Polyolefin Separator Crystallinity and Layering for Battery Safety
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Solution Overview
Problem
Polyolefin microporous membranes used in lithium ion secondary batteries face challenges in maintaining mechanical strength and shutdown properties while preventing electrolyte depletion, especially when complexed with heat-resistant porous layers, and are affected by the volume change of high-capacity electrode materials during charging and discharging.
Innovation Solution
A polyolefin microporous membrane with specific crystal parameters, including a degree of crystallinity of 65-85%, a lamellar crystal/crystal ratio of 30-85%, crystal length of 5-50 nm, and amorphous length of 3-30 nm, is developed, along with a heat-resistant porous layer containing inorganic fillers like aluminum hydroxide, to enhance mechanical strength and shutdown properties, and an adhesive porous layer with vinylidene fluoride resin is applied to improve electrolyte retention.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a polyolefin microporous membrane is covered with a heat resistant porous layer to improve heat resistance, then heat resistance is improved, but the shutdown function is restrained
Solution Approach 1:
The separator is divided into multiple functional layers: a polyolefin microporous membrane layer (providing shutdown function) and a heat resistant porous layer (providing heat resistance). Each layer independently performs its specific function without interfering with the other, resolving the contradiction between shutdown function and heat resistance
Solution Approach 2:
The separator uses a composite structure combining polyolefin material (for shutdown function) with heat resistant porous material (for heat resistance). This composite approach allows both shutdown function and heat resistance to coexist by leveraging the complementary properties of different materials
2Reliability
If the composition of the polyolefin microporous membrane is made to have high flowability to improve shutdown function, then shutdown function is improved, but mechanical strength decreases
Solution Approach 1:
The separator structure separates the shutdown function (handled by polyolefin composition and melting characteristics) from the mechanical strength support (handled by the heat resistant porous layer and overall structural design). This allows optimization of each function independently
Solution Approach 2:
The invention optimizes specific parameters of the polyolefin microporous membrane including degree of crystallinity (30-80%), porosity (30-80%), and pore size (0.01-1 μm) to achieve both adequate mechanical strength and improved shutdown function through controlled melting behavior
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 provides a polyolefin microporous membrane with improved mechanical strength, shutdown properties, and electrolyte retention, ensuring safety and battery performance by preventing electrolyte depletion and maintaining stability during temperature increases and electrode volume changes.
Implementation Method 1
when the temperature of the battery abnormally increases, polyolefin melts and holes of a porous membrane are blocked
Implementation Method 2
inorganic particulates such as alumina are contained in a heat resistant porous layer, to improve heat resistance
Implementation Method 3
a lithium ion secondary battery in which a lithium-containing transition metal oxide such as lithium cobaltate is used as a positive electrode, and carbon material which is capable of doping and dedoping lithium is used as a negative electrode
Data Source
AI summary
A polyolefin microporous membrane, the membrane having, when measured by DSC, a degree of crystallinity of from 65 to 85%, a lamellar crystal/crystal ratio of from 30 to 85%, a crystal length of from 5 nm to 50 nm and an amorphous length of from 3 nm to 30 nm, and a polyolefin microporous membrane, the membrane having, when measured by X-ray diffractometry, crystal size of from 12.5 nm to 13.5 nm and a degree of crystallinity of from 64 to 68%.