Crosslinked Polyolefin Separator for Controlled Battery Shutdown
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Solution Overview
Problem
Existing lithium secondary batteries face safety issues due to a large difference between shutdown and meltdown temperatures, leading to potential fires or processing difficulties, and performance degradation from increased resistance in crosslinked polyolefin porous membranes.
Innovation Solution
A lithium secondary battery with a crosslinked polyolefin porous membrane that maintains a specific numerical range between shutdown and meltdown temperatures, controlled by siloxane or peroxide crosslinking bonds, and uses a Ni-rich positive electrode active material to manage self-heating temperatures, ensuring safety and processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If silane-modified polyethylene crosslinking reaction is promoted in the extruder using a crosslinking catalyst master batch, then crosslinking reaction 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: first, silane grafting occurs during extrusion to modify the polyethylene; second, the actual crosslinking reaction occurs after membrane formation through moisture exposure or heat treatment. This segmentation prevents resin aggregation during extrusion while achieving the desired crosslinked structure in the final product, thereby maintaining both production efficiency and membrane homogeneity.
Solution Approach 2:
The silane modification is performed as a preliminary action during the extrusion process, preparing the polyethylene for subsequent crosslinking without completing the full crosslinking reaction at that stage. This preliminary grafting allows the resin to remain processable during membrane formation, while the crosslinking structure develops later to provide the required thermal and mechanical properties without causing aggregation issues.
2Quantity of substance
If membrane thickness is reduced to achieve smaller battery cell sizes, then energy density is improved, but membrane rupture resistance at high temperature deteriorates
Solution Approach 1:
The separator is constructed as a composite material combining polyethylene base resin with silane-modified crosslinked structures. This composite approach creates a three-dimensional network within the thin membrane that significantly enhances mechanical strength and thermal stability. The crosslinked gel fraction (30-80%) provides structural reinforcement that compensates for the reduced thickness, enabling membranes of 15 µm or less to maintain rupture temperatures above 200°C while achieving high energy density.
Solution Approach 2:
The crosslinked gel structure is distributed throughout the membrane matrix to provide localized reinforcement. This creates regions of enhanced strength and thermal resistance within the thin membrane structure, allowing the overall membrane to maintain integrity at high temperatures despite reduced thickness. The gel fraction acts as a distributed reinforcement network that prevents catastrophic failure.
3Temperature
If crosslinking reaction is enhanced to improve heat resistance, then membrane rupture temperature is improved, but radical generation increases causing electrolyte decomposition
Solution Approach 1:
The crosslinking chemistry is changed from conventional peroxide-based systems to silane-based moisture-curing or condensation crosslinking. This parameter change in the chemical reaction mechanism achieves the desired crosslinked network structure and high membrane rupture temperature (above 200°C) while significantly reducing harmful radical generation. The silane crosslinking proceeds through hydrolysis and condensation reactions that are less prone to generating free radicals compared to peroxide decomposition.
Solution Approach 2:
The silane crosslinking system creates a more chemically inert crosslinking environment compared to peroxide systems. The moisture-curing or thermal condensation mechanism of silane crosslinking avoids the aggressive radical chemistry of peroxide decomposition, thereby protecting the electrolyte from degradation while still achieving the necessary thermal and mechanical properties through the crosslinked gel structure.
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 battery achieves improved safety and processing by controlling the temperature differences, maintaining safety through controlled shutdown and meltdown temperatures, and enhancing resistance, while using a Ni-rich positive electrode for high capacity and cycling characteristics.
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
heat release due to interior battery short circuiting is inhibited by a shutdown function
Data Source
Figure 1
Figure 2(a)~2(b)
Figure 3(a)~3(b)
AI summary
The present disclosure relates to a lithium secondary battery comprising a positive electrode, a negative electrode and a separator interposed between the positive electrode and the negative electrode, wherein the separator includes a crosslinked polyolefin porous membrane having a shutdown temperature of 125 to 145°C, a difference between the shutdown temperature and a meltdown temperature ranging between 20 and 80°C, and a meltdown temperature that is higher by 5 to 35°C than a self-heating temperature of the positive electrode. The lithium secondary battery according to the present disclosure has improvements in both safety and processing.