Fluorophosphate-Based Nonflammable Concentrated Electrolytes with a Designed Lithium-Ion-Ordered Structure: Relationship between the Bulk Electrolyte and Electrode Interface Structures

Abstract

We propose a molecular design for lithium (Li)-ion-ordered complex structures in nonflammable concentrated electrolytes that facilitates the Li-ion battery (LIB) electrode reaction to produce safer LIBs. The concentrated electrolyte, composed of Li bis­(fluorosulfonyl)­amide (FSA) salt and a nonflammable tris­(2,2,2-trifluoroethyl) phosphate (TFEP) solvent, showed no electrode reaction (i.e., no Li-ion intercalation into the negative graphite electrode); however, introducing a small molecular additive (acetonitrile [AN]) into concentrated TFEP-based electrolytes is shown to improve the battery electrode reaction, leading to reversible charge/discharge behavior. Combined high-energy X-ray total scattering experiments incorporating all-atom molecular dynamics simulations were used to visualize Li-ion complexes at the molecular level and revealed that (1) Li ions form mononuclear complexes in a concentrated LiFSA/TFEP (without additives) owing to solvation steric effects arising from the molecular size of TFEP and (2) adding a small-sized additive, AN, reduces the steric effect and triggers a change in Li-ion structures, i.e., the formation of a specific Li-ion-ordered structure linked via FSA anions. These Li-ion-ordered complexes stabilize the energy of the lowest unoccupied molecular orbital (LUMO) on FSA anions, which is key to producing an anion-derived solid electrolyte interphase (SEI) at the graphite electrode. We performed in situ surface-enhanced infrared absorption spectroscopy and discussed the electrode/electrolyte interface and SEI formation mechanisms in TFEP-based concentrated electrolyte systems.

2,2,2-Trifluoroethyl Acetate as an Electrolyte Solvent for Lithium-Ion Batteries: Effect of Weak Solvation on Electrochemical and Structural Characteristics

“…It has recently been established that the concentration of salts and the above-mentioned solvent parameters affect the design of LIB electrolytes; namely, highly concentrated electrolyte solutions widen the electrochemical window to establish 4 V-class LIBs. − In concentrated electrolyte solutions with c Li above ∼3.0 mol dm –3 , metal ions (e.g., Li ion in the case of the LIB electrolyte) exist as ion-pair complexes in solutions; however, they form a specific ordered Li-ion structure based on multiple ion pairs between Li ions and counteranions to provide a high-voltage LIB system. ,− The formation of the specific Li-ion structure in concentrated electrolytes may originate from the balance between the solvation power due to the solvent molecules and the electrostatic interaction due to anions. However, sufficient knowledge to control these contributions has not yet been reported at the molecular level.…”

Polyether-based solid electrolytes with a homogeneous polymer network: effect of the salt concentration on the Li-ion coordination structure

“…The experimental procedures are similar to those described in previous reports. [34][35][36] Cyclic voltammetry (CV) was performed by using CR2032 coin-type cells equipped with Ni as the working electrode and Li foil as the counter electrode at a scan rate of 1 mV s À1 . The sample cell was kept at 333 K in the thermostat-controlled container at least 24 h for stabilization, which was followed by CV.…”

A structural and electrochemical study of lithium-ion battery electrolytes using an ethylene sulfite solvent: from dilute to concentrated solutions

“…This characteristic has been attributed mostly to forming certain Li-ion coordination structures, i.e., ordered Li-ion complexes linked via counter anions, yielding no free solvent in bulk. [21][22][23][24] In LIB applications, ethylene sulfite (ES, C 2 H 4 SO 3 ) is an SEIforming agent that can be used instead of cyclic carbonates. [25][26][27][28][29][30] According to Li et al, adding a small amount of ES (0.3 wt%) as an additive into carbonate-based LIB electrolytes results in successful charge-discharge performance, which is due to the ES-derived SEI formation on the negative electrode; in detail, the ES molecules in the electrolyte solutions reductively decompose at B2.0 V during a cathodic scan to form a stable passivation film, similar to that from the decomposed EC.…”