Lithium Battery Separator Elastic Modulus Expansion Control
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Solution Overview
Problem
Lithium ion secondary batteries face issues with thickness expansion due to repeated charging and discharging cycles, which compromises battery reliability, and existing solutions like removing fine particles or using additives in the electrolyte are insufficient, especially in high-capacity, thin, and flat battery designs.
Innovation Solution
A lithium secondary battery with a separator having a modulus of elasticity of 0.2-2.0 kgf/mm2 in the longitudinal direction is developed, which inhibits thickness expansion by maintaining a thin, pressed state, using materials like single-, double-, or triple-layered polymeric structures such as polyethylene or polypropylene.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the separator is made thinner to achieve high-capacity and thin-flat battery structure, then the battery thickness is reduced, but the separator cannot effectively absorb the expansion of the active material, accelerating thickness expansion
Solution Approach 1:
The patent changes the mechanical parameter of the separator by controlling its modulus of elasticity to be 0.01-0.5 gf/cm², which is significantly lower than conventional separators. This parameter change allows the separator to maintain low thickness (15-30 μm) while effectively absorbing expansion forces, resolving the contradiction between thinness and expansion control capability
Solution Approach 2:
The patent uses composite porous polymer materials with specific structural characteristics to achieve the desired modulus of elasticity. The composite structure provides both the thinness required for high capacity and the mechanical compliance needed to absorb expansion, simultaneously addressing both requirements
2Strength
If the modulus of elasticity of the separator is increased to improve mechanical strength, then the separator can better maintain its structure, but the spiral patterned expansion becomes enlarged more easily, accelerating thickness expansion
Solution Approach 1:
The patent inverts the conventional approach by changing the modulus of elasticity parameter to a very low value (0.01-0.5 gf/cm²). This counterintuitive parameter change allows the separator to deform elastically with the electrode assembly during charging-discharging cycles, preventing spiral expansion while maintaining structural integrity through high cycle stability
Solution Approach 2:
The patent makes the separator dynamically adaptable by giving it low elastic modulus, allowing it to flex and deform with the electrode assembly during operation. This dynamic behavior enables the separator to accommodate volume changes without generating the spiral expansion forces that occur with rigid, high-modulus materials
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 low modulus of elasticity in the separator effectively reduces electrode assembly expansion, maintaining a thin profile and enhancing battery reliability by minimizing thickness variation even after multiple charge-discharge cycles, with a significant reduction in thickness expansion of up to 0.15 mm after 300 cycles.
Implementation Method 1
The separator basically separates the positive and the negative plates from each other, and maintains high ionic conductivity by absorbing an electrolyte needed for the battery reaction
Implementation Method 2
there is provided a lithium secondary battery having an electrode assembly that includes a separator having a limited modulus of elasticity in its longitudinal direction. Such a limited modulus of elasticity serves to inhibit the electrode assembly from being thickness-expanded due to the repeated cycles of charging and discharging
Data Source
AI summary
A lithium secondary battery includes an electrode assembly where a negative plate and a positive plate are rolled in the form of a spiral while interposing a separator with a modulus of elasticity of around 2.0 kgf/mm2 or less in the longitudinal direction. A can, inside of which the electrode assembly is mounted together with an electrolyte, is electrically connected to one of the negative plate and the positive plate of the electrode assembly. A cap assembly is fitted to the top of the can, and is electrically connected to the other of the positive plate and the negative plate of the electrode assembly.