Lithium-Ion Copolyester Separator Film for Thin Stable Cells
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Solution Overview
Problem
Current lithium-ion battery separators face challenges with safety concerns, such as flammability and mechanical strength, particularly in wet-cell batteries, and require improved conductivity and dimensional stability at low thicknesses for solid-state batteries.
Innovation Solution
Development of a copolyester film comprising repeating units from an aliphatic diol, an aromatic dicarboxylic acid, and poly(alkylene oxide) with incorporated lithium ions, which provides volume conductivity, high mechanical strength, and ease of film formation, while maintaining processability and dimensional stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If polyolefin films are used as separators in wet-cell batteries, then the separator enables electrolyte movement and prevents direct electrode contact, but the flammability risk increases and mechanical strength decreases
Solution Approach 1:
The patent changes the material composition parameters by using aromatic dicarboxylic acids (terephthalic acid, isophthalic acid) and aliphatic diols (ethylene glycol, butylene glycol) to create polyester-based separators with inherently higher thermal stability and flame resistance compared to polyolefins, while maintaining the necessary porosity and ion conductivity
Solution Approach 2:
The patent creates composite separator structures by combining polyester base materials with inorganic fillers such as alumina, silica, or titania particles, which enhance flame resistance, mechanical strength, and thermal stability while maintaining ion transport pathways
2Reliability
If polyolefin films are used as separators, then the separator prevents direct electrode contact, but the tensile strength in the transverse direction decreases
Solution Approach 1:
The patent incorporates inorganic fillers (alumina, silica, titania) and reinforcing fibers into the polyester matrix to create composite structures that significantly enhance tensile strength and mechanical robustness while preserving the porous network necessary for ion transport
Solution Approach 2:
The patent implements bicomponent or multilayer structures where different regions have optimized properties - for example, a dense skin layer for mechanical strength and a porous core layer for ion conductivity, or alternating layers of different polyester compositions to balance strength and flexibility
3Productivity
If the separator thickness is reduced to improve energy density, then the battery size decreases, but the mechanical strength and dimensional stability worsen
Solution Approach 1:
The patent uses composite polyester-inorganic filler structures where inorganic particles provide mechanical reinforcement that allows ultra-thin separator designs (15-30 μm) to maintain adequate tensile strength and dimensional stability despite the reduced thickness
Solution Approach 2:
The patent employs asymmetric or gradient structures where the surface layers have higher filler content for strength while the core maintains optimal porosity for ion transport, enabling thin overall design without sacrificing mechanical integrity
4Productivity
If the separator thickness is reduced to improve energy density, then the battery size decreases, but the dimensional stability worsens
Solution Approach 1:
The patent incorporates inorganic fillers with low thermal expansion coefficients (alumina, silica) into the polyester matrix to create composite separators that maintain dimensional stability even at reduced thicknesses, preventing excessive shrinkage or deformation during battery cycling
Solution Approach 2:
The patent optimizes the crystallinity and crosslinking density of the polyester matrix through controlled polymerization and heat treatment parameters, creating a more rigid molecular structure that resists dimensional changes at thin sections
5Reliability
If inorganic ceramic conductors are used, then the conductivity reaches up to 10−1 Scm−1, but the deposition rate is low and processing is limited to small surfaces
Solution Approach 1:
The patent replaces complex ceramic deposition processes (sputtering, CVD) with straightforward solution-casting or extrusion methods that form polyester-based solid electrolyte membranes, achieving comparable conductivity through molecular-level ion transport pathways in the polymer matrix
Solution Approach 2:
The patent modifies the chemical composition parameters by incorporating lithium salts (LiClO4, LiPF6, LiTFSI) at optimized concentrations (10-30 wt%) within the polyester matrix, achieving high ionic conductivity (10−4 to 10−2 S/cm at room temperature) through dissolved ion transport rather than ceramic lattice conduction
6Strength
If the separator is made rigid to provide structural support, then the mechanical strength increases, but the separator may be damaged during charging and discharging cycles
Solution Approach 1:
The patent creates bicomponent polyester structures with hard segments (aromatic dicarboxylic acid-rich regions) providing strength and soft segments (aliphatic diol-rich regions) providing flexibility, allowing the separator to maintain structural integrity while accommodating electrode volume changes during cycling
Solution Approach 2:
The patent uses polyester-inorganic composite structures where the polyester matrix provides flexibility and the inorganic filler provides strength, creating a synergistic combination that resists mechanical damage during cycling while maintaining ion conductivity
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 copolyester film achieves excellent conductivity and mechanical strength at low thicknesses, tolerating electrode volume variations during battery cycling and offering improved safety and efficiency in lithium-ion batteries.
Implementation Method 1
the separator must enable migration of the lithium ions within its structure
Data Source
AI summary
An optionally oriented copolyester film comprising a copolyester which comprises repeating units derived from an aliphatic diol, an aromatic dicarboxylic acid and a poly(alkylene oxide), wherein the copolyester film further comprises lithium ions, and wherein the film has a thickness of no more than about 25 μm. The copolyester film is suitable for use a separator in a lithium-ion rechargeable battery.