Elsevier

Electrochimica Acta

Volume 212, 10 September 2016, Pages 416-425
Electrochimica Acta

Al2O3/PVdF-HFP-CMC/PE separator prepared using aqueous slurry and post-hot-pressing method for polymer lithium-ion batteries with enhanced safety

https://doi.org/10.1016/j.electacta.2016.07.016Get rights and content

Abstract

A composite separator is prepared to improve the safety of lithium ion batteries (LIBs) based on a gravure coating aqueous slurry and post-hot-pressing method. An environmentally friendly aqueous slurry with an Al2O3 ceramic holder and polyvinylidene difluoride-hexafluoropropylene (PVdF-HFP) and carboxymethyl cellulose (CMC) dual-binders is coated on a polyolefin (PE) substrate to form a prototype Al-PHC/PE separator. Then, after assembling the separator in the batteries and hot-pressing at 70 °C and 0.8 MPa for 3 h, the granular PVdF-HFP is transformed into a colloidal structure containing an electrolyte, which can binds the Al2O3 nanoparticles together and increases the battery hardness. Compared with a PE separator (9 μm), the Al-PHC/PE-2 separator (12 μm, with a Al2O3/PVdF-HFP weight ratio of 7/3) displays a comparative ionic conductivity of 9.3 × 10−4 S cm−2 and exhibits no obvious thermal deformation at 110 °C for 60 min. All of the batteries assembled with Al-PHC/PE-2 separators passed nail penetration and impact tests. In addition, the capacity retention increases from 83.4% to 87.6% when the battery is assembled with Al-PHC/PE-2 instead of a PE separator and charge-discharged at 0.7C/1.0C for 350 cycles. The enhanced safety and cycle performance indicate the promising prospect of Al-PHC/PE separators in LIBs.

Introduction

As a result of the safety performance, high energy density and acceptable lifetimes of lithium ion batteries (LIBs), they have become the dominant power source for portable electronic devices over the past two decades [1], [2], [3]. However, with the increasing demand for a higher energy density and capacity, especially for the application as the power sources for hybrid electric or all-electric vehicles (HEVs or EVs), the safety of LIBs has recently become a significant concern [4]. As an indispensable part of LIBs, separators can prevent internal short circuits, store electrolytes, and provide pathways for the migration of lithium ions. Thus, the technical improvement of the separator is urgently needed to meet the safety requirement for large scale applications of LIBs.

Until now, micro-porous polyolefin (PE) separators have been widely adopted in commercialized LIBs because of their favorable integrated properties and low-cost. However, PE separators shrink, soften, or even melt at elevated temperatures, which can cause short circuiting or even combustion. Therefore, improvements in the thermal resistance are critical for batteries with PE-based separators. In practice, pure PE separators usually exhibit inherently hydrophobic properties, low melting points (∼135 °C) and poor wettability, resulting in poor electrolyte uptake and low thermal stabilities [5], [6], [7]. To solve these problems, PE separators are generally modified through the anchoring of other functional compositions to improve the thermal-resistances and porosities of polymer LIBs. Jeong et al. coated acrylonitrile (AN) and methyl methacrylate co-polymer on a commercial PE membrane to improve the electrolyte affinity of the membrane; an adhesive gel formed and provided the efficient transport of lithium ions through the solid electrolyte interphase [8]. Roh et al. adopted a polyarylester (PAR) solution to coat a PE membrane to prepare a porous separator using tetrahydrofuran (THF) as the solvent through a non-solvent induced phase separation (NIPS) method, decreasing the ionic conductivity of the separator markedly compared to the liquid electrolyte resulting from the formation of a gel layer after absorbing the electrolyte [9]. Later, a ceramic coating technology was developed to relieve the limitations of organic polymer-modified PE separators using inorganic nano-materials (Al2O3 and SiO2) as supports because of their chemical inertness, high pore-forming abilities, good wettability and good thermal stability [10], [11]. In addition, the Al2O3-nanoparticles were usually used as supports to modify the surface of polyethylene separators [12], [13]. The separators exhibited a good wettability in non-aqueous liquid electrolytes and a better capacity retention than LIBs with PE separators. Jeong et al. developed a composite separator layer consisting of alumina nanoparticles and polyvinylidene difluoride-hexafluoropropylene (PVdF-HFP) using a phase inversion technique to balance the thermal shrinkage performance and good interface performance [14], [15]. Still, the phase inversion of PVdF-HFP needed a considerable amount of acetone and high explosion-proof device; an enhanced interfacial resistance was also found during the long cycling process. Furthermore, the coating operation must adhere to rigorous requirements on for the equipment and slurry parameters, and the high cost for the equipment and production also hinders its wide application in commercial LIBs.

