An enhanced poly(vinylidene fluoride) matrix separator with TEOS for good performance lithium-ion batteries
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The low crystallinity poly(vinylidene fluoride)/tetraethyl orthosilicate silane (PVDF/TEOS) composite separator with a finger-like pore structure for lithium-ion battery has been successfully prepared by non-solvent-induced phase separation (NIPS) technique. The PVDF/TEOS composite separator shows the excellent wettability and electrolyte retention properties compared with Celgard 2320 separator. AC impedance spectroscopy results indicate that the novel PVDF/TEOS composite separator has ion conductivity of 1.22 mS cm−1 at 25 °C, higher than that of Celgard 2320 separator (0.88 mS cm−1). The lithium-ion transference number of PVDF composite separator added 0.7% TEOS was 0.481, better than that of Celgard 2400 (0.332). What is more, the lithium-ion batteries assembled with PVDF/TEOS composite separator show good cycling performance and rate capability.
KeywordsSeparator Phase inversion PVDF/TEOS Lithium-ion battery
Scientists and engineers have long believed in the promise of batteries to change the world. The ubiquitous battery has already come a long way, especially lithium-ion batteries. Owing to their high energy density and outstanding cycle life, lithium-ion batteries are now widely employed in commercial electronics, and also furthered into surging markets such as electrical vehicles . Separator, which is sandwiched between the cathode and the anode to prevent the physical contact of the electrodes while enabling free ionic transport and electronic flow, directly affects the interface structure , the internal resistance [3, 4, 5], the battery capacity [2, 3, 6, 7, 8, 9, 10, 11], cycle characteristics , and safety performance. Essentially, a separator should be chemically and electrochemically stable toward the electrolyte and electrode materials and must be mechanically strong enough to withstand handing during battery assembly; moreover, it should have sufficient porosity to absorb liquid electrolyte for high ionic conductivity. The most common separator materials used in lithium-ion batteries are polyethylene (PE) and polypropylene (PP). They have good mechanical stress property and polypropylene (PP) separator has good thermodynamic stability. In addition, the SiO2/PVDF composite separator combined with the good mechanical strength of PP nonwoven has been prepared and applied to lithium-ion batteries . The addition of nanocrystalline cellulose (NCC) has improved the electrolyte retention and storage modulus of PVDF-HFP composite separator . What’s more, the single ion polymer electrolyte which is comprised of polymeric lithium tartaric acid borate salt (PLTB) and PVDF-HFP has been used to improve cellulose composite separator. The enhanced composite separator shows higher ionic conductivity, good flame retardancy, and superior thermal resistance compared to the commercial polypropylene (PP) separator . However, the polyethylene (PE) and polypropylene (PP) separators have low porosity, poor wettability with polar liquid electrolyte, and large thermal shrinkage at high temperatures. These drawbacks affect cell resistance, energy density, rate capability, and safety of lithium-ion batteries [12, 16, 17, 18].
A promising way to overcome these problems is using room temperature ionic liquids (RTILs), which has several attractive features, such as chemical and thermal stability, non-volatility, and intrinsic high ionic conductivity at room temperature which results in the improved safety in case of overcharging. Recently, gel polymer-based electrolytes (GPE), including polyethylene oxide (PEO) , cellulose , polyacrylonitrile (PAN) , poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) , and poly(vinylidene fluoride) (PVDF) , have been widely investigated, because of their high ionic conductivity, electrochemical stability, and electrolyte wettability.
But the poor strength of GPE hindered the application in batteries . Due to the strong electron-withdrawing functionality in C–F in the molecular structure, PVDF becomes one of the typical polymers which are used as the gel polymer separator . The connectivity of pore and crystallinity of PVDF separators contribute to the lithium-ion conductivity, because the well-interconnected pore can reduce the distance of ion migration and the lower crystallinity can improve the area of amorphous domain . However, the mechanical stress and ion conductivity could not afford the requirements of power batteries.
