Silane-Modified Polyolefin Separator for Shutdown and Rupture Resistance
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Solution Overview
Problem
Existing separators for lithium ion batteries face challenges in achieving thin, homogeneous membranes with high-temperature membrane rupture resistance and shutdown functions, while also ensuring stable production and safety.
Innovation Solution
A method for producing a separator that involves extruding a silane graft-modified polyolefin, polyethylene, and plasticizer into a sheet, followed by biaxial stretching, porous body formation, heat treatment, affinity treatment, crosslinking, and washing and drying steps to create a microporous membrane with specific 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 deteriorates
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 deteriorates
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.
3Reliability
If shutdown temperature is lowered below 150° C. for safety, then shutdown function is improved, but membrane rupture temperature must be maintained as high as possible
Solution Approach 1:
The separator employs functionally differentiated layers: a shutdown layer made of non-crosslinked polyethylene that melts at low temperature (below 150°C) to close pores and stop ion transport, and a silane crosslinked polyolefin layer that maintains dimensional stability and prevents rupture at high temperatures (200°C or higher). Each layer performs its specific thermal function independently.
Solution Approach 2:
The multilayer composite combines materials with different thermal behaviors: non-crosslinked polyethylene for low-temperature shutdown and silane crosslinked polyolefin for high-temperature structural support. This material combination enables the separator to exhibit both low shutdown temperature and high rupture temperature characteristics simultaneously.
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 achieves both shutdown function and high-temperature membrane rupture properties, improving the output, cycle characteristics, and safety of lithium ion batteries, while 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).