Herein, based on an analysis of the preceding studies, an Al2O3-PVdF-HFP-CMC/PE composite (Al-PHC/PE) separator has been prepared for polymer LIBs with enhanced safety based on an aqueous slurry and post-hot-pressing method. Notably, with the addition of hydrophilic carboxymethyl cellulose (CMC) binder, the fluid and viscosity of the aqueous slurry can be easily adjusted to facilitate the coating operation. In the first step, an Al-PHC/PE composite separator prototype is prepared using a PE film as the substrate, ceramic Al2O3 nanoparticles as the support, PVdF-HFP as the primary binder, and CMC as an accessory binder and slurry adjuster. Al2O3 nanoparticles are chosen as the support because of their good heat resistance. The PVdF-HFP has a high ionic conductivity, which can improve the migration speed of lithium ions. A self-made micro gravure coating machine is adopted to ensure the uniformity of the coating layers, and the key is the replacement of the organic acetone solvent with water to mix the ceramic Al2O3 nanoparticles and PVdF-HFP. In contrast to the earlier organic method, the PVdF-HFP existing in the composite separator prototype appears in a granular state to improve the space for the immersion of electrolyte into a PE substrate, and each island-like particle looks like an irregular spherical landmine. These landmine-like defects are disadvantageous and can even compromise the safety of LIBs. Therefore, in the second step, a hot-pressing process used in LIB fabrication is carried out to eliminate the defects during the electrochemical formation process to solve this problem. The temperature and pressure parameters are accurately controlled to ensure the mines can be cleared efficiently. During the hot-pressing post-treatment, the PVdF-HFP particles melt and transformed into colloidal PVdF-HFP. Meanwhile, more Al2O3 nanoparticles can be bound together by the colloidal PVdF-HFP and steadily anchored on the PE substrate. The results show that the Al-PHC/PE separator displays an excellent thermal stability and a high ionic conductivity compared to conventional PE separators. In addition, all the batteries with an Al-PHC/PE separator pass the nail penetration and impact tests; furthermore, they exhibit an enhanced cycle performance and rate capability compared to PE/LIBs, indicating the great potential of Al-PHC/PE separators for practical LIB applications. In this paper, Al-PHC/PE separators are prepared, and the evaluation of their safety and performances are presented.

Section snippets

Materials and chemicals

A PE separator with a thickness of 9 μm and porosity of 41% (W-SCOPE, Korea) is used as a substrate. The Al2O3 particle with an average size of 0.5 μm (LianLian Chemical Group, China), polyvinylidene fluoride particles (PVdF-HFP, LBG, Arkema, France), and carboxymethyl cellulose sodium (CMC, Mw = 400000, Daicel, Japan) were all purchased. The electrolyte was 1 M LiPF6 dispersed in the mixture of ethylene carbonate, diethylcarbonate, propylene carbonate and methyl ethyl carbonate (EC:DEC:PC:EMC = 

Preparation and characterization of the Al-PHC/PE separator

A self-made machine was used to coat the aqueous slurry of Al2O3, PVdF-HFP and CMC on both sides of the PE substrate via a gravure coating method. To restrict the space of the separators in the LIBs, the coated thickness on each face of the PE substrate was controlled to approximately 1.5 μm. The coating roll, with a diameter of approximately 60 mm, was etched by overlapping the curve on the surface of the roller. The operational directions of the anilox roll and the PE substrate were opposite.

Conclusions

A composite Al-PHC/PE separator was designed based on a gravure coating aqueous slurry and post-hot-pressing method to improve the safety of lithium ion batteries. An environmentally friendly aqueous slurry with an Al2O3 ceramic holder and polyvinylidene difluoride-hexafluoropropylene (PVdF-HFP) and carboxymethyl cellulose (CMC) binders was coated on polyolefin (PE) substrate to form a prototype Al-PHC/PE separator. The colloidal PVdF-HFP held more Al2O3 nanoparticles together and maintained

Acknowledgements

This work was financially supported by the science and technology projects of Guangdong Province (2013B090500025, 2015A040404043, 2016A050502054), the science and technology projects of Guangzhou (2014J4100027, 2014Y2-00012), the innovation team project of Guangdong Province (2013N079) and the scientific research foundation of graduate school of South China Normal University (2015lkxm29).

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