Composite separators combine the advantages of GPE and polyolefin separators. They have good mechanical stress property, wettability, and electrolyte retention properties. Researchers work on the preparation of composite separator and investigate the properties of different composite separators, but the structure of composite layers has not been investigated. In this paper, PVDF/Celgard 2320 composite separator with a novel finger-like pore structure has been prepared by non-solvent-induced phase separation (NIPS) method. In addition, PVDF composite layer with low crystallinity is obtained by adding tetraethyl orthosilicate silane (TEOS).
Preparation of PVDF/TEOS composite separator
Twelve grams pristine PVDF (PVDF, HSV900, Arkema Inc. Mw = 1000,000 g mol−1) and 1 g polyvinyl pyrrolidone (PVP, K30, Damao Chemical Reagent Co., Ltd., Tianjin, China) were mixed in 86.8 g N,N-Dimethylacetamide (DMAc, Sinopharm Chemical Reagent Co., Ltd., Ningbo, China) in a conical flask with continuously stirring. Then, 0.2 g TEOS (Sinopharm Chemical Reagent Co., Ltd., Ningbo, China) was added into the conical flask. The mixture was continuously stirred at 70 °C for 12 h to obtain the homogeneous casting solution. Then the conical flask containing the homogeneous casting solution was stoppered and placed in a vacuum oven at 70 °C overnight to eliminate the air bubbles. Composite separator was prepared by coating process; thereafter, the casting solution was cast on Celgard 2320 by a home-made casting knife. The thickness of PVDF layer was about 10 μm. The wet composite separator was left in air for 10 s at 25 °C and was then immersed into the mixing coagulation bath (Wt deionized water:Wt DMAc = 8:2) for 12 h. The resulting separator was immersed into deionized water for 3 days to remove the residue solvent. Finally, the composite separator was dried in vacuum oven at 50 °C for 12 h to remove the residue water.
Electrode preparation and cell assembly
A cathode slurry comprising 80% LiCoO2 (Hunan Shanshan Battery Materials Co. Ltd., China), 10% PVDF (Aladdin, Mw = 150,000 g mol−1), 10 wt% carbon black (Super P, Timcal), and N-methyl-1-pyrrolidone (NMP, Aldrich) were casted on aluminum foil by using a doctor blade and dried at 60 °C for 12 h. A lithium metal was used as counter electrode in this study. Half cells (CR2025-type coin cell) were assembled by sandwiching the prepared membranes between a LiCoO2 cathode and a lithium metal within an argon-filled glove box (Universal 2440/750) with a low moisture level (< 1 mg L−1). The liquid electrolyte was 1.0 M LiPF6–ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1:1:1, v/v/v) (Duoduo Reagent Co. Ltd., China).
Morphology of composite separators after cycled at 1 C rate for 300 cycles was studied by scanning electron microscope (SEM, JSM5600L, Japan). Crystallinity of composite separators was investigated by X-ray diffraction (XRD, Rint2000, Japan).
Thermogravimetric analysis (TGA) of the separators was carried out by 704 F1 Phoenix. The samples were tested in an argon atmosphere at a heating rate of 5 °C min−1.
The diffuse reflectance infrared spectrometry (DRIFT) was employed to measure the infrared spectra (IR) of the samples in the range of 400–4000 cm−1 with the resolution 2 cm−1 on a BRUKER VECTOR-22 spectrometer.
The half cells were assembled in an argon-filled glove box and sealed in 2025-type coin cells. Galvanostatic charge/discharge cycling was performed at voltage range of 3.0–4.3 V in a Neware Battery Test System (Shenzhen Neware New Energy Tech. Co. Ltd).
Results and discussion
Ionic conductivity and liquid uptake of different separators
PVDF composite separator
PVDF/TEOS composite separator
Liquid uptake (%)
Ionic conductivity (mS cm−1)
PVDF/TEOS composite separator with the novel finger-like structure was successfully prepared via NIPS. The PVDF/TEOS composite separator has ion conductivity of 1.22 mS cm−1 at 25 °C and the lithium-ion batteries assembled with the PVDF/TEOS composite separator exhibit good cycling performance and rate capability compared with the commercial separator. Hence, the facile preparation of PVDF/TEOS composite separator provides a method to modify the commercial separator for lithium-ion battery.
The authors received financial support from The National Key R&D Program of China (Grant no. 2018YFB0104203).
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