Rivalry at the Interface: Ion Desolvation and Electrolyte Degradation in Model Ethylene Carbonate Complexes of Li+, Na+, and Mg2+ with PF6 - on the Li4Ti5O12 (111) Surface

Hristo Rasheev; Radostina Stoyanova; Alia Tadjer

doi:10.1021/acsomega.1c04161

Rivalry at the Interface: Ion Desolvation and Electrolyte Degradation in Model Ethylene Carbonate Complexes of Li⁺, Na⁺, and Mg²⁺ with PF₆ ^- on the Li₄Ti₅O₁₂ (111) Surface

ACS Omega. 2021 Oct 26;6(44):29735-29745. doi: 10.1021/acsomega.1c04161. eCollection 2021 Nov 9.

Authors

Hristo Rasheev^{1

2}, Radostina Stoyanova², Alia Tadjer^{1

2}

Affiliations

¹ Faculty of Chemistry and Pharmacy, University of Sofia, Sofia 1164, Bulgaria.
² Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria.

Abstract

Spinel lithium titanate, Li₄Ti₅O₁₂ (LTO), emerges as a "universal" electrode material for Li-ion batteries and hybrid Li/Na-, Li/Mg-, and Na/Mg-ion batteries functioning on the basis of intercalation. Given that LTO operates in a variety of electrolyte solutions, the main challenge is to understand the reactivity of the LTO surface toward single- and dual-cation electrolytes at the molecular level. This study first reports results on ion desolvation and electrolyte solvent/salt degradation on an LTO surface by means of periodic DFT calculations. The desolvation stages are modeled by the adsorption of mono- and binuclear complexes of Li⁺, Na⁺, and Mg²⁺ with a limited number of ethylene carbonate (EC) solvent molecules on the oxygen-terminated LTO (111) surface, taking into account the presence of a PF₆ ^- counterion. Alongside cation adsorption, several degradation reactions are discussed: surface-catalyzed dehydrogenation of EC molecules, simultaneous dehydrogenation and fluorination of EC, and Mg²⁺-induced decay of PF₆ ^- to PF₅ and F^-. Data analysis allows the rationalization of existing experimentally established phenomena such as gassing and fluoride deposition. Among the three investigated cations, Mg²⁺ is adsorbed most tightly and is predicted to form a thicker fluoride-containing film on the LTO surface. Gassing, characteristic for carbonate-based electrolytes with LTO electrodes, is foreseen to be suppressed in dual-cation batteries. The latter bears promise to outperform the single-ion ones in terms of durability and safety.