Lithium Battery Separator Thermal Shrinkage Control
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Solution Overview
Problem
Conventional lithium rechargeable batteries face thermal instability issues due to separator shrinkage, leading to potential short-circuits and explosions, especially under external impacts or high temperatures, as existing materials with high elongation tend to contract and lose mechanical strength.
Innovation Solution
A separator with a maximum thermal shrinkage rate of less than 30% in both vertical and horizontal directions, and a ratio of horizontal to vertical shrinkage between 0.8 to 1.3, is developed to enhance thermal stability, using a polyethylene sheet characterized by its minimal contraction when exposed to high temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a separator with high elongation is used to improve mechanical strength, then the separator can better resist external impacts and dendrite penetration, but the separator tends to contract and lose mechanical strength at high temperatures
Solution Approach 1:
The patent changes the physical and chemical parameters of the separator by controlling the molecular weight distribution (specifically using a bimodal or multimodal distribution with specific weight average molecular weights and polydispersity indices) and adjusting the crystallization conditions to achieve the desired balance between mechanical strength and thermal stability. This allows the separator to maintain its dimensional integrity at high temperatures while retaining adequate mechanical strength.
Solution Approach 2:
The patent employs a composite molecular weight distribution approach, combining different polymer chains with varying molecular weights in a bimodal or multimodal distribution. This composite structure allows the lower molecular weight components to provide mechanical strength and the higher molecular weight components to maintain thermal stability, effectively resolving the contradiction between these two properties.
2Strength
If the separator is thickened to improve mechanical strength and prevent short-circuits, then the separator can better resist external impacts, but the battery's energy density decreases
Solution Approach 1:
The patent optimizes the thickness parameter within a specific range (15-30 μm) and combines it with controlled molecular weight distribution and crystallization parameters to achieve the desired mechanical strength. This allows for a thinner separator compared to conventional designs, thereby maintaining higher energy density while still providing adequate mechanical protection against short-circuits.
3Stability of the object's composition
If a separator with high thermal shrinkage resistance is used to prevent short-circuits, then the separator can maintain its shape at high temperatures, but the separator's ability to provide shutdown function at lower temperatures is reduced
Solution Approach 1:
The patent creates different functional zones within the separator by controlling the molecular weight distribution. The lower molecular weight fraction provides the shutdown function at lower temperatures (around the melting point of the lower MW component), while the higher molecular weight fraction maintains thermal shrinkage resistance at higher temperatures. This local differentiation of properties within the same separator structure resolves the contradiction between shutdown function and thermal stability.
4Quantity of substance
If the separator film is made thinner to increase energy density, then more active material can be packed in the battery, but the mechanical strength and thermal stability of the separator decrease
Solution Approach 1:
The patent compensates for the reduced mechanical strength of thinner films by optimizing the molecular weight distribution parameters, specifically using a bimodal or multimodal distribution with appropriate weight average molecular weights and polydispersity indices. This allows the separator to achieve the necessary mechanical strength and thermal stability at reduced thickness, thereby enabling higher energy density without sacrificing safety.
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 improved separator design significantly increases the thermal stability of lithium rechargeable batteries, delaying the onset of fire or explosion when subjected to high temperatures, as demonstrated by thermal stability tests showing extended time before failure.
Implementation Method 1
the problem of a separator of high elongation is that it tends to contract
Data Source
Figure 1
AI summary
The present invention relates to a lithium rechargeable battery which employs a separator to minimize a short-circuit inside the battery, and has an improved thermal stability. The separator according to the present invention has a maximum thermal shrinkage of vertical direction and horizontal direction within 30%, and the ratio of maximum thermal shrinkage of horizontal direction against vertical direction ranges from 0.8 to 1.3. Therefore, the battery with highly improved thermal stability can be obtained by employing the separator not having high thermal shrinkage characteristics.