Defect Thermodynamics and Diffusion Mechanisms in Li2CO3 and Implications for the Solid Electrolyte Interphase in Li-Ion Batteries

Abstract

Understanding and improving Li transport through crystalline Li 2 CO 3, a stable component of the solid electrolyte interphase (SEI) films in Li-ion batteries, is critical to battery rate performance, capacity drop, and power loss. Identification of the dominant diffusion carriers and their diffusion pathways in SEI films coated on anode and cathode surfaces is a necessary step toward the development of methods to increase Li conductivity. In this paper, we identify the dominant Li diffusion carriers in Li 2 CO 3 over a voltage range (0–4.4 V) that includes typical anode and cathode materials by computing and comparing thermodynamics of all seven Li-associated point defects with density functional theory (DFT). The main diffusion carriers in Li 2 CO 3 below 0.98 V are excess Li-ion interstitials; however, above 3.98 V, Li-ion vacancies become the dominant diffusion carrier type, and they have the same concentration in the voltage range of 0.98–3.98 V. Diffusion mechanisms of the main diffusion carriers were computed via the climbing image nudged elastic band method (CI-NEB). On the basis of the computed diffusion carrier concentration and diffusion barriers, the Li ionic conductivity was computed as a function of voltage, and it varies by ∼5 orders of magnitude. Insights gained from our calculations allow us to suggest different dopants to enhance Li conductivity in SEI films and propose a new mechanism for electron leakage through an SEI film into the electrolyte. Li 2 CO 3 is the one of the main stable components in the solid electrolyte interphase (SEI) in lithium-ion batteries and can be found in SEI films grown on both cathode and anode surfaces. On the basis of the concentrations and diffusion barriers computed for each of the identified point defects from first-principles calculations, the Li conductivity in Li 2 CO 3 is mapped as a function of battery voltage. It is found that the diffusion carriers in the Li 2 CO 3 on a negative electrode (graphite) are Li + interstitials but changed to Li + vacancies in the Li 2 CO 3 coated on cathode materials. While Li + vacancies diffuse via a typical direct hopping mechanism, the excess Li + interstitials diffuse via repetitive knock-off with the Li + ions on lattice sites. The Li + interstitial conductivity in Li 2 CO 3 on graphite is ∼5 orders of magnitude faster than that of a Li + vacancy in Li 2 CO 3 on the cathode. This suggests that Li transport through an SEI film on a cathode material can be dramatically different from Li transport through an SEI film on an anode material, despite the fact that SEI component chemistries appear to be similar in both cases. Thus, different doping strategies for the SEI on the anode and cathode could be taken to increase diffusion carrier concentrations in order to increase Li conductivity in an SEI film.