Lithium ion battery using crosslinkable separator

A separator for an electricity storage device comprising a polyolefin microporous membrane, wherein:

• in solid viscoelasticity measurement of the separator for an electricity storage device at a temperature of −50° C. to 250° C.,
• the minimum of the storage modulus is 1.0 MPa to 10 MPa, the maximum of the storage modulus is 100 MPa to 10,000 MPa, and
• the minimum of the loss modulus is 0.1 MPa to 10 MPa and the maximum of the loss modulus is 10 MPa to 10,000 MPa,
• the conditions for the solid viscoelasticity measurement to measure the storage modulus and loss modulus being 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:
• 200 μm to 400 μm (with the proviso that when the membrane thickness of the sample alone is less than 200 μm, the dynamic viscoelasticity measurement is carried out by stacking multiple samples so that their total thickness is 200 μm to 400 μm)
• Measuring temperature range:
• −50° C. to 250° C.
• Temperature-elevating rate:
• 10° C./min
• Measuring frequency:
• 1 Hz
• Transform mode:
• sine wave tension mode (linear tension)
• Initial static tensile load:
• 0.2 N
• Initial gap distance (at 25° C.):
• 10 mm
• Auto strain adjustment:
• Disabled;
• (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 0.1%, 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 5%, and when the sine wave load is 0.1 N or lower, the vibrational stress is measured with the static tensile load fixed at 0.1 N;
• (iv) the storage modulus and loss modulus are calculated from the relationship between the obtained sine wave load and amplitude value, and the following formulas:
• σ=σ0·Exp[i(ωt+δ)],
• ε*=ε0·Exp(iωt),
• 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.