Electrolyte Composition for Lithium-Ion Cells
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Solution Overview
Problem
Lithium-ion rechargeable electrochemical elements have a limited lifespan when used at high temperatures (above 80°C) or low temperatures (below -10°C), experiencing rapid degradation of constituents leading to short circuits, increased internal resistance, and reduced cycling performance due to the dissolution of the passivation layer on the negative electrode.
Innovation Solution
An electrolyte composition comprising a tetrafluorinated or hexafluorinated lithium salt, lithium bis(fluorosulfonyl)imide, vinylene carbonate, ethylene sulfate, and lithium difluorophosphate, with specific ratios and concentrations to stabilize the passivation layer and maintain lithium ion availability across a wide temperature range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the ratio of ethylene sulfate to vinylene carbonate is increased to improve low-temperature performance, then low-temperature operability is improved, but the passivation layer dissolves rapidly leading to decreased cycling performance at high temperatures
Solution Approach 1:
The patent optimizes the ESA/VC mass ratio to be greater than 0.5 and less than 2, and controls the LiPO2F2 content to be 0.5-2% by mass. This parameter optimization prevents excessive dissolution of the passivation layer while maintaining low-temperature performance, resolving the contradiction between low-temperature operability and high-temperature cycling stability
Solution Approach 2:
The electrolyte uses a composite formulation combining ethylene sulfate, vinylene carbonate, and lithium difluorophosphate in specific proportions. This composite approach creates a balanced electrolyte system that maintains passivation layer stability across both low and high temperature conditions, addressing the contradiction through material composition optimization
2Reliability
If the concentration of lithium difluorophosphate is increased to approach the solubility limit to improve performance, then performance is enhanced, but LiPO2F2 crystals appear limiting electrolyte filling quality
Solution Approach 1:
The patent precisely controls the LiPO2F2 concentration to be within 0.5-2% by mass of the electrolyte, which is below the solubility limit. This parameter control prevents crystal formation while maintaining the performance benefits of LiPO2F2, resolving the contradiction between performance enhancement and filling uniformity
3Reliability
If the electrolyte composition is optimized for high-temperature cycling to prevent passivation layer dissolution, then high-temperature cycling performance is improved, but low-temperature performance may be compromised
Solution Approach 1:
The patent optimizes the ESA/VC mass ratio to be greater than 0.5 and less than 2, which prevents excessive passivation layer dissolution at high temperatures while maintaining adequate low-temperature fluidity and ionic conductivity, thus resolving the contradiction between high-temperature cycling performance and low-temperature operability
Solution Approach 2:
The electrolyte employs a composite system of ethylene sulfate, vinylene carbonate, and lithium difluorophosphate where each component contributes different properties: ESA provides high-temperature stability, VC ensures low-temperature performance, and LiPO2F2 enhances overall cycling stability. This composite approach balances performance across the temperature range
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 electrolyte composition extends the lifespan of lithium-ion electrochemical elements by maintaining capacity retention over 200 cycles with minimal capacity loss, even at extreme temperatures, and reduces electrolyte decomposition and gas formation, enhancing the element's performance and stability.
Implementation Method 1
The formation of a new passivation layer to replace the dissolved one consumes lithium ions from the electrolyte
Implementation Method 2
Vinylene carbonate, ethylene sulfate, and lithium difluorophosphate act as additives that contribute to the stabilization of the passivation layer
Implementation Method 3
They can also represent 30% to 99% of the total lithium ions in the electrolyte composition
Implementation Method 4
The electrolyte composition includes at least one organic solvent in which the following compounds are dissolved
Implementation Method 5
lithium difluorophosphate act as additives that contribute to the stabilization of the passivation layer (SEI, or Solid Electrolyte Interface) that forms on the surface of the negative electrode
Implementation Method 6
The presence of lithium difluorophosphate reduces the chemical decomposition of the electrolyte when the element is stored at high temperatures
Data Source
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AI summary
An electrolyte composition for the lithium-ion electrochemical element, comprising: - at least one tetrafluorinated or hexafluorinated lithium salt, - lithium bis(fluorosulfonyl)imidide salt LiFSI, - vinylene carbonate, - ethylene sulfate, - lithium difluorophosphate, - at least one organic solvent selected from the group consisting of cyclic or linear carbonates, cyclic or linear esters, cyclic or linear ethers and a mixture thereof, the ratio of the mass of ethylene sulfate to the mass of vinylene carbonate before addition to the solvent being strictly less than 1, the mass percentage of lithium difluorophosphate representing less than 1% of the mass of the whole consisting of said at least one tetrafluorinated or hexafluorinated lithium salt, lithium bis(fluorosulfonyl)imidide salt and said at least one organic solvent.The use of this composition in a lithium-ion electrochemical element increases the lifespan of the element, particularly under low and high temperature cycling conditions.