Toward a quantitative interfacial description of solvation for Li metal battery operation under extreme conditions

Abstract

The future application of Li metal batteries (LMBs) at scale demands electrolytes that endow improved performance under fast-charging and low-temperature operating conditions. Recent works indicate that desolvation kinetics of Li + plays a crucial role in enabling such behavior. However, the modulation of this process has typically been achieved through inducing qualitative degrees of ion pairing into the system. In this work, we find that a more quantitative control of the ion pairing is crucial to minimizing the desolvation penalty at the electrified interface and thus the reversibility of the Li metal anode under kinetic strain. This effect is demonstrated in localized electrolytes based on strongly and weakly bound ether solvents that allow for the deconvolution of solvation chemistry and structure. Unexpectedly, we find that maximum degrees of ion pairing are suboptimal for ultralow temperature and high-rate operation and that reversibility is substantially improved via slight local dilution away from the saturation point. Further, we find that at the optimum degree of ion pairing for each system, weakly bound solvents still produce superior behavior. The impact of these structure and chemistry effects on charge transfer are then explicitly resolved via experimental and computational analyses. Lastly, we demonstrate that the locally optimized diethyl ether-based localized-high-concentration electrolytes supports kinetic strained operating conditions, including cycling down to −60 °C and 20-min fast charging in LMB full cells. This work demonstrates that explicit, quantitative optimization of the Li + solvation state is necessary for developing LMB electrolytes capable of low-temperature and high-rate operation.

Revealing the Molecular Origin of Driving Forces and Thermodynamic Barriers for Li+ Ion Transport to Electrode-Electrolyte Interfaces

“…9,16 Few experimental investigations [17][18][19][20][21][22][23][24][25] and computational studies 8,9,26,27 have attempted to obtain an understanding of the Li + ion adsorption process, which includes its dependence on various factors such as the electric double layer (EDL) structure, 16 cation-anion and cation-solvent dissociation energies, 28 and solvation environments. 29 Using AC impedance spectroscopy, Yamada et al 17 showed that the activation energy of the interfacial Li + ion transfer on a highly oriented pyrolytic graphite surface is proportional to the solvation energy of Li + ions, which is a proxy for the energy required for the desolvation of Li + ions from the solvent molecules. In another study, using Raman spectroscopy and AC impedance spectroscopy, Uchida et al 30 reported that increasing the association between Li + ions and anions in the electrolyte reduces the activation energy for Li + ion adsorption from the electrolyte into the graphite.…”

Fast Interfacial Defluorination Kinetics Enables Stable Cycling of Low-Temperature Lithium Metal Batteries

“…On one hand, strong solvation strength of solvent molecules with high donor numbers can facilitate the formation of high-concentration electrolytes (HCEs), thus reducing the free molecules and their side reactions on lithium metal anode . On the other hand, recent efforts have been dedicated to developing weakly solvating electrolytes (WSEs), , which can help create inorganic-rich SEIs on lithium metal anode surfaces that are essential for suppressing Li dendrite growth and improving the cycling performance of LMBs. − Although substantial efforts have been dedicated, the progress in developing low-temperature LMBs is still limited due to the lack of suitable electrolytes . Currently, a mechanistic understanding of the intricate interplay between solvation structure and SEI formation is imperative to further improve the performance of low-temperature LMBs.…”

Revealing the Molecular Origin of Driving Forces and Thermodynamic Barriers for Li+ Ion Transport to Electrode–Electrolyte Interfaces

“…Few experimental investigations − and computational studies ,,, have attempted to obtain an understanding of the Li + ion adsorption process, which includes its dependence on various factors such as the electric double layer (EDL) structure, cation–anion and cation–solvent dissociation energies, and solvation environments . Using AC impedance spectroscopy, Yamada et al showed that the activation energy of the interfacial Li + ion transfer on a highly oriented pyrolytic graphite surface is proportional to the solvation energy of Li + ions, which is a proxy for the energy required for the desolvation of Li + ions from the solvent molecules.…”