Functional Ionic Liquid Electrolytes for High-Voltage Li-Ion Cells
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Solution Overview
Problem
Conventional organic carbonate solvent-based electrolytes in lithium-ion batteries face issues such as oxidative decomposition, low Coulombic efficiency, rapid capacity fade, flammability, and safety concerns, especially at high potentials or high voltages, and struggle with lithium dendrite formation and reactivity with Li metal anodes.
Innovation Solution
Development of ionic liquids with nitrogen or phosphorus-based cations, including functional substituents like fluoro, cyano, alkenyl, and alkynyl groups, which form stable solid electrolyte interfaces (SEI) and exhibit lower viscosity, enhancing voltage stability and safety by suppressing flammability and electrolyte decomposition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional organic carbonate solvent-based electrolytes are used to increase energy density, then operational potential and voltage can be elevated, but oxidative decomposition occurs at the cathode surface causing gassing, low Coulombic efficiency, transition metal ion dissolution, and rapid capacity fade
Solution Approach 1:
The patent changes the chemical composition parameters of the electrolyte by introducing ionic liquids with specific cations (imidazolium, pyridinium, pyrrolidinium, piperidinium, phosphonium) and anions (FSO3-, CF3SO3-, BF4-, PF6-, TFSI-, FSI-). This compositional parameter change enables the electrolyte to maintain stability at elevated operational potentials up to 4.7V while preventing oxidative decomposition and capacity fade.
Solution Approach 2:
The patent creates a composite electrolyte system combining ionic liquid components with specific functional groups (fluoro, cyano, carbonate ester, alkenyl, alkynyl) to achieve synergistic effects. The composite structure provides both high voltage stability and suppressed decomposition, resolving the contradiction between elevated operational potential and capacity retention.
2Power
If conventional organic carbonate electrolytes are used to achieve high voltage operation, then cell voltage can be increased, but flammability and safety issues arise due to high vapor pressure and low flashpoint
Solution Approach 1:
The patent fundamentally changes the physical-chemical parameters of the electrolyte by using ionic liquids instead of conventional organic carbonates. This parameter change results in negligible vapor pressure and significantly elevated flash points, eliminating flammability hazards while maintaining high voltage operation capability.
Solution Approach 2:
The ionic liquid electrolyte creates an inherently safer, more inert chemical environment compared to conventional flammable organic carbonates. The ionic liquid's molecular structure and interionic interactions create a stable, non-volatile, and non-flammable medium that safely supports high voltage cell operation.
3Reliability
If room-temperature ionic liquids are used as electrolytes to address flammability and stability issues, then vapor pressure decreases and electrochemical window widens, but high viscosity and poor wettability limit their applications
Solution Approach 1:
The patent optimizes the molecular structure parameters of the ionic liquid cations by selecting specific ring structures (imidazolium, pyridinium, pyrrolidinium, piperidinium) and controlling alkyl chain lengths. These structural parameter changes reduce interionic interactions and molecular size, thereby decreasing viscosity and improving wettability while preserving electrochemical stability.
Solution Approach 2:
The patent introduces functional substituents (fluoro, cyano, carbonate ester, alkenyl, alkynyl groups) at specific local positions on the cation structures. These localized functional groups modify the local chemical properties to reduce viscosity and enhance electrode interface compatibility, while the overall ionic liquid structure maintains wide electrochemical window and stability.
4Reliability
If ionic liquids with improved viscosity and SEI formation are developed, then wettability and cycling performance can be enhanced, but structural complexity increases
Solution Approach 1:
The patent systematically varies specific structural parameters of the ionic liquid cations, such as the type of heterocyclic ring (5-membered vs. 6-membered), the length of alkyl chains (methyl, ethyl, propyl), and the position of functional substituents. These controlled parameter changes enable optimization of viscosity and SEI formation properties while maintaining reasonable molecular complexity.
Solution Approach 2:
The patent employs ionic liquids with symmetric or near-symmetric cation structures (e.g., 1,3-disubstituted imidazolium, 4,4-disubstituted pyridinium) where the substituents are positioned symmetrically around the heterocyclic ring. This structural homogeneity simplifies the molecular architecture, reduces viscosity, and promotes uniform SEI formation, thereby enhancing cycling performance without excessive structural complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The new ionic liquids demonstrate improved cycling performance, high voltage stability, and reduced viscosity, enabling stable lithium-ion battery operation with enhanced safety and capacity retention, particularly in LiNi0.5Mn0.3Co0.2O2 oxide-lithium cells.
Implementation Method 1
these ion-liquid electrolyte systems often suffer from high viscosity, poor wettability and poor SEI formation issue
Implementation Method 2
these conventional electrolytes are extremely flammable due to their high vapor pressure and low flashpoint
Data Source
AI summary
The ionic liquids disclosed herein are salts comprising a nitrogen or phosphorus such as a quaternary ammonium ion, a quaternary phosphonium ion, or an N-alkylated nitrogen heterocycle, and which include at least one functional substituent, e.g., a fluoro, cyano, carbonate ester, an alkenyl group, or an alkynyl group bonded to a carbon atom the cation. In a preferred embodiment, the cation is represented by the structure of Formula (I) as described herein.