Gel Electrolyte Stability for High-Voltage Lithium-Ion Cells
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Solution Overview
Problem
Lithium-ion cells with high-voltage cathodic materials and lithiated titanium oxide anodes face issues with electrolyte stability, oxidation, reduction, and chemical species migration, leading to reduced cycling performance and lifetime.
Innovation Solution
A gel-type electrolyte comprising a poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix impregnated with a liquid mixture of lithium salt and linear carbonate solvents, which enhances stability against oxidation and reduction across the operating voltage range and prevents chemical species migration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a liquid electrolyte containing cyclic carbonates is used in high-voltage lithium-ion cells, then the cell can operate at high voltage (≥4.5V), but oxidation of the electrolyte solvent occurs at the cathode leading to degraded cycling performance
Solution Approach 1:
The patent changes the chemical composition parameters of the electrolyte by replacing cyclic carbonate solvents with linear carbonate solvents (dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate) and using a gel polymer matrix (PVDF, P(VdF-HFP), PMMA) to immobilize the electrolyte. This parameter change prevents oxidation at high voltage while maintaining ionic conductivity.
Solution Approach 2:
The patent creates a composite gel-type electrolyte system combining polymer matrices (PVDF, P(VdF-HFP), or PMMA) with linear carbonate solvents and lithium salts. This composite structure provides both mechanical stability and electrochemical stability at high voltages, resolving the contradiction between high-voltage operation and electrolyte stability.
2Object-affected harmful factors
If lithium titanium oxide is used as anode material to prevent lithium dendrites, then safety under strong current is improved, but reduction of electrolyte compounds occurs at the anode forming soluble passivation layers
Solution Approach 1:
The patent changes the electrolyte composition from cyclic carbonates to linear carbonates in a gel matrix, which alters the reduction potential and reaction kinetics at the anode. This prevents the formation of soluble passivation layers while maintaining the protective function against lithium dendrites enabled by lithium titanium oxide.
Solution Approach 2:
The gel matrix acts as a stable, non-consuming medium that replaces the short-lived cyclic carbonate molecules which decompose at the anode. The gel-type structure with linear carbonates provides long-term stability without forming soluble degradation products.
3Power
If high-voltage cathodic material is used to offset voltage drop, then cell voltage is improved, but oxidation and reduction reactions of electrolyte over cycling degrade the electrolyte
Solution Approach 1:
The patent employs a composite gel electrolyte system combining stable polymer matrices (PVDF, P(VdF-HFP), PMMA) with linear carbonate solvents. This composite structure provides robust resistance against both oxidation at high voltage cathodes and reduction at anodes, enabling sustained cycling performance while maintaining high cell voltage.
Solution Approach 2:
The gel matrix creates an inert chemical environment that protects the linear carbonate solvents from oxidation and reduction reactions. The polymer network immobilizes the electrolyte components, preventing their participation in parasitic reactions at the electrodes during cycling.
4Productivity
If manganese and titanium species migrate between electrodes, then chemical species cross-talk occurs, but this leads to degradation of electrodes and reduced cell lifetime
Solution Approach 1:
The gel electrolyte acts as an intermediary medium between the anode and cathode. The polymer matrix and linear carbonate composition create a chemically stable environment that prevents migration and cross-talk of manganese and titanium species, while still allowing efficient lithium ion transport for high charge-discharge rates.
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 solution provides extended cycling lifetime, improved safety, reduced impedance, and decreased gas generation, while maintaining performance from ambient to 60°C, thus addressing the stability and migration issues in lithium-ion cells.
Implementation Method 1
A gel-type electrolyte comprising a poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix in which is embedded a liquid mixture comprising at least one lithium salt and a solvent comprising at least one linear carbonate
Implementation Method 2
oxidation is observed at the cathode of some compounds of the electrolyte solvent, in particular cyclic carbonates... the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix representing 5 to 95% by weight relative to the weight of the gel-type electrolyte, and the liquid mixture representing 95 to 5% by weight relative to the weight of the gel-type electrolyte
Implementation Method 3
when the electrolyte is subjected to a low voltage e.g. in the range of 1 to 1.5 V versus the Li+/Li couple, reducing of some electrolyte compounds is seen at the anode
Implementation Method 4
The migration is also observed of titanium species contained in Li4Ti5O12 of the anode towards the cathode. This cross-talk of chemical species between the anode and cathode leads to degradation of the anode and cathode
Data Source
AI summary
A gel-type electrolyte comprising a matrix which is a poly(vinylidene fluoride-co-hexafluoropropylene) polymer in which is embedded a liquid mixture comprising at least one lithium salt and a solvent comprising at least one linear carbonate, the poly(vinylidene fluoride-co-hexafluoropropylene) polymer matrix representing 5 to 95% by weight in relation to the weight of the gel-type electrolyte and the liquid mixture representing 95 to 5% by weight in relation to the weight of the gel-type electrolyte. This electrolyte exhibits increased stability with respect to oxidation and reduction.