Fibrillated Cellulose Separator for Battery Internal Resistance
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Solution Overview
Problem
Current separators for electrochemical elements, such as lithium ion secondary batteries and capacitors, face issues with low mechanical strength, high internal resistance, and poor processability due to insufficient electrolyte-retaining ability and adhesive strength between layers, leading to defects like internal short-circuits and poor discharge characteristics.
Innovation Solution
A porous sheet separator made from fibrillated solvent spun cellulose fibers with an average length of 0.40 to 1.10 mm, produced using a paper-making method with wet press treatment and heated cylinder roll processing, achieving an average internal bond strength of 60 mJ or more and lowest internal bond strength of 30 mJ or more in the thickness direction, enhancing mechanical strength and processability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a film-shaped porous film made of polyolefin is used as a separator, then the separator structure is simple and easy to manufacture, but the electrolyte-retaining ability is low and ion conductivity is low
Solution Approach 1:
The patent uses a composite structure combining polyolefin fibers and cellulose fibers to create a nonwoven fabric separator. The polyolefin provides structural integrity and ease of manufacture, while the cellulose fibers enhance electrolyte retention and ion conductivity, resolving the contradiction between manufacturing simplicity and electrolyte-retaining ability.
Solution Approach 2:
The patent applies different fiber types in specific proportions (polyolefin 30-70 wt%, cellulose 30-70 wt%) to create regions with different properties within the separator. This local differentiation allows the separator to simultaneously achieve good manufacturability from polyolefin and high electrolyte retention from cellulose regions.
2Device complexity
If nonwoven separators made of synthetic fiber are used, then the separator structure is simple, but the internal resistance becomes high and discharge characteristics are poor
Solution Approach 1:
The patent creates a composite nonwoven fabric combining synthetic polyolefin fibers and natural cellulose fibers. This composite structure maintains the simplicity of nonwoven construction while the cellulose component significantly improves ion conductivity and discharge characteristics, reducing internal resistance without increasing structural complexity.
Solution Approach 2:
The patent optimizes the fiber composition parameters by controlling the weight ratio of polyolefin to cellulose fibers (30-70 wt% each) and adjusting fiber diameter and length parameters. These parameter changes enable the separator to achieve low internal resistance and excellent discharge characteristics while maintaining a simple nonwoven structure.
3Length of stationary object
If the separator is formed by laminating two or more fiber layers, then the separator thickness is reduced, but the adhesive strength between layers is insufficient
Solution Approach 1:
The patent uses a composite of polyolefin and cellulose fibers within a single nonwoven layer, eliminating the need for multiple laminated layers. This approach achieves thin separator thickness while maintaining high internal bond strength through the synergistic interaction between fiber types, avoiding the delamination issues of laminated structures.
Solution Approach 2:
The patent extracts the need for multi-layer lamination by achieving the desired thin profile and strength within a single nonwoven fabric layer. By utilizing the composite fiber structure, the separator achieves sufficient mechanical strength and low thickness without requiring additional adhesive layers or complex lamination processes.
4Adaptability or versatility
If the separator needs to be cut as necessary, then the separator can be adapted to different applications, but fiber naps generated at the cut section deteriorate characteristics
Solution Approach 1:
The patent's composite fiber structure with controlled internal bond strength (30-100 mJ) ensures that when the separator is cut, the fibers hold together well at the cut edges. This reduces fiber napping and maintains the integrity of cut sections, allowing the separator to be adapted to different applications without compromising reliability.
Solution Approach 2:
The patent optimizes the internal bond strength parameter of the nonwoven fabric to a specific range (30-100 mJ) that balances cuttability with edge integrity. This parameter optimization allows the separator to be easily cut and adapted to different applications while minimizing fiber naps and maintaining cut section quality.
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 separator exhibits improved mechanical strength, reduced fiber napping and falling during cutting, and enhanced electrolyte-retaining ability, resulting in lower internal resistance and better discharge characteristics for electrochemical elements.
Implementation Method 1
a porous sheet that contains as a main fiber, a fibrillated solvent spun cellulose fiber... has an average internal bond strength of 60 mJ or more... enhanced electrolyte-retaining ability
Implementation Method 2
produced using a paper-making method with wet press treatment and heated cylinder roll processing
Data Source
AI summary
A separator for electrochemical element, comprising a porous sheet that contains as a main fiber, a fibrillated solvent spun cellulose fiber with an average fiber length of 0.40 to 1.10 mm, has an average internal bond strength of 60 mJ or more in a thickness direction, and has a lowest internal bond strength of 30 mJ or more in the thickness direction, a process for producing the separator, and an electrochemical element using the separator. The separator for electrochemical element has strong mechanical strength and excellent processability.