltage during delithiation. Finally, we will also present data showing that NPC can be sodiated. Sodiation processes mimic those of lithiation, with sodium storage capacity increasing with decreasing NPC mass-density.

We thank Lyle Brunke for assistance growing NPC films and Carlos Gutierrez for programmatic guidance. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and 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-NA-0003525. SAND2019-4730 A