Fluoropolymer Separator for Heat-Resistant Secondary Battery
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Solution Overview
Problem
Lithium secondary batteries face challenges with heat resistance and ion conductivity, as existing separators are prone to heat-shrinkage and swelling in high-temperature environments, leading to potential short-circuits and reduced battery performance due to inadequate bonding between inorganic particles and binder polymers.
Innovation Solution
A separator using a specific fluoropolymer with a high content of vinylidene fluoride and tetrafluoroethylene, combined with metal oxide or metal hydroxide particles, provides excellent heat resistance and ion conductivity, ensuring firm bonding and stability even at high temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a heat-resistant layer formed from vinylidene fluoride/hexafluoropropylene copolymer and inorganic particles is stacked on the separator, then the dimension stability of the separator at high temperatures is improved, but the binder polymer dissolves in the electrolyte solution at 80°C or higher, causing the heat-resistant layer to collapse and fall
Solution Approach 1:
The patent changes the chemical composition parameters of the binder polymer by selecting specific fluorinated polymers (polytetrafluoroethylene, polyvinylidene fluoride, or their copolymers) that maintain stability in carbonate electrolyte solutions at high temperatures. This parameter change resolves the dissolution issue while preserving dimension stability.
Solution Approach 2:
The patent creates a composite heat-resistant layer combining fluorinated binder polymer with inorganic particles (alumina, silica, titania, zirconia, magnesia). This composite structure provides both the dimensional stability from inorganic particles and the chemical stability from fluorinated polymers, preventing layer collapse.
2Reliability
If the separator uses polyolefin porous membranes to prevent electrode contact, then the basic separation function is achieved, but the separator is easily heat-shrunk and has poor dimension stability at high temperatures
Solution Approach 1:
The patent stacks a heat-resistant composite layer containing inorganic particles and fluorinated binder polymer on the polyolefin separator. This composite layer maintains the separator's separation function while providing heat resistance and dimensional stability, preventing heat-shrinkage at high temperatures.
Solution Approach 2:
The patent applies the heat-resistant layer specifically on the surface of the separator where thermal exposure occurs. This local application provides heat resistance where needed while maintaining the overall structure and separation function of the separator.
3Reliability
If the separator blocks microporous membrane to achieve shutdown function, then the safety function is improved, but the separator is heat-shrunk and broken causing internal short-circuit at high temperatures
Solution Approach 1:
The patent forms a composite heat-resistant layer with inorganic particles and fluorinated binder polymer on the separator. This composite structure reinforces the separator, preventing heat-shrinkage and structural breakdown that would cause internal short-circuits, while preserving the shutdown function.
Solution Approach 2:
The heat-resistant layer acts as a protective cushioning layer that prevents the separator from direct thermal damage. This beforehand protection maintains structural integrity during thermal events, preventing catastrophic failure and internal short-circuits.
4Stability of the object's composition
If aramid polymer with low swellability is used in the heat-resistant layer, then the dimension stability is improved, but the polar functional group causes oxidative decomposition at the interface between positive electrode and separator
Solution Approach 1:
The patent changes the chemical composition by selecting fluorinated polymers with specific properties that resist both swelling and oxidation. The carbon-fluorine bonds in these polymers provide exceptional oxidation resistance while maintaining dimensional stability, eliminating the harmful oxidative decomposition effect.
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 enhances the durability and high-rate performance of lithium secondary batteries, maintaining stability and preventing internal short-circuits, even under high-temperature and high-voltage conditions, with improved adhesiveness and resistance retention after repeated charge-discharge cycles.
Implementation Method 1
a composite porous membrane formed from a fluoropolymer and inorganic particles... the fluoropolymer being present in an amount of 50 mass% or less in the composite porous membrane
Implementation Method 2
the separator blocks the microporous membrane to achieve the shutdown function... preventing an excessive current flow by cutting off a current when an abnormal current passes through the battery
Implementation Method 3
to enable the flow of the electrolyte solution through the pores of the separator so as to form a channel for ionic conduction between the electrodes
Data Source
AI summary
The present invention aims to provide a separator for secondary batteries which has excellent heat resistance and ion conductivity, which is less likely to be heat-shrunk, which is less likely to swell with an electrolyte solution even at high temperatures, and in which a composite porous membrane including a binder polymer and at least one type of inorganic particles selected from the group consisting of metal oxide particles and metal hydroxide particles firmly bonds to a porous body. The present invention relates to a separator for secondary batteries, including a porous substrate; and a composite porous membrane disposed on or above the porous substrate. The composite porous membrane includes: a fluoropolymer that includes a polymerized unit based on vinylidene fluoride and a polymerized unit based on tetrafluoroethylene; and at least one type of inorganic particles selected from the group consisting of metal oxide particles and metal hydroxide particles. The fluoropolymer includes 40 mol% or more of the polymerized unit based on vinylidene fluoride in all the polymerized units and having a weight average molecular weight of 200,000 to 2,000,000. The fluoropolymer is present in an amount of 50 mass% or less in the composite porous membrane.