Microporous Battery Separator Resin Control for Fewer Defects
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Solution Overview
Problem
Existing microporous membranes for power storage devices suffer from defects during material mixing and extrusion steps, leading to aggregation and reduced cycle characteristics, and existing methods do not adequately address the impact of resin materials on these processes.
Innovation Solution
A method involving sheet casting, stretching, microporous membrane formation, heat treatment, and winding, using polypropylene and polyethylene with controlled molecular weights and crystallite sizes, and specific loading methods to produce a separator with reduced defects and improved cycle characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional polyolefin resins are used in mixing and extrusion steps, then production is simplified, but defects occur leading to aggregation and reduced cycle characteristics
Solution Approach 1:
The invention changes the molecular weight parameter of the polyolefin resin to 500,000 or greater, and controls the weight-average molecular weight to number-average molecular weight ratio (Mw/Mn) to be 3 or greater. These parameter changes resolve the contradiction by enabling complete melting and uniform dispersion during extrusion (reducing defects) while maintaining excellent adhesion to negative electrodes and inhibiting SEI growth (improving cycle characteristics).
Solution Approach 2:
The invention uses a composite resin composition comprising polyolefin resin and non-polyolefin resin, where the polyolefin component has specifically controlled molecular weight characteristics. This composite approach allows the polyolefin to provide defect-free extrusion and excellent electrode adhesion, while the non-polyolefin resin contributes to microporous structure formation, collectively resolving both manufacturing precision and reliability requirements.
2Reliability
If high molecular weight polyolefin resin is used to improve strength and adhesion, then cycle characteristics improve, but mixing and extrusion become difficult due to incomplete melting
Solution Approach 1:
The invention optimizes two critical parameters simultaneously: (1) increases the weight-average molecular weight to 500,000 or greater for excellent adhesion and cycle characteristics, and (2) controls the Mw/Mn ratio to be 3 or greater to ensure uniform melting and dispersion. This dual parameter control resolves the contradiction by making high molecular weight resin processable while maintaining its superior adhesion properties.
Solution Approach 2:
The invention performs preliminary mixing of the polyolefin resin with non-polyolefin resin and liquid paraffin before extrusion. This preliminary action ensures uniform distribution of components and facilitates complete melting during extrusion, even for high molecular weight polyolefin resin, thereby resolving the manufacturing difficulty while preserving the resin's excellent adhesion characteristics.
3Reliability
If separator films undergo repetitive compression during charge-discharge, then ion permeability decreases due to crushed pores, but capacity maintenance is required
Solution Approach 1:
The invention changes the molecular weight parameters of the polyolefin resin (weight-average molecular weight ≥500,000 and Mw/Mn ≥3) to create a resin composition that forms a robust microporous structure. This structure maintains its pore diameter integrity under repetitive compression during charge-discharge cycles, preventing crushing while ensuring excellent capacity maintenance through superior adhesion to negative electrodes.
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 method results in a separator with reduced defects and improved cycle characteristics, enabling better adhesion to negative electrodes and inhibiting excessive solid electrolyte interface growth, thus enhancing the performance of power storage devices.
Implementation Method 1
a microporous membrane forming step in which the plasticizer is extracted from the stretched sheet to form a microporous membrane
Implementation Method 2
a heat treatment step in which the microporous membrane is subjected to heat treatment
Implementation Method 3
the polyethylene has a weight-average molecular weight (Mw) of 100,000 to 9,700,000, and the ratio (Mw/Mn) of the weight-average molecular weight (Mw) to number-average molecular weight (Mn) is 3 to 12
Data Source
AI summary
Provided is a method for producing a separator for a power storage device, the method including: a step for extruding powdered polyethylene, pelletized polypropylene, and a plasticizer into a sheet form using an extruder to form a molded body; and a step for making the molded body porous by a wet method.