The intrinsic electrostatic dielectric behaviour of graphite anodes in Li-ion batteries -- across the entire functional range of charge

Abstract

Lithium-graphite intercalation compounds (Li-GICs) are the most common anode material for modern Li-ion batteries. However, the dielectric response of this material in the electrostatic limit (and its variation with the state of charge (SOC)) has not been investigated to a satisfactory degree, especially not for the higher SOC range. Nevertheless, said dielectric behaviour is a highly desired property, particularly as an input parameter for charged kinetic Monte Carlo simulations, one of the most promising modeling techniques for energy materials. In this work, we employ our recent DFTB parametrization for Li-GICs based on a machine-learned repulsive potential in order to overcome the computational hurdles of sampling the long-ranged Coulomb interactions within this material, as experienced by the charge carriers within. This approach is rather novel due to computational cost, but best suited for investigating our specific property of interest. For the first time, we discover a mostly linear dependency of the relative permittivity on the SOC, from ca. 7 at SOC 0% to ca. 25 at SOC 100%. In doing so, we also present a straightforward approach that can be used in future research for other intercalation compounds -- once sufficiently fast and long-ranged computational methods, e.g. linear-scaling DFT, a good DFTB parametrization, or atomic potentials with inbuilt electrostatics, become available. However, while the presented qualitative behaviour is robust and our results compare favourably with the few experimental studies available, we stress that quantitative results are strongly dependent on our estimation of the partial charge transfer from the intercalated Li-ion to the carbon host structure, and need to be verified by further experiments and calculations. Yet, our research shows that in principle, two measurements -- one at low and one at high SOC -- should suffice for that purpose.