Silane-Modified Battery Separator for Shutdown and Rupture Resistance
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Solution Overview
Problem
Existing separators for lithium ion batteries face challenges in achieving both high-temperature membrane rupture resistance and low-temperature shutdown functionality, while maintaining stable cycle charge-discharge performance and ensuring battery safety.
Innovation Solution
A method for producing a separator that involves constructing a microporous membrane using a silane graft-modified polyolefin and polyethylene, followed by specific steps such as biaxial stretching, plasticizer extraction, heat treatment, affinity treatment, and crosslinking using an organometallic catalyst, to achieve the desired properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If silane-modified polyethylene crosslinking reaction is promoted in the extruder using a crosslinking catalyst master batch, then crosslinking efficiency is improved, but resin aggregates are generated and homogeneity of physical properties is lowered
Solution Approach 1:
The crosslinking process is divided into two separate stages: (1) silane grafting to polyethylene during extrusion to form silane-modified polyethylene, and (2) subsequent crosslinking reaction in a moisture environment after extrusion. This segmentation prevents premature crosslinking that would cause resin aggregates, while ensuring homogeneous distribution of silane groups before crosslinking occurs.
Solution Approach 2:
The silane modification is performed as a preliminary action during the extrusion process, preparing the polyethylene with reactive silane groups that will crosslink later. This preliminary grafting ensures uniform distribution of crosslinking sites throughout the resin before the actual crosslinking reaction takes place in the moisture environment, preventing localized aggregation.
2Productivity
If the separator membrane is made thinner to reduce battery cell size, then energy density is improved, but membrane rupture resistance at high temperature is reduced
Solution Approach 1:
A multilayer composite structure is employed combining different polyolefin layers with distinct functions: a shutdown layer (non-crosslinked polyethylene) for safety shutdown at low temperature, and a silane crosslinked polyolefin layer for high-temperature mechanical strength. This composite structure enables thin membrane design while maintaining rupture resistance through the crosslinked layer's thermal stability.
Solution Approach 2:
Different regions of the separator are assigned different properties: the shutdown layer provides low-temperature pore closure for safety, while the silane crosslinked layer provides high-temperature structural integrity. This local differentiation of material properties allows the thin separator to simultaneously achieve both safety shutdown function and high-temperature rupture resistance.
3Temperature
If gel fraction and crosslinked sections are increased to improve high-temperature membrane rupture properties, then rupture temperature is improved, but shutdown function may be compromised
Solution Approach 1:
The separator is segmented into functionally distinct layers: a shutdown layer with controlled gel fraction (3-20%) for low-temperature pore closure, and a silane crosslinked layer with high gel fraction for high-temperature structural support. This segmentation allows each layer to optimize its specific function without compromising the other.
Solution Approach 2:
Different gel fraction levels are localized to different layers: the shutdown layer has lower gel fraction (3-20%) to maintain pore openness at normal temperatures for ion transport, while the silane crosslinked layer has high gel fraction to provide thermal stability and prevent rupture at elevated temperatures. This spatial differentiation of gel fraction resolves the contradiction between shutdown function and rupture resistance.
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 resulting separator exhibits improved high-temperature membrane rupture resistance, enhanced shutdown functionality, and increased safety and stability for lithium ion batteries, while also inhibiting the generation of resin aggregates during production.
Implementation Method 1
it has gradually come to light by experimentation that high-temperature membrane rupture properties can be exhibited by constructing silane crosslinked sections (a gel structure) in a polyolefin separator
Implementation Method 2
constructing silane crosslinked sections (a gel structure) in a polyolefin separator
Implementation Method 3
In order to ensure battery safety, separators must have both an active shutdown function and high membrane rupture temperature
Implementation Method 4
the methods described in PTLs 4, 5 and 6 propose providing a plasticizer extraction step
Data Source
AI summary
There is provided a method for producing a separator for an electricity storage device that includes a step of contacting a porous body formed from a silane-modified polyolefin-containing molded sheet with a base solution or acid solution, and a separator for an electricity storage device comprising a microporous film with a melted film rupture temperature of 180° C. to 220° C. as measured by thermomechanical analysis (TMA).