Crosslinkable Li-Ion Battery Separator for Shutdown and Rupture Resistance
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Solution Overview
Problem
Current lithium ion battery separators face challenges in achieving high-temperature membrane rupture resistance, shutdown function, and maintaining cycle stability and safety, particularly due to limitations in silane crosslinking reactions and the formation of resin aggregates.
Innovation Solution
A separator for electricity storage devices is developed, comprising a silane-modified polyolefin that crosslinks during contact with the electrolyte solution, allowing for controlled crosslinking timing and avoiding production defects. This separator also features a crosslinked structure formed in the amorphous portion of the polyolefin, which enhances mechanical strength and thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If silane crosslinking is performed during separator production, then high-temperature membrane rupture resistance is improved, but production defects occur due to resin aggregate formation
Solution Approach 1:
The silane modification is performed preliminarily during resin preparation, but the actual crosslinking reaction is delayed until after separator production. The silane groups are pre-introduced into the polyolefin structure, but remain dormant during production and only activate later through moisture or catalyst exposure during battery assembly or initial charging, thus achieving high-temperature resistance without production defects
Solution Approach 2:
The patent introduces silane-modified polyolefin as a precursor that cushions against future high-temperature risks. The silane groups are prepared in advance but do not crosslink during production, providing a latent protective mechanism that activates only when needed (after production), preventing both production defects and future thermal runaway
2Temperature
If silane crosslinked sections are constructed in polyolefin separator, then high-temperature membrane rupture properties are improved, but shutdown function may be compromised
Solution Approach 1:
The separator is designed with heterogeneous structure where only specific regions contain crosslinked gel sections while other regions remain as uncrosslinked polyolefin. This local quality differentiation allows the uncrosslinked regions to perform shutdown function at low temperatures while crosslinked regions provide high-temperature membrane rupture resistance
Solution Approach 2:
The patent carefully controls the gel fraction parameter within specific ranges (5-60% or 10-50%) to balance two opposing requirements. By adjusting this parameter, the separator achieves optimal combination of shutdown function (requiring low crosslinking) and membrane rupture resistance (requiring high crosslinking)
3Strength
If crosslinking reaction occurs during separator production, then mechanical strength is improved, but internal stress and deformation increase
Solution Approach 1:
The separator structure and dimensions are established preliminarily during production in the uncrosslinked state, avoiding internal stress. The crosslinking action is delayed until after the separator is assembled into the battery, at which point the separator is constrained by electrodes and cannot deform, thus achieving strength enhancement without stress-induced deformation
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 proposed separator achieves both low-temperature shutdown and high-temperature membrane rupture resistance, ensuring improved safety, output, and cycle stability of electricity storage devices, while also reducing internal stress and deformation during production and use.
Implementation Method 1
a crosslinked structure formed by contact between a silane-modified polyolefin-containing separator and water
Implementation Method 2
crosslinks during contact with the electrolyte solution
Implementation Method 3
an active shutdown function
Implementation Method 4
shutdown function when the degree of crystallinity and gel fraction are in specific ranges
Data Source
Figure 1~2
Figure 3
Figure 4(a)~4(b)
AI summary
A separator for an electricity storage device, comprising 5 to 40 weight% of a silane-modified polyolefin and 60 to 95 weight% of a polyolefin other than the silane-modified polyolefin, wherein the storage modulus change ratio (RΔE') is 1.5 to 20, as defined by the following formula (1): RΔE′=E′S/E′j where E'j is the storage modulus measured at 160°C to 220°C for the separator for an electricity storage device before crosslinking reaction of the silane-modified polyolefin, E'S is the storage modulus measured at 160°C to 220°C for the separator for an electricity storage device after crosslinking reaction of the silane-modified polyolefin, and the measuring conditions for the storage modulus E' (E'j or E'S) are specified by the following (i) to (iv): (i) the dynamic viscoelasticity measurement is carried out under the following conditions: • Measuring apparatus: RSA-G2 (TA Instruments) • Sample thickness: from 5 µm to 50 µm • Measuring temperature range: -50 to 225°C • Temperature-elevating rate: 10°C/min • Measuring frequency: 1 Hz • Transform mode: sine wave tension mode (linear tension) • Initial static tensile load: 0.5 N • Initial gap distance (at 25°C): 25 mm • Auto strain adjustment: Enabled (range: 0.05 to 25% amplitude, 0.02 to 5 N sine wave load); (ii) the static tensile load is the median value of the maximum stress and minimum stress for each periodic motion, and the sine wave load is the vibrational stress centered on the static tensile load; (iii) the sine wave tension mode is measurement of the vibrational stress while carrying out periodic motion at a fixed amplitude of 0.2%, wherein in sine wave tension mode, the vibrational stress is measured while varying the gap distance and static tensile load so that the difference between the static tensile load and the sine wave load is within 20%, and when the sine wave load is 0.02 N or lower, the vibrational stress is measured while amplifying the amplitude value so that the sine wave load is no greater than 5 N and the increase in the amplitude value is no greater than 25%; and (iv) the storage modulus E'is calculated from the relationship between the obtained sine wave load and amplitude value, and the following formulas: σ*=σ0⋅Expiωt+δ, ε*=ε0⋅Expiωt, σ*=E*⋅ε* E*=E′+iE" where σ ∗ : vibrational stress, ε ∗: strain, i: imaginary number unit, ω: angular frequency, t: time, δ: phase difference between vibrational stress and strain, E ∗: complex modulus, E': storage modulus, E": loss modulus, vibrational stress: sine wave load/initial cross-sectional area static tensile load: load at minimum point of vibrational stress for each period (minimum point of gap distance for each period), and sine wave load: difference between measured vibrational stress and static tensile load.