Understanding Ionic Diffusion through SEI Components for Lithium-Ion and Sodium-Ion Batteries: Insights from First-Principles Calculations

Abstract

The insufficient understanding of the physical and chemical phenomena taking place at the electrode−electrolyte interface is the main roadblock for improvement of current battery technologies and development of new ones. Of particular interest is the solid−electrolyte interphase (SEI) layer because many aspects of the battery performance depend on its quality. Recently we have shown that a stable SEI layer can be designed in specific Li-or Na-based electrolytes. In this paper, we continue exploring this concept by identifying the interactions that take place at the lithiated (or sodiated) carbon−electrolyte interface and discussing the transport mechanisms of Li and Na ions through the most commonly found SEI layer inorganic components. For the ab initio molecular dynamics (AIMD) simulations, we considered the case of the sodiated hard carbon structure. The simulations show the decomposition of ethylene carbonate on the edge of the graphite layers leading to products such as CO and other organic fragments. The decomposition of the PF 6 − anion is a precursor step for the formation of NaF layers. Regarding the Li-and Naion transport through the SEI, the results show that the energy to create defects is lowest when Li ions are guests at an interstitial position in NaF and lattice positions in Na 2 CO 3 . For the LiF and Li 2 CO 3 crystals, the energy to create defects is lowest when Na ions substitute Li. This lower energy cost for Li-ion defects in Na-based components is due to the smaller size of the Li ion when compared to the Na ion. Regarding diffusion barriers, the Na ions in Li-based SEI components show a preference for the vacancy diffusion and knock-off mechanisms as the preferred pathways to migrate through the SEI while Li ions in Na-based SEI components prefer a mechanism involving the migration of the interstitial ion through the knock-off or direct hopping mechanism. This work also emphasizes the interplay between the crystallographic orientation of the SEI components and the direction dependent ion migration guiding the controlled design of efficient artificial SEI layers.

Influence of lattice dynamics on lithium-ion conductivity: A first-principles study

“…By performing NEB calculations, we estimate a zerotemperature energy barrier for lithium vacancy diffusion of E 0 a = 0.68 eV (Fig.5a), which is in very good agreement with previous DFT calculations [42,57,58]. We note that in this case we could neither find a metastable interstitial configuration, hence ionic conductivity in LiF in principle appears to be vacancy mediated.…”

“…LiF, for example, illustrates very well the convenience of performing finite-temperature simulations. This material appears to be a very poor lithium-ion conductor [42,57,58], however, a relatively moderate migration energy barrier of E 0 a = 0.68 eV is calculated for it with T = 0 K methods. Such a value turns out to be quite similar to the E 0 a obtained for other promising ionic conductors (e.g., 0.78 eV for LiGaO 2 [41]), hence one could easily arrive at the wrong conclusion that LiF is a good superionic material.…”

Hard Carbon as Sodium‐Ion Battery Anodes: Progress and Challenges

“…The stable solid–electrolyte interphase (SEI) generated by EC enabled a wide voltage window, high specific capacity, and long cycling stability of graphite without exfoliation concerns, and it finally became the state‐of‐the‐art anode for LIBs . Recently, with knowledge of the similarities and differences between sodium and lithium ions, scientists started to carefully reconsider the electrolyte systems, the role of the SEI, and other properties required for high‐performance HC anodes for SIBs . A fundamental understanding of the HC structure, ion‐transfer and storage mechanisms, and interaction with different electrolytes is essential for improving the performance of HC and has attracted unprecedented attention .…”

“…[12] Recently,w ith knowledge of the similarities and differences between sodium and lithium ions, scientists started to carefullyr econsider the electrolyte systems, the role of the SEI, and other properties required for high-performance HC anodes for SIBs. [13][14][15][16][17][18][19] Af undamental understanding of the HC structure, ion-transfer and storage mechanisms, and interaction with different electrolytes is essentialf or improving the performance of HC and hasa ttracted unprecedented attention. [4,13,14,20] Herein,w ef ocus on recent progress and future challenges in understanding three scientific problems relatedt oH Ca sS IB anodes: 1) HC structure and the limit in capacity;2 )the ion-storage mechanism in HC;a nd 3) the SEI andelectrolyte system.…”

Re‐weighted random series path integral simulations of molecular clusters: Applications to lithium solvated by a mixed Stockmayer cluster

“…Even though the body of knowledge gathered to date on clusters is vast, we find surprisingly little work dedicated to the study of Stockmayer and ion‐Stockmayer clusters in the literature. The present goals are of fundamental importance for the research and development of lithium ion batteries, as a practical application. Lithium ion batteries are complex bulk systems, and their direct simulations will likely require more realistic models to extend the scope of the present investigation.…”