Design of Block‐Copolymer Nanoporous Membranes for Robust and Safer Lithium‐Ion Battery Separators

Abstract Lithium‐ion batteries (LIBs) suffer from unsatisfied performance and safety risks mainly because of the separators. Herein, a block copolymer (BCP) composed of robust and electrolyte‐affinitive polysulfone (PSF) and Li+‐affinitive polyethylene glycol (PEG) is rationally designed to prepare a new type of LIB separator. The copolymer is subjected to selective swelling, producing nanoporous membranes with PEG chains enriched along the pore walls. Intriguingly, when used as LIB separators, thus‐produced BCP membranes efficiently integrate the merits of both PSF and PEG chains, endowing the separators thermal resistance as high as 150 °C and excellent wettability. Importantly, the nanoporous separator is able to close the pores with a temperature of 125 °C, offering the battery a thermal shutdown function. The membrane exhibits ultrahigh electrolyte uptake up to 501% and a prominent ionic conductivity of 10.1 mS cm−1 at room temperature. Batteries assembled with these membranes show excellent discharge capacity and C‐rate performance, outperforming batteries assembled from other separators including the extensively used Celgard 2400. This study demonstrates a facile strategy, selective swelling of block copolymer, to engineer high‐performance and safer LIB separators, which is also applicable to produce advanced copolymer‐based separators for other types of batteries.


Preparation of SFEG separators
The SFEG separators were prepared by knife-coating and selective swelling process.
Then the solution was mechanically stirred for 4 h at room temperature to ensure polymers dissolve completely. Subsequently, 10-15 mL SFEG solution was dropped onto a clean glass and coated at a gate height of 100 μm by knife. After coating, the wet SFEG film was dried at 120°C for 5 min and separated from the glass by immersing in DI water. The SFEG membranes were then vacuum dried at 80°C for 12 h to remove residual solvent and water. Thus-produced dense SFEG self-supporting film was immersed into 60°C acetone/n-propanol (1:4 wt/wt) mixture solution for 4 h to generate pores. After swelling, the SFEG separator was vacuum dried at 60°C for 5 h and then stored at room temperature before further use.

Characterization
The surface and cross-sectional morphologies of SFEG separators were observed using a field-emission scanning electron microscope (SEM, Hitachi S4800, Japan). The samples were soaked into liquid nitrogen and quickly fractured to obtain cross-sectional morphology.
Before examination, the samples were vacuum sputtered a thin layer of Pt/Au alloy to enhance their conductivities. A software namely NanoMeasurer was used to statistically analyze the pore size of the SFEG membranes.
The wettability of SFEG separators and Celgard 2400 separators were measured by dropping a same volume liquid electrolyte on the sample surface and compared the wetting situation of separators after 5 s. Water contact angle tests were performed on a goniometer (DropMeter A100P, Maist).
Fourier transform infrared spectrometer (FTIR, Nicolet 8700, Thermo Fisher Scientific) was applied to reveal the composition of SFEG membranes before and after selective swelling, and the test was conducted at attenuated total reflection mode. A universal testing machine (CMT-6203, MTS) was used to test the tensile strength of the SFEG membrane. Thermal gravimetric analysis was obtained by a Netzsch STA 409PC thermal analyzer with the temperature ranging from 25 to 800°C under a heating rate of 10°C min -1 in N 2 .
The porosity (ε) of SFEG membranes and Celgard 2400 was calculated by the following equation: where ρ (g cm -3 ) is the density of pure water, A (cm 2 ) is the membrane area, and l (cm) is the membrane thickness. W 1 and W 0 are the weight of the membrane before and after saturating in pure water. Please note that before weighing the excessive water on the membrane surface is carefully removed after soaking in water for a sufficient duration.
The liquid electrolyte uptake of separators was measured by the weight difference of separators before and after liquid electrolyte soaking for a certain time.
where P (%) is the electrolyte uptake of separators, W 0 (g) is the weight of dry separators and W (g) is the weight of wet separators after soaking into liquid electrolyte.
The thermal shrinkage of SFEG separators and Celgrad 2400 separators was measured by heating at different temperatures (25, 50, 75, 100, 125 and 150°C) for 1 h. The separators were fixed on the glass slide to prevent the curl of separators during heating process. The thermal shrinkage is calculated by the following equation: where η (%) is the thermal shrinkage of separators, S (m 2 ) and S 0 (m 2 ) are the area of separators before and after heating.
The ionic conductivity of the SFEG separators and Celgard 2400 separators was measured by AC impedance measurement using an electrochemical workstation (CHI 604D, Shanghai Chenhua Apparatus, China). The separator was sandwiched between two stainless steel (SS) blocking electrodes and assembled in a CR2032 coin cell. The impedance was measured in the frequency ranging from 1 Hz to 10 5 Hz at an amplitude of 5 mV. The ionic conductivity of separators was calculated by the following equation: the thickness and effective area of the separators, respectively.
The electrochemical stability of the SFEG separators and Celgard 2400 separators was examined through linear sweep voltammeter (LSV) in a cell of lithium foil/separators/SS at the scanning rate of 2 mV/s over the potential ranging from 3 to 5.5 V.
To measure the electrochemical performances, the prototype lithium-ion batteries were assembled through sandwiching fabricated SFEG separators (or Celgard 2400 separators) between LiFePO 4 cathode and lithium metal anode in CR2032 coin cells. Previous to assemble cells, separators were dried at 70°C for 24 h under vacuum to remove water absorbed in separators. All cells were assembled inside argon-filled glove box with oxygen and water content <0.1 ppm to avoid absorbing water. Charge-discharge cycle performance was tested at room temperature in a battery test system (LAND CT2001A, Wuhan Lanhe, China) between 2.5 V and 4.2 V at 0.2 C rate. The C-rate performance of lithium-ion batteries with SFEG separators/Celgard 2400 separators was measured in a potential range of 2.5-4.2 V at different current densities (0.2, 0.5, 1.0 and 5.0 C). The long-cycle test was also performed by using the SFEG membrane prepared with a gate height of 200 μm.    Cycle number Figure S6. Long-cycle discharge capacity of LIBs with SFEG membranes at 1C. Table S1. Fabrication methods of polymeric separators for LIBs.

Fabrication method Preparation process
Electrospinning 1. Polymer is dissolved in organic solvent to prepare solution with a certain of concentration.
2. The polymer solution is electrospun at high voltage with appropriate spinning distance and feed rate.
3. Solvent, concentration, voltage and distance are considerably important factors for the electrospinning method.
4. The electrospinning process is carried at a fixed temperature and humidity.
5. Additionally, thermal treatment or mechanical pressing is carried out to increase the mechanical and stability of membrane.