Lithium and Sodium Ion Binding Mechanisms and Diffusion Rates in Lignin-Based Hard Carbon Models

Abstract

Hard carbons are the primary candidate for the anode of nextgeneration sodium-ion batteries for large-scale energy storage, as they are sustainable and can possess high charge capacity and long cycle life. These properties along with diffusion rates and ion storage mechanisms are highly dependent on nanostructures. This work uses reactive molecular dynamics simulations to examine lithium and sodium ion storage mechanisms and diffusion in lignin-based hard carbon model systems with varying nanostructures. It was found that sodium will preferentially localize on the surface of curved graphene fragments, while lithium will preferentially bind to the hydrogen dense interfaces of crystalline and amorphous carbon domains. The ion storage mechanisms are explained through ion charge and energy distributions in coordination with snapshots of the simulated systems. It was also revealed that hard carbons with small crystalline volume fractions and moderately sized sheets of curved graphene will yield the highest sodium-ion diffusion rates at ∼10 −7 cm 2 /s. Self-diffusion coefficients were determined by mean square displacement of ions in the models with extension through a confined random walk theory.

Combining Experimental and Theoretical Techniques to Gain an Atomic Level Understanding of the Defect Binding Mechanism in Hard Carbon Anodes for Sodium Ion Batteries

“…[78] A Lennard-Jones potential yielded an atomic interaction of Na and C as opposed to the more representative ionic interaction of Na + and C x − , which could be approximated using reactive potentials. [27][28][29][30] However, in light of other modeling of Na binding mechanisms in HC, its use was a straightforward approach which enabled the authors' detailed structural analysis in a dilute Na-C x model. [29] Although random, the initial position of the Na probe atom was restricted to being further than 1.0 Å from every carbon atom and <1.8 Å from the nearest carbon atom.…”

“…However, the same analysis shows it does not impact the Na binding mechanism. [27][28][29][30] The interaction between a model which has been appropriately decorated with heteroatoms and would afford an opportunity to study electrochemistry in HCs further and should be pursued.…”

“…This of course excludes the formation of metallic clusters, which have been shown to have lower energies, experimentally and theoretically. [29,31,54] Our dense HC was designed to eliminate contributions from metallic Na formation eliminating it from the GCD data and this feature would not be represented by our probe atom analysis. However, the best understood features, the turbostratic nanodomains and the plateau region, are not as straightforward as has been previously discussed.…”

“…procedures to understand electrochemical processes such as capacitance [24][25][26] and Li-ion binding, [27][28][29][30] but there are few studies that examine in sufficient detail, the structure of HC in relation to Na-ion binding. [29,30] In this study, we synthesized dense, low porosity HC materials derived from sucrose in line with our previous work.…”

“…procedures to understand electrochemical processes such as capacitance [24][25][26] and Li-ion binding, [27][28][29][30] but there are few studies that examine in sufficient detail, the structure of HC in relation to Na-ion binding. [29,30] In this study, we synthesized dense, low porosity HC materials derived from sucrose in line with our previous work. [31][32][33] These materials were characterized using X-ray diffraction (XRD), Raman, nitrogen sorption analysis, neutron total scattering and pair distribution function (PDF) analysis, various methods of density analysis, and galvanostatic charge/discharge (GCD) allowing for a differential look at how the structural, physical, and electrochemical properties evolve with increasing pyrolysis temperature.…”